Catalytic power of enzymes decreases with temperature: New insights for understanding soil C cycling and microbial ecology under warming

Most current models of soil C dynamics predict that climate warming will accelerate soil C mineralization, resulting in a long‐term CO₂ release and positive feedback to global warming. However, ecosystem warming experiments show that CO₂ loss from warmed soils declines to control levels within a few years. Here, we explore the temperature dependence of enzymatic conversion of polymerized soil organic C (SOC) into assimilable compounds, which is presumed the rate‐limiting step of SOC mineralization. Combining literature review, modelling and enzyme assays, we studied the effect of temperature on activity of enzymes considering their thermal inactivation and catalytic activity. We defined the catalytic power of enzymes (E_power) as the cumulative amount of degraded substrate by one unit of enzyme until its complete inactivation. We show a universal pattern of enzyme's thermodynamic properties: activation energy of catalytic activity (EA_cat) < activation energy of thermal inactivation (EA_inact). By investing in stable enzymes (high EA_inact) having high catalytic activity (low EA_cat), microorganisms may maximize the E_power of their enzymes. The counterpart of such EAs’ hierarchical pattern is the higher relative temperature sensitivity of enzyme inactivation than catalysis, resulting in a reduction in E_power under warming. Our findings could explain the decrease with temperature in soil enzyme pools, microbial biomass (MB) and carbon use efficiency (CUE) reported in some warming experiments and studies monitoring the seasonal variation in soil enzymes. They also suggest that a decrease in soil enzyme pools due to their faster inactivation under warming contributes to the observed attenuation of warming effect on soil C mineralization. This testable theory predicts that the ultimate response of SOC degradation to warming can be positive or negative depending on the relative temperature response of E_power and microbial production of enzymes.     

The priming potential of environmentally weathered pyrogenic carbon during land‐use transition to biomass crop production

Since land‐use change (LUC) to lignocellulosic biomass crops often causes a loss of soil organic carbon (SOC), at least in the short term, this study investigated the potential for pyrogenic carbon (PyC) to ameliorate this effect. Although negative priming has been observed in many studies, most of these are long‐term incubation experiments which do not account for the interactions between environmentally weathered PyC and native SOC. Here, the aim was to assess the impact of environmentally weathered PyC on native SOC mineralization at different time points in LUC from arable crops to short rotation coppice (SRC) willow. At eight SRC willow plantations in England, with ages of 3–22 years, soil amended 18–22 months previously with PyC was compared with unamended control soil. Cumulative CO₂ flux was measured weekly from incubated soil at 0–5 cm depth, and soil‐surface CO₂ flux was also measured in the field. For the incubated soil, cumulative CO₂ flux was significantly higher from soil containing weathered PyC than the control soil for seven of the eight sites. Across all sites, the mean cumulative CO₂ flux was 21% higher from soil incubated with weathered PyC than the control soil. These results indicate the potential for positive priming in the surface 5 cm of soil independent of changes in soil properties following LUC to SRC willow production. However, no net effect on CO₂ flux was observed in the field, suggesting this increase in CO₂ is offset by a contrasting PyC‐induced effect at a different soil depth or that different effects were observed under laboratory and field conditions. Although the mechanisms for these contrasting effects remain unclear, results presented here suggest that PyC does not reduce LUC‐induced SOC losses through negative priming, at least for this PyC type and application rate.     

