Increasing microbial carbon use efficiency with warming predicts soil heterotrophic respiration globally

The degree to which climate warming will stimulate soil organic carbon (SOC) losses via heterotrophic respiration remains uncertain, in part because different or even opposite microbial physiology and temperature relationships have been proposed in SOC models. We incorporated competing microbial carbon use efficiency (CUE)–mean annual temperature (MAT) and enzyme kinetic–MAT relationships into SOC models, and compared the simulated mass‐specific soil heterotrophic respiration rates with multiple published datasets of measured respiration. The measured data included 110 dryland soils globally distributed and two continental to global‐scale cross‐biome datasets. Model–data comparisons suggested that a positive CUE–MAT relationship best predicts the measured mass‐specific soil heterotrophic respiration rates in soils distributed globally. These results are robust when considering models of increasing complexity and competing mechanisms driving soil heterotrophic respiration–MAT relationships (e.g., carbon substrate availability). Our findings suggest that a warmer climate selects for microbial communities with higher CUE, as opposed to the often hypothesized reductions in CUE by warming based on soil laboratory assays. Our results help to build the impetus for, and confidence in, including microbial mechanisms in soil biogeochemical models used to forecast changes in global soil carbon stocks in response to warming.     

Reduced carbon use efficiency and increased microbial turnover with soil warming

Global soil carbon (C) stocks are expected to decline with warming, and changes in microbial processes are key to this projection. However, warming responses of critical microbial parameters such as carbon use efficiency (CUE) and biomass turnover (rB) are not well understood. Here, we determine these parameters using a probabilistic inversion approach that integrates a microbial‐enzyme model with 22 years of carbon cycling measurements at Harvard Forest. We find that increasing temperature reduces CUE but increases rB, and that two decades of soil warming increases the temperature sensitivities of CUE and rB. These temperature sensitivities, which are derived from decades‐long field observations, contrast with values obtained from short‐term laboratory experiments. We also show that long‐term soil C flux and pool changes in response to warming are more dependent on the temperature sensitivity of CUE than that of rB. Using the inversion‐derived parameters, we project that chronic soil warming at Harvard Forest over six decades will result in soil C gain of <1.0% on average (1st and 3rd quartiles: 3.0% loss and 10.5% gain) in the surface mineral horizon. Our results demonstrate that estimates of temperature sensitivity of microbial CUE and rB can be obtained and evaluated rigorously by integrating multidecadal datasets. This approach can potentially be applied in broader spatiotemporal scales to improve long‐term projections of soil C feedbacks to climate warming.     

Microbial carbon use efficiency in different ecosystems: A meta-analysis based on a biogeochemical equilibrium model

Soil microbial carbon use efficiency (CUE) is a crucial parameter that can be used to evaluate the partitioning of soil carbon (C) between microbial growth and respiration. However, general patterns of microbial CUE among terrestrial ecosystems (e.g., farmland, grassland, and forest) remain controversial. To address this knowledge gap, data from 41 study sites (n = 197 soil samples) including 58 farmlands, 95 forests, and 44 grasslands were collected and analyzed to estimate microbial CUEs using a biogeochemical equilibrium model. We also evaluated the metabolic limitations of microbial growth using an enzyme vector model and the drivers of CUE across different ecosystems. The CUEs obtained from soils of farmland, forest, and grassland ecosystems were significantly different with means of 0.39, 0.33, and 0.42, respectively, illustrating that grassland soils exhibited higher microbial C sequestration potentials (p < .05). Microbial metabolic limitations were also distinct in these ecosystems, and carbon limitation was dominant exhibiting strong negative effects on CUE. Exoenzyme stoichiometry played a greater role in impacting CUE values than soil elemental stoichiometry within each ecosystem. Specifically, soil exoenzymatic ratios of C:phosphorus (P) acquisition activities (EEA_C:P) and the exoenzymatic ratio of C:nitrogen (N) acquisition activities (EEA_C:N) imparted strong negative effects on soil microbial CUE in grassland and forest ecosystems, respectively. But in farmland soils, EEA_C:P exhibited greater positive effects, showing that resource constraints could regulate microbial resource allocation with discriminating patterns across terrestrial ecosystems. Furthermore, mean annual temperature (MAT) rather than mean annual precipitation (MAP) was a critical climate factor affecting CUE, and soil pH as a major factor remained positive to drive the changes in microbial CUE within ecosystems. This research illustrates a conceptual framework of microbial CUEs in terrestrial ecosystems and provides the theoretical evidence to improve soil microbial C sequestration capacity in response to global change.     

