增温对青藏高原高寒草原生态系统碳交换的影响

碳交换是影响草地生态系统碳汇功能的关键过程,对气候变暖极为敏感。青藏高原分布着大面积的高寒草原,其碳汇功能对气候变暖的响应对区域碳循环过程具有重要的影响。为探究高寒草原生态系统碳交换过程对增温的响应,2012—2014年,在青藏高原班戈县进行了模拟增温对高寒草原生态系统碳交换过程影响的研究。结果表明,增温对高寒草原碳交换各组分的影响存在年际差异,但总体上对碳交换存在负面影响。3年平均结果显示,增温显著降低了高寒草原地上生物量、总生态系统生产力(GEP)、生态系统呼吸(ER)和净生态系统碳交换量(NEE)(P0.05),平均降幅分别为15.1%、36.8%、19.2%和51.5%。增温条件下3年平均土壤呼吸(SR)较对照无显著变化(P0.05),但2013年增温显著降低了SR(P0.05),降幅达18.1%。增温对SR与ER的比值具有一定的促进作用,最高增幅达到40.0%。GEP、ER、SR和NEE与土壤温度和土壤水分无显著相关(P0.05),而GEP、ER和NEE与空气温度呈显著的负相关关系(P0.05)。增温引起的干旱胁迫以及地上生物量降低是导致高寒草原NEE降低的主要原因。研究表明,全球变暖会一定程度降低青藏高原高寒草原的碳汇功能。     

Priming effect stimulates carbon release from thawed permafrost

Climate warming leads to widespread permafrost thaw with a fraction of the thawed permafrost carbon (C) being released as carbon dioxide (CO₂), thus triggering a positive permafrost C-climate feedback. However, large uncertainty exists in the size of this model-projected feedback, partly owing to the limited understanding of permafrost CO₂ release through the priming effect (i.e., the stimulation of soil organic matter decomposition by external C inputs) upon thaw. By combining permafrost sampling from 24 sites on the Tibetan Plateau and laboratory incubation, we detected an overall positive priming effect (an increase in soil C decomposition by up to 31%) upon permafrost thaw, which increased with permafrost C density (C storage per area). We then assessed the magnitude of thawed permafrost C under future climate scenarios by coupling increases in active layer thickness over half a century with spatial and vertical distributions of soil C density. The thawed C stocks in the top 3 m of soils from the present (2000–2015) to the future period (2061–2080) were estimated at 1.0 (95% confidence interval (CI): 0.8–1.2) and 1.3 (95% CI: 1.0–1.7) Pg (1 Pg = 10¹⁵ g) C under moderate and high Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5, respectively. We further predicted permafrost priming effect potential (priming intensity under optimal conditions) based on the thawed C and the empirical relationship between the priming effect and permafrost C density. By the period 2061–2080, the regional priming potentials could be 8.8 (95% CI: 7.4–10.2) and 10.0 (95% CI: 8.3–11.6) Tg (1 Tg = 10¹² g) C year⁻¹ under the RCP 4.5 and RCP 8.5 scenarios, respectively. This large CO₂ emission potential induced by the priming effect highlights the complex permafrost C dynamics upon thaw, potentially reinforcing permafrost C-climate feedback.     

青藏高原气候变化与高寒草地

综述了近五十年来青藏高原气候和高寒草地的变化趋势,阐述了气候变化对高寒草地的可能影响。气候变化主要通过水、热过程及其诱导的环境变化对青藏高原高寒草地产生显著的影响。主要过程包括：气候变化对气候带、植被带、植物、植物群落、农业生产以及生态系统固碳潜力等的影响。从目前的观测和研究结果来看,有关青藏高原气候变化及其对高寒草地的可能影响都还很难得出一致的结论。因此,如何科学评价气候变化及其预测和评价对高寒草地结构和功能的潜在影响,以及如何将已经发生的变化纳入到全球变化模型或评价体系中,以便更加精确地评估气候变化的长期影响,将成为必须要回答的关键科学问题。     

