The carbon-quality temperature hypothesis: Fact or artefact?

Thermal Compensatory Response of Soil Heterotrophic Respiration Following Wildfire

Frequent wildfires pose a serious threat to carbon (C) dynamics of forest ecosystems under a warming climate. Yet, how wildfires alter the temperature sensitivity (Q10) of soil heterotrophic respiration (Rh) as a critical parameter determining the C efflux from burned landscapes remains unknown. We conducted a field survey and two confirmatory experiments in two fire-prone regions of China at <1, 3, 6, and 12 months after wildfires (n = 160 soil samples). We found that wildfire generally reduced the Q10 for soil organic and mineral horizons within the first year after wildfire mainly due to substrate depletion, which was confirmed by a uniform inoculation experiment. Mineral protection of organic matter in the mineral horizon rich in iron/aluminum (hydr)oxides and a near-neutral pH in organic horizons of postfire soils further suppressed the Q10. Decreased Q10 persisted in organic horizons even after removing substrate limitation, reflecting the dominance of a thermally adapted, r-strategist microbial community in postfire soils. Moreover, fire-induced low C quality increased Q10, which supported the C quality-temperature hypothesis, but a C-limited condition restricted this stimulatory effect. This study illustrates that a thermal compensatory response of Rh will help maintain C stocks in forest ecosystems after wildfires in a warming world.

Factors Regulating the Potential for Freshwater Mineral Soil Wetlands to Function as Natural Climate Solutions

There are increasing global efforts and initiatives aiming to tackle climate change and mitigate its impacts via natural climate solutions (NCS). Wetlands have been considered effective NCS given their capacity to sequester and retain atmospheric carbon dioxide (CO2) while also providing a myriad of other ecosystem functions that can assist in mitigating the impacts of climate change. However, wetlands have a dual impact on climate, influencing the atmospheric concentrations of both CO2 and methane (CH4). The cooling effect associated with wetland CO2 sequestration can be counterbalanced by the warming effect caused by CH4 emissions from wetlands. The relative ability of wetlands to sequester CO2 versus emit CH4 is dependent on a suite of interacting physical, chemical, and biological factors, making it difficult to determine if/which wetlands are considered important NCS. The fact that wetlands are embedded in landscapes with surface and subsurface hydrological connections to other wetlands (i.e., wetlandscapes) that flow over and through geochemically active soils and sediments adds a new layer of complexity and poses further challenges to understanding wetland carbon sequestration and greenhouse gas fluxes at large spatial scales. Our review demonstrates how additional scientific advances are required to understand the driving mechanisms associated with wetland carbon cycling under different environmental conditions. It is vital to understand wetland functionality at both wetland and wetlandscape scales to effectively implement wetlands as NCS to maximize ecological, social, and economic benefits.

Short-term warming supports mineral-associated carbon accrual in abandoned croplands

Effective soil organic carbon (SOC) management can mitigate the impact of climate warming. However, the response of different SOC fractions to warming in abandoned croplands remains unclear. Here, categorizing SOC into particulate and mineral-associated organic carbon (POC and MAOC) with physical fractionation, we investigate the responses of POC and MAOC content and temperature sensitivity (Q10) to warming through a 3-year in situ warming experiment (+1.6 °C) in abandoned croplands across 12 sites in China (latitude: 22.33-46.58°N). Our results indicate that POC content remains unchanged while MAOC content significantly increases under warming. POC and MAOC content changes are mainly influenced by root biomass and microbial necromass carbon changes, respectively. The Q10 of MAOC is significantly lower than that of POC regardless of the warming or control treatment, suggesting that MAOC represents the most persistent and least vulnerable carbon fraction within SOC. Collectively, the sequestration of stable soil carbon can be enhanced in abandoned croplands under short-term warming.

