Atmospheric Deposition Of Nitrogen Compounds In Assam (India)
Abstract
The study comprises estimate of wet deposited nitrogenous compounds in Assam (India). Deposition has been estimated from a survey works (2010 -11) at urban and peri -urban areas of Assam. Air samples were collected by clinical syringe (10 cm3) for Oxides of Nitrogen (NO, NO 2: NOx) and reduced Nitrogen (NH 3, NH 4+: NHy). The samples were diffused into 10 cm3 each of distilled water and 0.1N HCl respectively for estimation of mean concentration of weighted hydrogen (µeql-1), quantity of elemental nitrogen (N) or nitrate (NO 3-1) and ammonium (NH 4+) ions expressed in m g l-1 or kg ha-1y-1. The measured concentrations of the nitrogenous compounds were interpolated with a properly used Kriging Technique on a 1km x 1km grid covering districts characterised by varying congestions of population, vehicular transport and of industrial evidences. There were many fold variations of these air quality parameters among the major sites and locations of the pollutants e.g. nitrogen deposited through aerosol of its oxides ranged from 6.0 -38 kg ha-1yr-1, whereas nitrogen accumulation from the reduced aerosol was 7 -24 kg ha-1yr-1. Tissue nitrogen in some indicator plant species (e.g. Pinus longifolia , Ficus benjamina), collected from the square grids of polluted areas was also elevated. Thus, the hypothesis that the North -east India, especially Assam is also facing with enrichment of nitrogenous pollution due to anthropogenic activities, mass vehicular and industrial growth, was tested.
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Introduction
The inputs of reactive N (termed here as NH x:NH 3 and NH 4+, NO x :(NO + NO 2), and nitrate (NO 3-) and nitrite (N 2O) are through increased biological fixation of atmospheric N 2 in certain crops, combustion of fossil fuels and release of NO x, and production of synthetic fertilizer nitrogen (Galloway et al., 1994, 1995). There is an additional release of reactive N from stable soil organic matter as a result of soil disturbance for crop production that is not readily quantifiable (Edu Dorland et al., 2013). Nitrogen ox ides have increased in importance in recent decades as atmospheric pollutants in rapidly growing urban and its surrounding areas of India (Bharali et al. , 2012; Vitousek et. al., 1997). Deposition of gaseous Ammonia and particulate NH4+ may also contribute to the acidification of the ecosystem (Mohan and Kumar, 1998; McClean et. al . 2011). The wet and dry deposited oxides of N redistribute nitrogen throughout the environment. Ammonia as an atmospheric pollutant can contribute to a substantial portion of tot al deposition of nitrogen (Sutton, Moncrieff and Fowler, 1992). In fact, emissions of ammonia and nitrogen oxides have been strongly increased globally since the 1950s (Galloway et. al. 2008) . The source of volatilised Ammonia is the intensive agricultural systems related to dairy farming, animal husbandry, whereas nitrogen oxides are linked to mainly anthropogenic activities viz., burning of fossil fuel by traffic, industry and households. So, many natural and semi -natural ecosystems across the world are deposited with the atmospheric nitrogen as a result of short - and long-range transport of the nitrogenous pollutants (Bobbink et. al. 2010). Since 1990s, areas recognised historically with high atmospheric N deposition (20 –100kgNha−1year−1) have been the central and western Europe, eastern USA, eastern Asia and India. Estimated background inputs (pre -1900s) ranged between 1 and 3 kgNha−1 year−1 (e.g. Asman et. al. 1998; Dentener et al. 2006; Galloway and Cowling 2002). Thus, atmospheric nitrogen deposition and climate variability are both measured as major components of global change (Gaudnik et al. 2011). The exploration for possibilities of current deposition of nitrogen in the province of Assam by virtue of any reason needs a strong attention.
Conclusion
The wet deposited nitrogen (NOx, NHy and their compounds) was estimated following a square grid (1km x1km) resolution in a few major districts of Assam. The deposition had been assessed from the measured and modeled concentrations of acidifying compounds in the atmosphere and the precipitations with reference to UKRGR (1997). Net deposition of Nitrogen through its oxides ranged from 6.0 -38kgha-1yr-1, whereas net deposition of nitrogen by ammonia was in between 7 -24 kgha- 1yr-1. A total maximum deposition of N through oxidized and reduced forms of Nitrogen together is about 18 -46 kg ha-1yr-1. The hypothesis that Assam (especially Guwahati and its adjoining areas) is also facing with enrichment of nitrogenous pollution due to anthropogenic activities and mass vehicular and industrial growth was tested positive.