Experimental warming increases CO2 saturation in a shallow prairie pond

There is an urgent need to understand the effect of climate warming on the carbon dynamics of lakes and ponds in order to assess contributions to global carbon budgets. Currently, we are unable to predict how the exchange of carbon gases (i.e. CO₂) across the air–water boundary and organic carbon storage in the sediments will be altered with realistic warming scenarios downscaled from climatic models. Given the prevalence of shallow systems and tight atmospheric coupling, we conducted a mesocosm experiment to test the impacts of warming on CO₂ saturation in a shallow prairie pond. We outline and test three possible scenarios for the effect of warming on the CO₂ saturation of ponds, resulting in either an increase, decrease or no net effect for CO₂ saturation. We show that with approximately a two-degree (ºC) increase in average water temperature, the pCO₂ of the warmed mesocosms was nine times greater than the ambient temperature mesocosms by the end of the 5-week experiment. Changes in water colour (a measure of dissolved organic carbon) in warmed systems indicate that decomposition of organic matter in the sediments and water column was the main contributor to the increase in pCO₂ in the warmed mesocosms. Our results show that with warming, the release of CO₂ from shallow ponds to the atmosphere will increase and carbon storage in the sediments will decrease, altering the current functioning of shallow prairie ponds and influencing the contribution of ponds to the global carbon cycle.     

土壤呼吸的温度敏感性——全球变暖正负反馈的不确定因素

基于模型模拟结果表明,全球变暖与大气CO2浓度增加将形成正反馈关系,这种正反馈效应将明显加速21世纪的气候变暖。然而,这些模拟模型都基于一个重要假设,即不同平均驻留时间的土壤有机质分解具有相同的温度敏感性(Q10)。这一假设与酶动力学理论相悖,而且不同学者对不同质量土壤有机质分解温度敏感性的差异的认识存在严重分歧,所以,全球变暖与大气CO2浓度增加的正反馈关系的显著性仍值得商榷。围绕土壤呼吸的温度敏感性问题进行了讨论和评述,涉及1)土壤有机质分解温度敏感性争论的焦点问题;2)通过经验模型曲线拟合估计Q10值存在的分歧及Q10变异的机理解释;3)实验室土壤培养实验估计Q10值存在的问题;4)土壤培养实验中Q10值计算方法的改进。进一步深化有关土壤有机质分解温度敏感性不确定性的认识,将为今后土壤呼吸及其对气候变化响应的相关研究提供参考。     

Will changes in soil organic carbon act as a positive or negative feedback on global warming?

The world's soils contain about 1500 Gt of organic carbon to a depth of 1m and a further 900 Gt from 1--2m. A change of total soil organic carbon by just 10% would thus be equivalent to all the anthropogenic CO₂ emitted over 30 years. Warming is likely to increase both the rate of decomposition and net primary production (NPP), with a fraction of NPP forming new organic carbon. Evidence from various sources can be used to assess whether NPP or the rate of decomposition has the greater temperature sensitivity, and, hence, whether warming is likely to lead to an increase or decrease in soil organic carbon.Evidence is reviewed from laboratory-based incubations, field measurements of organic carbon storage, carbon isotope ratios and soil respiration with either naturally varying temperatures or after experimentally increasing soil temperatures. Estimates of terrestrial carbon stored at the Last Glacial Maximum are also reviewed. The review concludes that the temperature dependence of organic matter decomposition can be best described as: d(T) = exp[3.36 (T – 40)/(T + 31.79)] where d(T) is the normalised decomposition rate at temperature T (in °C). In this equation, decomposition rate is normalised to 1 at 40 °C.The review concludes by simulating the likely changes in soil organic carbon with warming. In summary, it appears likely that warming will have the effect of reducing soil organic carbon by stimulating decomposition rates more than NPP. However, increasing CO₂ is likely to simultaneously have the effect of increasing soil organic carbon through increases in NPP. Any changes are also likely to be very slow. The net effect of changes in soil organic carbon on atmospheric CO₂ loading over the next decades to centuries is, therefore, likely to be small.     