Chemical changes in organic matter after fungal colonization in a nitrogen fertilized and unfertilized Norway spruce forest

Background and aims

Mineralization of soil organic matter (SOM) constitutes a major carbon flux to the atmosphere. The carbon use efficiency (CUE) of the saprotrophic microorganisms mineralizing SOM is integral for soil carbon dynamics. Here we investigate how the CUE is affected by temperature, metabolic conditions, and the molecular complexity of the substrate.

Methods

We incubated O-horizon soil samples (with either ¹³C–glucose or ¹³C–cellulose) from a boreal coniferous forest at 4, 9, 14, and 19 °C, and calculated CUEs based on the amount of ¹³C–CO₂ and ¹³C–labelled microbial biomass produced. The effects of substrate, temperature, and metabolic conditions (representing unlimited substrate supply and substrate limitation) on CUE were evaluated.

Results

CUE from metabolizing glucose was higher as compared to cellulose. A slight decrease in CUE with increasing temperature was observed in glucose amended samples (but only in the range 9–19 °C), but not in cellulose amended samples. CUE differed significantly with metabolic conditions, i.e. CUE was higher during unlimited growth conditions as compared to conditions with substrate limitation.

Conclusions

We conclude that it is integral to account for both differences in CUE during different metabolic phases, as well as complexity of substrate, when interpreting temperature dependence on CUE in incubation studies.

     

Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses

Increasing temperatures can accelerate soil organic matter decomposition and release large amounts of CO₂ to the atmosphere, potentially inducing positive warming feedbacks. Alterations to the temperature sensitivity and physiological functioning of soil microorganisms may play a key role in these carbon (C) losses. Geothermally active areas in Iceland provide stable and continuous soil temperature gradients to test this hypothesis, encompassing the full range of warming scenarios projected by the Intergovernmental Panel on Climate Change for the northern region. We took soils from these geothermal sites 7 years after the onset of warming and incubated them at varying temperatures and substrate availability conditions to detect persistent alterations of microbial physiology to long-term warming. Seven years of continuous warming ranging from 1.8 to 15.9 °C triggered a 8.6–58.0% decrease on the C concentrations in the topsoil (0–10 cm) of these sub-arctic silt-loam Andosols. The sensitivity of microbial respiration to temperature (Q₁₀) was not altered. However, soil microbes showed a persistent increase in their microbial metabolic quotients (microbial respiration per unit of microbial biomass) and a subsequent diminished C retention in biomass. After an initial depletion of labile soil C upon soil warming, increasing energy costs of metabolic maintenance and resource acquisition led to a weaker capacity of C stabilization in the microbial biomass of warmer soils. This mechanism contributes to our understanding of the acclimated response of soil respiration to in situ soil warming at the ecosystem level, despite a lack of acclimation at the physiological level. Persistent increases in the respiratory costs of soil microbes in response to warming constitute a fundamental process that should be incorporated into climate change-C cycling models.     

Soil carbon storage,respiration potential,and organic matter quality across an age and climate gradient in southwestern Greenland

