Experimental soil warming effects on CO2 and CH4 flux from a low elevation spruce–fir forest soil in Maine, USA

The effect of soil warming on CO₂ and CH₄ flux from a spruce–fir forest soil was evaluated at the Howland Integrated Forest Study site in Maine, USA from 1993 to 1995. Elevated soil temperatures (～5 °C) were maintained during the snow-free season (May – November) in replicated 15 × 15-m plots using electric cables buried 1–2 cm below the soil surface; replicated unheated plots served as the control. CO₂ evolution from the soil surface and soil air CO₂ concentrations both showed clear seasonal trends and significant (P < 0.0001) positive exponential relationships with soil temperature. Soil warming caused a 25–40% increase in CO₂ flux from the heated plots compared to the controls. No significant differences were observed between heated and control plot soil air CO₂ concentrations which we attribute to rapid equilibration with the atmosphere in the O horizon and minimal treatment effects in the B horizon. Methane fluxes were highly variable and showed no consistent trends with treatment.     

Carbon losses due to soil warming: Do autotrophic and heterotrophic soil respiration respond equally?

Global warming has the potential to increase soil respiration (R_S), one of the major fluxes in the global carbon (C) cycle. R_S consists of an autotrophic (R_A) and a heterotrophic (R_H) component. We combined a soil warming experiment with a trenching experiment to assess how R_S, R_A, and R_H are affected. The experiment was conducted in a mature forest dominated by Norway spruce. The site is located in the Austrian Alps on dolomitic bedrock. We warmed the soil of undisturbed and trenched plots by means of heating cables 4 °C above ambient during the snow‐free seasons of 2005 and 2006. Soil warming increased the CO₂ efflux from control plots (R_S) by ∼45% during 2005 and ∼47% during 2006. The CO₂ efflux from trenched plots (R_H) increased by ∼39% during 2005 and ∼45% during 2006. Similar responses of R_S and R_H indicated that the autotrophic and heterotrophic components of R_S responded equally to the temperature increase. Thirty‐five to forty percent or 1 t C ha⁻¹ yr⁻¹ of the overall annual increase in R_S (2.8 t C ha⁻¹ yr⁻¹) was autotrophic. The remaining, heterotrophic part of soil respiration (1.8 t C ha⁻¹ yr⁻¹), represented the warming‐induced C loss from the soil. The autotrophic component showed a distinct seasonal pattern. Contribution of R_A to R_S was highest during summer. Seasonally derived Q₁₀ values reflected this pattern and were correspondingly high (5.3–9.3). The autotrophic CO₂ efflux increase due to the 4 °C warming implied a Q₁₀ of 2.9. Hence, seasonally derived Q₁₀ of R_A did not solely reflect the seasonal soil temperature development.     

Impacts of an anomalously warm year on soil CO2 efflux in experimentally manipulated tallgrass prairie ecosystems

Modeling analyses suggest that an increase in growth rate of atmospheric CO₂ concentrations during an anomalously warm year may be caused by a decrease in net ecosystem production (NEP) in response to increased heterotrophic respiration (R_h). To test this hypothesis, 12 intact soil monoliths were excavated from a tallgrass prairie site near Purcell, Oklahoma, USA and divided among four large dynamic flux chambers (Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs)). During the first year, all four EcoCELLs were subjected to Oklahoma air temperatures. During the second year, air temperature in two EcoCELLs was increased by 4°C throughout the year to simulate anomalously warm conditions. This paper reports on the effect of warming on soil CO₂ efflux, representing the sum of autotrophic respiration (R_a) and R_h. During the pretreatment year, weekly average soil CO₂ efflux was similar in all EcoCELLs. During the late spring, summer and early fall of the treatment year, however, soil CO₂ efflux was significantly lower in the warmed EcoCELLs. In general, soil CO₂ efflux was correlated with soil temperature and to a lesser extent with moisture. A combined temperature and moisture regression explained 64% of the observed variation in soil CO₂ efflux. Soil CO₂ efflux correlated well with a net primary production (NPP) weighted greenness index derived from digital photographs. Although separate relationships for control and warmed EcoCELLs showed better correlations, one single relationship explained close to 70% of the variation in soil CO₂ efflux across treatments and years. A strong correlation between soil CO₂ efflux and canopy development and the lack of initial response to warming indicate that soil CO₂ efflux is dominated by R_a. This study showed that a decrease in soil CO₂ efflux in response to a warm year was most likely dominated by a decrease in R_a instead of an increase in R_h.     

