Carbon quality and soil microbial property control the latitudinal pattern in temperature sensitivity of soil microbial respiration across Chinese forest ecosystems

Carbon and nitrogen addition-derived enzyme activities in topsoil but nitrogen availability in subsoil controls the response of soil organic carbon decomposition to warming

Subsoil stores the majority of soil organic carbon (SOC), and plays a vital role in the global carbon cycle in terrestrial ecosystems and in regulating climate change. Response of SOC decomposition to temperature warming (TR) is a crucial parameter to predict SOC dynamics under global warming. However, it remains unknown how TR varies across the whole soil profile and responds to exogenous C and N inputs. To assess this, we designed a novel incubation system to measure SOC-derived CO2 efflux across the whole soil column (i.e., 60 cm length), allowing manual addition of 13C-labeled glucose and ammonium nitrate, and incubated it under ambient or warmed temperatures (+4 °C). We found that C addition significantly increased TR in 0-20 cm, 20-40 cm and 40-60 cm by 64.3 %, 68.1 % and 57.2 %, respectively. However, the combined addition of C and N decreased TR by 11.1 % - 15.3 % compared to without anything addition (CK) in the whole soil profile. The effect of N on TR ranged from -22.8 % to -40.4 % in the whole soil profile, and was significantly lower in topsoil than in subsoil. Furthermore, sole N addition significantly promoted TR compared to CK by 79.0 % and 94.7 % in 20-40 cm and 40-60 cm subsoil, only 9.8 % in 0-20 cm topsoil. These results together suggested that TR is sensitive to increasing C availability in the whole soil profile and increasing N availability in 20-60 cm subsoil. Random forest model indicated that soil enzyme activities (explained 21.3 % of the variance) and DOC (explained 11.1 % of the variance) dominantly governed TR in topsoil, but N availability displayed a predominant control of TR in subsoil. Overall, our results suggested that increased C and N availability under climate warming scenarios could further increase the risk of carbon loss especially in subsoil with substrate deficiency, but labile C (e.g., root exudation) input under climate warming and N enrichment could reduce SOC decomposition and benefit for C sequestration by decreasing TR.

Greater influences of nitrogen addition on priming effect in forest subsoil than topsoil regardless of incubation warming

Subsoil (below 20 cm), storing over 50 % of soil organics carbon (SOC) within the 1 m depth, plays a critical role in regulating climate and ecosystem function. However, little was known on the changes in SOC decomposition induced by exogenous C input (i.e., priming effect) across the whole soil profile under nitrogen (N) enrichment and climate warming. We designed an incubation system of soil columns with minor physical disturbance, which allows the manual additions of exogenous C and N and incubation under ambient or elevated temperature. A negative priming effect by glucose was observed in all layers of ambient soil, while the negative priming effect was enhanced by soil depth but inhibited by warming. Nitrogen addition shifted the priming effect from negative to positive under ambient temperature, and decreased the magnitude of negative priming effect under elevated temperature. Nitrogen uplift effect on priming effect was more pronounced in subsoil compared to topsoil, while this effect diminished with rising temperature. Soil microbial activity (e.g., the CO2 production within 3 days) and acid phosphatase activity had important roles in regulating the variations in priming effect across the soil profile. Our results indicated that increase in labile substrate (e.g., exogenous C input) input would not lead to native SOC destabilization in subsoil, N addition shifted the priming effect from negative to positive, increasing the SOC decomposition under ambient temperature, while labile C input together with N addition benefited SOC sequestration by inducing negative priming effects in forest soil under warming climate.

Soil and stone terraces offset the negative impacts of sloping cultivation on soil microbial diversity and functioning by protecting soil carbon

Cultivation of sloping land is a main cause for soil erosion. Conservation practices, such as soil and stone terraces, may reduce the impacts of erosion but their impacts on soil microbial diversity and functioning related to carbon (C) and nutrient metabolisms remain unclear. This study was conducted to evaluate the effects of slope gradients (5°, 8°, 15°, 25°) and conservation practices (cultivated, uncultivated, soil terrace, and stone terrace) on bacterial and fungal diversities, metagenomic and metabolomic functioning associated with basic soil properties. Our results showed that steep slopes at 25° significantly decreased soil pH, silt percentage, and bacterial and fungal abundances, but that soil and stone terraces increased soil organic C (SOC), silt and clay contents, and fungal abundance compared to sloping cultivated lands. In addition, soil and stone terraces increased both bacterial and fungal alpha diversities, and relative abundances of Crenarchaeota, Nitrospirota, and Latescibacterota, but reduced the proportions of Actinobacteriota and Patescibacteria, thus shifting microbial beta diversities, which were significantly associated with increased SOC and silt content. For metagenomics, soil and stone terraces greatly increased the relative abundance of functional genes related to Respiration, Virulence, disease and defense, Stress response, and nitrogen and potassium metabolisms, such as Denitrification and Potassium homeostasis. For soil metabolomics, a total of 22 soil metabolites was enriched by soil and stone terraces, such as Lipids and lipid-like molecules (Arachidonic acid, Gamma-Linolenic acid, and Pentadecanoic acid), and Organoheterocyclic compounds (Adenine, Laudanosine, Methylpyrazine, and Nicotinic acid). To sum up, soil and stone terraces could reduce some of the negative impacts of steep slope cultivation on soil microbial diversity as well as their metagenomic and metabolomic functioning related to C and nutrient metabolism useful for soil health improvement, potentially bolstering the impact of sustainable practices in erosion hotspots around the world.

Direct Evidence for Microbial Regulation of the Temperature Sensitivity of Soil Carbon Decomposition

Soil physicochemical protection, substrates, and microorganisms are thought to modulate the temperature sensitivity of soil carbon decomposition (Q10), but their regulatory roles have yet to be distinguished because of the confounding effects of concurrent changes of them. Here, we sought to differentiate these effects through microorganism reciprocal transplant and aggregate disruption experiments using soils collected from seven sites along a 5000-km latitudinal transect encompassing a wide range of climatic conditions and from a 4-year laboratory incubation experiment. We found direct microbial regulation of Q10, with a higher Q10 being associated with greater fungal:bacterial ratios. However, no significant direct effects of physicochemical protection and substrate were observed on the variation in Q10 along the latitudinal transect or among different incubation time points. These findings highlight that we should move forward from physicochemical protection and substrate to microbial mechanisms regulating soil carbon decomposition temperature sensitivity to understand and better predict soil carbon-climate feedback.

