Short‐term soil carbon sink potential of oil palm plantations

Oil palm plantations cover ≈14.6 million ha worldwide and the total area under cultivation is expected to increase during the 21st century . Indonesia and Malaysia together account for 87% of global palm oil production and the combined harvested area in these countries has expanded by 6.5 million ha since 1990. Despite this, soil C cycling in oil palm systems is not well quantified but such information is needed for C budget inventories. We quantified soil C storage (root biomass, soil organic matter (SOM) and microbial biomass) and losses [potential soil respiration (R_s) and soil surface CO₂ flux (F_s)] in mineral soils from an oil palm plantation chronosequence (11–34 years since planting) in Selangor, Malaysia. There were no significant effects of plantation age on SOM, microbial biomass, R_s or F_s, implying soil C was in dynamic equilibrium over the chronosequence. However, there was a significant increase in root biomass with plantation age, indicating a short‐term C sink. Across the chronosequence, R_s was driven by soil moisture, soil particle size, root biomass and soil microbial biomass N but not microbial biomass C. This suggests that the nutrient status of the microbial community may be of equal or greater importance for soil CO₂ losses than substrate availability and also raises particular concerns regarding the addition of nitrogenous fertilizer, i.e. increased yields will be associated with increased soil CO₂ emissions. To fully assess the impact of oil palm plantations on soil C storage, initial soil C losses following land conversion (e.g. from native forest or other previous plantations) must be accounted for. If initial soil C losses are large, our data show that there is no accumulation of stable C in the soil as the plantation matures and hence the conversion to oil palm would probably represent a net loss of soil C.     

Temperature Response of Soil Respiration in a Chinese Pine Plantation: Hysteresis and Seasonal vs. Diel Q 10

Although the temperature response of soil respiration (R_s) has been studied extensively, several issues remain unresolved, including hysteresis in the R_s–temperature relationship and differences in the long- vs. short-term R_s sensitivity to temperature. Progress on these issues will contribute to reduced uncertainties in carbon cycle modeling. We monitored soil CO₂ efflux with an automated chamber system in a Pinus tabulaeformis plantation near Beijing throughout 2011. Soil temperature at 10-cm depth (T_s) exerted a strong control over R_s, with the annual temperature sensitivity (Q ₁₀) and basal rate at 10°C (R_{s 10}) being 2.76 and 1.40 µmol m⁻² s⁻¹, respectively. Both R_s and short-term (i.e., daily) estimates of R_{s 10} showed pronounced seasonal hysteresis with respect to T_s, with the efflux in the second half of the year being larger than that early in the season for a given temperature. The hysteresis may be associated with the confounding effects of microbial population dynamics and/or litter input. As a result, all of the applied regression models failed to yield unbiased estimates of R_s over the entire annual cycle. Lags between R_s and T_s were observed at the diel scale in the early and late growing season, but not in summer. The seasonality in these lags may be due to the use of a single T_s measurement depth, which failed to represent seasonal changes in the depth of CO₂ production. Daily estimates of Q ₁₀ averaged 2.04, smaller than the value obtained from the seasonal relationship. In addition, daily Q ₁₀ decreased with increasing T_s, which may contribute feedback to the climate system under global warming scenarios. The use of a fixed, universal Q ₁₀ is considered adequate when modeling annual carbon budgets across large spatial extents. In contrast, a seasonally-varying, environmentally-controlled Q ₁₀ should be used when short-term accuracy is required.     

No overall stimulation of soil respiration under mature deciduous forest trees after 7 years of CO2 enrichment

