Recently developed in recent decades, the carbon isotope tracing technology is one of the most reliable methods, which has been widely used in the study of carbon (C) cycling in terrestrial ecosystems due to its high specificity and sensitivity. Here, the principle, analysis method and application process of C isotope tracing technology in C cycling in terrestrial ecosystem have been reviewed. Four different methods are currently being used in laboratory or field conditions, including natural abundance method, Free-Air Concentration Enrichment (FACE) technology coupling with ¹³C dilution method, pulse and continuous labeling with ¹³C enriched CO₂, and labeling with ¹³C enriched substrates. Results of field experiments and lab incubation experiments employing carbon isotope tracing technology were combined in order to quantify the transformation and distribution of photosynthetic C in plant-soil system. Furthermore, these techniques also help to understand the contribution of plant photosynthetic C to soil organic matter, the stabilization of soil organic matter and its microbial mechanism, to illustrate the dynamic changes of soil organic carbon (SOC), evaluate the contribution of new and old organic C to soil C storage, and estimate the micromechanism of SOC input, conversion and the stabilization in terrestrial ecosystems. Carbon cycle is affected by climate, vegetation, human activities and other factors, and therefore it is imperative to further develop a sensitive, accurate, multiscale and multidirectional isotope tracing system by combining carbon isotopes with mass spectrometry, spectroscopy and molecular biological technology. We have summarized the coupled application of carbon isotope tracing technology and the insitu detection involving molecular and biological approaches, and discussed the existing issues of carbon isotope tracing technology.

Climate warming is one key issue of global change and plays an important role on carbon cycling in terrestrial ecosystems. This paper reviews the recent advance in our understanding of global warming and its impacts on terrestrial carbon cycling and underlying mechanisms. We also discuss the state-of-the art in ecosystem modeling and its applications to ecosystem assessment. Climate warming will influence terrestrial carbon cycling in several aspects: 1) net primary productivity (NPP) will decrease in low-latitude region, and increase in mid- and high-latitude zones, totally show an increase on the global scale; 2) soil respiration will increase at the initial stage and then keep relatively stable because of biotic adaptability; 3) plant carbon storage will increase in high-latitude region, and remain stable or even decrease in low-latitude zone, and show a slight increase on the global level; 4) the production and decomposition rate of litterfall will increase; 5) the decomposition rate of soil organic carbon will increase and thus decrease soil carbon stock, meanwhile, soil carbon stock will increase for more carbon input from plant litter. These two processes will trade off in certain degree, resulting in different results for varied ecosystems. On the global scale, soil carbon stock will show a decrease; 6) although the different performances of diverse ecosystems, the global terrestrial ecosystem acts as a weak carbon source. Biophysical, biogeographical and biogeochemical models were developed in the past decades for global change research. In the future research, there are an urgent need to address the interaction among climate warming and other factors including elevated CO₂, O₃, drought, fire disturbance. The big challenge we are facing is how to deal with the complexity with multi-factors and multi-scales by using experimental and modeling approaches.

The regional characteristics of carbon storage and carbon dioxide fluxes of major Chinese forest ecosystems were studied from the points of internal biological cycle, based on published data regarding forest biomass, productivity, the organic carbon content of soil profile, stand and annual weight of the litter, soil respiration etc. The results are as follows: the average carbon density of Chinese forest ecosystem is 258.83 t·hm^-2, showing a generally increasing trend with increasing latitude; carbon density of the vegetation, soil and litter is 57.07 t·hm^-2,193.55 t·hm^-2, and 8.21 t·hm^-2, respectively; the characteristics of the carbon density of these three fractions (vegetation, soil, litter) were also analyzed; from the recent areal data provided by the Ministry of Forestry of China in 1989-1993 the total carbon storage of Chinese forests was estimated to be 281.16 ×10⁸ t, in which the vegetation, soil and litter were 62.00×10⁸ t, 210.23×10⁸ t,8.92×10⁸ t, making up 22.2% ,74.6%, 3.2%, respectively of the total, the carbon storage of deciduous broad-leaved forests, warmer temperate coniferous forests, evergreen/evergreen-deciduous broad-leaved forests, Picea-Abies forests, and Larix forests were the major carbon pool of the forest, making up 87% of the total; in China the net flux between the forest ecosystem and the atmosphere is 4.80×10⁸ t·a^-1 , and the forest ecosystem acts as a carbon sink when exchanged with the atmosphere, absorbing 48.7% of the carbon dioxide from burning of biomass, fossil fuel and human respiration (9.87×10⁸ t·a^-1). Generally, the carbon dioxide fixing capacity of the deciduous forest is higher than the coniferous forests, decreasing with increasing latitude.

Land use/cover change (LUCC) is one of the most concerned environmental problems by scientists, land managers and policy makers. LUCC can affect energy flow, biogeochemical and hydrological cycling in terrestrial ecosystems through altering land surface and species composition. Ecosystem carbon cycling responds differently to various LUCC types, showing a pattern of CO₂ release into the atmosphere when LUCC from a high-biomass forest to low-biomass grassland, cropland or urban area. Previous reports indicated that global terrestrial ecosystem released 2.21 Pg C (1 Pg C=10¹⁵ g C) per year induced by LUCC during the 1990s, which explains about 25% of the global C emission per year in the same period; and in the last two centuries, the released C from LUCC accounts for 50% of the C emission from fossil fuel combustion. The LUCC patterns are totally diversified for regions around the world, which cause obviously different C fluxes among them. The reports showed that LUCC in the tropics is a C source, while it is a C sink in the middle and high latitude regions in the northern hemisphere, which possibly explain a large part of the “missing carbon sink" in the terrestrial ecosystems. Currently, modeling is the most popular way to simulate LUCC-induced changes in ecosystem C cycling. The quantitative relationship between LUCC patterns and their related processes and ecosystem carbon cycling remains uncertain. This uncertainty causes great discrepancies in the estimation of terrestrial ecosystem CO₂ fluxes from land use/cover changes. In the near future, except for carrying on long-term experiments to determine these quantitative relationships, model development by integrating LUCC with vegetation dynamic model and ecosystem process model will be essential for making an accurate estimation of C fluxes induced by LUCC. Sound land management can greatly increase C storage in the terrestrial ecosystems during LUCC processes. However, the quantification of land management effects is not well-established yet and land management is thus not included in most simulation models of LUCC impacts, which needs more researches in the future.

Aims Drought effects on terrestrial ecosystems are a key issue in global change research. This study was designed to 1) analyze effects of drought on carbon exchange in a subtropical coniferous plantation; 2) elucidate the sensitivity of carbon exchange to different degree of water deficit and the critical values when the ecosystem converts from carbon sink to source and 3) investigate the main factors that control ecosystem carbon exchange when drought occurs.

Methods The CEVSA2 model, which incorporated several significant modifications based on the CEVSA process-based ecosystem model and has been tested by using eddy covariance observation in different forest ecosystems, was parameterized by using site-specific ecophysiological measurements. Drought scenarios were designed to analyze effects on annual carbon budget and to elucidate the main control factors.

Important findings Drought decreases ecosystem production and carbon exchange significantly. Compared with simulation of no drought effect scenario, the droughts in 2003 and 2004 decrease annual net ecosystem production (NEP) by 63% and 47%, respectively. Ecosystem photosynthesis and respiration respond to drought differently, and the more rapid decrease of gross ecosystem production (GEP) than ecosystem respiration (Re) lead to the decrease of NEP when drought occurs. As daily average vapor pressure deficit (VPD) rises above 1.5 kPa, GEP, Re and NEP begin to decrease; When VPD rises above 2.5 kPa and relative soil water content (RSW; soil water content/saturated soil water content) decreases below 40%, the ecosystem converts from a carbon sink to source. Soil water deficit, which is the main factor controlling the ecosystem carbon exchange, accounts for 46% to the decrease of total annual NEP in 2003 and 2004, and atmospheric drought accounts for only 4%.

Aims The groundwater table has changed and air pollution has been reducing solar radiation on the southern periphery of China’s Gurbantonggut Desert. Our objective was to investigate the response and adaptation of Tamarix ramosissima, a native dominant desert shrub in Central Asia, towards variation in groundwater and photosynthetically active radiation (PAR), in terms of ecophysiological activities, morphological characteristics and community carbon/water balances.