Differential responses of temperature sensitivity of greenhouse gases emission to seasonal variations in plateau riparian zones

Riparian zones, regarded as hotspots for greenhouse gas (GHG) emissions, where the variation in temperature sensitivity (Q10) of GHG emissions is crucial for assessing GHG budgets under global warming. However, the seasonal Q10 of GHG emissions from high-elevation riparian zones and underlying microbial mechanisms are poorly documented. This study focuses on seasonal Q10 patterns of GHG emissions from riparian zones along the Lhasa River on the Tibetan Plateau. CO2 and CH4 emissions from riparian soils were more sensitive to temperature in spring than in summer. The opposite trend was observed for Q10 of N2O emissions. Soil organic carbon (SOC) had relatively large direct effects on the Q10-CO2 value in summer, whereas soil nitrate nitrogen (SNO3--N) was the determinant of Q10-CO2 value in spring. mcrA:pmoA and soil microbial biomass C (SMBC) had strong direct effects on the Q10 of CH4 emissions in summer; the Q10-CH4 value in spring was significantly affected by the mcrA abundance. SMBC and the nirK + nirS abundance were key factors affecting the Q10-N2O value. Q10-CO2 and Q10-CH4 values exhibited strong seasonalities in the lower reaches of riparian soils, mainly due to the seasonalities of SNO3--N and mcrA:pmoA, respectively. The Q10-N2O value in the middle and upper reaches of riparian soils presented seasonality, which was largely due to the seasonalities of soil ammonia nitrogen and microbial biomass carbon. During thawing, plant productivity increased, substrate carbon was sufficient, microbial biomass increased, and inorganic nitorgen and denitrifier abundance decreased, causing 29.67% and 37.47% decreases in the Q10-CO2 and Q10-CH4 values, respectively, and a 70.85% increase in the Q10-N2O value, indicating that the potential release of N2O from riparian zones along the plateau river was more susceptible to seasonal variations. Our findings are conducive to accurately evaluating the potential contribution of GHG emissions from high-elevation riparian zones to global warming.

Carbon availability and microbial activity manipulate the temperature sensitivity of anaerobic degradation in a paddy soil profile

Soil contains a substantial amount of organic carbon, and its feedback to global warming has garnered widespread attention due to its potential to modulate atmospheric carbon (C) storage. Temperature sensitivity (Q10) has been widely utilized as a measure of the temperature-induced enhancement in soil organic carbon (SOC) decomposition. It is currently rare to incorporate Q10 of CO2 and CH4 into the study of waterlogged soil profiles and explore the possibility of artificially reducing Q10 in rice fields. To investigate the key drivers of Q10, we collected 0-1 m paddy soil profiles, and stratified the soil for submerged anaerobic incubation. The relationship between SOC availability, microbial activity, and the Q10 of CO2 and CH4 emissions was examined. Our findings indicate that as the soil layer deepens, soil C availability and microbial activity declined, and the Q10 of anaerobic degradation increased. Warming increased C availability and microbial activity, accompanied by weakened temperature sensitivity. The Q10 of CO2 correlated strongly with soil resistant C components, while the Q10 of CH4 was significantly influenced by labile substrates. The temperature sensitivity of CH4 (Q10 = 3.99) was higher than CO2 emissions (Q10 = 1.78), indicating the need for greater attention of CH4 in predicting warming's impact on anaerobic degradation in rice fields. Comprehensively assessing CO2 and CH4 emissions, the 20-40 cm subsurface soil is the most temperature-sensitive. Despite being a high-risk area for C loss and CH4 emissions, management of this soil layer in agriculture has the potential to reduce the threat of global warming. This study underscores the importance of subsurface soil in paddy fields, advocating greater attention in scientific simulations and predictions of climate change.

New insights on ecological roles of waste activated sludge in nutrient-stressed co-digestion

There have been extensive applications of waste activated sludge (WAS) in anaerobic co-digestion (AcoD). Nonetheless, mechanisms through which AcoD systems maintain stability, particularly under nutrient-stressed conditions, are under-appreciated. In this study, the role of WAS in a nutrient-stressed WAS-food waste AcoD system was re-evaluated. Our findings demonstrated that WAS-based co-digestion increased methane production (by 20-60%) as WAS bolsters such systems' resilience via establishing a core niche-based microbial balance. The carbon utilization investigation suggested a microbial niche balance is attainable if two conditions are satisfied: 1) hydrolysis efficiency is greater than 50%; and 2) both the acidogenesis-to-hydrolysis and acetogenesis-to-hydrolysis efficiencies surpass 0.5. Metagenomic assembly genome (MAG) analysis indicated that the versatile metabolic characteristics strengthened the microbial niche balance, rendering the system resilient and efficient through a syntrophic mode, contributing to both acidogenesis and acetogenesis. The findings of this study provide new insights into the ecological effects of WAS on AcoD.

Identification of the soil physicochemical and bacterial indicators for soil organic carbon and nitrogen transformation under the wheat straw returning

Wheat straw returning is widely practiced in agriculture; therefore, it is critical to determine the physicochemical and bacterial indicators in soil for the organic carbon storage, accumulative C mineralization, total nitrogen improvement, and nitrogen mineralization in various soil types after wheat straw returning. This study evaluated the influenced indicators of wheat straw addition on soil organic carbon and nitrogen transformation in diverse soil types. For this purpose, an incubation experiment was conducted to analyze the carbon and nitrogen transformation in soil from eight Chinese provinces treated with the same dry weight of wheat straw. The results indicated that the primary physicochemical and bacterial indicators that predict the carbon and nitrogen transformations in the acidic and alkaline soils were different. Of all the natural physicochemical properties of soil, cation exchange capacity and clay content were significantly correlated with organic carbon, mineralized carbon, total nitrogen, and mineralized nitrogen in the alkaline soil. In the acidic soil, the initial C/N ratio of soil was the most significant indicator of carbon and nitrogen transformation. From the perspective of the carbon- and nitrogen-relating bacterial communities, Proteobacteria were largely responsible for the accumulative C mineralization in both types of soil. Furthermore, Proteobacteria strongly regulated the organic carbon storage in the acidic soil after wheat straw addition, whereas Gemmatimonadetes was the main predicted indicator in the alkaline soil. Additionally, total nitrogen and mineralized nitrogen levels were largely explained by Bifidobacterium and Luteimonas in the alkaline soil and by Nitrospira and Bdellovibrio in the acidic soil. Soil physicochemical and biological properties significantly influence soil carbon and nitrogen transformation, which should be considered crucial indicators to guide the rational regulation of straw return in several areas.

Key Role of Vegetation Cover in Alleviating Microplastic-Enhanced Carbon Emissions

Microplastics (MPs) are considered to influence fundamental biogeochemical processes, but the effects of plant residue-MP interactions on soil carbon turnover in urban greenspaces are virtually unknown. Here, an 84-day incubation experiment was constructed using four types of single-vegetation-covered soils (6 years), showing that polystyrene MP (PSMP) pollution caused an unexpectedly large increase in soil CO2 emissions. The additional CO2 originating from highly bioavailable active dissolved organic matter molecules (<380 °C, predominantly polysaccharides) was converted from persistent carbon (380-650 °C, predominantly aromatic compounds) rather than PSMP derivatives. However, the priming effect of PSMP derivatives was weakened in plant-driven soils (resistivity: shrub > tree > grass). This can be explained from two perspectives: (1) Plant residue-driven humification processes reduced the percentage of bioavailable active dissolved organic matter derived from the priming effects of PSMPs. (2) Plant residues accelerated bacterial community succession (dominated by plant residue types) but slowed fungal community demise (retained carbon turnover-related functional taxa), enabling specific enrichment of glycolysis, the citric acid cycle and the pentose phosphate pathway. These results provide a necessary theoretical basis to understand the role of plant residues in reducing PSMP harm at the ecological level and refresh knowledge about the importance of biodiversity for ecosystem stability.

