Nitrate (NO3): Release, Transport and Attenuation into Soils from Agricultural Activities - Assignment Example

Table of Contents Introduction 1 Sources of Nitrate in groundwater 2 The release, transport of Nitrate in groundwater 3 In particular consider the conditions that will release sediment-bound, naturally occurring Nitrate 2 4 The transport and dispersion properties of the pollutant in groundwater 2 5 Major processes that effect of Nitrate in groundwater 3 5.1 Sorption and desorption 3 5.2 Attenuation of Nitrate in groundwater 3 5.3 Nitrate oxidation and reduction 4 5.4Seasonal effect 5 6 Major conditions that effect of Nitrate in groundwater 6 6.1 pH redox potential (Eh or pe) 6 7 Fate of the contaminant 7 References 11 Nitrate (NO3-) release, transport and attenuation into soils from agricultural activities INTRODUCTION: Nitrogen is one among the most important elements of the world. Next to carbon cycle, the nitrogen cycle is more important for all the living organisms. The nitrogen compounds are very diverse as that of the carbon. Nitrogen is an important constituent of the earth. The nitrogen is present in the 78% of the total atmospheric air composition. Nitrogen is believed to have originated from the soil mainly from the fundamental rocks of the earth’s crust and the mantle. Nitrogen is also present in the aquatic systems as inorganic Nitrogen, Nitrate, Nitrite and Ammonium and as particulate matters. 1. SOURCES OF NITRATE IN GROUND WATER: The denitrifying bacteria are able to reduce the biological oxygen demand present in the water using the nitrites and nitrates. Some of the important denitrifying bacteria are Pseudomonas, Alcaligens and Bacillus. The Enterobacter and Escherichia coli can use only nitrate source and Alcaligens can use only the nitrite sources. (Gerardi 2002). 2. THE RELEASE, TRANSPORT OF NITRATE IN GROUNDWATER: Nitrate leaching occurs whenever the water flows through the soil containing the nitrate. If the percentage of leaching is higher, then the high concentration of the nitrate will affect the quality of the water. The maximum concentration of the nitrate permissible in the drinking water is 10 mg/l in Canada. These nitrogen losses can be minimized by minimizing the leaching loses by decreasing the volume of runoff and reducing the erosion. The use of the more efficient nitrogen cultivars, avoiding the over fertilization for the crops, encouraging the glyphosate application on the legumes are some of the important measures that can be taken to reduce the over flow of the nitrates into the ground water. (Sedlak 1991). (Horan, Lowe and Stentiford 1994) 3. SEDIMENT-BOUND, NATURALLY OCCURRING NITRATE: Nitrate is a commonly occurring pollutant and it does not come from the naturally occurring rocks or minerals. The nitrate release is mainly due to the result of the human activities. These nitrate minerals are highly soluble and as a result the current resource of these is very less. These nitrates that are present in the soil are washed off by the rivers and they get settled in the water. 4. THE TRANSPORT AND DISPERSION PROPERTIES OF THE POLLUTANT IN GROUNDWATER: The nitrogen is brought to the ground from the atmosphere by the natural reactions of atmospheric nitrogen with the rain water and reaches the earth as nitrate and ammonium salts. The nitrate gets accumulated in mainly in the ground water. (OECD). This nitrate accumulation is mainly due to the natural processes of nitrogen cycle and anthropogenic sources. The anthropogenic sources include agricultural processes, septic tanks and application of the nitrogen rich fertilizers in the land. The increase in the level of nitrate is alarming the world’s environment. Nitrate, nitrous oxide and Ammonia are the major environment affecting factors. Of these compounds the nitrate are of great concern because of their leaching ability in to the ground water source. (Revbech 1990). 5 MAJOR PROCESSES THAT EFFECT OF NITRATE IN GROUNDWATER: 5.1 SORPTION AND DISORPTION: The ammonia and other nitrogen sources are adsorbed well into the soil. These adsorption mechanisms were of two types. Physical adsorption and chemisorption. It was found that the chemisorptions will take place first followed by the physical adsorption. The Hydrogen bonding was found to be the best source of physical sorption. The physical adsorption was found to be very weak and found to have a short duration because of the drop of the pH quickly in the first week and thus enables the diffusion of the ammonia and influence the reduction of the concentration in the soil. (Schepers and Raun 2008). 5.2 ATTENUATION OF NITRATE IN GROUNDWATER: The rate of presence of nitrate in ground water is found to play a critical role in the nitrate pollution. The process of reducing the concentration of nitrate in the soil is done by the nitrifying bacteria. The presence of the bacteria is influenced by many factors. The presence of the oxygen and the carbon concentration are also found to influence the denitrification rates. They include nutrient availability, pH, temperature, the concentration of the nitrate, the presence of the toxins and the presence of other micro organisms etc. Influence of Carbon on dentirifcation: As most of the soil bacteria are chemoorganotrophs, the availability of the soil carbon is found to influence the dentirifcation process of the bacteria. Many studies have proved that the concentration of the carbon plays an important role on denitrification. It was found that if the concentration of the soil carbon is higher, then denitrification was higher and if the concentration of the soil carbon is lower, the gentrification was lower. The denitrification was also found to be related to the water soluble carbon and reducing sugars. The correlation rate between the mineralized carbon and the water soluble carbon was found to be greater. The available carbon was found to be less in the soil because of wetting and the drying cycles. (Postgate 1999). 