4.1. Direct effects of DWC on soil carbon mineralization
As we hypothesized, the strong DWC with intensified drought had a stronger impact on C emission, showing that compared to the constant-moisture control treatment (60% WHC), the 100-20% WHC treatment decreased cumulative C mineralization during the DWC period in OF and BF soils and increased it in BF+ soil, while the 80-40% WHC treatment did not significantly change the C emission of all three soils (Fig. 4a). This is because the 80-40% WHC treatment might not cause as much osmotic shock and cell lyses as the strong DWC treatment (100-20% WHC in this study) during the drying period (Guo et al., 2012). Also, the 80-40% WHC treatment might not reach the drought threshold, and the accessible C from physical and physiological mechanisms after the mild drought (to 40% WHC) may not contribute much to the subsequent CO2 pulse (Barnard et al., 2020; Slessarev et al., 2020).
Although the 100-20% WHC treatment decreased cumulative C mineralization during the entire DWC period (0-90 d) in OF and BF soils, this reduction in C emission of the 100-20% WHC treatment only occurred in two cycles and did not significantly affect C mineralization in other cycles (Table S2). This result was consistent with a meta-analysis of Zhang et al. (2020), which showed that the rewetting-driven CO2 pulse can fully compensate the reduced CO2 emission during the drying period.
For BF+ soil, the 100-20% WHC treatment increased cumulative C mineralization by 16% relative to the mean constant-moisture control treatment (Table S2). Since few active SOC in the BF+ soil was left after 25-year BF treatment and 815-d laboratory incubation, the microbial community in the BF+ soil is more dominated by fungi (Fig. 5c). Fungi are considered more resistant to drought and osmotic stress than bacteria due to their multicellular hyphal networks allowing remote access to water in the soil (Barnard et al., 2013). Therefore, the reduction in C mineralization of the soil with more fungi (BF+ soil) during the drying period may be smaller, and the cumulative C mineralization may be higher during the whole DWC period compared to the soil dominated by bacteria (OF soil). Moreover, DWC can result in a microbial composition shift toward more desiccation tolerant. For example, sensitive microorganisms die or become inactive during drought and are difficult to recover, thereby supplying substrates for more drought-tolerant surviving microorganisms (Meisner et al., 2021).