Stronger compensatory thermal adaptation of soil microbial respiration with higher substrate availability

Ongoing global warming is expected to augment soil respiration by increasing the microbial activity, driving self-reinforcing feedback to climate change. However, the compensatory thermal adaptation of soil microorganisms and substrate depletion may weaken the effects of rising temperature on soil respiration. To test this hypothesis, we collected soils along a large-scale forest transect in eastern China spanning a natural temperature gradient, and we incubated the soils at different temperatures with or without substrate addition. We combined the exponential thermal response function and a data-driven model to study the interaction effect of thermal adaptation and substrate availability on microbial respiration and compared our results to those from two additional continental and global independent datasets. Modeled results suggested that the effect of thermal adaptation on microbial respiration was greater in areas with higher mean annual temperatures, which is consistent with the compensatory response to warming. In addition, the effect of thermal adaptation on microbial respiration was greater under substrate addition than under substrate depletion, which was also true for the independent datasets reanalyzed using our approach. Our results indicate that thermal adaptation in warmer regions could exert a more pronounced negative impact on microbial respiration when the substrate availability is abundant. These findings improve the body of knowledge on how substrate availability influences the soil microbial community-temperature interactions, which could improve estimates of projected soil carbon losses to the atmosphere through respiration.

Habitat-specific responses of soil organic matter decomposition to Spartina alterniflora invasion along China's coast

Plant invasions cause a fundamental change in soil organic matter (SOM) turnover. Disentangling the biogeographic patterns and key drivers of SOM decomposition and its temperature sensitivity (Q10 ) under plant invasion is a prerequisite for making projections of global carbon feedback. We collected soil samples along China's coast across saltmarshes to mangrove ecosystems invaded by the smooth cordgrass (Spartina alterniflora Loisel.). Microcosm experiments were carried out to determine the patterns of SOM decomposition and its thermal response. Soil microbial biomass and communities were also characterized accordingly. SOM decomposition constant dramatically decreased along the mean annual temperature gradient, whereas the cordgrass invasion retarded this change (significantly reduced slope, p < 0.05). The response of Q10 to invasion and the soil microbial quotient peaked at midlatitude saltmarshes, which can be explained by microbial metabolism strategies. Climatic variables showed strong negative controls on the Q10 , whereas dissolved carbon fraction exerted a positive influence on its spatial variance. Higher microbial diversity appeared to weaken the temperature-related response of SOM decomposition, with apparent benefits for carbon sequestration. Inconsistent responses to invasion were exhibited among habitat types, with SOM accumulation in saltmarshes but carbon loss in mangroves, which were explained, at least in part, by the SOM decomposition patterns under invasion. This study elucidates the geographic pattern of SOM decomposition and its temperature sensitivity in coastal ecosystems and underlines the importance of interactions between climate, soil, and microbiota for stabilizing SOM under plant invasion.

Temperature-Related N2O Emission and Emission Potential of Freshwater Sediment

Nitrous oxide (N2O) is a major radiative forcing and stratospheric ozone-depleting gas. Among natural sources, freshwater ecosystems are significant contributors to N2O. Although temperature is a key factor determining the N2O emissions, the respective effects of temperature on emitted and dissolved N2O in the water column of freshwater ecosystems remain unclear. In this study, 48 h incubation experiments were performed at three different temperatures; 15 °C, 25 °C, and 35 °C. For each sample, N2O emission, dissolved N2O in the overlying water and denitrification rates were measured, and N2O-related functional genes were quantified at regular intervals. The highest N2O emission was observed at an incubation of 35 °C, which was 1.5 to 2.1 factors higher than samples incubated at 25 °C and 15 °C. However, the highest level of dissolved N2O and estimated exchange flux of N2O were both observed at 25 °C and were both approximately 2 factors higher than those at 35 °C and 15 °C. The denitrification rates increased significantly during the incubation period, and samples at 25 °C and 35 °C exhibited much greater rates than those at 15 °C, which is in agreement with the N2O emission of the three incubation temperatures. The NO3− decreased in relation to the increase of N2O emissions, which confirms the dominant role of denitrification in N2O generation. Indeed, the nirK type denitrifier, which constitutes part of the denitrification process, dominated the nirS type involved in N2O generation, and the nosZ II type N2O reducer was more abundant than the nosZ I type. The results of the current study indicate that higher temperatures (35 °C) result in higher N2O emissions, but incubation at moderate temperatures (25 °C) causes higher levels of dissolved N2O, which represent a potential source of N2O emissions from freshwater ecosystems.

Evaluation of carbon mineralization and its temperature sensitivity in different soil aggregates and moisture regimes: A 21-year tillage experiment

Characterizing soil organic carbon (SOC) mineralization and its temperature sensitivity (Q₁₀) under different soil moisture in tillage systems is crucial for determining global carbon balance under climate warming and increasing precipitation. Aggregate protection can potentially govern SOC mineralization and its Q₁₀. However, how tillage and aggregate sizes affect SOC mineralization and its Q₁₀, especially under varying soil moisture, remains unclear. Soil samples (0-10 cm and 10-20 cm) were collected from a 21-year field study with four tillage treatments: conventional tillage (CT), reduced tillage (RT), no-tillage (NT), and subsoiling (SS). Bulk soil and dry-sieved aggregates were incubated at 15°C and 25°C at low, medium, and high moistures (i.e., 40%, 70%, and 100% water-holding capacity, respectively). Macro-aggregates (> 0.25 mm) had lower SOC mineralization relative to micro-aggregates (< 0.25 mm) across all soil temperatures, moistures, and depths (P < 0.01), which was attributed to their lower SOC quality (i.e., higher ratio of SOC to total nitrogen and lower ratio of dissolved organic carbon to SOC). Moreover, NT and SS promoted macro-aggregation relative to CT and RT, and thereby decreased mineralization (P < 0.001). However, Q₁₀ was higher in macro-aggregates than in micro-aggregates at low and medium moistures. The Q₁₀ was negatively correlated with the SOC quality in macro-aggregates (P < 0.05). The macroaggregate-associated SOC quality was lower under NT and SS than under CT and RT, which resulted in a greater Q₁₀ under NT and SS at low and medium moistures, suggesting that NT and SS may accelerate SOC losses under global warming. Furthermore, increased soil moisture could lower Q₁₀, and no differences among tillage practices were observed at high moisture levels (P > 0.05). Overall, our findings indicated that NT and SS decreased SOC mineralization but increased Q₁₀ because of their large amounts of macro-aggregates with low SOC quality, and the improvement of Q₁₀ was constrained by increasing soil moisture.

