Heterotrophic soil microbial respiration – one of the main processes of carbon loss from soils to the atmosphere – is sensitive to temperature in the short-term. However, how this sensitivity is affected by long-term thermal regimes is uncertain. There is an expectation that soil microbial respiration rates adapt to the ambient thermal regime, but whether this adaptation magnifies or reduces respiration sensitivities to temperature fluctuations remains unresolved. This gap in our understanding is particularly pronounced for drylands as most studies conducted so far have focused on mesic systems. Here, we conducted an incubation study using soils from 110 global drylands encompassing a wide gradient in mean annual temperature. We tested how mean annual temperature affects soil respiration rates at three assay temperatures while controlling for substrate depletion and microbial biomass. Estimated soil respiration rates at the mean microbial biomass were lower in sites with higher mean annual temperatures across the three assayed temperatures. The patterns observed are consistent with expected evolutionary trade-offs in the structure and function of enzymes under different thermal regimes. Our results therefore suggest that soil microbial respiration adapts to the ambient thermal regime in global drylands.

Anthropogenic warming is expected to accelerate global soil organic carbon (SOC) losses via microbial decomposition, yet there is still no consensus on the loss magnitude. In this Perspective, we argue that despite the mechanistic uncertainty underlying these losses, there is confidence that a strong, positive land carbonclimate feedback can be expected. Two major lines of evidence support net global SOC losses with warming via increases in soil microbial metabolic activity: the increase in soil respiration with temperature and the accumulation of SOC in low mean annual temperature (MAT) regions. Warming-induced SOC losses are likely to be of a magnitude relevant for emission negotiations, and necessitate more aggressive emission reduction targets to limit climate change to 1.5 o C by 2100. We suggest microbial community−temperature interactions, and how they are influenced by substrate availability, are promising research areas to improve the

Extracellular enzymes catalyze rate-limiting steps in soil organic matter decomposition, and their activities (EEAs) play a key role in determining soil respiration (SR). Both EEAs and SR are highly sensitive to temperature, but their responses to climate warming remain poorly understood. Here, we present a meta-analysis on the response of soil cellulase and ligninase activities and SR to warming, synthesizing data from 56 studies. We found that warming significantly enhanced ligninase activity by 21.4% but had no effect on cellulase activity. Increases in ligninase activity were positively correlated with changes in SR, while no such relationship was found for cellulase. The warming response of ligninase activity was more closely related to the responses of SR than a wide range of environmental and experimental methodological factors. Furthermore, warming effects on ligninase activity increased with experiment duration. These results suggest that soil microorganisms sustain long-term increases in SR with warming by gradually increasing the degradation of the recalcitrant carbon pool.

The degree to which climate warming will stimulate soil organic carbon (SOC) losses via heterotrophic respiration remains uncertain, in part because different or even opposite microbial physiology and temperature relationships have been proposed in SOC models. We incorporated competing microbial carbon use efficiency (CUE)mean annual temperature (MAT) and enzyme kinetic-MAT relationships into SOC models, and compared the simulated mass-specific soil heterotrophic respiration rates with multiple published datasets of measured respiration. The measured data included 110 dryland soils globally distributed and two continental to global-scale cross-biome datasets. Model-data comparisons suggested that a positive CUE-MAT relationship best predicts the measured mass-specific soil heterotrophic respiration rates in soils distributed globally. These results are robust when considering models of increasing complexity and competing mechanisms driving soil heterotrophic respiration-MAT relationships (e.g., carbon substrate availability). Our findings suggest that a warmer climate selects for microbial communities with higher CUE, as opposed to the often hypothesized reductions in CUE by warming based on soil laboratory assays. Our results help to build the impetus for, and confidence in, including microbial mechanisms in soil biogeochemical models used to forecast changes in global soil carbon stocks in response to warming. K E Y W O R D S CO 2 efflux, global warming, microbe, soil carbon stock, soil respiration | 3355 YE Et al. S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section at the end of the article. How to cite this article: Ye J-S, Bradford MA, Dacal M, Maestre FT, García-Palacios P. Increasing microbial carbon use efficiency with warming predicts soil heterotrophic respiration globally. Glob Change Biol. 2019;25:3354-3364.