Responses of soil respiration to simulated precipitation pulses in semiarid steppe under different grazing regimes

Aims Precipitation pulses and different land use practices (such as grazing) play important roles in regulating soil respiration and carbon balance of semiarid steppe ecosystems in Inner Mongolia. However, the interactive effects of grazing and rain event magnitude on soil respiration of steppe ecosystems are still unknown. We conducted a manipulative experiment with simulated precipitation pulses in Inner Mongolia steppe to study the possible responses of soil respiration to different precipitation pulse sizes and to examine how grazing may affect the responses of soil respiration to precipitation pulses.Methods Six water treatments with different precipitation pulse sizes (0, 5, 10, 25, 50 and 100 mm) were conducted in the ungrazed and grazed sites, respectively. Variation patterns of soil respiration of each treatment were determined continuously after the water addition treatments.Important findings Rapid and substantial increases in soil respiration occurred 1 day after the water treatments in both sites, and the magnitude and duration of the increase in soil respiration depended on pulse size. Significantly positive relationships between the soil respiration and soil moisture in both sites suggested that soil moisture was the most important factor responsible for soil respiration rate during rain pulse events. The ungrazed site maintained significantly higher soil moisture for a longer time, which was the reason that the soil respiration in the ungrazed site was maintained relatively higher rate and longer period than that in the grazed site after a rain event. The significant exponential relationship between soil temperature and soil respiration was found only in the plots with the high water addition treatments (50 and 100 mm). Lower capacity of soil water holding and lower temperature sensitivity of soil respiration in the grazed site indicated that degraded steppe due to grazing might release less CO₂ to the atmosphere through soil respiration under future precipitation and temperature scenarios.     

The dependence of respiration on photosynthetic substrate supply and temperature: integrating leaf, soil and ecosystem measurements

Interactions between photosynthetic substrate supply and temperature in determining the rate of three respiration components (leaf, belowground and ecosystem respiration) were investigated within three environmentally controlled, Populus deltoides forest bays at Biosphere 2, Arizona. Over 2 months, the atmospheric CO₂ concentration and air temperature were manipulated to test the following hypotheses: (1) the responses of the three respiration components to changes in the rate of photosynthesis would differ both in speed and magnitude; (2) the temperature sensitivity of leaf and belowground respiration would increase in response to a rise in substrate availability; and, (3) at the ecosystem level, the ratio of respiration to photosynthesis would be conserved despite week‐to‐week changes in temperature. All three respiration rates responded to the CO₂ concentration‐induced changes in photosynthesis. However, the proportional change in the rate of leaf respiration was more than twice that of belowground respiration and, when photosynthesis was reduced, was also more rapid. The results suggest that aboveground respiration plays a key role in the overall response of ecosystem respiration to short‐term changes in canopy photosynthesis. The short‐term temperature sensitivity of leaf respiration, measured within a single night, was found to be affected more by developmental conditions than photosynthetic substrate availability, as the Q₁₀ was lower in leaves that developed at high CO₂, irrespective of substrate availability. However, the temperature sensitivity of belowground respiration, calculated between periods of differing air temperature, appeared to be positively correlated with photosynthetic substrate availability. At the ecosystem level, respiration and photosynthesis were positively correlated but the relationship was affected by temperature; for a given rate of daytime photosynthesis, the rate of respiration the following night was greater at 25 than 20°C. This result suggests that net ecosystem exchange did not acclimate to temperature changes lasting up to 3 weeks. Overall, the results of this study demonstrate that the three respiration terms differ in their dependence on photosynthesis and that, short‐ and medium‐term changes in temperature may affect net carbon storage in terrestrial ecosystems.     

