How can we mitigate it?

Did you know that the soil that nourishes our food is simultaneously poisoning the air we breathe?  Nitrogen fertilisers, widely applied on agricultural lands across the Indo-Gangetic Plain and Himalayan Foothills (IGP-HF), is among the major hidden pollution sources silently intensifying fine particulate matter (PM_2.5) pollution in the region’s airshed, without leaving any smoke trails or visible signatures.

While vehicular pollution, emissions from brick kilns and crop residue burning dominate headlines, the adverse impact of nitrogen fertiliser emissions remain overlooked.  How does this damage occur?

From soil to smog: the N ¹ to PM_2.5 chain

Let us take the case of urea, a nitrogenous fertiliser predominately applied in the Indo Gangetic region due to historical politico-economic reasons such as subsidy regimes, price controls, and entrenched agricultural practices.  

Air pollution from urea occurs by a two-step process. Firstly, by hydrolysis, a process in which soil enzymes break down urea into ammonium and carbon dioxide, temporarily raising the alkalinity ^[2] of the soil; and secondly, by ammonia volatilisation, a process by which the ammonium in the soil shifts into un-ionised ammonia gas (NH₃), given the alkaline condition of the soil from hydrolysis and intensifying under warm weather conditions.

The soil ammonia gas thus released, reacts with atmospheric sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from other combustion sources, like vehicles and power plants, to form fine particulate matters (PM_2.5). The PM_2.5 thus formed are known as “secondary inorganic aerosols” (SIAs), as these are not emitted directly from the pollution source but created secondarily through chemical reactions of ammonia and gaseous precursors like SO₂ and NOₓ.

Figure 1: The Nitrogen PM_2.5 Chain: How urea applied in rural fields generates secondary PM_2.5 that pollutes cities hundreds of kilometres away

STEP 1 - FIELD

Urea applied to crops

More than 50% of applied nitrogen is never absorbed. Excess sits on the soil surface under hot, alkaline conditions.

STEP 2 - AIR

NH₃ volatilises

Ammonia gas rises and drifts across state and national borders invisible but now measurable from space via satellite.

STEP 3 - CITY LUNGS

Secondary PM_2.5 forms

NH3 + SO2 + NOx forms ammonium sulphate & nitrate: fine particles small enough to lodge deep in human lungs.

Although the formation of SIAs is invisible, their impacts are sweeping and severe. Unlike primary pollutants, SIAs can persist in the atmosphere for days to weeks, exerting regional effects that often outweigh those of long‑lived greenhouse gases. Their most visible consequence is the exacerbation of regional haze across IGP-HF. More insidious are their gradual impacts in altering regional climate patterns by scattering and absorbing solar radiation; and in increasing cardiovascular and pulmonary health risks due to their deep penetration into the human respiratory system.

Yet nitrogen fertiliser emissions hardly find place in either the public or the policy discourse on plummeting air quality in the IGP-HF airshed.

IGP’s nitrogen use efficiency (NUE) puzzle: Where does our soil nitrogen go?

Measured in terms of crop productivity or yield^[3] per kilogram of nitrogen applied, nitrogen use efficiency (NUE) is a critical indicator of the agricultural nitrogen balance or imbalance in a region. A high NUE reflects efficient fertiliser use, with most of the applied nitrogen absorbed by crops. Conversely, a low NUE signals inefficiency, with large portions of nitrogen lost through leaching, runoff, ammonia volatilisation, or denitrification.

Several field studies reveal wide variability in NUE across cereal systems in the IGP‑HF, with more than half of applied nitrogen often unabsorbed by crops. One major driver of this inefficiency is the over‑application of nitrogen fertilisers. A practice rooted in the Green Revolution of the mid‑20th century and reinforced over time by the subsidised pricing of urea and the entrenched perception among farmers that “more urea means higher yield.”

Contrary to this perception, the region is now caught in a paradox of diminishing returns from rising nitrogen applications. Instead of sustained productivity gains, over‑application has exposed the inefficiencies of nitrogen use and unleashed mounting environmental ramifications, including plummeting soil health, contamination of groundwater table, ammonia volatilisation, PM_2.5 pollution and nitrous oxide emissions in the region.

Figure 2: Where does all the nitrogen go? Nitrogen use inefficiency and air pollution in IGP-HF

The over-application rate is palpably alarming in the Indian states like Punjab and Haryana, where farmers routinely apply twice the NPK (nitrogen: phosphorous: potassium) ratio recommended by national guidelines. In countries like Bangladesh, Nepal and Pakistan, where comparable national guidelines are lacking, one can assume the extent of overuse to be more severe, being largely left to farmers’ discretion and the market forces.

Estimates from an atmospheric modelling exercise using the International Institute of Systems Analysis’s (IIASA) Greenhouse Gas and Air Pollution Interactions and Synergies (GAINS) framework show that annual ambient PM_2.5 from nitrogen fertiliser emissions hovers between 1.4 and 3.4 micrograms per cubic metre (μg/m³)across various IGP states, reaching  seasonal peaks as high as 7.6 μg/m³ during intensive fertiliser application periods in the crop cycles. These are not trivial numbers in a region where even marginal reductions in particulate matter pollution can prevent thousands of premature deaths each year.

The situation is even more concerning with fertiliser-linked PM_2.5 emissions projected to dampen some of the air quality gains expected from reduced crop residue burning by 2035, as stricter enforcement and alternative crop-management technologies begin to take hold.