Ocean warming enhances respiration and carbon demand of coastal microbial plankton

The increase of anthropogenic CO₂ during this century is expected to cause warming of large regions of the ocean. Microbes lead the biological role in the CO₂ balance of marine ecosystems, their activity is known to be influenced by temperature, and it is important to constrain and quantify these effects on bacterial carbon use. Furthermore, if warming were to enhance the carbon demand (production + respiration) of planktonic microbes but would maintain their efficiency low (as it generally is), then most carbon consumed would end up respired. We designed a strategy in which we measured bacterial production and respiration throughout a seasonal cycle in a coastal Mediterranean site, and determined experimentally the effects of ca. 2.5 °C on these processes. We show that warming will increase nearly 20% the total carbon demand of coastal microbial plankton without any effect on their (commonly low) growth efficiency, which could generate a positive feedback between coastal warming and CO₂ production.     

Responses of Soil CO2 Fluxes to Short-Term Experimental Warming in Alpine Steppe Ecosystem,Northern Tibet

Soil carbon dioxide (CO₂) emission is one of the largest fluxes in the global carbon cycle. Therefore small changes in the size of this flux can have a large effect on atmospheric CO₂ concentrations and potentially constitute a powerful positive feedback to the climate system. Soil CO₂ fluxes in the alpine steppe ecosystem of Northern Tibet and their responses to short-term experimental warming were investigated during the growing season in 2011. The results showed that the total soil CO₂ emission fluxes during the entire growing season were 55.82 and 104.31 g C m^-2 for the control and warming plots, respectively. Thus, the soil CO₂ emission fluxes increased 86.86% with the air temperature increasing 3.74°C. Moreover, the temperature sensitivity coefficient (Q ₁₀) of the control and warming plots were 2.10 and 1.41, respectively. The soil temperature and soil moisture could partially explain the temporal variations of soil CO₂ fluxes. The relationship between the temporal variation of soil CO₂ fluxes and the soil temperature can be described by exponential equation. These results suggest that warming significantly promoted soil CO₂ emission in the alpine steppe ecosystem of Northern Tibet and indicate that this alpine ecosystem is very vulnerable to climate change. In addition, soil temperature and soil moisture are the key factors that controls soil organic matter decomposition and soil CO₂ emission, but temperature sensitivity significantly decreases due to the rise in temperature.     

Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 °N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO₂ concentration stimulate vegetation productivity, taking up CO₂. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 ± 0.3 Pg C yr^?1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 ± 20 Pg C over 1901 levels, of which 17 ± 8 Pg C comes from vegetation (43%) and 21 ± 16 Pg C from the soil (8%). Both CO₂ fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO₂ fertilization effect, permafrost, and the role of soil moisture. Unlike the CO₂ fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid‐latitudes. These effects are further enhanced as a result of positive carbon cycle–climate feedbacks due to additional CO₂ and warming.     

Decomposition of Organic Carbon in Fine Soil Particles Is Likely More Sensitive to Warming than in Coarse Particles: An Incubation Study with Temperate Grassland and Forest Soils in Northern China

It is widely recognized that global warming promotes soil organic carbon (SOC) decomposition, and soils thus emit more CO₂ into the atmosphere because of the warming; however, the response of SOC decomposition to this warming in different soil textures is unclear. This lack of knowledge limits our projection of SOC turnover and CO₂ emission from soils after future warming. To investigate the CO₂ emission from soils with different textures, we conducted a 107-day incubation experiment. The soils were sampled from temperate forest and grassland in northern China. The incubation was conducted over three short-term cycles of changing temperature from 5°C to 30°C, with an interval of 5°C. Our results indicated that CO₂ emissions from sand (>50 µm), silt (2–50 µm), and clay (<2 µm) particles increased exponentially with increasing temperature. The sand fractions emitted more CO₂ (CO₂-C per unit fraction-C) than the silt and clay fractions in both forest and grassland soils. The temperature sensitivity of the CO₂ emission from soil particles, which is expressed as Q₁₀, decreased in the order clay>silt>sand. Our study also found that nitrogen availability in the soil facilitated the temperature dependence of SOC decomposition. A further analysis of the incubation data indicated a power-law decrease of Q₁₀ with increasing temperature. Our results suggested that the decomposition of organic carbon in fine-textured soils that are rich in clay or silt could be more sensitive to warming than those in coarse sandy soils and that SOC might be more vulnerable in boreal and temperate regions than in subtropical and tropical regions under future warming.     