Predominant role of water in regulating soil and microbial respiration and their responses to climate change in a semiarid grassland

Climate change can profoundly impact carbon (C) cycling of terrestrial ecosystems. A field experiment was conducted to examine responses of total soil and microbial respiration, and microbial biomass to experimental warming and increased precipitation in a semiarid temperate steppe in northern China since April 2005. We measured soil respiration twice a month over the growing seasons, soil microbial biomass C (MBC) and N (MBN), microbial respiration (MR) once a year in the middle growing season from 2005 to 2007. The results showed that interannual variations in soil respiration, MR, and microbial biomass were positively related to interannual fluctuations in precipitation. Laboratory incubation with a soil moisture gradient revealed a constraint of the temperature responses of MR by low soil moisture contents. Across the 3 years, experimental warming decreased soil moisture, and consequently caused significant reductions in total and microbial respiration, and microbial biomass, suggesting stronger negatively indirect effects through warming‐induced water stress than the positively direct effects of elevated temperature. Increased evapotranspiration under experimental warming could have reduced soil water availability below a stress threshold, thus leading to suppression of plant growth, root and microbial activities. Increased precipitation significantly stimulated total soil and microbial respiration and all other microbial parameters and the positive precipitation effects increased over time. Our results suggest that soil water availability is more important than temperature in regulating soil and microbial respiratory processes, microbial biomass and their responses to climate change in the semiarid temperate steppe. Experimental warming caused greater reductions in soil respiration than in gross ecosystem productivity (GEP). In contrast, increased precipitation stimulated GEP more than soil respiration. Our observations suggest that climate warming may cause net C losses, whereas increased precipitation may lead to net C gains in the semiarid temperate steppe. Our findings highlight that unless there is concurrent increase in precipitation, the temperate steppe in the arid and semiarid regions of northern China may act as a net C source under climate warming.     

Shifts of growing‐season precipitation peaks decrease soil respiration in a semiarid grassland

Changing precipitation regimes could have profound influences on carbon (C) cycle in the biosphere. However, how soil C release from terrestrial ecosystems responds to changing seasonal distribution of precipitation remains unclear. A field experiment was conducted for 4 years (2013–2016) to examine the effects of altered precipitation distributions in the growing season on soil respiration in a temperate steppe in the Mongolian Plateau. Over the 4 years, both advanced and delayed precipitation peaks suppressed soil respiration, and the reductions mainly occurred in August. The decreased soil respiration could be primarily attributable to water stress and subsequently limited plant growth (community cover and belowground net primary productivity) and soil microbial activities in the middle growing season, suggesting that precipitation amount in the middle growing season is more important than that in the early, late, or whole growing seasons in regulating soil C release in grasslands. The observations of the additive effects of advanced and delayed precipitation peaks indicate semiarid grasslands will release less C through soil respiratory processes under the projected seasonal redistribution of precipitation in the future. Our findings highlight the potential role of intra‐annual redistribution of precipitation in regulating ecosystem C cycling in arid and semiarid regions.     

短期增温和增加降水对内蒙古荒漠草原土壤呼吸的影响

利用红外辐射增温装置模拟短期持续增温和降水增加交互作用对内蒙古荒漠草原土壤呼吸作用的影响, 结果表明: 土壤含水量对月土壤呼吸的影响显著大于土壤温度增加的影响, 生长旺季的月土壤呼吸显著大于生长末季; 土壤温度和水分增加都显著影响日土壤呼吸, 但二者的交互作用对土壤呼吸无显著影响。荒漠草原7‒8月平均土壤呼吸速率为1.35 μmol CO₂·m ^-2·s ^-1, 7月份为2.08 μmol CO₂·m ^-2·s ^-1, 8月份为0.63 μmol CO₂·m ^-2·s ^-1。土壤呼吸与地下各层根系生物量呈幂函数关系, 0‒10 cm土层的根系生物量对土壤呼吸的解释率(79.2%)明显高于10‒20 cm土层的解释率(31.6%)。0-10 cm土层的根系生物量是根系生物量的主体, 根系生物量对土壤呼吸的影响具有层次性。在未来全球变暖和降水格局变化的情景下, 荒漠草原土壤水分含量是影响生物量的主导环境因子, 而根系生物量的差异是造成土壤呼吸异质性的主要生物因素, 土壤含水量可通过影响根系生物量控制土壤呼吸的异质性。     