Soil organic carbon decomposition responding to warming under nitrogen addition across Chinese vegetable soils

Chemical fertilization in excess and warming disrupt the soil microbes and alter resource stoichiometry, particularly in intensive vegetable soils, while the effects of these variables on the temperature sensitivity of soil organic carbon (SOC) decomposition (Q₁₀) and SOC stability remain elusive. Thus, we collected six long-term vegetable soils along a climatic gradient to examine the microbial mechanisms and resource stoichiometry effects on fluctuations in Q₁₀ and SOC stability induced by warming and fertilization from vegetable soils. Our results showed that the SOC decomposition was dominated by microbes and regulated by stoichiometry. Compared to cold sites, higher Q₁₀ of SOC decomposition was observed in warm sites, accompanied by lower enzyme activities, microbial CUE, and C:N ratio. In this context, warming reduced SOC stability as evidenced by up to 31.8% greater Q₁₀ (1.45) at warm sites than at cold sites (1.10) owing to less richness of microbial communities and lower microbial CUE. The relatively lower pH and labile organic C value restricted the development of microbial richness, and decreased C- and N-related enzyme activities and a lower C:N ratio resulted in microbial CUE reduction. Additionally, N fertilization altered the C:N imbalance and enhanced SOC stability in vegetable soils, exhibiting an increase of Q₁₀ values, particularly of great importance in warm sites. Collectively, our findings emphasize the importance of the microbial mechanism and resource stoichiometry in predicting variations in Q₁₀ and fluctuations in SOC stability, and provide theoretical advice on improving management policies in the context of warming and fertilization from vegetable soils.

Low carbon availability in paleosols nonlinearly attenuates temperature sensitivity of soil organic matter decomposition

Temperature sensitivity (Q₁₀ ) of soil organic matter (SOM) decomposition is an important parameter in models of the global carbon (C) cycle. Previous studies have suggested that substrate quality controls the intrinsic Q₁₀ , whereas environmental factors can impose large constraints. For example, physical protection of SOM and its association with minerals attenuate the apparent Q₁₀ through reducing substrate availability and accessibility ([S]). The magnitude of this dampening effect, however, has never been quantified. We simulated theoretical Q₁₀ changes across a wide range of [S] and found that the relationship between Q₁₀ and the log₁₀ -transformed [S] followed a logistic rather than a linear function. Based on the unique Holocene paleosol chronosequence (7 soils from ca. 500 to 6900 years old), we demonstrated that the Q₁₀ decreased nonlinearly with soil age up to 1150 years, beyond which Q₁₀ remained stable. Hierarchical partitioning analysis indicated that an integrated C availability index, derived from principal component analysis of DOC content and parameters reflecting physical protection and mineral association, was the main explanatory variable for the nonlinear decrease of Q₁₀ with soil age. Microbial inoculation and ¹³ C-labelled glucose addition showed that low C availability induced by physical protection and minerals association attenuated Q₁₀ along the chronosequence. A separate soil incubation experiment indicated that Q₁₀ increased exponentially with activation energy (E_a ) in the modern soil, suggesting that SOM chemical complexity regulates Q₁₀ only when C availability is high. In conclusion, organic matter availability strongly decreased with soil age, whereas Michelis-Menten kinetics defines the Q₁₀ response depending on C availability, but Arrhenius equation describes the effects of increasing substrate complexity.

Microbial assemblies associated with temperature sensitivity of soil respiration along an altitudinal gradient

Identifying the drivers of the response of soil microbial respiration to warming is integral to accurately forecasting the carbon-climate feedbacks in terrestrial ecosystems. Microorganisms are the fundamental drivers of soil microbial respiration and its response to warming; however, the specific microbial communities and properties involved in the process remain largely undetermined. Here, we identified the associations between microbial community and temperature sensitivity (Q₁₀) of soil microbial respiration in alpine forests along an altitudinal gradient (from 2974 to 3558 m) from the climate-sensitive Tibetan Plateau. Our results showed that changes in microbial community composition accounted for more variations of Q₁₀ values than a wide range of other factors, including soil pH, moisture, substrate quantity and quality, microbial biomass, diversity and enzyme activities. Specifically, co-occurring microbial assemblies (i.e., ecological clusters or modules) targeting labile carbon consumption were negatively correlated with Q₁₀ of soil microbial respiration, whereas microbial assemblies associated with recalcitrant carbon decomposition were positively correlated with Q₁₀ of soil microbial respiration. Furthermore, there were progressive shifts of microbial assemblies from labile to recalcitrant carbon consumption along the altitudinal gradient, supporting relatively high Q₁₀ values in high-altitude regions. Our results provide new insights into the link between changes in major microbial assemblies with different trophic strategies and Q₁₀ of soil microbial respiration along an altitudinal gradient, highlighting that warming could have stronger effects on microbially-mediated soil organic matter decomposition in high-altitude regions than previously thought.

Highly spatial variation of soil microbial respiration and temperature sensitivity in a subtropical forest

Quantifying the spatial variation and drivers of microbe-driven soil carbon (C) decomposition (also called soil microbial respiration, MR) and its temperature sensitivity (Q₁₀) is crucial for reducing the uncertainty in modelling the terrestrial C cycle under global warming. To this end, most previous studies sampled soils from multiple sites at regional scales and incubated them at the same temperature level in the laboratory. However, this unified incubation temperature is too warm to the cold sites, and too cold to the warm sites, thus causing a large bias in the MR and Q₁₀ estimations. Here, we conducted fine scale intensive sampling (194 soil samples) and measurements within a 4-ha subtropical forest plot to examine the underlying mechanisms driving the spatial pattern of MR and Q₁₀. Our results showed that both MR and Q₁₀ varied spatially within subtropical forests. The fine scale variation of MR was dominated by soil nitrogen concentration and slope position, and Q₁₀ was dominated by soil fungi abundance. Overall, the 35 investigated biotic and abiotic factors explained 38% of the spatial variation of MR and 9% of the spatial variation of Q₁₀ in the subtropical forest. This suggests that the fine scale variation of soil C dynamics is much more complex than that at the regional scale reported in previous studies, which should be considered in the assessments of terrestrial soil C cycles.

