Environmental Footprint of Reactive Nitrogen in Indian Agricultural Sector: An Extended Input-Output Analysis

Food, feed, and fuel production are vital for human well-being. Yet current agricultural practices have resulted in extensive multi-media damages, primarily due to reactive nitrogen (Nr) emissions (NH₃, N₂O, NO_x). Managing Nr sustainably to alleviate food and feed insecurity has been identified as a Grand Engineering Challenge. Systematically analysing source contributions, flows, and impacts of Nr is crucial for an agro-dominant country like India that faces the dual challenge of food and environmental security for 1.6 billion people by 2050. Here, we construct an Environmentally Extended Input-Output model for Nr in the Indian agriculture sector (cropland + livestock) for 2000–2020. Our findings indicate an increase in total N input to cropland from 23 Tg-N to 33 Tg-N (2000–2020), largely attributed to synthetic fertilizers (62%), biological N fixation (17%), atmospheric nitrogen deposition (11%), and livestock manure use (7%). Despite these increases, nitrogen use efficiency has only improved marginally (45% in 2000 to 57% in 2020). Nr losses to hyrosphere constitute 55%-60% of total N, with atmospheric emissions accounting for 40%-45% of total N. Key pollutants include nitrites/nitrates lost through runoff (40%), NH₃ emissions (34%), and NO₃ leakage to groundwater (20%). Noteworthy are NH₃ emissions from fertilizer (55%) and manure (28%) application, and nitrogen deposition (12%).

Flows from the cropland sector serves as an input to the livestock sector, e.g., the production of grain and straw as feed to turn plant protein into animal protein, with efficiency varying from 4% to 10%. The type of animal and manure management systems and practices influences the N flow outputs from the livestock sector. Nitrogen within the remaining fraction (90–96%) is found in urine and dung, leading to potential nitrogen losses, i.e., 13.7TgN in the year 2020 due to the volatilization, leaching, and runoff as a result of application on cropland and manure management system of manure. Bovine animals have the largest share in manure N production, i.e., non-dairy cattle (37%), dairy (20%), and buffalo (26%), which constitute 83% of the total manure N production. Of the total produced manure (17Tg-N), 80% is produced in agriculture, and 20% is in pastoral areas. Of the agriculturally produced region, 30% undergo manure management system for treatment, i.e., 4TgN, while 70% are used for fuel combustion. Manure subjected to treatment is reintegrated into cropland at a rate of 52%, with approximately half being environmentally lost. Of this loss, 27% is attributed to atmospheric dissemination, comprising 22% as NH₃ resulting from volatilization and 5% through direct emissions of N₂O. Furthermore, 22% of the nitrogen is lost to the hydrosphere, distributed as 19% through runoff (0.8TgN) and 3% (0.1TgN) via NO₃- leaching.   Opportunities to alleviate N losses and boost feed conversion efficiency involve refining animal feed composition and the herd's genetic potential. However, a challenge remains in upgrading manure management practices. Our study constraints national-scale inputs, accumulation, and flows of Nr in Indian agriculture to enable a holistic approach to co-develop agriculture and environmental policies while identifying levers to enable greener agricultural production practices.