氮和水分添加对内蒙古荒漠草原放牧生态系统土壤呼吸的影响

土壤呼吸是生态系统碳循环的重要组成部分, 同时也是评价生态系统健康状况的重要指标, 对于评估退化草地恢复过程中生态系统功能具有重要意义。该研究在内蒙古四子王旗短花针茅(Stipa breviflora)荒漠草原长期放牧实验平台上进行, 该平台设置对照(CK)、轻度(LG)、中度(MG)和重度(HG) 4个放牧强度。通过在4个放牧处理区设置氮、水添加实验处理, 探讨不同放牧强度背景下, 氮、水补充对荒漠草原土壤呼吸过程的影响。结果表明: (1)历史放牧强度除2015年对土壤呼吸无显著影响, 2016和2017年都有显著影响, 放牧区3年平均土壤呼吸速率基本都高于对照区。此外, 氮和水分添加显著增加了MG区土壤呼吸速率, HG区氮、水同时添加对土壤呼吸速率有显著增加作用; (2)无论是历史放牧强度, 还是氮、水添加处理, 都没有改变荒漠草原生长季土壤呼吸速率的季节动态变化趋势, 土壤呼吸速率基本表现为单峰曲线模式, 峰值出现在水热同期的7月份; (3)不同年份生长季土壤呼吸速率对氮、水处理的响应并不相同, 氮添加至第3年产生显著影响。水分添加在平水年份(2015和2017年)对土壤呼吸产生显著影响, 但在丰水年份(2016年)无显著影响。氮、水共同添加分别在CK、LG和HG区3年平均土壤呼吸速率显著高于单独加水处理, 说明氮添加的有效性依赖于水分条件, 两者表现为协同作用; (4)不同处理下荒漠草原土壤呼吸的温度敏感性(Q₁₀)值介于1.13-2.41之间, 平均值为1.71。在无氮、水添加时, 放牧区的Q₁₀值都小于CK区, 总体表现为CK 大于 MG 大于 LG 大于 HG; 加水和氮水共同添加处理后, Q₁₀值都有明显增加, 其中NW处理下Q₁₀值都增加到2.0以上。上述结果说明在过去受不同放牧强度影响的荒漠草原在停止放牧后的恢复过程中, 土壤水分仍是影响土壤呼吸的主导环境因子, 外源氮添加只有在满足一定水分供给的基础上才起作用, 尤其是过去的重度放牧区土壤呼吸速率对氮、水补充的响应最为强烈。该研究结果可以为评估荒漠草原恢复过程中土壤呼吸速率受养分和水分影响提供基础资料和依据。     

Contrasting soil respiration in young and old-growth ponderosa pine forests

Three years of fully automated and manual measurements of soil CO₂ efflux, soil moisture and temperature were used to explore the diel, seasonal and inter‐annual patterns of soil efflux in an old‐growth (250‐year‐old, O site) and recently regenerating (14‐year‐old, Y site) ponderosa pine forest in central Oregon. The data were used in conjunction with empirical models to determine which variables could be used to predict soil efflux in forests of contrasting ages and disturbance histories. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. Soil CO₂ efflux at both sites showed large inter‐annual variability that could be attributed to soil moisture availability in the deeper soil horizons (O site) and the quantity of summer rainfall (Y site). Seasonal patterns of soil CO₂ efflux at the O site showed a strong positive correlation between diel mean soil CO₂ efflux and soil temperature at 64 cm depth whereas diel mean soil efflux at the Y site declined before maximum soil temperature occurred during summer drought. The use of diel mean soil temperature and soil water potential inferred from predawn foliage water potential measurements could account for 80% of the variance of diel mean soil efflux across 3 years at both sites, however, the functional shape of the soil water potential constraint was site‐specific. Based on the similarity of the decomposition rates of litter and fine roots between sites, but greater productivity and amount of fine litter detritus available for decomposition at the O site, we would expect higher rates of soil CO₂ efflux at the O site. However, annual rates were only higher at the O site in one of the 3 years (597 ± 45 vs. 427 ± 80 g C m^?2). Seasonal patterns of soil efflux at both sites showed influences of soil water limitations that were also reflected in patterns of canopy stomatal conductance, suggesting strong linkages between above and below ground processes.     