Geological factors influence biological cycling of organic carbon in soils but are not well represented in our understanding of Arctic carbon dynamics. Landscape age, for instance, directly affects quantity and quality of soil carbon, which are two strong controls of the temperature sensitivity of soil organic matter. We investigated soil carbon storage, respiration potential, and organic matter quality for microbial decomposition across a climate and landscape age gradient in southwest Greenland that deglaciated during the Holocene. We measured soil respiration during a 370-day laboratory incubations of active layer soils collected from four study areas across this gradient (ages 1.8 × 10², 6.8 × 10³, and 1.0 × 10⁴, coinciding with a climate gradient from drier inland to wetter coastal terrain) and used a soil respiration model comparison approach to assess the substrate quality of stored organic matter for microbial decomposers. Soils store more than three times greater organic carbon at the 10,000-year-old, maritime climate study areas than the 180-year-old, continental climate study areas. Respiration rates were highest in the surface soils of the coastal areas. Model comparisons reveal important heterogeneity in the quality of organic matter for microbial decomposition between areas: coastal soils were best modeled by both one- and two-pooled models, and inland soils were best represented by one-pooled respiration models. Together, the measures of carbon quality (C:N, CO₂ production, and model parameters estimating initial CO₂ production rates from different organic matter pools) show that shallow soils at the southern coastal area, Kobbefjord, had the highest respiration rates from the recalcitrant carbon pool. This study reveals differences in carbon storage and turnover associated with landscape age and climate factors in western Greenland. When applied to thermodynamic theory, which predicts that temperature sensitivity increases with carbon recalcitrance, our findings suggest that carbon stored in coastal soils may be more sensitive to climate warming than inland soils.     

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.     

Drought-induced dynamics of carbon and water use efficiency of global grasslands from 2000 to 2011

Drought is frequently recorded as a result of climate warming and elevated concentration of greenhouse gases, which affect the carbon and water cycles in terrestrial ecosystems, particularly in arid and semi-arid regions. To identify the drought in grassland ecosystems and to determine how such drought affects grassland ecosystems in terms of carbon and water cycles across the globe, this study evaluated the drought conditions of global grassland ecosystems from 2000 to 2011 on the basis of the remotely sensed Drought Severity Index (DSI) data. The temporal dynamics of grassland carbon use efficiency (CUE) and water use efficiency (WUE), as well as their correlations with DSI, were also investigated at the global scale. Results showed that 57.04% of grassland ecosystems experienced a dry trend over this period. In general, most grassland ecosystems in the northern hemisphere (N.H.) were in near normal condition, whereas those in the southern hemisphere (S.H.) experienced a clear drying and wetting trend, with the year 2005 regarded as the turning point. Grassland CUE increased continually despite the varied drought conditions over this period. By contrast, WUE increased in the closed shrublands and woody savannas but decreased in all the other grassland types. The drought conditions affected the carbon and water use mainly by influencing the primary production and evapotranspiration of grass through photosynthesis and transpiration process. The CUE and WUE of savannas was most sensitive to droughts among all the grassland types. The areas of grassland DSI that showed significant correlations with CUE and WUE were 52.92% and 22.11% of the total grassland areas, respectively. Overall, droughts sufficiently explained the dynamics of grassland CUE, especially in the S.H. In comparison with grassland CUE, the grassland WUE was less sensitive to drought conditions at the global scale.     

Short-term responses of ecosystem carbon fluxes to experimental soil warming at the Swiss alpine treeline

Climatic warming will probably have particularly large impacts on carbon fluxes in high altitude and latitude ecosystems due to their great stocks of labile soil C and high temperature sensitivity. At the alpine treeline, we experimentally warmed undisturbed soils by 4 K for one growing season with heating cables at the soil surface and measured the response of net C uptake by plants, of soil respiration, and of leaching of dissolved organic carbon (DOC). Soil warming increased soil CO₂ effluxes instantaneously and throughout the whole vegetation period (+45%; +120 g C m y^?1). In contrast, DOC leaching showed a negligible response of a 5% increase (NS). Annual C uptake of new shoots was not significantly affected by elevated soil temperatures, with a 17, 12, and 14% increase for larch, pine, and dwarf shrubs, respectively, resulting in an overall increase in net C uptake by plants of 20–40 g C m^?2y^?1. The Q ₁₀ of 3.0 measured for soil respiration did not change compared to a 3-year period before the warming treatment started, suggesting little impact of warming-induced lower soil moisture (?15% relative decrease) or increased soil C losses. The fraction of recent plant-derived C in soil respired CO₂ from warmed soils was smaller than that from control soils (25 vs. 40% of total C respired), which implies that the warming-induced increase in soil CO₂ efflux resulted mainly from mineralization of older SOM rather than from stimulated root respiration. In summary, one season of 4 K soil warming, representative of hot years, led to C losses from the studied alpine treeline ecosystem by increasing SOM decomposition more than C gains through plant growth.     