Belowground carbon responses to experimental warming regulated by soil moisture change in an alpine ecosystem of the Qinghai–Tibet Plateau

Recent studies found that the largest uncertainties in the response of the terrestrial carbon cycle to climate change might come from changes in soil moisture under the elevation of temperature. Warming‐induced change in soil moisture and its level of influence on terrestrial ecosystems are mostly determined by climate, soil, and vegetation type and their sensitivity to temperature and moisture. Here, we present the results from a warming experiment of an alpine ecosystem conducted in the permafrost region of the Qinghai–Tibet Plateau using infrared heaters. Our results show that 3 years of warming treatments significantly elevated soil temperature at 0–100 cm depth, decreased soil moisture at 10 cm depth, and increased soil moisture at 40–100 cm depth. In contrast to the findings of previous research, experimental warming did not significantly affect NH ₄ ⁺‐N, NO ₃ ⁻‐N, and heterotrophic respiration, but stimulated the growth of plants and significantly increased root biomass at 30–50 cm depth. This led to increased soil organic carbon, total nitrogen, and liable carbon at 30–50 cm depth, and increased autotrophic respiration of plants. Analysis shows that experimental warming influenced deeper root production via redistributed soil moisture, which favors the accumulation of belowground carbon, but did not significantly affected the decomposition of soil organic carbon. Our findings suggest that future climate change studies need to take greater consideration of changes in the hydrological cycle and the local ecosystem characteristics. The results of our study will aid in understanding the response of terrestrial ecosystems to climate change and provide the regional case for global ecosystem models.     

Seasonal and Spatial Controls over Nutrient Cycling in a Pacific Northwest Prairie

West Coast prairies in the US are an endangered ecosystem, and effective conservation will require an understanding of how changing climate will impact nutrient cycling and availability. We examined how seasonal patterns and micro-heterogeneity in edaphic conditions (% moisture, total organic carbon, % clay, pH, and inorganic nitrogen and phosphorus) control carbon, nitrogen, and phosphorus cycling in an upland prairie in western Oregon, USA. Across the prairie, we collected soils seasonally and measured microbial respiration, net nitrogen mineralization, net nitrification, and phosphorus availability under field conditions and under experimentally varied temperature and moisture treatments. The response variables differed in the degree of temperature and moisture limitation within seasons and how these factors varied across sampling sites. In general, we found that microbial respiration was limited by low soil moisture year-round and by low temperatures in the winter. Net nitrogen mineralization and net nitrification were never limited by temperature, but both were limited by excessive soil moisture in winter, and net nitrification was also inhibited by low soil moisture in the summer. Factors that enhanced microbial respiration tended to decrease soil phosphorus availability. Edaphic factors explained 76% of the seasonal and spatial variation in microbial respiration, 35% of the variation in phosphorus availability, and 29% of the variation in net nitrification. Much of the variation in net nitrogen mineralization remained unexplained (R ² = 0.19). This study, for the first time, demonstrates the complex seasonal controls over nutrient cycling in a Pacific Northwest prairie.     

Diurnal, seasonal and annual variation in net ecosystem CO2 exchange of an alpine shrubland on Qinghai-Tibetan plateau