Conversion effects of farmland to Zanthoxylum bungeanum plantations on soil organic carbon mineralization in the arid valley of the upper reaches of Yangtze River, China

Farmland conversion to forest is considered to be one of the effective measures to mitigate climate change. However, the impact of farmland conversion to forest land or grassland on soil CO2 emission in arid areas is unclear due to the lack of comparative information on soil organic carbon (SOC) mineralization of different conversion types. The SOC mineralization in 0-100 cm soil layer in farmland (FL), abandoned land (AL) and different ages (including 8, 15, 20 and 28 years) of Zanthoxylum bungeanum plantations were measured by laboratory incubation. The size and decomposition rate of fast pool (Cf) and slow pool (Cs) in different land-use types and soil layers were estimated by double exponential model. The results showed that: 1) Farmland conversion increased the cumulative CO2-C release (Cmin) and SOC mineralization efficiency, and those indexes in AL were higher than that in Z. bungeanum plantations. The Cmin and SOC mineralization efficiency of 0-100 cm soil increased with the ages of Z. bungeanum plantation. Both Cmin and SOC mineralization efficiency decreased with the increase of soil depth; 2) Both soil Cf and Cs increased after farmland converted to Z. bungeanum plantations and AL. The Cs in the same soil layer increased with the ages of Z. bungeanum plantation, and the Cf showed a "V" type with the increased ages of Z. bungeanum plantation. The Cf and Cs decreased with the increase of soil depth in all land-use types; 3) Farmland conversion increased the decomposition rate of Cf (k1) in all soil layer by 0.008-0.143 d-1 and 0.082-0.148 d-1 in Z. bungeanum plantations and AL, respectively. The k1 was obviously higher in the 0-20 cm soil layer than that in other soil layers, while the decomposition rate of Cs (k2) was not affected by FL conversion and soil depth; and 4) The initial soil chemical properties and enzyme activity affected SOC mineralization, especially the concentrations of total organic nitrogen (TON), SOC, easily oxidizable organic carbon (EOC) and microbial biomass carbon (MBC). It indicated that the conversion of farmland to Z. bungeanum plantations and AL increases SOC mineralization, especially in deeper soils, and it increased with the ages. The conversion of farmland to Z. bungeanum plantation is the optimal measure when the potential C sequestration of plant-soil system were taken in consideration.

Phosphorus Limitation of Trees Influences Forest Soil Fungal Diversity in China

Fungal-biogeography studies have shown global patterns of biotic interactions on microbial biogeography. However, the mechanisms underlying these patterns remain relatively unexplored. To determine the dominant factors affecting forest soil fungal diversity in China, soil and leaves from 33 mountain forest reserves were sampled, and their properties were measured. We tested three hypotheses and established the most realistic one for China. The results showed that the soil fungal diversity (Shannon index) varied unimodally with latitude. The relative abundance of ectomycorrhizae was significantly positively correlated with the leaf nitrogen/phosphorus. The effects of soil available phosphorus and pH on fungal diversity depended on the ectomycorrhizal fungi, and the fungal diversity shifted by 93% due to available phosphorus, potassium, and pH. Therefore, we concluded that latitudinal changes in temperature and the variations in interactions between different fungal guilds (ectomycorrhizal, saprotrophic, and plant pathogenic fungi) did not have a major influence. Forest soil fungal diversity was affected by soil pH, available phosphorus, and potassium, which are driven by the phosphorus limitation of trees.

Soil microbial trait-based strategies drive metabolic efficiency along an altitude gradient

Trait-based approaches provide a candidate framework for linking soil microbial community to ecosystem processes, yet how the trade-offs in different microbial traits regulate the community-level metabolic efficiency remains unknown. Herein we assessed the roles of the microbial taxa with particular trait strategies in mediating soil microbial metabolic efficiency along an altitude gradient on the Tibetan Plateau. Results showed that soil microbial metabolic efficiency declined with increasing altitude, as indicated by the increasing metabolic quotient (microbial respiration per unit biomass, qCO2) and decreasing carbon use efficiency (CUE). Both qCO2 and CUE were predominantly predicted by microbial physiological and taxonomic attributes after considering key environmental factors including soil pH, substrate quantity and quality. Specifically, the reduced metabolic efficiency was associated with higher investment into nutrient (particularly for phosphorus) acquisitions via enzymes. Furthermore, we identified key microbial assemblies selected by harsh environments (low substrate quality and temperature) as important predictors of metabolic efficiency. These results suggest that particular microbial assemblies adapted to nutrient limited and cold habitats, but at the expense of lower metabolic efficient at higher altitude. Our findings provide a candidate mechanism underlying community-level metabolic efficiency, which has important implications for microbial-mediated processes such as carbon dynamics under global climate changes.

Temperature sensitivity of SOM decomposition is linked with a K-selected microbial community

Temperature sensitivity (Q₁₀ ) of soil organic matter (SOM) decomposition is a crucial parameter to predict the fate of soil carbon (C) under global warming. Nonetheless, the response pattern of Q₁₀ to continuous warming and the underlying mechanisms are still under debate, especially considering the complex interactions between Q₁₀ , SOM quality, and soil microorganisms. We examined the Q₁₀ of SOM decomposition across a mean annual temperature (MAT) gradient from -1.9 to 5.1°C in temperate mixed forest ecosystems in parallel with SOM quality and bioavailability, microbial taxonomic composition, and functional genes responsible for organic carbon decomposition. Within this temperature gradient of 7.0°C, the Q₁₀ values increased with MAT, but decreased with SOM bioavailability. The Q₁₀ values increased with the prevalence of K-strategy of soil microbial community, which was characterized by: (i) high ratios of oligotrophic to copiotrophic taxa, (ii) ectomycorrhizal to saprotrophic fungi, (iii) functional genes responsible for degradation of recalcitrant to that of labile C, and (iv) low average 16S rRNA operon copy number. Because the recalcitrant organic matter was mainly utilized by the K-strategists, these findings independently support the carbon quality-temperature theory from the perspective of microbial taxonomic composition and functions. A year-long incubation experiment was performed to determine the response of labile and recalcitrant C pools to warming based on the two-pool model. The decomposition of recalcitrant SOM was more sensitive to increased temperature in southern warm regions, which might attribute to the dominance of K-selected microbial communities. It implies that climate warming would mobilize the larger recalcitrant pools in warm regions, exacerbating the positive feedback between increased MAT and CO₂ efflux. This is the first attempt to link temperature sensitivity of SOM decomposition with microbial eco-strategies by incorporating the genetic information and disentangling the complex relationship between Q₁₀ and soil microorganisms.

