Drainage intensity in paddy rice fields : nitrogen flows, salinity, rice yield and greenhouse gas emissions

Abstract

Managing drainage intensity is important in controlling soil moisture and nutrient losses and improving crop yields. This thesis evaluated the effects of drainage intensity on nitrogen losses, salinity and rice grain yield in three cropping seasons, and on gaseous emissions of methane (CH4) and nitrous oxide (N2O) from rice flowering to ripening in one season, on a marshland in semiarid region of Rwanda. Three drainage treatments were compared in a randomised complete block design: drainage to 0.6 m depth, weir open four times per week (S4); drainage to 1.2 m depth, weir open four times per week (D4); and drainage to 1.2 m, weir open twice per week (D2). In seasons 1 and 3, treatment D4 had higher drainage outflow and higher salt loads than treatments D2 and S4, but in season 2 treatment D2 had higher drainage outflow and higher salt loads than D4 and S4. Drainage water salinity (ECwd) decreased by around 41-57% from season 1 to season 2, and by 29-37% from season 2 to season 3. Soil salinity decreased by one electrical conductivity (EC) unit (dS m-1) from season 1 to season 2, and by a similar amount from season 2 to season 3. Nitrogen uptake and rice grain yield were significantly greater in the deep drainage treatments (D4, D2) compared with shallow drainage (S4). Deep drainage (D4, D2) reduced CH4 emissions but had no marked effect on N2O emissions. These findings suggest that deep drainage performs better than shallow drainage in semi-arid paddy fields, as it enables a balance between maintaining water in the soil and having sufficient drain outflow to leach salts, reduce CH4 emissions and achieve high rice yield.