易分解有机碳对不同恢复年限森林土壤激发效应的影响

土壤有机碳库作为陆地生态系统最大的碳库,其微小的改变都将引起大气CO_2浓度的急剧改变。易分解有机碳的输入可以通过正/负激发效应加快/减缓土壤有机碳(SOC)的矿化,并最终影响土壤碳平衡。以长汀县不同恢复年限森林(裸地、5年、15年、30年马尾松林以及天然林)土壤为研究对象,通过室内培养向土壤中添加~(13)C标记葡萄糖研究易分解有机碳输入对不同恢复阶段森林土壤激发效应的影响。研究结果表明,易分解有机碳输入引起的土壤激发效应的方向和强度因不同恢复阶段而异。易分解有机碳输入的初期对各恢复阶段森林土壤均产生正的激发效应,然而随着时间的推移,15年、30年马尾松林以及天然林相继出现负的激发效应。从整个培养期(59 d)来看,易分解有机碳的输入促进了裸地与5年生马尾松林土壤有机碳的矿化,有机碳的矿化量分别提高了131%±27%与25%±5%;但是减缓了15年生马尾松林土壤有机碳的矿化,使其矿化量减少了10%±1%;然而,易分解有机碳输入对30年生马尾松林及天然林土壤有机碳的矿化则无明显影响。土壤累积激发碳量与葡萄糖添加前后土壤氮素的改变百分比呈显著正相关关系(R~2=0.44,P0.05),表明易分解有机碳输入诱导的土壤激发效应受土壤氮素可利用性的调控,土壤微生物需要通过分解原有土壤有机碳释放的氮素来满足自身的需求。     

Temporal response of soil organic carbon after grassland‐related land‐use change

The net flux of CO₂ exchanged with the atmosphere following grassland‐related land‐use change (LUC) depends on the subsequent temporal dynamics of soil organic carbon (SOC). Yet, the magnitude and timing of these dynamics are still unclear. We compiled a global data set of 836 paired‐sites to quantify temporal SOC changes after grassland‐related LUC. In order to discriminate between SOC losses from the initial ecosystem and gains from the secondary one, the post‐LUC time series of SOC data was combined with satellite‐based net primary production observations as a proxy of carbon input to the soil. Globally, land conversion from either cropland or forest into grassland leads to SOC accumulation; the reverse shows net SOC loss. The SOC response curves vary between different regions. Conversion of cropland to managed grassland results in more SOC accumulation than natural grassland recovery from abandoned cropland. We did not consider the biophysical variables (e.g., climate conditions and soil properties) when fitting the SOC turnover rate into the observation data but analyzed the relationships between the fitted turnover rate and these variables. The SOC turnover rate is significantly correlated with temperature and precipitation (p < 0.05), but not with the clay fraction of soils (p > 0.05). Comparing our results with predictions from bookkeeping models, we found that bookkeeping models overestimate by 56% of the long‐term (100 years horizon) cumulative SOC emissions for grassland‐related LUC types in tropical and temperate regions since 2000. We also tested the spatial representativeness of our data set and calculated SOC response curves using the representative subset of sites in each region. Our study provides new insight into the impact grassland‐related LUC on the global carbon budget and sheds light on the potential of grassland conservation for climate mitigation.     

Carbon sequestration potential of soils in southeast Germany derived from stable soil organic carbon saturation

Sequestration of atmospheric carbon (C) in soils through improved management of forest and agricultural land is considered to have high potential for global CO₂ mitigation. However, the potential of soils to sequester soil organic carbon (SOC) in a stable form, which is limited by the stabilization of SOC against microbial mineralization, is largely unknown. In this study, we estimated the C sequestration potential of soils in southeast Germany by calculating the potential SOC saturation of silt and clay particles according to Hassink [Plant and Soil 191 (1997) 77] on the basis of 516 soil profiles. The determination of the current SOC content of silt and clay fractions for major soil units and land uses allowed an estimation of the C saturation deficit corresponding to the long‐term C sequestration potential. The results showed that cropland soils have a low level of C saturation of around 50% and could store considerable amounts of additional SOC. A relatively high C sequestration potential was also determined for grassland soils. In contrast, forest soils had a low C sequestration potential as they were almost C saturated. A high proportion of sites with a high degree of apparent oversaturation revealed that in acidic, coarse‐textured soils the relation to silt and clay is not suitable to estimate the stable C saturation. A strong correlation of the C saturation deficit with temperature and precipitation allowed a spatial estimation of the C sequestration potential for Bavaria. In total, about 395 Mt CO₂‐equivalents could theoretically be stored in A horizons of cultivated soils – four times the annual emission of greenhouse gases in Bavaria. Although achieving the entire estimated C storage capacity is unrealistic, improved management of cultivated land could contribute significantly to CO₂ mitigation. Moreover, increasing SOC stocks have additional benefits with respect to enhanced soil fertility and agricultural productivity.     