T4-like Phages Reveal the Potential Role of Viruses in Soil Organic Matter Mineralization

Viruses are the most abundant biological entities in the world, but their ecological functions in soil are virtually unknown. We hypothesized that greater abundance of T4-like phages will increase bacterial death and thereby suppress soil organic carbon (SOC) mineralization. A range of phage and bacterial abundances were established in sterilized soil by reinoculation with 10^-3 and 10^-6 dilutions of suspensions of unsterilized soil. The total and viable 16S rRNA gene abundance (a universal marker for bacteria) was measured by qPCR to determine bacterial abundance, with propidium monoazide (PMA) preapplication to eliminate DNA from non-viable cells. Abundance of the g23 marker gene was used to quantify T4-like phages. A close negative correlation between g23 abundance and viable 16S rRNA gene abundance was observed. High abundance of g23 led to lower viable ratios for bacteria, which suggested that phages drove microbial necromass production. The CO₂ efflux from soil increased with bacterial abundance but decreased with higher abundance of T4-like phages. Elimination of extracellular DNA by PMA strengthened the relationship between CO₂ efflux and bacterial abundance, suggesting that SOC mineralization by bacteria is strongly reduced by the T4-like phages. A random forest model revealed that abundance of T4-like phages and the abundance ratio of T4-like phages to bacteria are better predictors of SOC mineralization (measured as CO₂ efflux) than bacterial abundance. Our study provides experimental evidence of phages' role in organic matter turnover in soil: they can retard SOC decomposition but accelerate bacterial turnover.

High microbial diversity stabilizes the responses of soil organic carbon decomposition to warming in the subsoil on the Tibetan Plateau

Soil microbes are directly involved in soil organic carbon (SOC) decomposition, yet the importance of microbial biodiversity in regulating the temperature sensitivity of SOC decomposition remains elusive, particularly in alpine regions where climate change is predicted to strongly affect SOC dynamics and ecosystem stability. Here we collected topsoil and subsoil samples along an elevational gradient on the southeastern Tibetan Plateau to explore the temperature sensitivity (Q₁₀ ) of SOC decomposition in relation to changes in microbial communities. Specifically, we tested whether the decomposition of SOC would be more sensitive to warming when microbial diversity is low. The estimated Q₁₀ value ranged from 1.28 to 1.68, and 1.80 to 2.10 in the topsoil and subsoil, respectively. The highest Q₁₀ value was observed at the lowest altitude of forests in the topsoil, and at the highest altitude of alpine meadow in the subsoil. Variations in Q₁₀ were closely related to changes in microbial properties. In the topsoil the ratio of gram-positive to gram-negative bacteria (G+:G-) was the predominant factor associated with the altitudinal variations in Q₁₀ . In the subsoil, SOC decomposition showed more resilience to warming when the diversity of soil bacteria (both whole community and major groups) and fungi was higher. Our results partly support the positive biodiversity-ecosystem stability hypothesis. Structural equation modeling further indicates that variations in Q₁₀ in the subsoil were directly related to changes in microbial diversity and community composition, which were affected by soil pH. Collectively our results provide compelling evidence that microbial biodiversity plays an important role in stabilizing SOC decomposition in the subsoil of alpine montane ecosystems. Conservation of belowground biodiversity is therefore of great importance in maintaining the stability of ecosystem processes under climate change in high-elevation regions of the Tibetan Plateau.

Temperature adaptation of soil microbial respiration in alpine, boreal and tropical soils: An application of the square root (Ratkowsky) model

Warming is expected to stimulate soil microbial respiration triggering a positive soil carbon-climate feedback loop while a consensus remains elusive regarding the magnitude of this feedback. This is partly due to our limited understanding of the temperature-adaptive response of soil microbial respiration, especially over broad climatic scales. We used the square root (Ratkowsky) model to calculate the minimum temperature for soil microbial respiration (T_min , which describes the temperature adaptation of soil microbial respiration) of 298 soil samples from alpine grasslands on the Tibetan Plateau and forest ecosystems across China with a mean annual temperature (MAT) range from -6°C to +25°C. The instantaneous soil microbial respiration was determined between 4°C and 28°C. The square root model could well fit the temperature effect on soil microbial respiration for each individual soil, with R² higher than 0.98 for all soils. T_min ranged from -8.1°C to -0.1°C and increased linearly with increasing MAT (R²  = 0.68). MAT dominantly regulated T_min variation when accounting simultaneously for multiple other drivers (mean annual precipitation, soil pH and carbon quality); an independent experiment showed that carbon availability had no significant effect on T_min . Using the relationship between T_min and MAT, soil microbial respiration after an increased MAT could be estimated, resulting in a relative increase in respiration with decreasing MAT. Thus, soil microbial respiration responses are adapted to long-term temperature differences in MAT. We suggest that T_min  = -5 + 0.2 × MAT, that is, every 1°C rise in MAT is estimated to increase T_min of respiration by approximately 0.2°C, could be used as a first approximation to incorporate temperature adaptation of soil microbial respiration in model predictions. Our results can be used to predict future changes in the response of soil microbial respiration to temperature over different levels of warming and across broad geographic scales with different MAT.

Effects of natural vegetation restoration on dissolved organic matter (DOM) biodegradability and its temperature sensitivity

Biodegradation of dissolved organic matter (DOM) plays a key role in regulating both production of greenhouse gases and accumulation and stabilisation of soil organic matter (SOM). However, the mechanisms by which natural vegetation restoration affects the extent, rate, and temperature sensitivity of DOM biodegradation are poorly understood. Elucidating these mechanisms is important for SOM management, especially in light of future climate warming scenarios. In this study, a laboratory DOM solution incubation experiment was conducted to comprehensively investigate the effects of temperature and natural vegetation restoration spanning a period of 160 y on DOM biodegradation in the Loess Plateau, China. The results indicated that dissolved organic C (DOC) biodegradation significantly decreased with vegetation restoration after an incubation period of 60 d. Further, biodegradation of dissolved organic N (DON) and dissolved organic P (DOP) significantly decreased after farmland abandonment. Specifically, the lowest values were observed in pioneer (Populus davidiana) and mingled (Populus davidiana and Quercus liaotungensis) forests. Generally, an increase in temperature significantly promoted the biodegradation of DOC, DON, and DOP by enhancing the microbial utilisation efficiencies of recalcitrant humic substrates (i.e., low-molecular-weight humic materials). Our results suggest that DOM biodegradability and its temperature sensitivity were regulated by DOM substrate quality (i.e, recalcitrant humic materials), and microbial properties (i.e., gram-negative bacterial and fungal PLFA, enzyme activities). Additionally, our results suggest that climax forest communities (Quercus liaotungensis) played a vital role in reducing DOC and DOP losses. This could be attributed to the low Q10 of the DOC and DOP biodegradation rates.

Rising Temperature May Trigger Deep Soil Carbon Loss Across Forest Ecosystems

Significantly more carbon (C) is stored in deep soil than in shallow horizons, yet how the decomposition of deep soil organic C (SOC) will respond to rising temperature remains unexplored on large scales, leading to considerable uncertainties to predictions of the magnitude and direction of C-cycle feedbacks to climate change. Herein, short-term temperature sensitivity of SOC decomposition (expressed as Q 10) from six depths within the top 1 m soil from 90 upland forest sites (540 soil samples) across China is reported. Results show that Q 10 significantly increases with soil depth, suggesting that deep SOC is more vulnerable to loss with rising temperature in comparison to shallow SOC. Climate is the primary regulator of shallow soil Q 10 but its relative influence declines with depth; in contrast, soil C quality has a minor influence on Q 10 in shallow soil but increases its influence with depth. When considering the depth-dependent Q 10 variations, results further show that using the thermal response of shallow soil layer for the whole soil profile, as is usually done in model predictions, would significantly underestimate soil C-climate feedbacks. The results highlight that Earth system models need to consider multilayer soil C dynamics and their controls to improve prediction accuracy.