Vulnerability of wetland soil carbon stocks to climate warming in the perhumid coastal temperate rainforest

The perhumid coastal temperate rainforest (PCTR) of southeast Alaska has some of the densest soil organic carbon (SOC) stocks in the world (>300 Mg C ha^?1) but the fate of this SOC with continued warming remains largely unknown. We quantified dissolved organic carbon (DOC) and carbon dioxide (CO₂) yields from four different wetland types (rich fen, poor fen, forested wetland and cedar wetland) using controlled laboratory incubations of surface (10 cm) and subsurface (25 cm) soils incubated at 8 and 15 °C for 37 weeks. Furthermore, we used fluorescence characterization of DOC and laboratory bioassays to assess how climate-induced soil warming may impact the quality and bioavailability of DOC delivered to fluvial systems. Soil temperature was the strongest control on SOC turnover, with wetland type and soil depth less important in controlling CO₂ flux and extractable DOC. The high temperature incubation increased average CO₂ yield by ~40 and ~25% for DOC suggesting PCTR soils contain a sizeable pool of readily biodegradable SOC that can be mineralized to DOC and CO₂ with future climate warming. Fluxes of CO₂ were positively correlated to both extractable DOC and percent bioavailable DOC during the last few months of the incubation suggesting mineralization of SOC to DOC is a strong control of soil respiration rates. Whether the net result is increased export of either carbon form will depend on the balance between the land to water transport of DOC and the ability of soil microbial communities to mineralize DOC to CO₂.     

Rate of warming affects temperature sensitivity of anaerobic peat decomposition and greenhouse gas production

Temperature sensitivity of anaerobic carbon mineralization in wetlands remains poorly represented in most climate models and is especially unconstrained for warmer subtropical and tropical systems which account for a large proportion of global methane emissions. Several studies of experimental warming have documented thermal acclimation of soil respiration involving adjustments in microbial physiology or carbon use efficiency (CUE), with an initial decline in CUE with warming followed by a partial recovery in CUE at a later stage. The variable CUE implies that the rate of warming may impact microbial acclimation and the rate of carbon‐dioxide (CO₂) and methane (CH₄) production. Here, we assessed the effects of warming rate on the decomposition of subtropical peats, by applying either a large single‐step (10°C within a day) or a slow ramping (0.1°C/day for 100 days) temperature increase. The extent of thermal acclimation was tested by monitoring CO₂ and CH₄ production, CUE, and microbial biomass. Total gaseous C loss, CUE, and MBC were greater in the slow (ramp) warming treatment. However, greater values of CH₄–C:CO₂–C ratios lead to a greater global warming potential in the fast (step) warming treatment. The effect of gradual warming on decomposition was more pronounced in recalcitrant and nutrient‐limited soils. Stable carbon isotopes of CH₄ and CO₂ further indicated the possibility of different carbon processing pathways under the contrasting warming rates. Different responses in fast vs. slow warming treatment combined with different endpoints may indicate alternate pathways with long‐term consequences. Incorporations of experimental results into organic matter decomposition models suggest that parameter uncertainties in CUE and CH₄–C:CO₂–C ratios have a larger impact on long‐term soil organic carbon and global warming potential than uncertainty in model structure, and shows that particular rates of warming are central to understand the response of wetland soils to global climate change.     