Tree photosynthesis modulates soil respiration on a diurnal time scale

To estimate how tree photosynthesis modulates soil respiration, we simultaneously and continuously measured soil respiration and canopy photosynthesis over an oak‐grass savanna during the summer, when the annual grass between trees was dead. Soil respiration measured under a tree crown reflected the sum of rhizosphere respiration and heterotrophic respiration; soil respiration measured in an open area represented heterotrophic respiration. Soil respiration was measured using solid‐state CO₂ sensors buried in soils and the flux‐gradient method. Canopy photosynthesis was obtained from overstory and understory flux measurements using the eddy covariance method. We found that the diurnal pattern of soil respiration in the open was driven by soil temperature, while soil respiration under the tree was decoupled with soil temperature. Although soil moisture controlled the seasonal pattern of soil respiration, it did not influence the diurnal pattern of soil respiration. Soil respiration under the tree controlled by the root component was strongly correlated with tree photosynthesis, but with a time lag of 7–12 h. These results indicate that photosynthesis drives soil respiration in addition to soil temperature and moisture.     

Differential responses of auto‐ and heterotrophic soil respiration to water and nitrogen addition in a semiarid temperate steppe

Evaluating how autotrophic (SR_A), heterotrophic (SR_H) and total soil respiration (SR_TOT) respond differently to changes of environmental factors is critical to get an understanding of ecosystem carbon (C) cycling and its feedback processes to climate change. A field experiment was conducted to examine the responses of SR_A and SR_H to water and nitrogen (N) addition in a temperate steppe in northern China during two hydrologically contrasting growing seasons. Water addition stimulated SR_A and SR_H in both years, and their increases were significantly greater in a dry year (2007) than in a wet year (2006). N addition increased SR_A in 2006 but not in 2007, while it decreased SR_H in both years, leading to a positive response of SR_TOT in 2006 but a negative one in 2007. The different responses of SR_A and SR_H indicate that it will be uncertain to predict soil C storage if SR_TOT is used instead of SR_H to estimate variations in soil C storage. Overall, N addition is likely to enhance soil C storage, while the impacts of water addition are determined by its relative effects on carbon input (plant growth) and SR_H. Antecedent water conditions played an important role in controlling responses of SR_A, SR_H and the consequent SR_TOT to water and N addition. Our findings highlight the predominance of hydrological conditions in regulating the responses of C cycling to global change in the semiarid temperate steppe of northern China.     

Plant carbon substrate supply regulated soil nitrogen dynamics in a tallgrass prairie in the Great Plains,USA: results of a clipping and shading experiment

Aims Land use management affects plant carbon (C) supply and soil environments and hence alters soil nitrogen (N) dynamics, with consequent feedbacks to terrestrial ecosystem productivity. The objective of this study was to better identify mechanisms by which land-use management (clipping and shading) regulates soil N in a tallgrass prairie, OK, USA.Methods We conducted 1-year clipping and shading experiment to investigate the effects of changes in land-use management (soil microclimates, plant C substrate supply and microbial activity) on soil inorganic N (NH 4 + ? N and NO 3 ? ? N), net N mineralization and nitrification in a tallgrass prairie.Important findings Land-use management through clipping and/or shading significantly increased annual mean inorganic N, possibly due to lowered plant N uptake and decreased microbial N immobilization into biomass growth. Shading significantly increased annual mean mineralization rates (P < 0.05). Clipping slightly decreased annual mean N nitrification rates whereas shading significantly increased annual mean N nitrification rates. Soil microclimate significantly explained 36% of the variation in NO 3 ? ? N concentrations (P = 0.004). However, soil respiration, a predictor of plant C substrate supply and microbial activity, was negatively correlated with NH 4 + ? N concentrations (P = 0.0009), net N mineralization (P = 0.0037) and nitrification rates (P = 0.0028) across treatments. Our results suggest that change in C substrate supply and microbial activity under clipping and/or shading is a critical control on NH 4 + ? N, net N mineralization and nitrification rates, whereas clipping and shading-induced soil microclimate change can be important for NO 3 ? ? N variation in the tallgrass prairie.     

