Reducing nitrogen fertilizer applications mitigates N2O emissions and maintains sugarcane yields in South China

Fertilizer N application combined with crop residue retention and a warm and humid climate increases nitrous oxide (N2O) emissions from sugarcane systems. However, the effects of high fertilizer N application rates on N2O emissions from sugarcane fields in China and the N2O reduction potential are still unclear. This study measured N2O emissions, the delta 15N values for N2O and soil mineral N (NH4+ and NO3-), and yields during three continuous growing seasons in a sugarcane field in subtropical South China when four fertilizer N application rates: 0 (N0), 300 (N300), 400 (N400), and the local conventional rate of 500 kg N ha-1 (N500), were applied. The results showed that N2O emissions peaked in the first 4 weeks after fertilizer N application and the peak amplitude was highest after applying the N500 rate. The greatest soil N2O fluxes occurred when soil NH4+ and NO3- contents were high, the soil pore spaces contained large amounts of water (around 60 %), and there was a high soil surface temperature (around 35 degrees C). The cumulative N2O emissions over the whole season (1.3-64.8 kg N ha-1) and emission factor values (2.6-11.1 %) both strongly increased with fertilizer N rate. The delta 15N values for N2O and soil NH4+ and NO3- indicated that N2O production after N fertilization was dominated by denitrification and the microbial types varied with time and N fertilization rate. The principal component analysis showed that the factors associated with denitrification and nitrification explained 53.53 % and 30.53 % of the variation in N2O fluxes, respectively. Sugarcane yields, N uptake, and sugar contents were not affected by the reduced fertilizer N rates (N400 and N300) compared to that of N500. A reasonable N fertilizer rate of around 340 kg N ha-1 could reduce N2O emissions by more than 65 % compared to the conventional N500 rates while maintaining sugarcane yields. This study quantified the N2O emissions induced by different fertilizer N rates in a sugarcane system in China and highlighted the considerable potential for reducing N2O emissions through optimization of the fertilizer N application rate.