The anthropogenic rise in atmospheric CO₂ is expected to impact carbon (C) fluxes not only at ecosystem level but also at the global scale by altering C cycle processes in soils. At the Swiss Canopy Crane (SCC), we examined how 7 years of free air CO₂ enrichment (FACE) affected soil CO₂ dynamics in a ca. 100‐year‐old mixed deciduous forest. The use of ¹³C‐depleted CO₂ for canopy enrichment allowed us to trace the flow of recently fixed C. In the 7th year of growth at ～550 ppm CO₂, soil respiratory CO₂ consisted of 39% labelled C. During the growing season, soil air CO₂ concentration was significantly enhanced under CO₂‐exposed trees. However, elevated CO₂ failed to stimulate cumulative soil respiration (R_s) over the growing season. We found periodic reductions as well as increases in instantaneous rates of R_s in response to elevated CO₂, depending on soil temperature and soil volumetric water content (VWC; significant three‐way interaction). During wet periods, soil water savings under CO₂‐enriched trees led to excessive VWC (>45%) that suppressed R_s. Elevated CO₂ stimulated R_s only when VWC was ≤40% and concurrent soil temperature was high (>15 °C). Seasonal Q₁₀ estimates of R_s were significantly lower under elevated (Q₁₀=3.30) compared with ambient CO₂ (Q₁₀=3.97). However, this effect disappeared when three consecutive sampling dates of extremely high VWC were disregarded. This suggests that elevated CO₂ affected Q₁₀ mainly indirectly through changes in VWC. Fine root respiration did not differ significantly between treatments but soil microbial biomass (C_mic) increased by 14% under elevated CO₂ (marginally significant). Our findings do not indicate enhanced soil C emissions in such stands under future atmospheric CO₂. It remains to be shown whether C losses via leaching of dissolved organic or inorganic C (DOC, DIC) help to balance the C budget in this forest.     

Source components and interannual variability of soil CO2 efflux under experimental warming and clipping in a grassland ecosystem

Partitioning soil CO₂ efflux into autotrophic (R_A) and heterotrophic (R_H) components is crucial for understanding their differential responses to climate change. We conducted a long‐term experiment (2000–2005) to investigate effects of warming 2°C and yearly clipping on soil CO₂ efflux and its components (i.e. R_A and R_H) in a tallgrass prairie ecosystem. Interannual variability of these fluxes was also examined. Deep collars (70 cm) were inserted into soil to measure R_H. R_A was quantified as the difference between soil CO₂ efflux and R_H. Warming treatment significantly stimulated soil CO₂ efflux and its components (i.e. R_A and R_H) in most years. In contrast, yearly clipping significantly reduced soil CO₂ efflux only in the last 2 years, although it decreased R_H in every year of the study. Temperature sensitivity (i.e. apparent Q₁₀ values) of soil CO₂ efflux was slightly lower under warming (P>0.05) and reduced considerably by clipping (P<0.05) compared with that in the control. On average over the 4 years, R_H accounted for approximately 65% of soil CO₂ efflux with a range from 58% to 73% in the four treatments. Over seasons, the contribution of R_H to soil CO₂ efflux reached a maximum in winter (∼90%) and a minimum in summer (∼35%). Annual soil CO₂ efflux did not vary substantially among years as precipitation did. The interannual variability of soil CO₂ efflux may be mainly caused by precipitation distribution and summer severe drought. Our results suggest that the effects of warming and yearly clipping on soil CO₂ efflux and its components did not result in significant changes in R_H or R_A contribution, and rainfall timing may be more important in determining interannual variability of soil CO₂ efflux than the amount of annual precipitation.     

Accelerated soil CO<Subscript>2</Subscript> efflux after conversion from secondary oak forest to pine plantation in southeastern China

Soil respiration (R _s) is an important component of the carbon cycle in terrestrial ecosystems, and changes in soil respiration with land cover alteration can have important implications for regional carbon balances. In southeastern China (Xiashu Experimental Forest, Jiangsu Province), we used an automated LI-8100 soil CO₂ flux system to quantify diurnal variation of soil respiration in a secondary oak forest and a pine plantation. We found that soil respiration in the pine plantation was significantly higher than that in the secondary oak forest. There were similar patterns of soil respiration throughout the day in both the secondary oak forest and the pine plantation during our 7-month study (March–September 2005). The maximum of R _s occurred between 4:00 pm and 7:00 pm. The diurnal variations of R _s were usually out of phase with soil surface (0.5 cm) temperature (T _g). However, annual variation in R _s correlated with surface soil temperature. Soil respiration reached to a maximum in June, and decreased thereafter. The Q₁₀ of R _s in the secondary oak forest was significantly higher than that in the pine plantation. The higher Q₁₀ value in the secondary oak forest implied that it might release more CO₂ than the pine plantation under a global-warming scenario. Our results indicated that land-use change from secondary forest to plantation may cause a significant increase in CO₂ emission, and reduce the temperature sensitivity of soil respiration in southeastern China.     