不同植被被覆下温性草原土壤养分分异特征

以青海湖区4种不同植被被覆下温性草原为对象,研究在自然及放牧因素影响下土壤异质性分布格局.结果表明:速效养分(速效氮、速效磷、速效钾)具有明显的分层特征,表层土壤含量最高,随土层加深含量逐渐降低.紫花针茅退化样地各层土壤速效养分含量普遍低于其他3个样地,恢复时间较长的样地(早熟禾样地)和有外来物质输入的样地(赖草样地)含量较高.全量养分表现不同,全氮含量表现出分层现象,退化和恢复时间短样地(紫花针茅退化样地、垂穗披碱草样地)表层(0～10 cm)和第二层(10～20 cm)全氮含量高,下层含量迅速降低;早熟禾样地和赖草样地各层全氮含量都较高;全磷含量随土层降低没有出现显著差异(P＞0.05),紫花针茅退化样地0～40 cm土层全磷含量都显著低于其他样地(P＜0.05),其全钾和有机质含量也普遍低于其他样地;有机质与全量养分、速效养分均呈现极显著相关(P＜0.01).随土层加深土壤容重增高,退化使土壤pH值升高.退化温性草原在恢复6a后土壤基本得到恢复,人类扰动和自然因素都影响到土壤养分状况.     

Rhizospheric and heterotrophic components of soil respiration in six Chinese temperate forests

Partitioning soil respiration (R_S) into heterotrophic (R_H) and rhizospheric (R_R) components is an important step for understanding and modeling carbon cycling in forest ecosystems, but few studies on R_R and R_H exist in Chinese temperate forests. In this study, we used a trenching plot approach to partition R_S in six temperate forests in northeastern China. Our specific objectives were to (1) examine seasonal patterns of soil surface CO₂ fluxes from trenched (R_T) and untrenched plots (R_UT) of these forests; (2) quantify annual fluxes of R_S components and their relative contributions in the forest ecosystems; and (3) examine effects of plot trenching on measurements of R_S and related environmental factors. The R_T maximized in early growing season, but the difference between R_UT and R_T peaked in later summer. The annual fluxes of R_H and R_R varied with forest types. The estimated values of R_H for the Korean pine (Pinus koraiensis Sieb. et Zucc.), Dahurian larch (Larix gmelinii Rupr.), aspen‐birch (Populous davidiana Dode and Betula platyphylla Suk.), hardwood (Fraxinus mandshurica Rupr., Juglans mandshurica Maxim. and Phellodendron amurense Rupr.), Mongolian oak (Quercus mongolica Fisch.) and mixed deciduous (no dominant tree species) forests averaged 89, 196, 187, 245, 261 and 301 g C m⁻² yr⁻¹, respectively; those of R_R averaged 424, 209, 628, 538, 524 and 483 g C m⁻² yr⁻¹, correspondingly; calculated contribution of R_R to R_S (RC) varied from 52% in the larch forest to 83% in the pine forest. The annual flux of R_R was strongly correlated to biomass of roots <0.5 cm in diameter, while that of R_H was weakly correlated to soil organic carbon concentration at A horizon. We concluded that vegetation type and associated carbon metabolisms of temperate forests should be considered in assessing and modeling R_S components. The significant impacts of changed soil physical environments and substrate availability by plot trenching should be appropriately tackled in analyzing and interpreting measurements of R_S components.     

