Aims Soil respiration may have distinct dynamic patterns at different temporal scales since it is affected by diverse abiotic and biotic factors. Seasonal variation in soil respiration is largely controlled by abiotic factors such as temperature and soil moisture, whereas the regulation of diurnal variation is likely physiological rhythms of plants. Our objectives were to compare seasonal and diurnal patterns of soil respiration and to evaluate relationships between soil respiration and temperature at annual and diurnal scales.
Methods We examined seasonal variations of soil respiration using infrared gas analyzers at monthly or bimonthly intervals from April 2004 to March 2005, and diurnal variations in July, September and November 2004 as well as in January, March and May 2005 in a montane evergreen broad-leaved forest in Ailao Mountains, China. Soil temperature, air temperature, soil water content and air humidity were measured at the same time. We evaluated Q₁₀ values of soil respiration and correlations between soil respiration and soil temperature.
Important findings Soil respiration fluctuated with distinct seasonal and diurnal patterns. Soil respiration was higher in the wet season (May through October) than in the dry season (November through April). Diurnal patterns of soil respiration varied among seasons. The mean rate of soil respiration was higher in nighttime than in daytime in July, September, January and March, but lower in November and May. On the whole-year basis, soil respiration correlated strongly with soil temperature and soil water content. However, on a diurnal scale, these regressions were not significant. Q₁₀ values were 4.48, 7.17 and 2.34 for the whole year, dry season and wet season, and their corresponding soil temperature ranges were 5.9-16.6, 5.9-11.0 and 10.3-16.6 ℃, respectively. Our results demonstrate that biotic and abiotic factors have distinct impacts on soil respiration at different temporal scales in the forest. Estimation on daily and annual carbon fluxes based on instantaneous measurements of soil respiration, rather than 24-hour measurements, may cause severe deviation from actual values because of the lack of diurnal correlation between soil respiration and temperature.

Stable isotopes are used as both natural integrators and tracers of complicated biological, ecological and biogeochemical processes, and their responses to environmental changes at different spatial and temporal scales. In this article, the application of stable isotopes and the Keeling plot approach to carbon and water exchange studies of terrestrial ecosystems were reviewed. We focused mainly on the current applications and potential development of stable isotope techniques and the Keeling plot approach in conjunction with concentration and flux measurements of CO₂ and water in terrestrial ecosystems. For these applications it is critical to know the isotopic identities of specific ecosystem components, such as the isotopic compositions of CO₂, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant-atmosphere continuum. Based on the principle of mass conservation, the Keeling plot approach combines measurements of stable isotope ratios and concentrations of CO₂, water or other trace gases, and allows the identification of the contributions of various ecosystems, or ecosystem components, to the net exchange fluxes between the terrestrial biosphere and atmosphere, and the estimation of net ecosystem isotopic discrimination and disequilibrium effect. Net ecosystem carbon fluxes can be partitioned into C uptake during photosynthesis and C release during respiration or evapotranspiration into leaf transpiration and soil evaporation by the Keeling plot technique. This approach also allows partitioning urban CO₂ sources into gasoline combustion, natural gas combustion and biogenic respiration. Recent modifications of the Keeling plot approach permit examination of CO₂ recycling in forest ecosystems. At the global scale, we can estimate relative contributions of terrestrial and ocean ecosystems to the global carbon cycle by combining stable isotope techniques, the Keeling plot approach and terrestrial ecosystem models. However, applications of stable isotope techniques and the Keeling plot approach to ecological research are sometimes constrained by the heterogeneity of terrestrial ecosystems. In addition, selection of suitable isotopic sampling protocols is another factor that we should consider in its application. Nevertheless, with new improvements in analytic protocols in the near future, stable isotope techniques and the Keeling plot approach will become one of the most effective techniques for understanding carbon and water relationships in terrestrial ecosystems.

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 Our objective was to understand seasonal and diurnal variations in CO₂ exchange of a grassland ecosystem by clarifying the carbon cycle and its affecting factors for a planted pasture ecosystem in the Sanjiangyuan Region of the Qinghai-Tibetan Plateau.

Methods We used the eddy covariance method to measure net ecosystem CO₂ exchange (NEE) and environmental factors in the planted pasture (Elymus nutans) ecosystem in 2006.

Important findings Daily maximum uptake and release of CO₂ were 6.56 and -4.87 g CO₂·m^-2·d^-1, respectively. Maximum rates of NEE uptake and release were -0.35 and 0.22 mg CO₂·m^-2·s^-1, respectively. Annual gross primary production (GPP) was 1 761 g CO₂·m^-2·a^-1, of which more than 90% was consumed by ecosystem respiration (R_eco). Annual NEE was -111 g CO₂·m^-2. In the growing season, maximum and minimum R_eco/GPP values were 90% in May and 79% in June, respectively. The Q₁₀ was 4.81, which is higher than in other ecosystems. The NEE was mostly influenced by photosynthetic photon flux density (PPFD), temperature and vapor pressure deficit (VPD). The R_eco was mainly affected by soil temperature at 5 cm depth (T_s).

Tropical forests play an important role in altering the carbon budgets of terrestrial ecosystems. We examined patterns of diurnal and seasonal net ecosystem CO₂ exchange (NEE) in a tropical seasonal rainforest of Xishuangbanna on clear days between November 2003 and October 2004. We found that the diurnal dynamics of NEE showed a single_peaked curve. During daytime throughout the year, NEE increased with solar radiation after sunrise, but fluctuated after sunset. Values of NEE (absolute values) in the foggy_cool and wet seasons were greater than those in the dry_hot season during daytime. During the night, NEE values were dominated by soil temperature and soil water content and were highest in the wet season, followed by those in the dry_hot season and lowest in the foggy_cool season. NEE, water vapor deficit (VPD) and air temperature (T_a) curves all showed pronounced seasonal variation, but photosynthetically available radiation (PAR) did not vary significantly. Maximum photosynthesis rates (P_max) and dark respiration rates (R_e) were greater when VPD≥16 hPa than whenVPD<16 hPa, whereas photon density (α) was the opposite. R_e was also greater when T_a≥25 ℃ than whenT_a<25 ℃ in the three seasons.α was reduced when T_a≥25 ℃ in the dry_hot and wet seasons. TheP_max increased in the dry_hot season and decreased in the wet season when T_a≥25 ℃. Our data suggested thatPAR is the main factor influencing NEE diurnal dynamics, whereas both VPD and T_a play a major role in regulating NEE seasonal dynamics.

Aims Agroecosystems are influenced strongly by human activity and climate change. In order to scientifically manage agroecosystems under climate change, it is important to understand water exchange and energy transfer between agroecosystems and the atmosphere. We analyze latent and sensible heat fluxes of a maize agroecosystem as a case study.

Methods Latent and sensible heat fluxes were measured in a maize agroecosystem using a 3.5 m eddy covariance tower from June 2004 to December 2005 at the Jinzhou maize agricultural ecosystem field observation station (Liaoning Province, China). Meteorological factors were recorded using sensors at 2.3 and 4.1 m using a micrometeorological tower.

