植物源和土壤有机质源土壤呼吸组分的拆分原理、方法与应用进展

区分土壤呼吸组分并揭示其与环境因素的相关关系，对于准确评估土壤碳过程及其环境影响机制至关重要。根据底物来源和作用机制的差异，土壤呼吸主要包括根系呼吸、根际微生物呼吸、凋落物分解、自然条件下和激发效应下土壤有机质（SOM）分解。现有土壤呼吸组分拆分方法可以分为基于植物源CO₂测定或土壤有机质源CO₂测定的差分拆分方法，以及基于土壤呼吸组分同位素信号差异的拆分方法。土壤呼吸组分拆分研究可以解决不同土壤呼吸组分对环境变化的响应机制、植物光合碳输入与地下土壤呼吸组分的交互作用、土壤呼吸组分变化对土壤碳库周转的影响机制等科学问题，但其理论假设、观测技术方法、潜在的误差来源等仍需要继续关注并系统研究。     

土壤各组分呼吸区分方法研究进展

土壤呼吸分为自养型呼吸(根呼吸)和异养型呼吸(微生物和动物呼吸),区分各组分呼吸可了解在全球变化条件下土壤碳循环和碳平衡的动态。本文综述了3种主要区分自养呼吸和异养呼吸的方法:①组分法;②根去除术;③同位素法。其中同位素法对根和土壤的影响最小,是最可靠的一种方法;综合各方面考虑,根去除法是最切实可行的方法。     

黄土高原典型植物根际对土壤微生物生物量碳、氮、磷和基础呼吸的影响

选择黄土高原7种典型植物的根际与非根际土壤为研究对象,对土壤的养分含量、微生物生物量碳、氮、磷和基础呼吸的影响进行了初步研究。结果表明,7种不同植物根际土壤与非根际土壤的养分含量、微生物生物量和基础呼吸均存在显著差异;除冷蒿的土壤微生物生物量磷以外,其他各种植物的根际土壤的养分含量、微生物生物量和基础呼吸均比非根际土壤的高;土壤有机碳、全氮与土壤微生物生物量碳、氮及基础呼吸之间均具有极显著或显著相关关系,表明了土壤微生物生物量碳、氮可以作为判断土壤肥力状况的生物学指标,同时也可为提高土壤肥力水平和土壤培肥效果提供依据。     

氮肥对玉米生长季土壤呼吸的影响

在玉米生长季,采用温室盆栽试验,利用分根箱法和根去除法,研究了氮肥对土壤呼吸、土壤基础呼吸、根系呼吸和根际微生物呼吸的影响.试验设4个处理:不种植玉米不施氮肥(CKO)、不种植玉米施氮肥(CKN)、种植玉米不施氮肥(MO)和种植玉米施氮肥(MN).结果表明:不种植玉米处理(CKN和CKO)土壤呼吸速率(土壤基础呼吸)为13.41～77.27 mg C·m-2·h-1,施用氮肥对土壤基础呼吸没有显著影响;种植玉米条件下,施氮处理(MN)的平均土壤呼吸速率为138.54 mg C·m-2·h-1,显著高于不施氮处理(MO),增幅达17.7%,尤其在玉米的抽穗期和开花期增幅明显.施氮肥处理土壤基础呼吸、根系呼吸和根际微生物呼吸对土壤呼吸的贡献率分别为36.2%、45.9%和17.9%,而不施氮肥处理分别为35.5%、36.9%和37.6%.     

浓度升高对森林土壤微生物呼吸与根（际）呼吸的影响

根呼吸与微生物呼吸的作用底物不同,二者对高浓度CO₂的响应机理及敏感程度亦不同。在大气CO₂浓度升高的背景下,精确区分根呼吸与微生物呼吸是构建森林生态系统碳循环模型和预测森林生态系统碳源/汇关系所必需的。根（际）呼吸与微生物呼吸对高浓度CO₂的响应呈增加、降低或无明显变化等不同趋势,根（际）呼吸变化主要与根生物量明显相关,细根的作用大于粗根;土壤微生物呼吸变化存在较大的不确定性,微生物量和微生物活性与土壤微生物呼吸相关或不相关。根系统对高浓度CO₂的响应会潜在地影响微生物的代谢底物,进而影响微生物呼吸强度。凡影响土壤总呼吸的生物与非生物因子都会直接或间接地影响根呼吸与土壤微生物呼吸。     