Methods During the growing season from 2005 to 2007, we carried out experiments in the original habitat of T. ramosissima, where the groundwater table fluctuated from 2.9 to 4.5 m. Photosynthesis, transpiration, leaf water potential, water-use efficiency and root distribution were examined to reveal the water-use strategy of the species, and CO₂ and H₂O fluxes above an undisturbed T. ramosissima ecosystem were measured by eddy covariance method to evaluate net carbon assimilation, water loss and leaf area index (LAI).

Important findings Physiological activity and community carbon uptake of T. ramosissima did not respond to sustained drought in upper soil or rainfall pulses, and its photosynthetic consistency is achieved by its water-use pattern. Special stomatal behavior and root distribution are two main mechanisms. Tamarix ramosissima tends to maximize its carbon gain at the cost of higher water consumption, attributable to its phreatophytic root system that ensures sufficient groundwater supply and avoids the effects of water deficiency in upper soil. Tamarix ramosissima can adapt to moderate fluctuation of groundwater table, but severe decline will threaten its survival, and hence the overexploitation of groundwater will cause severe degradation of Tamarix-dominated perennial vegetation and disturb the original ecohydrological processes in this arid region. PAR is another important environmental factor positively correlating with community carbon uptake. The LAI indicates that the seasonal pattern in community carbon assimilation represents the combined effects of groundwater table and PAR on the phenological photosynthesis capacity. It shows that the integrated study on different scales is an effective approach to further the understanding of desert shrub adaptive strategies and ecosystem processes under variable environmental conditions.

Coarse woody debris (CWD) in forest ecosystems provides critical habitat for many organisms, maintains a healthy forest structure, and is important in the biogeochemical cycling of carbon and nutrients. However, the characteristics and ecological functions of CWD are poorly documented and understood in the subtropical forests of southern China. In this study, the amount and characteristics of CWD in three typical forest ecosystem types in southern China were investigated at the Dinghushan Nature Reserve. These forests were selected to form a successional sequence with a Pinus massoniana forest, a mixed coniferous broad-leaved forest, and a monsoon evergreen broad-leaved forest representing early-, mid-, and advanced-successional stages, respectively. Both the Pinus massoniana and the mixed coniferous broad-leaved forests developed on artificial Pinus massoniana plantations planted in the 1930s. Nevertheless, these two forests were at different successional stages. The Pinus massoniana forest was harvested for leaf/needle litterfall, CWD, and undergrowth until 1990 whereas human interventions were excluded in the mixed coniferous broad-leaved forest. Results indicated that human disturbance dramatically altered the successional process of the Pinus massoniana forest and its ecological functions. Total aboveground biomass was just 35% of that of the mixed coniferous broad-leaved forest. The number of tree species that contributed to CWD increased along the successional sequence with 7, 18, and 29 species in the Pinus massoniana, mixed coniferous broad-leaved, and monsoon evergreen broad-leaved forest sites, respectively. There was almost no CWD (0.1 Mg C·hm^-2) in the Pinus massoniana forest, while CWD amounted to 8.7 Mg C·hm^-2 in the mixed coniferous broad-leaved forest and 13.2 Mg C·hm^-2 in the monsoon evergreen broad-leaved forest, representing 9.1% and 11.3% of the total aboveground biomass, respectively. Only about 10% of the CWD was standing in the mixed coniferous broad-leaved and the monsoon evergreen broad-leaved forests, suggesting that sudden forest canopy gaps created by falling or snapping of trees might be more important than gradual gaps formed by standing dead trees in the succession of these forests in this region. Although the decomposition rate of CWD was relatively fast, it was still comparable to that of the soil organic carbon in the region, suggesting that CWD can play an important role in the global carbon cycle. Keeping CWD on the forest floor is a critical strategy for maintaining forest productivity and implementing sustainable forest management in southern China.

Background and Aims Many reports on global change have predicted major change in the temporal and spatial pattern of precipitation, which may have significant effects on temperate grasslands in arid and semi-arid regions. The responses of grasslands to environmental changes, especially amount and timing of precipitation, can be very different. Some studies indicate that drought may result in degradation of ecosystem function in NEE, even changing the ecosystem from a carbon sink to a carbon source.

Methods In order to quantify net ecosystem carbon exchange in Leymus chinensis steppe and its response to precipitation, we used the eddy covariance technique to measure carbon dioxide flux during the 2005 growing season in Xilin River Basin of Inner Mongolia Plateau in 2005. Only 126 mm precipitation fell during this growing season, far less than average; therefore, the steppe was in an extreme drought condition.

Key Results The daily pattern of CO₂ uptake in this drought year was consistent bimodal, with peaks at 8∶00 and 18∶00. In normal years, the bimodal pattern occurred only when soil water stress occurred. Maximum half-hourly average CO₂ uptake was -0.38 mg·m^-2·s^-1 in 2005, which was half that in typical growing seasons. Moreover, the ecosystem was a CO₂ source most of the growing season, releasing about 0.05 mg CO₂·m^-2·s^-1 at nighttime.

Conclusions The seasonal pattern of CO₂ uptake closely followed that of aboveground biomass and was strongly affected by soil temperature and soil water content. The ecosystem emitted 372.56 g CO₂·m^-2 during the growing season in 2005. The partial explanation is that much plant litter accumulated on the ground surface due to enclosure of the grassland since 1979, and this litter decomposed and resulted in a net release of CO₂ to atmosphere.

Forests are the world’s largest carbon (C) pool and sink among the terrestrial ecosystems. The amount of C in vegetation plays an important role in the global C cycle and balance. Our objectives were to assess C density and sequestration capacity in seven typical forest types of the Xiaoxing’an Mountains, Heilongjiang Province, China and to understand the implication of the C sink to the regional C budget and future forest C management.

Field surveys were combined with laboratory analysis and allometric equations for obtaining data for a variety of variables. Seven typical forest types in the Xiaoxing’an Mountains were studied based on age groups and plant functional groups (trees, shrubs, herbaceous and litter), including Pinus koraiensis, Larix gmelinii, Pinus sylvestris var. mongolica, Picea-Abies, Betula platyphylla, Quercus mongolica, and Populus davidiana forests. Surveys were made on C density and annual carbon gains in trees, understory shrubs, herbaceous plants and litter for each forest type. The forest stands were classified into age groups for estimating the biomass and C density of the study area.

The C density of the seven forest types in different age groups varied widely. The C density per unit area for young, middle-aged, near mature and mature forests of each forest type were as follows: 31.4, 74.7, 118.4 and 130.2 t·hm^-2 in Pinus koraiensis; 28.9, 44.3, 74.2 and 113.3 t·hm^-2 in L. gmelinii; 22.8, 52.0, 71.1 and 92.6 t·hm^-2 in Pinus sylvestris var. mongolica; 23.1, 44.1, 77.6 and 130.3 t·hm^-2 in Picea-Abies; 18.8, 35.3, 66.6 and 88.5 t·hm^-2 in B. platyphylla; 25.0, 20.0, 47.4 and 68.9 t·hm^-2 in Q. mongolica; and 19.8, 28.7, 43.7 and 76.6 t·hm^-2 in Populus davidiana forests, respectively. These results show that biomass C stocks in the Xiao- xing’an Mountains play an important role in the C cycle and regional C balance. Different forest types and stands of different age groups varied greatly in C stocks. Because most growth in the seven forest types occurs in the young and middle-aged forest stands, these age groups are considered to have a great potential to increase the biomass C density. This significant C sink will be further enhanced in the Xiaoxing’an Mountains with the development and restoration designed to provide specific ecological services including C sequestration.

Aims Climate change is expected to cause changes of carbon cycling in forest ecosystems, and prediction research suggests there is dramatic spatial heterogeneity and uncertainty in responses of carbon balance of forest ecosystems to climate change. The goals of our research were to predict and analyze impacts of climate change on the carbon cycling of warm temperate forests in Beijing mountain area in the next 100 years and to understand the heterogeneity and uncertainty on the ecosystem scale with LPJ-GUESS model.