Controls on microbial CO2 production: a comparison of surface and subsurface soil horizons

Although a significant amount of the organic C stored in soil resides in subsurface horizons, the dynamics of subsurface C stores are not well understood. The objective of this study was to determine if changes in soil moisture, temperature, and nutrient levels have similar effects on the mineralization of surface (0–25 cm) and subsurface (below 25 cm) C stores. Samples were collected from a 2 m deep unsaturated mollisol profile located near Santa Barbara, CA, USA. In a series of experiments, we measured the influence of nutrient additions (N and P), soil temperature (10–35°C), and soil water potential (?0.5 to ?10 MPa) on the microbial mineralization of native soil organic C. Surface and subsurface soils were slightly different with respect to the effects of water potential on microbial CO₂ production; C mineralization rates in surface soils were more affected by conditions of moderate drought than rates in subsurface soils. With respect to the effects of soil temperature and nutrient levels on C mineralization rates, subsurface horizons were significantly more sensitive to increases in temperature or nutrient availability than surface horizons. The mean Q₁₀ value for C mineralization rates was 3.0 in surface horizons and 3.9 in subsurface horizons. The addition of either N or P had negligible effects on microbial CO₂ production in surface soil layers; in the subsurface horizons, the addition of either N or P increased CO₂ production by up to 450% relative to the control. The results of these experiments suggest that alterations of the soil environment may have different effects on CO₂ production through the profile and that the mineralization of subsurface C stores may be particularly susceptible to increases in temperature or nutrient inputs to soil.     

Greenhouse gas mitigation potential in crop production with biochar soil amendment—a carbon footprint assessment for cross‐site field experiments from China

Biochar soil amendment (BSA) had been advocated as a promising approach to mitigate greenhouse gas (GHG) emissions in agriculture. However, the net GHG mitigation potential of BSA remained unquantified with regard to the manufacturing process and field application. Carbon footprint (CF) was employed to assess the mitigating potential of BSA by estimating all the direct and indirect GHG emissions in the full life cycles of crop production including production and field application of biochar. Data were obtained from 7 sites (4 sites for paddy rice production and 3 sites for maize production) under a single BSA at 20 t/ha^?1 across mainland China. Considering soil organic carbon (SOC) sequestration and GHG emission reduction from syngas recycling, BSA reduced the CFs by 20.37–41.29 t carbon dioxide equivalent ha^?1 (CO₂‐eq ha^?1) and 28.58–39.49 t CO₂‐eq ha^?1 for paddy rice and maize production, respectively, compared to no biochar application. Without considering SOC sequestration and syngas recycling, the net CF change by BSA was in a range of ?25.06 to 9.82 t CO₂‐eq ha^?1 and ?20.07 to 5.95 t CO₂‐eq ha^?1 for paddy rice and maize production, respectively, over no biochar application. As the largest contributors among the others, syngas recycling in the process of biochar manufacture contributed by 47% to total CF reductions under BSA for rice cultivation while SOC sequestration contributed by 57% for maize cultivation. There was a large variability of the CF reductions across the studied sites whether in paddy rice or maize production, due likely to the difference in GHG emission reductions and SOC increments under BSA across the sites. This study emphasized that SOC sequestration should be taken into account the CF calculation of BSA. Improved biochar manufacturing technique could achieve a remarkable carbon sink by recycling the biogas for traditional fossil‐fuel replacement.     