Thus far, grassland ecosystem research has mainly been focused on low‐lying grassland areas, whereas research on high‐altitude grassland areas, especially on the carbon budget of remote areas like the Qinghai‐Tibetan plateau is insufficient. To address this issue, flux of CO₂ were measured over an alpine shrubland ecosystem (37°36′N, 101°18′E; 325 above sea level [a. s. l.]) on the Qinghai‐Tibetan Plateau, China, for 2 years (2003 and 2004) with the eddy covariance method. The vegetation is dominated by formation Potentilla fruticosa L. The soil is Mol–Cryic Cambisols. To interpret the biotic and abiotic factors that modulate CO₂ flux over the course of a year we decomposed net ecosystem CO₂ exchange (NEE) into its constituent components, and ecosystem respiration (R_eco). Results showed that seasonal trends of annual total biomass and NEE followed closely the change in leaf area index. Integrated NEE were ?58.5 and ?75.5 g C m^?2, respectively, for the 2003 and 2004 years. Carbon uptake was mainly attributed from June, July, August, and September of the growing season. In July, NEE reached seasonal peaks of similar magnitude (4–5 g C m^?2 day^?1) each of the 2 years. Also, the integrated night‐time NEE reached comparable peak values (1.5–2 g C m^?2 day^?1) in the 2 years of study. Despite the large difference in time between carbon uptake and release (carbon uptake time < release time), the alpine shrubland was carbon sink. This is probably because the ecosystem respiration at our site was confined significantly by low temperature and small biomass and large day/night temperature difference and usually soil moisture was not limiting factor for carbon uptake. In general, R_eco was an exponential function of soil temperature, but with season‐dependent values of Q₁₀. The temperature‐dependent respiration model failed immediately after rain events, when large pulses of R_eco were observed. Thus, for this alpine shrubland in Qinghai‐Tibetan plateau, the timing of rain events had more impact than the total amount of precipitation on ecosystem R_eco and NEE.     

Carbon uptake in Eurasian boreal forests dominates the high-latitude net ecosystem carbon budget

Arctic-boreal landscapes are experiencing profound warming, along with changes in ecosystem moisture status and disturbance from fire. This region is of global importance in terms of carbon feedbacks to climate, yet the sign (sink or source) and magnitude of the Arctic-boreal carbon budget within recent years remains highly uncertain. Here, we provide new estimates of recent (2003–2015) vegetation gross primary productivity (GPP), ecosystem respiration (R_eco), net ecosystem CO₂ exchange (NEE; R_eco − GPP), and terrestrial methane (CH₄) emissions for the Arctic-boreal zone using a satellite data-driven process-model for northern ecosystems (TCFM-Arctic), calibrated and evaluated using measurements from >60 tower eddy covariance (EC) sites. We used TCFM-Arctic to obtain daily 1-km² flux estimates and annual carbon budgets for the pan-Arctic-boreal region. Across the domain, the model indicated an overall average NEE sink of −850 Tg CO₂-C year⁻¹. Eurasian boreal zones, especially those in Siberia, contributed to a majority of the net sink. In contrast, the tundra biome was relatively carbon neutral (ranging from small sink to source). Regional CH₄ emissions from tundra and boreal wetlands (not accounting for aquatic CH₄) were estimated at 35 Tg CH₄-C year⁻¹. Accounting for additional emissions from open water aquatic bodies and from fire, using available estimates from the literature, reduced the total regional NEE sink by 21% and shifted many far northern tundra landscapes, and some boreal forests, to a net carbon source. This assessment, based on in situ observations and models, improves our understanding of the high-latitude carbon status and also indicates a continued need for integrated site-to-regional assessments to monitor the vulnerability of these ecosystems to climate change.     

A minimally managed switchgrass ecosystem in a humid subtropical climate is a source of carbon to the atmosphere

Bioenergy has been identified as a key component of climate change mitigation. Therefore, quantifying the net carbon balance of bioenergy feedstocks is crucial for accurate projections of climate mitigation benefits. Switchgrass (Panicum virgatum) has many characteristics of an ideal bioenergy crop with high yields, low maintenance, and deep roots with potential for belowground carbon sequestration. However, the assessments of net annual carbon exchange between switchgrass fields and the atmosphere are rare. Here we present observations of net carbon fluxes in a minimally managed switchgrass field in Virginia (Ameriflux site US-SB2) over 5 years (3–7 years since establishment). Average annual net ecosystem exchange (NEE) of carbon was near zero (60 g C m^?2 year^?1) but the net ecosystem carbon balance that includes harvested carbon (HC) was a net source of carbon to the atmosphere (313 g C m^?2 year^?1). The field alternated between a large and small source of carbon annually, with the interannual variability most strongly correlated with the day of the last frost and the interaction of temperature and precipitation. Overall, the consistent source of carbon to the atmosphere at US-SB2 differs substantially from other eddy covariance studies that report switchgrass fields to be either neutral or a sink of carbon when accounting for both NEE and HC. This study illustrates that predictions of net carbon climate benefits from bioenergy crops cannot assume that the ecosystem will be a net sink of carbon from the atmosphere. Background climate, management, and land-use history may determine whether widespread deployment of switchgrass as a bioenergy feedstock results in realized climate change mitigation.     

