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

Abstract

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 CO2 eq. ha−1) and GHG intensity (GHGI, kg CO2 eq. kg−1 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 ha−1) was achieved at 112–119 kg N ha−1, 50% higher than the yield of the control (0 kg N ha−1). Increasing N rate increased the seasonal N2O flux by 4.56–7.11 g N2O kg−1 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 ha−1. Thus, GWP also showed a quadratic response to N rate, peaking at 122–130 kg N ha−1. The GHGI decreased as N rate increased and was the lowest (1.10–1.28 kg CO2-eq. kg−1 grain yield) at 104–112 kg N ha−1, 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).