Long-term effect of no-tillage on soil organic carbon and nitrogen in an irrigated rice-based cropping system

A 10-year field experiment was conducted to determine the effect of no-tillage (NT) on soil organic carbon (SOC) and nitrogen (N) in an irrigated rice-based cropping system. The results showed that concentrations of SOC, soil total N (STN), and soil organic N (SON) were higher at 0-5 cm depth but lower at 5-10 cm depth under NT than conventional tillage (CT), while at 10-20 cm depth the differences were not significant. Consequently, stocks of SOC, STN, and SON under NT were higher at 0-5 cm depth but lower at 5-10 cm depth than those under CT. However, no significant differences were observed in stocks of SOC, STN, and SON at 0-20 cm depth between NT and CT, indicating that the lower st ocks of SOC, STN, and SON at 5-10 cm depth could offset the greater stocks of them at 0-5 cm depth under NT. In addition, there were no significant differences in soil inorganic N concentration and soil C:N ratio between NT and CT at all the three depths. Our study suggests that (1) NT farming does not necessarily store SOC and N more than CT soils for the upper soil layer and (2) converting from CT to NT farming may not cause a significant change in soil N mineralization in the irrigated rice-based cropping systems.