Differential responses of production and respiration to temperature and moisture drive the carbon balance across a climatic gradient in New Mexico

Southwestern North America faces an imminent transition to a warmer, more arid climate, and it is critical to understand how these changes will affect the carbon balance of southwest ecosystems. In order to test our hypothesis that differential responses of production and respiration to temperature and moisture shape the carbon balance across a range of spatio‐temporal scales, we quantified net ecosystem exchange (NEE) of CO₂ and carbon storage across the New Mexico Elevational Gradient, which consists of six eddy‐covariance sites representing biomes ranging from desert to subalpine conifer forest. Within sites, hotter and drier conditions were associated with an increasing advantage of respiration relative to production such that daily carbon uptake peaked at intermediate temperatures – with carbon release often occurring on the hottest days – and increased with soil moisture. Across sites, biotic adaptations modified but did not override the dominant effects of climate. Carbon uptake increased with decreasing temperature and increasing precipitation across the elevational gradient; NEE ranged from a source of ～30 g C m^?2 yr^?1 in the desert grassland to a sink of ～350 g C m^?2 yr^?1 in the subalpine conifer forest. Total aboveground carbon storage increased dramatically with elevation, ranging from 186 g C m^?2 in the desert grassland to 26 600 g C m^?2 in the subalpine conifer forest. These results make sense in the context of global patterns in NEE and biomass storage, and support that increasing temperature and decreasing moisture shift the carbon balance of ecosystems in favor of respiration, such that the potential for ecosystems to sequester and store carbon is reduced under hot and/or dry conditions. This implies that projected climate change will trigger a substantial net release of carbon in these New Mexico ecosystems (～3 Gt CO₂ statewide by the end of the century), thereby acting as a positive feedback to climate change.     

The impacts of climate change and human activities on biogeochemical cycles on the Qinghai‐Tibetan Plateau

With a pace of about twice the observed rate of global warming, the temperature on the Qinghai‐Tibetan Plateau (Earth's ‘third pole’) has increased by 0.2 °C per decade over the past 50 years, which results in significant permafrost thawing and glacier retreat. Our review suggested that warming enhanced net primary production and soil respiration, decreased methane (CH₄) emissions from wetlands and increased CH₄ consumption of meadows, but might increase CH₄ emissions from lakes. Warming‐induced permafrost thawing and glaciers melting would also result in substantial emission of old carbon dioxide (CO₂) and CH₄. Nitrous oxide (N₂O) emission was not stimulated by warming itself, but might be slightly enhanced by wetting. However, there are many uncertainties in such biogeochemical cycles under climate change. Human activities (e.g. grazing, land cover changes) further modified the biogeochemical cycles and amplified such uncertainties on the plateau. If the projected warming and wetting continues, the future biogeochemical cycles will be more complicated. So facing research in this field is an ongoing challenge of integrating field observations with process‐based ecosystem models to predict the impacts of future climate change and human activities at various temporal and spatial scales. To reduce the uncertainties and to improve the precision of the predictions of the impacts of climate change and human activities on biogeochemical cycles, efforts should focus on conducting more field observation studies, integrating data within improved models, and developing new knowledge about coupling among carbon, nitrogen, and phosphorus biogeochemical cycles as well as about the role of microbes in these cycles.     

Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics

Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO₂ and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (R_h) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO₂ concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO₂ concentration but to decrease with warming. As CO₂ concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased R_h, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO₂ and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year^?1 for boreal forests and 0–23% year^?1 in the tundra between 1960 and 1990.     

