Optimum nitrogen fertiliser rates maintain yields and minimise nitrous oxide emissions

Abstract

Key findings •• Fertilising above the optimum plant nitrogen (N) demand level reduced N-use efficiency and increased environmental impact. •• Despite receiving 8–9 irrigations as well as several intense rainfall events in a season, there were typically only 3–4 significant nitrous oxide (N2O) flux periods during the season, with most occurring early in the season, i.e. irrigations 1 and 2. •• Other significant N2O emissions events occurred mostly in response to adding in-crop N fertiliser followed by irrigation and/or intense rainfall. •• Initially, higher N2O emissions came from the beds where N fertiliser had been applied pre-sowing. Later in the season, emissions often peaked in the non-irrigated furrows after rainfall and irrigation events, particularly after additional N was added in the irrigation water. •• Farmers’ N rates were at least 25% higher than necessary to achieve the same lint yield in 2014–15, i.e. 50–100 kg N/ha. Less N fertiliser could have produced the same amount of cotton. •• Increasing N rates increased total N2O emissions, especially in prolonged wet conditions.