Temperature structuring of microbial communities on a global scale

Temperature is a fundamental physical constraint regulating key aspects of microbial life. Protein binding, membrane fluidity, central dogma processes, and metabolism are all tightly controlled by temperature, such that growth rate profiles across taxa and environments follow the same general curve. An open question in microbial ecology is how the effects of temperature on individual traits scale up to determine community structure and function at planetary scales. Here, we review recent theoretical and experimental efforts to connect physiological responses to the outcome of species interactions, the assembly of microbial communities, and their function as temperature changes. We identify open questions in the field and define a roadmap for future studies.

Temperature dependence of leaf breakdown in streams differs between organismal groups and leaf species

Increased temperatures are altering rates of organic matter (OM) breakdown in stream ecosystems with implications for carbon (C) cycling in the face of global change. The metabolic theory of ecology (MTE) provides a framework for predicting temperature effects on OM breakdown, but differences in the temperature dependence of breakdown driven by different organismal groups (i.e., microorganisms vs. invertebrate detritivores) and litter species remain unresolved. Over two years, we conducted 12 60-day leaf litterbag incubations in 20 headwater streams in the southern Appalachian Mountains (USA). We compared temperature dependence (as activation energy, Ea) between microbial and detritivore-mediated breakdown, and between a highly recalcitrant (Rhododendron maximum) and a relatively labile (Acer rubrum) leaf species. Detritivore-mediated breakdown had a higher Ea than microbial breakdown for both leaf species (Rhododendron: 1.48 > 0.56 eV; Acer: 0.97 > 0.29 eV), and Rhododendron breakdown had a higher Ea than Acer breakdown for both organismal groups. Similarly, the Ea of total (coarse-mesh) Rhododendron breakdown was higher than the Ea of total Acer breakdown (0.89 > 0.52 eV). These effects for total breakdown were large, implying that the number of days to 95% mass loss would decline by 40% for Rhododendron and 26% for Acer between 12°C (our mean temperature value) and 16°C (+4°C, reflecting projected increases in global surface temperature due to climate change). Despite patterns in Ea, overall breakdown rates were higher for microbes than detritivores, and for Acer than Rhododendron over most of our temperature gradient. Additionally, the Ea for a subset of the microbial breakdown data declined from 0.40 to 0.22 eV when fungal biomass was included as a model predictor, highlighting the key role of fungi in determining the temperature dependence of litter breakdown. Our results imply that, as streams warm, routing of leaf litter C to detritivore-mediated fates will increase faster than predicted by previous studies and MTE, especially for labile litter. As temperatures rise, earlier depletion of autumn-shed, labile leaf litter combined with rapid breakdown rates of recalcitrant litter could exacerbate seasonal resource limitation and alter carbon storage and transport dynamics in temperate headwater stream networks.

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.

Reconciling carbon quality with availability predicts temperature sensitivity of global soil carbon mineralization

Soil organic carbon (SOC) mineralization is a key component of the global carbon cycle. Its temperature sensitivity Q10 (which is defined as the factor of change in mineralization with a 10 °C temperature increase) is crucial for understanding the carbon cycle-climate change feedback but remains uncertain. Here, we demonstrate the universal control of carbon quality-availability tradeoffs on Q10. When carbon availability is not limited, Q10 is controlled by carbon quality; otherwise, substrate availability controls Q10. A model driven by such quality-availability tradeoffs explains 97% of the spatiotemporal variability of Q10 in incubations of soils across the globe and predicts a global Q10 of 2.1 ± 0.4 (mean ± one SD) with higher Q10 in northern high-latitude regions. We further reveal that global Q10 is predominantly governed by the mineralization of high-quality carbon. The work provides a foundation for predicting SOC dynamics under climate and land use changes which may alter soil carbon quality and availability.