Supply-side controls on soil respiration among Oregon forests

To test the hypothesis that variation in soil respiration is related to plant production across a diverse forested landscape, we compared annual soil respiration rates with net primary production and the subsequent allocation of carbon to various ecosystem pools, including leaves, fine roots, forests floor, and mineral soil for 36 independent plots arranged as three replicates of four age classes in three climatically distinct forest types. Across all plots, annual soil respiration was not correlated with aboveground net primary production (R²=0.06, P>0.1) but it was moderately correlated with belowground net primary production (R²=0.46, P<0.001). Despite the wide range in temperature and precipitation regimes experienced by these forests, all exhibited similar soil respiration per unit live fine root biomass, with about 5 g of carbon respired each year per 1 g of fine root carbon (R²=0.45, P<0.001). Annual soil respiration was only weakly correlated with dead carbon pools such as forest floor and mineral soil carbon (R²=0.14 and 0.12, respectively). Trends between soil respiration, production, and root mass among age classes within forest type were inconsistent and do not always reflect cross‐site trends. These results are consistent with a growing appreciation that soil respiration is strongly influenced by the supply of carbohydrates to roots and the rhizosphere, and that some regional patterns of soil respiration may depend more on belowground carbon allocation than the abiotic constraints imposed on subsequent metabolism.     

Decrease in winter respiration explains 25% of the annual northern forest carbon sink enhancement over the last 30 years

Aim Winter snow has been suggested to regulate terrestrial carbon (C) cycling by modifying microclimate, but the impacts of change in snow cover on the annual C budget at a large scale are poorly understood. Our aim is to quantify the C balance under changing snow depth. Location Non‐permafrost region of the northern forest area. Methods Here, we used site‐based eddy covariance flux data to investigate the relationship between depth of snow cover and ecosystem respiration (R_eco) during winter. We then used the Biome‐BGC model to estimate the effect of reductions in winter snow cover on the C balance of northern forests in the non‐permafrost region. Results According to site observations, winter net ecosystem C exchange (NEE) ranged from 0.028 to 1.53 gC·m^?2·day^?1, accounting for 44 ± 123% of the annual C budget. Model simulation showed that over the past 30 years, snow‐driven change in winter C fluxes reduced non‐growing season CO₂ emissions, enhancing the annual C sink of northern forests. Over the entire study area, simulated winter R_eco significantly decreased by 0.33 gC·m^?2·day^?1·year^?1 in response to decreasing depth of snow cover, which accounts for approximately 25% of the simulated annual C sink trend from 1982 to 2009. Main conclusion Soil temperature is primarily controlled by snow cover rather than by air temperature as snow serves as an insulator to prevent chilling impacts. A shallow snow cover has less insulation potential, causing colder soil temperatures and potentially lower respiration rates. Both eddy covariance analysis and model‐simulated results show that both R_eco and NEE are significantly and positively correlated with variation in soil temperature controlled by variation in snow depth. Overall, our results highlight that a decrease in winter snow cover restrains global warming as less C is emitted to the atmosphere.     

Effects of permafrost degradation on ecosystems

Permafrost, covering approximately 25% of the land area in the Northern Hemisphere, is one of the key components of terrestrial ecosystem in cold regions. As a product of cold climate, permafrost is extremely sensitive to climate change. Climate warming over past decades has caused degradation in permafrost widely and quickly. Permafrost degradation has the potential to significantly change soil moisture content, alter soil nutrients availability and influence on species composition. In lowland ecosystems the loss of ice-rich permafrost has caused the conversion of terrestrial ecosystem to aquatic ecosystem or wetland. In upland ecosystems permafrost thaw has resulted in replacement of hygrophilous community by xeromorphic community or shrub. Permafrost degradation resulting from climate warming may dramatically change the productivity and carbon dynamics of alpine ecosystems. This paper reviewed the effects of permafrost degradation on ecosystem structure and function. At the same time, we put forward critical questions about the effects of permafrost degradation on ecosystems on Qinghai–Tibetan Plateau, included: (1) carry out research about the effects of permafrost degradation on grassland ecosystem and the response of alpine ecosystem to global change; (2) construct long-term and located field observations and research system, based on which predict ecosystem dynamic in permafrost degradation; (3) pay extensive attention to the dynamic of greenhouse gas in permafrost region on Qinghai–Tibetan Plateau and the feedback of greenhouse gas to climate change; (4) quantitative study on the change of water-heat transport in permafrost degradation and the effects of soil moisture and heat change on vegetation growth.     

