Wet deposition of atmospheric inorganic reactive nitrogen (Nr) across an urban-industrial-rural transect of Nr emission hotspot (India)

Abstract

The present study comprehensively reports the simultaneous measurement of wet deposition of total inorganic nitrogen (TIN; which is the sum of the NH₄⁺-N and NO₃⁻-N) at three different sites in Nr emission hotspot of Indo-Gangetic plain (IGP) over a year-long temporal scale from October 2017 to September 2018. At rural Meetli (MTL) site, urban Baraut (BRT) site and industrial Loni (LNI) site, the annual wet deposition of NH₄⁺-N was estimated as 21.87, 19.48 and 7.43 kg N ha⁻¹ yr⁻¹, respectively; the annual wet deposition NO₃⁻-N was estimated as 12.96, 12.17 and 4.44 kg N ha⁻¹ yr⁻¹, respectively; and the annual wet deposition of TIN was estimated as 34.83, 31.64 and 11.87 kg N ha⁻¹ yr⁻¹, respectively. NH₄⁺-N was dominantly contributing species in annual, monsoon and non-monsoon-time wet deposition of TIN at all sites. The spatial gradient (variability) in percent contribution of NH₄⁺ to total annual volume-weighted mean (VWM) concentration of all analyte ions was observed as MTL (43.23%) > BRT (37.90%) > LNI (30%). On the other hand, the spatial gradient in percent contribution of NO₃⁻ to total annual VWM concentration of all analyte ions was observed as MTL (7.45%) > BRT (6.89%) > LNI (5.32%). The extremely narrow range of NH₄⁺-N/NO₃⁻-N ratios (ranging from 1.60 at BRT site to 1.69 at LNI site) showed the approximately equal relative abundance of oxidized and reduced nitrogen (N) deposition across all sites. Inferences from enrichment factor analysis, principal component analysis and Pearson’s correlation coefficient analysis suggested that across all sites, virtually all NH₄⁺-N and NO₃⁻-N depositions were originated anthropogenically. The annual wet deposition of TIN measured in this study showed ≥ 6865%, ≥ 6228% and ≥ 2274% increment than the natural N deposition rate at MTL, BRT and LNI site, respectively. These empirically measured annual wet depositions of TIN also emanated theoretical transgression of critical N load threshold across all sites therefore signifying probable undermining of long-term elastic stability and resilience of ecosystems against stressor in the study domain.