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。     

Winter soil respiration during soil-freezing process in a boreal forest in Northeast China

Aims Boreal forest is the largest and contains the most soil carbon among global terrestrial biomes. Soil respiration during the prolonged winter period may play an important role in the carbon cycles in boreal forests. This study aims to explore the characteristics of winter soil respiration in the boreal forest and to show how it is regulated by environmental factors, such as soil temperature, soil moisture and snowpack.Methods Soil respiration in an old-growth larch forest (Larix gmelinii Ruppr.) in Northeast China was intensively measured during the winter soil-freezing process in 2011 using an automated soil CO₂ flux system. The effects of soil temperature, soil moisture and thin snowpack on soil respiration and its temperature sensitivity were investigated.Important findings Total soil respiration and heterotrophic respiration both showed a declining trend during the observation period, and no significant difference was found between soil respiration and heterotrophic respiration until the snowpack exceeded 20cm. Soil respiration was exponentially correlated with soil temperature and its temperature sensitivity (Q 10 value) for the entire measurement duration was 10.5. Snow depth and soil moisture both showed positive effects on the temperature sensitivity of soil respiration. Based on the change in the Q 10 value, we proposed a 'freeze–thaw critical point' hypothesis, which states that the Q 10 value above freeze–thaw critical point is much higher than that below it (16.0 vs. 3.5), and this was probably regulated by the abrupt change in soil water availability during the soil-freezing process. Our findings suggest interactive effects of multiple environmental factors on winter soil respiration and recommend adopting the freeze–thaw critical point to model soil respiration in a changing winter climate.     

亚热带毛竹人工林土壤呼吸组分动态变化及其影响因素

利用Li-8100土壤碳通量测量系统,研究了2013年4月—2014年3月浙江临安市毛竹人工林土壤呼吸、异养呼吸和自养呼吸速率的动态变化规律.结果表明：毛竹人工林土壤总呼吸速率、异养呼吸速率和自养呼吸速率均呈现出明显的季节变化特征,最高值出现在7月,最低值出现在1月,年平均值分别为2.93、1.92和1.01 μmol CO₂·m^-2·s^-1.毛竹林土壤总呼吸、异养呼吸和自养呼吸年累积CO₂排放量分别为37.25、24.61和12.64 t CO₂·hm^-2·a^-1.土壤呼吸各组分均与土壤5 cm温度呈显著指数相关,土壤总呼吸、异养呼吸和自养呼吸的温度敏感系数Q₁₀值分别为2.05、1.95和2.34.土壤总呼吸速率、异养呼吸速率与土壤水溶性有机碳(WSOC)含量均呈显著相关,而自养呼吸与WSOC无显著相关性;土壤呼吸各组分与土壤含水〖JP2〗量以及微生物生物量碳均无显著相关性.土壤温度是影响毛竹人工林土壤呼吸及其组分季节变化的主要驱动因子,土壤WSOC含量是影响土壤总呼吸和异养呼吸的重要环境因子.     

增温和刈割对高寒草甸土壤呼吸及其组分的影响

评估土壤呼吸及其组分对增温等全球变化的响应对于预测陆地生态系统碳循环至关重要。本研究利用红外线辐射加热器(Infrared heater)装置在青藏高原高寒草甸生态系统设置增温和刈割野外控制实验。通过测定2018年生长季(5—9月)土壤呼吸和异养呼吸,探究增温和刈割对土壤呼吸及其组分的影响。研究结果表明：(1) 单独增温使土壤呼吸显著增加31.65% (P<0.05),异养呼吸显著增加27.12% (P<0.05),土壤自养呼吸没有显著改变(P>0.05);单独刈割对土壤呼吸和自养呼吸没有显著影响(P>0.05),单独刈割刺激异养呼吸增加32.54% (P<0.05);(2) 增温和刈割之间的交互作用对土壤呼吸和异养呼吸没有显著影响(P>0.05),但是对自养呼吸的影响是显著的(P<0.05),土壤呼吸和异养呼吸的季节效应显著(P<0.05);(3)土壤呼吸及其组分与土壤温度均成显著指数关系,与土壤湿度呈显著的正相关关系(P<0.05),处理影响它们的响应敏感性。本研究表明青藏高原东缘高寒草甸土壤碳排放与气候变暖存在正反馈。     

