Effects of long-term conservation tillage on soil aggregate stability and carbon and nitrogen in paddy field

Abstract: Soil structure stability, aggregate-associated C and N play an important role in soil conservation and nutrient supply. Tillage practices can affect the soil aggregate stability and C and N distribution, thus affecting the farmland ecological security. To estimate the effects of tillage practices on soil aggregate stability and its C and N distribution of double paddy field in Southern China, a long-term field experiment initiated from 2005 with four treatments (no-till with residue retention, NTS), rotary tillage with residue retention, RTS), plow tillage with residue retention, CTS), and plow tillage with residue removed, CT)) was conducted in a double rice cropping system in Ningxiang, Hunan. After 12-years of the experiment, the soil water-stable aggregates, stability, and C, N concentration were determined from four soil depths of 0-5, 5-10, 10-20, and 20-30 cm. The results showed that there were significant positive correlations between soil C, N and aggregate stability (P <0.05). The percentage of soil aggregate decreased with the particle size decreases in paddy fields. It mainly composed of macro-aggregate (>0.25 mm), accounting for 66.90%-87.82%, of which >2 mm part accounted for 35.02%-64.44% in 0-30 cm soil layers under different tillage practices. For >2 mm soil aggregate, NTS was significantly higher than RTS (P <0.05) in the 5-30 cm soil layers.NTS was significantly higher than CTS at 5-20 cm (P < 0.05), but NTS, RTS and CTS were not significant in the 0-5 cm soil layer. The <0.25 mm soil aggregate accounted for 12.18%-33.10% in 0-30 cm soil layers under different tillage practices. In terms of aggregate stability, NTS was significantly higher than RTS (10-30 cm) and CTS (5-20 cm), but NTS, RTS, and CTS were not significant in the 0-5 cm soil layer. The contribution rate of macro-aggregate to soil C, N in paddy fields were 76.58%-90.62% and 72.28%-89.76%, respectively, and the contribution rates of >2 mm aggregates to C and N were52.12% and 52.16%, respectively. Compared with straw removal, the contribution rate of >2 mm aggregate treated with straw returning to the soil C, N increased by 8.20 percentage point and 7.35 percentage point, while the contribution of 0.25-2 mm aggregate decreased by 4.96 percentage point and 4.84 percentage point, respectively. Further analysis of the relationship between soil C and N and aggregate stability showed that SOC and GMD (geometric mean diameter), TN and GMD were significantly positively correlated. Thus, straw returning was conducive to the transformation of micro-aggregate to macro-aggregate. Compared with CTS and RTS, NTS significantly increased the C, N content in soil surface and promoted the stable macro-aggregate formation, which had significant effects on improving aggregate stability in paddy fields (especially 0-20 cm) (P <0.05). Therefore, no-till with residue retention is an effective measure to maintain and improve soil performance of the paddy field in Southern China.