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.     

Responses of Soil,Heterotrophic, and Autotrophic Respiration to Experimental Open-Field Soil Warming in a Cool-Temperate Deciduous Forest

How global warming will affect soil respiration (R _S) and its source components is poorly understood despite its importance for accurate prediction of global carbon (C) cycles. We examined the responses of R _S, heterotrophic respiration (R _H), autotrophic respiration (R _A), nitrogen (N) availability, and fine-root biomass to increased temperature in an open-field soil warming experiment. The experiment was conducted in a cool-temperate deciduous forest ecosystem in northern Japan. As this forest is subjected to strong temporal variation in temperature, on scales ranging from daily to seasonal, we also investigated the temporal variation in the effects of soil warming on R _S, R _H, and R _A. Soil temperature was continuously elevated by about 4.0°C from 2007 to 2014 using heating wires buried in the soil, and we measured soil respiratory processes in all four seasons from 2012 to 2014. Soil warming increased annual R _S by 32–45%, but the magnitude of the increase was different between the components: R _H and R _A were also stimulated, and increased by 39–41 and 17–18%, respectively. Soil N availability during the growing season and fine-root biomass were not remarkably affected by the warming treatment. We found that the warming effects varied seasonally. R _H increased significantly throughout the year, but the warming effect showed remarkable seasonal differences, with the maximum stimulation in the spring. This suggests that warmer spring temperature will produce a greater increase in CO₂ release than warmer summer temperatures. In addition, we found that soil warming reduced the temperature sensitivity (Q ₁₀) of R _S. Although the Q ₁₀ of both R _H and R _A tended to be reduced, the decrease in the Q ₁₀ of R _S was caused mainly by a decrease in the response of R _A to warming. These long-term results indicate that a balance between the rapid and large response of soil microbes and the acclimation of plant roots both play important roles in determining the response of R _S to soil warming, and must be carefully considered to predict the responses of soil C dynamics under future temperature conditions.     

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.     

Soil carbon stocks and fluxes in a warm-temperate oak chronosequence in China

Soil respiration (R_S) and soil carbon stocks, as well as stand properties were investigated in a warm-temperate oak chronosequence in order to understand the age effect on soil CO₂ efflux. The chronosequence consisted of three 40-year-old, 48-year-old, 80-year-old, and 143-year-old oak stands, respectively. R_S measurements were conducted using a Li-8100 soil CO₂ flux system from October 2008 to October 2009. Temporal variations of R_S of all the four forests largely depended on soil temperature of 5 cm depth (T₅) (R²?=?0.738?C0.825). The mean R_S for 40-year-old, 48-year-old, 80-year-old, and 143-year-old forests were 2.37, 2.59, 2.99, and 3.32 ??mol CO₂ m^-2 s^-1 respectively. Both top soil organic carbon (SOC) and light fraction organic carbon (LFOC) stocks were significantly correlated to R_S variation, while only significant different LFOC among stands was found. This indicated that cumulated labile organic carbon was a better indicator on R_S variation, which was further illustrated by a better relationship between R ₁₀ and LFOC than that of R₁₀ and SOC. We found that the variation of mean R_S among stands was well correlated with basal area (BA). Marginal correlation between R_S and fine root biomass (FR) demonstrated the relationship between R_S and belowground metabolism. We also found total porosity (TP) negatively influenced the mean R_S and this negative effect may mainly be attributed to the capillary porosity (CP). Forest growth and yield could be contributed to R_S variation among stands. Forest succession also changed soil labile carbon stock and soil physical properties that influenced the CO₂ efflux.     

Differential effects of grazing,water, and nitrogen addition on soil respiration and its components in a meadow steppe

Background and aims

Understanding the influences of environmental variation and anthropogenic disturbance on soil respiration (R_S) is critical for accurate prediction of ecosystem C uptake and release. However, surprisingly, little is known about how soil respiration and its components respond to grazing in the context of global climate change (i.e., precipitation or nitrogen deposition increase).

Methods

We conducted a field manipulative grazing experiment with water and nitrogen addition treatments in a meadow grassland on the Songnen Plain, China, and assessed the combined influences of grazing and global change factors on R_S, autotrophic respiration (R_A), and heterotrophic respiration (R_H).

