Soil organic carbon (SOC) pool is the largest carbon pool in terrestrial ecosystems and plays an important role in regulating the global carbon cycle and climate change. The inputs of nitrogen (N) and phosphorus (P) induced by anthropogenic activities and atmospheric deposition of N and P increase the availabilities of N and P in terrestrial ecosystems, which in turn will have important impacts on SOC dynamics via regulating plant growth and microbial activity. At present, many field-manipulation experiments regarding the effects of N addition and/or P addition on the dynamics of SOC have been conducted worldwide, and some breakthroughs and progress have been made, but a systematic and comprehensive review and summary of them is still lacking. By taking the effects of N addition and/or P addition on the inputs and outputs of soil carbon as the starting point, we systematically reviewed the effects of N addition and/or P addition on SOC and the potential mechanisms from three aspects: the size, fraction and molecular composition of SOC. According to the results of previous studies, N addition, P addition, and combined N and P (N + P) addition generally stimulate the size of SOC pool. The stimulation effect of N is caused by the decreased carbon outputs from microbial decomposition and/or the enhanced carbon inputs of plant above- and/or below-ground under N addition. However, the stimulation effect of P may be dominated by the enhanced carbon inputs of plant above- and/or below-ground under P addition. As for the fractions of SOC separated by particle-size or density fractionation, N addition promotes both labile fractions (particulate organic carbon or light fraction carbon) and stable fractions (mineral-associated organic carbon or heavy fraction carbon) of SOC, but reduces the proportion of stable carbon fractions to total SOC. In addition, the effects of N addition on the molecular composition of SOC are complex and diverse, and are regulated by environmental and experimental factors such as soil N availability, N addition rate, and N fertilizer form. Compared with N addition, studies on the effects of P addition and N + P addition on the fraction and molecular composition of SOC are very limited, and the associated mechanisms for the effects of P addition and N + P addition on these variables are still unclear. To improve our understanding, we propose four aspects of studies that need to be strengthened in the future, including the effects of P addition on SOC in different types of ecosystems (especially tropical forests), the role and relative contribution of plants and microorganisms in regulating the changes of SOC and its fractions under N addition and/or P addition, the effects of long-term N addition and/or P addition and their interactions on SOC, and the effects of N addition and/or P addition on SOC in deep soils (below 20 cm).

Aims To further understand how community functional diversity drives biomass change following nitrogen (N) addition, a nitrogen addition experiment was conducted in an alpine grassland.

Methods Species composition of community and six functional traits of common species were measured in a short-term N addition experiment in Bayanbulak alpine grassland of Tianshan Mountains. We compared the response patterns of species diversity, functional diversity, and community level traits, and quantified the relative contribution of those factors to community biomass variation.

Important findings Both aboveground and belowground biomass increased following short-term N addition, with higher proportional enhancement of aboveground biomass. N addition reduced functional diversity, but did not affect species diversity. At the community level, height and leaf carbon content increased following N addition, whereas specific leaf area, seed mass, and leaf phosphorus content decreased. The variations of species diversity contributed less to the variations of community biomass change, whereas functional diversity and community level traits explained most of the variation of community biomass. Our results support the mass ratio hypothesis. In conclusion, community level functional traits and functional diversity were sensitive to short-term N addition, and played a key role in driving community biomass.

Aims The aim of this study was to explore the effects of native tree species on soil microbial biomass carbon (MBC) and nitrogen (MBN) content in subtropical Sichuan.

Methods In the present study, Cinnamomum japonicum, C. longepaniculatum, C. austrosiense, Alnus cremastogyne, C. camphora, Toona ciliata and T. sinensis in a common garden were selected as the research objects; whereas the abandoned land as the control. The effects of tree species on soil MBC content and MBN at different depths (0-10, 10-20, and 20-30 cm) were analyzed using the method of common garden.

Important findings (1) Tree species significantly affected the content of soil MBC and MBN, as well as their ratio. Compared with the abandoned land, tree species exhibited positive or neutral effects and the effects of tree species were particularly obvious in C. japonicum.For example, the contents of MBC and MBN in 0-10 cm soil layer were 108.2% and 139.6% higher than those in the abandoned land, respectively. (2) The content of soil MBC and MBN in the both tree and abandoned land generally decreased with an increase with soil depth; however, the characteristics of MBC:MBN varied with tree species. (3) The content of soil MBC and MBN varied with tree species and soil layer. The variations caused by tree species were stronger than that caused by soil layer. Compared with other species, C. japonicum was more conducive to the growth and reproduction of soil microorganisms.

Aims This study aimed to examine the practicability of eddy covariance method in a conifer-broadleaf mixed forest ecosystem in Jinyun Mountain of Chongqing, China, and to analyze the dynamics of water vapor flux in this forest ecosystem. Meanwhile, the main environmental factors that influence water vapor flux was also discussed. Our results may provide a case for such study in forest water vapor budget.

