Current World Environment

Introduction

The biogeochemical cycling of nitrogen is of vital importance due to the role of nitrogen in both aquatic and terrestrial ecosystems. However, global nitrogen cycle is being disturbed by the unchecked increase in food and energy production resulting in the accumulation of Reactive Nitrogen (Nr) in various environmental matrices. Generally, Nr refers to any formof nitrogen compound that is biologically active, photo chemically reactive and radioactively active in the biosphere and atmosphere of theearth.^1-2Nr includes inorganic reduced form of N (NH₃, NH₄⁺), inorganic oxidized forms (NO_x, NO₃^-, N₂O, HONO) & organic nitrogen compounds.³However, these Nr species are extremely important in atmospheric chemistry. Henceforth, the monitoring of Agricultural nitrogen air emissions and understanding their behaviour in atmosphere is crucial due to impending environmental and human health concern of the latter.One of the major sources of the incessant increase in the nitrogen air emissions is the agriculture activities followed by livestock, human waste, transport sector. Ammonia is one of the most important reactive Nitrogen (Nr) compounds emitted from agricultural sources. Other minor anthropogenic sources ofatmospheric NH₃emissions include animal manure, slash burning and industries⁴along with the natural sources such as forest fires, soils and oceans.^5-6. In India, IGP has been reported as hotspot of both NH₃ and NH₄⁺ emissions due to intense agricultural activities in the region.⁷ The major contributor for NH₃ emissions in agricultural systems includes inorganic fertilizers, livestock manure, increased biological N-fixation, biomass burning and residue added to the field after crop harvesting.For instance, as reported byMisselbrook et al.,⁸agricultural sector alone is responsible for about 90% of the total atmospheric NH₃ emissions. According to Vitousek et al.,⁹human interventions have approximately doubled the fluxes of Nr species viz NH₃, NOx,NH₄⁺, NO₃⁻, and N₂O¹⁰ .Human activities contribute approximately 170 Tg year^-1 of Nr into agroecosystems, where inorganic fertilizer additions account for about 80 Tg Nr annually.¹¹ For Indian Scenario, Aneja et al.,¹² reported the highest contribution of fertilizer application and livestock towards NH₃ as 2696.6 Gg and 1704.8 Gg respectively. Among various inorganic fertilizers, urea isthe most widely used N fertilizer in south Asia.¹³ Furthermore,urea is the highest contributor (~ 94%) of NH₃ because of its large scale use and nitrogen content is maximum among all other fertilizers being used in India.¹⁴ Urea alone contributes 2481.5 Gg NH₃whereas di-ammonium phosphate (DAP), NPK, ammonium phosphate contributes 124.8 Gg, 89.7 Gg and 0.6 Gg NH₃respectively, annually. Also, the rate of increase of urea in agricultural fields is estimated to be 92% per year, thus adding to the Nr budget.¹³45-55% of these Nr inputs are recovered by crop biomass whereas remaining is lost from agricultural systems via. leaching, erosion (32–45 Tg N year⁻¹) and denitrification(26–60 Tg N year⁻¹).^10-11Hence directly or indirectly influences the Nitrogen cycle.

A remarkable portion of N-fertilizer or excessreactive soil N (Nr) is lost through volatilization in the form of ammonia(NH₃) and surface run off which significantly contributes to the Nitrogen loss. Bouwman et al., ¹⁵ reported that 10–30% of the applied N fertilizer is lost through volatilization process. Once NH₃ is applied to the soil by fertilizer addition, NH₄⁺ gets retained on the exchange sites. NH₄⁺ is either nitrified to nitrate (NO₃^-), or decomposed to NH₃, depending on soil and environmental conditions. High temperature inhibits the nitrification process as reported by Grunditz and Dalhammar,¹⁶and hence, ammonia volatilization increases with increase in temperature.In agroecosystems, NH₃ emissions can be reduced by minimizing the N input, increasing the efficiency of N fertilizers.¹⁷

Some studies have been carried out by research groups for NH₃ inventories in Asian region¹⁸ which mainly relied on emission factors. However, no country-specific emission factor for NH₃ is available, there is high uncertainty in the emission estimates of NH₃ in India.¹⁹ As reported by Parashar et al ²⁰NH₃ emissions from fertilizers and livestock are estimated to be 1175 Gg and 1433 Gg, respectively. According to the estimates given by Yang et al,²¹ the concentration of gaseous ammonia varied from 1.3 to 17.2 μg m⁻³ throughout the year. These concentrations were observed at an agricultural field site which was remarkably higher in the summer than winter.

