ABSTRACT Paddy rice production based on traditional soil management emits large amounts of methane (CH4) into the atmosphere. This study assessed the potential of no-tillage (NT) and winter cover crops (WCC) to mitigate net greenhouse gas (GHG) emissions in a subtropical paddy rice ecosystem. A long-term (20-yrs) experiment was evaluated regarding the effect of NT combined with winter fallow or three WCC (ryegrass, white oat, and birdsfoot trefoil) on seasonal CH4 -C and nitrous oxide (N2O-N) emissions and on soil organic carbon (SOC) stocks in comparison to conventional tillage (CT) under winter fallow in a Gleysol of Southern Brazil. The changes in SOC were used as a proxy for annual net carbon dioxide (CO2) exchanges in the soil-atmosphere, taking the CT treatment as a reference. The GHG balance (summation of CH4 , N2O and CO2 emissions multiplied by their global warming potential of 34, 298, and 1, respectively) and emissions intensity of GHG emissions were calculated. Across winter managements, NT decreased 25 % of GHG emissions in comparison to CT system. This effect was mainly related to the decrease of seasonal CH4 -C emissions (31-113 kg ha-1) and by promoting SOC accumulation (0.45-0.65 Mg ha-1yr-1) in comparison to CT system, since soil N2 O-N emission was not affected by management practices. Increased soil CH4 -C emissions offset the positive effect of WCC on SOC accumulation compared with winter fallow. Based on our findings, NT mitigates net GHG emissions in subtropical paddy rice ecosystems, but no additional effect is observed combining NT with WCC.

ABSTRACT Soil management and crop rotation are key factors in controlling the accumulation of C and N in the soil profile, but their long-term effect remains poorly understood for deep soil layers, especially in subtropical conditions. Using a long-term experiment (26-years), this study aimed to evaluate the effect of different soil management systems associated with different winter cover crops on C and N accumulation in a very clayey (72 % clay) soil up to 1 m deep. Two tillage systems [conventional tillage (CT) and no-tillage (NT)] were cultivated with eight winter cover crops (black oat, rye, common vetch, hairy vetch, oilseed radish, wheat, blue lupine, and fallow) in a subtropical Oxisol from Southern Brazil. Soil samples were taken in eight soil layers up to 1.00 m soil depth after 26 years of experiment and, also from an adjacent native forest. After forest clearing, the C stock in the 0.00-0.20 m soil layer was reduced by 45 % in only 10 years (from 1976 to 1986) of soil tillage. Twenty-six years after the beginning of the experiment, C and N stock in 0.00-0.20 m soil layer were 13 and 20 % higher in NT compared to CT, with the greatest differences in C and N content observed in the 0.00-0.05 m layer. When associated with winter cover crops, NT accumulated 0.6 and 0.06 Mg ha-1 yr-1 more C and N than CT with winter fallow in the 0.00-0.20 m soil layer. No-tillage and CT recovered 95 and 83 %, respectively, of the C stock found in the 0.00-0.20 m layer from the native forest. However, in the 0.00-1.00 m soil layer, the positive effect of NT on soil C accumulation compared to CT was diluted, and no clear effect of NT was verified. Moreover, no difference in winter cover crops on soil C and N stocks were observed in all soil layers, possibly due to their similar residues input (3.3-4.9 Mg ha-1 yr-1). No-tillage associated with high biomass input through winter cover crops promoted a faster recovery of soil C and N stock than in CT and, therefore, is an efficient tool to improve soil C and N accumulation even in Oxisols with high clay content.