米槠和杉木人工林土壤呼吸及其组分分析

区分森林土壤呼吸组分是了解生态系统碳循环的重要环节。该文以福建省三明市格氏栲自然保护区米槠(Castanopsis carlesii)人工林和邻近的杉木(Cunninghamia lanceolata)人工林为研究对象, 于2012年8月至2013年7月, 采用LI-8100开路式土壤碳通量系统, 通过挖壕沟方法, 测定了土壤呼吸及异养呼吸的速率, 同时测定了5 cm深处的土壤温度和0-12 cm深处的土壤含水量。利用指数模型和双因素模型, 分析土壤呼吸及其组分与土壤温度和土壤含水量的关系, 同时计算了土壤呼吸各组分在土壤呼吸中所占的比例, 并分析了不同森林类型对土壤呼吸及其组分的影响。结果表明: 米槠人工林和杉木人工林土壤呼吸及其组分的季节变化显著, 均呈单峰型曲线, 与5 cm深处的土壤温度呈极显著正相关关系。土壤温度可以分别解释米槠人工林土壤呼吸、自养呼吸和异养呼吸变化的70.3%、73.4%和58.2%, 可以解释杉木人工林土壤呼吸、自养呼吸和异养呼吸变化的77.9%、65.7%和79.2%。土壤呼吸及其组分与土壤含水量没有相关关系。米槠和杉木人工林自养呼吸的年通量分别为4.00和2.18 t C·hm^-2·a^-1, 占土壤呼吸年通量的32.5%和24.1%; 异养呼吸年通量分别为8.32和6.88 t C·hm^-2·a^-1, 分别占土壤呼吸年通量的67.5%和75.9%, 米槠人工林土壤呼吸及其组分的年通量都大于杉木人工林。     

Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas

Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO₂ efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO₂ efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the ‘autotrophic’ root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO₂ efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO₂ flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO₂ flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO₂ flux component represents an overflow ‘CO₂ tap’ through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO₂.     

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.     

三源区分土壤呼吸组分研究

三源区分土壤呼吸组分是指将土壤呼吸区分为纯根呼吸、根际微生物呼吸和土壤有机质呼吸3个部分。土壤有机质呼吸、纯根呼吸和根际微生物呼吸是3种不同的生物学过程,这3种呼吸对环境变化具有不同的响应机制。区分土壤呼吸中由根系引起的自养和异养呼吸组分的研究对定量评价陆地生态系统碳平衡具有重要的意义。论述了三源区分土壤呼吸组分的意义、方法和应用,分析了不同条件下土壤呼吸组分区分的研究结果。实验室纯根和根际微生物呼吸占根源呼吸比重约为45%和55%;野外条件下约为60%和40%。最后对本研究未来的发展方向进行了展望。     

Effects of nitrogen addition on soil respiration in shrublands in Mt. Dongling,Beijing, China