Soil respiration in six temperate forests in China

Scaling soil respiration (R_S), the major CO₂ source to the atmosphere from terrestrial ecosystems, from chamber‐based measurements to ecosystems requires studies on variations and correlations of R_S from various biomes and across geographic regions. However, few studies on R_S are available for Chinese temperate forest despite the importance of this forest in the national and global carbon budgets. In this study, we conducted 18‐month R_S measurements during 2004–2005 in six temperate forest types, representing the typical secondary forest ecosystems across various site conditions in northeastern China: Mongolian oak (Quercus mongolica Fisch.), aspen‐birch (Populous davidiana Dode and Betula platyphylla Suk.), mixed deciduous (no dominant tree species), hardwood (dominated by Fraxinus mandshurica Rupr., Juglans mandshurica Maxim., and Phellodendron amurense Rupr.) forests, Korean pine (Pinus koraiensis Sieb. et Zucc.) and Dahurian larch (Larix gmelinii Rupr.) plantations. Our specific objectives were to: (1) explore relationships of R_S against soil temperature and water content for the six forest ecosystems, (2) quantify annual soil surface CO₂ flux and its relations to belowground carbon storage, (3) examine seasonal variations in R_S and related environmental factors, and (4) quantify among‐ and within‐ecosystem variations in R_S. The R_S was positively correlated to soil temperature in all forest types, and was significantly influenced by the interactions of soil temperature and water content in the pine, larch, and mixed deciduous forests. The sensitivity of R_S to soil temperature at 10 cm depth (Q₁₀) ranged from 2.61 in the oak forest to 3.75 in the aspen‐birch forests. The Q₁₀ tended to increase with soil water content until reaching a threshold, and then decline. The annual R_S for the larch, pine, hardwood, oak, mixed deciduous, and aspen‐birch forests averaged 403, 514, 781, 785, 786, and 813 g C m^?2 yr^?1, respectively. The annual R_S of the broadleaved forests was 72% greater than that of the coniferous forests. The annual R_S was positively correlated to soil organic carbon (SOC) concentration at O horizon (R²=0.868) and total biomass of roots <0.5 cm in diameter (R²=0.748). The coefficient of variation (CV) of R_S among forest types averaged 25% across the 18‐month measurements. The CV of R_S within plots varied from 20% to 27%, significantly (P<0.001) greater than those among plots (9–15%), indicating the importance of the fine‐scaled heterogeneity in R_S. This study emphasized that variations in soil respiration and potential sampling bias should be appropriately tackled for accurate soil CO₂ flux estimates.     

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.     

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.     

Seasonal changes in the contribution of root respiration to total soil respiration in a cool-temperate deciduous forest

A trenching method was used to determine the contribution of root respiration to soil respiration. Soil respiration rates in a trenched plot (R _trench) and in a control plot (R _control) were measured from May 2000 to September 2001 by using an open-flow gas exchange system with an infrared gas analyser. The decomposition rate of dead roots (R _D) was estimated by using a root-bag method to correct the soil respiration measured from the trenched plots for the additional decaying root biomass. The soil respiration rates in the control plot increased from May (240–320 mg CO₂ m^–2 h^–1) to August (840–1150 mg CO₂ m^–2 h^–1) and then decreased during autumn (200–650 mg CO₂ m^–2 h^–1). The soil respiration rates in the trenched plot showed a similar pattern of seasonal change, but the rates were lower than in the control plot except during the 2 months following the trenching. Root respiration rate (R _r) and heterotrophic respiration rate (R _h) were estimated from R _control, R _trench, and R _D. We estimated that the contribution of R _r to total soil respiration in the growing season ranged from 27 to 71%. There was a significant relationship between R _h and soil temperature, whereas R _r had no significant correlation with soil temperature. The results suggest that the factors controlling the seasonal change of respiration differ between the two components of soil respiration, R _r and R _h.     

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.     

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.     