荒漠草原区土壤粒径组成对柠条根际土壤微生物数量及酶活性的影响

柠条(Caragana korshinskii)是荒漠草原区主要的造林绿化树种,研究其根际土壤微生物和酶活性与不同土壤类型土壤粒径组成的关系有重要意义,然而土壤粒径对荒漠草原柠条根际土壤微生物数量和酶活性的影响知之甚少,探讨土壤颗粒组分与微生物数量、土壤酶活性之间的关系,以及土壤颗粒组成对荒漠草原区固沙灌木植物柠条根际土壤微生物数量及酶活性的影响,可为揭示荒漠草原土壤退化及生态修复提供参考。以宁夏荒漠草原区土壤粒径组成差异显著的灰钙土、红黏土、风沙土环境下栽植的柠条为研究对象,研究不同土壤颗粒组成对根际土壤微生物数量及酶活性的相互关系与影响。结果表明:土壤微生物的数量表现为细菌放线菌真菌。根际土壤中的细菌、真菌数量显著高于非根际,且在3种不同类型的土壤中随着细砂粒的增多,真菌和放线菌数量逐渐降低,而细菌数量呈先增大后减小的趋势;根际与非根际土壤的蔗糖酶、碱性磷酸酶及过氧化氢酶活性均呈现出灰钙土红黏土风沙土的趋势,红黏土根际土壤中的脲酶活性显著高于灰钙土与风沙土;除过氧化氢酶外,土壤酶活性表现为根际高于非根际,在3种不同类型的土壤中随着细砂含量的增加,土壤酶活性均呈递减趋势。土壤颗粒组成与微生物数量之间没有明显的相关性,而与土壤酶活性之间显著相关,土壤酶活性与黏粒、粉粒呈正相关,与细砂、中砂呈负相关关系,根际土壤中酶活性更高,能够为植物及微生物提供更多的营养。     

根际沉积及其在植物-土壤碳循环中的作用

植物根际沉积是一种重要的植物与土壤交换的界面过程,在土壤碳周转方面具有重要的作用;根际碳的沉积也是联系植物、土壤及微生物的桥梁.本文就近年来关于根际沉积中碳平衡、碳循环等相关研究,阐述了根际碳沉积的机制,探讨了相关试验中存在的问题,以及不同植物品种、种类和生育期根际沉积的差异和根际沉积物与土壤呼吸的关系,指出了根际沉积在植物 土壤体系中碳循环的重要作用.在此基础上,提出了未来的研究领域及方向.     

CO2浓度升高对森林土壤微生物呼吸与根（际）呼吸的影响

 根呼吸与微生物呼吸的作用底物不同,二者对高浓度CO₂的响应机理及敏感程度亦不同。在大气CO₂浓度升高的背景下,精确区分根呼吸与微生物呼吸是构建森林生态系统碳循环模型和预测森林生态系统碳源/汇关系所必需的。根（际）呼吸与微生物呼吸对高浓度CO₂的响应呈增加、降低或无明显变化等不同趋势,根（际）呼吸变化主要与根生物量明显相关,细根的作用大于粗根;土壤微生物呼吸变化存在较大的不确定性,微生物量和微生物活性与土壤微生物呼吸相关或不相关。根系统对高浓度CO₂的响应会潜在地影响微生物的代谢底物,进而影响微生物呼吸强度。凡影响土壤总呼吸的生物与非生物因子都会直接或间接地影响根呼吸与土壤微生物呼吸。     

广东省本地油茶和引种油茶根际土壤微生物群落特征

【背景】近年来,油茶低效林面积较大,根际土壤微生物影响林木抗性和生长,对林业可持续发展具有重要意义。【目的】了解广东省本地油茶和引种油茶根际土壤微生物群落特征。【方法】利用高通量测序分析油茶根际土壤微生物群落组成。【结果】油茶根际土壤细菌有26门77纲201目377科593属676种,真菌有14门50纲121目266科502属631种。油茶根际土壤中的优势细菌为酸杆菌门和变形菌门,优势真菌为子囊菌门和担子菌门。两种油茶根际土壤微生物组成差异显著,本地油茶根际土壤的细菌多样性显著高于引种油茶。在门水平上,脱硫杆菌门细菌和罗兹菌门、被孢霉门真菌的相对丰度在两种油茶间差异显著,Amorphotheca在本地油茶根际土壤中特异性富集。两种油茶根际土壤细菌碳代谢相对丰度差异显著,真菌以腐生营养型为主,其次为病理营养型和共生营养型。本地油茶根际土壤中显著富集土壤腐生菌,而共生营养型真菌(尤其是丛枝菌根真菌)相对丰度(6.43%)显著低于引种油茶中(21.83%)。此外,有机质和养分含量是影响油茶根际土壤微生物群落的关键因子。【结论】本地油茶和引种油茶根际土壤微生物群落组成和结构差异显著,Amorp...     