Fate of atmospheric ammonia (NH₃) is highly variable.NH₃ is the most abundant alkaline constituent in theatmosphere²²and a precursor molecule for secondary aerosol formation causing severe air pollution in East and South Asia by contributing ambient levels of fine particulate matter (PM_2.5).²³Transformation of atmospheric NH₃ into NH₄⁺via chemical processes of precursor gases (NH₃, SO₂, NO_x) occurs either by condensation or by direct nucleation. The major inorganic compounds formed through the gas-to-particle formation process are ammonium bisulfate (NH₄HSO₄), ammonium sulfate (NH₄)₂SO₄, ammonium nitrate (NH₄NO₃) and ammonium chloride (NH₄Cl).Due to the substantial increment in atmospheric NH₃ emissions and its transformation, more ammonium (NH₄⁺) is returned to terrestrial and aquatic ecosystems via dry and wet deposition mechanisms.In temperate regions, precipitation is the main scavenging mechanism whereas in dry regions (with no or little rainfall) dry deposition is the dominating mechanism for the removal of atmospheric pollutants. Since, in India~ 90 % of precipitation occurs during monsoon period (June to September), dry deposition is the dominating mechanism for the scavenging of atmospheric pollutants throughout the year.²⁴ Moreover, the fate of Nr species is determined by atmospheric acidity, particulate loading, land use dynamics, and photochemistry of the atmosphere.²⁵The deposition of NH₃ and NH₄⁺in various compartments across the globe causes a cascade of environmental problems such as eutrophication of terrestrial and aquatic ecosystem, biodiversity loss,forest damage, water and soil acidification.^26-27Hence its role in atmospheric phenomenon such as neutralisation of cloud water, precipitation and aerosol formation typically in the fine particle size range is pivotal.²⁸

In addition, the formation of ammonium particulates increases the residence time of NH₃ in the atmosphere thereby influencing the geographic distribution of acidic species. Also,deposition of these fine particles deep into the lungs causes morbidity and mortality in humans, alterations in visibility and climate.²⁹Henceforth,monitoring of atmospheric NH₃in agricultural fields is one of the decisive parameters needs to be considered. For this the study was conducted at a rural site in Jhajjar district of Haryana with the following objectives- i). to quantify the NH₃ concentration and its variation during different growing stages of crops sown in our study area and ii).to study the effect of meteorological parameters in order to identify the possible transformations of NH₃.

NH₃ emission rate from various agricultural activities and its downwind concentration mainly depends on meteorological conditions. There exists a positive relationship between incident solar radiation/air temperature and NH₃ emission from surface-applied fertilizers.^30-32 Solar radiation increases NH₃emission by increasing the atmospheric turbulence and hence,NH₃evaporates from the surface. Apart from that evaporation of water due to increasing air temperature increases the total N concentration at the surface. Consequently, the NH₃ at the surface is transported upwards by atmospheric turbulence and sideways, by advection. Hence, NH₃ emission levels are related to wind speed, relative humidity and temperature.³²Higher humidity and lower temperature favours the formation of NH₄⁺ aerosols in presence of NO₂ and SO₂.

Methodology

Site Description

A study was conducted at a rural site in Chhuchhakwas village of Jhajjar district in Haryana (Fig. 1). It lies between 28°22’ - 28°49’ North latitudes, and 76°18’ -76°59’ East longitudes. The village is located at a distance of 13 kms from Jhajjar district which lies in the south-east part of the Haryana state covering total geographical area of 1834 sq.km. The climate characteristics of this area are hot summer, cold winter and moderate rainfall of about 444 mm. This area comprises approximately 87.03% land under agriculture and 6.77% build up area, indicating very less development in terms of urbanization. The district covers an forest area of 41 km², net sown area of 1670 km² and cultivable area of 1760 km² (Agriculture department Jhajjar). The sampling site represents a typical rural atmosphere surrounded by agricultural fields in all directions.

Figure 1: Map of the Sampling Site Click here to view figure

Meteorological Data

Temperature, relative humidity and wind data of the study region for every month (July to October) were downloaded from theworldweatheronline.com.Table 1 shows the mean values of all above mentioned meteorological parameters during the sampling period.

Table 1: Monthly Variation of Temperature, Wind Speed and Relative Humidity During the Sampling Period

Experimental Setup

Collection of gaseous NH₃samples was performed on 6 hr basis during both daytime and night time (8am-2pm and 10pm-4am) from July to October 2017 using a sampling assembly (Fig. 2) consisting of a low volume pump operating at a flow rate of 1 LPM. Ammonia gas was absorbed in absorbing solution (20 ml of 25 mM H₂SO₄) in a standard impinger for 6hours.³³ The aerosol samples were collected on thePTFE filters (diameter 47 mm and pore size 0.2µm) placed upstream to the impingers. Filter pack was connected to the impinger through silicon tubing. Collected gaseous NH₃ samples in absorbing solution were transferred into centrifuge tubes, preserved in refrigerator and analyzedby catalyzed Indophenol-blue method. It includes photometric determination of NH₃ which is based on reaction with phenol and hypochlorite producing Indophenol, intensely blue in alkaline medium which is further analysed using spectrophotometer at 630nm.The collection efficiency of impinger technique for NH₃capture was estimated by using two impingers in series (Fig. 2), using the formula:

Collection efficiency = η₂∗100/(η₁ + η₂)                                 - 1)

where η₁ and η₂ are the values of optical density in impingers 1 and 2, respectively (Fig. 2)

Figure 2: Flow Diagram of Sampling Assembly Click here to view figure

Hence, collection efficiency of the NH₃ capture using this method was estimated as 67%.Furthermore, certainty and reliability in data collection was maintained by rejection of daily samples if any disruption was noted or post-sampling flow rate was reduced by more than 10% of the pre-sampling flow rate.

Agriculture and Cropping Pattern

Thefarming practices in Jhajjar district are dominated by agriculture activities and animal husbandry. The main crops sown in this region are Kharif cropsand Rabi crops.The word “Kharif” is Arabic for autumn since the season coincides with the beginning of autumn, also known as monsoon crops. These are the crops that are cultivated at the onset of the monsoon season around June and harvested by September or Octobereg.Oryza Sativa (Paddy), pearl millet (bajra), gossypium (cotton), Cyamopsistetragonoloba (guar) etc. and Rabi crops are agricultural crops that are sown in winter in month of November and harvested in Marcheg. Triticum aestivum (Wheat), Brassica juncea(mustard),Hordeum Vulgare(barley), etc.

Results and Discussion

Concentration of NH₃ Over the Study Period

Table 2 gives the concentration of NH₃ over the various periods (growing, post-fertilizer, grain filling, post-harvest) of the kharif crop(mainly pearl millet in our study area). Growing time in the month of July shows the least NH₃ concentration with an average value 29.68 µg m^-3 as compared to all other time phases. These values can be assumed as background values. Nitrogen fertiliser can either be incorporated into the soil (basal dressing) before growing the crop or applied to the soil surface (top dressing). In basal dressing, nitrogen fertiliser is placed below the soil layer which limits the rates of nitrification, denitrification, volatilization and therefore, enhance efficiency of N fertilizer use.³⁴ As Diammonium phosphate (DAP) fertilizer is mainly used in basal dressing (30 kg/hectare) prior to growing, the levels were very less as compared to the levels after top dressing (commonly urea) fertilizer application. The levels increased markedly after urea was applied on the soil surface on 1^st August with an application rate~100kg/hectare. Among various fertilizers being used in India, the NH₃concentrations peaked in conjunction with the application of urea. Contribution from urea is highest not only because of its excessive usage in Indian agriculture but also due to the large emission factor for NH₃ emissions.³⁵The average concentrations during day and night time were measured as 96 µg m^-3and 149 µg m^-3respectively.The levels of NH₃ are very high during grain filling and post-harvest period.As plant uptake of soil-Ndecreases towards maturity, NH₃ volatilization from soil increases. This may be responsible for higher NH₃ emissiontowards maturity.For further analysis during these months,variation of NH₃ was related with wind directions as described below (Fig. 7).

Table 2: Day-Night Concentration of NH₃ Over Various Time Periods of the Kharif Crop (Pearl Millet in our Study Area)

Diurnal Variation of NH₃ Over the Study Period

Fig.3  shows the diurnal variability in NH₃ concentration over the study period. NH₃ concentration is higher during night time as compared to the day time. Similar variation in NH₃ emissions were also reported by other researchers.³⁶ During day time NH₃ concentration varied from 1 to 187µgm^-3 and during night time it lies in the range 88 to 249 µgm^-3. The mean concentration of NH₃ for the day and night time was found to be 89 and 168µgm^-3 respectively. Fig.4 shows the variation of average day and night time concentration of NH₃ over the sampling months. Higher NH₃ concentrations observed during night time might be due to stable atmospheric conditions (less turbulence) which resulted in reduced dispersion of gaseous NH₃ in the atmosphere because of the accumulation and inefficient vertical mixing within a relative shallow boundary. This pattern is in consistent with the studies by other researchers.^37-38

Figure 3: Diurnal Variation of NH3 Concentration Over the Study Period. Click here to view figure

Figure 4: Average Day- Night Time Variation of NH3 Over the Sampling Months. Click here to view figure

Meteorological Data

Mean temperature during the sampling periods over the month of July, August, September, October were observed to be 37.4°C, 35.83°C, 35.28°C and 34.86^oC respectively whereas the average relative humidity levels were found to be 39.46%, 52.125%, 38.06% and 32.40% respectively.The prevailing wind speed and directions were mostly in the range 2–7 mphwith distinct patterns in W, E, NW and ESE directions.

