3.3. Dynamics of soil carbon decomposition
During the DWC period, soil C mineralization rates declined rapidly after the first cycle (0-10 d) for all water regimes, and then decreased slowly with the increasing number of DWC (Fig. 3a-c). For OF soils, cumulative soil C mineralization in 60%, 80-40%, and 100-20% WHC treatments were 28.0, 27.1, and 25.7 mg CO2-C g-1 SOC, respectively. For BF soils, cumulative soil C mineralization in 60%, 80-40%, and 100-20% WHC treatments were 24.0, 22.3, and 21.6 mg CO2-C g-1 SOC, respectively. For BF+ soils, cumulative soil C mineralization in 60%, 80-40%, and 100-20% WHC treatments were 10.8, 11.9, and 12.5 mg CO2-C g-1 SOC, respectively (Fig. 3d-f, Fig. 4a). Compared to the constant-moisture control treatment (60% WHC), the 100-20% WHC treatment decreased cumulative C mineralization of OF and BF soils by 8% and 10%, respectively, but increased that of BF+ soil by 16%. The 80-40% WHC treatment did not significantly change cumulative C mineralization of all three soils relative to the constant-moisture control treatment (Fig. 4a and Table S2). There was also an interactive effect of water regime and soil treatment on cumulative C mineralization during the DWC period (P =0.001, Table S3).
During the extended period, cumulative soil C mineralization of 60%, 80-40%, and 100-20% WHC treatments in OF soil were 5.0, 5.9, and 5.8 mg CO2-C g-1 SOC, respectively. For BF soil, cumulative soil C mineralization in 60%, 80-40%, and 100-20% WHC treatments were 5.8, 3.8, and 4.0 mg CO2-C g-1 SOC, respectively (Fig. 4b and Table S2). For BF+ soil, cumulative soil C mineralization in 60%, 80-40%, and 100-20% WHC treatments were 4.9, 2.0, and 1.3 mg CO2-C g-1 SOC, respectively (Fig. 4b and Table S2). Compared to the constant-moisture control treatment, both DWC treatments (80-40% and 100-20% WHC) increased the cumulative C mineralization in OF soil but decreased it in BF and BF+ soils. Specifically, the 80-40% WHC treatment increased C mineralization by 18% in OF soil and decreased it by 35% and 59% in BF and BF+ soils, respectively; The 100-20% WHC treatment increased C mineralization by 16% in OF soil and decreased it by 31% and 74% in BF and BF+ soils, respectively (Fig. 4b and Table S2). There was also an interactive effect of water regime and soil treatment on cumulative C mineralization during the extended period (P =0.001, Table S3).
During the entire incubation period (0-90 d and 100-128 d), the 80-40% and 100-20% WHC treatments decreased cumulative C mineralization of BF soil by 12% and 14%, respectively, but did not significantly change cumulative C mineralization of OF and BF+ soils compared with the constant-moisture control treatment (Fig. 4c and Table S2).