Methods The ecosystem model was used to learn how forest ecosystem productivity and carbon bal-ance change in a long-term time scale and to learn about differences of carbon balance among various ecosystems by comparing ecosystem components of carbon balance. The dynamic vegetation model (LPJ-GUESS), used for the first time in China and driven by A2 and B2 greenhouse gas emission sce-narios of the Special Report on Emissions Scenarios (SRES) of IPCC, projected climate change impacts on carbon balance for three typical warm temperate forest ecosystems (oak, birch and Chinese pine forests) in the Dongling Mountain area of Beijing, China.

Important findings Net ecosystem primary productivity (NPP) and heterotrophic respiration (Rh) will increase in the three forests, and the A2 scenario was associated with larger changes in NPP and Rh than the B2 scenario. Because of differences in species composition among the forests, increases in NPP and Rh were different and changes in net ecosystem exchange (NEE) were different among the forests: oakforest switched from a sink to a small source of carbon, birch forest remained as a smaller sink of carbon, and Chinese pine forest became a larger carbon sink over the next 100 years. Also, carbon biomass will generally increase in the forests by 2100. Comparing SRES A2 with B2, there was larger carbon storage in the relative lower emission scenario (B2). Projected differences in carbon balance among these forests in the same area were more dependent on species composition than climate factors (A2 and B2 scenarios) under future climate change.

Terrestrial ecosystems are a large carbon density and play an important role in the global carbon budget and mitigating global warming by carbon sequestration. To encourage additional carbon sequestration and storage in global vegetation and soils, we need to be clear about the distribution patterns of carbon density and the factors that influence these patterns. Therefore, the characteristics of the distribution patterns of carbon density in terrestrial ecosystems of the world and China are reviewed in this paper. At the global scale, the distribution of carbon in vegetation corresponds with the spatial pattern of biomass and generally decreases from low-latitude to high-latitude with the exception of boreal forests. In contrast, soil organic pools of carbon increase along the same gradient. The largest stores of carbon in biomass are in tropical and boreal forests, and the largest stores of soil carbon are in the high latitude ecosystems (boreal forest and tundra). Total carbon density of both vegetation and soils are highest in tropical and boreal forests. In the tropics more carbon is stored in vegetation than in soils, while in the boreal region far more carbon is stored in the soils. At the regional scale, these patterns might vary due to differences in climate, topography and human influence among regions. Several major factors, including climatic conditions, soil nutrients, biodiversity, climate and atmospheric CO2 changes, land use and cover changes, all contribute to the storage and maintenance of carbon. For example, carbon density will be high in regions where temperature and precipitation are favorable for abundant plant growth. The enhanced carbon sequestered in response to elevated levels of CO2 or nitrogen, or their combination, is less in species-poor than in species-rich regions. In a word, they can raise the carbon density in terrestrial ecosystems by directly or indirectly accelerating net primary production, or constraining respiration and decomposition. However, in spite of its great significance for explaining the present distribution patterns of carbon pools and estimating future changes, the mechanisms by which these processes occur are not fully understood. It is critical that we strengthen our research effort in this area of study. Although a great deal of effort has been put into the carbon density researches, there still remains much uncertainty regarding data collection, mechanistic explanations, and model construction in the relevant studies. In the future, we should establish a standardized and unified system to estimate carbon density, produce more accurate and useful models, and perform integrated studies at multi-scales and multi-resolutions levels.

Aims Carbon density, net production and carbon stock were estimated using data from natural spruce forests of Northwest Subalpine Sichuan.

Methods We harvested biomass, litter and soil carbon and calculated net production by dividing biomass by forest age.

Important findings The mean biomass of spruce forest is 230.37×10³ kg·hm^-2, with the tree layer accounting for 212.77×10³ kg·hm^-2 (92.36%). The percentage of carbon density in tree organs are stem 57.85%, bark 47.12%, branch 51.22%, leaf 48.27%, and root 52.39%. The percentage of carbon density in different strata are shrub 49.91%, herb 46.34%, duff 43.21%, litter 39.44%, and soil 1.41%. Carbon density declines with increased soil depth. The carbon stock is 273.79×10³ kg·hm^-2, divided among the tree layer with 109.30×10³ kg·hm^-2 (39.92%), shrub 5.69×10³ kg·hm^-2 (2.08%), herb 1.26×10³ kg·hm^-2 (0.46%), duff 0.60×10³ kg·hm^-2 (0.22%), litter 0.83×10³ kg·hm^-2 (0.30%), and soil (0-100 cm) 156.11×10³ kg·hm^-2 (57.01%). Therefore, the carbon stocks are ordered: soil (0-100 cm) > tree layer > shrub > herb > litter > duff. Mean net production is 6 838.5 kg·hm ^-2·a^-1, with the tree layer accounting for 4 676 kg·hm^-2·a^-1 (68.38%). Mean annual carbon sequestration is 3 584.98 kg·hm^-2·a^-1, with the tree layer accounting for 2 552.99 kg·hm^-2·a^-1 (71.21%).

Aims Grassland is an important component of the global terrestrial ecosystem and plays a significant role in the global carbon cycle. Knowledge of the spatial distribution of biomass and carbon density and their constraining environmental factors in different types of grasslands is crucial for revealing the variations of grassland carbon pool and understanding the grassland ecosystem carbon sequestration in China. The objective of this study was to determine the spatial patterns of biomass and carbon density distribution in natural grasslands of Hebei Province, China.

Methods The aboveground biomass, root biomass, litter mass, and their carbon densities were investigated in 390 grassland plots from 78 sites representative of six different types of natural grasslands based on vegetation, soil and climate from 2011 to 2013. The grassland types include temperate steppe, temperate meadow, temperate mountain meadow, low-land saline meadow, warm-temperate tussock and warm-temperate shrub tussock.

Important findings There were significant differences (p < 0.05) in the total biomass among the six grassland types, with the highest value of 2770.2 g·m^-2 in the low-land saline meadow and lowest value of 747.6 g·m^-2 in the temperate steppe. The low-land saline meadow also had the highest value in the aboveground biomass (285.0 g·m^-2), followed by the warm-temperate shrub tussock (235.1 g·m^-2) and the temperate mountain meadow (203.1 g·m^-2); the lowest value in aboveground biomass was found in the temperate steppe (110.6 g·m^-2). The litter mass was largest in the lowland saline meadow (584.0 g·m^-2), followed by the temperate mountain meadow (187.9 g·m^-2) and the warm-temperate shrub tussock (91.0 g·m^-2). The values of root biomass were 1.9-4.3 times greater than that of aboveground biomass across the six types of grasslands, resulting in average root:shoot ratio of 3.1. The root biomass was largest in the lowland saline meadow (1901.3 g·m^-2), and smallest in the temperate steppe with only 1/3 of that in the former. In terms of carbon density, lowland saline meadow also displayed the largest values among all the types of grasslands. The values of carbon density in the aboveground vegetation, litter and root were respectively 132.7, 81.2, and 705.9 g C·m^-2. In all grassland types, the biomass of aboveground vegetation and root, litter mass, and total biomass decreased initially and then increased with elevation (p < 0.05). With the increasing accumulative temperatures >10 °C, the root biomass and the total biomass decreased initially and then increased (p < 0.01). In this study, the warm-temperate shrub tussock mostly distributes in the rocky mountain area where the soil layer is very thin, leading to the lower biomass relatively to the temperate meadow. Therefore, climate, soil and geographical factors should be comprehensively considered when comparing the biomass among different grassland types in large area.

About 7.0 Pg (1 Pg=109 t) of carbon is annually released to the atmosphere from fossil fuel combustion and the buring and clearance of tropical forests, of which 3-3.4 PgC of the carbon adds to the atmospheric carbon pool and about 2.0 PgC is uptaken by oceans. The terrestrial biosphere is considered to hold its carbon dynamic in balance with approximately equal rates of sequestration and emission. Therefore 1.6 to 2.0 PgC per year is unattributed and this is known as the ’missing sink’. Many studies, including the monitoring of atmospheric components, analysis of forest inventories, CO2 flux measurements and modeling simulations, have suggested that the mid- and high latitude terrestrial ecosystems of the Northern Hemisphere are functioning as a significant carbon sink, though with a large uncertainty and considerablly spatio-temporal heterogeneity. Global warming, CO2 fertilization, increasing nitrogen and phosphorus deposition, and the expansion and re-growth of forests are major factors impacting the size and distribution of these carbon sinks. Study on the role of soils in the carbon cycles-as well as long-term monitoring and improvement of existing carbon model simulations is a critical step required to reduce uncertainty in the size of this sink.