5.3 NITRATE OXIDATION AND REDUCTION: Apart from the water source, nitrogen is found at a larger concentration in the soil also. Nitrogen is present mainly in the organic form in the soil. (Gobat, Aragno and Matthey 2004). Nitrogen is believed to have originated from the soil mainly from the fundamental rocks of the earth’s crust and the mantle. Nitrification is the process of converting the reduced nitrogen forms that are present in the soil into oxidized forms. (Bardgett, Hopkins and Usher 2005).The reduced nitrogen forms are generally Ammonia (NH3) and Ammonium (NH4) and the oxidized nitrogen forms are Nitrite ( NO2) and Nitrate (NO3). In most of the agricultural lands, the Ammonium (NH4+) ions present are converted in to Nitrate (NO3-). The conversion of the cation into anion enables the movement of the ions inside the soil matrix and thus it determines the concentration of the nitrogen in the soil. These nitrate ions then moves to the root of the plants, where they are used by the plants and release free nitrogen into the atmosphere. In the nitrogen cycle, the bacteria that are present in the soil convert the nitrogen into nitrate and these are taken up by the plants and incorporated into the tissues. These tissues upon death reach the soil through the microbial degradation. Thus the nitrogen cycle continues. Here the cycle continues and the nitrogen is not excessively accumulated anywhere. But now, due to the human activities, the environmental nitrate concentration is increased. The increased concentration of the nitrate in the ground water has resulted in the health effects such as Diabetes, Goitre, Neurologic disorders and vascular diseases. The nitrates that are taken in are converted into nitrosoamine one of the compounds found to be related to the cancer. (Bardgett, Hopkins and Usher 2005). The removal of these nitrates from the soil is possible through the process of dentirification. In the denitrification process, the nitrate ion is the substrate and the molecular nitrogen is the final product. Under anaerobic conditions these nitrates are converted into nitrogen oxides which are released into the atmosphere. But when aerobic and anaerobic conditions are compared, the aerobic condition was found to produce more energy than the anaerobic process. (Hyman 2001). 5.4 SEASONAL EFFECT: The crop production was found to be severely affected by these processes. Hence several measures were aimed and taken to reduce the N losses in the field crop. As the nitrate concentration in the soil is mainly due to the addition of the fertilizers and the plant residues, the steps were taken to reduce rate of addition of the fertilizer into the field. The other methods include improved irrigation techniques and the use of the fertigation systems. Similarly the use of the slow release fertilizers is recommended for these tasks. Similarly the use of the anion exchangers was practiced. The nitrate specific resins were used to remove the nitrate from the soil; these reduced the nitrate concentration in the soils. Though this technique was found to be more effective, the usage was limited because of the high costs involved. Apart from this, these resins when used will decay in the soil and create some unwanted compounds. A good technique for the removal of the nitrates is the use of the nitrate absorbing anionic clay material. The addition of this clay reduced the movement of the ammonium in the soil through leaching. These are the methods that were proposed for reducing the nitrate concentration in the soil. 6. MAJOR CONDITIONS THAT EFFECT OF NITRATE IN GROUNDWATER: The nitrates that are present in the soil, are carried away by the water and are poured into the streams, as a result they increase the concentration of the nitrogen in the streams. These nitrates are found as sediment in the water stream soil and found to produce nitrous oxide. The nitrous oxide concentration thus increases if the stream nitrate concentration increases. This reveals that there exists a relationship between the nitrous oxide production and the nitrate concentration in the streams. (Beaulieu, Arango and Tank 2009). (Mulholland etal, 2008). 6.1 pH – The optimum pH for the denitrification process was found to be around 6-8. The denitrification process can occur even at more acidic and more basic pH. The studies have proved that the denitrification rates were found to increase with the increase in the pH from 4 to 8. (Schepers and Raun 2008). It was also found that the temperature and pH had an interactive effect on the nitrite and nitrate reduction in the marsh soils. The concentration of the denitrifying bacteria and their rate of denitrification was found to increase when the soil pH was increased from 4 to 7. the concentration of the denitrifying bacteria increased by 1000 fold. Here the total incubation period was 30 hours. There are two types of denitrifiers, The first type can grow best on the acid soils and the second type on the neutral pH. For those bacteria that grew on the acidic pH failed to denitrify at their greater pH, because the sustainability of the bacteria was lost at greater pH. Similarly the pH of the soil also affected the distribution of the end products. (Payne 1981). 