Topography and plant community structure contribute to spatial heterogeneity of soil respiration in a subtropical forest

Soil respiration is the largest carbon (C) flux from terrestrial ecosystems into the atmosphere. Accurate estimates of the magnitude and distribution of soil respiration are critically important to models of global C cycling and predictions of future climate change. One of the greatest challenges to accurate large-scale estimation of soil respiration is its great spatial heterogeneity at the site level. Our study explored how soil respiration varies in space and the drivers that lead to this variance in a natural subtropical evergreen broadleaf forest in Southern China. We conducted a two-year soil respiration measurement for 168 randomly selected sampling points in a 4 ha plot. We measured the spatial variance of soil respiration and tested its correlation to a variety of abiotic and biotic factors including topography, aboveground plant community structure, soil environmental factors, soil organic matter, and microbial community structure. We found that soil respiration was highly varied across the study plot, with a spatial variation coefficient (CV) of 32.75%. The structural equation modeling (SEM) analysis showed that elevation influenced tree species diversity, productivity, and soil water content, which in turn affected soil respiration via soil C content, clay content, fungal:bacterial ratio, annual litterfall, and fine root biomass. 31% of the total spatial variation of soil respiration was accounted for in the SEM, mostly by elevation, soil C content, annual litterfall biomass, tree species diversity as estimated by the Simpson's index, and soil water content, with standardized total effects of 0.31, -0.31, 0.29, 0.19, and -0.18, respectively. Our data demonstrated that soil respiration was highly spatially varied at the fine scale, and was primarily regulated by factors of topography and plant community structure. More studies investigating the spatial variation of soil respiration are therefore needed to better understand and assess terrestrial ecosystem C cycling.

Blind spots in global soil biodiversity and ecosystem function research

Soils harbor a substantial fraction of the world's biodiversity, contributing to many crucial ecosystem functions. It is thus essential to identify general macroecological patterns related to the distribution and functioning of soil organisms to support their conservation and consideration by governance. These macroecological analyses need to represent the diversity of environmental conditions that can be found worldwide. Here we identify and characterize existing environmental gaps in soil taxa and ecosystem functioning data across soil macroecological studies and 17,186 sampling sites across the globe. These data gaps include important spatial, environmental, taxonomic, and functional gaps, and an almost complete absence of temporally explicit data. We also identify the limitations of soil macroecological studies to explore general patterns in soil biodiversity-ecosystem functioning relationships, with only 0.3% of all sampling sites having both information about biodiversity and function, although with different taxonomic groups and functions at each site. Based on this information, we provide clear priorities to support and expand soil macroecological research.

Biogeographic variation in temperature sensitivity of decomposition in forest soils

Determining soil carbon (C) responses to rising temperature is critical for projections of the feedbacks between terrestrial ecosystems, C cycle, and climate change. However, the direction and magnitude of this feedback remain highly uncertain due largely to our limited understanding of the spatial heterogeneity of soil C decomposition and its temperature sensitivity. Here we quantified C decomposition and its response to temperature change with an incubation study of soils from 203 sites across tropical to boreal forests in China spanning a wide range of latitudes (18°16' to 51°37'N) and longitudes (81°01' to 129°28'E). Mean annual temperature (MAT) and mean annual precipitation primarily explained the biogeographic variation in the decomposition rate and temperature sensitivity of soils: soil C decomposition rate decreased from warm and wet forests to cold and dry forests, while Q_10-MAT (standardized to the MAT of each site) values displayed the opposite pattern. In contrast, biological factors (i.e. plant productivity and soil bacterial diversity) and soil factors (e.g. clay, pH, and C availability of microbial biomass C and dissolved organic C) played relatively small roles in the biogeographic patterns. Moreover, no significant relationship was found between Q_10-MAT and soil C quality, challenging the current C quality-temperature hypothesis. Using a single, fixed Q_10-MAT value (the mean across all forests), as is usually done in model predictions, would bias the estimated soil CO₂ emissions at a temperature increase of 3.0°C. This would lead to overestimation of emissions in warm biomes, underestimation in cold biomes, and likely significant overestimation of overall C release from soil to the atmosphere. Our results highlight that climate-related biogeographic variation in soil C responses to temperature needs to be included in next-generation C cycle models to improve predictions of C-climate feedbacks.

Watershed geomorphology modifies the sensitivity of aquatic ecosystem metabolism to temperature

The regulation of aquatic carbon cycles by temperature is a significant uncertainty in our understanding of how watersheds will respond to climate change. Aquatic ecosystems transport substantial quantities of carbon to the atmosphere and ocean, yet we have limited understanding of how temperature modifies aquatic ecosystem metabolic processes and contributions to carbon cycles at watershed to global scales. We propose that geomorphology controls the distribution and quality of organic material that forms the metabolic base of aquatic ecosystems, thereby controlling the response of aquatic ecosystem metabolism to temperature across landscapes. Across 23 streams and four years during summer baseflow, we estimated variation in the temperature sensitivity of ecosystem respiration (R) among streams draining watersheds with different geomorphic characteristics across a boreal river basin. We found that geomorphic features imposed strong controls on temperature sensitivity; R in streams draining flat watersheds was up to six times more temperature sensitive than streams draining steeper watersheds. Further, our results show that this association between watershed geomorphology and temperature sensitivity of R was linked to the carbon quality of substrates that changed systematically across the geomorphic gradient. This suggests that geomorphology will control how carbon is transported, stored, and incorporated into river food webs as the climate warms.

Temperature sensitivity of soil respiration across multiple time scales in a temperate plantation forest

Soil respiration (Rs) is the largest carbon (C) flux from terrestrial ecosystems to the atmosphere. Predictions of Rs and associated feedback to climate change remain largely uncertain, in part due to the high temporal heterogeneity of temperature sensitivity (apparent Q₁₀) of Rs under a changing climate. Therefore, it is of critical importance to provide better insight into how Q₁₀ varies across multiple temporal scales. We investigated the diurnal, seasonal, and annual variabilities in the Q₁₀ of Rs using continuous Rs measurements (at hourly intervals) over six growing seasons in a mature temperate larch plantation in North China. We found that night-time values of Q₁₀ were slightly lower than daytime values. Large seasonal and annual fluctuations of Q₁₀ were observed, as illustrated by high coefficients of variation of 15.0% and 21.8%, respectively. The higher Q₁₀ in spring and autumn were primarily regulated by fine root growth and higher soil moisture after snow melt in spring, and leaf senescence in autumn. Lower Q₁₀ in summer may have been caused by limitations in substrate availability and microbial activity resulting from drought, which also caused a decoupling of Rs from soil temperature in summer. Furthermore, a bivariate nonlinear model incorporating both soil temperature and soil moisture best explained Q₁₀ variability. Generally, lower soil temperature and higher soil moisture lead to higher values of Q₁₀, indicating that climate warming could exert a negative effect on Q₁₀, partially offsetting the warming-induced increase in soil C loss. We provide long-term field experimental evidence that it would be inappropriate to estimate Rs on a multiyear scale using a fixed Q₁₀ value or a value obtained from one season and/or one year. Thus, we emphasize the importance of incorporating the seasonal and annual heterogeneities of Q₁₀ into C cycle model simulations under future climate change scenarios.

