Wet deposition of total dissolved nitrogen in Indo-Gangetic Plain (India)

Very limited information on the magnitude and environmental impacts of both inorganic and organic forms of nitrogen (N) wet deposition is available in India. Molar concentrations of inorganic (NH4+ and NO3-) and organic N in rainwater were monitored at three different land use sites in Indo-Gangetic Plain (IGP) during the monsoon period (June–September) of 2017. It has been observed that dissolved organic N (DON) contributed significantly to the total dissolved N (TDN) ranging from 5 to 60%. Dissolved inorganic N (DIN = NH4+ + NO3-) concentration was recorded as high as 221.0 μmol L−1 at urban site to as low as 65.9 μmol L−1 at the rural site. A similar pattern was also observed for DON. NH4+ contribution to TDN had the order urban megacity (65%) > urban (70%) > rural (75%). Agriculture and animal husbandry are the primary sources of NH4+ emissions in the rural site. However, NO3- has shown a contrasting trend at these sites (25%, 15%, and 8%, respectively). Wet deposition fluxes of atmospheric TDN were observed to be higher at urban sites. This can be attributed to a variety of local sources such as vehicular emission, microbial emissions, biomass burning, human excreta due to higher population density, and transportation from surrounding areas, as observed from concentration weighted trajectories (CWT) model and cluster analysis. Upwind region of IGP has experienced major influence of air mass transported from agriculturally rich northwest part of India. However, both the downwind sites have experienced by-and-large the influence of south-westerly air masses originated over the Arabian Sea. This study has found that the DON contributes significantly to TDN, and therefore, its inclusion for nitrogen budget assessment in South Asia is emphasized.