Aims Solar radiation can affect net ecosystem exchange (NEE) of carbon dioxide of forests, because cloud cover alters solar radiation, which in turn alters other environmental factors such as temperature and vapor pressure deficit. Our objective was to analyze the effects of these changes on NEE of broadleaved-Korean pine (Pinus koraiensis) mixed forest in Changbai Mountain.

Methods Our analysis was based on 30-min flux data and routine meteorology data for mid-growing season (June to August) for 2003-2006.

Important findings Cloud cover significantly increased NEE. The light-saturated maximum photosynthetic rate was enhanced 34%, 25%, 4% and 11% on cloudy days compared with clear days in the four years of study. Relative irradiance and clearness index (k_t) were important in quantifying the effects of cloud cover, cloud shape and cloud thickness on solar radiation. When k_twas about 0.5, NEE reached its maximum. When the relative irradiance was over the critical relative irradiance of 37%, NEE was enhanced; maximum NEE occurred at about 75%. Enhancement ofNEE was ascribed to increased canopy assimilation and decreased above-ground respiration, which resulted from increased diffuse radiation and decreased air temperature and vapor pressure deficit with increased cloudiness.

Aims Our objective was to use multiple terrestrial carbon observations to improve existing terrestrial ecosystem models.
Methods We conducted a Bayesian probabilistic inversion to estimate the key parameter (i.e., carbon residence time) of a terrestrial ecosystem model (TECO) by using biometric and eddy covariance flux data measured at a temperate broad-leaved Korean pine forest in Changbai Mountain (CBS) of China from 2003 to 2005. We then estimated carbon stocks, carbon fluxes and uncertainties with posterior estimates of parameters. Biometric measurements consisted of foliage biomass, fine root biomass, woody biomass, litterfall, soil organic matter (SOM) and soil respiration.
Important findings Residence times of carbon for most pools can be constrained by eddy covariance flux and biometric measurements, except for the passive soil organic matter pool. Estimated residence times of C ranged from 2 to 6 months for litter and microbial biomass pools, 1 to 2 years for foliage and fine root biomass, 8 to 16 years for slow SOM pool and 77-109 and 409-1 879 years for woody biomass and passive SOM pools, respectively. Model results showed that the prediction uncertainties of carbon stocks and accumulated carbon fluxes increased with time. When air temperature increased 10% and 20%, annual gross primary productivity (GPP) increased 6.5% and 9.9%, but annual net ecosystem productivity (NEP) changed with soil temperature. If soil temperature is constant, annual NEP increased 11.4%-21.9% and 17.6%-33.1%, while if soil temperature increased 10% and 20%, annual NEP decreased to a level that was lower than that under ambient temperature. Given the same climate condition and seasonal variation for leaf area index during 2003-2005, annual NEP and soil respiration in 2020 would be 163±12 and 721±14 g C·m^-2·a^-1. Markov Chain Monte Carlo method is an effective way to estimate model parameters and to evaluate model prediction uncertainties. However, more studies are needed on a) estimation of residence time of C for passive soil organic matter, b) uncertainty analysis of input data and model structure and c) model-data fusion methods so as to improve the prediction accuracy of terrestrial ecosystem models.

Conversion of terrestrial ecosystems resulting from land-use practice plays an important role in global carbon cycling. The studies on the effects of forest felling and the conversions of forest and grassland into farmland on terrestrial carbon budget are

We applied CENTURY model (version 4.0) to simulate the impacts of forestry management practices on the carbon (C) balance of L． gmelini (Dahurian larch) forest. The model was validated using the observed data of L． gmelini forest ecosystem. The results in

Carbon is the main element of biological material, especially of plant matter. The carbon content accounts for half of the total dry biomass, so the biosphere is a big carbon pool and the carbon content of terrestrial ecosystems has significant effects on carbon biogeochemical cycling. When the carbon content of the biosphere decreases, most of the carbon is released to the atmosphere and causes the concentration of carbon dioxide in the atmosphere to increase greatly. CO2 is a green house gas; increased atmospheric concentrations will cause increased warming of the climate. In recent years, with increasing usage of fossil fuel and the destruction of natural ecosystems, the increasing release of CO2 from those processes caused the temperature of the earth’s surface to rise and then gradually level at a higher standard. The carbon cycling in forest ecosystems is closely related to CO2 concentration in atmosphere, has an obvious effect on composition of atmospheric elements, and so can influence global climate change. This study focused on the carbon distribution among each component of a typical temperate forest ecosystem of the Beijing area, China. The forest belongs to the warm temperate zone, with a continental monsoon climate which shows clear seasonal periods, dry and cold in winter, warm and humid in summer. The mean annual temperature averages 4.8℃, -10.1 ℃ in January and 18.3 ℃ in July. The annual precipitation amounts to 611.9 mm, 78 percent occurring in June, July and August. The soil is dominated by mountain brown earth. At the research site, the forest is dominated by Quercus liaotungensis, and also includes Betula dahurica, Acer mono, Populus davidiana, Betula platyphylla, Fraxinus rhynchophylla and other related species.We attempt to set up a numerical estimation of forest carbon cycling that will provide parameters of a future computer model. The carbon cycle includes many processes of a forest ecosystem, and so it is difficult to accurately estimate the amount of forest ecosystem carbon. A numerical model of the carbon cycle in this forest ecosystem was established based on studies of this warm temperate deciduous broad-leaved forest ecosystem over 10 years. The data we used in this paper are forest biomass, productivity, biogeochemical cycle, and related published results. We processed, analyzed, integrated, and transformed all the data. Finally we used a comparative method to get more insight on the carbon cycle among different forest ecosystems.Our results showed that a typical forest ecosystem in warm temperate region absorbed 10.3 t·hm-2·a-1 carbon from the atmosphere through photosynthesis. Carbon released to the atmosphere from plant respiration was 5.5 t·hm-2·a-1, with 4.8 t·hm-2·a-1 accumulated in dry plant biomass, and 2.46 t·hm-2·a-1 emitted to the atmosphere by decomposing plant litter. Most carbon assimilated by forests was respired and decomposed to the atmosphere, with little retention in living or dead biomass. Investigation on C storage of warm temperate forests found that carbon-standing amount was 165.05 t·hm-2, with 61.2 t·hm-2 in living biomass, 104.05 t·hm-2 in dead biomass (including soil carbon), and 96 t·hm-2 stored in the soil. The soil C storage in the forest studied accounted for 58% of total forest carbon, and therefore we concluded that carbon in the forest soil is the main storage pool of the forest. C storage change in the forest soil will definitely affect C change of forests in the entire region greatly.

Aims Our objective was to investigate the soil organic carbon stock, fine root turnover and its contribution to soil carbon cycling of Reaumuria soongorica community in an arid ecosystem in Xinjiang Uygur Autonomous Region, China.
Methods We selected a R. soongorica community, a typical local community, in natural areas around the Ecological Station of Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences. We investigated the distribution and dynamics of soil organic carbon, monthly fine root dynamics, production and turnover rate throughout the 2010 growing season by means of sequential coring and ingrowth core. We calculated the soil organic carbon pool, fine roots carbon stocks and contribution of fine roots.
Important findings Soil water content increased with depth, but soil organic carbon decreased. The mean fine root mass in the community was 54.51 g·m^-2, and annual fine root production was estimated to be 82.76-136.21 g·m^-2·a^-1. Fine root turnover rate in the community was 2.08 times·a^-1, and annual inputs from fine root mortality to belowground soil organic carbon was 17 g·m^-2·a^-1. These results demonstrate that due to the high turnover rate of shrub fine root in the arid region, shrub fine root carbon is a crucial portion of soil organic carbon inputs.