Soil respiration of forest ecosystems in Japan and global implications

Within terrestrial ecosystems, soil respiration is one of the largest carbon flux components. We discuss the factors controlling soil respiration, while focusing on research conducted at the Takayama Experimental Site. Soil respiration was affected by soil temperature, soil moisture, rainfall events, typhoons, and root respiration. We consider the temporal and spatial variability of soil respiration at the Takayama Experimental Site and review the variability of annual soil respiration in Japanese forests. In the 26 compiled studies, the values of annual soil respiration ranged from 203 to 1,290 g C m⁻² year⁻¹, with a mean value of 669 g C m⁻² year⁻¹ (SD=264, CV=40). We note the need for more studies and data synthesis for the accurate prediction of soil respiration and soil carbon dynamics in Japanese forests. Finally, several methods for measuring soil respiration rates are compared and the implications of soil respiration rates for global climate change are discussed.     

Decade of experimental permafrost thaw reduces turnover of young carbon and increases losses of old carbon,without affecting the net carbon balance

Thicker snowpacks and their insulation effects cause winter‐warming and invoke thaw of permafrost ecosystems. Temperature‐dependent decomposition of previously frozen carbon (C) is currently considered one of the strongest feedbacks between the Arctic and the climate system, but the direction and magnitude of the net C balance remains uncertain. This is because winter effects are rarely integrated with C fluxes during the snow‐free season and because predicting the net C balance from both surface processes and thawing deep layers remains challenging. In this study, we quantified changes in the long‐term net C balance (net ecosystem production) in a subarctic peat plateau subjected to 10 years of experimental winter‐warming. By combining ²¹⁰Pb and ¹⁴Cdating of peat cores with peat growth models, we investigated thawing effects on year‐round primary production and C losses through respiration and leaching from both shallow and deep peat layers. Winter‐warming and permafrost thaw had no effect on the net C balance, but strongly affected gross C fluxes. Carbon losses through decomposition from the upper peat were reduced as thawing of permafrost induced surface subsidence and subsequent waterlogging. However, primary production was also reduced likely due to a strong decline in bryophytes cover while losses from the old C pool almost tripled, caused by the deepened active layer. Our findings highlight the need to estimate long‐term responses of whole‐year production and decomposition processes to thawing, both in shallow and deep soil layers, as they may contrast and lead to unexpected net effects on permafrost C storage.     

Carbon balances in US croplands during the last two decades of the twentieth century

Carbon (C) added to soil as organic matter in crop residues and carbon emitted to the atmosphere as CO₂ in soil respiration are key determinants of the C balance in cropland ecosystems. We used complete and comprehensive county-level yields and area data to estimate and analyze the spatial and temporal variability of regional and national scale residue C inputs, net primary productivity (NPP), and C stocks in US croplands from 1982 to 1997. Annual residue C inputs were highest in the North Central and Central and Northern Plains regions that comprise ~70% of US cropland. Average residue C inputs ranged from 1.8 (Delta States) to 3.0 (North Central region) Mg?C?ha^?1?year^?1, and average NPP ranged from 3.1 (Delta States) to 5.4 (Far West region) Mg?C?ha^?1?year^?1. Residue C inputs tended to be inversely proportional to the mean growing season temperature. A quadratic relationship incorporating the growing season mean temperature and total precipitation closely predicted the variation in residue C inputs in the North Central region and Central and Northern Plains. We analyzed the soil C balance using the crop residue database and the Introductory Carbon Balance regional Model (ICBMr). Soil C stocks (0–20?cm) on permanent cropland ranged between 3.07 and 3.1?Pg during the study period, with an average increase of ~4?Tg?C?year^?1, during the 1990s. Interannual variability in soil C stocks ranged from 0 to 20?Tg?C (across a mean C stock of 3.08?±?0.01?Pg) during the study period; interannual variability in residue C inputs varied between 1 and 43?Tg C (across a mean input of 220?±?19?Tg). Such interannual variation has implications for national estimates of CO₂ emissions from cropland soils needed for implementation of greenhouse gas (GHG) mitigation strategies involving agriculture.     

