Effects of conservation tillage on aggregate stability of paddy soil in hilly region

Long-term conservation tillage is an effective agricultural management to improve soil structure in severely destroyed paddy soil by increase of soil organic carbon (SOC). More work is still needed to understand the impacts of conservation tillage on the mechanisms of aggregates stability. Soil samples in 0-20 cm layer were adopted from the long-term tillage experiment: (1) paddy-upland rotation and plain culture (rice- rape) (PU-R-RR), (2)no-tillage and ridge culture (rice-fallow) (NT-R-RF), (3) no-tillage and ridge culture (rice-rape) (NT-R-RR) and (4) no-tillage and plain culture (rice- rape) (NT-P-RR). Different pretreatments, such as slaking in fast wetting, wetting and subsequent slaking, were applied to simulate the breakdown mechanisms of aggregates in paddy soil. The results showed that no significant difference of aggregate stability between slaking and wetting pretreatments was observed, while tillage patterns affected greatly aggregate stability. The aggregate stability under slaking and wetting pretreatment ranked in the order of NT-R-RR > NT-P-RR > NT-R-RF > PU-R-RR and NT-P-RR > NT-R-RR > NT-R-RF > PU-R-RR, respectively. The organic carbon concentrations in aggregates were significantly positive related to the aggregates stability under wetting（r=0.626, p<0.01), while low correlation linear relationships were observed under slaking treatment (r=0.432, p>0.05). The results suggest that long-term conservation tillage favorably led to the increase of SOC concentrations in aggregates and the enhancement of aggregates water stabilization.