Thermal acclimation in widespread heterotrophic soil microbes

Respiration by plants and microorganisms is primarily responsible for mediating carbon exchanges between the biosphere and atmosphere. Climate warming has the potential to influence the activity of these organisms, regulating exchanges between carbon pools. Physiological ‘down‐regulation’ of warm‐adapted species (acclimation) could ameliorate the predicted respiratory losses of soil carbon under climate change scenarios, but unlike plants and symbiotic microbes, the existence of this phenomenon in heterotrophic soil microbes remains controversial. Previous studies using complex soil microbial communities are unable to distinguish physiological acclimation from other community‐scale adjustments. We explored the temperature‐sensitivity of individual saprotrophic basidiomycete fungi growing in agar, showing definitively that these widespread heterotrophic fungi can acclimate to temperature. In almost all cases, the warm‐acclimated individuals had lower growth and respiration rates at intermediate temperatures than cold‐acclimated isolates. Inclusion of such microbial physiological responses to warming is essential to enhance the robustness of global climate‐ecosystem carbon models.     

Increased topsoil carbon stock across China's forests

Biomass carbon accumulation in forest ecosystems is a widespread phenomenon at both regional and global scales. However, as coupled carbon–climate models predicted, a positive feedback could be triggered if accelerated soil carbon decomposition offsets enhanced vegetation growth under a warming climate. It is thus crucial to reveal whether and how soil carbon stock in forest ecosystems has changed over recent decades. However, large‐scale changes in soil carbon stock across forest ecosystems have not yet been carefully examined at both regional and global scales, which have been widely perceived as a big bottleneck in untangling carbon–climate feedback. Using newly developed database and sophisticated data mining approach, here we evaluated temporal changes in topsoil carbon stock across major forest ecosystem in China and analysed potential drivers in soil carbon dynamics over broad geographical scale. Our results indicated that topsoil carbon stock increased significantly within all of five major forest types during the period of 1980s–2000s, with an overall rate of 20.0 g C m^?2 yr^?1 (95% confidence interval, 14.1–25.5). The magnitude of soil carbon accumulation across coniferous forests and coniferous/broadleaved mixed forests exhibited meaningful increases with both mean annual temperature and precipitation. Moreover, soil carbon dynamics across these forest ecosystems were positively associated with clay content, with a larger amount of SOC accumulation occurring in fine‐textured soils. In contrast, changes in soil carbon stock across broadleaved forests were insensitive to either climatic or edaphic variables. Overall, these results suggest that soil carbon accumulation does not counteract vegetation carbon sequestration across China's forest ecosystems. The combination of soil carbon accumulation and vegetation carbon sequestration triggers a negative feedback to climate warming, rather than a positive feedback predicted by coupled carbon–climate models.     

氮沉降增加情景下植物-土壤-微生物交互对自然生态系统土壤有机碳的调控研究进展

大气氮沉降增加倾向于促进受氮限制陆地生态系统地上生物量,但是对地下碳过程和土壤碳截存的影响结果迥异,导致陆地生态系统“氮促碳汇”的评估存在很大的不确定性。大气氮沉降输入直接影响微生物活性或间接影响底物质量,改变凋落物和土壤有机质(SOM)的分解速率和分解程度,进而影响土壤有机碳(SOC)的积累与损耗过程。过去相关研究主要集中在土壤碳转化过程和碳储量动态方面,缺乏植物-微生物-SOM交互作用的理解,对土壤碳截存调控的生物化学和微生物学机理尚不清楚。本文以地下碳循环过程为主线,分别综述了氮沉降增加对植物地下碳分配、SOC激发效应、微生物群落碳代谢过程的影响,深入分析SOM化学稳定性与微生物群落动态的关系。该领域研究的薄弱环节体现在:(1)增氮倾向于降低根系的生长和周转,对根际沉积碳分配(数量和格局)的影响及驱动因素不明确;(2)虽然认识到氮素有效性影响土壤激发效应的方向和强度,但是氧化态NO-3和还原态NH+4输入对有机质激发效应的差异性影响及潜在机理知之甚少;(3)微生物碳利用效率(CUE)是微生物群落碳代谢的关键表征,能够很好地解释土壤碳的积累与损耗过程;由于缺乏适宜的测定方法,难以准确量化土壤微生物的CUE及微生物生物量的周转时间;(4)增氮会抑制土壤真菌群落及其胞外酶活性,对细菌群落组成的影响尚未定论,有关SOM化学质量与土壤微生物群落活性、组成之间的耦合关系尚不清楚。未来研究应基于长期的氮添加控制实验平台,结合碳氧稳定性同位素示踪、有机质化学、分子生物学和宏基因组学等方法,深入分析植物同化碳的地下分配规律、微生物碳代谢和周转、有机质化学结构与功能微生物群落的耦合关系等关键环节。上述研究将有助于揭示植物-土壤-微生物交互作用对SOC动态的调控机制,完善陆地生态系统碳-氮耦合循环模型,有效降低区域陆地碳汇评估的不确定性,并可为陆地生态系统应对全球变化提供科学依据。     

