UC Davis

Flooded rice systems are critical for global food security but contribute significantly to anthropogenic greenhouse gas (GHG) emissions due to high methane (CH4) production in anerobic soils. Herbicide use in conventional rice systems has also created selection pressure for herbicide resistant aquatic weed species that threaten yields. Dry seeding (DS) rice, which in California includes early season drainage events, has been shown to reduce CH4 emissions and shift weed species emergence for improved control compared to continuously flooded water-seeded systems (WS). The effect of these drainage events on nitrogen (N) fertilizer losses and nitrous oxide (N2O) emissions, however, are not well understood. In a two-year study we quantified the effects of early season drainage events utilized in DS rice on global warming potential (GWP) (CH4 + N2O in CO2 eq.), nitrate (NO3-) accumulation, and N fertilizer losses as measured by the difference in crop N-uptake compared to a WS control. Despite 1.06 kg ha-1 more N2O emissions in the DS system the GWP was 4,610 CO2 eq. kg ha-1, a 42% reduction compared to 7,983 CO2 eq. kg ha-1 in the WS system. This was due to a 46% reduction in CH4 in the DS (126 CH4 kg ha-1) relative to the WS (235 CH4 kg ha-1) system. Nitrate accumulation in the DS system amounted to 25.9 kg N ha-1, and subsequent N losses via denitrification likely contributed to the 22.4 kg N ha-1 less crop N-uptake in the DS system. These results suggest that DS rice has potential for improved environmental impact via GWP reductions. Future research should consider the effects of increased pre-plant N application rates and timing for improved N management, a quantification of annual GWP including CO2 emissions, and changes in soil organic carbon stocks in DS rice.