Aims Soil respiration from terrestrial ecosystems is an important component of terrestrial carbon budgets. Compared to forests, natural or semi-natural shrublands are mostly distributed in nutrient-poor sites, and usually considered to be relatively vulnerable to environmental changes. Increased nitrogen (N) input to ecosystems may remarkably influence soil respiration in shrublands. So far the effects of N deposition on shrubland soil respiration are poorly understood. The aim of this study is to investigate the soil respiration of Vitex negundo var. heterophylla and Spiraea salicifolia shrublands and their response to N deposition. Methods We carried out a N enrichment experiment in V. negundo var. heterophylla and S. salicifolia shrublands in Mt. Dongling, Beijing, with four N addition levels (N₀, control, 0; N₁, low N, 20 kg N·hm^-2·a^-1; N₂, medium N, 50 kg N·hm^-2·a^-1 and N₃, high N, 100 kg N·hm^-2·a^-1). Respiration was measured from 2012-2013 within all treatments.Important findings Under natural conditions, annual total and heterotrophic respiration were 5.91 and 4.23, 5.76 and 3.53 t C·hm^-2·a^-1 for the V. negundo var. heterophylla and S. salicifolia shrublands, respectively and both were not affected by short-term N addition. In both shrubland types, soil respiration rate exhibited significant exponential relationships with soil temperature. Temperature sensitivity (Q₁₀) of total soil respiration in V. negundo var. heterophylla and S. salicifolia shrublands ranged from 1.44 to 1.58 and 1.43 to 1.98, and Q₁₀ of heterotrophic soil respiration ranged from 1.38 to 2.11 and 1.49 to 1.88, respectively. Short-term N addition decreased only autotrophic respiration rate during the growing season, but had no significant effects on total and heterotrophic soil respiration in V. negundo var. heterophylla shrubland. In contrast, N addition enhanced the heterotrophic soil respiration rate and did not influence autotrophic and total soil respiration in S. salicifolia shrubland.     

Mycorrhizal Status Influences the Rate but not the Temperature Sensitivity of Soil Respiration

Mycorrhizal fungi, which can produce a large portion of total soil respiration, respond strongly to global changes such as elevated CO₂, N-deposition, and land-use change. Predictions of future ecosystem C sequestration hinge on respiration budgets, but the mycorrhizal influence on total soil respiration remains unknown. In this study, sunflowers (Helianthus annuus) were subjected to various mycorrhizal treatments, and their root and soil systems were enclosed in chambers that continuously monitored belowground (root + mycorrhizal + heterotrophic) CO₂ production during plant growth, death, and decomposition. Rhizocosms with high mycorrhizal colonization exhibited higher soil respiration rates as plants matured, an increase that was in proportion to the mycorrhizal stimulation of plant growth. Living mycorrhizal plants behaved like nonmycorrhizal ones in that total rhizocosm respiration had the same relationship to plant mass and the same temperature sensitivity as nonmycorrhizal plants. Upon removal of the shoots though, mycorrhizal plants exhibited the largest relative reduction in respiration resulting in a unique relationship of soil respiration with plant mass. The mycorrhizal influence on heterotrophic respiration merits as much attention from experimenters and modelers as the mycorrhizal contribution to autotrophic respiration.     

寒温带岛状林沼泽土壤呼吸速率和季节变化

2011年生长季内利用静态箱-气相色谱法,研究了寒温带典型湿地白桦(Betula platyphylla)岛状林沼泽、兴安落叶松(Larix gmelinii)岛状林沼泽土壤呼吸速率的季节动态及其主要环境因子,利用壕沟隔断法对土壤呼吸各组分间的差异进行研究。结果表明:生长季白桦和兴安落叶松岛状林沼泽土壤呼吸速率具有明显的季节性规律,土壤呼吸总速率分别为368.60和312.46 mg m-2h-1,异养呼吸速率分别为300.57和215.70 mg m-2h-1,占土壤呼吸总速率的81.5%和69.0%;自养呼吸速率为68.03和96.76 mg m-2h-1,占土壤呼吸总速率的18.5%和31.0%。不同处理条件下的土壤呼吸在季节变化上表现基本一致,高峰期都发生在夏季;土壤呼吸与温度呈极显著相关性,但与土壤湿度的相关性较差。生长季白桦和兴安落叶松岛状林沼泽土壤呼吸总量分别为12.64和10.61 t/hm2。     