Important findings Diurnal and annual variations of latent heat fluxes and sensible heat fluxes had the same characteristics as net radiation and could be expressed as hyperbola curves. Their peak values appeared at 12∶00-13∶00. The maximum latent heat flux was about 655 w·m^-2 (at 13∶00 July 8,2004), and the maximum sensible heat flux was 369 w·m^-2 (at 13∶00 May 31,2004). The intensities of latent heat fluxes and sensible heat fluxes had close relationships with environmental factors. Latent heat flux was negatively correlated with atmospheric pressure, and sensible heat flux was positively correlated with air temperature. Fluxes of latent and sensible heat were very sensitive to precipitation. Latent heat flux was greatly affected by the intensity and timing of precipitation, regardless of seasonal and daily changes. Energy of the maize agroecosystem was unbalanced with the loss of about 15.5%, possibly because of lack of knowledge of 0-5 cm soil heat reserve and canopy heat reserve. The energy balance had obvious differences between cloudy and rainy days, with the loss of energy much less on rainy than cloudy days, especially with a surplus of energy on rainy days in August.

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.

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.

Aims The two main methods for estimating CO₂ flux from forests are the eddy covariance micrometeorological method and the ecophysiological component summation method. Eddy covariance is a standard method for long-term, direct measurement of forest CO₂ and is used in studying large-scale terrestrial carbon budgets, while the ecophysiological method can estimate each component (e.g., stem, leaves, branches, roots, as well as soil microbes) of total CO₂ flux of forests. Because forest CO₂ flux study, including eddy covariance measurement, is a recent development in China, it is important to compare results from these two methods for understanding scaling-up of forest carbon budgets. We did a preliminary comparison during a typical month of the strongest sink capacity (June 2002). Our aim was to determine how the methods differed in carbon budget estimation and evaluate implications for future research.

Methods A micrometeorological tower with the eddy covariance system was used to directly estimate net ecosystem exchange of a larch (Larix gmelinii) plantation at Laoshan station (45°20' N, 127°34' E). Ecophysiological measurements by a Li-6400 system were used to measure leaf photosynthesis and respiration of the tree canopy and herbaceous understory, stem respiration, branch respiration and soil respiration. Root respiration, soil microbe respiration and litter respiration were measured by the pre-installed trenched box and litter exclusion method. We converted each photosynthesis and respiration value from an organ-area base to a soil-area base using leaf area index measured by LAI-2000 and stem area index and branch area index estimated by standard tree sampling.

Important findings Energy balance was estimated to be 75% using half-hourly flux data, but improved when 5 days of accumulated data were used, indicating that the eddy covariance method is suitable for this site. In relative cloudy weather (mean photosynthetic active radiation, PAR<400 μmol·m^-2·s^-1), light use efficiency was much higher than on days with a mean PAR>500 μmol·m^-2·s^-1. This may be related to diffuse light on cloudy days. Expressed on a soil area base, gross primary productivity (GPP) of the larch plantation was 20-50 μmol·m ^-2·s^-1 estimated by the eddy covariance method. This value was much higher than the total photosynthetic capacity of dominant canopy leaves of 9.8-23.4 μmol·m ^-2·s^-1 (mean of 16.2 μmol·m ^-2·s^-1); however, it was equivalent to the summation of dominant canopy and understory photosynthesis, indicating the critical importance of understory photosynthesis in the carbon balance of the studied plantation. Ecosystem respiration estimated by eddy covariance on a windy night was 3-9 μmol·m ^-2·s^-1, which is about 50% lower than estimated by the ecophysiological method (14.2 μmol·m ^-2·s^-1). This large discrepancy between the two methods would lead to a large difference in carbon sink estimation. Therefore, methods of estimating respiration need additional study.

Aims Methane (CH₄) plays an important role in the greenhouse effects. Our objectives were to evaluate the CH₄ budget, understand seasonal variation of CH₄, and explore effects of temperature and moisture on CH₄ flux in a mid-subtropical pine plantation to provide data for estimating the influence of subtropical forest ecosystems on greenhouse effects.
Methods We analyzed CH₄ flux from soils in the Qianyanzhou red earth hill region of China for 16 months from September 2004 to December 2005, using a static chamber-gas chromatograph technique.
Important findings The soil of this type of pine plantation was a sink of CH₄ to the atmosphere as a whole; annual CH₄ flux ranged from 7.67 to -67.17 μg·m^-2·h^-1 with average of -15.530 μg·m^-2·h^-1 under a forest soil treatment and from 9.31 to -90.36 μg·m^-2·h^-1 with average of -16.53 μg·m^-2·h^-1 under a litter-free treatment. CH₄ absorption had similarly seasonal variations with a sequence of autumn > summer > spring > winter for both treatments, but differed in variation ranges and time. The litter-free soil had larger ranges of seasonal variations, maximum CH₄ sink was in October and minimum sink was in March. Meanwhile, the corresponding maximum and minimum CH₄ sinks in the forest soil were in the September and February, respectively, a month earlier than litter-free treatment. Analysis of correlations between CH₄ flux and temperature and moisture showed that CH₄ flux had a significant positive correlation to soil temperature at 5 cm depth and a significant negative correlation to soil water content at 5 cm depth. Partial correlations showed the combined effects of moisture and temperature on CH₄ flux in different seasons. Temperature was a limiting factor for soil absorption of CH₄ during winter (December to February), but soil absorption increased during the rainy season (March to May). From July to August, CH₄ absorption increased with the declining soil moisture but was restricted by high temperature. During the fall (September to November), CH₄ absorption reached the maximum value for suitable combined effects of temperature and moisture. In summary, CH₄ absorption increased with soil temperature and decreased with soil water content, but was restricted by high temperature.

Aims Our objective was to develop a new method for simultaneously measuring evapotranspiration and CO₂ absorption in plant communities using the LI-6262 CO₂/H₂O analyzer for regional integration of water and CO₂ dynamics.

Methods We measured evapotranspiration and CO₂ absorption of some plant communities in the typical steppe of the Xilin River Basin, China.

Important findings Our method enabled us to concurrently measure evapotranspiration, photosynthesis and respiration, the important ecological processes of communities and therefore obtain a series of valuable measurements of community characteristics. Our method is precise, the instruments are easily portable, and the method is adaptable for plant communities of steppe, sand lands and wetlands. The method has important practical value for study of the impacts of global change on ecological functions of steppe.

