Nitrogen is the most limiting nutrient controlling the primary production of all agricultural systems. Intensively cultivated systems, therefore, require exogenous application of nitrogen both in inorganic and organic forms. Worldwide, use efficiency of external nitrogen supplied is as low as 33 per cent (J K Ladha, et al., 2005, ‘Efficiency of fertiliser nitrogen in cereal production: retrospects and prospects’, Journal of Advances in Agronomy), while the unaccounted for 67 per cent represents loss of nitrogen fertiliser from agricultural systems and is a cause of serious concern. Soil nitrogen dynamics is an integral component of the global nitrogen cycle. A single nitrogen molecule (N2) introduced anywhere in system can have cascading effects in various parts of the environment, after it has been converted to reactive nitrogen forms, such as ammonia (NH3), nitrogen oxides (NOx), nitrous oxide (N2O), nitrate (NO3-), urea, amines, proteins and nucleic acids. Nitrous oxide, as a greenhouse gas is responsible for global warming and climate change (Fig. 1).
The objective of this article is to assess the role of nitrogen use in gaseous-N emission from Indian agriculture and assess its mitigation options.
Emission of gaseous-N
Agricultural activities have greatly altered the global nitrogen cycle and produced nitrogenous gases of environmental significance. There are a variety of sources of nitrogen in agricultural systems that are anthropogenic. These include (a) synthetic fertilisers, (b) animal manures, (c) nitrogen derived from enhanced biological N-fixation through N2-fixing crops, (d) crop residue returned to the field after harvest and (e) human sewage sludge application etc. Although some part of the nitrogen from animal manures, crop residue and sewage may have come from previous application of synthetic fertiliser, the re-entry of this nitrogen into soil system again renders it susceptible to microbial processes of transformation. A major consequence of this human-driven change in global nitrogen cycle is the increased emission of N-based trace gases that impacted regional and global atmospheric chemistry.
The N2O is emitted into the atmosphere from both natural (water bodies and soils) as well as from anthropogenic activities like agriculture, transport, industries and waste management practices. One of the main controlling factors of N2O in air is the availability of inorganic nitrogen in the soil.
Nitrogen oxide (NOx) emission
The release of NOx has accelerated during the last few decades through primarily the increase in fossil fuel combustion and biomass burning. The high temperatures during combustion are generally responsible for the formation of NOx as it helps in breaking down the molecular nitrogen and oxygen of the air which recombine to form NOx which includes both nitric oxide (NO) and nitrogen dioxide (NO2). Vehicular exhaust is the largest contributor of NOx emission in India. The NOx emission from the soil is primarily a result of NO production through nitrification and denitrification.
Ammonia (NH3) emission
Use of fertilisers in the agricultural sector and rearing of livestock are mainly responsible for NH3 emission. The emission estimates of NH3 are highly uncertain in India as no country specific emission factor is available as yet. The emission varied from 0.98 to 25.8 kg N ha-1 depending upon soil type, crop and fertiliser material. Some studies have shown that the leaching loss of N from soils in the Indo-Gangetic Plains is 10-15 kg N ha-1 while the ammonia volatilisation loss is 20-30 kg N ha-1 with application of 120 kg N ha-1 in rice and wheat.
Intensive agricultural systems, envisaging high productivity, high input use, high rainfall or irrigated systems, are widely recognised as major sources of nitrous oxide emission. It is argued that there is substantial scope of mitigating nitrous oxide emission from such systems. Appropriate crop management practices, which lead to increased N-use efficiency and higher yield by optimising the crop’s natural ability to compete with the N-loss processes, hold the key.
The most efficient management practices are site-specific nutrient management and use of chemical inhibitors of nitrification such as nitrapyrin and dicyandiamide (Fig 2). There are some plant-derived organics such as neem oil, neem cake and karanj seed extract which can also act as nitrification inhibitors. Using a leaf colour chart nitrous oxide emission may be reduced and global warming potential may be mitigated by about 10 per cent (Bhatia et al., 2012, ‘Greenhouse gas mitigation in rice-wheat system with leaf colour chart-based urea application’, Environmental Monitoring and Assessment Journal). By using controlled release fertiliser in synchrony with plant growth it should be possible to provide sufficient N in a single application to satisfy the plant requirements. The gaseous loss would be small because of the limited substrate. Many different slow-release forms of nitrogen have been suggested for application but their adoption by the farmers is poor because of the extra cost involved and less marginal profit.
Nitrous oxide emission not only depends on the type of fertiliser nitrogen used but also on the mode of its application. The application of urea in plough layers leads to less emission of nitrous oxide than band application of urea. The emission of different amounts of nitrous oxide from different forms of fertiliser nitrogen suggests that controlling this factor is an efficient way of limiting the emission.
Methane and carbon dioxide are the other two green house gases emitted from agricultural fields. Methane fluxes are strongly influenced by the type, method and the rate of fertiliser application. In fact sulphate-containing fertilisers reduce methane emission. It has been shown that a nitrification inhibitor, encapsulated calcium carbide (a slow release source of acetylene), reduces carbon dioxide emission and appears to be effective in minimising methane emission in flooded rice.
Nitrogen management for climate change adaptation: The most common strategies suggested for climate change adaptation includes alternate land-use management, intensification of agriculture, crop diversification and conservation agriculture. All these strategies require a suitable nitrogen management strategy, which might be different from the conventional methods. Management of nitrogen can also help in improving crop quality, which is likely to deteriorate under climate change scenarios. As suggested, because of the higher levels of carbon dioxide concentration in the atmosphere and lower nitrogen availability in soil under climate change scenarios, crops would have higher carbohydrate content and lower protein content. Additional and efficient N application under elevated carbon dioxide can compensate the deteriorating quality of crop.