Rapid loss of an ecosystem engineer: Sphagnum decline in an experimentally warmed bog

Sphagnum mosses are keystone components of peatland ecosystems. They facilitate the accumulation of carbon in peat deposits, but climate change is predicted to expose peatland ecosystem to sustained and unprecedented warming leading to a significant release of carbon to the atmosphere. Sphagnum responses to climate change, and their interaction with other components of the ecosystem, will determine the future trajectory of carbon fluxes in peatlands. We measured the growth and productivity of Sphagnum in an ombrotrophic bog in northern Minnesota, where ten 12.8‐m‐diameter plots were exposed to a range of whole‐ecosystem (air and soil) warming treatments (+0 to +9°C) in ambient or elevated (+500 ppm) CO₂. The experiment is unique in its spatial and temporal scale, a focus on response surface analysis encompassing the range of elevated temperature predicted to occur this century, and consideration of an effect of co‐occurring CO₂ altering the temperature response surface. In the second year of warming, dry matter increment of Sphagnum increased with modest warming to a maximum at 5°C above ambient and decreased with additional warming. Sphagnum cover declined from close to 100% of the ground area to <50% in the warmest enclosures. After three years of warming, annual Sphagnum productivity declined linearly with increasing temperature (13–29 g C/m² per °C warming) due to widespread desiccation and loss of Sphagnum. Productivity was less in elevated CO₂ enclosures, which we attribute to increased shading by shrubs. Sphagnum desiccation and growth responses were associated with the effects of warming on hydrology. The rapid decline of the Sphagnum community with sustained warming, which appears to be irreversible, can be expected to have many follow‐on consequences to the structure and function of this and similar ecosystems, with significant feedbacks to the global carbon cycle and climate change.     

陆地生态系统土壤CO2排放对模拟增温的响应特征及影响因素

全球变暖已经成为不争的事实,陆地生态系统碳循环的研究受到了各界广泛关注,是当前全球变化研究中的重点。土壤CO₂排放是陆地生态系统与大气间二氧化碳交换的最大通量之一,当前陆地生态系统中土壤CO₂排放如何响应全球气候变暖及其影响因素仍不清楚,限制了对土壤碳循环过程及影响机制的深入认识。旨在明确全球变暖背景下陆地生态系统中土壤CO₂排放格局及影响因素。基于Web of Science、PubMed和中国知网等中英文期刊数据库,充分收集全球范围内的相关野外试验文献81篇,提取出65个研究位置和213组相关研究数据,采用Meta分析方法探讨陆地生态系统土壤CO₂排放对增温的响应特征,分析其与海拔、气候、土壤含水量、容重(BD)、pH、全氮(TN)和土壤有机碳(SOC)的相关关系。结果表明:陆地生态系统中土壤CO₂排放对增温整体有显著的正向响应,在农、林、草生态系统中,增温使土壤CO₂排放分别显著增加13.1%、18.0%、5.9% (P<0.05),森林生态系统对增温响应的正效应最强烈;增温能在短时期内促进土壤呼吸,但随着增温持续时间增加,土壤呼吸对温度的敏感性会降低,对温度变化产生适应性,从而使其对增温的响应能力减弱;响应特征受到环境因子、土壤特性以及其他试验条件等的影响,绝大多数条件下对增温表现出显著的正响应特征,不同影响因子之间共同作用、相互影响。增温通常能够改变植物生物量、土壤养分含量及微生物数量和活性,从而影响到植被根际呼吸和土壤呼吸速率。相关分析表明,海拔对土壤CO₂排放有显著负向影响,而年均气温、年均降水量、土壤含水量和仪器嵌入土壤深度则对土壤CO₂排放产生显著正向影响。这些结果对于理解全球土壤CO₂排放的时空变化格局有重要意义,也为准确评价全球变暖背景下土壤碳汇功能及其持续性提供理论依据。     