Figure 3:  Emissions from nitrogen fertilisers vs crop residue burning in the IGP-HF: Actual emissions in 2021 vs projected emissions in 2035 based on the GAINS model estimations

Lacking fertiliser emissions management in IGP: A gap in design or in governance?

Although several national governments across the IGP-HF have invested significantly in air-quality, such as India’s National Clean Air Programme, the ambitions of such programs, as reflected in India’s Clean Air Action Plan targets, remain essentially urban-centric. Rural and peri‑urban landscapes, which are the source sites of fertiliser emissions, continue to fall outside the scope of systematic management.  

On the other hand, the nodal agriculture ministries, which control fertiliser policy, have historically had no mandate to worry about urban air quality.

In countries like Nepal, Bangladesh, and Pakistan, fertiliser emissions management is even more precarious as the structural blind spots are intrinsically compounded by their less developed national regulatory infrastructures.

While the absence of nitrogen fertiliser governance comes across as a flaw in policy design, it cascades into a glaring implementation gap and is further reinforced by weak or absent enforcement mechanisms.

In addressing this dual gap in policy design and governance, one must remember that efficient nitrogen fertiliser use is not merely an environmental imperative. It is also an equally compelling economic mandate.

When unabsorbed nitrogen fertilisers are lost to air or water, the consequences extend beyond degraded ecosystems and public health. It also means that the millions of dollars spent on fertiliser subsidies across IGP‑HF countries are effectively wasted - draining government exchequers but failing to deliver commensurate economic gains.

Figure 4: The triple burden of nitrogen misgovernance: Soil nitrogen lost in air or water, imposes an environmental burden, triggers associated health consequences, and results in fiscal losses from the wasted fertiliser subsidies

Towards efficient nitrogen management: Innovative incentives, responsive science, ambitious target‑setting

Smarter incentives for smarter nitrogen use:

• China offers the most instructive precedent for the IGP‑HF. A decade ago, it applied nearly 300 kg of nitrogen per hectare, among the world’s highest rates in smallholder paddy‑wheat systems under heavy subsidies. In 2015, Beijing introduced its ‘Zero Growth’ policy, shifting the measure of agricultural performance from fertiliser sales to nitrogen use efficiency.

• Within India, an innovative example comes from Andhra Pradesh, where farmers receive about USD 8 for every bag of urea they forgo. Under the Prime Minister’s Programme for Restoration, Awareness Generation, Nourishment and Amelioration of Mother Earth (PM‑PRANAM) scheme’s Green Direct Benefit Transfer model, this policy overturns decades of fertiliser subsidies by rewarding farmers nearly three times the fertiliser cost to not use it.

Science and technology enabled NUE:

• Neem-coated urea, already mandatory in India, reduces ammonia volatilisation by slowing soil enzyme (urease) activity at the soil surface. The next step is ‘Neem Plus’, urea coated with pharmaceutical-grade urease inhibitors^[4] such as N‑(n‑butyl) thiophosphoric triamide (NBPT), which can reduce ammonia emissions by up to 70% without any change in how farmers handle or apply their fertiliser.
• GIS‑based soil mapping and site‑specific nutrient management initiatives are being piloted in the Indian state of Uttar Pradesh. These pilots aim to move towards precision nutrient management (PNM), with the explicit goal of improving NUE.
• A region‑specific fertiliser efficiency Package of Practice (PoP) is urgently needed. Such a package should build on the science‑policy‑praxis interface by leveraging state agricultural universities and government extension networks to design, pilot^[5], monitor^[6], and scale practices^[7]. Farmer collectives must be harnessed to overcome adoption barriers and extend outreach^[8], ensuring that locally grounded solutions translate into real efficiency gains.

Figure 5: Pathways to nitrogen fertiliser management: the six priority areas of intervention spanning technology, policy, institutions, and monitoring.

Vision 35-35: Transforming NUE from a technical goal to a clean air mission

The IGP-HF region has set an ambitious clean-air target of reducing PM_2.5 concentrations 35 µg/m³ by 2035. This ‘35-by-35’ goal is achievable if the region tackles every major source of particulate pollution, including agricultural ammonia.

What is needed now is a systemic response on multiple fronts, beginning with an airshed approach that treats the IGP-HF airshed as a shared common. Countries can also set up national nitrogen authorities to bring agriculture, environment, and finance ministries into one conversation, and craft fertiliser policies that reward efficiency rather than volume.

The moment is ripe: policy windows are open, state-level clean-air projects are being scaled, satellite monitoring is democratising data, enabling innovation and R&D beyond government actors. What is needed now is stronger political will to connect these dots and transform fertiliser efficiency to a public‑good mission for cleaner skies, healthier communities, and resilient yields.

To know more about the targeted management of agricultural nitrogen in the IGP-HF region, read our technical report here.

^[1] Symbol of nitrogen

^[2] Ability of soil to neutralise acids and in effect maintain basic nutrient balance; also called the buffering capacity of soil.

^[3] The volume of crop per hectare of land

^[4] An inhibitor slows the breakdown of urea in soil, thereby reducing ammonia volatilisation and NUE. NBPT is considered the most effective and commercially successful urease inhibitor globally

^[5] Test PoPs in high‑emission agro‑climatic zones to generate field‑level evidence

^[6] Implement monitoring systems that apply the 4Rs (right source, right rate, right time, and right place) of nutrient management and track yields and emissions.

^[7] Adopt PoPs nationally once evidence shows productivity is maintained and pollution reduced.

^[8] Use Custom Hiring Centres and machinery pools to overcome equipment barriers and outreach to women farmers.