Evolution of soil carbon stocks under Miscanthus × giganteus and Miscanthus sinensis across contrasting environmental conditions

Miscanthus is a C4 bioenergy perennial crop characterized by its high potential yield. Our study aimed to compare the carbon storage capacities of Miscanthus sinensis (M. sinensis) with that of Miscanthus × giganteus (M. × giganteus) in field conditions in different types of soils in France. We set up a multi‐environment experimental network. On each trial, we tested two treatments: M. × giganteus established from rhizomes (Gr) and M. sinensis transplanted seedlings (Sp). We quantified the soil organic carbon (SOC) stock at equivalent soil mass for both genotypes in 2014 and 2019 and for two sampling depths: L1 (ca. 0–5 cm) and L1‐2 (ca. 0–30 cm). We also calculated the total and annual variation of the SOC stock and investigated factors that could explain the variation and the initial state of the SOC stock. ANOVAs were performed to compare the SOC stock, as well as the SOC stock variation rates across treatments and soil layers. Results showed that the soil bulk density did not vary significantly between 2014 and 2019 for both treatments (Gr and Sp). The SOC concentration (i.e. SOC expressed in g/kg) increased significantly between 2014 and 2019 in L1, whereas no significant evolution was found in L2 (ca. 5–30 cm). The SOC stock (i.e. SOC expressed in t/ha) increased significantly in the superficial layer L1 for M. × giganteus and M. sinensis, by 0.48 ± 0.41 and 0.54 ± 0.25 t ha^?1 year^?1 on average, respectively, although no significant change was detected in the layer L1‐2 for both genotypes. Moreover, SOC stocks in 2019 did not differ significantly between M. × giganteus and M. sinensis in the soil layers L1 and L1‐2. Lastly, our results showed that the initial SOC stock was significantly higher when miscanthus was grown after set‐aside than after annual crops.     

Measured and modelled effect of land‐use change from temperate grassland to Miscanthus on soil carbon stocks after 12 years

Soil organic carbon (SOC) is an important carbon pool susceptible to land‐use change (LUC). There are concerns that converting grasslands into the C₄ bioenergy crop Miscanthus (to meet demands for renewable energy) could negatively impact SOC, resulting in reductions of greenhouse gas mitigation benefits gained from using Miscanthus as a fuel. This work addresses these concerns by sampling soils (0–30 cm) from a site 12 years (T₁₂) after conversion from marginal agricultural grassland into Miscanthus x giganteus and four other novel Miscanthus hybrids. Soil samples were analysed for changes in below‐ground biomass, SOC and Miscanthus contribution to SOC (using a ¹³C natural abundance approach). Findings are compared to ECOSSE soil carbon model results (run for a LUC from grassland to Miscanthus scenario and continued grassland counterfactual), and wider implications are considered in the context of life cycle assessments based on the heating value of the dry matter (DM) feedstock. The mean T₁₂ SOC stock at the site was 8 (±1 standard error) Mg C/ha lower than baseline time zero stocks (T₀), with assessment of the five individual hybrids showing that while all had lower SOC stock than at T₀ the difference was only significant for a single hybrid. Over the longer term, new Miscanthus C₄ carbon replaces pre‐existing C₃ carbon, though not at a high enough rate to completely offset losses by the end of year 12. At the end of simulated crop lifetime (15 years), the difference in SOC stocks between the two scenarios was 4 Mg C/ha (5 g CO₂‐eq/MJ). Including modelled LUC‐induced SOC loss, along with carbon costs relating to soil nitrous oxide emissions, doubled the greenhouse gas intensity of Miscanthus to give a total global warming potential of 10 g CO₂‐eq/MJ (180 kg CO₂‐eq/Mg DM).     

Decadally cycling soil carbon is more sensitive to warming than faster‐cycling soil carbon

The response of soil organic carbon (SOC) pools to globally rising surface temperature crucially determines the feedback between climate change and the global carbon cycle. However, there is a lack of studies investigating the temperature sensitivity of decomposition for decadally cycling SOC which is the main component of total soil carbon stock and the most relevant to global change. We tackled this issue using two decadally ¹³C‐labeled soils and a much improved measuring system in a long‐term incubation experiment. Results indicated that the temperature sensitivity of decomposition for decadally cycling SOC (>23 years in one soil and >55 years in the other soil) was significantly greater than that for faster‐cycling SOC (<23 or 55 years) or for the entire SOC stock. Moreover, decadally cycling SOC contributed substantially (35–59%) to the total CO₂ loss during the 360‐day incubation. Overall, these results indicate that the decomposition of decadally cycling SOC is highly sensitive to temperature change, which will likely make this large SOC stock vulnerable to loss by global warming in the 21st century and beyond.     

