Global climate change is expected to increase the frequency of drought and heavy precipitation, which could create more frequent drying-rewetting cycles (DWC) in the soils. Although DWC effects on SOC decomposition has been widely studied, the effect of DWC and the subsequent legacy effect on the decomposition of different SOC pools is still unclear. We conducted a 128-d laboratory incubation to investigate the DWC effects by using soils from old-field for 15 years (OF, representing active SOC), bare-fallow for 15 years (BF), and bare-fallow for 23 years plus extra 815-d incubation (BF+, representing relatively resistant SOC). The experiment included nine 10-d DWC of three treatments: 1) constant-moisture at 60% WHC, 2) mild DWC with 10-d drying to 40% WHC and rewetting to 80% WHC, and 3) strong DWC with 10-d drying to 20% WHC and rewetting to 100% WHC. Following DWC period, there was a 10-d stabilization period (adjusting all treatments to 60% WHC), and then a 28-d extended incubation. During DWC period, the strong DWC had strong effect on CO2 release compared with the constant-moisture control, reducing the SOC decomposition from OF by 8% and BF by 10%, while increasing the SOC decomposition of BF+ by 16%. During extended period, both mild and strong DWC significantly increased SOC mineralization of OF, but decreased that of BF and BF+. This legacy effect compensated the changes in CO2 release during DWC period, resulting in the minor response of SOC decomposition of OF and BF+ to the DWC during the entire incubation.

In recent decades, global warming under rising CO 2 significantly influences external organic carbon (EOC) input and drying-rewetting processes in terrestrial ecosystems. However, little is known about how soil CO 2 emissions respond to these perturbations, which provides us with a chance to explore potential factors and variability. In this study, a meta-analysis on the responses of CO 2 emissions with or without EOC input in the soil drying-rewetting cycles (DWC) based on 291 observations (included 33 study sites and 11 variables) has been conducted. The results indicated that i) CO 2 emission with the increase of EOC by 284% relative to without EOC under DWC; ii) The effect size of CO 2 emission was the smallest in the forest (+15%) and the largest in the grassland (+1468%); iii) The CO 2 emission effect sizes were substantially greater in complex substrates (+288%) than in simple substrates (+132%), and iv) longer drought period in a DWC can induce more CO 2 emission. The study suggests that terrestrial CO 2 emission may be multi folds in the long drought-rapid rewetting processes under large input of EOC.