In this article, I first briefly introduce the concept of “Carbon Neutrality”, and then discuss the vital role of ecosystem carbon sinks in achieving the carbon neutrality target. It is assertive that any efforts to achieve the carbon neutrality target depend unavoidably on both reducing carbon emissions and enhancing carbon sequestrations. There are four key factors in reducing carbon emissions, including lowering energy consumption in human activities, restructuring fossil energy consumptions especially decreasing coal consumption, promoting energy use efficiency, and developing clean and low-carbon energy. Enhancing carbon sequestration relies inclusively on restoration, construction, and better management of the ecosystems. Benefited from enhanced vegetation growth and ecological engineering practice, Chinese terrestrial ecosystem has acted and will continue to act the significant role in the carbon sequestration. To improve the ecosystem carbon sequestration, I propose the “three-optimization principles”, i.e., optimal ecosystem arrangement, optimal species setting, and optimal ecosystem management. In addition, I also state some viewpoints on potential problems and challenges in the “post-carbon neutrality” era. It may be crucial to proactively and rationally think about the possibilities of declining global vegetation productivity and relevant new environmental issues caused by a decrease in the CO₂ concentration rising in the era.

Aims Forest fire is a major disturbance factor for forest ecosystems and an important pathway of decreasing vegetation- and soil-carbon storage. Scientifically and effectively measuring carbonaceous gases emission from forest fire is important in understanding the significance of forest fire in carbon balance and climate change. However, carbon emissions from forest fire remain unclear. Our objective was to estimate carbon emissions from forest fires from 1965 to 2010 in Daxing’an Mountains of Heilongjiang Province, China.

Methods We used a geographic information system (GIS) based modeling approach to generate emission estimates using a two-step procedure. First, we calculated total carbon released from forest fires in Daxing’an Mountains for selected years between 1965 and 2010 by merging and analyzing several measurement parameters. Second, we calculated amounts of four carbonaceous gases released during the burns, carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), and nonmethane hydrocarbon (NMHC), using several different experimentally derived emission factors. The origin of each of the inputs used in our models is based on a combination of analysis of forest fire statistics, forest resources inventory, field research and laboratory experiments.

Important findings Direct total carbon emissions from forest fires in Daxing’an Mountains during 1965-2010 are about 2.93 × 10⁷t, and mean annual carbon emissions are about 6.38 × 10 ⁵t per year, accounting for 5.64% of the direct total carbon emissions from forest fires in China. Carbon atmospheric emissions of CO₂, CO, CH₄ and NMHC from forest fires were 1.02 × 10⁸t, 9.41 × 10 ⁶t, 5.41 × 10 ⁵t and 2.11 × 10 ⁵t, respectively, and mean annual emissions of CO₂, CO, CH₄, and NMHC from forest fires were 2.22 × 10⁶t, 2.05 × 10 ⁵t, 1.18 × 10 ⁴t and 4.59 × 10 ³t, respectively, accounting for 5.46%, 7.56%, 10.54% and 4.06% of the amounts of four carbonaceous gases released from forest fires in China, respectively. Our results indicate that combustion efficiency of coniferous broad-leaved mixed forests is lower than other forest types, and burned area of coniferous broad-leaved mixed forests accounts for 21.23% of total burned area, but carbon emissions accounts for 7.81% of total carbon emissions.

Aims As one of the major terrestrial ecosystems of the world, a small fluctuation of grassland soil carbon (C) would affect the carbon cycle of the terrestrial ecosystem and ecosystem multifunctionlity (EMF). The carbon accumulation rate (CAR) of aboveground community well reflects the capacity and efficiency of carbon sequestration in a field from the start to the peak of a growing season. The changes in plant CAR could influence the ability of above- and below-ground community. Currently, the majority of studies have primarily focused on the relationship between community diversity and EMF, while the linkages of CAR with EMF were understudied. We aimed to explore the process and underlying mechanism of how CAR affecting EMF in alpine grassland community. Our results would improve the understanding of EMF maintenance mechanism and provide theoretical support for alpine ecosystem management.
Methods We conducted a field transect survey which consists of a total of 115 sample sites of alpine grasslands on the Qingzang Plateau from July to August 2015. The ecosystem multifunctionality index (M) was calculated from 13 key ecosystem parameters including soil organic carbon content, total nitrogen content, total phosphorus content above- and belowground biomass etc. The normalized difference vegetation index (NDVI, 1982-2013) was adopted to obtain the phenology in 2015. We calculated the CAR value. To explore the underlying mechanism of how CAR affecting EMF, the annual total precipitation and temperature were extracted by the method of thin disk smooth spline interpolation based on observations of meteorological stations from 2011-2015.
Important findings Belowground biomass, soil organic carbon content, total phosphorus content and microbial biomass carbon content had high weighting for CAR (0.58, 0.80, 0.83 and 0.79) and M (1.05, 0.98, 1.02 and 0.97). There was a significantly positive correlation between CAR and M (R² = 0.45, p < 0.01). Our findings suggested that the synergism of plant community and soil elements affected CAR and further regulated EMF under the influences of precipitation and temperature.

Owing to the high carbon capture and storage capacity, salt marshes are considered an effective blue carbon sink for mitigating global warming. In addition, salt marshes are likely to increase their carbon sink capacity in the future in response to climate warming and sea level rise. Therefore, the blue carbon sink function of salt marshes has received increasing attention from the international research community. This study reviewed the five aspects comprising the key processes of blue carbon formation, photosynthetic carbon allocation, burial fluxes and sources of sedimentary organic carbon, stability of soil carbon pools and the associated microbial mechanisms, and the simulation and assessment of blue carbon sequestration potentials in salt marshes. On this basis, concerning the main knowledge gaps, this paper proposes further research on the effect of vegetation distribution pattern along the land-to-sea hydrologic gradient on photosynthetic carbon fixation and allocation, the response of soil organic carbon deposition and burial to global change, the stability of soil carbon pools and its lateral exchange, blue carbon simulation and assessment of blue carbon sink potential in the context of climate change and sea level rise, and technologies and approaches of blue carbon sequestration in salt marshes. Prioritizing these research topics may elucidate the formation processes and mechanisms of blue carbon, predict the changing trend of blue carbon sequestration potential under global changes, and offer new insights into achieving the goal of “carbon peak and carbon neutrality”.

Aims The carbon exchange between ecosystems and the atmosphere and its response to environmental factors is the focus of current research. The aim of this study was to examine the effects of fencing on ecosystem carbon exchange at meadow steppe in the northern slope of Tianshan Mountains.

Methods The static box method with a LI-840 CO₂/H₂O infrared analyzer was used to evaluate daily and seasonal changes of ecosystem carbon exchange and their relationship with environmental factors in the inside fence and outside fence after 9 years fencing.

Important findings We found the ecosystem carbon exchange inside the fence was significantly (p < 0.05) higher than that in outside the fence. The ecosystem carbon exchange had obvious daily and seasonal variation both in inside and outside the fence, which showed a unimodal curve during the plant growing season. The minimum net ecosystem CO₂ exchange (NEE) in the inside and outside of the fences were -7.62 and -6.63 μmol·m^-2·s^-1, respectively; the maximum ecosystem respiration (ER) were 8.55 and 7.04 μmol·m^-2·s^-1, respectively; and the maximum gross ecosystem productivity (GEP) were -14.66 and -13.89 μmol·m^-2·s^-1, respectively. Due to the protection of fence, the vegetation in the fence was flourished with higher photosynthesis, and thus resulted in lower NEE. Meanwhile, organic carbon input enhanced ecosystem respiration. Besides, the ecosystem carbon exchange significantly correlated with the air temperature and soil temperature of 0 to 10 cm depth, and the correlation with the air temperature was higher than soil temperature of 0 to 10 cm depth. Also, the correlation in the inside of the fence was higher than that in the outside of the fence. Ecosystem carbon exchange had correlation with soil water content, but the correlation was slightly lower than that with soil temperature.