Stem respiration is an important part of the annual carbon balance of forest ecosystems and consumes ca. 11%-33% of total net daytime carbon assimilation. Because of difficulties in measurement, little attention was paid to stem respiration studies in the past. However, with increasing atmospheric CO₂ concentration, studies of stem respiration have become popular. Several methods were applied in earlier studies, including gas exchange measurements and closed method. An open flow system is employed in recent studies. Results from recent research show that the diurnal pattern of stem respiration is bimodal with a midday depression and that rates are the greatest in the growing season. Controlling factors include meteorological factors (e.g., stem temperature, CO₂ concentration and humidity) and biological factors (tree species, tree age, diameter at breast height, sapwood size and nitrogen content in stem). Latitude, altitude and topographic factors indirectly influence respiration rates through meteorological or biological factors, in particular stem temperature. Stem respiration rate is positively correlated with stem temperature. The mechanism of stem respiration and its controlling factors will continue to be subjects of future research. Integration of meteorological and biological factors into models of stem respiration will provide insight into contribution of stem respiration to the carbon balance of forest ecosystems, role of forest ecosystems in reducing CO₂ concentration elevation in the atmosphere, response of forest ecosystems to global changes, and development of carbon cycle models of forest ecosystems. These issues and measurement techniques remain challenging and fruitful areas for future research.

Aims Using field data from an eddy-covariance (EC) flux tower and sap-flow sensors installed in a poplar (Populus euramericana) plantation, we investigated the magnitudes and changes of evapotranspiration (ET) under different soil moisture and climatic conditions. Our objectives were to quantify the energy partitioning and energy balance, explore the dynamic process and regulatory mechanisms on ET, and understand primary biophysical regulations, especially soil moisture.
Methods An open path EC system, sap-flow sensors, soil water balance monitoring system, and microclimatic station were installed to record various components of energy fluxes and water budget at an 11-year-old poplar plantation in Daxing District, Beijing, China. We used data collected at 30-min intervals in the growing season of 2006 in this study.
Important findings The overall energy closure of the study site was high (86%) during the growing season, but with notable dependence on soil water conditions. The ratio between sensible heat and net radiation (Hs:Rn) was much higher during dry conditions than that during moist conditions. With dry soils, net radiation and soil physical properties played important roles in transpiration, which was less than evaporation prior to rain events. In contrast, transpiration exceeded evaporation when the soil water content in the deep layers was more abundant following rain events. The total ET rates quantified by the soil water balance and sap-flow methods were comparable and lower than that determined by the EC method. The ratio between transpiration and ET appeared to be more dependent on net radiation and vapor pressure deficit (VPD) during wet periods than that during dry periods.

Measurements of soil respiration, soil temperature and rice biomass were made during the rice growing season in the hill region of the central Sichuan Basin from April to September 2003. Characteristics of the daily and seasonal variations of soil respiration and their controlling factors are presented. The results showed that daily variations of soil respiration could be modeled with a single peak curve. The minimum and maximum soil respiration values from rice fields occurred at 7∶00 and 15∶00, respectively. Daily soil respiration rates were highly correlated with 5 cm depth soil temperature measurements. The mean rate of soil respiration was 121.76 mg·m^-2·h^-1, ranging from 18.00 to 269.69 mg·m^-2·h^-1 during the growing season. Rice root biomass and 5 cm depth soil temperatures were the major factors influencing soil CO₂ emissions during the entire growing season. There was a significant relationship between the rate of soil respiration and root biomass of rice during the initial growing season. From the middle/late stages to mature stage, 5 cm depth soil temperatures played a key role in regulating soil CO₂ emissions from rice fields, while the relationship between the rate of soil respiration and water levels in the rice fields was not obvious.

In order to understand more about mechanisms of and factors that influence CH₄ and N₂O production in wetlands, fluxes of CH₄ and N₂O were measured using static-chamber and gas-chromatography methods in a marsh wetland, located at the Honghe Farm in eastern part of Heilongjiang Province, China (47°35'17.8″ N, 133°37'48.4″ E), from June to September,2003. Three plant communities, Carex pseudocuraica, Carex lasiocarpa and Deyeuxia angustifolia, were selected to measure fluxes of CH₄ and N₂O to contrast the variance of the emission rates of both greenhouse gases in these different plant zones. Air temperature and soil temperature at 5 cm depth, soil redox potential (0-100 cm), and standing water depth at each site also were measured to determine the main factors that control CH₄ and N₂O emissions within and among plant zones.

The wetland was a source of both CH₄ and N₂O during the growing season and emissions showed conspicuous temporal and spatial variations. Similar temporal variations of CH₄ and N₂O fluxes were observed in the C. pseudocuraica and C. lasiocarpa sites. Emission rates of CH₄ were higher in July and August while emissions of N₂O were higher in July and September. However, the highest emissions of CH₄ and N₂O in the C. angustifolia site occurred about one month earlier than in the C. pseudocuraica and C. lasiocarpa sites. The highest CH₄ emissions observed in the wetland were in the C. pseudocuraica site on July 19 with a rate of 696.24 mg·m^-2·d^-1, and the highest N₂O emissions were in the D. angustifolia site on June 12 with a rate of 2.53 mg·m^-2·d^-1. The average CH₄ flux from the C. pseudocuraica site was 273.6 mg·m^-2·d^-1, the highest among the three sites over the growing season but was not significantly different from 259.2 mg·m^-2·d^-1 of the C. lasiocarpa site. However, both were significantly higher than the 38.16 mg·m^-2·d^-1 measured in the D. angustifolia site (p<0.000 1). These results showed that average CH₄ fluxes in submerged wetlands were higher than in seasonal wetlands. N₂O fluxes from the C. pseudocuraica, C. lasiocarpa and D. angustifolia sites were not significantly different (p=0.967) with an average flux of 0.969, 0.932 and 0.983 mg·m^-2·d^-1, respectively, suggesting that submerged and seasonal wetlands had similar rates of N₂O emissions.

Air temperature, soil temperature, soil redox potential and standing water depth were important factors influencing emission rates of CH₄ and N₂O from the wetlands. Relationship analysis showed that CH₄ fluxes were correlated weakly with air temperature and soil temperature at 5 cm depth within a site (0.201<r<0.560) but not correlated with standing water depth ((0.100<r<0.176). Strong correlations were found between N₂O fluxes and standing water depth (r₁=-0.701; r₂=-0.528), but no correlation between N₂O fluxes and air temperature and soil temperature at 5 cm depth in the C. pseudocuraica and C. lasiocarpa sites (-0.089<r<0.211) was found. However, in theD. angustifolia site, there were no correlations between N₂O fluxes and the three factors (r<0.344). These results indicated that temperature was more important in influencing CH₄ emissions in the seasonal and submerged wetlands whereas standing water depth was more important in influencing N₂O emissions in the submerged wetlands. Furthermore, standing water table was the main control of the difference in CH₄ emissions among plant zones. However, there appeared to be similar rates of N₂O emissions among plant zones in the wetlands with strongly anaerobic conditions.

Aims Evapotranspiration (ET) plays an important role in arid and semiarid temperate grassland where water availability is a major limiting factor for ecosystem functions. Understanding temporal variation of ET can help explain the surface-atmosphere interaction and its ecological function in grassland ecosystems. Partitioning total ET into its components of evaporation from soil (E) and transpiration from plants (T) is important for understanding the biotic and abiotic factors that control water balance. Our objectives were to simulate the seasonal and interannual variations of ET and its components, analyze the contribution of the components to ETand analyze influencing factors.