Impacts of tropospheric ozone and climate change on net primary productivity and net carbon exchange of China's forest ecosystems

Aim We investigated how ozone pollution and climate change/variability have interactively affected net primary productivity (NPP) and net carbon exchange (NCE) across China's forest ecosystem in the past half century. Location Continental China. Methods Using the dynamic land ecosystem model (DLEM) in conjunction with 10‐km‐resolution gridded historical data sets (tropospheric O₃ concentrations, climate variability/change, and other environmental factors such as land‐cover/land‐use change (LCLUC), increasing CO₂ and nitrogen deposition), we conducted nine simulation experiments to: (1) investigate the temporo‐spatial patterns of NPP and NCE in China's forest ecosystems from 1961–2005; and (2) quantify the effects of tropospheric O₃ pollution alone or in combination with climate variability and other environmental stresses on forests' NPP and NCE. Results China's forests acted as a carbon sink during 1961–2005 as a result of the combined effects of O₃, climate, CO₂, nitrogen deposition and LCLUC. However, simulated results indicated that elevated O₃ caused a 7.7% decrease in national carbon storage, with O₃‐induced reductions in NCE (Pg C year^?1) ranging from 0.4–43.1% among different forest types. Sensitivity experiments showed that climate change was the dominant factor in controlling changes in temporo‐spatial patterns of annual NPP. The combined negative effects of O₃ pollution and climate change on NPP and NCE could be largely offset by the positive fertilization effects of nitrogen deposition and CO₂. Main conclusions In the future, tropospheric O₃ should be taken into account in order to fully understand the variations of carbon sequestration capacity of forests and assess the vulnerability of forest ecosystems to climate change and air pollution. Reducing air pollution in China is likely to increase the resilience of forests to climate change. This paper offers the first estimate of how prevention of air pollution can help to increase forest productivity and carbon sequestration in China's forested ecosystems.     

中国草原土壤呼吸作用研究进展

中国草原面积约占国土面积的40%, 且大都位于生态脆弱区, 对气候和环境变化十分敏感, 在未来大气CO₂调控中有着重要的作用。为增进对中国草原土壤呼吸作用的理解, 该文综述了近10年来中国草原土壤呼吸作用的最新研究进展, 指出中国草原土壤呼吸作用的研究主要集中在东北平原、内蒙古高原和青藏高原。草原土壤呼吸作用日动态的主导控制因子是温度, 季节动态的主导控制因子可以是温度、水分或二者的交互作用, 取决于研究地点的限制性环境因子, 而年际动态的主导控制因子为水分。草原土壤呼吸作用还存在着巨大的空间变异, 年降水和土壤全氮含量是不同类型草原土壤呼吸作用空间异质性的主导控制因子。土壤呼吸作用对全球变化的响应比较复杂, 取决于各因子之间相互影响的贡献。现有的土壤呼吸作用模型大多只考虑了水热因子, 很少包含土壤因子和生物因子及其协同作用的影响。在此基础上, 指出未来中国草原土壤呼吸作用拟加强的研究重点: 1)温带荒漠草原土壤呼吸作用研究; 2)非生长季土壤呼吸作用研究; 3)多时空尺度草原土壤呼吸作用的比较研究; 4)草原土壤呼吸作用过程模拟研究; 5)草原土壤呼吸作用的遥感监测评估研究。