8 - Nitrogen Inputs From Biological Nitrogen Fixation in Indian Agriculture

Abstract

The fertilizer requirements of agriculture are increasing steadily across the world and the production of nitrogenous fertilizer through Haber-Bosch process is about 120Tgyear−1. It is expected that it may reach to 165Tgyear−1 (1Tg=1million tons or 109kg) by 2050, bringing with it all the attendant environmental consequences, particularly the increased production of reactive nitrogen species (Nr) and nitrate leaching to groundwater. Cultivation of crops adds N inputs though biological nitrogen fixation (BNF) amounting to 55–60Tgyear−1, which although environmentally benign to produce, yet results in production of reactive nitrogen albeit to a much lesser extent. The rate of Haber-Bosch mediated Nr increase is much faster than agrosystem-based BNF-Nr, and hence legumes are gaining greater attention. Reduction of the use of synthetic fertilizer nitrogen and improving the contribution from BNF in farming systems will benefit both agriculture and environment. By adopting the agricultural cropping area data of 2014, the BNF input added by various crops of India was estimated. BNF inputs by legumes were calculated from Ndfa (experimentally derived values of nitrogen derived from air) in the total N harvest (2.24Tg) which is lower than the estimate derived from legume acreage using mean gross BNF estimates of various legumes (2.81Tg). In these estimates, the contribution of pulses was 0.94 and 1.39Tg as per the two methods. However, for rice, wheat, and maize, BNF estimate based on nitrogen fraction using top-down global N budget (1.51Tg) was similar to the estimate derived from acreage using gross global BNF rate (1.50Tg) but higher when using gross BNF rates derived from long-term experiments in India (1.09Tg). In the calculations on gross BNF rate basis, the coarse cereals added 0.13Tg; sugarcane added 0.20Tg; and fodder legumes and agroforestry contributed 0.61 and 0.40Tg, respectively. A conservative estimate of BNF in Indian agriculture was computed as 5.20 to 5.76Tg (9.5–10.6% of global agricultural BNF). The contribution of cereal crops was 32% and grain legumes 43% of the cultivation-induced BNF of 5.2Tgyear−1. More extensive surveys, precise quantification in various N-fixing systems and regional scaling will help narrowing the variations in the estimates and may predict the future trends in BNF more accurately under various agroecological subregions. Such efforts along with integrated nutrient management involving microbial inoculation strategies would contribute to enhancing the BNF inputs, reducing the inputs of chemical N fertilizers, and improving the sustainability of food production and ecosystem health.