Methods We used flux data derived from eddy covariance technology over Inner Mongolia steppe (43°32′ N, 116°40′ E), measuredLAIand MODIS data from 2003 to 2005 and parameterized VIP (Vegetation interface processes) model to simulate ET of the grassland. The results were validated using half-hourly latent heat fluxes (LE) and net radiation (R_n) estimated from eddy covariance measurements.

Important findings VIP model can effectively simulate latent heat fluxes of the grassland (R²=0.80). In 2003 and 2004, precipitation (P) was near average and annual ET was 337 and 338 mm, respectively, which were greater than P. In the drier year of 2005, annual ET was 223 mm, which was higher than P. On average, E andT made relatively equivalent contributions to ET. About 83% of annual EToccurred during the growing season. Ewas the primary component of ET before June and was exceeded by Tafter that. The monthly totals of both ET and Treached maxima in July and August. Total ETduring July and August accounted for 43% of the annual amount. ETwas strongly correlated with LAI and moderately correlated with P. E changed little during the growing season, and the difference in ET was accounted for T.

Aims Soil CO₂ flux is driven primarily by the CO₂ diffusion gradient across the soil surface. Ideally, the soil CO₂ flux measurement should be made without affecting the diffusion gradient across the soil surface. With the closed chamber system, the soil CO₂ diffusion rate (∂c/∂t) is required to estimate the soil CO₂ flux. To obtain the ∂c/∂t, the chamber CO₂ concentration must be allowed to rise. Consequently, the ∂c/∂t will be affected by the CO₂ diffusion gradient across the soil surface because of the decreased CO₂ diffusion gradient in the soil chamber. Additionally, the ∂c/∂t will also be affected by the diurnal variation of the CO₂ concentration across the soil surface. Our objective was to compare linear and exponential fitting methods to estimate ∂c/∂t.
Methods Currently, the ∂c/∂t is commonly estimated using linear fitting regression. Instead of using the linear fitting method, an exponential fitting method is used to fit the time series of chamber CO₂ concentration adopted in the LI-8100 automated soil CO₂ flux system.
Important findings The ∂c/∂t estimated from the linear slopes was consistently underestimated as compared to that from exponential initial slopes. Nighttime ∂c/∂t was significantly negatively correlated with soil surface CO₂ concentration, suggesting that the decreased CO₂ diffusion gradient across the soil surface strongly influences the ∂c/∂t. For the closed-chamber method, linear curve fitting significantly underestimated the ∂c/∂t rate during the nighttime. These results demonstrated the importance of estimating the ∂c/∂t at ambient soil surface CO₂ concentration. The response of the ∂c/∂t to air temperature exhibited significant asymmetry characteristic, showing that it is a better way for exponential fitting to make long-term and continuous soil CO₂ flux measurement to elucidate the magnitudes and processes of soil CO₂ flux in the typical terrestrial ecosystem.

Aims Stable isotopes technique and Keeling Plot relationship offer great promise for partitioning evapotranspiration (ET), which can help us better understand the hydrologic cycle within terrestrial ecosystems. Our objectives are to evaluate the Keeling Plot method in ET partitioning using in situ continuous δ¹⁸O data and find the fractional contribution of crop transpiration to total ET in a winter wheat (Triticum aestivum) field.

Methods Field experiments were conducted at Luancheng Agro-ecology Station, Chinese Academy of Sciences. A hydrogen and oxygen isotopes in situ measurement system based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) was used to obtain the continuous atmospheric vapor δ¹⁸O data. Other measurements were made with the eddy covariance technique, cryogenic vacuum distillation and stable isotope ratio mass spectrometry.

Important findings An analysis on the Keeling Plot relationships based on data from different time intervals in one daytime showed that the Keeling Plot would be better when using the midday time interval data to build Keeling Plot, which inferred that the plant transpiration isotopic steady-state (ISS) can be more easily obtained during midday when plant transpiration flux is generally largest. ISS was not always satisfied in field conditions, especially when mature wheat suffered from water stress. Using isotopic partitioning, we estimated transpiration contributed roughly 94%-99% to the total ET during the field measurement period, which indicated plant transpiration dominated local ET.

Micrometeorological measurements depend on knowing the spatial resolution of the heterogeneous surface, defined as the source area. Based on 2002-2003 flux data at the Mid-Subtropical forest area (Qianyanzhou Station), the flux source area was examined using the FSAM analytical model. Evidence suggested that the source area changed with measurement height, time and atmospheric stability, i.e., under the same conditions, a higher measurement and more stable atmospheric layer created a larger flux source area and farther minimum horizontal distance (a) away from the tower. Seasonal variation of wind direction influenced the distribution direction of the area. A higher measurement height and more stable atmospheric layer caused a stronger pertinence of d/Z 0 and S v/u *, which was independent of wind direction. Because of the big fetch, the measurements were representative of the specific characteristics of the instrument location, i.e., the surface of the underlying forest. Moreover, plotting the wind direction and flux data (including the CO 2 flux, the sensible heat flux and the latent heat flux) on a synchronous basis indicated that when the Z/L data satisfy the model requirements, the flux data almost converge at the same wind direction. The results were obtained under conditions of horizontally homogeneous turbulence, which meets the basic need of the model. In Qianyanzhou Station, the FSAM model can only simulate the flux source area under conditions of day-and-night stable atmospheric layer and inactive turbulence. Future work needs to study the turbulent characteristics within and above the forest and improve the model so that it can be applied more widely in the station.

Aims In light of increasing interest in understanding carbon fluxes of terrestrial ecosystems under changing climate and escalating human influences, our study examined the dependence of carbon fluxes on abiotic and biotic factors and explored the effects of conversion of grassland to cropland on ecosystem C fluxes.

Methods Our study was carried out in Duolun (42^o02′ N, 116^o17′ E; 1 350 m asl), a semiarid agriculture-pasture transition region in southeastern Inner Mongolia, China. We used the chamber method during the growing season.

Important findings There was no difference in net ecosystem exchange of carbon dioxide (NEE) between wheat field and steppe at the beginning of the growing season. NEE of wheat field became higher than that of steppe in late June. No differences were found between the two ecosystems from mid-July to August 1st. NEE of wheat field became significantly lower than that of steppe starting in mid-August. During the growing season, the maximum of NEE in steppe was -11.26 µmol CO ₂·m^-2·s^-1, while that in wheat field was -12.29 µmol CO ₂·m^-2·s^-1. Mean NEE in steppe (-5.33 µmol CO ₂·m^-2·s^-1) was a little lower than that in wheat field (-7.66 µmol CO ₂·m^-2·s^-1). Leaf area index (LAI) was the main factor controlling NEE of the two ecosystems. Poor soil nutrient levels also might limit NEE in these ecosystems. Because of characteristics of wheat, the sensitivity of NEE response to LAIwas lower in wheat field than steppe in the middle and late growing season. Lower soil volume water content (10 cm depth) in wheat field limited total ecosystem respiration (TER) and decreased the sensitivity of TER to temperature.