Vascular plants promote ancient peatland carbon loss with climate warming

Northern peatlands have accumulated one third of the Earth's soil carbon stock since the last Ice Age. Rapid warming across northern biomes threatens to accelerate rates of peatland ecosystem respiration. Despite compensatory increases in net primary production, greater ecosystem respiration could signal the release of ancient, century‐ to millennia‐old carbon from the peatland organic matter stock. Warming has already been shown to promote ancient peatland carbon release, but, despite the key role of vegetation in carbon dynamics, little is known about how plants influence the source of peatland ecosystem respiration. Here, we address this issue using in situ ¹⁴C measurements of ecosystem respiration on an established peatland warming and vegetation manipulation experiment. Results show that warming of approximately 1 °C promotes respiration of ancient peatland carbon (up to 2100 years old) when dwarf‐shrubs or graminoids are present, an effect not observed when only bryophytes are present. We demonstrate that warming likely promotes ancient peatland carbon release via its control over organic inputs from vascular plants. Our findings suggest that dwarf‐shrubs and graminoids prime microbial decomposition of previously ‘locked‐up’ organic matter from potentially deep in the peat profile, facilitating liberation of ancient carbon as CO₂. Furthermore, such plant‐induced peat respiration could contribute up to 40% of ecosystem CO₂ emissions. If consistent across other subarctic and arctic ecosystems, this represents a considerable fraction of ecosystem respiration that is currently not acknowledged by global carbon cycle models. Ultimately, greater contribution of ancient carbon to ecosystem respiration may signal the loss of a previously stable peatland carbon pool, creating potential feedbacks to future climate change.     

Effects of experimental warming of air,soil and permafrost on carbon balance in Alaskan tundra

The carbon (C) storage capacity of northern latitude ecosystems may diminish as warming air temperatures increase permafrost thaw and stimulate decomposition of previously frozen soil organic C. However, warming may also enhance plant growth so that photosynthetic carbon dioxide (CO₂) uptake may, in part, offset respiratory losses. To determine the effects of air and soil warming on CO₂ exchange in tundra, we established an ecosystem warming experiment – the Carbon in Permafrost Experimental Heating Research (CiPEHR) project – in the northern foothills of the Alaska Range in Interior Alaska. We used snow fences coupled with spring snow removal to increase deep soil temperatures and thaw depth (winter warming) and open‐top chambers to increase growing season air temperatures (summer warming). Winter warming increased soil temperature (integrated 5–40 cm depth) by 1.5 °C, which resulted in a 10% increase in growing season thaw depth. Surprisingly, the additional 2 kg of thawed soil C m^?2 in the winter warming plots did not result in significant changes in cumulative growing season respiration, which may have been inhibited by soil saturation at the base of the active layer. In contrast to the limited effects on growing‐season C dynamics, winter warming caused drastic changes in winter respiration and altered the annual C balance of this ecosystem by doubling the net loss of CO₂ to the atmosphere. While most changes to the abiotic environment at CiPEHR were driven by winter warming, summer warming effects on plant and soil processes resulted in 20% increases in both gross primary productivity and growing season ecosystem respiration and significantly altered the age and sources of CO₂ respired from this ecosystem. These results demonstrate the vulnerability of organic C stored in near surface permafrost to increasing temperatures and the strong potential for warming tundra to serve as a positive feedback to global climate change.     

Estimating the oxidative ratio of the global terrestrial biosphere carbon

The oxidative ratio (OR) is the amount of CO₂ sequestered in the terrestrial biosphere for each mol of O₂ produced. The OR governs the efficiency of a terrestrial biome’s O₂ production and it has been used to calculate the balance of terrestrial and oceanic carbon sinks across the globe. However, the value used in carbon cycle calculations comes from only one study of one environment. Here we perform a meta-analysis of studies of soil organic matter and vegetation composition to calculate the first global ecosystem OR value. We use data from 138 samples across 31 studies covering 9 USDA global soil orders, 7 global biomes and 5 continents and combine this information as a weighted average based upon biome land area or organic carbon content of the soil order. Organic matter fractions could not be shown to be reliable proxies for whole soil or vegetation OR. The resulting analysis suggests that although the presently used value of 1.1 is within the range of natural occurrence, it is not the most accurate choice, representing between the 97th and 99th percentile value. Our study yields a global terrestrial OR = 1.04 ± 0.03. This value of OR means that the sink of anthropogenic carbon fluxes to land has been underestimated (and the sink to the ocean overestimated) by up to 14 %. Recalculating with our OR value, the fossil fuel carbon flux to land is 1.48 ± 0.04 Gt C/year and flux to oceans is 2.02 ± 0.03 Gt C/year.     