降水时间对内蒙古温带草原地上净初级生产力的影响

全球气候变化下降水时间的改变将深刻影响草原生态系统地上净初级生产力(ANPP),而草原生态系统ANPP是区域碳循环的重要过程.利用1998-2007年的SPOT-VEG NDVI数据并结合111个样点的ANPP地面样方调查数据,获得了内蒙古温带草原1998-2007年的ANPP区域数据,依此分析了中国内蒙古温带草原以及区域内的3种植被类型(荒漠草原、典型草原、草甸草原)降水时间对ANPP的影响.研究结果表明,对于整个内蒙古温带草原来说,一个水分年内(从上一年9月份到当年地上生物量达最大值时的8月份)影响ANPP较为重要的降水月份为2-7月份,其中,5-7月份降水尤为重要.具体到每个月降水的影响,研究发现,7月份降水最重要,而仍处于生长季的8月份降水相对于其他生长季降水作用最小;影响不同草地类型最重要的降水时期存在一定差异,对荒漠草原和典型草原地区来说,ANPP达最大值前3个月(5-7月份)的生长季降水最重要,而8月份降水影响较小,而草甸草原地区8月份和非生长季的3、4月份降水最重要,但各个降水时期降水对ANPP的影响都较荒漠草原和典型草原小,大部分地区降水对ANPP的影响不显著.     

控制降水梯度对荒漠草原优势植物叶功能性状及土壤养分的影响

探究植物叶功能性状随降水梯度的变化规律,对揭示干旱区优势植物对环境变化的响应和适应策略至关重要。以盐池荒漠草原为研究对象,采用遮雨棚和喷灌系统控制降水梯度,分析了优势植物蒙古冰草（Agropyron mongolicum）、短花针茅（Stipa breviflora）及达乌里胡枝子（Lespedeza davurica）叶功能性状变异,以及土壤水分、养分和微生物特性与响应性状间的相关关系。结果表明：HW（增水50%）处理下3个植物LA（叶面积）、LDMC（干物质含量）显著增大,LW处理（减水50%）下短花针茅和达乌里胡枝子LA、LDMC显著减小,降水处理对短花针茅和达乌里胡枝子SLA（比叶面积）影响不显著,LW处理显著提高了蒙古冰草SLA;LW处理显著提高了蒙古冰草和短花针茅LNC（叶氮含量）和LPC（叶磷含量）;HW显著降低了土壤C、N含量,LW和HW均显著减少了真菌数量,而放线菌数量、微生物生物量C、N显著增加;3种优势植物LA均与土壤水分显著正相关,蒙古冰草和短花针茅通过提高SLA、LNC及LNP来适应干旱生境,蒙古冰草和短花针茅LNC及LNP是表征土壤P、微生物生物量有效性的关键指标,达乌里胡枝子通过自我调节养分利用策略来适应C、N、P含量和微生物活性较低的生境,从而决定其在群落中的优势地位。     

Differential controls by climate and substrate over the heterotrophic and rhizospheric components of soil respiration