Aims It is important to the study of the carbon cycle and ecological issues to understand seasonal variation in CO₂ flux and the influence of environmental factors on the artificial grassland in the source region of the three rivers on the Qinghai-Tibetan Plateau.

Methods We utilized the eddy covariance method to observe net ecosystem CO₂ exchange (NEE) and biological and environmental factors and their variation at the Elymus nutans artificial grassland from September 1, 2005 to August 31, 2006.

Important findings The daily maximum uptake of CO₂ was 2.38 g C·m^-2·d^-1 on July 30. The ratio of daily uptake and emission in August were observed, -6.82 and 2.95 μmol CO ₂·m^-2·s^-1, respectively. In the growing seasons, daily NEE was dominated by the variation of photosynthetically active radiation (PAR). At the same time, daily NEE combined with leaf area and community diversity to control photosynthetic rate and photosynthetic efficiency. Maximum photosynthetic rate was 2.46-10.39 μmol CO ₂·m^-2·s^-1, and the apparent quantum yield (denoting the maximum efficiency of light utilization in photosynthesis) was 0.013-0.070 μmol CO ₂·μmol^-1 PAR. The influence of temperature, Q₁₀ (1.8) in the growing season was less than in the non-growing season. The respiration of the ecosystem was mainly dominated by temperature and leaf area. Carbon absorption was not dominated by the larger temperature difference of the day and night in the growing season. Our study proved that the artificial grassland ecosystem was a carbon sink with a carbon absorption of -49.35 g C·m^-2·a^-1. Our study also proved that the source and sink function of carbon was influenced by the amount, intensity and seasonal allocation of annual precipitation, as well as by plant community diversity.

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.

Forest ecosystems in northeastern China play an important role in both local and national carbon budgets because of their large area extent and huge amount of carbon storage. The spatial and temporal changes in soil surface CO₂ flux (R_S), the major CO₂ source to the atmosphere from terrestrial ecosystems, directly influence the local and regional carbon budgets. However, few data on R_S were available for this region. In this study, we used an infrared gas exchange analyzer (LI_COR 6400) to measure the R_S and related biophysical factors, and examined soil temperature and moisture effects on soil respiration for six secondary temperate forest ecosystem types: Mongolian oak (dominated by Quercus mongolica), poplar_birch (dominated by Populus davidiana and Betula platyphylla), mixed_wood (no dominant tree species), hard_wood forests (dominated by Fraxinus mandshurica, Juglans mandshurica and Phellodendron amurense), Korean pine (Pinus koraiensis) and Dahurian larch (Larix gmelinii) plantations. Our specific objectives were to: 1) compare the soil temperature, soil moisture, R_S, and Q₁₀ (temperature coefficient) of the six forest types; 2) quantify the seasonality of R_S and related environmental factors; and 3) determine the environmental factors affecting the R_S, and construct models of R_S against the related environmental factors.

Soil temperature, soil moisture and their interactions significantly (p < 0.01) influenced the R_S, but their effects depended on forest type and soil depth. These factors could explain 67.5%-90.6% of the variations in the R_S data. During the growing season, the soil temperature at 10 cm depth in the different forest types did not differ significantly but soil moisture did. The R_S for the oak, pine, larch, hardwood, mixed_wood, and poplar_birch stands varied from 1.89-5.23, 1.09-4.66, 0.95-3.52, 1.13-5.97, 1.05-6.58, and 1.11-5.76 μmol CO ₂·m^-2·s^-1, respectively; the Q₁₀ values for those stands were 2.32, 2.76, 2.57, 2.94, 3.55 and 3.54, correspondingly. The seasonality of R_S was driven mainly by soil temperature and moisture, and was roughly consistent with that of soil temperature. The broad_leaved forests had a higher soil respiration rate than those of coniferous forests probably because of a more suitable soil thermal and moisture regimes and other biological factors.

The temperature sensitivity coefficient of soil respiration (Q₁₀) showed a convex_type curve along a soil moisture gradient. The Q₁₀ tended to increase when soil moisture increased from 30.19 to 40.7, and then declined probably because the extremely high soil moisture content in the hardwood forest may impede activities of soil microbes and plant roots, and thus decrease decomposition rates and soil CO₂ emission. Our study strongly recommended that estimation of soil surface CO₂ flux from forest ecosystems should consider the comprehensive effects of both soil temperature and moisture on soil respiration so as to reduce uncertainties of ecosystem carbon budget studies in this region.

Carbon (C) and water cycles are the most critical processes in terrestrial ecosystems, which links the materials and energy flows through the pedosphere-biosphere-atmosphere integration. Most attention has been paid to the responses of C and water and their feedbacks to global climate change. Flux observation is the basic pathway to quantify the rate of material and energy exchange across soil-plant-atmosphere continuum. As an only technique can directly measure the carbon, water and energy fluxes between vegetation and atmosphere, eddy covariance (EC) technique has been considered as a standard method for flux observation internationally. With broad applications of EC technique on global C and water cycles, long-term flux observations provide scientific data on assessing ecosystem C sequestration capability, water and energy balance, and ecosystem feedback to climate change; optimizing and validating models on regional and global scales; and understanding responses of ecosystem functions to extreme events. Based on long-term flux observation in individual site, scientists have described the seasonal and inter-annual dynamics, and quantified the baseline rates of ecosystem carbon and water fluxes across different climate and vegetation types. With the development of regional and global flux networks, researchers further understood the spatial patterns of ecosystem carbon and water fluxes and their climatic control mechanisms at regional and global scales. This paper briefly introduces the basic principles, hypothesis and instrument system composition, summarizes the major applications of EC observation on C and water fluxes in terrestrial ecosystems, and finally discusses future directions of EC observation network.

Aims Soil respiration is the largest carbon flux in forest ecosystems except for canopy photosynjournal, but the effect of litter on soil respiration in Cunninghamia lanceolata plantations is poorly understood. Our objectives were to examine quantitative differences in changes in soil respiration induced by litter exclusion and addition and to determine how litter manipulation affects soil CO₂ flux in C. lanceolata plantation ecosystems.
Methods We measured soil respiration with an infrared gas exchange analyzer (Li-6400-09) in Tianjiling National Forestry Park, Changsha, Hunan, China from January to December 2007. In the litter exclusion treatment, all ground litter was removed and aboveground litter input was excluded. In the litter addition treatment, litter removed from litter exclusion plots was added to produce double litter. Soil temperature and moisture were measured at 5 cm depth at the same time as soil respiration measurements.
Important findings The treatments of litter exclusion and addition had significantly different seasonal patterns of CO₂ flux processes. Average soil respiration rates of litter exclusion and addition plots were 159.2 and 216.8 mg CO₂·m^-2·h^-1, respectively. The soil respiration rate with litter exclusion was 15.0% lower than the control (180.9 mg CO₂·m^-2·h^-1) and with litter addition was 17.0% higher than the control. Significant exponential relationships were found between soil temperature and soil respiration rate under both treatments, and soil temperature could explain 85.3% and 89.6% of the seasonal changes in soil respiration in the litter exclusion and litter addition plots, respectively. The relationship between soil respiration rate (y) and soil temperature (t) was described by the regression equations: y=27.33e^{0.087 2t}(R²=0.853, p＜0.001) and y=37.25e^{0.088 8t}(R²=0.896, p＜0.001) in the litter exclusion and addition plots, respectively. The Q₁₀ values in the litter exclusion and litter addition plots were 2.39 and 2.43, higher than the control (2.26). The results indicated that litter-fall is an important factor affecting soil CO₂ efflux in forests.

