The effect of rice straw incorporation into paddy soil on carbon sequestration and emissions in the double cropping rice system.

BACKGROUND: Soil organic carbon (SOC) sequestration, methane emission, and the net carbon sink represented by rice straw incorporated into soil (RIS) were studied using long-term experimentation with rice straw incorporated into soil (LRIS) and short-term experimentation with different patterns of rice straw incorporated into soil (SPRIS).
RESULTS: Soil organic carbon could be improved by RIS combined with soil ploughing. The increased rate of SOC deposition per cultivated layer was 0.10 t C ha(-1) for 2.625 t ha(-1) straw incorporated each season in LRIS and 0.36 t C ha(-1) for 4.5 t straw ha(-1) season(-1) incorporated in SPRIS; the apparent SOC conversion by rice straw (stubble) was reduced as the amount of incorporated straw increased. However, RIS methane emission from paddy fields also significantly exacerbated the CH(4) emission flux observed during the early and late rice growing seasons, which was increased by 75.0% (P < 0.01) and 251.5% (P < 0.01), respectively, compared with combined application of nitrogen, phosphorus and potassium fertiliser (NPK). The apparent methane conversion of straw was almost uniform with a similar rice yield and soil cultivating mode. Among the patterns of RIS, methane emission was significantly reduced under straw covering untilled land, and this property led to the lowest apparent methane conversion.
CONCLUSION: RIS with ploughing and tilling resulted in negative carbon sequestration because of increased methane emissions. A combined NPK application with only rice stubble incorporation may be sustainable for a higher rice yield, but this approach has a reduced rate of negative carbon sequestration in the paddy field. Straw covering with no tillage was the best measure to realise high yield and low carbon emission for RIS.