The partitioning of soil respiration rates into the component processes of rhizospheric respiration (because of live roots and those microorganisms that subsist on root exudations) and heterotrophic respiration (because of decomposer microorganisms that subsist on the oxidation of soil organic matter) is difficult to accomplish through experimental observation. In order to minimize disturbance to the soil and maximize preservation of the natural relationships among roots, rhizospheric microorganisms, and decomposers, we conducted a girdling experiment in a subalpine forest dominated by lodgepole pine trees. In two separate years, we girdled trees in small forest plots (5–7 m in diameter) and trenched around the plots to sever invading roots in order to experimentally stop the transport of photosynthate from needles to roots, and eliminate rhizospheric respiration. Soil respiration rates in plots with trees girdled over 1 year prior to measurement were higher than those in plots with trees girdled 2–3 months prior to measurement. These results suggest that any stimulation of respiration because of the experimental artifact of fine root death and addition of labile carbon to the pool of decomposer substrates is slow, and occurs beyond the first growing season after girdling. Compared with control plots with nongirdled trees, soil respiration rates in plots with girdled trees were reduced by 31–44% at the mid‐summer respiratory maximum. An extreme drought during one of the 2 years used for observations caused greater reductions in the heterotrophic component of soil respiration compared with the rhizospheric component. In control plots, we observed a pulse in K₂SO₄‐extractable carbon during the spring snowmelt period, which was absent in plots with girdled trees. In control plots, soil microbial biomass increased from spring to summer, coincident with a seasonal increase in the rhizospheric component of soil respiration. In plots with girdled trees, the seasonal increase in microbial biomass was lower than in control plots. These results suggest that the observed seasonal increase in rhizospheric respiration rate in control plots was because of an increase in rhizospheric microbial biomass following ‘soil priming’ by a spring‐time pulse in dissolved organic carbon. Winter‐time, beneath‐snow microbial biomass was relatively high in control plots. Soil sucrose concentrations were approximately eight times higher during winter than during spring or summer, possibly being derived from the mechanical damage of shallow roots that use sucrose as protection against low‐temperature extremes. The winter‐time sucrose pulse was not observed in plots with girdled trees. The results of this study demonstrate that (1) the rhizospheric component of soil respiration rate at this site is significant in magnitude, (2) the heterotrophic component of soil respiration rate is more susceptible to seasonal drought than the rhizospheric component, and (3) the trees in this ecosystem exert a major control over soil carbon dynamics by ‘priming’ the soil with sugar exudates during the late‐spring snowmelt period and releasing high concentrations of sucrose to the soil during winter.     

Effects of seasonal precipitation change on soil respiration processes in a seasonally dry tropical forest

Precipitation is projected to change intensity and seasonal regime under current global projections. However, little is known about how seasonal precipitation changes will affect soil respiration, especially in seasonally dry tropical forests. In a seasonally dry tropical forest in South China, we conducted a precipitation manipulation experiment to simulate a delayed wet season (DW) and a wetter wet season (WW) over a three‐year period. In DW, we reduced 60% throughfall in April and May to delay the onset of the wet season and irrigated the same amount water into the plots in October and November to extend the end of the wet season. In WW, we irrigated 25% annual precipitation into plots in July and August. A control treatment (CT) receiving ambient precipitation was also established. Compared with CT, DW significantly increased soil moisture by 54% during October to November, and by 30% during December to April. The treatment of WW did not significantly affect monthly measured soil moisture. In 2015, DW significantly increased leaf area index and soil microbial biomass but decreased fine root biomass. In contrast, WW significantly decreased fine root biomass and forest floor litter stocks. Soil respiration was not affected by DW, which could be attributed to the increased microbial biomass offsetting the decrease in fine root biomass. In contrast, WW significantly increased soil respiration from 3.40 to 3.90 μmol m^?2 s^?1 in the third year, mainly due to the increased litter decomposition and soil pH (from 4.48 to 4.68). The present study suggests that both a delayed wet season and a wetter wet season will have significant impacts on soil respiration‐associated ecosystem components. However, the ecosystem components can respond in different directions to the same change in precipitation, which ultimately affected soil respiration.     

Substrate availability regulates the suppressive effects of Canada goldenrod invasion on soil respiration

基质有效性调节加拿大一枝黄花入侵对土壤呼吸的抑制作用 外来植物入侵不仅会降低河边近岸湿地生态系统植被多样性,而且会改变湿地生态系统的地下碳过程。外来入侵植物加拿大一枝黄花(Solidago canadensis L.)已广泛入侵我国东南部地区,但加拿大一枝黄花入侵对入侵地生态系统地下土壤碳循环过程的影响却知之甚少。本研究通过野外原位观测实验和温室模拟入侵实验,探究外来植物加拿大一枝黄花入侵对入侵地土壤呼吸的影响规律及其驱动因素。野 外原位观测实验开展于2018年7月21日至12月15日,期间每周测定样地土壤呼吸。温室模拟入侵实验开展于2019年7月15日至12月15日,期间每月1日与15日上午测定土壤呼吸、自养呼吸和异养呼吸。土壤呼吸、自养呼吸和异养呼吸通过静态箱结合深埋根系隔离法测定。野外原位观测实验和温室模拟入侵实验结果均显示,加拿大一枝黄花的入侵降低了土壤二氧化碳的排放通量。加拿大一枝黄花入侵对土壤呼吸的抑制作用可能归因于其入侵引起的土壤可利用底物质量与数量的变化,表明外来入侵植物加拿大一枝黄花可通过改变植物释放基质以及与本地植物和/或土壤微生物争夺土壤有效基质而影响土壤碳循环。这些研究结果对于评估外来入侵植物对入侵地地下碳动态的影响以及对全球变暖的贡献具有重要意义。     

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.     