Seasonal and spatial variability of soil respiration in four Sitka spruce stands

We investigated the causes for the seasonal and spatial variation of soil respiration in a first rotation Sitka spruce chronosequence composed of four age classes (10, 15, 31, and 47 year old) in Central Ireland. The study aimed at identifying easily determinable environmental parameters that explained the variation in soil respiration rates. The variation in temperature and soil water content influenced the seasonal trend observed in the spatial variability of soil respiration. The highest coefficients of variation in soil respiration were observed during autumn drought, while lower coefficients were generally observed during periods with highest soil respiration rates. On average, the sampling strategy of 30 sampling points per stand was adequate to obtain an average rate of soil respiration within 20% of its actual value at the 95% confidence level. Significantly higher soil respiration rates were observed at locations with high accumulation of organic matter and in collars established in close vicinity to tree stems. The organic layer thickness was the only variable that yielded significant regressions for explaining spatial variation in soil respiration in all the stands. Correlation analyses between the studied variables and soil respiration suggested the relative importance of heterotrophic and autotrophic components differed in their annual contribution to total soil respiration at each forest stand. Multiple regression analyses were used to assess the relative importance of primary temporal and spatial controls over soil respiration. Soil temperature and organic layer thickness explained most of the variance of soil respiration for the different sampling periods, while soil water content had a weaker effect as well as a different influence on soil respiration depending on the time of the year. The strong linear correlation between forest floor carbon and soil carbon stock further confirmed organic layer thickness as an integrative factor encompassing the effect of soil carbon pools on soil respiration. Moreover, its inclusion in the multiple regression analyses overrode the influence of both distance and fine root biomass. Overall, a multiple linear regression model driven by easily determinable environmental variables such as soil temperature, organic thickness, soil water content, soil bulk density, and soil organic carbon concentration allowed us to explain 54% of total variance of soil respiration over the different stand ages for the entire year (P < 0.05). Our results show that the adoption of an adequate sampling strategy, and the determination of some key environmental variables may help to explain a large proportion of total variation of soil respiration over the entire rotation length of afforested ecosystems.     

帽儿山3种森林生态系统土壤动物与土壤呼吸及其相互关系研究

土壤呼吸是森林生态系统碳循环的关键过程,土壤动物可通过自身代谢及影响微生物活动调控土壤呼吸,因此研究土壤动物与土壤呼吸的相互关系对进一步揭示生态系统碳循环的规律和机理具有重要意义。通过野外定点,以帽儿山3种森林生态系统的土壤呼吸及土壤动物为研究对象,探讨不同森林生态系统的土壤呼吸、土壤动物个体密度和生物量的时间变化规律及二者相互关系。结果表明:(1)3种森林生态系统土壤总呼吸速率与土壤异养呼吸速率均呈现先增强后减弱的时间动态变化(P<0.05),且不同森林生态系统土壤异养呼吸速率差异显著(P<0.05),表现为硬阔叶林最高,红松人工林最低;(2)3种森林生态系统土壤动物生物量也具有显著的时间动态变化(P<0.05),均在9月份达到最大,且不同森林生态系统土壤动物个体密度显著不同(P<0.05),蒙古栎林土壤动物个体密度显著小于红松人工林与硬阔叶林;(3)通过回归分析可得,土壤动物数量及生物量的增加抑制了土壤呼吸速率,尤其在生长季初期、末期。研究表明土壤动物可通过抑制微生物生命活动和降低根系呼吸从而对土壤总呼吸及异养呼吸产生负反馈作用,三者是不可分割的整体,与土壤温度、水分等环境因子共同调控着土壤呼吸。     

Separating root and soil microbial contributions to soil respiration: A review of methods and observations

Forest soil respiration is the sum of heterotrophic (microbes, soil fauna) and autotrophic (root) respiration. The contribution of each group needs to be understood to evaluate implications of environmental change on soil carbon cycling and sequestration. Three primary methods have been used to distinguish hetero- versus autotrophic soil respiration including: integration of components contributing to in situ forest soil CO₂ efflux (i.e., litter, roots, soil), comparison of soils with and without root exclusion, and application of stable or radioactive isotope methods. Each approach has advantages and disadvantages, but isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots. Published data from all methods indicate that root/rhizosphere respiration can account for as little as 10 percent to greater than 90 percent of total in situ soil respiration depending on vegetation type and season of the year. Studies which have integrated percent root contribution to total soil respiration throughout an entire year or growing season show mean values of 45.8 and 60.4 percent for forest and nonforest vegetation, respectively. Such average annual values must be extrapolated with caution, however, because the root contribution to total soil respiration is commonly higher during the growing season and lower during the dormant periods of the year.     

