Responses of soil organic carbon decomposition to warming depend on the natural warming gradient

Abstract

Inconsistent responses of soil organic carbon (SOC) decomposition to climate warming have been observed in field experiments. Responses vary with the duration of experimental warming; however, even the longest warming experiment could not account for possible future climate-driven changes in SOC decomposition. Here, we investigated the responses of SOC decomposition to natural warming in soil samples (at soil depths of 0–10, 10–20, and 20–30 cm) collected at two altitudes (lower-elevation, warm points; higher-elevation, cool points) with an approximate 1 °C difference in mean soil temperature, representing an infinite natural warming duration in the Qinghai–Tibetan Plateau. We incubated these soil samples at three temperatures (i.e., 5, 10, and 15 °C) and analyzed the CO2 emission rate to assess the responses of SOC decomposition to warming. The results revealed higher CO2 emission rates from topsoil (0–10 and 10–20 cm) samples collected at lower-elevation (warm points) than higher-elevation (cool points). By contrast, subsoil (20–30 cm) samples showed higher CO2 emission rates at higher-elevation points. Soil microbial biomass carbon (MBC) exhibited similar patterns in topsoil and subsoil, indicative of the regulation SOC decomposition by MBC. Overall, CO2 emission rates were higher in subsoil, possible the result of the high vulnerability of carbon and decreased microbial carbon use efficiency in subsoil. For all sampling points, the largest CO2 emission rates and MBC increases were observed at 5 °C incubation temperature, demonstrating that cold-tolerant microbes may undergo adaptations that enable their tolerance to cold conditions. Taken together, our findings provide evidence of the natural warming gradient using a climosequence approach to illustrate the importance of microbial-mediation in controlling SOC decomposition. Moreover, models should consider different mechanisms of soil carbon dynamics in topsoil and subsoil when predicting carbon–climate feedback.