Responses of soil respiration to soil management changes in an agropastoral ecotone in Inner Mongolia,China

Studying the responses of soil respiration (R_s) to soil management changes is critical for enhancing our understanding of the global carbon cycle and has practical implications for grassland management. Therefore, the objectives of this study were (1) quantify daily and seasonal patterns of R_s, (2) evaluate the influence of abiotic factors on R_s, and (3) detect the effects of soil management changes on R_s. We hypothesized that (1) most of daily and seasonal variation in R_s could be explained by soil temperature (T_s) and soil water content (S_w), (2) soil management changes could significantly affect R_s, and (3) soil management changes affected R_s via the significant change in abiotic and biotic factors. In situ R_s values were monitored in an agropastoral ecotone in Inner Mongolia, China, during the growing seasons in 2009 (August to October) and 2010 (May to October). The soil management changes sequences included free grazing grassland (FG), cropland (CL), grazing enclosure grassland (GE), and abandoned cultivated grassland (AC). During the growing season in 2010, cumulative R_s for FG, CL, GE, and AC averaged 265.97, 344.74, 236.70, and 226.42 gC m^?2 year^?1, respectively. The T_s and S_w significantly influenced R_s and explained 66%–86% of the variability in daily R_s. Monthly mean temperature and precipitation explained 78%–96% of the variability in monthly R_s. The results clearly showed that R_s was increased by 29% with the conversion of FG to CL and decreased by 35% and 11% with the conversion of CL to AC and FG to GE. The factors impacting the change in R_s under different soil management changes sequences varied. Our results confirm the tested hypotheses. The increase in Q₁₀ and litter biomass induced by conversion of FG to GE could lead to increased R_s if the climate warming. We suggest that after proper natural restoration period, grasslands should be utilized properly to decrease R_s.     

Large seasonal variation of soil respiration in a secondary tropical moist forest in Puerto Rico

Tropical forests are the largest contributors to global emissions of carbon dioxide (CO₂) to the atmosphere via soil respiration (R_s). As such, identifying the main controls on R_s in tropical forests is essential for accurately projecting the consequences of ongoing and future global environmental changes to the global C cycle. We measured hourly R_s in a secondary tropical moist forest in Puerto Rico over a 3‐year period to (a) quantify the magnitude of R_s and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of R_s. Across 3 years of measurements, mean R_s was 7.16 ± 0.02 μmol CO₂ m^‐2 s^‐1 (or 2,710 g C m^‐2 year^‐1) and showed significant seasonal variation. Despite small month‐to‐month variation in temperature (~4°C), we found significant positive relationships between daily and monthly R_s with both air and soil temperature, highlighting the importance of temperature as a driver of R_s even in warm ecosystems, such as tropical forests. We also found a significant parabolic relationship between mean daily volumetric soil moisture and mean daily R_s, with an optimal moisture value of 0.34 m³ m^‐3. Given the relatively consistent climate at this site, the large range in mean monthly R_s (~7 μmol CO₂ m^‐2 s^‐1) was surprising and suggests that even small changes in climate can have large implications for ecosystem respiration. The strong positive relationship of R_s with temperature at monthly timescales particularly stands out, as moisture is usually considered a stronger control of R_s in tropical forests that already experience warm temperatures year‐round. Moreover, our results revealed the strong seasonality of R_s in tropical moist forests, which given its high magnitude, can represent a significant contribution to the seasonal patterns of atmospheric (CO₂) globally.     

