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

Abstract

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.

Keywords: Ammonia volatilization; Lowland rice ecosystem; Neem coated urea; Nitrate leaching; Nitrogen use efficiency; Nitrous oxide.