3.1 |Carbon inputs
Compared with the CK, the accumulative input of plant-derived biomass was significantly enhanced by 72.1% and 61.6% under NT and RT, respectively (Fig. 1a; Table S1; bothP < 0.001). Straw return increased the accumulative input of plant-derived biomass by 90.3% under SR relative to the SR0 (P < 0.001). No-tillage interacted with straw return to influence the accumulative input of plant-derived biomass (P < 0.001). Compared with the CK, the NT and RT increased the accumulative input of plant-derived biomass by 57.5% and 46.4%, respectively, in SR plots, while increased it by 106.8% and 97.1%, respectively, in SR0 plots (all P < 0.01). Compared with the SR0, the SR promoted the accumulative input of plant-derived biomass by 80.7%, 76.5%, and 137.3% in NT, RT, and CK plots, respectively (all P < 0.001). Compared with the CK-SR0, the accumulative input of plant-derived biomass in RT-SR0, NT-SR0, CK-SR, RT-SR, and NT-SR gradually increased significantly by 97.1%, 106.8%, 137.3%, 247.8%, and 273.7%, respectively (allP < 0.01).
The accumulative input of plant-derived C exhibited a trend that similar to those of the accumulative inputs of plant-derived biomass (Fig. 1a). The higher inputs of plant-derived biomass resulted in higher inputs of plant-derived C ranging from 22.7 Mg ha-1 in CK-SR0 to 84.8 Mg ha-1 in NT-SR. The total proportions of plant-derived C inputs derived from stubble and root in CK-SR0 were 23.9% and 76.1%, respectively (Fig. 1b). In other treatments, the total proportions of plant-derived C inputs derived from straw, stubble, and root were 41.0-63.0%, 9.0-14.2%, and 28.0-44.8%, respectively.
3.2 |Soil organic carbon contents and stocks
Over the 8-yr study, the SOC contents showed similar dynamic changing trends in all treatments (Fig. 2a). Compared with the CK, the NT and RT increased SOC contents by 7.4% and 5.5%, respectively (Fig. 2b; Table 2; P < 0.001). The SOC content was 6.5% higher under SR than under SR0 (P < 0.001). However, there were no interaction effects of tillage and straw managements on SOC contents (Table 2; P > 0.05). The largest SOC contents were 13.15 g kg−1 (2015) in CK-SR, 12.25 g kg−1 (2012) in CK-SR0, 12.84 g kg−1(2015) in RT-SR0, 13.65 kg−1 (2015) in RT-SR, 14.12 kg−1 (2016) in NT-SR, and 12.79 (2016) in NT-SR0, respectively (Fig. 2a). The average SOC content in NT-SR was 2.7% and 8.0% higher than in RT-SR and CK-SR, respectively. After the 8-yr study, SOC stock was enhanced by 10.5% and 9.6% under NT and RT, respectively, relative to the CK, and was higher by 9.5% under SR than under SR0 (P < 0.01; Table 3 & S1). No interaction effects of tillage and straw managements existed on SOC contents (P > 0.05). Soil organic C stocks increased by 34.1% in NT-SR, following by RT-SR (33.9%), CK-SR (22.3%), NT-SR0 (21.3%), RT-SR0 (20.8%), and CK-SR0 (9.1%) compared to initial SOC stock in 2008 (Table 3).