Differential effects of warming and nitrogen fertilization on soil respiration and microbial dynamics in switchgrass croplands

The mechanistic understanding of warming and nitrogen (N) fertilization, alone or in combination, on microbially mediated decomposition is limited. In this study, soil samples were collected from previously harvested switchgrass (Panicum virgatum L.) plots that had been treated with high N fertilizer (HN: 67 kg N ha^?1) and those that had received no N fertilizer (NN) over a 3‐year period. The samples were incubated for 180 days at 15 °C and 20 °C, during which heterotrophic respiration, δ¹³C of CO₂, microbial biomass (MB), specific soil respiration rate (R_s: respiration per unit of microbial biomass), and exoenzyme activities were quantified at 10 different collections time. Employing switchgrass tissues (referred to as litter) with naturally abundant ¹³C allowed us to partition CO₂ respiration derived from soil and amended litter. Cumulative soil respiration increased significantly by 16.4% and 4.2% under warming and N fertilization, respectively. Respiration derived from soil was elevated significantly with warming, while oxidase, the agent for recalcitrant soil substrate decomposition, was not significantly affected by warming. Warming, however, significantly enhanced MB and R_s indicating a decrease in microbial growth efficiency (MGE). On the contrary, respiration derived from amended litter was elevated with N fertilization, which was consistent with the significantly elevated hydrolase. N fertilization, however, had little effect on MB and R_s, suggesting little change in microbial physiology. Temperature and N fertilization showed minimal interactive effects likely due to little differences in soil N availability between NN and HN samples, which is partly attributable to switchgrass biomass N accumulation (equivalent to ~53% of fertilizer N). Overall, the differential individual effects of warming and N fertilization may be driven by physiological adaptation and stimulated exoenzyme kinetics, respectively. The study shed insights on distinct microbial acquisition of different substrates under global temperature increase and N enrichment.     

内蒙古农牧交错区不同土地利用方式下土壤呼吸速率及其温度敏感性变化

2009年8-10月, 采用动态气室法观测了内蒙古农牧交错区多伦县农田、弃耕和围封3种土地利用方式下, 土壤呼吸速率从6:00到18:00的变化规律, 分析了不同深度的土壤温度与土壤含水量对土壤呼吸速率的控制作用。结果表明, 空间尺度上, 不同土地利用方式的土壤呼吸速率由高到低依次为: 农田>弃耕>围封; 时间尺度上, 土壤呼吸速率在6:00-18:00的变化趋势为单峰曲线, 在12:00-15:00达到峰值, 随后降低, 在18:00基本恢复到6:00左右的呼吸水平, 同时, 土壤呼吸速率在9、10月显著降低。利用Van’t Hoff指数模型研究不同深度土壤温度对土壤呼吸速率的影响发现, 10-15 cm深度的土壤温度对土壤呼吸速率的影响最为显著, 其中, 土壤呼吸温度敏感性由高到低分别为: 农田>围封>弃耕。相反, 由于8-10月土壤含水量变化较小, 故土壤含水量与土壤呼吸速率间的相关性不显著, 土壤含水量不能解释该时段土壤呼吸速率的变化。     

Impact of rainfall manipulations and biotic controls on soil respiration in Mediterranean and desert ecosystems along an aridity gradient

Spatially heterogeneous ecosystems form a majority of land types in the vast drylands of the globe. To evaluate climate‐change effects on CO₂ fluxes in such ecosystems, it is critical to understand the relative responses of each ecosystem component (microsite). We investigated soil respiration (R_s) at four sites along an aridity gradient (90–780 mm mean annual precipitation, MAP) during almost 2 years. In addition, R_s was measured in rainfall manipulations plots at the two central sites where ～30% droughting and ～30% water supplementation treatments were used over 5 years. Annual R_s was higher by 23% under shrub canopies compared with herbaceous gaps between shrubs, but R_s at both microsites responded similarly to rainfall reduction. Decreasing precipitation and soil water content along the aridity gradient and across rainfall manipulations resulted in a progressive decline in R_s at both microsites, i.e. the drier the conditions, the larger was the effect of reduction in water availability on R_s. Annual R_s on the ecosystem scale decreased at a slope of 256/MAP g C m^?2 yr^?1 mm^?1 (r²=0.97). The reduction in R_s amounted to 77% along the aridity gradient and to 16% across rainfall manipulations. Soil organic carbon (SOC) decreased with declining precipitation, and variation in SOC stocks explained 77% of the variation in annual R_s across sites, rainfall manipulations and microsites. This study shows that rainfall manipulations over several years are a useful tool for experimentally predicting climate‐change effects on CO₂ fluxes for time scales (such as approximated by aridity gradients) that are beyond common research periods. Rainfall reduction decreases rates of R_s not only by lowering biological activity, but also by drastically reducing shrub cover. We postulate that future climate change in heterogeneous ecosystems, such as Mediterranean and deserts shrublands will have a major impact on R_s by feedbacks through changes in vegetation structure.     

