The role of nitrogen in agriculture and pollution by nitrate

Nitrogen (N) is an essential element for plants and with phosphorus (P) and potassium (K) make up the three macronutrients (NPK) most important in plant nutrition. At the same time, as a result of the agricultural and livestock activity, it also participates in a set of reaction that may affect the environment and/or to people's health. These reactions are as follows:

-Nitrogen monoxide (NO) is formed from molecular nitrogen (N₂) when reacting with molecular oxygen (or₂):

₂ N +₂ > 2 not

-In the air, nitrogen monoxide (NO) oxidises fairly easily giving (not₂) nitrogen dioxide. NO gas and NO₂, usually are oxides of nitrogen (NO_x) called:

2 +₂ > 2 NO₂

-The ammonium ion (NH₄⁺) as a result of the acid-base reaction:

NH₃ + H⁺ <> NH₄⁺

-Through the ' synthesis of having ', the nitrogen from the air (N₂) reacts with hydrogen (H₂) at high pressure and temperature to give ammonia (NH₃):

N₂ + 3 H₂ <>2 NH₃

-Ammonia (NH₃) is oxidized to nitrite (not₂):

2 NH₃ + 3 O₂ > 2 NO₂^- + 2 H⁺ + 2 H₂O

-The ammonium ion (NH₄⁺) is also oxidized to nitrite (NO₂):

2 NH₄⁺ + 2 OH^- + 3 O₂ <> 2 H⁺ + 2 NO₂^- + 4 H₂O

-Ion nitrite (not₂) is oxidized to nitrate (not₃):

2 NO₂^- + O₂ > 2 NO₃^-

The cycle of the N (Figure 1) has several stages and the bacteria take part in four of them:

· Attachment: Consists of the incorporation of the atmospheric N plants, using the enzyme nitrogenase present in some bacteria and cyanobacteria which live in the soil and aquatic environments, in the absence of oxygen. To this end, the microorganisms (Rhizobium SP.) that carry out this transformation (N₂ > NH₃ > NO₃) living in the interior of nodules, and legumes species host in which usually inhabit leg-haemoglobin red pigment which characterized its activities. According to some authors, the amount of N fixed by these bacteria in the biosphere is of the order of 200 million tonnes per year.

· Nitrification and mineralization: the roots of the plants we grow only can absorb two forms of N: nitrate (not₃) and ammonium (NH₄⁺). Ammonium is converted to nitrate via nitrification. The transformation of the ammonium nitrate increases with temperature (> 10 ° C) and the pH (5.5 - 6.5) of the medium. The process is carried out in two stages: a) transformation of ammonium (NH₄⁺) in nitrite (not₂) by bacteria present in the soil (nitrosomonas and nitrococcus); (and b) transformation of the (not₂) nitrite to nitrate (not₃) by other bacteria also present in the soil (Nitrobacter). It is actually two reactions of oxidative type, of which the bacteria obtain energy.

· Assimilation: Has place when plants absorb nitrate (not₃) or ammonium (NH₄⁺) through their roots. In the interior of the plant, these molecules are metabolized and properly combined with sugars and other molecules from the photosynthetic activity, the N was finally incorporated into amino acids, proteins and nucleic acids (DNA and RNA) or other substances specific to each species. Animals consume these substances and transformed them into other similar animal in nature.

· Ammonification: Nitrogen compounds (proteins and nucleic acids etc) whether of plant or animal, as well as other wastes that contain nitrogen, such as from urine urea, uric acid, from birds or dead organismsthey break down by bacteria present in the environment, freeing up N, in the form of ammonium - hence the name-previous step by cornflakes compounds (proteins, Peptones and ultimately amino acids).

· Immobilization: Is the process opposite to the mineralization, by means of which forms inorganic (NH₄⁺ and₃) are converted to not assimilable organic N.

· Denitrification. The reduction of the N2 (not₃) nitrate and ammonium (NH₄⁺) to ammonia is named after denitrification. He is done by denitrifying bacteria that reversed the action of fixing bacteria of N, with which the N is returned to the atmosphere as a gas (N₂). This process results in a loss of nitrogen to the ecosystem; occurs where accumulates organic matter under anaerobic conditions and high pH. Conditions of humidity in the soil, lack of oxygen requires certain micro-organisms to use nitrate instead of oxygen for breathing (get energy).