植被类型变化对土壤微生物碳利用效率的影响研究进展

植被类型变化强烈影响着土壤碳循环。土壤微生物碳利用效率（CUE）是微生物将从环境中获取的碳分配给自身生长的比例,是土壤碳循环的综合指标。研究植被类型变化对CUE的影响有助于从微生物视角理解该过程中的土壤碳动态,可以为评估植被类型变化对土壤质量及生态系统碳循环的影响提供基础,具有重要的理论及实际价值。通过系统查阅相关文献,综述了植被类型变化导致的CUE变化情况,以及该过程中影响CUE的因子与机制。目前,相关研究主要涉及以林地、草地和农业用地为起点或终点的植被变化类型。天然林（原生林、次生林）变化为人工林、林地变化为草地后CUE普遍下降,随终点植被的发展CUE可能恢复至起点水平。植被成熟度越高,发生转变时CUE变化越剧烈。植被类型变化以农业用地为起点或终点时,CUE变化方向的不确定性及幅度的变异性均增加。植被类型变化导致的CUE变化主要受到植被、土壤、微生物因子及其交互作用的驱动,指示CUE的指标、采样季节和土层也会一定程度上影响CUE的变化。今后相关研究应采用直接的CUE测定方法,拓宽研究气候区及植被变化类型,关注植被变化过程中CUE变化的土层差异及动态监测,深入对植被类型变化导致的生态环境因子变化与CUE的关系及作用机制的研究。     

罗浮栲林土壤微生物碳利用效率对短期氮添加的响应

作为调节土壤碳矿化过程的重要参数,微生物碳利用效率(CUE)对理解陆地生态系统中的碳循环至关重要。本研究在戴云山罗浮栲林设置对照(0 kg N·hm^-2·a^-1)、低氮(40 kg N·hm^-2·a^-1)和高氮(80 kg N·hm^-2·a^-1) 3个氮添加水平以模拟氮沉降,测定了表层(0～10 cm)土壤基本理化性质、有机碳组分、微生物生物量和酶活性;并利用¹⁸O标记水方法测定土壤微生物CUE,以更好地理解氮沉降加剧对微生物CUE的影响及其影响因素。结果表明: 短期氮添加显著降低了土壤微生物的呼吸速率、碳和氮获取酶活性,但显著增加了土壤微生物CUE。β-N-乙酰氨基酸葡糖苷酶(NAG)/微生物生物量碳(MBC)、微生物呼吸速率、β-葡萄糖苷酶(BG)/MBC、纤维素水解酶(CBH)/MBC和土壤有机碳含量是影响CUE的主要因素,且CUE与NAG/MBC、微生物呼吸速率、BG/MBC和CBH/MBC呈显著负相关,与土壤有机碳呈显著正相关。综上,短期氮添加导致土壤微生物获取碳和氮的成本降低,减少微生物呼吸,从而提高了土壤微生物CUE,这将有助于提高罗浮栲林土壤碳固存潜力。     

Microbial ecoenzyme stoichiometry,nutrient limitation,and organic matter decomposition in wetlands of the conterminous United States