Temperature and peat type control CO2 and CH4 production in Alaskan permafrost peats

Controls on the fate of ~277 Pg of soil organic carbon (C) stored in permafrost peatland soils remain poorly understood despite the potential for a significant positive feedback to climate change. Our objective was to quantify the temperature, moisture, organic matter, and microbial controls on soil organic carbon (SOC) losses following permafrost thaw in peat soils across Alaska. We compared the carbon dioxide (CO₂) and methane (CH₄) emissions from peat samples collected at active layer and permafrost depths when incubated aerobically and anaerobically at ?5, ?0.5, +4, and +20 °C. Temperature had a strong, positive effect on C emissions; global warming potential (GWP) was >3× larger at 20 °C than at 4 °C. Anaerobic conditions significantly reduced CO₂ emissions and GWP by 47% at 20 °C but did not have a significant effect at ?0.5 °C. Net anaerobic CH₄ production over 30 days was 7.1 ± 2.8 μg CH₄‐C gC^?1 at 20 °C. Cumulative CO₂ emissions were related to organic matter chemistry and best predicted by the relative abundance of polysaccharides and proteins (R² = 0.81) in SOC. Carbon emissions (CO₂‐C + CH₄‐C) from the active layer depth peat ranged from 77% larger to not significantly different than permafrost depths and varied depending on the peat type and peat decomposition stage rather than thermal state. Potential SOC losses with warming depend not only on the magnitude of temperature increase and hydrology but also organic matter quality, permafrost history, and vegetation dynamics, which will ultimately determine net radiative forcing due to permafrost thaw.     

Warming Reduces Carbon Losses from Grassland Exposed to Elevated Atmospheric Carbon Dioxide

The flux of carbon dioxide (CO₂) between terrestrial ecosystems and the atmosphere may ameliorate or exacerbate climate change, depending on the relative responses of ecosystem photosynthesis and respiration to warming temperatures, rising atmospheric CO₂, and altered precipitation. The combined effect of these global change factors is especially uncertain because of their potential for interactions and indirectly mediated conditions such as soil moisture. Here, we present observations of CO₂ fluxes from a multi-factor experiment in semi-arid grassland that suggests a potentially strong climate – carbon cycle feedback under combined elevated [CO₂] and warming. Elevated [CO₂] alone, and in combination with warming, enhanced ecosystem respiration to a greater extent than photosynthesis, resulting in net C loss over four years. The effect of warming was to reduce respiration especially during years of below-average precipitation, by partially offsetting the effect of elevated [CO₂] on soil moisture and C cycling. Carbon losses were explained partly by stimulated decomposition of soil organic matter with elevated [CO₂]. The climate – carbon cycle feedback observed in this semiarid grassland was mediated by soil water content, which was reduced by warming and increased by elevated [CO₂]. Ecosystem models should incorporate direct and indirect effects of climate change on soil water content in order to accurately predict terrestrial feedbacks and long-term storage of C in soil.     

Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas-fir terracosms

Although numerous studies indicate that increasing atmospheric CO₂ or temperature stimulate soil CO₂ efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO₂ and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO₂ and elevated temperature on these components of soil CO₂ efflux in Douglas-fir terracosms. We measured both soil CO₂ efflux rates and the ¹³C and ¹⁸O isotopic compositions of soil CO₂ efflux in 12 sun-lit and environmentally controlled terracosms with 4-year-old Douglas fir seedlings and reconstructed forest soils under two CO₂ concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO₂ efflux. In most cases, litter decomposition was the dominant component of soil CO₂ efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO₂ concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO₂, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual-isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.     

Water- and Plant-Mediated Responses of Ecosystem Carbon Fluxes to Warming and Nitrogen Addition on the Songnen Grassland in Northeast China

Background

Understanding how grasslands are affected by a long-term increase in temperature is crucial to predict the future impact of global climate change on terrestrial ecosystems. Additionally, it is not clear how the effects of global warming on grassland productivity are going to be altered by increased N deposition and N addition.

Methodology/Principal Findings

In-situ canopy CO₂ exchange rates were measured in a meadow steppe subjected to 4-year warming and nitrogen addition treatments. Warming treatment reduced net ecosystem CO₂ exchange (NEE) and increased ecosystem respiration (ER); but had no significant impacts on gross ecosystem productivity (GEP). N addition increased NEE, ER and GEP. However, there were no significant interactions between N addition and warming. The variation of NEE during the four experimental years was correlated with soil water content, particularly during early spring, suggesting that water availability is a primary driver of carbon fluxes in the studied semi-arid grassland.