· Contributions by rain: rain brings varying amounts of N in the form of ammonium, nitrate, and nitrogen oxides, which is an important source of N for some natural ecosystems. This contribution ranges between 5 and 15 kg of N per hectare per year. However, for many agricultural systems, this value is insufficient when compared with the needs that are usually covered with chemical fertilizers.

The optimal dose of nitrogen (Fig. 2) that we must bring to a crop depends on three factors: to) the cultivation; (b) the "fertility" of the plot at the time of the application; (and c) the objective that we want to achieve. Therefore, in most cases, the decision to use a certain dose can not be taken from the mere calculation of extractions which carried out the crop, as it has been done for many yearsfollowing the recommendations of the classic treaties of fertilization in agriculture.

Agronomic fertilization recommendations are based on experimental work and are valid for specific areas and crops. When we compare (Fig. 2) the increase in production that experiences a culture based on the amount of nitrogen applied per hectare (AU, arbitrary units), we find that gross output (E) ceases to grow from a particular point (1); It's the best agronomic. If you look at the production function (D) which is obtained when we explain exclusion applied to N (A) costs to gross output (E), we obtain another optimal; It is the economic optimum (2) and finally, if we were able to estimate the environmental cost that represents the application of N (B) subtracting from the gross production (and) we would get another optimal; in this case, it would be the optimal environmental (3). Two conclusions can be drawn from the above: to) that the optimum implementation of N clearly depends on the objective that we want to achieve (agronomic, economic or environmental); (and b) that the best three are closely interrelated (environmental N < economic N < agronomic N).

The management of the nitrogen in a farm

In the 'information technique 147', prepared by D. Manuel Gil Martinez and published in 2004 by the Department of agriculture and food of the Government of Aragon, the aspects of management and control of the N must be considered in a farm are listed:

· Individualized management: Exploitation and homogeneous pedology plots.

· Management of irrigated and independent irrigation: irrigation will be deemed the irrigation way to calculate the dosage and frequency of Subscriber.

· Management variables: You should consider the amount and the frequency of applied N (mineral and organic) and their ability to release.

· Soil organic matter: It is an essential periodic control variable.

· Nitrogen dissolved in the water for irrigation: It may be relevant for plots of irrigated land, especially in end of the basin.

· Remains of crop and vegetable covers: They regenerate the soil organic matter and reduce the uncontrolled movement of the N in the soil.

· Legume: We must consider the N set and the contribution made through the organic matter that remains in the soil.

· Mineralization: The mineralization of organic N in the soil depends on provided organic matter and a constant farms.

· Farms: The amount of organic matter depends on the medium of the cattle Census and its state of development.

· Cattle manure: Cattle manure pollution depends on the location and the operating system.

· With rules: Each community's own legal regulations must be known and respected.

· Good practices: Good agricultural or livestock practices should be applied at all times.

According to the rules in the field of nitrogen fertilization, especially in the so-called vulnerable areas to nitrogen (ZVN), before taking the decision to apply a certain amount of N to a culture we must take stock. Is calculating the needs posed by the crop, according to the expected production and subtracting all the contributions that will be carried out, as a result of: to) the previous crop; (b) the amount of manure applied; (c) organic matter that will be mineralizada; (d), the plant remains that can be incorporated; (e) water for irrigation; (and f) other contributions.

In table 2, we present an outline of the balance sheet which could be used for this purpose. To properly develop a subscriber to a crop, in addition to the foregoing in accordance with the regulations in force, it is necessary to address some practical recommendations.

For several years, there are compact systems that allow to know the concentration of nitrogen (N) or nitrate (not₃) that we have in the soil, irrigation water or even crop with sufficient precision and reliability in the domestic market. In this article deal with the first one of these aspects: the measure of the concentration of N in the soil. The results we show (table 3) come from the comparative measurements that, for several months and in different soil types, we have made in the Department of plant production (breeding), of engineers surveyors school technique Superior, of the Polytechnic University of Madrid (UPM)using a parser of Hanna Instruments.