Soil respiration is stimulated by elevated CO2 and reduced by summer drought: three years of measurements in a multifactor ecosystem manipulation experiment in a temperate heathland (CLIMAITE)

This study investigated the impact of predicted future climatic and atmospheric conditions on soil respiration (R_S) in a Danish Calluna‐Deschampsia‐heathland. A fully factorial in situ experiment with treatments of elevated atmospheric CO₂ (+130 ppm), raised soil temperature (+0.4 °C) and extended summer drought (5–8% precipitation exclusion) was established in 2005. The average R_S, observed in the control over 3 years of measurements (1.7 μmol CO₂ m^?2 sec^?1), increased 38% under elevated CO₂, irrespective of combination with the drought or temperature treatments. In contrast, extended summer drought decreased R_S by 14%, while elevated soil temperature did not affect R_S overall. A significant interaction between elevated temperature and drought resulted in further reduction of R_S when these treatments were combined. A detailed analysis of short‐term R_S dynamics associated with drought periods showed that R_S was reduced by ~50% and was strongly correlated with soil moisture during these events. Recovery of R_S to pre‐drought levels occurred within 2 weeks of rewetting; however, unexpected drought effects were observed several months after summer drought treatment in 2 of the 3 years, possibly due to reduced plant growth or changes in soil water holding capacity. An empirical model that predicts R_S from soil temperature, soil moisture and plant biomass was developed and accounted for 55% of the observed variability in R_S. The model predicted annual sums of R_S in 2006 and 2007, in the control, were 672 and 719 g C m^?2 y^?1, respectively. For the full treatment combination, i.e. the future climate scenario, the model predicted that soil respiratory C losses would increase by ~21% (140–150 g C m^?2 y^?1). Therefore, in the future climate, stimulation of C storage in plant biomass and litter must be in excess of 21% for this ecosystem to not suffer a reduction in net ecosystem exchange.     

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.     

Trends and methodological impacts in soil CO2 efflux partitioning: A metaanalytical review

Partitioning soil carbon dioxide (CO₂) efflux (R_S) into autotrophic (R_A; including plant roots and closely associated organisms) and heterotrophic (R_H) components has received considerable attention, as differential responses of these components to environmental change have profound implications for the soil and ecosystem C balance. The increasing number of partitioning studies allows a more detailed analysis of experimental constraints than was previously possible. We present results of an exhaustive literature search of partitioning studies and analyse global trends in flux partitioning between biomes and ecosystem types by means of a metaanalysis. Across all data, an overall decline in the R_H/R_S ratio for increasing annual R_S fluxes emerged. For forest ecosystems, boreal coniferous sites showed significantly higher (P<0.05) R_H/R_S ratios than temperate sites, while both temperate or tropical deciduous forests did not differ in ratios from any of the other forest types. While chronosequence studies report consistent declines in the R_H/R_S ratio with age, no difference could be detected for different age groups in the global data set. Different methodologies showed generally good agreement if the range of R_S under which they had been measured was considered, with the exception of studies estimating R_H by means of root mass regressions against R_S, which resulted in consistently lower R_H/R_S estimates out of all methods included. Additionally, the time step over which fluxes were partitioned did not affect R_H/R_S ratios consistently. To put results into context, we review the most common techniques and point out the likely sources of errors associated with them. In order to improve soil CO₂ efflux partitioning in future experiments, we include methodological recommendations, and also highlight the potential interactions between soil components that may be overlooked as a consequence of the partitioning process itself.     

Responses of soil respiration to elevated CO2, air warming, and changing soil water availability in a model old-field grassland