Results

Compared with the control plots, R_S, R_A and R_H all exhibited positive responses to water or nitrogen addition in the wet year, while a similar effect occurred only for R_H in the dry year. The responses of R_S to precipitation regimes were dominated by both frequency and amount. However, grazing significantly inhibited both soil respiration and its components in all subplots. Further analysis demonstrated that the plant root/shoot ratio, belowground biomass and microbial biomass played dominant roles in shaping these C exchange processes.

Conclusion

These findings suggest that changes in precipitation regimes, nitrogen deposition, and land utilization may significantly alter soil respiration and its component processes by affecting local carbon users (roots and soil microorganism) and carbon substrate supply in meadow steppe grasslands. The future soil carbon sequestration in the studied meadow steppe will be benefited more by the moderate grazing disturbance.

     

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.     

帽儿山不同年龄森林土壤呼吸速率的影响因子

为探明东北温带森林恢复过程中土壤呼吸(R_S)的变化趋势及其影响因子,在帽儿山选取皆伐后天然更新恢复的4个年龄(1a、10a、25a和56a)林分进行了1年的野外原位测定。结果表明:(1)皆伐后天然更新恢复1年、10年、25年和56年林分的年R_S通量差异显著(P0.05),分别为686.5、639.7、733.3、762.3g C m~(-2)a~(-1);其中生长季(5月─10月)和非生长季的R_S通量也存在显著差异,均呈现出随林龄增加先减后增的趋势。全年、生长季和非生长季R_S随林龄变化的变异系数分别为7.6%、6.3%和21.1%,表明非生长季R_S通量的变异性加大了全年R_S通量的差异。(2)4个年龄林分的Rs季节变化趋势相似,且其主控因子均随季节而变:6月─8月Rs与土壤含水率呈二次函数关系(R~2波动在56%─79%之间),其余时段则与土壤温度呈指数函数关系(R~2波动在85%─93%之间)。(3)不同年龄林分生长季R_S与0─20cm土层有机碳(SOC)密度呈正相关关系(R~2=0.434,P0.05),而非生长季R_S与同期土壤5cm温度呈正相关关系(R~2=0.959,P0.01)。本研究区森林皆伐导致R_S降低,随皆伐后森林恢复R_S不断增加,其主导驱动因子是SOC密度的增加和非生长季土壤温度的变化。     

Integration of Process-based Soil Respiration Models with Whole-Ecosystem CO2 Measurements

We integrated soil models with an established ecosystem process model (SIPNET, simplified photosynthesis and evapotranspiration model) to investigate the influence of soil processes on modelled values of soil CO₂ fluxes (R _Soil). Model parameters were determined from literature values and a data assimilation routine that used a 7-year record of the net ecosystem exchange of CO₂ and environmental variables collected at a high-elevation subalpine forest (the Niwot Ridge AmeriFlux site). These soil models were subsequently evaluated in how they estimated the seasonal contribution of R _Soil to total ecosystem respiration (TER) and the seasonal contribution of root respiration (R _Root) to R _Soil. Additionally, these soil models were compared to data assimilation output of linear models of soil heterotrophic respiration. Explicit modelling of root dynamics led to better agreement with literature values of the contribution of R _Soil to TER. Estimates of R _Soil/TER when root dynamics were considered ranged from 0.3 to 0.6; without modelling root biomass dynamics these values were 0.1–0.3. Hence, we conclude that modelling of root biomass dynamics is critically important to model the R _Soil/TER ratio correctly. When soil heterotrophic respiration was dependent on linear functions of temperature and moisture independent of soil carbon pool size, worse model-data fits were produced. Adding additional complexity to the soil pool marginally improved the model-data fit from the base model, but issues remained. The soil models were not successful in modelling R _Root/R _Soil. This is partially attributable to estimated turnover parameters of soil carbon pools not agreeing with expected values from literature and being poorly constrained by the parameter estimation routine. We conclude that net ecosystem exchange of CO₂ alone cannot constrain specific rhizospheric and microbial components of soil respiration. Reasons for this include inability of the data assimilation routine to constrain soil parameters using ecosystem CO₂ flux measurements and not considering the effect of other resource limitations (for example, nitrogen) on the microbe biomass. Future data assimilation studies with these models should include ecosystem-scale measurements of R _Soil in the parameter estimation routine and experimentally determine soil model parameters not constrained by the parameter estimation routine.     

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.     

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.