Optimum N rate for grain yield coincides with minimum greenhouse gas intensity in flooded rice fields

Excessive application of N fertilizer to rice results in water and atmospheric pollution including greenhouse gas (GHG) emissions. Therefore, N fertilizer management needs to be optimized taking into account grain yield, global warming potential (GWP, Mg CO 2 eq. haˉ1) and GHG intensity (GHGI, kg CO 2 eq. kgˉ1 grain). However, the tradeoffs between the effects of N rate on rice grain yield, GWP and GHGI have not been adequately evaluated. Therefore, field experiments to determine the effect of N rate (as urea) on yield, GWP and GHGI were conducted in a typical flooded, transplanted rice paddy in a temperate environment. Methane (CH₄) and nitrous oxide (N₂O) emission rates were determined throughout the entire year (both during growing and fallow seasons) over two years. Rice grain yield showed a quadratic response to N rate, and the maximum yield (6.7-6.8 t haˉ1) was achieved at 112-119 kg N haˉ1, 50% higher than the yield of the control (0 kg N haˉ1). Increasing N rate increased the seasonal N₂O flux by 4.56-7.11 g N₂O kgˉ1 N, but N₂O flux contributed less than 7% of the total GWP. The GWP was mainly determined by the CH₄ flux, which showed a relatively flat quadratic response to N rate, peaking at 124-138 kg N haˉ1. Thus, GWP also showed a quadratic response to N rate, peaking at 122-130 kg N haˉ1. The GHGI decreased as N rate increased and was the lowest (1.10-1.28 kg CO 2 -eq. kg −1 grain yield) at 104-112 kg N haˉ1, approximately 20% lower than GHGI in the 0 N treatment. In conclusion, the N rate for maximum yield was similar to the N rate for minimum GHGI, mainly because of the small effect of N rate on CH₄ emissions and the low magnitude of N₂O emissions. Thus, GHGI was largely driven by grain yield, so the N rate for maximum grain yield was similar to the N rate for maximum GHGI. Proper N fertilization is essential in rice farming systems to increase crop productivity and reduce the global warming impact (GWP and GHGI).