Reducing the environmental impact of rice production in subtropical India by minimising reactive nitrogen loss

The future of reactive nitrogen (N) for subtropical lowland rice to be characterised under diverse N-management to develop adequate sustainable practices. It is a challenge to increase the efficiency of N use in lowland rice, as N can be lost in various ways, e.g., through nitrous oxide (N2O) or dinitrogen (N2) emissions, ammonia (NH3) volatilization and nitrate (NO3−) leaching. A field study was carried out in the subsequent wet (2021) and dry (2022) seasons to assess the impacts of different N management strategies on yield, N use efficiency and different N losses in a double-cropped rice system. Seven different N-management practices including application of chemical fertilisers, liquid organic fertiliser, nitrification inhibitors, organic nutrient management and integrated nutrient management (INM) were studied. The application of soil test-based neem-coated urea (NCU) during the wet season resulted in the highest economic yield, while integrated nutrient management showed the highest economic yield during the dry season. Total N losses by volatilization of NH3, N2O loss and leaching were 0.06–4.73, 0.32–2.14 and 0.25–1.93 kg ha−1, corresponding to 0.06–5.84%, 0.11–2.20% and 0.09–1.81% of total applied N, respectively. The total N-uptake in grain and straw was highest in INM (87–89% over control) followed by the soil test-based NCU (77–82% over control). In comparison, recovery efficiency of N was maximum from application of NCU + dicyandiamide during both the seasons. The N footprint of paddy rice ranged 0.46–2.01 kg N-eq. t−1 during both seasons under various N management. Ammonia volatilization was the process responsible for the largest N loss, followed by N2O emissions, and NO3− leaching in these subtropical lowland rice fields. After ranking the different N management practices on a scale of 1–7, soil test-based NCU was considered the best N management approach in the wet year 2021, while INM scored the best in the dry year 2022.