Nitrogen deposition raises temperature sensitivity of soil organic matter decomposition in subtropical forest

Subtropical ecosystems are strongly affected by nitrogen (N) deposition, impacting soil organic matter (SOM) availability and stocks. Here we aimed to reveal the effects of N deposition on i) the structure and functioning of microbial communities and ii) the temperature sensitivity (Q10) of SOM decomposition. Phosphorus (P) limited evergreen forest in Guangdong Province, southeastern China, was selected, and N deposition (factor level: N (100 kg N ha-1 y-1 (NH4NO3)) and control (water), arranged into randomized complete block design (n = 3)) was performed during 2.5 y. After that soils from 0 to 20 cm were collected, analyzed for the set of parameters and incubated at 15, and 25, and 35 °C for 112 days. N deposition increased the microbial biomass N and the content of fungal and Gram-positive bacterial biomarkers; activities of beta-glucosidase (BG) and acid phosphatase (ACP) also increased showing the intensification of SOM decomposition. The Q10 of SOM decomposition under N deposition was 1.66 and increased by 1.4 times than under control. Xylosidase (BX), BG, and ACP activities increased with temperature under N but decreased with the incubation duration, indicating either low production and/or decomposition of enzymes. Activities of polyphenol-(PPO) and peroxidases (POD) were higher under N than in the control soil and were constant during the incubation showing the intensification of recalcitrant SOM decomposition. At the early incubation stage (10 days), the increase of Q10 of CO2 efflux was explained by the activities of BX, BQ, ACP, and POD and the quality of the available dissolved organic matter pool. At the later incubation stages (112 days), the drop of Q10 of CO2 efflux was due to the depletion of the labile organic substances and the shift of microbial community structure to K-strategists. Thus, N deposition decoupled the effects of extracellular enzyme activities from microbial community structure on Q10 of SOM decomposition in the subtropical forest soil.

Variation in Temperature Dependences across Europe Reveals the Climate Sensitivity of Soil Microbial Decomposers

Temperature is a major determinant of biological process rates, and microorganisms are key regulators of ecosystem carbon (C) dynamics. Temperature controls microbial rates of decomposition, and thus warming can stimulate C loss, creating positive feedback to climate change. If trait distributions that define temperature relationships of microbial communities can adapt to altered temperatures, they could modulate the strength of this feedback, but if this occurs remains unclear. In this study, we sampled soils from a latitudinal climate gradient across Europe. We established the temperature relationships of microbial growth and respiration rates and used these to investigate if and with what strength the community trait distributions for temperature were adapted to their local environment. Additionally, we sequenced bacterial and fungal amplicons to link the variance in community composition to changes in temperature traits. We found that microbial temperature trait distributions varied systematically with climate, suggesting that an increase in mean annual temperature (MAT) of 1°C will result in warm-shifted microbial temperature trait distributions equivalent to an increase in temperature minimum (T_min) of 0.20°C for bacterial growth, 0.07°C for fungal growth, and 0.10°C for respiration. The temperature traits for bacterial growth were thus more responsive to warming than those for respiration and fungal growth. The microbial community composition also varied with temperature, enabling the interlinkage of taxonomic information with microbial temperature traits. Our work shows that the adaptation of microbial temperature trait distributions to a warming climate will affect the C-climate feedback, emphasizing the need to represent this to capture the microbial feedback to climate change. IMPORTANCE One of the largest uncertainties of global warming is if the microbial decomposer feedback will strengthen or weaken soil C-climate feedback. Despite decades of research effort, the strength of this feedback to warming remains unknown. We here present evidence that microbial temperature relationships vary systematically with environmental temperatures along a climate gradient and use this information to forecast how microbial temperature traits will create feedback between the soil C cycle and climate warming. We show that the current use of a universal temperature sensitivity is insufficient to represent the microbial feedback to climate change and provide new estimates to replace this flawed assumption in Earth system models. We also demonstrate that temperature relationships for rates of microbial growth and respiration are differentially affected by warming, with stronger responses to warming for microbial growth (soil C formation) than for respiration (C loss from soil to atmosphere), which will affect the atmosphere-land C balance.