Microbial respiration (R_m) and ecoenzyme activities (EEA) related to microbial carbon, nitrogen, and phosphorus acquisition were measured in 792 freshwater and estuarine wetlands (representing a cumulative area of 217,480 km²) across the continental United States as part of the US EPA’s 2011 National Wetland Condition Assessment. EEA stoichiometry was used to construct models for and assess nutrient limitation, carbon use efficiency (CUE), and organic matter decomposition (? k). The wetlands were classified into ten groups based on aggregated ecoregion and wetland type. The wetlands were also assigned to least, intermediate, and most disturbed classes, based on the extent of human influences. Ecoenzyme activity related to C, N and P acquisition, R_m, CUE, and ? k differed among ecoregion–wetland types and, with the exception of C acquisition and ? k, among disturbance classes. R_m and EEA were positively correlated with soil C, N and P content (r = 0.15–0.64) and stoichiometry (r = 0.15–0.48), and negatively correlated with an index of carbon quality (r = ? 0.22 to ? 0.39). EEA stoichiometry revealed that wetlands were more often P- than N-limited, and that P-limitation increases with increasing disturbance. Our enzyme-based approach for modeling C, N, and P acquisition, and organic matter decomposition, all rooted in stoichiometric theory, provides a mechanism for modeling resource limitations of microbial metabolism and biogeochemical cycling in wetlands. Given the ease of collecting and analyzing soil EEA and their response to wetland disturbance gradients, enzyme stoichiometry models are a cost-effective tool for monitoring ecosystem responses to resource availability and the environmental drivers of microbial metabolism, including those related to global climate changes.     

土壤微生物碳素利用效率研究进展

土壤微生物碳素利用效率(CUE)是指微生物将吸收的碳(C)转化为自身生物量C的效率,也称为微生物的生长效率。土壤微生物CUE是生态系统C循环中的重要生理生态学参数,影响着生态系统的C固持、周转、土壤矿化以及温室气体排放等过程。在全球环境变化背景下,认识土壤微生物CUE的变异及其影响机制,对于更好的认识生态系统C循环过程及其对全球变化的响应具有重要意义。概述了CUE的定义及其测定方法,重点综述和分析土壤微生物CUE的变异及影响因素取得的研究进展。基于现有研究的分析得出,土壤微生物CUE通常表示为微生物的生长与吸收的比值,分为基于微生物生长速率、微生物生物量、底物吸收速率和底物浓度变化等方法进行测定。土壤微生物CUE在0.2—0.8的范围内变化,这种变异主要受到来自热力学、生态环境因子、底物养分质量和有效性、化学计量平衡以及微生物群落组成的影响。今后土壤微生物CUE的研究应加强对微量代谢组分的定量分析,生物和环境要素交互影响的调控机理解析,以及微生物动态生理响应过程的碳循环模型优化。     

Does declining carbon‐use efficiency explain thermal acclimation of soil respiration with warming?

Enhanced soil respiration in response to global warming may substantially increase atmospheric CO₂ concentrations above the anthropogenic contribution, depending on the mechanisms underlying the temperature sensitivity of soil respiration. Here, we compared short‐term and seasonal responses of soil respiration to a shifting thermal environment and variable substrate availability via laboratory incubations. To analyze the data from incubations, we implemented a novel process‐based model of soil respiration in a hierarchical Bayesian framework. Our process model combined a Michaelis–Menten‐type equation of substrate availability and microbial biomass with an Arrhenius‐type nonlinear temperature response function. We tested the competing hypotheses that apparent thermal acclimation of soil respiration can be explained by depletion of labile substrates in warmed soils, or that physiological acclimation reduces respiration rates. We demonstrated that short‐term apparent acclimation can be induced by substrate depletion, but that decreasing microbial biomass carbon (MBC) is also important, and lower MBC at warmer temperatures is likely due to decreased carbon‐use efficiency (CUE). Observed seasonal acclimation of soil respiration was associated with higher CUE and lower basal respiration for summer‐ vs. winter‐collected soils. Whether the observed short‐term decrease in CUE or the seasonal acclimation of CUE with increased temperatures dominates the response to long‐term warming will have important consequences for soil organic carbon storage.     