Effects of temperature,soil substrate,and microbial community on carbon mineralization across three climatically contrasting forest sites

How biotic and abiotic factors influence soil carbon (C) mineralization rate (R_S) has recently emerged as one of the focal interests in ecological studies. To determine the relative effects of temperature, soil substrate and microbial community on R_s, we conducted a laboratory experiment involving reciprocal microbial inoculations of three zonal forest soils, and measured R_S over a 61‐day period at three temperatures (5, 15, and 25°C). Results show that both R_s and the cumulative emission of C (R_cum), normalized to per unit soil organic C (SOC), were significantly affected by incubation temperature, soil substrate, microbial inoculum treatment, and their interactions (p < .05). Overall, the incubation temperature had the strongest effect on the R_S; at given temperatures, soil substrate, microbial inoculum treatment, and their interaction all significantly affected both R_s (p < .001) and R_cum (p ≤ .01), but the effect of soil substrate was much stronger than others. There was no consistent pattern of thermal adaptation in microbial decomposition of SOC in the reciprocal inoculations. Moreover, when different sources of microbial inocula were introduced to the same soil substrate, the microbial community structure converged with incubation without altering the overall soil enzyme activities; when different types of soil substrate were inoculated with the same sources of microbial inocula, both the microbial community structure and soil enzyme activities diverged. Overall, temperature plays a predominant role in affecting R_s and R_cum, while soil substrate determines the mineralizable SOC under given conditions. The role of microbial community in driving SOC mineralization is weaker than that of climate and soil substrate, because soil microbial community is both affected, and adapts to, climatic factors and soil matrix.     

A global meta‐analysis of soil organic carbon response to corn stover removal

Corn (Zea mays L.) stover is a global resource used for livestock, fuel, and bioenergy feedstock, but excessive stover removal can decrease soil organic C (SOC) stocks and deteriorate soil health. Many site‐specific stover removal experiments report accrual rates and SOC stock effects, but a quantitative, global synthesis is needed to provide a scientific base for long‐term energy policy decisions. We used 409 data points from 74 stover harvest experiments conducted around the world for a meta‐analysis and meta‐regression to quantify removal rate, tillage, soil texture, and soil sampling depth effects on SOC. Changes were quantified by: (a) comparing final SOC stock differences after at least 3 years with and without stover removal and (b) calculating SOC accrual rates for both treatments. Stover removal generally reduced final SOC stocks by 8% in the upper 0–15 or 0–30 cm, compared to stover retained, irrespective of soil properties and tillage practices. A more sensitive meta‐regression analysis showed that retention increased SOC stocks within the 30–150 cm depth by another 5%. Compared to baseline values, stover retention increased average SOC stocks temporally at a rate of 0.41 Mg C ha^?1 year^?1 (statistically significant at p < 0.01 when averaged across all soil layers). Although SOC sequestration rates were lower with stover removal, with moderate (<50%) removal they can be positive, thus emphasizing the importance of site‐specific management. Our results also showed that tillage effects on SOC stocks were inconsistent due to the high variability in practices used among the experimental sites. Finally, we conclude that research and technological efforts should continue to be given high priority because of the importance in providing science‐based policy recommendations for long‐term global carbon management.     

The effect of soil water content,soil temperature,soil pH-value and the root mass on soil CO<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript> efflux – A modified model