Temperate and tropical oxic soils usually exhibit low levels of atmospheric CH4 oxidation, which are estimated to consume about 10% of the atmospheric CH4. It has been reported that the semi-arid grasslands represent a significant global sink for CH4 and

Aims Our objective was to determine the impact of temperature and soil water content on soil respiration in Haloxylon ammodendron, Anabasis aphylla and Halostachys caspica desert communities.
Methods We measured soil respiration in the 2005 and 2006 growing seasons using an automated CO₂ efflux system (Li-Cor 8100). Air temperature (at 50 cm in height) and soil temperature (every 5 cm from 0 to 50 cm depth) were monitored at three points adjacent to the chamber using a digital thermometer at each site. Gravimetric soil moisture at 0-5, 5-15, 15-30, and 30-50 cm depths at three points was measured using the oven-drying method at 105 °C for 48 h. Water was added for artificial precipitation using plastic watering cans.
Important findings Soil respiration showed an asymmetric daytime pattern, with the minimum at 8:00 and the maximum at 12:00-14:00. The seasonal variation of soil respiration was characterized by a minimum in October and a maximum in June or July, which generally followed that of air temperature. The mean soil respiration rate in the growing season was 0.76, 0.52 and 0.46 μmol CO₂·m^-2·s^-1 in Haloxylon ammodendron, Anabasis aphylla and Halostachys caspica communities, respectively. Air temperature explained >35%, 51% and 65% of seasonal variations of soil respiration in Haloxylon ammodendron, Anabasis aphylla and Halostachys caspica communities, respectively. Q₁₀ values increased in Haloxylon ammodendron (1.35), Anabasis aphylla (1.41) and Halostachys caspica (1.52) communities, and R₁₀ decreased 0.45, 0.30 and 0.22 μmol CO₂·m^-2·s^-1 in each site, respectively. Significant power and quadratic relationships existed between normalized soil respiration and soil water content in the Haloxylon ammodendron and Anabasis aphylla communities, but not in the Halostachys caspica community. Two-dimensional equations based on temperature and soil water content explained most of temporal variations of soil respiration: 71%-93% in Haloxylon ammodendron, 79%-82% in Anabasis aphylla and 70%-80% in Halostachys caspica. Following artificial precipitation, the rate of soil respiration decreased, increased and then quickly decreased again, a pattern consistent with changes in soil temperature.

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 Seasonal drought frequently occurs in the mid-subtropical region of China and commonly combines with high temperature. Our objectives were to test the sensitivity of carbon exchange to this seasonal drought and discuss the influence of seasonal drought on carbon assimilation.

Methods We used flux measurements obtained from eddy covariance technology since October 2002 over a human-planted forest ecosystem at Qianyanzhou (QYZ) (26°44' N, 115°03' E, 110.8 m als.). The EALCO (ecological assimilation of land and climate observations) model is parameterized to simulate the ecosystem carbon exchange process in the human-planted evergreen forest. Simulation results were validated using half-hourly carbon fluxes and daily and annualGPP (gross primary production), NEP (net ecosystem production) and TER (total ecosystem respiration) estimated from eddy covariance measurements.

Important findings In general, the model can effectively simulate the two years' carbon fluxes among soil-plant-atmosphere on hourly, daily and annual scales. Both simulations and observations showed strong impact of drought on GPP in 2003. Compared with 2004, the annual GPP in 2003 was 12.9% lower according to observations (1 610 vs. 1 865 g C·m^-2) and 11.2% lower according to model results (1 637 vs. 1 844 g C·m^-2). The diurnal variations of NEP from both observations and simulations during the period of soil water deficit showed asymmetric format, i.e., the peak value of carbon exchange accrued at a certain time in the morning and then decreased with time. Modeling results indicated that water stress has more influence on photosynthesis than TER, which led to the decrease of NEP. Further analysis suggested that deep soil water content controls canopy photosynthesis in sunny days before noon during soil water stress. Afternoon, both high temperature and deep soil water content eliminate the GPP, and their elimination percents are equal. On cloudy days, radiation and deep soil water content primarily determine the photosynthesis, and temperature becomes a generally minor controlling factor.

Aims Our major objectives were to quantify methane fluxes and variations during growing season and understand key factors controlling methane fluxes in five natural forested swamps.

Methods We measured methane fluxes from natural forested swamps in Xiaoxing’an Mountains from early June to late October 2007, using the static opaque chamber and gas chromatography technique.

Important findings We observed large seasonal variations in methane fluxes in Alnus sibirica swamp, Larix gmelinii-moss swamp and Larix gmelinii-Sphagnum spp. swamp, but smaller variations in Betula platyphylla swamp and Larix gmelinii-Carex schmidtii swamp. Episodic flux was detected in Larix gmelinii-Sphagnum spp. swamp, which greatly influenced methane fluxes during the growing season. Alnus sibirica swamp, Larix gmelinii-moss swamp and Larix gmelinii-Sphagnum spp. swamp were sources of atmospheric methane, but Betula platyphylla swamp and Larix gmelinii-Carex schmidtii swamp were sinks, and the average methane emission rates during the growing season were (56.08 ± 200.38), (15.34 ± 14.89), (0.64 ± 0.88), (-0.88 ± 1.76) and (-0.94 ± 3.00 ) mg·m-2·d-1, respectively. Average methane emission rates were higher with higher water table among the forested swamps, except for Larix gmelinii-Sphagnum spp. swamp. There was a critical point of atmospheric methane source or sink when the water table was at -34.5 ~ -30.8 cm; forested swamps with average water table below this value were atmospheric methane sinks. Effects of temperature on methane fluxes were complex, as temperature may show positive or negative and significant or non-significant correlations. Aboveground biomass of trees may be the best indicator of methane emissions from these forested swamps, because there were strong negative correlations between them.

Soil respiration is an important issue in research on regional carbon budget and global change. Rainfall, which acts as an important disturbance to soil respiration, leads to large uncertainties in estimating carbon exchange between soil and the atmosphere, especially in arid and semiarid regions. Although significant progress on the response of soil respiration to rainfall has been made, considerable controversies on its mechanism still exist. There are two different mechanisms to interpret the “Birch effect”, which is characterized by a strong soil CO₂ emission soon after a rainfall event: “the substrate supply change mechanism” and “microbial stress mechanism”. We review progress in the study of the response of soil respiration to rainfall and summarize the responses of different components of soil respiration to the changes induced by rainfall, including physical replacement and blockage, substrate supply change, activity change of root system and microbes and changes in the structure and function of the microbial community. We also point out four important aspects to be considered in the future: 1) evaluating the function of “substrate supply change mechanism” and “microbial stress mechanism” to the “Birch effect”, 2) quantifying the response of soil respiration to rainfall based on different components, 3) modeling and estimating the response of soil respiration to rainfall on different temporal and spatial scales, and 4) evaluating the possible effects of N and H⁺ from rainfall on soil respiration.