间作不同作物对栀子根际土壤微生态的影响

【背景】栀子为多年生常绿灌木,经过连续多年种植会导致土壤微生态环境恶化、病虫害加剧、品质降低等问题。研究发现间作可以改善土壤微生物区系、土壤养分及酶活性,是生产中常用的有效栽培措施。【目的】通过对不同间作模式下栀子根际土壤微生物区系、酶活性及养分的动态变化进行研究,为揭示栽培措施改良土壤生态环境及提升栀子产量的土壤微生态学机理提供理论依据。【方法】为了解不同间作模式对栀子根际微生态的影响,本试验选择3年生栀子进行了大田试验,采用随机区组设计,设置栀子单作及栀子/白及、栀子/金钱草、栀子/射干3种间作处理,以栀子根际土壤为研究对象进行全生育期取样。采用Illumina高通量测序技术测定细菌16S rRNA基因V3-V4区序列及真菌rDNAITS1-ITS2区序列,并测定各时期土壤的理化性质,以明确栀子间作不同作物对其根际微生物群落及土壤理化性质随栀子生育期的动态变化。【结果】在栀子整个生育过程中,根际细菌群落中变形菌门和酸杆菌门的相对丰度分别为39%和18%,为细菌优势菌门。真菌群落中子囊菌门、担子菌门和被孢霉门相对丰度所占比例依次为51%、22%和19%,为主要真菌类群。在果实膨大期,与单作栀子相比,栀子/射干和栀子/白及可以显著增加土壤细菌群落Shannon指数,增幅分别为6.55%和3.45%(P0.05),其他时期差异不显著。盛花期,栀子/射干间作不会显著降低根际真菌的多样性,而与金钱草或白及间作则显著降低其多样性;果实膨大期,栀子/射干和栀子/白及间作均可以显著提高根际土壤真菌群落的Shannon指数,增幅分别为29.19%和9.12%。土壤养分方面,不同间作模式下根际土壤理化性质存在一定差异。单作栀子根际土壤仅有机质、全氮、速效磷的含量较高,而碱解氮、速效钾含量均低于3种间作处理。土壤酶方面,单作栀子除酸性蛋白酶外,其余几项土壤酶活性均处偏下水平。土壤理化性质与根际微生物多样性的相关性分析显示,细菌多样性指数Shannon与根际土壤有机质、速效磷含量呈显著正相关,与pH值呈极显著正相关(P0.01);真菌多样性指数Shannon与根际土壤全钾、脲酶、过氧化氢酶呈极显著负相关,与速效钾、蔗糖酶、酸性磷酸酶、酸性蛋白酶活性呈极显著正相关。【结论】合理间作可以改善根际微生物群落结构,同时提高土壤综合肥力。     

Partitioning sources of soil-respired CO2 and their seasonal variation using a unique radiocarbon tracer

Soil respiration is derived from heterotrophic (decomposition of soil organic matter) and autotrophic (root/rhizosphere respiration) sources, but there is considerable uncertainty about what factors control variations in their relative contributions in space and time. We took advantage of a unique whole‐ecosystem radiocarbon label in a temperate forest to partition soil respiration into three sources: (1) recently photosynthesized carbon (C), which dominates root and rhizosphere respiration; (2) leaf litter decomposition and (3) decomposition of root litter and soil organic matter >1–2 years old. Heterotrophic sources and specifically leaf litter decomposition were large contributors to total soil respiration during the growing season. Relative contributions from leaf litter decomposition ranged from a low of ～1±3% of total soil respiration (6± 3 mg C m^?2 h^?1) when leaf litter was extremely dry, to a high of 42±16% (96± 38 mg C m^?2 h^?1). Total soil respiration fluxes varied with the strength of the leaf litter decomposition source, indicating that moisture‐dependent changes in litter decomposition drive variability in total soil respiration fluxes. In the surface mineral soil layer, decomposition of C fixed in the original labeling event (3–5 years earlier) dominated the isotopic signature of heterotrophic respiration. Root/rhizosphere respiration accounted for 16±10% to 64±22% of total soil respiration, with highest relative contributions coinciding with low overall soil respiration fluxes. In contrast to leaf litter decomposition, root respiration fluxes did not exhibit marked temporal variation ranging from 34±14 to 40±16 mg C m^?2 h^?1 at different times in the growing season with a single exception (88±35 mg C m^?2 h^?1). Radiocarbon signatures of root respired CO₂ changed markedly between early and late spring (March vs. May), suggesting a switch from stored nonstructural carbohydrate sources to more recent photosynthetic products.     