Sampling is essential for the interpretation of results is correct; Hence the need to exercise caution when taking the sample of soil.

-Taking of samples: for the determination of the concentration of N in the soil must be based on samples that are representative of the plot. To this end, it should be recalled that the concentration of N varies with the natural soil horizons or the different layers that, as a result of the tillage or other cultural practices, have been able to establish. It therefore tends to be reasonable to take samples at least at two different depths: between 0 and 20 - 30 cm (A sample) and below 20 - 30 cm (sample B). In the case that in the case of non homogeneous parcels, please divide the plot in as many subparcelas as different areas can be identified. Therefore the plot should be visited in zig-zag to the length and breadth of its area, taking small fractions of samples A and B, placing them in containers separated, to get 1 kg of sample, approximately, for each one of them.

-Preparation of the saturation extract. In a 500 ml container will add 200 g of the sample of soil that we want to analyze. Slowly and kneading with a spatula is adding distilled water up to the point of saturation, which appears when the paste shines and flows slightly when leans the container. Once reached the saturation point score the ml of distilled water used in the operation of saturation. The sample is left to rest for an hour and verified the State of saturation, add more distilled water if necessary.

The saturated pasta is transferred with the same spatula that has been used for processing, to a Buchner funnel with filter policarbonate at its base paper and applies vacuum. Filtering, which should leave almost colorless, reflected in the glass called kitasato vessel and is the saturation extract. To prevent the formation of a precipitate with calcium carbonate, you can add a drop of a solution of sodium hexametafosfato (1 g·100 ml^{- 1}) for each 25 ml of extract.

-Measure of the concentration of nitrate. With a photometer multiparametric Hanna Instruments (83214 HI), the extent of the nitrate concentration (not₃) can be directly on the sample if you are between 0 and 30 mg·l^-1. In the case it is higher, the sample should be diluted with distilled water and the final result which shows the screen of the computer must be multiplied by the dilution factor used; for example, if 75 ml of distilled water added to 25 ml of extract, until reaching a final volume of 100 ml of dissolution, the dilution factor is 4.

The photometer multiparametric HI 83214 works with a tungsten lamp and filter of interference that lets the electromagnetic radiation of 420 nm. When the sample contains nitrate, the addition of the reagents that incorporates the method generates a characteristic yellow colour whose intensity is proportional to the amount of nitrate that contains the sample. The resolution of how we get is 0.1 mg·l^-1. As is the case with other methods, the presence of ion concentrations then listed above may be subject to interference: barium (Ba^{+ 2}), 1 mg·l^-1; chloride (Cl), 1000 mg·l^{- 1} and nitrite (not₂), 50mg·l^-1.

Results and conclusions

The Fig. 3 comparative displays the results achieved by comparing the concentration of nitrate with extracts of saturation of six samples of soils, characterised by its texture and previous addition of nitrogen fertilizers. The results obtained with the photometer multiparametric Hanna Instruments HI 83214 have been compared with those obtained when the same samples are analysed using ion chromatography MetronMH (mod.) (850 IC) from properly calibrated patterns. The obtained results allow to conclude that:

-When are followed carefully the instructions for sample preparation, the results that are achieved by measuring the concentration of nitrate (not₃) in saturation extracts obtained from samples of soil, with the spectrophotometer multiparametric Hanna Instruments HI 83214, are comparable with those obtained to analyse samples in a specialized laboratory, using ion chromatography, as well calibrated patterns.

-The differences obtained can be explained by the fact that the method using the HI 83214 team is 'technique sensitive', i.e. how to shake the vial containing the sample with the reagents (number of agitations and frequency) should be performed always in the same way andIf possible, by the same operator. The presence of chloride (Cl) in the extract of saturation, with a concentration exceeding 1000 mg·l^-1, may also cause interference.

-Regardless of the remaining applications that can be carried out, the spectrophotometer multiparametric Hanna Instruments HI 83214 turns out to be a highly recommended tool for the determination of the content in nitrate (not₃) on plots of farmland, especially within the so-called areas vulnerable to nitrogen.