Aims Soil respiration shows spatio-temporal differences at all scales because it is affected by diverse abiotic and biotic factors. Our objectives were to understand the seasonal dynamics and regional patterns of soil respiration in subtropical forests of eastern China and explore the possible underlying reasons.
Methods We examined seasonal variations of soil respiration using a LI-8100 Soil Respiration System in 8 subtropical forests, belonging to three regions of eastern China, from August 2009 to October 2010. Soil temperatures at 5 cm depth were measured at the same time. We evaluated apparent Q₁₀ values and annual CO₂ efflux from soil to atmosphere at each site using the exponential relationship between soil respiration and soil temperature.
Important findings Seasonal differences in soil respiration appeared at all sites, ranging from 2.64 to 6.24 μmol CO₂·m^-2·s^-1. Soil respiration was higher in summer and lower in winter, following the seasonal dynamic of soil temperature. Soil temperature can explain 58.3%-90.2% of the variance of soil respiration for the year. Annual Q₁₀ values from different sites ranged from 1.56 to 3.27. Annual soil CO₂ efflux ranged from 1 077 to 2 058 g C·m ^-2·a^-1 among all research sites in 2010, a high level of ecosystem CO₂ efflux at the global scale.

Aims Responses of soil respiration to global change play an important role in global carbon cycling, but the effects of increasing atmospheric carbon dioxide concentration ([CO₂]), nitrogen (N) deposition and precipitation on soil respiration in subtropical China are unclear. Our objective was to test the effects of increased [CO₂], N deposition and precipitation on soil respiration and to determine how they influence soil respiration in subtropical China.
Methods A model forest ecosystem was constructed of six tree species native to South China. The species were exposed to four experimental treatments in open-top chambers beginning March 2005. Three chambers were used for elevated [CO₂] (CC), two for high N treatment (NN) and the control (CO) and one for elevated precipitation (HR). The CC treatment was achieved by supplying additional CO ₂ from a tank until the chambers had a concentration of (700 ± 20) μmol CO ₂·mol^-1. The NN treatment was achieved by spraying seedlings once a week for a total amount of NH₄NO₃ of 100 kg N·hm^-2·a^-1. The HR treatment was achieved by weekly irrigation with 100 L water.
Important findings For two years, soil respiration displayed strong seasonal patterns with higher values observed in the wet season (April to September) and lower values in the dry season (October to March) in the control chambers (CO) and the CC and NN treatments (p<0.001). There was no seasonal difference in the HR treatment (p>0.05). The CC enrichment affected soil respiration significantly (p<0.05), and there were no significant differences in annual CO₂effluxes between CO and the other two treatments. The annual CO₂ effluxes reached 4 241.7, 3 400.8, 3 432.0 and 3 308.4 g CO₂·m^-2·a^-1 in the CC, NN, HR and CO treatments, respectively. Soil respiration showed diverse responses between dry and wet seasons under different treatments. Higher soil respiration in the CC and NN treatments occurred in the wet season (p<0.05). In the dry season, soil respiration increased in the HR treatment (p<0.05) and decreased in the NN treatment (p<0.05). We found significant exponential relationships between soil respiration rates and soil temperature and significant linear relationships between soil respiration rates and soil moisture (below 15%).

Aims Although temperate broadleaved deciduous forest accounts for two-thirds of the forest area in northeastern China, its spatio-temporal variations of CO₂ concentration ([CO₂]) have not been quantified. Our objectives were to quantify diurnal and seasonal variations and vertical gradients of [CO₂] and explore controlling factors.

Methods A [CO₂] flux tower with an 8-level [CO₂] profile system (at 0.5, 2.0, 4.0, 8.0, 16.0, 20.0, 28.0 and 36.0 m) was installed at the Maoershan Forest Ecosystem Research Station in Heilongjiang Province (45°24′ N, 127°40′ E) in 2007. [CO₂] at each level was measured with a LI-COR LI-840 infrared gas analyzer (IRGA) by drawing it from each level with a sample pump through tubes of equal length. The IRGA was controlled and data were collected with a datalogger. Automatic calibration was done for the IRGA once a day. A Vaisala GMP343 was installed at 36.0 m to monitor ambient [CO₂] for quality control of the [CO₂] profile data. We simultaneously measured micrometeorological variables, including wind speed, air temperature, relative humidity, photosynthetically active radiation, vapor pressure, soil temperature and water content.

Important findings At a daily scale, maximum [CO₂] occurred at night or sun rise, while the minimum occurred in the afternoon at all levels. This pattern was predominant in the summer. The diurnal course of the [CO₂] was “V”-shaped in winter but “U”-shaped in other seasons. [CO₂] decreased with increasing height, particularly on summer nights. During the daytime of summer, daily mean [CO₂] within the canopy was substantially lower than the ambient [CO₂], suggesting that the vegetation acted as a CO₂ sink due to its photosynthesis. Daily mean [CO₂] above the canopy peaked in spring and autumn and reached a minimum in summer, while that near the forest floor showed a unimodal seasonal pattern with its maximum in summer. The diurnal dynamics of [CO₂] and their vertical gradients during the growing season were jointly controlled by the atmospheric boundary layer (ABL) and forest carbon metabolism, while those during the dormant season were controlled mainly by ABL. The seasonal dynamics of the [CO₂] near the forest floor were determined mainly by soil respiration, while those above the canopy were jointly controlled by canopy photosynthesis and ecosystem respiration.

Aims Our objectives were to evaluate the CO2, CH4 and N2O budget for exploring the relationship of source or sink of carbon and nitrogen, to understand seasonal variations of CO2, CH4 and N2O and to explore the effects of temperature and moisture on CO2, CH4 and N2O fluxes in a marsh ecosystem over a growing season.

Methods We analyzed temporal fluxes and factors that influenced CO2, CH4 and N2O fluxes in a Carex schmidtii marsh of Xiaoxing’an Mountains from June to October 2007, using a static chamber and gas chromatograph technique.

Important findings CO2 flux was the highest (99.61%), followed by CH4 (0.39%) and N2O (0.000 7%) in greenhouse gases emission from the marsh. The marsh was a sink of carbon (53.93%) and nitrogen (0.04%). Average CO2, CH4 and N2O fluxes were 487.89, 1.88 and 0.004 mg·m-2·h-1, respectively, and displayed large seasonal variations. The highest emissions of CO2 and N2O were observed in summer (from June 24 to August 14 and from July 14 to August 14, respectively), and the highest emissions of CH4 were in summer and fall (from August 24 to September 24). The CO2 fluxes were significantly correlated with temperature (p < 0.05) (air temperature and soil temperature at 0, 5, 10, 15 and 20 cm), the CH4 fluxes were significantly correlated with air temperature (p < 0.01) and the N2O fluxes were negatively significantly correlated with standing water depth (p < 0.05).