6.2 Temperature – The relationship between the temperature and the denitrification process was much affected. The dentirification process was found to occur in the temperature range of 4 to 60 degree Celsius. It was also found that the denitrifying bacteria were found to grow over a wide range of temperature. (Lehr, Hyman and Gass 2002). 6.3 Redox - The nitrites and the nitrates that are present in the soil are used for the bacterial degradation. The redox potential of about +50 to -50 mV is required. Similarly if the denitrification occurs in the sewage system, then the cBOD is heavily reduced in the cell concentrations. If the substrate concentration is lower in the medium, the growth rate is also reduced much. (Werner 2005). 7. FATE OF THE CONTAMINANT: Though the rate of influence of the plant roots on the denitrification process is complex, they have both positive and negative effects. The roots exudates carbon and at the same time increases the heterotrophic denitrifier growth. The roots are found to create micro sites by the process of Oxygen consumption and they are also found to increase ammounium ion reduction and also excrete the mineralizable organic nitrogen. The roots provide air when the region is flooded and they also increase the oxygen diffusion during the dry conditions. The rhizospheres are the special roots that need to be mentioned here for nitrification process. The root zone was found to be responsible for the 50 – 70% dentirification process at the lake sediment regions. Soluble carbon and the plant residues: It was found that the plant residues have a lot of nitrogen organic load and this was found to increase the denitrification process. The plant residues of the plant alfalfa (Medicago sativa) had 60% greater nitrogen evolution when the concentration of the residue qwas 1500 mg against 200 mg of residue. Similarly the primary and the secondary waste water effluents were found to have very less amount of nitrogen because of the presence of the low amounts of glucose, methanol and succinate. But this waste water treatment was found to be very useful for the land application of the waste waters. The carbon concentration in the potato was found to increase the process of dentirification .Similarly the straw and the other crop residues are also found to have readily available carbon for denitrification process. (Dorante 2007). The C/N ratio is very important for the mineralization, immobilization and the denitrification process. The C/N ratios should be around 11 to 66. In the desert systems, the nitrogen is found to be accumulated, though N is the required by the plants. Di and Cameron (2002) in their paper have described that the nitrate leaching occurs in the ground and surface waters from various land usage systems varying from forest eco systems, grazed postures, organic cropping, arable cropping, mixed cropping and cut grass land. If the human consume nitrate accumulated water, then they are prone for a number of diseases such as blue-baby syndrome, stomach cancer and methemoglobinemia. (Huang 1995). A number of agricultural by products including the animal manures, crop residues, municipal bio-solids, composts, yard trimming and other agri related industrial by products are highly diverse in the amount that is generated. The most important advantage among all these products is that all the by products have the similar physical, chemical and biological properties. The crop residues that returns to the soil and are the primary sources of the nitrogen and the crops that feeds on these remaining nitrates for their growth. These crops act as the catch crops to trap the nitrate residue of the previous crops and they reduce the potential for reducing the nitrate leaching in to the ground waters. When these catch crops are planted care must be taken to know whether the crops are planted at the right time for the removal of the excess nitrate from the residue or not. The available carbons from these residues when dropped in the soil have the greater ability to immobilize it with the Nitrogen and the soil minerals. These mineralization and the demineralization activities in the soil are controlled by the activity of the micro organisms. (Wang 2009). The volatilization process also promotes the high free Ammonia concentration in the soil. The factors that enhance this operation are high pH, warm and moist conditions in the soil. (Stevenson and Cole 1999). When denitrification occurs in the primary clarifiers, the solid particles present in the sample raises to the surface of the clarifier. These solid particles are then collected and transferred to the appropriate tanks for the dewatering, thickening and finally disposal. Denitrification operation occurs in the secondary clarifiers. Because of this process, a thin slugging blanket is formed on the surface and the bacterial number decreases heavily. Most of the coli form bacteria are lost with in the secondary clarifiers. References: Bardgett RD, 2005, The biology of soil: a community and ecosystem approach, Oxford University press. Bardgett RD, Hopkins, DW and Usher, MB 2005, Biological diversity and function in soils, Cambridge University press. Beaulieu, JJ, Arango, CP and Tank JL 2009, The Effects of Season and Agriculture on Nitrous Oxide Production in Headwater Streams, Journal of Environmental Quality, vol. 38, no. 2, pp. 637 Di, HJ & Cameron, KC 2002, Nitrate leaching in temperate agroecosystems: sources, factors and mitigating strategies, Nutrient Cycl. 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