Conclusion/Significance

Ecosystem carbon fluxes in grassland ecosystems are sensitive to warming and N addition. In the studied water-limited grassland, both warming and N addition influence ecosystem carbon fluxes by affecting water availability, which is the primary driver in many arid and semiarid ecosystems. It remains unknown to what extent the long-term N addition would affect the turn-over of soil organic matter and the C sink size of this grassland.     

Effects of soil moisture on the temperature sensitivity of heterotrophic respiration vary seasonally in an old‐field climate change experiment

Microbial decomposition of soil organic matter produces a major flux of CO₂ from terrestrial ecosystems and can act as a feedback to climate change. Although climate‐carbon models suggest that warming will accelerate the release of CO₂ from soils, the magnitude of this feedback is uncertain, mostly due to uncertainty in the temperature sensitivity of soil organic matter decomposition. We examined how warming and altered precipitation affected the rate and temperature sensitivity of heterotrophic respiration (R_h) at the Boston‐Area Climate Experiment, in Massachusetts, USA. We measured R_h inside deep collars that excluded plant roots and litter inputs. In this mesic ecosystem, R_h responded strongly to precipitation. Drought reduced R_h, both annually and during the growing season. Warming increased R_h only in early spring. During the summer, when R_h was highest, we found evidence of threshold, hysteretic responses to soil moisture: R_h decreased sharply when volumetric soil moisture dropped below ~15% or exceeded ~26%, but R_h increased more gradually when soil moisture rose from the lower threshold. The effect of climate treatments on the temperature sensitivity of R_h depended on the season. Apparent Q₁₀ decreased with high warming (~3.5 °C) in spring and fall. Presumably due to limiting soil moisture, warming and precipitation treatments did not affect apparent Q₁₀ in summer. Drought decreased apparent Q₁₀ in fall compared to ambient and wet precipitation treatments. To our knowledge, this is the first field study to examine the response of R_h and its temperature sensitivity to the combined effects of warming and altered precipitation. Our results highlight the complex responses of R_h to soil moisture, and to our knowledge identify for the first time the seasonal variation in the temperature sensitivity of microbial respiration in the field. We emphasize the importance of adequately simulating responses such as these when modeling trajectories of soil carbon stocks under climate change scenarios.     

Warming intensify CO2 flux and nutrient release from algal wrack subsidies on sandy beaches

Algal wrack subsidies underpin most of the food web structure of exposed sandy beaches and are responsible of important biogeochemical processes that link marine and terrestrial ecosystems. The response in decomposition of algal wrack deposits to global warming has not been studied in ocean‐exposed sandy beaches to date. With this aim, passive open top chambers (OTCs) were used to increase soil temperature within the range predicted by the IPCC for western Europe (between 0.5 and 1.5°C), following the hypothesis that the biogeochemical processing of macroalgal wrack subsidies would accelerate in response to temperature increase. The effect of temperature manipulation on three target substrates: fresh and aged macroalgae, and bare sand, was tested. Results indicated that a small warming (<0.5°C) affected the wrack decomposition process through traceable increases in soil respiration through CO₂ flux, inorganic nutrients within the interstitial environment (N and P), sediment organic contents measured through the amount of proteins and microbial pool through the total soil DNA. The different responses of soil variables in the studied substrates indicated that the decomposition stage of stranded macroalgae influences the biogeochemical processing of organic matter in sandy beaches. Thus, CO₂ fluxes, releases of organic and inorganic nutrients and microbial activity intensify in aged wrack deposits. Our results predict that expected global warming will increase the release of inorganic nutrients to the coastal ocean by 30% for the N (21 Gg/year) and 5.9% for P (14 Gg/year); that increase for the flow of C to the atmosphere as CO₂ was estimated in 8.2% (523 Gg/year). This study confirms the key role of sandy beaches in recycling ocean‐derived organic matter, highlighting their sensitivity to a changing scenario of global warming that predicts significant increases in temperature over the next few decades.