Methods The eddy covariance method was used to continuously observe the vapor fluxes and meteorological factors from September 2019 to August 2020 in a conifer-broadleaf mixed forest. The original data of water vapor flux was corrected and interpolated by Eddy Pro software. We used these data to analyze the energy closure and variation of water vapor fluxes, and as well as environmental factors.

Important findings (1) The energy closure rate in our study forest is 0.77. The direction of the high contribution area of flux footprints in such forest is similar to the annual main wind direction (northeast), indicating that the method of vorticity related technology is practicable and reliable in this kind of forest. (2) In our study forest, the annual water vapor flux is over zero, and the monthly average daily variation is -0.001-6.623 mmol·m^-2·s^-1, suggesting that this forest is a source of water vapor in study area. There is a single peak trends for monthly average daily variation and seasonal variation of water vapor fluxes. By contrast, the average value of water vapor fluxes is the highest (4.620 mmol·m^-2·s^-1) in summer with strong fluctuations, and the lowest (2.077 mmol·m^-2·s^-1) in winter with weak fluctuations. (3) The total annual evapotranspiration (792.40 mm) in this forest accounts for 53.12% of the total precipitation (1 489.18 mm), and the summer evapotranspiration (325.53 mm) and precipitation (680.52 mm) are the highest, accounting for the annual evapotranspiration and precipitation respectively 41% and 46%. Compared with other ecosystem sites, we found that total annual evapotranspiration was less in our study forest than in wetland, but more than in farmland and grassland. (4) The water vapor flux was positively correlated with net radiation, air temperature and vapor pressure deficit. Such correlations (R²) were the highest in summer, and reached to 0.85, 0.53 and 0.60, respectively. Conversely, the water vapor flux was negatively correlated with wind speed, and the R² equal to 0.61 in the summer. It seems likely that net radiation and air temperature are the main drivers in water circulating at our study forest.

Aims Diffuse radiation is one of the important factors affecting forest carbon uptake. However, the response of gross primary productivity (GPP) of planted forest ecosystems to diffuse radiation in China is still unclear. We explored the effects of diffuse radiation on GPP at 6 plantation ecosystems in eastern China during the growing season.

Methods Based on carbon flux data and meteorological data during the growing season of 2019-2020, we estimated the diffuse radiation and identified the direct and diffuse conditions. The important light response parameters of plantation ecosystems were obtained by the rectangular hyperbolic curve. Meanwhile, we quantified the variations of GPP responding to diffuse and direct radiation. The contribution of light and environmental factors to the diurnal variation of GPP was analyzed by partial correlation method.

Important findings Diffuse radiation can effectively promote canopy photosynthesis. The values of light response parameter canopy quantum efficiency (α) and photosynthesis at photosynthetically active radiation of 1 000 µmol·m^-2·s^-1 (P₁₀₀₀)increased by 47%-150% and 2%-65%, respectively. Compared with direct sky conditions, GPP increased by 0.86%-1.70% in response to 1 μmol·m^-2·s^-1 enhancement of photosynthetically active radiation (PAR) under diffuse sky conditions, which was affected by forest type and vegetation phenology. In diffuse skies, the increment of the variation of GPP under increasing per unit PAR (0.86%-1.00%) at Pinus sylvestrisvar. mongolica and P. tabuliformissites with lower normalized difference vegetation index (NDVI) value was significantly lower than other plantation sites (1.04%-1.70%), and there was a significant positive correlation between NDVI and P₁₀₀₀. Under low light level, PAR controlled the averaged gross primary productivity (GPP_a), but diffuse fraction (DF) mainly regulated GPP_ain middle and high light level. The photosynthesis corresponding to diffuse radiation under moderate light was roughly equal to photosynthesis corresponding to total radiation under high light. Under middle light conditions, the GPP_a value in medium and high DF (≥0.5) at Cunninghamia lanceolata, Populusspp., Quercus variabilis and Larix gmeliniiwas about 27%-50% higher than under low DF condition (<0.5), and the GPP_a value at high DF was about 2% more than under low DF conditions at Pinus sylvestrisvar. mongolica and P. tabuliformis sites. Under diffuse radiation conditions, diffuse photosynthetically active radiation (PAR_dif) explained 16%-45% of the variation of GPP. Air temperature (T_a) and vapor pressure deficit (VPD) explained 10%-19% of the variation of GPP at Cunninghamia lanceolata, Quercus variabilis and Larix gmelinii sites. Under diffuse radiation conditions, the P₁₀₀₀ will be the highest when T_a is 15-25 °C and VPD is 0-1 kPa.