Partitioning sources of soil respiration in boreal black spruce forest using radiocarbon

Separating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ¹⁴C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently‐fixed C that fuels plant or microbial metabolism has Δ¹⁴C values close to that of current atmospheric CO₂, while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ¹⁴C of C respired by recently excised black spruce roots averaged 14‰ greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. The Δ¹⁴C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60‰ higher than the contemporary atmosphere Δ¹⁴CO₂, indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ¹⁴C end members with measurements of the Δ¹⁴C of total soil respiration, we calculated that 47–63% of soil CO₂ emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for high Δ¹⁴C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration.     

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.     

土壤呼吸组分分离技术研究进展

分离土壤呼吸组分是理解陆地生态系统碳循环的重要步骤,研究农田生态系统土壤呼吸组分的呼吸过程和机理对促进农业温室气体减排和碳汇增加、气候变化适应、保障粮食安全以及推动农业可持续发展都具有积极意义。本文综述了近年来土壤呼吸组分分离的理论依据、主要技术及分类,系统比较了现有技术优势、劣势和应用领域,并总结了土壤呼吸组分分离技术在国内外农田生态系统中的应用情况。由于多数分离技术在森林生态系统的相关研究中发展而来,它们在农田生态系统的应用十分有限,目前应用以同位素法、根分离法和回归法为主。由于土壤呼吸理论划分和分离方法的差异,不同研究结果之间往往难以比较。分离技术的发展有赖于土壤呼吸源分离理论的进一步发展,未来土壤呼吸组分分离研究的主要方向在于：（1）利用现有观测技术促进组分集成分析法和根分离法在农田生态系统中的应用,强化土壤呼吸组分和环境因子的同步观测,准确评估农田碳收支;（2）利用定位观测数据开展大尺度模型研究,改进和重构现有全球碳模型的碳氮过程,并在其中考虑重要的土壤呼吸过程;（3）利用FACE试验评估气候变化对土壤呼吸组分的影响和土壤-植物碳循环的适应机制;（4）分析呼吸组分与植物-土壤-养分的交互作用,评估农田管理措施的综合影响。     

Allocation and residence time of photosynthetic products in a boreal forest using a low-level 14C pulse-chase labeling technique

Much of our understanding about how carbon (C) is allocated in plants comes from radiocarbon (¹⁴C) pulse‐chase labeling experiments. However, the large amounts of ¹⁴C required for decay‐counting mean that these studies have been restricted for the most part to mesocosm or controlled laboratory experiments. Using the enhanced sensitivity for ¹⁴C detection available with accelerator mass spectrometry (AMS), we tested the utility of a low‐level ¹⁴C pulse‐chase labeling technique for quantifying C allocation patterns and the contributions of different plant components to total ecosystem respiration in a black spruce forest stand in central Manitoba, Canada. All aspects of the field experiment used ¹⁴C at levels well below regulated health standards, without significantly altering atmospheric CO₂ concentrations. Over 30 days following the label application in late summer (August and September), we monitored the temporal and spatial allocation patterns of labeled photosynthetic products by measuring the amount and ¹⁴C content of CO₂ respired from different ecosystem components. The mean residence times (MRT) for labeled photosynthetic products to be respired in the understory (feather mosses), canopy (black spruce), and rhizosphere (black spruce roots and associated microbes) were <1, 6, and 15 days, respectively. Respiration from the canopy and understory showed significantly greater influence of labeled photosynthates than excised root and intact rhizosphere respiration. After 30 days,∼65% of the label assimilated had been respired by the canopy,∼20% by the rhizosphere, and∼9% by the understory, with∼6% unaccounted for and perhaps remaining in tissues. Maximum ¹⁴C values in root and rhizosphere respiration were reached 4 days after label application. The label was still detectable in root, rhizosphere and canopy respiration after 30 days; these levels of remaining label would not have been detectible had a ¹³C label been applied. Our results support previous studies indicating that a substantial portion of the C fueling rhizosphere respiration in the growing season may be derived from stored C pools rather than recent photosynthetic products.     