Nitrogen availability mediates soil carbon cycling response to climate warming: A meta-analysis

Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO₂ release that offset the increased root C input. Also, reactive N input alleviated warming-induced C loss from soil, but elevated atmospheric CO₂ or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C-climate models may improve the prediction of soil C cycling under future global warming scenarios.     

Experimental fire increases soil carbon dioxide efflux in a grassland long‐term multifactor global change experiment

Numerous studies have demonstrated that soil respiration rates increase under experimental warming, although the long‐term, multiyear dynamics of this feedback are not well constrained. Less is known about the effects of single, punctuated events in combination with other longer‐duration anthropogenic influences on the dynamics of soil carbon (C) loss. In 2012 and 2013, we assessed the effects of decadal‐scale anthropogenic global change – warming, increased nitrogen (N) deposition, elevated carbon dioxide (CO₂), and increased precipitation – on soil respiration rates in an annual‐dominated Mediterranean grassland. We also investigated how controlled fire and an artificial wet‐up event, in combination with exposure to the longer‐duration anthropogenic global change factors, influenced the dynamics of C cycling in this system. Decade‐duration surface soil warming (1–2 °C) had no effect on soil respiration rates, while +N addition and elevated CO₂ concentrations increased growing‐season soil CO₂ efflux rates by increasing annual aboveground net primary production (NPP) and belowground fine root production, respectively. Low‐intensity experimental fire significantly elevated soil CO₂ efflux rates in the next growing season. Based on mixed‐effects modeling and structural equation modeling, low‐intensity fire increased growing‐season soil respiration rates through a combination of three mechanisms: large increases in soil temperature (3–5 °C), significant increases in fine root production, and elevated aboveground NPP. Our study shows that in ecosystems where soil respiration has acclimated to moderate warming, further increases in soil temperature can stimulate greater soil CO₂ efflux. We also demonstrate that punctuated short‐duration events such as fire can influence soil C dynamics with implications for both the parameterization of earth system models (ESMs) and the implementation of climate change mitigation policies that involve land‐sector C accounting.     

森林土壤有机碳分解对模拟增温的响应

由化石燃料燃烧和土地利用变化引起的全球气候变暖是地球上最严重的人为干扰之一,对陆地生态系统结构和功能产生重要的影响。土壤有机碳（SOC）是陆地生态系统最大的碳库,其微小变化都会影响全球碳平衡和气候变化。近30年来,国内外学者在不同森林生态系统相继开展了野外模拟增温对SOC分解的影响及其调控机制研究。基于在全球建立的26个野外模拟气候变暖实验平台,系统分析增温对森林生态系统SOC分解的影响格局和潜在机制,发现增温通常促进森林SOC的分解,对气候变暖产生正反馈作用。然而,因增温方式和持续时间、土壤微生物群落结构和功能的多样性、SOC结构和组成的复杂性、植物-土壤-微生物之间相互作用以及森林类型等不同而存在差异,导致人们对森林SOC分解响应气候变暖的程度及时空格局变化缺乏统一的认识,且各类生物和非生物因子的相对贡献尚不清楚。基于已有研究,从土壤微生物群落结构和功能、有机碳组分以及植物-土壤-微生物互作3个方面构建了气候变暖影响SOC分解的概念框架,并进一步阐述了今后的重点研究方向,以期深入理解森林生态系统碳-气候反馈效应,为制定森林生态系统管理措施和实现"碳中和"提供科学依据。1）加强模拟增温对不同森林生态系统（特别是热带亚热带森林生态系统） SOC分解的长期观测研究,查明SOC分解的时空动态特征;2）加强土壤微生物功能群与SOC分解之间关系的研究,揭示SOC分解对增温响应的微生物学机制;3）形成统一的SOC组分研究方法,揭示不同碳组分对增温的响应特征和机制;4）加强森林生态系统植物-土壤-微生物间相互作用对模拟增温的响应及其对SOC分解调控的研究;5）加强模拟增温与其他全球变化因子（例如降水格局变化、土地利用变化、大气氮沉降）对SOC分解的交互作用,为更好评估未来全球变化背景下森林土壤碳动态及碳汇功能的维持提供理论基础。