Deep Drainage Lowers Methane and Nitrous Oxide Emissions from Rice Fields in a Semi-Arid Environment in Rwanda

Few studies have explored greenhouse gas (GHG) emissions from arable land in sub-Saharan Africa (SSA), and particularly from rice paddy fields, which can be a major source of methane (CH4) and nitrous oxide (N2O) emissions. This study examined the effect of drainage on CH4 and N2O emissions from rice fields in Rwanda under shallow drainage to 0.6 m, with the drain weir open four times per week, and deep drainage to 1.2 m with the weir open four times or two times per week. CH4 and N2O fluxes from the soil surface were measured on nine occasions during rice flowering and ripening, using a closed chamber method. Measured fluxes made only a minor contribution to total GHG emissions from rice fields. However, drainage depth had significant effects on CH4 emissions, with shallow drainage treatment giving significantly higher emissions (~0.8 kg ha−1 or ~26 kg CO2-equivalents ha−1) than deep drainage (0.0 kg) over the 44-day measurement period. No treatment effect was observed for N2O fluxes, which ranged from low uptake to low release, and were generally not significantly different from zero, probably due to low nitrogen (N) availability in soil resulting from low N fertilization rate (in the region). Overall, the results suggest that deep drainage can mitigate CH4 emissions compared with traditional shallow drainage, while not simultaneously increasing N2O emissions

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

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

Effects of drainage intensity on water and nitrogen use efficiency and rice grain yield in a semi-arid marshland in Rwanda

Drainage management is important in intensification of irrigated paddy rice production. This study assessed the effects of drainage intensity on water and nitrogen use efficiency and rice grain yield in a field experiment conducted during three seasons in Rwanda. The experiment comprised 12 plots with four blocks and three treatments: DS0.6(0.6 m deep drain), DD1(1.2)(1.2 m deep drain, control structure open four times per week), and DD2(1.2)(1.2 m deep drain, control structure open two times per week). Outflow was calculated from water balance. Nitrogen (N) content in drainage water was determined weekly. Crop yield and N uptake were determined in grain and straw. In all seasons, grain yield was 61-131% higher, crop N uptake was 24-90% higher, harvest index (HI) was 24-65% higher and water use efficiency (WUE) was 50-150% higher in treatments DD1(1.2)and DD2(1.2)than in DS0.6. There was a decrease in soil carbon/nitrogen ratio at the end of Seasons 2 and 3. Recirculating straw to fields is thus necessary to replenish SOC for long-term soil fertility. A practical implication of the study is that managed deep drainage systems could enhance water use efficiency and rice grain yield in poorly drained paddy fields

Deep Drainage Lowers Methane and Nitrous Oxide Emissions from Rice Fields in a Semi-Arid Environment in Rwanda

Few studies have explored greenhouse gas (GHG) emissions from arable land in sub-Saharan Africa (SSA), and particularly from rice paddy fields, which can be a major source of methane (CH4) and nitrous oxide (N2O) emissions. This study examined the effect of drainage on CH4 and N2O emissions from rice fields in Rwanda under shallow drainage to 0.6 m, with the drain weir open four times per week, and deep drainage to 1.2 m with the weir open four times or two times per week. CH4 and N2O fluxes from the soil surface were measured on nine occasions during rice flowering and ripening, using a closed chamber method. Measured fluxes made only a minor contribution to total GHG emissions from rice fields. However, drainage depth had significant effects on CH4 emissions, with shallow drainage treatment giving significantly higher emissions (~0.8 kg ha−1 or ~26 kg CO2-equivalents ha−1) than deep drainage (0.0 kg) over the 44-day measurement period. No treatment effect was observed for N2O fluxes, which ranged from low uptake to low release, and were generally not significantly different from zero, probably due to low nitrogen (N) availability in soil resulting from low N fertilization rate (in the region). Overall, the results suggest that deep drainage can mitigate CH4 emissions compared with traditional shallow drainage, while not simultaneously increasing N2O emissions