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.