林木根呼吸及测定方法进展

 森林土壤呼吸的近2/3是由林木根呼吸产生的,林木根呼吸对估计森林C吸存及构建森林生态系统碳动态模型有重要意义,是全球碳循环研究的一个重要组成部分。林木根呼吸包括生长呼吸和维持呼吸,不同森林生态系统林木根呼吸对土壤呼吸的贡献大多在40%～60%范围内,林木根呼吸在生长季节较高而休眠季节较低。测定林木根呼吸的主要方法有排除根法、离体根法、同位素法和原位PVC管气室法,前两者相对简单、成本低,常用于森林生态系统中;同位素法可原位测定根呼吸,对土壤干扰较小,但不易操作,且成本高。根呼吸受土壤温度、根直径大小、根组织N浓度、环境CO2浓度、土壤湿度、养分有效性等因素的影响。今后的研究应集中在以下方面：1）探讨和比较不同条件下测定根呼吸组成（生长呼吸、维持呼吸）的最合适方法;2）加大在野外条件下使用有效方法分离根呼吸和根际微生物呼吸的力度;3）对森林生态系统根呼吸动态进行长期的定位研究;4）进一步加强研究不同气候带,不同森林类型林木根呼吸,并将研究尺度从气室扩大到区域或全球水平;5）加强林木根呼吸对全球变化的响应及机制的研究;6)对林木根呼吸进行多学科合作研究将为全球C循环做出新的贡献。     

Partitioning of soil respiration components and evaluating the mycorrhizal contribution to soil respiration in a semiarid grassland

土壤呼吸组分的区分对于理解地下碳循环过程非常重要。而菌根真菌在地下碳循环过程中扮演着重要的角色, 但是有关菌根呼吸在草原生态系统中的研究相对较少。该研究在内蒙古半干旱草原应用深浅环网孔法, 结合浅环、深环(排除根系)和一个带有40 μm孔径窗口的土壤环(排除根系但是有菌根菌丝体)将根和菌丝物理分离, 来区分不同的呼吸组分。结果表明: 异养呼吸对总呼吸的贡献比例为51%, 根呼吸的贡献比例为26%, 菌根呼吸的贡献比例为23%, 菌根呼吸的比例3年变化范围为21%-26%。与国内外研究相比, 此方法提供了一个相对稳定的菌根呼吸测量精度范围, 在草原生态系统中切实可行。对菌根呼吸的准确定量将有助于预测草原生态系统土壤碳释放过程对未来气候变化的响应。     

半干旱草原土壤呼吸组分区分与菌根呼吸的贡献

土壤呼吸组分的区分对于理解地下碳循环过程非常重要。而菌根真菌在地下碳循环过程中扮演着重要的角色, 但是有关菌根呼吸在草原生态系统中的研究相对较少。该研究在内蒙古半干旱草原应用深浅环网孔法, 结合浅环、深环(排除根系)和一个带有40 μm孔径窗口的土壤环(排除根系但是有菌根菌丝体)将根和菌丝物理分离, 来区分不同的呼吸组分。结果表明: 异养呼吸对总呼吸的贡献比例为51%, 根呼吸的贡献比例为26%, 菌根呼吸的贡献比例为23%, 菌根呼吸的比例3年变化范围为21%-26%。与国内外研究相比, 此方法提供了一个相对稳定的菌根呼吸测量精度范围, 在草原生态系统中切实可行。对菌根呼吸的准确定量将有助于预测草原生态系统土壤碳释放过程对未来气候变化的响应。     

Different responses of soil respiration and its components to nitrogen addition among biomes: a meta‐analysis

