Abstract:
Food security for more than half the world population highly depends on the ability of the world to produce rice (Oryza sativa). Rice paddies are known to be one of the main anthropogenic sources of the greenhouse gas methane (CH4), and use more water than all other staple food crops. Previous studies have examined the effectiveness of different water management regimes on mitigating GHG emissions and conserving water, however none of these were on smallholder rice farms in sub-Saharan Africa (SSA). In the SSA region, about 20% of the cultivated rice area is under flooded conditions also known as paddy rice fields. The area under rice cultivation in this region is forecast to rise to meet rice demand which has increased considerably than anywhere in the world. No previous research has investigated GHG emissions in rice production systems in Kenya and how soil type and water management regimes influences emissions. This study was therefore conducted during the rice growing season of 2017 (July – December) to address the paucity of data on GHG emissions from rice production in Kenya. Two rice water management systems (continuous flooding (CF) and alternate wetting and drying (AWD)) in two different soil types (Vertisols (VS) and Nitisols (NS)) were established in Mwea irrigation scheme (MIS) in central Kenya. The GHG fluxes were measured weekly (or more frequently depending on field management) using static GHG manual chambers. Alternate wetting and drying (AWD) water management regime greatly influenced GHG emissions (P < 0.001) during the rice growing season. AWD showed seasonal cumulative CH4 emissions values of 2.19 and 0.90 kg CH4-C ha-1, 88% and 84% lower CH4 emissions; while increasing N2O emissions by 72% and 50% (0.31 and 1.29 Kg N2O-N ha-1), compared to CF in VS and NS respectively. With CH4 and N2O emissions expressed as CO2 equivalents for a 100-yr horizon, AWD in the VS and NS soils lowered global warming potential (GWP) by 76% and 8%, respectively. Soil type had a significant (P < 0.001) effect on the GHG emissions with the VS soil having higher CH4 and lower N2O emissions compared to the NS soil. Interaction of water management regime and soil type greatly influenced (P < 0.001) the GHG emissions. Considering grain yield and GHG emissions together, AWD allowed for lower yield-scaled GWP. Higher water productivity was achieved under AWD in both soils. These findings suggest that AWD could be the best option for not only reducing GHG but also increasing irrigation water productivity.