Aims
Plantations play important roles in modifying regional carbon budget and maintaining regional carbon balance. In this study, we assessed larch plantation (Larix gmelinii var. principis-rupprechtii) carbon dynamics in Weichang County from a perspective of the forest biomass-soil-wood-products chain. Our objectives were to elucidate the carbon sink capacity of larch plantation and the influences of biomass, soil and wood product pools on carbon balance.
Methods
CO2FIX model was used to evaluate the carbon storage and flow of larch plantation over a time span of 120 years. Input data for model were derived from practical investigations and published papers. We validated the simulated results and found that this model was suitable in the region and the simulated results were reliable.
Important findings
(1) Soil was the largest carbon pool for larch plantation and the wood product pool had the smallest carbon storage. Meanwhile, carbon storage in wood products gradually increased with time. (2) In a rotation of 50 years from secondary poplar-birch forest to larch plantation, 250 t C·hm^-2 was sequestrated by the larch plantation. 70% of the carbon was transferred into soil in the form of litter and logging slash and the other 30% was transferred into wood products. (3) Larch plantation was a carbon sink during most of its growing period and turned to temporary carbon source when it was harvested. Larch plantation could sequestrate about 0.3 t C·hm^-2·a^-1 in the long term. Our results indicated the importance of wood product carbon pool in carbon dynamics of plantation, which facilitated our understanding in the carbon dynamics and capacity of plantation.

Aims
Estimating soil organic carbon (SOC) density and influence factors of tropical virgin forests in Hainan Island provide new insight in basic data for SOC pool estimation and its dynamics study.
Methods
The main distribution areas of tropical virgin forests in Jianfengling (JFL), Bawangling (BWL), Wu- zhishan (WZS), Diaoluoshan (DLS), Yinggeling (YGL) of Hainan Island were selected, and soil samples (0-100 cm) were sampled and analyzed. SOC density was estimated by soil vertical fitting method and soil stratification method to discover the distribution characteristics of soil organic carbon in tropical virgin forests of Hainan Island.
Important findings
Results showed that: (1) The average SOC density using soil vertical fitting method in JFL, BWL, WZS, DLS and YGL was 14.98, 18.46, 16.48, 18.81, 16.66 kg·m^-2, respectively, which was significantly higher (p < 0.05) than the estimated average SOC density using soil stratification method in these areas (14.73, 16.24, 15.50, 16.91, 15.03 kg·m^-2, respectively). It is better to use soil vertical fitting method for SOC density estimation when the soil was natural without disturbance. (2) The proportion of SOC content in the first 0-30 cm depth interval out of SOC in the whole 0-100 cm soil profiles in JFL, BWL, WZS, DLS and YGL was 50.50%, 48.56%, 43.49%, 47.37%, 42.88%, respectively. (3) SOC density was significantly negative correlated with Shannon-Wiener index, Simpson index, species richness, and soil bulk density; and was significantly positive correlated with altitude, soil porosity, and soil nitrogen. However, SOC density was not significantly correlated to slope, biomass, average diameter at breast height, or average height. (4) Our study area Hainan was located in low latitude area with high rainfall and high temperature, which accelerated the decomposition of organic matter and nutrient recycling, resulting in significantly lower SOC densities in this tropical virgin forests of Hainan Island than the average value in China.

Aims Grassland ecosystem is the largest terrestrial ecosystem type in China. The dynamic changes in carbon cycle play an important role in global carbon budget balance. Grazing is the main use of grassland ecosystem. Different grazing intensity has different effects on the grassland ecosystem.

Methods In the growing seasons (May to October) from 2014 to 2016, we used portable optical LI-6400 and the method of static chamber to measure net ecosystem carbon exchange in Stipa breviflora desert steppe with 3 different stocking rates (CK, no grazing control; LG, lightly grazing; HG, heavily grazing). At the same time, the soil temperature and moisture of 10 cm depth were also measured. The effects of stocking rate and hydrothermal factors on the carbon exchange were discussed.

Important findings Stocking rate had a significant impact on net ecosystem carbon exchange. With the increase of stocking rate, the net ecosystem carbon exchange, ecosystem respiration, gross ecosystem productivity decreased by 48.6%, 35.3% and 40.4% respectively. Heavily grazing significantly reduced grassland carbon sequestration, but lightly grazing had no significant effect. The inter-annual changes in net carbon exchange was mainly controlled by precipitation. Throughout the growing season, Stipa breviflora desert steppe were carbon sinks. The contribution of soil temperature to the variations of net ecosystem carbon exchange was higher than that of soil moisture.

Climate warming has significantly alerted the terrestrial carbon dynamics, resulting in enhanced vegetation productivity, especially in the northern hemisphere. However, most of the prior modeling studies have neglected the effects of nutrient availability, such as the phosphorus limitation, on carbon processes, which potentially leads to an overestimation of the capacity of terrestrial ecosystems to sequester additional carbon. Here, we reviewed recent progress in phosphorus limitation and its interactions with carbon dynamics in the context of climate change, with a focus on the process-based modeling approach. We comparatively analyzed quantitative representations of phosphorus-associated biological processes in some models (i.e., Carnegie-Ames-Stanford Approach (CASA), Community Land Model (CLM), and Jena Scheme for Biosphere-Atmosphere Coupling in Hamburg (JSBACH)), such as photosynthesis and distribution of assimilates, phosphorus uptake by plants, the transformation of phosphorus pools in soil, phosphorus inputs and outputs, etc. We also discussed the key characteristics of these models and summarized the mathematical representations of the terrestrial phosphorus cycle. In addition, we identified and discussed the limitations, uncertainties and future needs in process-based modeling in terms of nutrient and carbon dynamics. Our study highlighted the importance of including phosphorus limitation in regional carbon estimation and provided deep insights related to biogeochemical modeling at broad scales.

Coastal salt marshes are an effective blue carbon sink to mitigate climate warming, but their ecosystem stability and carbon sink function are threatened by the large amount of nitrogen input caused by coastal eutrophication. Under the action of regular tides, the high nitrogen content in the coastal waters will have a profound effect on the key processes of carbon cycle such as plant photosynthetic carbon fixation, carbon allocation in plant-soil system, and soil carbon release in the salt marsh. This study reviewed the effects of nitrogen input on plant photosynthetic carbon fixation, carbon allocation in plant-soil system, decomposition of soil organic carbon, formation and release of soil dissolved organic carbon (DOC), and carbon sequestration in the salt marsh. Based on the shortcomings of current research, this review proposed the directions of future research, including the effects of nitrogen input on plant photosynthetic carbon fixation and carbon allocation in plant-soil system, the microbial mechanism of soil organic carbon decomposition, production and lateral exchange of soil DOC, and the potential impact of different forms of nitrogen input on soil carbon sequestration in the salt marsh. Overall, this study aims to improve the understanding of impacts of nitrogen input on the key carbon processes and the mechanisms of carbon sequestration in a salt marsh, and to provide new ideas for assessing the potential changes of carbon pools under the influence of eutrophication of coastal waters in the salt marsh wetlands.

Aims As an immense carbon (C) stock, grassland ecosystem plays a crucial role in global C cycling. The objective of this research was to reveal the effects of enclosure on C density of the plant-soil system by comparing the aboveground biomass (AGB), belowground biomass (BGB) and soil C density in enclosure plots with those in grazing plots in the typical steppe (TS) and desert steppe (DS) in Nei Mongol, China.

Methods At each of the 19 study sites, we set up a 100 m × 100 m plot and 5 quadrats (1 m × 1 m) along the diagonal transect within each plot. At each quadrat, AGB was harvested first and then a soil core (0-100 cm depth, 7 cm inner diameter) was taken for BGB and soil C content measurement. Each soil core was divided into 7 depth increments (0-5 cm, 5-10 cm, 10-20 cm, 20-30 cm, 30-50 cm, 50-70 cm, 70-100 cm).

Important findings (1) Enclosure significantly increased C density of AGB and BGB in TS. In DS, enclosure significantly increased C density of AGB, but had no significant effect on the C density of BGB. (2) Enclosure significantly increased soil C density in TS, but had no significant impact in DS although there was an increasing trend. (3) For all increments along the soil profile, enclosure significantly increased BGB and soil C density compared to grazing plots in TS, but this effect was not found in DS. (4) Enclosure increased C density of the plant-soil system by 2.2 and 1.6 times in TS and DS, respectively. 65% and 89% C was stored in soil in TS and DS, respectively, and BGB C stock accounted for more than 90% of total biomass C in both TS and DS. Enclosure is an effective approach to improve C sequestration in grassland ecosystems.