Responses of soil respiration to atmospheric and climatic change will have profound impacts on ecosystem and global carbon (C) cycling in the future. This study was conducted to examine effects on soil respiration of the concurrent driving factors of elevated atmospheric CO₂ concentration, air warming, and changing precipitation in a constructed old‐field grassland in eastern Tennessee, USA. Model ecosystems of seven old‐field species were established in open‐top chambers and treated with factorial combinations of ambient or elevated (+300 ppm) CO₂ concentration, ambient or elevated (+3 °C) air temperature, and high or low soil moisture content. During the 19‐month experimental period from June 2003 to December 2004, higher CO₂ concentration and soil water availability significantly increased mean soil respiration by 35.8% and 15.7%, respectively. The effects of air warming on soil respiration varied seasonally from small reductions to significant increases to no response, and there was no significant main effect. In the wet side of elevated CO₂ chambers, air warming consistently caused increases in soil respiration, whereas in the other three combinations of CO₂ and water treatments, warming tended to decrease soil respiration over the growing season but increase it over the winter. There were no interactive effects on soil respiration among any two or three treatment factors irrespective of time period. Treatment‐induced changes in soil temperature and moisture together explained 49%, 44%, and 56% of the seasonal variations of soil respiration responses to elevated CO₂, air warming, and changing precipitation, respectively. Additional indirect effects of seasonal dynamics and responses of plant growth on C substrate supply were indicated. Given the importance of indirect effects of the forcing factors and plant community dynamics on soil temperature, moisture, and C substrate, soil respiration response to climatic warming should not be represented in models as a simple temperature response function, and a more mechanistic representation including vegetation dynamics and substrate supply is needed.     

A global relationship between the heterotrophic and autotrophic components of soil respiration?

Soil surface CO₂ flux (R_S) is overwhelmingly the product of respiration by roots (autotrophic respiration, R_A) and soil organisms (heterotrophic respiration, R_H). Many studies have attempted to partition R_S into these two components, with highly variable results. This study analyzes published data encompassing 54 forest sites and shows that R_A and R_H are each strongly (R²>0.8) correlated to annual R_S across a wide range of forest ecosystems. Monte Carlo simulation showed that these correlations were significantly stronger than any correlation introduced as an artefact of measurement method. Biome type, measurement method, mean annual temperature, soil drainage, and leaf habit were not significant. For sites with available data, there was a significant (R²=0.56) correlation between total detritus input and R_H, while R_A was unrelated to net primary production. We discuss why R_A and R_H might be related to each other on large scales, as both ultimately depend on forest carbon balance and photosynthate supply. Limited data suggest that these or similar relationships have broad applicability in other ecosystem types. Site‐specific measurements are always more desirable than the application of inferred broad relationships, but belowground measurements are difficult and expensive, while measuring R_S is straightforward and commonly done. Thus the relationships presented here provide a useful method that can help constrain estimates of terrestrial carbon budgets.     

皆伐火烧对亚热带森林不同深度土壤CO2通量的影响

评估不同深度土壤的CO_2通量是研究土壤碳动态的重要手段。目前有关皆伐火烧对森林土壤碳排放的影响研究仅局限于表层土壤,而对不同深度土壤碳排放影响鲜见报道。以米槠(Castanopsis carlesii)次生林(对照)及其皆伐火烧后林地为研究对象,利用非红外散射CO_2探头测定土壤CO_2浓度,并结合Fick第一扩散法则估算不同深度(0—80 cm)土壤CO_2通量。结果表明:(1)皆伐火烧改变土壤向大气排放的表观CO_2通量,在皆伐火烧后的2个月内土壤表观CO_2通量显著高于对照68%;2个月后,土壤表观CO_2通量低于对照37%。(2)皆伐火烧后,除10—20 cm的CO_2通量提高外,其余各深度(0—10、20—40、40—60 cm和60—80 cm)的CO_2通量均降低。同时,皆伐火烧改变不同土层对土壤呼吸的贡献率,降低0—10 cm土层的贡献率,提高10—20 cm土层的贡献率。(3)对照样地仅0—10 cm土壤CO_2通量与温度呈显著指数相关,10—40 cm深度CO_2通量则与土壤含水率呈显著线性相关。皆伐火烧后0—10 cm和10—20 cm处土壤的CO_2通量均与温度呈指数相关。说明皆伐火烧改变了不同深度土壤CO_2通量对于环境因子的响应。因此为准确评估和预测皆伐火烧对土壤与大气间碳交换的影响,应考虑皆伐火烧后不同时期土壤CO_2通量的变化,以及不同深度土壤CO_2通量对皆伐火烧的响应。     

Effects of soil moisture on the temperature sensitivity of heterotrophic respiration vary seasonally in an old‐field climate change experiment

