Widespread asymmetric response of soil heterotrophic respiration to warming and cooling

Soil is the largest organic carbon (C) pool in terrestrial ecosystems. Periodic changes in environmental temperature occur diurnally and seasonally; yet, the response of soil organic matter (SOM) decomposition to varying temperatures remains unclear. In this study, we conducted a modified incubation experiment using soils from 16 forest ecosystems in China with periodically and continuously varying incubation temperature to investigate how heterotrophic respiration (R_h) responds to different temperature patterns (both warming and cooling temperature ranging between 5 and 30°C). Our results showed a pronounced asymmetric response of R_h to temperature warming and cooling among the soils of all forest ecosystems, with R_h increasing more rapidly during the warming phase compared to the cooling phase. This asymmetric response of R_h to warming and cooling temperatures was widespread in all soils. In addition, the amplitude of this asymmetric response differed among different forest ecosystems, with subtropical and warm-temperate forest ecosystems exhibiting greater asymmetric responses. Path analyses showed that soil pH and the microbial community explained most of the variation in this asymmetric response. Furthermore, the widespread asymmetric response of R_h to warming and cooling temperatures suggests that accumulated SOM decomposition might be overestimated on average by 20% for warming alone when compared with admix warming and cooling. These findings provide new insights on the responses of R_h to natural shifts in temperature, emphasizing the need to consider this widespread asymmetric response of R_h to warming and cooling phases to predict C-climate feedback with great accuracy, especially under future non-uniform warming scenarios.