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

Abstract 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 (CH 4 ) 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 CH 4 emissions compared to water‐seeded (WS) systems. The effect of these early‐season flushes on nitrogen (N) fertilizer losses and nitrous oxide (N 2 O) 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) (CH 4 + N 2 O in CO 2 eq.), nitrate (NO 3 − ) accumulation during flushes, and crop N‐uptake. Despite 0.68 kg ha −1 more N 2 O–N emissions in the DS system, GWP was 3700 CO 2 eq. kg ha −1 , a 42% reduction compared to 6340 CO 2 eq. kg ha −1 in the WS system. This was due to a 46% reduction in CH 4 in the DS (94.5 CH 4 –C kg ha −1 ) relative to the WS (175.7 CH 4 –C kg ha −1 ) system. Nitrate accumulation in the DS system amounted to 26.2 kg NO 3 –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 N 2 O emissions.