Responses of soil carbon decomposition to drying-rewetting cycles: A meta-analysis

Drying-rewetting cycles (DWC) is an important climate change factor in this century. The rewetting of dry soil stimulates carbon dioxide emission into the atmosphere, which is called the Birch effect. Although a large number of studies have been conducted to quantify the effect of DWC on cumulative soil respiration (CSR), their results are often contradictory, which offers us an opportunity to explore potential influencing factors and variability among studies. Using a meta-analysis including 524 observations from studies across different ecosystems and soil types, we investigated the responses of CSR to DWC. Our results indicated that DWC led to 72% stimulation of CSR compared with lower constant-moisture control (LC), 25% inhibition compared with upper constant-moisture control (UC), but minimal difference with mean constant-moisture control (MC), respectively. The meta-regression analysis showed that the DWC effect on CSR depended on mean annual precipitation, soil properties (clay content, C/N ratio, TN, pH), and the duration of drying in the DWC treatment. In addition, the effect size of CSR relative to UC was positively correlated with the number of DWC, suggesting that the slowercycling soil carbon may be more vulnerable to DWC than the faster-cycling soil carbon and the "physical mechanism" may play a role in substrate release and CO2 emission after multiple DWC. Overall, our study provided a comprehensive analysis of the effects of DWC on CSR and showed that frequent drying-rewetting cycles induced by climate change might stimulate the loss of soil carbon to the atmosphere. Moreover, the similar CSR between DWC and MC (which have the same mean soil moisture content) suggests that the CO2 pulse after rewetting may be compensated by the low CO2 emission during the drying phase. Further research is needed to explore the responses of SOC pools with different turnover times to DWC and to investigate the DWC effects on soil respiration by in situ experiments and long-term studies.