To quantify the effects of soil temperature (T_soil), and relative soil water content (RSWC) on soil respiration we measured CO₂ soil efflux with a closed dynamic chamber in situ in the field and from soil cores in a controlled climate chamber experiment. Additionally we analysed the effect of soil acidity and fine root mass in the field. The analysis was performed on three meadow, two bare fallow and one forest sites. The influence of soil temperature on CO₂ emissions was highly significant with all land-use types, except for one field campaign with continuous rain. Where soil temperature had a significant influence, the percentage of variance explained by soil temperature varied from site to site from 13–46% in the field and 35–66% in the climate chamber. Changes of soil moisture influenced only the CO₂ efflux on meadow soils in field and climate chamber (14–34% explained variance), whereas on the bare soil and the forest soil there was no visible effect. The spatial variation of soil CO₂ emission in the field correlated significantly with the soil pH and fine root mass, explaining up to 24% and 31% of the variability. A non-linear regression model was developed to describe soil CO₂ efflux as a function of soil temperature, soil moisture, pH-value and root mass. With the model we could explain 60% of the variability in soil CO₂ emission of all individual field chamber measurements. Through the model analysis we highlight the temporal influence of rain events. The model overestimated the observed fluxes during and within four hours of the last rain event. Conversely, after more than 72h without rain the model underestimated the fluxes. Between four and 72 h after rainfall, the regression model of soil CO₂ emission explained up to 91% of the variance.     

Effects of vegetation coverage on the spatial distribution of soil nematode trophic groups

The spatial variability of total soil nematodes and trophic groups in bare and fallow plots in Shenyang Experimental Station of Ecology, Chinese Academy of Sciences was examined using geostatistics combined with classic statistics. Results showed that the soil pH value had a negative effect on plant-parasites in both bare and fallow plots; the mean number of total nematodes was significantly higher in fallow plots than in bare plots, which was 1485.3 and 464.0 individuals per 100 g dry soil in fallow and bare plots, respectively; the nugget (C ₀)/sill (C ₀+C) ratio of total nematodes, plant-parasites and bacterivores were lower in fallow plots (27.3%–45.6%) than in bare plots (49.5%–100%); the spatial distribution of total nematodes and trophic groups was found to be different between fallow and bare plots, which indicated that vegetation coverage had an effect on soil nematodes. __________ Translated from Chinese Journal of Applied Ecology, 2006, 17(2): 295–299 [译自: 应用生态学报]     

Hierarchical saturation of soil carbon pools near a natural CO2 spring

Soil has been identified as a possible carbon (C) sink to mitigate increasing atmospheric CO₂ concentration. However, several recent studies have suggested that the potential of soil to sequester C is limited and that soil may become saturated with C under increasing CO₂ levels. To test this concept of soil C saturation, we studied a gley and organic soil at a grassland site near a natural CO₂ spring. Total and aggregate‐associated soil organic C (SOC) concentration showed a significant increase with atmospheric CO₂ concentration. An asymptotic function showed a better fit of SOC and aggregation with CO₂ level than a linear model. There was a shift in allocation of total C from smaller size fractions to the largest aggregate fraction with increasing CO₂ concentration. Litter inputs appeared to be positively related to CO₂ concentration. Based on modeled function parameters and the observed shift in the allocation of the soil C from small to large aggregate‐size classes, we postulate that there is a hierarchy in C saturation across different SOC pools. We conclude that the asymptotic response of SOC concentration at higher CO₂ levels indicates saturation of soil C pools, likely because of a limit to physical protection of SOC.     

Effects of straw carbon input on carbon dynamics in agricultural soils: a meta‐analysis

Straw return has been widely recommended as an environmentally friendly practice to manage carbon (C) sequestration in agricultural ecosystems. However, the overall trend and magnitude of changes in soil C in response to straw return remain uncertain. In this meta‐analysis, we calculated the response ratios of soil organic C (SOC) concentrations, greenhouse gases (GHGs) emission, nutrient contents and other important soil properties to straw addition in 176 published field studies. Our results indicated that straw return significantly increased SOC concentration by 12.8 ± 0.4% on average, with a 27.4 ± 1.4% to 56.6 ± 1.8% increase in soil active C fraction. CO₂ emission increased in both upland (27.8 ± 2.0%) and paddy systems (51.0 ± 2.0%), while CH₄ emission increased by 110.7 ± 1.2% only in rice paddies. N₂O emission has declined by 15.2 ± 1.1% in paddy soils but increased by 8.3 ± 2.5% in upland soils. Responses of macro‐aggregates and crop yield to straw return showed positively linear with increasing SOC concentration. Straw‐C input rate and clay content significantly affected the response of SOC. A significant positive relationship was found between annual SOC sequestered and duration, suggesting that soil C saturation would occur after 12 years under straw return. Overall, straw return was an effective means to improve SOC accumulation, soil quality, and crop yield. Straw return‐induced improvement of soil nutrient availability may favor crop growth, which can in turn increase ecosystem C input. Meanwhile, the analysis on net global warming potential (GWP) balance suggested that straw return increased C sink in upland soils but increased C source in paddy soils due to enhanced CH₄ emission. Our meta‐analysis suggested that future agro‐ecosystem models and cropland management should differentiate the effects of straw return on ecosystem C budget in upland and paddy soils.     

Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2‐enrichment

Increased partitioning of carbon (C) to fine roots under elevated [CO₂], especially deep in the soil profile, could alter soil C and nitrogen (N) cycling in forests. After more than 11 years of free‐air CO₂ enrichment in a Liquidambar styraciflua L. (sweetgum) plantation in Oak Ridge, TN, USA, greater inputs of fine roots resulted in the incorporation of new C (i.e., C with a depleted δ¹³C) into root‐derived particulate organic matter (POM) pools to 90‐cm depth. Even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated [CO₂] at the conclusion of the experiment. Greater C inputs from fine‐root detritus under elevated [CO₂] did not result in increased net N immobilization or C mineralization rates in long‐term laboratory incubations, possibly because microbial biomass was lower in the CO₂‐enriched plots. Furthermore, the δ¹³CO₂ of the C mineralized from the incubated soil closely tracked the δ¹³C of the labile POM pool in the elevated [CO₂] treatment, especially in shallower soil, and did not indicate significant priming of the decomposition of pre‐experiment soil organic matter (SOM). Although potential C mineralization rates were positively and linearly related to total SOM C content in the top 30 cm of soil, this relationship did not hold in deeper soil. Taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated [CO₂] may increase the potential for long‐term soil C storage. However, C in deeper soil is likely to take many years to accrue to a significant fraction of total soil C given relatively smaller root inputs at depth. Expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric [CO₂].     

Response of soil carbon dioxide fluxes,soil organic carbon and microbial biomass carbon to biochar amendment: a meta‐analysis

Biochar as a carbon‐rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta‐analysis procedures to examine responses of soil carbon dioxide (CO₂) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO₂ fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO₂ fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N‐fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land‐use types. Responses of soil CO₂ fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO₂ fluxes and MBC content, respectively. Both soil CO₂ fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO₂ fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO₂ flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1–9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land‐use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.     

Using CO2 Efflux Rates to Indicate Below-Ground Growing Seasons by Land-use Treatment

CO₂ efflux rates are affected by vegetation type, temperature, and soil surface conditions, and serve as an indicator of the length of the below-ground biological and microbial growing season. This study determined the effect of three land-use treatments on CO₂ efflux and growing season lengths in Southeast Virginia on two forested mineral soil wetlands. CO₂ efflux, soil temperature, and soil moisture were measured 24 times in 18 months at plots representing forest, early successional field, and bare ground land-use treatments. CO₂ efflux differed (p < 0.05) by treatment in the order forest > field > bare ground. CO₂ efflux was higher in hardwood- than conifer-dominated forest and higher in bare ground plots that were not inundated. Appreciable CO₂ efflux took place even once leaves had fallen off deciduous trees, and most of the CO₂ efflux appeared to be from vegetation rather than microbial sources during that period. Variability in CO₂ efflux was best described by the interaction between soil temperature and soil moisture (R² = 0.32) (p < 0.05). The below-ground growing season indicated by appreciable CO₂ efflux was similar to that indicated by soil temperatures above 5°C measured at 50 cm, the regulatory reference depth. The CO₂ efflux growing season was 365 days in the forest but was 9–16 days shorter in the field and 21–78 days shorter in the bare ground land-use treatment plots. These data can be used to modify the regulatory growing season definition in forested thermic wetlands and to reflect the environmental variation caused by different land uses.