Drying and rewetting cycles increased soil carbon dioxide rather than nitrous oxide emissions: A meta-analysis.

The increased frequency of extreme weather variations worldwide has resulted in dramatic changes in the soil water content via pronounced drying and rewetting cycles (DWCs). A comprehensive exploration of carbon dioxide (CO) and nitrous oxide (NO) emissions in response to DWCs can help summarize the existing results and better estimate terrestrial greenhouse gas emissions under the intensified drought and precipitation variations. This meta-analysis based on soil emissions of CO (868 observations, 29 studies) and NO (52 observations, 19 studies) at the global scale investigated the direction and intensity of the changes in soil CO and NO emissions in response to DWCs as controlled by experimental variables including land use type, soil texture, soil nutrients, and frequency and duration of DWCs. The results showed that, compared to the constant soil water content, DWCs led to the increase in CO emissions by 35.7% (95% confidence intervals ranging from 0.300 to 0.415), whereas it had no significant effect on NO emissions (-0.2638 to 1.4751). The random-effects model indicated that soil water-filled pore space during wetting, soil clay content, days of drying and wetting, and frequency of DWCs significantly affected CO and NO emissions in response to DWCs. Furthermore, potential biotic and abiotic factors affecting soil CO and NO emissions under DWCs are also summarized, and it was proposed that mobility and availability of carbon substrate as well as enhanced microbial activity and abundance are the main drivers facilitating soil CO and NO emissions in response to DWCs. However, soil gas diffusion or oxygen availability also dominated soil NO emissions under DWCs. Overall, this study improves our understanding of soil CO and NO emissions in response to various DWC scenarios and facilitates the development of better greenhouse gas mitigation strategies against the background of a rapidly changing climate.