Contribution of fallow periods between rice crops to seasonal GHG emissions: effect of water and tillage management

Irrigated rice cultivation is a major source of greenhouse gas (GHG) emissions from agriculture. Methane (CH4) and nitrous oxide (N2O) are emitted not only throughout the growing season but also in the fallow period between crops. A study was conducted for two transition periods between rice crops (dry to wet season transition and wet to dry season transition) in the Philippines to investigate the effect of water and tillage management on CH4 and N2O emissions as well as on soil nitrate and ammonium. Management treatments between rice crops included (1) continuous flooding (F), (2) soil drying (D), (3) soil drying with aerobic tillage (D + T), and (4) soil drying and wetting (D + W). The static closed chamber method was used to measure CH4 and N2O fluxes. Soil nitrate accumulated and N2O was emitted in treatments with soil drying. Nitrate disappeared while ammonium gradually increased after the soil was flooded during land preparation, indicating net nitrogen mineralization. N2O emissions were highest in both transition periods in D + W (437 and 645 mu g N2O m(-2)h(-1)). Methane emissions were significant in only the F treatment. The highest global warming potential (GWP) in the transition between rice crops occurred in F, with CH4 contributing almost 100% to the GWP. The GWP from other treatments was lower than F, with about 60-99% of the GWP attributed to N2O emissions in treatments with soil drying. The GWP in the transition between rice crops represented up to 26% of the total GWP from harvest to harvest. This study demonstrates that the transition period can be an important source of GHG emissions with relative importance of CH4 and N2O depending on the soil water regime. Therefore, the transition period should not be disregarded when estimating GHG emissions for rice cropping systems.