Residue returning with subsoiling replacing rotary tillage improving aggregate and associated carbon

Abstract: Soil tillage and residue returning could exert significant impact on soil structure and nutrient turnover, which were also important driving factors for the distribution and update of soil aggregate. The objectives of this study were to estimate the effects of rotary tillage converting to subsoiling on distribution, stability and associated-C content of soil aggregate based on the 8-year experiment, which included rotary tillage with residue returning (RTS), rotary tillage with residue removing (RTA), rotary tillage converting to subsoiling with residue returning (RTS-STS) and rotary tillage converting to subsoiling with residue removing (RTA-STA). The contribution rate of aggregate-C to soil organic carbon (SOC) and their interaction during rotary tillage converting to subsoiling were also analyzed in this study. The results showed that, the distribution, stability and associated-C of soil aggregate fractions were significantly affected by rotary tillage converting to subsoiling combined with the returning of residue, which not only significantly increased the proportion of soil macroaggregate, but also promoted the stability of aggregate. For example, the aggregate fraction proportions of 1-2, 0.25-1 and <0.25 mm under RTS-STS treatment were significantly higher than those of other 3 treatments in the soil depth of 0-10 cm, and the mean weight diameter (MWD) for stability index of soil aggregate under RTS-STS treatment was also 6.1%, 65.4% and 87.8% higher than RTA-STA, RTS and RTA treatments respectively in this study. And the aggregate fraction proportions of 1-2, 0.25-1 and <0.25 mm under 4 treatments were increased in the soil depth of >30-50 cm compared with that in the soil depth of 0-30 cm. Whereas, the MWD values were significantly decreased under the RTS-STS and RTA-STA treatments in the soil depth of 30-50 cm, and the opposite result was found under RTS and RTA treatments. Meanwhile, RTS-STS treatment significantly increased aggregate associated-C and its contribution rate to SOC in the depth of 0-20 cm. Although there was no significant difference for aggregate associated-C between RTS-STS and RTS treatments in the depth of >20-30 and >30-50 cm, the contribution rate of aggregate associated-C to SOC was decreased under the RTS-STS treatment compared with that in >20-50 cm depth. In comparison to the residue removing, rotary tillage and rotary tillage converting to subsoiling combined with the returning of residue increased the contents of SOC and aggregate associated-C in the soil depth of 0-50 cm. The contribution rate of aggregate associated-C to SOC under RTS-STS treatment was 87.89%, which was 4.9%, 10.3% and 18.6% higher than RTS, RTA-STA and RTA treatments respectively in this study. Tillage method, residue returning and their interaction were important effect factors on the distribution and associated-C of soil aggregate under 4 treatments in the different soil depths according to the force analysis in this study. Additionally, there was significant positive correlation between the SOC and the stability and associated-C(P<0.01) of aggregate by the correlation analysis. The tillage converting combined with residue returning (RTS-STS) can improve the aggregate gathering and stabilizing in the 0-20 cm soil depth, and also increase the contribution rate of soil aggregate associated-C to SOC, which would significantly improve the SOC level in this region.