Microbial decomposition of soil organic matter produces a major flux of CO₂ from terrestrial ecosystems and can act as a feedback to climate change. Although climate‐carbon models suggest that warming will accelerate the release of CO₂ from soils, the magnitude of this feedback is uncertain, mostly due to uncertainty in the temperature sensitivity of soil organic matter decomposition. We examined how warming and altered precipitation affected the rate and temperature sensitivity of heterotrophic respiration (R_h) at the Boston‐Area Climate Experiment, in Massachusetts, USA. We measured R_h inside deep collars that excluded plant roots and litter inputs. In this mesic ecosystem, R_h responded strongly to precipitation. Drought reduced R_h, both annually and during the growing season. Warming increased R_h only in early spring. During the summer, when R_h was highest, we found evidence of threshold, hysteretic responses to soil moisture: R_h decreased sharply when volumetric soil moisture dropped below ~15% or exceeded ~26%, but R_h increased more gradually when soil moisture rose from the lower threshold. The effect of climate treatments on the temperature sensitivity of R_h depended on the season. Apparent Q₁₀ decreased with high warming (~3.5 °C) in spring and fall. Presumably due to limiting soil moisture, warming and precipitation treatments did not affect apparent Q₁₀ in summer. Drought decreased apparent Q₁₀ in fall compared to ambient and wet precipitation treatments. To our knowledge, this is the first field study to examine the response of R_h and its temperature sensitivity to the combined effects of warming and altered precipitation. Our results highlight the complex responses of R_h to soil moisture, and to our knowledge identify for the first time the seasonal variation in the temperature sensitivity of microbial respiration in the field. We emphasize the importance of adequately simulating responses such as these when modeling trajectories of soil carbon stocks under climate change scenarios.     

Diurnal, seasonal and annual variation in net ecosystem CO2 exchange of an alpine shrubland on Qinghai-Tibetan plateau

Thus far, grassland ecosystem research has mainly been focused on low‐lying grassland areas, whereas research on high‐altitude grassland areas, especially on the carbon budget of remote areas like the Qinghai‐Tibetan plateau is insufficient. To address this issue, flux of CO₂ were measured over an alpine shrubland ecosystem (37°36′N, 101°18′E; 325 above sea level [a. s. l.]) on the Qinghai‐Tibetan Plateau, China, for 2 years (2003 and 2004) with the eddy covariance method. The vegetation is dominated by formation Potentilla fruticosa L. The soil is Mol–Cryic Cambisols. To interpret the biotic and abiotic factors that modulate CO₂ flux over the course of a year we decomposed net ecosystem CO₂ exchange (NEE) into its constituent components, and ecosystem respiration (R_eco). Results showed that seasonal trends of annual total biomass and NEE followed closely the change in leaf area index. Integrated NEE were ?58.5 and ?75.5 g C m^?2, respectively, for the 2003 and 2004 years. Carbon uptake was mainly attributed from June, July, August, and September of the growing season. In July, NEE reached seasonal peaks of similar magnitude (4–5 g C m^?2 day^?1) each of the 2 years. Also, the integrated night‐time NEE reached comparable peak values (1.5–2 g C m^?2 day^?1) in the 2 years of study. Despite the large difference in time between carbon uptake and release (carbon uptake time < release time), the alpine shrubland was carbon sink. This is probably because the ecosystem respiration at our site was confined significantly by low temperature and small biomass and large day/night temperature difference and usually soil moisture was not limiting factor for carbon uptake. In general, R_eco was an exponential function of soil temperature, but with season‐dependent values of Q₁₀. The temperature‐dependent respiration model failed immediately after rain events, when large pulses of R_eco were observed. Thus, for this alpine shrubland in Qinghai‐Tibetan plateau, the timing of rain events had more impact than the total amount of precipitation on ecosystem R_eco and NEE.     

