Nitrous oxide is a greenhouse gas and a major modulator of global climate. We know that the use of chemical nitrogen fertilisers, though necessary for agricultural productivity, leads to nitrous oxide emissions into the atmosphere. But not many are aware that nitrous oxide accumulates in groundwater.
To understand the extent of nitrous oxide accumulation in groundwater in diverse agricultural regions in India, a research team from the regional centre of the CSIR-National Institute of Oceanography, Visakhapatnam, collaborated with the Indo-French Cell for Water Sciences at the Indian Institute of Science, Bengaluru as well as with researchers from France.
The team selected four contrasting regions: the banks of the Godavari Estuary, the Berambadi agricultural zone, the Indian coastal region, and the Mule-Hole reserved forest. These sites, representing different land uses, range from heavily farmed areas to natural, undisturbed environments. Such a selection allowed the researchers to make meaningful comparisons in water chemistry based on land use.
They collected groundwater samples from open wells in all four regions to analyse water chemistry. The nitrous oxide in the water samples ranged from less than three nanomolar in Mule-Hole to more than sixteen hundred nanomolar in the coastal and Berambadi regions. The high nitrous oxide concentrations in groundwater under agricultural areas suggested a strong link between fertiliser-driven nitrate pollution and nitrous oxide accumulation in groundwater.
The nitrate concentrations in the groundwater also varied widely, from 10 micrograms per litre in the Mule-Hole forest to more than 655 milligrams per litre in the Berambadi region, where intense fertiliser use is prevalent.
To verify whether the nitrous oxide in groundwater was indeed produced from nitrogen fertilisers, the researchers examined the isotope ratios of nitrogen and oxygen. The heavier isotope of nitrogen is lower in chemical fertilisers than in nitrates formed naturally in the soil. So isotope ratios provide clues about the sources of nitrous oxide and transformations, such as the nitrification of ammonia which takes up oxygen and the denitrification of nitrates by bacteria under low oxygen conditions. The team used an isotope ratio mass spectrometer to measure the isotopes of nitrogen and oxygen. This technique helped confirm that the nitrous oxide came from nitrogen fertilisers, and not from animal waste or natural sources.
The researchers estimated how much nitrate was removed from groundwater through denitrification, a process where bacteria convert nitrate into nitrous oxide and nitrogen under low oxygen conditions. The highest conversion of nitrates into nitrous oxide was near the Godavari estuary, followed by the Indian coastal region. Denitrification in Berambadi was one-fourth of what was found in the coastal region.
In normal circumstances, low concentrations of dissolved oxygen indicate denitrification by bacteria. So the researchers measured dissolved oxygen levels. The dissolved oxygen concentrations ranged from a few micrograms per litre in Mule-Hole to more than six milligrams per litre in Berambadi. Low dissolved oxygen levels reflected minimal biological activity in the groundwater under pristine environments. This pattern suggests a link between oxygen availability and denitrification activity, say the researchers.
To understand the influence of rainfall on changes in groundwater chemistry, the researchers had collected water samples on a monthly basis where possible and only in dry and wet seasons where the collection of samples was difficult. The concentration of oxygen in the groundwater, they found, was linked to the precipitation pattern and groundwater extraction rates. Hence, the amount of nitrate denitrified also followed the patterns of precipitation and groundwater use.
The team calculated the nitrous oxide emission factor, the ratio of nitrous oxide concentration to total nitrate in groundwater. This factor helped assess how much nitrate pollution was being converted into nitrous oxide. The emission factors were highest in intensive farming areas, particularly along the coast.
Regions with heavy fertiliser use, like Berambadi, showed the highest nitrous oxide concentrations, while the Mule-Hole forest, a pristine site, showed minimal nitrous oxide in groundwater. Thus the research confirmed a clear link between fertiliser use, nitrate pollution and increased nitrous oxide in groundwater.
Nitrous oxide in groundwater under agricultural lands in India is much lower than the global average. But this does not mean that India is free from this environmental threat. To reduce nitrous oxide emissions and to protect groundwater quality, it is crucial to improve nitrogen management, optimize fertiliser use and adopt sustainable farming practices, say the researchers.
ACS EST Water, 5: 1557−1568 (2025);
DOI: 10.1021/acsestwater.4c00625
D C Jhariya
NIT, Raipur
STEAMindiaReports 
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