An incubation study of temperature sensitivity of greenhouse gas fluxes in three land-cover types near Sydney, Australia

Greenhouse gas (GHG) fluxes play crucial roles in regulating the Earth surface temperature. However, our understanding of the effect of land-cover and soil depth on the potential GHG fluxes and their temperature sensitivities (Q₁₀) is limited, which consequently increases the uncertainty to predict GHG exchange between soils and the atmosphere. In the present study, we sampled soils with contrasting characteristics from three land-cover types (wetland, grassland, and forest) and soil depths (0-10, 10-20, and 20-30 cm) from the Cumberland Plain near Sydney, Australia, and incubated at optimal (60%) water holding capacity at three temperatures (15, 25, and 35 °C). Overall, GHG fluxes and Q₁₀ values differed significantly among land-cover types and soil depths. CO₂ and N₂O emissions were highest in wetland followed by grassland and forest soils, and they decreased with soil depth. In contrast, CH₄ uptake was highest in grassland followed by forest and wetland soils, and it increased with soil depth. Combining the three major GHGs, the global warming potential in soil from wetland was higher than that from grassland and forest. Moreover, Q₁₀ values of CO₂ and N₂O emissions were: wetland > grassland > forest, while Q₁₀ value of CH₄ uptake showed the opposite pattern. Q₁₀ values of CO₂ and N₂O emissions and CH₄ uptake all increased with soil depth, demonstrating that subsoil has a higher potential for CO₂ and N₂O emissions and CH₄ uptake in a warming climate. While these experiments were conducted under ideally controlled laboratory conditions, results suggest that the large carbon stocks in wetland soils are vulnerable to loss and thus may amplify climate warming; upland soils are crucial CH₄ sinks and thus potentially mitigate climate change. In addition, the greater temperature sensitivities of CO₂ and N₂O emissions and CH₄ uptake in subsoil should be accounted for in carbon and nitrogen cycling models.

The sensitivity of soil microbial respiration declined due to crop straw addition but did not depend on the type of crop straw

An incubation experiment was conducted to investigate whether the type of crop straw added to soil influenced the temperature sensitivity of soil microbial respiration. The soil for incubation was collected from a winter wheat-soybean rotation cropland. Five temperature levels (5, 10, 15, 20, and 25 °C), five crop straw types (soybean, peanut, rice, winter wheat, and maize), and a control (CK, no crop straw addition) were established. Soil microbial respiration rates were measured on days 1, 2, 3, 5, 7, 10, 14, 20, and 27 after crop straw addition using an infrared CO₂ analyser. Soil enzyme activities of invertase, urea, and catalase and the dissolved organic carbon (DOC) content were measured after incubation. Estimated Q₁₀ (temperature sensitivity of soil microbial respiration) ranged from 1.472 ± 0.045 to 1.970 ± 0.020 and showed no significant (P > 0.05) difference between straw addition treatments, but there was significantly (P < 0.001) higher temperature sensitivity (1.970 ± 0.020) for CK. A significant (P = 0.002) relationship was found between the Q₁₀ of cumulative soil microbial respiration and basal soil microbial respiration (soil microbial respiration at 0 °C). Moreover, a marginally significant (P < 0.1) relationship was found between the Q₁₀ at different incubation stages and basal soil microbial respiration. A quadratic function was used to explain the relationship between estimated basal microbial respiration and the lignin content. Soil microbial respiration was positively correlated with the activities of invertase, urease, and catalase and the dissolved organic carbon (DOC) content in all treatments. This study indicated that crop straw addition significantly (P < 0.001) reduced the Q₁₀ of soil microbial respiration and that the types of crop straw added to soil did not significantly (P > 0.05) change the Q₁₀ value.

High resolution characterisation of E. coli proliferation profiles in livestock faeces

Agricultural intensification can lead to high volumes of livestock faeces being applied to land, either as solid or liquid manures or via direct defecation, and can result in reservoirs of faecal indicator organisms (FIOs) persisting within farmland. Understanding the survival of FIOs, e.g. E. coli, in agricultural environments, and in particular within different livestock faeces, is key to developing catchment management practices for the protection of ecosystem services provided by clean water. Frequently, controlled laboratory studies, under constant temperature regimes, are used to determine the impact of environmental factors on E. coli persistence in livestock faeces; however, such studies oversimplify the diurnal variations and interactions of real world conditions. The aim of this study was to investigate the survival of E. coli using a controlled environment facility, which simulated diurnal variation of temperatures typically experienced during a British spring and summer. The approach provided a comparison of E. coli persistence profiles within faeces of sheep, beef cattle and dairy cattle to allow novel interpretations of E. coli regrowth patterns in contrasting livestock faeces in the period immediately post-defecation. Thus, the coupling of a tightly controlled environment facility with high resolution monitoring enabled the development of a new non-linear, asymptotic description of E. coli proliferation in livestock faeces, with increased potential for E. coli growth observed during warmer temperatures for all livestock types. While this study focused on temperatures typical of the UK, the occurrence of a phase of E. coli regrowth has implications for microbial water quality management worldwide.

Temperature sensitivity of different soil carbon pools under biochar addition

The objective of this study was to investigate the temperature sensitivity of labile and relatively recalcitrant forest soil carbon (C) pools amended with biochar during short-term incubation. Biochars were prepared using sugar cane residue under pyrolysis temperatures of 300 and 700 °C (i.e., BC300 and BC700), respectively. Coarse particulate organic matter and acid hydrolysis residue were separated from a forest soil and treated as the labile and recalcitrant C pools of the soil, respectively. Temperature sensitivity of the soil C pools was characterized using the Q₁₀ values (i.e., the proportional increase in respiration per 10 °C rise). The increased Q₁₀ values of treatments in the earlier stage were attributable to instantaneously increased aromatic C content. The following decreased Q₁₀ values were related to the consumption of labile C. However, the two types of biochars led to similar Q₁₀ values in the same C pools at the later stage of incubation, which was closely related to the nearly humic-like component content in the dissolved organic matter. The different temporal distributions of Q₁₀ values were attributable to changes of aromatic C content and continuous consumption of labile components.