Variation of NH₃ with temperature and relative humidity is as represented in Fig. 5. There was no significant correlation observed betweenNH₃ concentration with temperature and relative humidity in our catchment (Fig. 6). Levels of NH₃ are negatively correlated with temperature and relative humidity during daytime. In contrast, NH₃ concentration are positively correlated with temperature & negatively with relative humidity during night time.The relation can be explained on the fact that high temperature and low relative humidity conditionsfavours the formation of NH₃ from NH₄⁺. The reason being, in summer, particulate ammonium nitrate is volatile, and hence, NH₄NO₃ will be in gaseous phase.³⁹

Figure 5: Variation of NH3(µg m-3) with RH (%) and Temperature (0C) During Day Time and Night Time. Click here to view figure

Figure 6: Correlation of NH3 (µg m-3) with RH (%) and Temperature (0C) During Day Time and Night Time. Click here to view figure

Possible Sources of NH₃

Since, NH₃is either subjected to dry deposition or readily converted to NH₄⁺. Therefore, high levels of gaseous NH₃ are expected close to the surface and at less distance from the emission sources.^40-41 Hence, there might be some local emission sources near the observation site. Wind rose plots suggested higher concentration from the west direction of the sampling site in the month of July and August (Fig.7). Application of large amount of inorganic nitrogen fertilizers in the nearby agricultural fields might be contributing to ambient NH₃ concentration in these sampling months. Several researchers from various parts of the world also reported similar observations.^42-43During the grain filling period, in the month of September, the levels of ammonia were considerably high, and the prevailing wind direction was mainly northwest (Fig.7c). As this village is in the northwest direction of the sampling site, livestock population, solid waste generation might be contributing towards these high NH₃ levels. In addition, winds were very diverse majorly from E and ESE direction during post harvest period in October. In addition to biomass burning, there may be various sources like animal waste, solid waste, etc. from nearby areas.

Figure 7: Wind Rose Plot (a) July 2017, (b) August 2017, (c) September 2017 and (d)October 2017. Click here to view figure

It is observed that the NH₃ concentration during day & night time on 8 October, 2017 was 56µg m^-3& 176 µg m^-3 (Table 3). Wind rose diagrams were plotted to understand the significant gap between the diurnal values on this particular day (Fig.8). It can be inferred that there were only minor changes in wind directions during day and night time. Hence, the high NH₃ levels starting from night time (8 October, 2017) can be attributed to the onset of biomass burning during post harvest period. However, to rule out any other possibility further investigation is required.

Figure 8: Day and Night Wind Rose Plots (e, f) 8 October 2017, (g,h) 9 October 2017, (i,j)10October 2017, (k, l)11 October 2017, respectively. Click here to view figure

Table 3: Comparison of Ambient NH₃ Concentrations at Various Characteristic Sites Worldwide

Comparison of Concentrations of Gaseous NH₃ Withother Studies

Generally, levels of atmospheric NH₃ are high in tropical regions as compared to temperate regions. This is probably due to high temperature in tropical regions resulting in higher evaporation rates of gaseous NH₃ from agricultural soils, animal waste and other sources.In addition, abundance of alkaline dust in tropics favours the existence of alkaline NH₃ in its gaseous form. In contrast,dominance of H₂SO₄ in acidified atmosphere in most of the temperate areas results in the formation of NH₄⁺ and SO₄^2- aerosols.³⁸Table 4 gives a comparison of NH₃concentrations at various sites worldwide.

Conclusion

Average NH₃ level in the growing period,when DAP was incorporated into the soil was very less i.e. 29.68µgm^-3. This is because addition into the soil increases the efficiency of fertilizer use. The concentration reached to considerably high levels, 96.53µgm^-3 and 149.40µgm^-3 during day and night time respectively, after addition of N-fertilizer (urea) on the soil surface. Wind direction indicated nearby agricultural fields to be the main source of these high levels. Average NH₃concentration during day and night time were observed to be 138.41µg m^-3 and 144.01µg m^-3 respectively prior to crop harvesting. Such high levels might be due to the contribution from livestock, sewage, etc. as the winds were predominantly from North-west (village) direction during this sampling period. Ambient NH₃concentration reached its maxima at night and minima during midday.

The study suggested that in order to reduce anthropogenic forcing of N cycle globally, there is need to address the inefficiency of N fertilization. In addition, there is a need to examine the formation of ammonium aerosols in the air around the agricultural areas in a comprehensive manner in order to address the health and climate issues related to ammonium aerosols.

Acknowledgement

Our sincere thanks to the University Grants Commission (UGC) for awarding the fellowship to Sudesh. We thank the Department of Science and Technology (DST) for extending their financial support through DST-PURSE programme. This work has been a part of DRSNet-India network.

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