Anthropogenic activities have increased nitrogen (N) deposition by threefold to fivefold over the last century, which may considerably affect soil respiration (Rs). Although numerous individual studies and a few meta‐analyses have been conducted, it remains controversial as to how N addition affects Rs and its components [i.e., autotrophic (Ra) and heterotrophic respiration (Rh)]. To reconcile the difference, we conducted a comprehensive meta‐analysis of 295 published studies to examine the responses of Rs and its components to N addition in terrestrial ecosystems. We also assessed variations in their responses in relation to ecosystem types, environmental conditions, and experimental duration (DUR). Our results show that N addition significantly increased Rs by 2.0% across all biomes but decreased by 1.44% in forests and increased by 7.84% and 12.4% in grasslands and croplands, respectively (P < 0.05). The differences may largely result from diverse responses of Ra to N addition among biomes with more stimulation of Ra in croplands and grasslands compared with no significant change in forests. Rh exhibited a similar negative response to N addition among biomes except that in croplands, tropical and boreal forests. Methods of partitioning Rs did not induce significant differences in the responses of Ra or Rh to N addition, except that Ra from root exclusion and component integration methods exhibited the opposite responses in temperate forests. The response ratios (RR) of Rs to N addition were positively correlated with mean annual temperature (MAT), with being more significant when MAT was less than 15 °C, but negatively with DUR. In addition, the responses of Rs and its components to N addition largely resulted from the changes in root and microbial biomass and soil C content as indicated by correlation analysis. The response patterns of Rs to N addition as revealed in this study can be benchmarks for future modeling and experimental studies.     

Litter standing crop of shrubland ecosystems in southern China

Aims Litter is an important component of terrestrial ecosystems, which plays significant roles in carbon and nutrient cycles. Quantifying regional-scale pattern of litter standing crop would improve our understanding in the mechanism of the terrestrial ecosystem carbon cycle, also with help in predicting the responses of carbon cycle of terrestrial ecosystems to future climate change. Our objective was to examine variation in litter standing crop of shrublands along the environmental gradients in southern China.
Methods During 2011-2014, we investigated the litter standing crop at 453 shrublands sites by the stratified random sampling, reflecting climatic and soil attributes across southern China.
Important findings We found that the mean value of litter standing crop in these shrubland ecosystems across southern China was 0.32 kg·m^-2. It was 68% of forest litter standing crop (0.47 kg·m^-2) and was five times higher than that in grasslands (0.06 kg·m^-2) in China. Litter standing crop increased with latitude. Our results showed that litter standing crop was negatively correlated with mean annual temperature, soil total P and soil pH, but not significantly correlated with other environmental variables, including mean annual precipitation, soil carbon, nitrogen and soil organic matter. The conversion coefficient of carbon in litter standing crop was 0.41, which is significantly lower than that of vegetation in shrublands (0.50), resulting in an overestimate in carbon storage of litter standing crop in shrubland up to 22% by applying wrong conversion coefficient. We concluded that litter standing crop of shrublands is an important component in terrestrial ecosystems. Mean annual temperature was the most important environmental variable, accounting for the variation in litter standing crop of shrublands in southern China. To our best of knowledge, this is the first study to quantify variation in litter standing crop of shrublands at the regional scale. Therefore, our study will have important implications for assessing the carbon budget of terrestrial ecosystems in China.     

Diel patterns of autotrophic and heterotrophic respiration among phenological stages

Improved understanding of the links between aboveground production and allocation of photosynthate to belowground processes and the temporal variation in those links is needed to interpret observations of belowground carbon cycling processes. Here, we show that combining a trenching manipulation with high‐frequency soil respiration measurements in a temperate hardwood forest permitted identification of the temporally variable influence of roots on diel and seasonal patterns of soil respiration. The presence of roots in an untrenched plot caused larger daily amplitude and a 2–3 h delay in peak soil CO₂ efflux relative to a root‐free trenched plot. These effects cannot be explained by differences in soil temperature, and they were significant only when a canopy was present during the growing season. This experiment demonstrated that canopy processes affect soil CO₂ efflux rates and patterns at hourly and seasonal time scales, and it provides evidence that root and microbial processes respond differently to environmental factors.