Nonlinear CO2 flux response to 7 years of experimentally induced permafrost thaw

Rapid Arctic warming is expected to increase global greenhouse gas concentrations as permafrost thaw exposes immense stores of frozen carbon (C) to microbial decomposition. Permafrost thaw also stimulates plant growth, which could offset C loss. Using data from 7 years of experimental Air and Soil warming in moist acidic tundra, we show that Soil warming had a much stronger effect on CO₂ flux than Air warming. Soil warming caused rapid permafrost thaw and increased ecosystem respiration (R_eco), gross primary productivity (GPP), and net summer CO₂ storage (NEE). Over 7 years R_eco, GPP, and NEE also increased in Control (i.e., ambient plots), but this change could be explained by slow thaw in Control areas. In the initial stages of thaw, R_eco, GPP, and NEE increased linearly with thaw across all treatments, despite different rates of thaw. As thaw in Soil warming continued to increase linearly, ground surface subsidence created saturated microsites and suppressed R_eco, GPP, and NEE. However R_eco and GPP remained high in areas with large Eriophorum vaginatum biomass. In general NEE increased with thaw, but was more strongly correlated with plant biomass than thaw, indicating that higher R_eco in deeply thawed areas during summer months was balanced by GPP. Summer CO₂ flux across treatments fit a single quadratic relationship that captured the functional response of CO₂ flux to thaw, water table depth, and plant biomass. These results demonstrate the importance of indirect thaw effects on CO₂ flux: plant growth and water table dynamics. Nonsummer R_eco models estimated that the area was an annual CO₂ source during all years of observation. Nonsummer CO₂ loss in warmer, more deeply thawed soils exceeded the increases in summer GPP, and thawed tundra was a net annual CO₂ source.     

平茬对半干旱黄土丘陵区柠条林地土壤水分的影响

半干旱黄土丘陵区多年生柠条人工林地发生土壤旱化,研究柠条林平茬对土壤水分影响对于防治土壤旱化具有重要意义。采用中子仪测定土壤水分,对未平茬和平茬柠条林地土壤水分进行测定,分析了平茬对土壤水分的影响。结果表明:未平茬和平茬柠条林地降雨补给量(R₁,R₂)同降雨量(P)显著正相关(P<0.05)。定义降雨耗损量(林冠截留量和地表径流之和)占降雨量的百分比为降雨耗损率,未平茬林地降雨损耗率(L₁)和平茬柠条林地降雨损耗率(L₂)分别与其降雨前土壤表层(0-20 cm)含水量(S₁,S₂)呈明显指数关系(P<0.05):L₁=2.54exp(0.22S₁),L₂=2.40exp(0.27S₂),表层含水量相同时,平茬林地降雨损耗率明显高于未平茬林地。平茬后,林地降雨最大入渗深度减小,土壤水分利用深度减小;短时间内(2个月左右)林地20-160 cm含水量增加,之后平茬林地土壤含水量与未平茬林地土壤含水量接近;丰水年和丰水年后的第一年,平茬林地含水量低于未平茬林地,0-400 cm土壤储水量比未平茬林地最多低45.9 mm。平茬后200-400 cm土层土壤水分有少量增加,但是0-200 cm土层土壤含水量损失更严重。平茬3a后,平茬对柠条林地土壤水分的影响减弱。     

The response of heterotrophic CO2 flux to soil warming

In a forest ecosystem at steady state, net carbon (C) assimilation by plants and C loss through soil and litter decomposition by heterotrophic organisms are balanced. However, a perturbation to the system, such as increased mean soil temperature, will lead to faster decay, enhancing CO₂ release from decomposers, and thus upsetting the balance. Recent in situ experiments have indicated that the stimulation of soil respiration following a step increase in annual average soil temperature declines over time. One possible explanation for this decline may be changes in substrate availability. This hypothesis is examined by using the ecosystem model G'DAY, which simulates C and nitrogen (N) dynamics in plants and soil. We applied the model to observations from a soil‐warming experiment in a Norway spruce (Picea abies (L.) Karst.) stand by simulating a step increase of soil temperature. The model provided a good qualitative reproduction of the observed reduction of heterotrophic respiration (R_h) under sustained warming. The simulations showed how the combined effects of faster turnover and reduced substrate availability lead to a transient increase of R_h. The simulated annual increase in R_h from soil was 60% in the first year after perturbation but decreased to 30% after a decade. One conclusion from the analysis of the simulations is that R_h can decrease even though the temperature response function for decomposition remains unchanged. G'DAY suggests that acclimation of R_h to soil warming is partly an effect of substrate depletion of labile C pools during the first decade of warming as a result of accelerated rates of mineralization. The response is attributed mainly to changing levels of C in pools with short time constants, reflecting the importance of high‐quality soil C fractions. Changes of the structure or physiology of the decomposer community were not invoked. Therefore, it becomes a question of definition whether the simulated dynamics of the declining response of CO₂ release to the warming should be named acclimation or seen as a natural part of the system dynamics.