Evaluation of the use of a model rhizodeposition technique to separate root and microbial respiration in soil

A model rhizodeposition technique to estimate the root and microbial components of ¹⁴C soil/root respiration in pulse-labelling experiments is described. The method involves the injection of model rhizodeposits, consisting of ¹⁴C-labelled glucose, root extract or root cell wall material, into the rooted soil of an unlabelled plant, simultaneously with the pulse-labelling of a separate but similar plant with ¹⁴CO₂. In a growth chamber experiment with 30 day old wheat and barley the contribution of direct root respiration to ¹⁴C soil/root respiration over a 26 day period after labelling was estimated 89–95%. Estimates of direct root respiration in field-grown wheat and barley at different development stages in most cases accounted for at least 75% of ¹⁴C soil/root respiration over a 21 day period after labelling. The mineralization rate of injected ¹⁴C-glucose was positively correlated with the concentration of glucose-C established in soil. The use of the method in rhizosphere carbon budget estimations is evaluated. Communication No. 73 of the Dutch Programme on Soil Ecology of Arable Farming Systems. Communication No. 73 of the Dutch Programme on Soil Ecology of Arable Farming Systems.     

13C isotope fractionation during rhizosphere respiration of C3 and C4 plants

Stable carbon isotopes are used extensively to partition total soil CO₂ efflux into root-derived rhizosphere respiration or autotrophic respiration and soil-derived heterotrophic respiration. However, it remains unclear whether CO₂ from rhizosphere respiration has the same δ¹³C value as root biomass. Here we investigated the magnitude of ¹³C isotope fractionation during rhizosphere respiration relative to root biomass in six plant species. Plants were grown in a carbon-free sand-perlite medium inoculated with microorganisms from a farm soil for 62 days inside a greenhouse. We measured the δ¹³C value of rhizosphere respiration using a closed-circulation 48-hour CO₂ trapping method during 40~42 and 60~62 days after sowing. We found a consistent depletion in ¹³C (0.9~1.7‰) of CO₂ from rhizosphere respiration relative to root biomass in three C₃ species (Glycine max L. Merr., Helianthus annuus L. and Triticum aestivum L.), but a relatively large depletion in ¹³C (3.7~7.0‰) in three C₄ species (Amaranthus tricolor L., Sorghum bicolor (L.) Moench and Zea mays L. ssp. mays). Overall, our results indicate that CO₂ from rhizosphere respiration is more ¹³C-depleted than root biomass. Therefore, accounting for this ¹³C fractionation is required for accurately partitioning total soil CO₂ efflux into root-derived and soil-derived components using natural abundance stable carbon isotope methods.     

Measurement of rhizosphere respiration and organic matter decomposition using natural 13C

Due to the limitations in methodology it has been a difficult task to measure rhizosphere respiration and original soil carbon decomposition under the influence of living roots. ¹⁴C-labeling has been widely used for this purpose in spite of numerous problems associated with the labeling method. In this paper, a natural ¹³C method was used to measure rhizosphere respiration and original soil carbon decomposition in a short-term growth chamber experiment. The main objective of the experiment was to validate a key assumption of this method: the ¹³C value of the roots represents the ¹³C value of the rhizosphere respired CO₂. Results from plants grown in inoculated carbon-free medium indicated that this assumption was valid. This natural ¹³C method was demonstrated to be advantageous for studying rhizosphere respiration and the effects of living roots on original soil carbon decomposition.     

Effects of experimental drought on soil respiration and radiocarbon efflux from a temperate forest soil

Soil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5 × 5 m² plots excluded throughfall water during the summers of 2001 (168 mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53 mg C m^?2 h^?1 over 84 days in 2001, and by 68 mg C m^?2 h^?1 over 126 days in 2002, representing 10–30% of annual SR in this forest and 35–75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r²=0.69) and the Oe/Oa horizon (r²=0.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO₂ efflux and of CO₂ respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43–71% of the total C respired in the control plots and 41–80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO₂ efflux of radiocarbon‐rich substrates during a period of abundant growth of mushrooms. Our results suggest that prolonged summer droughts decrease primarily heterotrophic respiration in the O horizon, which could cause increases in the storage of soil organic carbon in this forest. However, the C stored during two summers of simulated drought was only partly released as increased respiration during the following summer of natural throughfall. We do not know if this soil C sink during drought is transient or long lasting. In any case, differential decomposition of the O horizon caused by interannual variation of precipitation probably contributes significantly to observed interannual variation of NEE in temperate forests.