The temperature dependence of the decomposition of soil organic matter is shaped by both microbial thermal traits and substrate quality

The net effect of temperature variations on soil organic carbon (SOC) budgets depends on the balance of carbon (C) losses via respiration and C stabilization. Respiration increases monotonically with temperature, whereas the response of C stabilization to temperature is less clear. Microbial residues, formed via microbial growth, can get stabilized in soil and thus contribute to SOC accumulation. Here we test how the temperature dependence of the microbial SOC use for respiration (proxy for C losses) and growth (proxy for C stabilization) varies across climatic, edaphic, and substrate quality gradients, and how it responds to experimental warming. We hypothesized that the temperature dependence of microbial decomposition of organic matter is primarily governed by two factors: (i) the thermal traits of microbial communities, and (ii) SOC quality. To test these hypotheses, we collated more than 200 paired growth and respiration thermal response curves from over 20 published studies spanning a wide range of climates. Thermal traits of microbial communities (eg. minimal temperature, Tmin) were derived from microbial growth response curves, and temperature sensitivity was estimated as the ratio of microbial uptake rates at two reference temperatures offset by 10°C (Q10). Environmental temperatures at sampling sites were used as a proxy for climatic forcing, and C uptake per unit SOC (i.e., microbial assimilability) at a reference temperature as a proxy of SOC quality. Preliminary results indicate that warmer climates select for warm-shifted microbial thermal traits (i.e., higher Tmin values), and that temperature sensitivities are higher for lower-quality SOC. In addition, experimental warming alters microbial thermal responses in ways consistent with thermal adaptation. These findings allow us to describe the relative contributions of microbial thermal traits and of substrate quality in shaping the temperature dependence of SOC decomposition, thereby improving predictions of soil carbon fluxes under future climate scenarios.