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.

Aims Our objective was to examine the effects of temperature, soil water content and photosynthesis on soil respiration in Populus and Ulmus pumila planting shelterbelts in China’s Junngar Basin.

Methods Soil respiration was measured during the growing seasons in 2005 and 2006 using an automated CO₂ efflux system (LI-8100). Air temperature (at 50 cm in height) and soil temperature (every 5 cm from 0 to 50 cm depth) were monitored at three points adjacent to the chamber using a digital thermometer (WMY-01C, Huachen Medical Instrument Inc., Shanghai, China) at each site. Gravimetric soil moisture at 0-5, 5-15, 15-30, and 30-50 cm depths at three points were measured using the oven-drying method at 105 °C for 48 h.

Important findings Soil respiration displayed irregular fluctuation of daytime pattern and significant single-peaked curve of seasonal pattern in the two woodlands. Seasonally, soil respiration was characterized by a maximum in July or August and a minimum in October or May, following the change of soil temperature. The rate of soil respiration was significantly higher in Populus woodland than that in U. pumila woodland with mean rates of 3.71 and 1.82 μmol CO₂·m^-2·s^-1in two growing seasons, respectively. Soil respiration was significantly correlated with temperature in exponential equation, but uncorrelated with soil water content in the two woodlands. Soil temperature at 50 and 30 cm depths could explain 78.5% and 64.4% of seasonal variations of soil respiration in Populus woodland and U. pumila woodland, respectively, which confirms the common explanation by temperature and soil water content. The difference in soil respiration between the woodlands was influenced by growth state of trees, photosynthesis and soil salinity. Our results suggested that there was significant seasonal variation of soil respiration in oasis shelterbelts in the arid region and soil temperature was the main regulating factor.

Using a static chamber technique, methane fluxes from sediments of five mangrove communities at four sites were studied. An average value of 0.81mg·m-2·d-1 was observed. Using polyethylene bags, methane fluxes through leaves of six mangrove species were also studied. It indicates that mangrove leaves generally absorbed atmospheric methane as an overall effect. Diurnal variations of methane fluxes from sediments of Bruguiera sexangula community at Changning site were related to tidal conditions in the forest while flat variations of methane fluxes from sediments of this community were related to soil water contents. There were two different seasonal patterns of methane flux from soils in the five mangrove communities.

Aims Alpine shrub-meadows and steppe-meadows are the two dominant vegetation types on the Qinghai-Xizang Plateau, and plays an important role in regional carbon cycling. However, little is known about the temporal-spatial patterns and drivers of CO₂ fluxes in these two ecosystem types.

Methods Based on five years of consecutive eddy covariance measurements (2004-2008) in an eastern alpine shrub-meadow at Haibei and a hinterland alpine steppe-meadow at Damxung, we investigated the seasonal and annual variation of net ecosystem productivity (NEP) and its components, i.e. gross primary productivity (GPP) and ecosystem respiration (R_e).

Important findings The CO₂ fluxes (NEP, GPP and R_e) were larger in the shrub-meadow than in the steppe-meadow during the study period. The shrub-meadow functioned as a carbon sink through the five years, with the mean annual NEP of 70 g C·m ^-2·a ^-1. However, the steppe-meadow acted as a carbon neutral, with mean annual NEP of -5 g C·m ^-2·a ^-1. The CO₂ fluxes of steppe-meadow exhibited large variability due to the inter-annual and seasonal variations in precipitation, ranging from a carbon sink (54 g C·m ^-2·a ^-1) in 2008 to a carbon source (-88 g C·m ^-2·a ^-1) in 2006. The differences in carbon budget between the two alpine ecosystems were firstly attributed to the discrepancy of normalized difference vegetation index (NDVI) because NDVI was the direct factor regulating the seasonal and inter-annual NEP. Secondly, the shrub-meadow had higher carbon use efficiency (CUE), which was substantially determined by annual precipitation (PPT) and NDVI. Our results also indicated that the environmental drivers of CO₂ fluxes were also different between these two alpine ecosystems. The structure equation model analyses showed that air temperature (T_a) determined the seasonal variations of CO₂ fluxes in the shrub-meadow, with NEP and GPP being positively correlated with T_a. By contrast, the seasonal CO₂ fluxes in the steppe-meadow were primarily co-regulated by soil water content (SWC) and T_a, and increased with the increase of SWC and T_a. In addition, the changes of R_e during the growing season in two ecosystems were directly affected by GPP and soil temperature at 5 cm depth (T_s), while R_e during non-growing season were determined by T_s. These results demonstrate that the synergy of soil water and temperature played crucial roles in determining NEP and GPP of the two alpine meadows on the Qinghai-Xizang Plateau.

Aims The exotic earthworm Ocnerodrilus occidentalis is widespread in plantation and abandoned areas in Guangdong, China. Its distribution is gradually expanding due to insensitivity to temperature, moisture, soil pH and soil organic matter. Study of the processes of soil carbon dynamics affected by O. occidentalis can provide new insights for the reduction of soil carbon emissions. Our objective was to investigate the short-term impacts of this exotic earthworm and native plants on soil CO₂ fluxes.
Methods A field experiment was conducted in an Acacia auriculaeformis plantation at the Heshan Hilly Land Interdisciplinary Experimental Station. CO₂ fluxes were measured for 15 days in situ using the static chamber technique and analyzed with a gas chromatogram.
Important findings Both O. occidentalis and Evodia lepta had no significant effects on soil CO₂ fluxes. The effects of plant physical processes (such as shading), plant biological processes (such as secretion of root exudates) and overall processes on soil CO₂ fluxes were -32.1%, 40.9% and 8.8%, respectively, in treatments without earthworm addition, and were -7.2%, 30.7% and 23.5%, respectively, in treatments with earthworm addition. Plant physical processes inhibited soil CO₂ emissions, but enhanced the effects of the earthworm on soil CO₂ emissions (increased by 39.3%). Plant biological processes enhanced soil CO₂ emissions, but inhibited the effects of earthworms on soil CO₂ emissions (decreased by 23.5%). Earthworm addition showed almost no significant impacts on most soil physical and chemical properties, but enhanced the activity of soil bacteria and led to a closer correlation between soil CO₂ fluxes and soil physical and chemical properties. Meanwhile, earthworm activities changed the relationships between soil CO₂ fluxes and soil hydrothermal factors. Hence, soil CO₂ fluxes were not only influenced by hydrothermal factors, but also regulated by above- and below-ground biological processes. Therefore, it was difficult to determine an effective way to reduce forest soil CO₂ emissions if only the amount of soil CO₂ is considered and the impact of plant biological processes on soil CO₂ fluxes is ignored. In order to reduce soil carbon efflux, it will be useful to note the potential of the independent and interactive effects of plant physical processes, plant biological processes and earthworm activity on soil CO₂ flux.