AimsGlobal warming could have profound effects on ecosystem carbon (C) fluxes in alpine ecosystems. The aim of our study is to examine the effects of gradient warming on net ecosystem carbon exchange (NEE).MethodsIn the Northern Tibetan Grassland Ecosystem Research Station (Nagqu station), Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, we conducted various levels of temperature increasing experiments (i.e., 2 °C and 4 °C increments). The warming was achieved using open-top chambers (OTCs). In total, there were three levels of temperature treatments (control, 2 °C and 4 °C increment), and four replicates for each treatment. The ecosystem NEE was monitored every five days during the growing season in 2015.Important findings Our findings highlight the importance of soil moisture in mediating the responses of NEE to climatic warming in alpine meadow ecosystem. The 4 °C warming significantly stimulated NEE,except for July measurements. The 2 °C warming had no effects on NEE during the growing season. Compared to the 2 °C warming, the 4 °C warming significantly stimulated NEE. The results showed that our targeted ecosystem acts as a carbon sink under 2 °C warming, whereas will act as a net carbon source under 4 °C warming in the future. This study provides basic data and theoretical basis for evaluating the alpine ecosystem’s responses to climate change.

Aims Vegetation restoration plays an important role in the accumulation and storage of soil organic carbon (SOC). Our objectives were to investigate the effects of vegetation restoration on SOC and to explain the underlying mechanisms of carbon sequestration during vegetation restoration in the mid-subtropical China.

Methods According to the disturbance intensity and the degree of restoration, we used the space-for-time substitution method by selecting four different types of vegetation communities, composed of Loropetalum chinense-Vaccinium bracteatum-Rhododendron simsii scrub-grass-land (LVR), Loropetalum chinense-Cunninghamia lanceolata-Quercus fabri shrubbery (LCQ), Pinus massoniana-Lithocarpus glaber-Loropetalum chinense coniferous-broad leaved mixed forest (PLL), and Lithocarpus glaber-Cleyera japonica-Cyclobalanopsis glauca evergreen broad-leaved forest (LAG) to represent the successional sequence in the secondary forests in Changsha County, Hunan Province, China. Permanent plots were established in each vegetation communities. Soil samples (0-40 cm) were collected and divided into four layers (0-10, 10-20, 20-30 and 30-40 cm). Soil organic carbon concentration (C_SOC) and soil organic carbon density (D_SOC) were measured. The main influencing factors on C_SOC and D_SOC were analyzed with Principal Component Analysis and Stepwise Regressions Analysis.

Important findings 1) Along vegetation restoration, C_SOC and D_SOC increased dramatically. The C_SOC was the highest in LAG, which was 12.5, 9.3 and 4.7 g·kg ^-1 higher than in LVR, LCQ and PLL in 0-40 cm soil depth, increasing by 248.5%, 113.1% and 58.5%, respectively. The increments of D_SOC in LAG at 0-40 cm soil depth were 67.1, 46.1 and 32.5 t C·hm ^-2, and increased by 182.0%, 79.7% and 45.6% compared to D_SOC in LVR, LCQ and PLL, respectively. 2) Correlation analysis showed that C_SOC and D_SOC were strongly and positively correlated with species diversity index, community total biomass, aboveground biomass, root biomass, existing biomass in litter layer, nitrogen (N), phosphorus (P) concentration in litter layer, soil total P, soil available P, soil C/N ratio (except C_SOC), soil C/P ratio, soil N/P ratio and percentage of soil clay (< 0.002 mm), but significantly and negatively correlated with C/N in litter layer (except D_SOC), C/P in litter layer, soil pH and soil bulk density, suggesting that the differences in C_SOC and D_SOC under different vegetation stages were related to both vegetation and soil properties. 3) The results of principal component analysis and stepwise regression analysis revealed that soil C/P, pH, concentration of soil clay (except C_SOC) and C/P in litter layer were the dominant factors affecting C_SOC and D_SOC during vegetation restoration. Among them, soil C/P ratio ranked first. These results indicated that the differences in soil C/P ratio, pH, soil clay concentration and C/P in litter layer were responsible for the changes in SOC during vegetation restoration.

The vineyard ecosystem is an important part of agro-ecosystem, and contiguous vineyards have important ecological values. The research on carbon source/sink in vineyard ecosystem is an indispensable content of our understanding of carbon cycling. The mechanisms of carbon cycling and the carbon sink function of vineyard ecosystem have become hot topics. We found that a large amount of carbon was fixed in vineyard ecosystem that was distributed in annual organs (fruit, etc), perennial organs (trunk, etc) and soil carbon pool. The carbon input flux of the vineyard ecosystem was greater than the carbon output flux, suggesting a carbon sink. Soil was the largest carbon pool of vineyard ecosystem, accounting for 70% of total carbon stock, especially the soil-vines interface. Covering and non-tillage can reduce carbon emissions and increase soil fertility in vineyards. In order to clarify the carbon sink functions of vineyard ecosystem, we reviewed the latest research progress in the field, including the mechanisms of carbon cycling, and the strategies of carbon emission reduction. This paper provides a theoretical basis and prospects for future research directions and application.

Aims Shrub recovery is identified as a major cause of an increase in carbon stocks in terrestrial ecosystems in China, and yet there is a great uncertainty in the contribution of shrubs to the carbon sink. Our objectives were to determine the biomass allocation pattern and carbon density in alpine shrubs.
Methods We conducted investigations in 14 shrub communities in eastern Qinghai-Xizang Plateau, at 3500 m above sea level. Plant samples were collected from each plot and measured for biomass in leaves, branches and stems, and roots in laboratory; the data were used to analyze the biomass allocation and carbon density.
Important findings The mean biomass was (5.38 ± 3.30) Mg·hm^-2 in the shrub layer. There were significant differences in biomass between different shrub types, with the mean of (7.28 ± 4.96) Mg·hm^-2 for the broadleaved deciduous shrubs and (4.32 ± 1.36) Mg·hm^-2 for the leathery-leaved shrubs. The indicators of individual feature and community structure were significantly correlated with biomass per unit land area. However, these relationships were developed based on multiple community structure factors; any single factor alone was insufficient to explain the patterns of biomass variations. The patterns of biomass allocation differed significantly between different shrub types. In this study, there was more allocation of photosynthetic products to roots. The mean total community biomass was (6.41 ± 3.86) Mg·hm^-2 and the shrub layer accounted for (83.18 ± 8.14)% of the total community biomass. There were significant correlations (p < 0.05) between shrub layer biomass and herb layer biomass, between shrub layer biomass and litter layer biomass, and between shrub layer biomass and the total community biomass. The biomass of various organs were also significantly correlated (p < 0.01) with the total community biomass. The mean biomass carbon density of the shrubs was estimated at (3.20 ± 1.93) Mg·hm^-2 across the 14 communities by using biomass conversion factor method.

There are substantial carbon exchange fluxes among soil, vegetation and atmosphere in the terrestrial ecosystems, which are highly relevant to global climate changes. Mycorrhizal fungi can form symbiotic associations with most terrestrial plants, linking the above- and below-ground ecosystems through mineral nutrient-carbon exchange; thus, mycorrhizal fungi play crucial roles in terrestrial carbon cycling. This review summarized the involvements of mycorrhizal fungi in the terrestrial carbon cycling processes, including the carbon input, and formation, stabilization, and decomposition of soil organic matter. Studies have demonstrated that mycorrhizal fungi markedly influence the terrestrial carbon input processes by alleviating plant nutrient deficiencies, improving plant stress resistance, influencing plant photosynthesis, and regulating plant diversity-productivity relationships, subsequently sustaining or improving primary productivity of terrestrial vegetation. A considerable proportion of photosynthetic carbon is channeled directly into the soil matrix via the fungal mycelial network, where it is partly converted into microbial-derived organic carbon, further changes the composition of soil organic carbon, and be stabilized through association with minerals and/or forming soil aggregates. Mycorrhizal fungi can affect the decomposition and transformation of soil organic matter mainly through two mechanisms: the rhizosphere priming effects and/or hyphosphere biogeochemical processes. These mechanisms involve the secretion of specific extracellular enzymes, shaping hyphosphere microbial communities, induction of chemical oxidation, and competition for limited resources (e.g., nutrients and water) with free-living saprotrophs. Considering the sensitivity of mycorrhizal fungi to environmental and climate changes, we also discuss the impact of global change factors on soil carbon cycling mediated by mycorrhizal fungi. Finally, we proposed future research directions, emphasizing a need for in-depth studies on the role of mycorrhizal fungi in terrestrial carbon cycling and their environmental dependence based on network experiments in typical ecosystems. Quantitative studies should be strengthened to integrate mycorrhizal fungi into ecosystem carbon cycling models, and mycorrhizal technologies should be developed and practiced in ecological restoration and agriculture to facilitate terrestrial carbon sequestration for achieving the national carbon neutrality goals and combating climate changes.