Response of soil respiration to temperature and soil moisture: Effects of different vegetation types on a small scale in the eastern Loess Plateau of China

Soil respiration is affected by vegetation and environmental conditions. The purpose of this study was to investigate the effect of vegetation type on soil respiration, temperature and water content, and their correlations on a small scale. We measured soil respiration rate (R_s) over a 3-year period at biweekly intervals in three plots in the eastern Loess Plateau of China, with the same soil texture but different vegetation types: pine forest, grassland, and shrub land. Simultaneously, soil temperature (T_s) at 10 cm depth and soil water content (W_s) within 10 cm depth were measured. The seasonal course of R_s and T_s showed a similar temporal variation in the three plots, with higher values in summer and autumn and lower values in winter and spring. No significant differences (P>0.05) were found between plots, except for W_s. The mean cumulative release of CO₂ efflux from March to December was 962.5, 1027.5, and 1166.5 g C m^? 2 a^? 1 for plots 1, 2, and 3, respectively, with no significant difference between plots. The fitted exponential equations of R_s versus T_s from the 3-year data-set were significant (P < 0.05) with an R² of 0.72, 0.64, and 0.72 for plots 1, 2, and 3, respectively. The calculated Q₁₀ from the parameters of the fitted equation was 3.57, 3.52, and 3.61, and the R₁₀ was 2.36, 2.03, and 2.37 μmol CO₂ m^? 2 s^? 1 for plots 1, 2, and 3, respectively. Compared with the T_s, the correlations between R_s and W_s were not significant for the three plots. However, if the T_s was above 10°C, then their correlation was significant, and W_s had an impact on R_s. Four combined regression equations including two variables of T_s and W_s could be well established to model correlations between R_s and both T_s and W_s. Our study demonstrated that the exponential and power model fitted best and no significant different correlations of combined equations existed between the three plots. These results show that vegetation type had little impact on R_s, T_s, W_s, and their correlations, as well as on related parameters such as Q₁₀ and R₁₀. Therefore, while doing R_s research in a horizontal patchy vegetation conditions on a small area, the sampling location of measurements should focus on vertical dominant vegetation and ignore patch vegetation so as to reduce field work load.     

Modeled interactive effects of precipitation, temperature, and [CO2] on ecosystem carbon and water dynamics in different climatic zones

Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO₂] (C) on net primary production (NPP), heterotrophic respiration (R_h), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three‐way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two‐way interactive effects on NPP, R_h, and NEP were generally positive (i.e. amplification of one factor's effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor's effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two‐way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, R_h, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.     

间伐和改变凋落物输入对华北落叶松人工林土壤呼吸的影响

作为森林生态系统的第二大碳通量,土壤呼吸在全球碳循环和气候变化中发挥着重要作用。通过探究土壤呼吸对间伐和改变凋落物的响应规律以及响应之间的联系,能够为准确评价森林碳循环提供依据。针对不同强度(对照、轻度、中度、重度)间伐后的华北落叶松人工林,2016年5月至10月采用LI-8100土壤碳通量测量系统对其原状、凋落物去除、凋落物加倍的土壤呼吸进行观测。结果表明:土壤呼吸在生长季的8月份达到最高值,呈现出明显的季节动态。不同林分间伐处理下,中度间伐显著促进了土壤呼吸,使平均土壤呼吸速率升高了15.66%,轻度间伐和重度间伐对土壤呼吸的影响不显著;不同凋落物处理下,去除凋落物使平均土壤呼吸速率降低了40.16%,加倍凋落物使平均土壤呼吸速率升高了16.06%。中度间伐使土壤呼吸生长季通量增加了55.06 g C/m~2;去除凋落物使土壤呼吸生长季通量减少了153.48 g C/m~2,加倍凋落物使土壤呼吸生长季通量增加了79.87 g C/m~2。土壤呼吸速率与土壤温度呈显著指数相关,而与土壤湿度无显著相关。不同林分间伐处理下,土壤呼吸的温度敏感性指数(Q10)为2.36—3.46,轻度间伐下Q10值最高;凋落物去除和加倍均降低了土壤呼吸的温度敏感性。土壤温湿度对土壤呼吸存在着显著影响,能够解释土壤呼吸28.7%—62.3%的季节变化。研究结果表明间伐和凋落物处理对华北落叶松人工林土壤CO_2释放的影响表现出一定的交互作用,中度间伐和加倍凋落物的交互作用对土壤呼吸的促进作用显著大于单一因子。可见,间伐作业通过改变土壤微环境和凋落物量,对土壤呼吸以及森林生态系统碳循环产生着重要影响。     