典型温带草原群落土壤呼吸温度敏感性与土壤水分的关系

工业革命以来,人类活动所导致的CO2等温室气体的浓度在大气中持续上升,全球表面温度因此不断升高。在全球温暖化的背景下,土壤呼吸与温室效应之间正反馈关系势必影响到未来陆地生态系统功能与全球变化的趋势,所以,关于土壤呼吸对温度变化响应的研究备受瞩目。土壤呼吸对温度依赖性的研究已经有许多报道,其关系可以用简单的指数方程表示。但是,土壤水分条件对于土壤呼吸温度敏感性(用Q10表示)的影响却研究得较少。采用碱液吸收法对内蒙古典型温带草原11个不同水分状况群落的土壤呼吸进行了测定,并分析了土壤呼吸的温度敏感性。结果显示土壤呼吸的温度敏感性存在一定程度的空间变异,各群落Q10值平均为1．65,变异系数为6．94％。其中,春小麦群落的Q10值最高(1．84),其次是湿生杂类草群落(Q10=1．78),而Q10最低的是冷蒿(aRMESIA FRIGIDA)-星毛萎陵菜(pOTENTILLA ACAULIS)群落(1．47)。用Spearman秩相关分析法对表层土壤(O～20cm)水分与Q10值之间的关系进行了分析,结果表明各群落Q10值与生长季土壤平均水分含量呈显著的正相关关系(R=0．64545,p=0．032),说明水分状况对土壤呼吸的温度敏感性有一定程度的影响。由此推断,在中国温带草原地区,温度升高对较湿润区域土壤呼吸的影响大于较干旱区域。全球变化导致的水分时空格局的变化可能对温带草原土壤呼吸有较大的影响。所以,模拟大尺度土壤CO2排放量时,水分因素必须作为一个重要的变量加以考虑。     

底座入土深度和面积对典型草原土壤呼吸测定结果的影响

生态学家对土壤呼吸开展了大量研究, 但很少评估“底座”对土壤呼吸测量结果的影响, 特别是底座入土深度和面积对土壤呼吸测定结果的影响。为此, 该研究在内蒙古典型草原设置了2个底座面积(15 cm × 15 cm和30 cm × 30 cm)和2个底座入土深度(2 cm和5 cm)处理, 采用气室法在植物生长季对土壤呼吸进行了测定, 分析评估了底座面积和入土深度对土壤呼吸测定结果的影响。结果显示: 与底座入土较浅和面积较小的处理相比, 底座入土较深和面积较大的处理, 土壤呼吸测定值分别降低了8.0%-9.7%和9.1%-10.8%; 这两个处理的底座内土壤温度显著升高, 土壤含水量显著下降, 地上净初级生产力显著降低。结构方程模型分析表明, 底座入土较深、面积较大的处理, 主要通过降低地上净初级生产力和土壤含水量, 增加土壤温度, 使土壤呼吸下降, 各因子共同解释了土壤呼吸变异的89%。研究发现, 在使用气室法测定土壤呼吸时, 底座入土深度和面积对土壤呼吸测定结果具有显著影响, 评估底座处理效应对准确测定土壤呼吸强度具有重要的意义。理论上, 适当降低底座入土深度和底座面积大小, 将有助于准确测定土壤呼吸。但在实践中, 由于土壤异质性的影响, 减少底座面积可能会增加新的测量误差, 只能考虑适当降低底座入土深度。