Drill-seeding rice reduces global warming potential but increases nitrogen loss potential compared to water-seeding

Flooded rice (Oryza sativa L.) systems are critical for global food security but contribute significantly to anthropogenic greenhouse gas (GHG) emissions due to high methane (CH4) emissions from anaerobic soils. Drill-seeding (DS) rice, which in California includes early-season irrigation flushes to establish the rice, has been shown to reduce CH4 emissions compared to water-seeded (WS) systems. The effect of these early-season flushes on nitrogen (N) fertilizer losses and nitrous oxide (N2O) emissions, however, is not well understood. In a 2-year study, we quantitatively compared DS to WS systems with respect to their global warming potential (GWP) (CH4 + N2O in CO2 eq.), nitrate (NO3-) accumulation during flushes, and crop N-uptake. Despite 0.68 kg ha-1 more N2O-N emissions in the DS system, GWP was 3700 CO2 eq. kg ha-1, a 42% reduction compared to 6340 CO2 eq. kg ha-1 in the WS system. This was due to a 46% reduction in CH4 in the DS (94.5 CH4-C kg ha-1) relative to the WS (175.7 CH4-C kg ha-1) system. Nitrate accumulation in the DS system amounted to 26.2 kg NO3-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 but will require increased N inputs. Future efforts should focus on reducing N losses, which have a negative economic impact for the farmer and contribute to N2O emissions.