Effects of elevated atmospheric CO2 in agro-ecosystems on soil carbon storage

Increasing global atmospheric CO₂ concentration has led to concerns regarding its potential effects on the terrestrial environment. Attempts to balance the atmospheric carbon (C) budget have met with a large shortfall in C accounting (≈1.4 × 10¹⁵ g C y^–1) and this has led to the hypothesis that C is being stored in the soil of terrestrial ecosystems. This study examined the effects of CO₂ enrichment on soil C storage in C3 soybean (Glycine max L.) Merr. and C4 grain sorghum (Sorghum bicolor L.) Moench. agro-ecosystems established on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with two crop species (soybean and grain sorghum) as the main plots and two CO₂ concentration (ambient and twice ambient) as subplots using open top field chambers. Carbon isotopic techniques using δ¹³C were used to track the input of new C into the soil system. At the end of two years, shifts in δ¹³C content of soil organic matter carbon were observed to a depth of 30 cm. Calculated new C in soil organic matter with grain sorghum was greater for elevated CO₂ vs. ambient CO₂ (162 and 29 g m^–2, respectively), but with soybean the new C in soil organic matter was less for elevated CO₂ vs. ambient CO₂ (120 and 291 g m^–2, respectively). A significant increase in mineral associated organic C was observed in 1993 which may result in increased soil C storage over the long-term, however, little change in total soil organic C was observed under either plant species. These data indicate that elevated atmospheric CO₂ resulted in changes in soil C dynamics in agro-ecosystems that are crop species dependent.     

Impacts of fire on sources of soil CO2 efflux in a dry Amazon rain forest

Fire at the dry southern margin of the Amazon rainforest could have major consequences for regional soil carbon (C) storage and ecosystem carbon dioxide (CO₂) emissions, but relatively little information exists about impacts of fire on soil C cycling within this sensitive ecotone. We measured CO₂ effluxes from different soil components (ground surface litter, roots, mycorrhizae, soil organic matter) at a large‐scale burn experiment designed to simulate a severe but realistic potential future scenario for the region (Fire plot) in Mato Grosso, Brazil, over 1 year, and compared these measurements to replicated data from a nearby, unmodified Control plot. After four burns over 5 years, soil CO₂ efflux (R_s) was ~5.5 t C ha^?1 year^?1 lower on the Fire plot compared to the Control. Most of the Fire plot R_s reduction was specifically due to lower ground surface litter and root respiration. Mycorrhizal respiration on both plots was around ~20% of R_s. Soil surface temperature appeared to be more important than moisture as a driver of seasonal patterns in R_s at the site. Regular fire events decreased the seasonality of R_s at the study site, due to apparent differences in environmental sensitivities among biotic and abiotic soil components. These findings may contribute toward improved predictions of the amount and temporal pattern of C emissions across the large areas of tropical forest facing increasing fire disturbances associated with climate change and human activities.     

Effect of soil water stress on soil respiration and its temperature sensitivity in an 18-year-old temperate Douglas-fir stand