Aims As an important potential carbon sink, shrubland ecosystem plays a vital role in global carbon balance and climate regulation. Our objectives were to derive appropriate regression models for shrub biomass estimation, and to reveal the biomass allocation pattern and carbon density in Rhododendron simsii shrubland.
Methods We conducted investigations in 27 plots, and developed biomass regression models for shrub species to estimate shrub biomass. The biomass of herb and litterfall were obtained through harvesting. Plant samples were collected from each plot to measure carbon content in different organs.
Important findings The results showed that the power and linear models were the most appropriate equation forms. The D and D²H (where D was the basal diameter (cm) and H was the shrub height (m)) were good predictors for organ biomass and total biomass of shrubs. All of the biomass models reached extremely significant level, and could be used to estimate shrub biomass with high accuracy. It was more difficult to predict leaf and annual branch biomass than stem biomass, because leaf and annual branch were susceptible to herbivores and inter-plant competition. The mean biomass of the shrub layer was 20.78 Mg·hm^-2, in which Rhododendron simsii and Symplocos paniculata biomass accounted for 93.63%. Influenced by both environment and species characteristics, the biomass of the shrub layer organs was in the order of stem > root > leaf > annual branch. The root:shoot ratio of the shrub layer was 0.32, which was less than other shrubs in subtropical regions. The relative higher aboveground biomass allocation reflected the adaptation of plants to the warm and humid environment for more photosynthesis. The mean total community biomass was 26.26 Mg·hm^-2, in which shrub layer, herb layer and litter layer accounted for 79.14%, 7.62% and 13.25%, respectively. Litter biomass was relatively high, which suggested that this community had high nutrient return. There were significant correlations among aboveground biomass, belowground biomass and total biomass of shrub layer and herb layer. The mean biomass carbon density of the community was 11.70 Mg·hm^-2 and the carbon content ratio was 44.55%. The carbon density was usually obtained using the conversion coefficient of 0.5 in previous studies, which could overestimate carbon density by 12.22%.

Aims Forest trees alter litter inputs, turnover and rhizospheric activities, modify soil physical, chemical and biological properties, and consequently affect soil organic carbon (SOC) storage and carbon sink strength. That how to select appropriate tree species in afforestation, reforestation and management practices is critical to enhancing forest carbon sequestration. The objective of this study was to determine the effects of tree species on SOC density and vertical distributions.Methods A common garden experiment with the same climate, soil, and management history was established in Maoershan Forest Ecosystem Station, Northeast China, in 2004. The experimental design was a completely randomized arrangement with twenty 25 m × 25 m plots, consisting of monocultures of five tree species, including white birch (Betula platyphylla), Manchurian walnut (Juglans mandshurica), Manchurian ash (Fraxinus mandshurica), Dahurian larch (Larix gmelinii), and Mongolian pine (Pinus sylvestris var. mongolica), each with four replicated plots. A decade after the establishment (2013-2014), we measured carbon density and related factors (i.e., bulk density, total nitrogen concentration, microbial biomass carbon, microbial biomass nitrogen, pH value) in soils of the 0-40 cm depth for these monocultures. Important findings Results showed that tree species significantly influenced the SOC density in the 0-40 cm depth (p < 0.05). SOC density in the 0-10 cm depth varied from 2.79 to 3.08 kg·m^-2, in the order of walnut > ash> birch > larch > pine, in the 10-20 cm depth from 1.56 to 2.19 kg·m^-2, in the order of pine > walnut > ash > birch > larch, in the 20-30 cm depth from 1.17 to 2.10 kg·m^-2, and in the 20-40 cm depth from 0.84 to 1.43 kg·m^-2. The greatest SOC density occurred in the birch stands in the 20-40 cm depth. The vertical distributions of SOC density varied with tree species. The percentage of SOC in the 0-10 cm depth over the total SOC in the soil profile was significantly higher in the walnut and larch stands than in others, while the percentage of SOC in the 20-40 cm depth over the total SOC was highest in the birch stands. SOC concentration and soil bulk density differed significantly among the stands of different tree species, and were negatively correlated. SOC density was positively correlated with soil microbial biomass and soil pH in the walnut, ash, and larch stands, and with total nitrogen density in all the stands. We conclude that tree species modifies soil properties and microbial activity, thereby influencing SOC density, and that different patterns of vertical distributions of SOC density among monocultures of different tree species may be attributed to varying SOC controls at each soil depth.

Aims Alpine meadow is widely distributed in the Qinghai-Xizang Plateau, playing an important role in regulating the regional carbon budget. Over the Qinghai-Xizang Plateau, precipitation generally shows an increasing trend during past the several decades, and is projected to increase during the 21st century. Alpine meadow is very susceptible to such climate change, but it remains unclear how its ecosystem carbon exchange responses to precipitation change. In this study, we aim to clarify the effects of altered precipitation on ecosystem carbon exchange in the alpine meadow by conducting a manipulative field experiment.

Methods We conducted a precipitation manipulation experiment at an alpine meadow site in the Namtso area of central Qinghai-Xizang Plateau during 2013 to 2014. A total of six treatments were established, with levels of water addition set for 0%, 20%, 40%, 60%, 80% and 100%, respectively, of equivalent increases in precipitation. We investigated the effects of water addition on gross ecosystem production (GEP), ecosystem respiration (ER), net ecosystem carbon exchange (NEE), and environmental conditions during the growing season.

Important findings The increasing water addition substantially increased soil moisture, but had no significant effect on soil temperature. Both GEP and NEE significantly increased with water addition equivalent to 20% of increases in precipitation, but were suppressed with further increases in the level of water addition. No significant difference was detected in ER across the water addition treatments. Our study suggests that: 1) The change in soil moisture significantly affected NEE and GEP but had a weak effect on ER in the alpine meadow; 2) CO₂ sequestration in the alpine meadow could be stimulated by moderate increases (e.g. 20%-40%) in precipitation.

Aims The expansion of shrublands is considered as one of the key reasons leading to the increase of carbon density in terrestrial ecosystems in China. In the present study, our aims were to explore the biomass allocation and carbon density of Sophora moorcroftiana shrublands in Xizang.
Methods We sampled the biomass of S. moorcroftiana shrubs from 18 sites in the middle reaches of Yarlung Zangbo River, Xizang. Using concentrations of different organs, we estimated the carbon density of different layers in S. moorcroftiana shrublands.
Important findings The plant cover rather than biomass volume (the product of cover and height) provided the best fit for aboveground biomass. The average of the total biomass was 5.71 Mg·hm^-2, ranging from 2.32 to 8.96 Mg·hm^-2. The average biomass of shrub layer, the main component of shrub ecosystem, was 4.08 Mg·hm^-2, accounting for 71% of the total biomass. The belowground biomass of shrub and herb layers was 2.08 and 0.86 Mg·hm^-2, respectively, which was higher than the corresponding aboveground biomass. The average biomass carbon density was 2.48 Mg·hm^-2. Shrub vegetation in the eastern part of the middle reaches has lower carbon density than that in the western part. The relatively high biomass allocation to roots to increase water and nutrient undertake as well as physical support for plants is an important strategy of S. moorcroftiana to cope with the arid environment on the Qinghai-Xizang Plateau. Moreover, the lower carbon density in the eastern part of the middle reaches might be due to the dry environment resulted from high temperature and evapotranspiration and enhanced human activities at low altitudes. The continuous decrease of evapotranspiration under scenarios of future climate change may lead to increase in carbon density in S. moorcroftiana shrublands.