Aims This study aims to determine the radial growth characteristics of natural secondary forests along an elevation gradient and their responses to climate change, and to identify the main factors affecting the radial growth of trees on mountain sites. The outcome of the study would be of great significance for understanding the impact of climate change on growth adaptation, succession and sustainable management in temperate forest ecosystems.

Methods Field plots were set up at 1 350, 1 550, 1 750, and 1 950 m above sea level in the study area to collect samples of wood cores and discs. Tree-ring climatology method was adopted to establish the standard chronology of Betula platyphylla in natural secondary forest, and the annual ring width index and climate factors were subjected to correlation analysis and multiple stepwise regression analysis.

Important findings From 1960 to 2018, climate in the study area showed a warming and drying trend, with 1960-1989 as a stable period and 1989-2018 as a period with rapid changes. A change occurred in the radial growth of B. platyphylla in 1989, with the annual ring width index displaying an “up-down” growth trend. During the stable period of climate change, the radial growth of B. platyphylla at the low-elevation sites (B1350, B1550) was positively correlated with temperature variables (i.e. average air temperature, maximum air temperature, minimum air temperature); whereas at the high-elevation sites (B1750, B1950), the growth was significantly and positively correlated with the precipitation of the previous year and during the growing season of current year. During the period of rapid climate change, the radial growth was negatively correlated with temperature and potential evapotranspiration (ET₀) during the growing season at the low-elevation sites (B1350, B1550), and negatively with ET₀ during the growing season and end of the growing season at the high-elevation sites (B1750, B1950). During the stable period of climate change, temperature explained 76%, 54%, and 51% of the variations in radial growth, and moisture explained 24%, 46%, and 49%, at the B1350, B1550, and B1750 sites, respectively. During the period of rapid climate change, temperature explained 58%, 41%, and 38% of the variations in radial growth, and moisture explained 42%, 59%, and 62%, at the B1350, B1550, and B1750 sites, respectively. The radial growth of trees at the high-elevation site B1950 was always controlled by the moisture during the study period.

Aims The interspecific interactions between plants are vital for species survival. For biological invaders, stronger competitive ability allows them to invade successfully when appearing with indigenous species, which is considered as an important mechanism underlying successful invasion. However, environmental changes may alter interspecific interactions, thus impacting invasion consequences. In this study, we aimed to explore the effect of acid rain, which is a seriously environmental problem worldwide, on the interactions between a Chinese invader common ragweed (Ambrosia artemisiifolia) and its accompanying indigenous forb (Bidens bipinnata) to further explore the role of an environmental disturbance in biological invasions.

Methods In March 2021, we performed a de Wit replacement competitive experiment with A. artemisiifolia and B. bipinnata at the campus of Beijing Normal University; meanwhile we also simulated acid rain through providing different concentrations of solutions (pH = 3, 4, 5, 7). Plant height after 24, 34, 45 days, final plant height, and aboveground biomass were determined for each individual plant. Relative neighbor effect (RNE) and replacement diagrams were used to estimate interspecific competition.

Important findings When the two species were planted separately, the medium level of an acid solution (pH = 4) promoted their early growth, and the high level of an acid solution (pH = 3) significantly inhibited their early growth but did not affect the final plant height. When grown together, the ragweed showed a decrease in plant height in the presence of the high level of an acid solution (pH = 3) after 34 and 45 days, but this negative effect disappeared at harvest. The plant height of bidens applied with the high level of an acid solution (pH = 3) decreased significantly at harvest compared with the control treatment. The RNE of bidens on ragweed was not significant under all the treatments, while the RNE of ragweed on bidens was significant without acid application or under the condition of acid treatments at a lower proportion of bidens. Replacement diagrams showed that the bidens had an advantage over ragweed when applied with the low level of an acid solution (pH = 5) at higher proportions, and the ragweed had an enhanced advantage over bidens when applied with the high level of an acid solution (pH = 3). Our study suggests that acid rain might affect the growth of ragweed and bidens as well as their interactions, and acid rain with low pH may boost the competitive advantage of invasive ragweed.

Aims Livestock grazing is one of the most important factors affecting grassland plant diversity. However, the information on the effects of different livestock types and their grazing behaviors on grassland plant diversity and community composition are less available. A better understanding of the changes in plant diversity and community composition in response to the grazing of various livestock types is essential to the management and preservation of grassland biodiversity.

Methods We conducted a grazing experiment in a typical steppe of Nei Mongol to examine the effects of different livestock species (cattle, goat, sheep) and their behaviors (forage selection and grazing aggregation) on plant diversity (i.e., α, β and γ diversity) and community composition.