We analyzed 17 months (August 2005 to December 2006) of continuous measurements of soil CO₂ efflux or soil respiration (R_S) in an 18‐year‐old west‐coast temperate Douglas‐fir stand that experienced somewhat greater than normal summertime water deficit. For soil water content at the 4 cm depth (θ) > 0.11 m³ m^?3 (corresponding to a soil water matric potential of ?2 MPa), R_S was positively correlated to soil temperature at the 2 cm depth (T_S). Below this value of θ, however, R_S was largely decoupled from T_S, and evapotranspiration, ecosystem respiration and gross primary productivity (GPP) began to decrease, dropping to about half of their maximum values when θ reached 0.07 m³ m^?3. Soil water deficit substantially reduced R_S sensitivity to temperature resulting in a Q₁₀ significantly < 2. The absolute temperature sensitivity of R_S (i.e. dR_S/dT_S) increased with θ up to 0.15 m³ m^?3, above which it slowly declined. The value of dR_S/dT_S was nearly 0 for θ < 0.08 m³ m^?3, thereby confirming that R_S was largely unaffected by temperature under soil water stress conditions. Despite the possible effects of seasonality of photosynthesis, root activity and litterfall on R_S, the observed decrease in its temperature sensitivity at low θ was consistent with the reduction in substrate availability due to a decrease in (a) microbial mobility, and diffusion of substrates and extracellular enzymes, and (b) the fraction of substrate that can react at high T_S, which is associated with low θ. We found that an exponential (van't Hoff type) model with Q₁₀ and R₁₀ dependent on only θ explained 92% of the variance in half‐hourly values of R_S, including the period with soil water stress conditions. We hypothesize that relating Q₁₀ and R₁₀ to θ not only accounted for the effects of T_S on R_S and its temperature sensitivity but also accounted for the seasonality of biotic (photosynthesis, root activity, and litterfall) and abiotic (soil moisture and temperature) controls and their interactions.     

Response of soil surface CO2 flux in a boreal forest to ecosystem warming

Soil surface carbon dioxide (CO₂) flux (R_S) was measured for 2 years at the Boreal Soil and Air Warming Experiment site near Thompson, MB, Canada. The experimental design was a complete random block design that consisted of four replicate blocks, with each block containing a 15 m × 15 m control and heated plot. Black spruce [Picea mariana (Mill.) BSP] was the overstory species and Epilobium angustifolium was the dominant understory. Soil temperature was maintained (～5 °C) above the control soil temperature using electric cables inside water filled polyethylene tubing for each heated plot. Air inside a 7.3‐m‐diameter chamber, centered in the soil warming plot, contained approximately nine black spruce trees was heated ～5 °C above control ambient air temperature allowing for the testing of soil‐only warming and soil+air warming. Soil surface CO₂ flux (R_S) was positively correlated (P < 0.0001) to soil temperature at 10 cm depth. Soil surface CO₂ flux (R_S) was 24% greater in the soil‐only warming than the control in 2004, but was only 11% greater in 2005, while R_S in the soil+air warming treatments was 31% less than the control in 2004 and 23% less in 2005. Live fine root mass (< 2 mm diameter) was less in the heated than control treatments in 2004 and statistically less (P < 0.01) in 2005. Similar root mass between the two heated treatments suggests that different heating methods (soil‐only vs. soil+air warming) can affect the rate of decomposition.     

Pathways regulating decreased soil respiration with warming in a biocrust‐dominated dryland

A positive soil carbon (C)‐climate feedback is embedded into the climatic models of the IPCC. However, recent global syntheses indicate that the temperature sensitivity of soil respiration (R_S) in drylands, the largest biome on Earth, is actually lower in warmed than in control plots. Consequently, soil C losses with future warming are expected to be low compared with other biomes. Nevertheless, the empirical basis for these global extrapolations is still poor in drylands, due to the low number of field experiments testing the pathways behind the long‐term responses of soil respiration (R_S) to warming. Importantly, global drylands are covered with biocrusts (communities formed by bryophytes, lichens, cyanobacteria, fungi, and bacteria), and thus, R_S responses to warming may be driven by both autotrophic and heterotrophic pathways. Here, we evaluated the effects of 8‐year experimental warming on R_S, and the different pathways involved, in a biocrust‐dominated dryland in southern Spain. We also assessed the overall impacts on soil organic C (SOC) accumulation over time. Across the years and biocrust cover levels, warming reduced R_S by 0.30 μmol CO₂ m^?2 s^?1 (95% CI = ?0.24 to 0.84), although the negative warming effects were only significant after 3 years of elevated temperatures in areas with low initial biocrust cover. We found support for different pathways regulating the warming‐induced reduction in R_S at areas with low (microbial thermal acclimation via reduced soil mass‐specific respiration and β‐glucosidase enzymatic activity) vs. high (microbial thermal acclimation jointly with a reduction in autotrophic respiration from decreased lichen cover) initial biocrust cover. Our 8‐year experimental study shows a reduction in soil respiration with warming and highlights that biocrusts should be explicitly included in modeling efforts aimed to quantify the soil C–climate feedback in drylands.     

