Reactive nitrogen in Indian agriculture: Inputs, use efficiency and leakages

The flows of reactive N in terrestrial, aquatic and atmospheric ecosystems in India are being increasingly regulated by inputs, use efficiency and leakages of reactive N from agriculture. In the last three decades, use of reactive N in the form of chemical fertilizers has kept pace with the production of foodgrains, although the consumption is concentrated in certain areas with intensive farming. As for cereal-based agriculture, recovery of N by rice and wheat at on-farm locations in India rarely exceeds the 50% mark. Agricultural activities in India account for more than 80% of the total N2O emissions, including 60% from the use of N fertilizers and 12% from burning of agricultural residues. In Asia, reactive N transfers to the atmosphere by NH3 volatilization are expected to reach 19 Tg N yr(-1) in the next three decades; 29% being India's contribution. Of the total anthropogenic emissions of NOx and N2O from Asian agriculture, about 68% is due to the combined contributions of India and China. Additionally, riverine discharge of dissolved inorganic N derived from N in river basins and leaching of nitrate-N to the surface and ground water bodies also contributes to the application of reactive N in agriculture. Integrated management of organic amendments and fertilizer N can improve efficiency of reactive N use by crop plants, while achieving targets of productivity and quality. The greatest challenge in improving N use efficiency lies in developing precision management of reactive N in time and space. Approaches to maximize synchrony between crop-N demand and the supply of mineral N from soil resources along with reactive N inputs in high-yielding agricultural systems are critical towards this end. Among a host of upcoming technologies aimed at improving N management strategies, leaf colour charts, chlorophyll meters and optical sensors, which allow in-season estimation of N requirement of crops, are the most promising.