Important findings Our results showed that: (1) cattle, goat, and sheep grazing all increased plant α, β, and γ diversity at moderate grazing intensity, and the increase was the largest and significant under cattle grazing. (2) Three livestock species all changed community structure; sheep grazing reduced the relative abundance of dominant short grass Cleistogenes squarrosa, which is in contrary to the changes in community structure induced by cattle and goat grazing. (3) Cattle and goat grazing significantly reduced the aboveground biomass of dominant species, including tall grasses Leymus chinensis and Stipa grandis and short grass C. squarrosa, while sheep grazing only decreased that of short grass C. squarrosa. Cattle grazing also had a lower spatial aggregation than that of goat and sheep. (4) Plant diversity decreased with the increase of the aboveground biomass of either tall or short dominant species, indicating that livestock grazing promoted plant species diversity by reducing plant aboveground biomass of dominant species. (5) Plant diversity decreased with the increase in spatial aggregation of livestock grazing, indicating a lower aggregation benefiting plant diversity maintenance. Overall, our study suggests that grazing animal types should be considered along with grazing intensity in the development of grazing management regime for better conservation and sustainable use of the grassland resources.

Aims This study aimed to understand whether the coupling between environmental factors and chlorophyll contents is related to the dominance of plant life forms in the grasslands.

Methods In July 2020, we measured chlorophyll contents in 185 plant samples from 50 forage species of 11 sampling plots of a typical steppe in Xilinhot. Through correlation analysis, single factor analysis of variance, redundancy analysis (RDA), stepwise regression analysis and path analysis, we analyzed the effects of multiple environmental factors on indices of plant chlorophyll and the mechanism underlying the competition advantage of different plant species.

Important findings 1) The contents of chlorophyll a and chlorophyll b were significantly positively correlated with that of total chlorophyll, with the correlation coefficients of 0.807 and 0.936, respectively. The contents of total chlorophyll were more affected by that of chlorophyll b. 2) The contents of chlorophyll a, chlorophyll b, and total chlorophyll of the four life forms followed an order of semi-shrubs > perennial grasses > annual or biennial plants > perennial forbs. 3) The variations of chlorophyll contents among plant life forms were differently explained by environmental factors. The RDA results showed that the explanatory degree of semi-shrubs was the highest (28.0%), followed by annual or biennial plants (18.3%) and perennial forbs (17.7%), and that of perennial grasses was the lowest (12.7%). 4) The chlorophyll index of plant life forms were affected by various environmental factors. The chlorophyll b contents in semi-shrubs were affected by soil organic carbon content and relative air humidity, while their total chlorophyll contents were mainly influenced by relative air humidity. Chlorophyll b contents of perennial forbs were mainly restricted by surface temperature. The contents of chlorophyll a, chlorophyll b, and total chlorophyll of annual or biennial species were mainly affected by soil pH. 5) Path analysis results showed that chlorophyll a contents of grasses were mainly affected by soil factors, chlorophyll b and total chlorophyll contents were mainly impacted by atmospheric factors in the grassland habitats. Comparatively, perennial grasses were not susceptible to environmental factors so that they become more tolerant to environmental changes than the other life forms. Therefore, these plants gradually became dominant during community succession and evolution.

Aims In order to explore the effects of different water and salt environments on the plant functional traits and their ecological stoichiometric characteristics at the lakeshore zone of Bosten Lake. The dominant plants and soil environmental factors in this area were selected to clarify the strategy of plant adaptation to the environments in this region.

Methods Eighteen sample plots were set up to investigate plant diversity. A total of 24 plant species including 8 shrubs and 16 herbaceous species were examined. The relationship between functional traits of plant leaves and soil environmental factors was tested using redundancy analysis method.

Important findings Our results showed that leaf functional traits varied considerably with the increasing water and salt content. Among plant traits, the chlorophyll content (SPAD), leaf thickness (LT) and specific leaf area (SLA) were the greatest in low water and salt environments, while the leaf water content (LWC), leaf dry matter content (LDMC) and leaf dry mass (LDM) were greater in medium and high water and salt environments. The content of carbon (C), nitrogen (N) and phosphorus (P) of plant leaves and their stoichiometric ratios were highly variable, with the C:N being 9.35-26.51 and the range of C:P being 50.13-228.95. The range of N:P was 2.31-11.99, with the largest variation in C:P. The Leaf C content was significantly and positively correlated with LT, LDMC and LDM, and leaf N content was significantly and positively correlated with SPAD and LT, while leaf P content was significantly and positively correlated with LWC. Whereas, C:N and C:P were both significantly and positively correlated with LDMC, while N:P was not correlated with any of the leaf functional traits. SLA was not correlated with any of the leaf ecological stoichiometric characteristics. The correlation between environmental factors and the functional traits of the dominant plant leaves revealed that the environmental factors affecting the functional traits of plants differed between species.