Global change belowground: impacts of elevated CO2, nitrogen,and summer drought on soil food webs and biodiversity

The world's ecosystems are subjected to various anthropogenic global change agents, such as enrichment of atmospheric CO₂ concentrations, nitrogen (N) deposition, and changes in precipitation regimes. Despite the increasing appreciation that the consequences of impending global change can be better understood if varying agents are studied in concert, there is a paucity of multi‐factor long‐term studies, particularly on belowground processes. Herein, we address this gap by examining the responses of soil food webs and biodiversity to enrichment of CO₂, elevated N, and summer drought in a long‐term grassland study at Cedar Creek, Minnesota, USA (BioCON experiment). We use structural equation modeling (SEM), various abiotic and biotic explanatory variables, and data on soil microorganisms, protozoa, nematodes, and soil microarthropods to identify the impacts of multiple global change effects on drivers belowground. We found that long‐term (13‐year) changes in CO₂ and N availability resulted in modest alterations of soil biotic food webs and biodiversity via several mechanisms, encompassing soil water availability, plant productivity, and – most importantly – changes in rhizodeposition. Four years of manipulation of summer drought exerted surprisingly minor effects, only detrimentally affecting belowground herbivores and ciliate protists at elevated N. Elevated CO₂ increased microbial biomass and the density of ciliates, microarthropod detritivores, and gamasid mites, most likely by fueling soil food webs with labile C. Moreover, beneficial bottom‐up effects of elevated CO₂ compensated for detrimental elevated N effects on soil microarthropod taxa richness. In contrast, nematode taxa richness was lowest at elevated CO₂ and elevated N. Thus, enrichment of atmospheric CO₂ concentrations and N deposition may result in taxonomically and functionally altered, potentially simplified, soil communities. Detrimental effects of N deposition on soil biodiversity underscore recent reports on plant community simplification. This is of particular concern, as soils house a considerable fraction of global biodiversity and ecosystem functions.     

Antecedent moisture and temperature conditions modulate the response of ecosystem respiration to elevated CO2 and warming

Terrestrial plant and soil respiration, or ecosystem respiration (R_eco), represents a major CO₂ flux in the global carbon cycle. However, there is disagreement in how R_eco will respond to future global changes, such as elevated atmosphere CO₂ and warming. To address this, we synthesized six years (2007–2012) of R_eco data from the Prairie Heating And CO₂ Enrichment (PHACE) experiment. We applied a semi‐mechanistic temperature–response model to simultaneously evaluate the response of R_eco to three treatment factors (elevated CO₂, warming, and soil water manipulation) and their interactions with antecedent soil conditions [e.g., past soil water content (SWC) and temperature (SoilT)] and aboveground factors (e.g., vapor pressure deficit, photosynthetically active radiation, vegetation greenness). The model fits the observed R_eco well (R² = 0.77). We applied the model to estimate annual (March–October) R_eco, which was stimulated under elevated CO₂ in most years, likely due to the indirect effect of elevated CO₂ on SWC. When aggregated from 2007 to 2012, total six‐year R_eco was stimulated by elevated CO₂ singly (24%) or in combination with warming (28%). Warming had little effect on annual R_eco under ambient CO₂, but stimulated it under elevated CO₂ (32% across all years) when precipitation was high (e.g., 44% in 2009, a ‘wet’ year). Treatment‐level differences in R_eco can be partly attributed to the effects of antecedent SoilT and vegetation greenness on the apparent temperature sensitivity of R_eco and to the effects of antecedent and current SWC and vegetation activity (greenness modulated by VPD) on R_eco base rates. Thus, this study indicates that the incorporation of both antecedent environmental conditions and aboveground vegetation activity are critical to predicting R_eco at multiple timescales (subdaily to annual) and under a future climate of elevated CO₂ and warming.