3.4 Estimation of annual CO2 flux and C budget
In all treatments, the estimated annual CO2 flux in the first year tended to be lower than that in the second year (Table 4). Cumulative CO2 flux as C output for the whole experimental period (27 months) was 2.4, 2.7, 4.0, and 3.7 Mg C ha−1 in the C, B, M, and BM treatments, respectively (Fig. 3 and Table S3). Cumulative CO2 flux in the M treatment was 1.6 Mg C ha−1 larger than that in the C treatment, while cumulative CO2 flux in the B treatment was 0.3 Mg C ha−1 larger than that in the C treatment. In addition, cumulative CO2 flux in the BM treatment was 0.3 Mg C ha−1 lower than that in the M treatment.
Surface SOC stock in all treatment plots except for the C treatment significantly increased from Sep 2017 to Dec 2019 (Fig. 3 and Table S3). In the C treatment, SOC stock decreased from 7.9 Mg C ha−1 (in Sep 2017) to 7.0 Mg C ha−1(in Dec 2019), although it was not significantly different. In the B and BM treatments, SOC stock increased significantly by 6.0–8.9 Mg C ha−1 (0–15 cm), while SOC stock in the M treatment increased significantly by 2.0 Mg C ha−1 (0–15 cm). These variations in SOC stock led to SOC increments in the B and BM treatments that were significantly higher than those in the C treatment. Additionally, BM treatment caused the largest SOC increment in this experiment.