Nitrogen Fertilizer Contributes To Acid Rain Quizlet
- November 19, 2021
Point-source pollution is easy to identify.The United States Environmental Protection Agency (EPA) defines point source pollution as any contaminant that enters the environment from an easily identified and confined place.Factories and power plants can be a source of point-source pollution, affecting both air and water.Effluent from a treatment plant can introduce nutrients and harmful microbes into waterways.It is a big problem in cities because of all the hard surfaces, including streets and roofs. .
Fertilizers and Soil Acidity
- Of all the major fertilizer nutrients, nitrogen is the main nutrient affecting soil pH , and soils can become more acidic or more alkaline depending on the type of nitrogen fertilizer used.- Nitrate-based products are the least acidifying of the nitrogen fertilizers, while ammonium-based products have the greatest potential to acidify soil.- Soil acidification due to use of phosphorus fertilizers is small compared to that attributed to nitrogen, due to the lower amounts of this nutrient used and the lower acidification per kg phosphorus.- Potassium fertilizers have little or no effect on soil pH .Soil acidification is a widespread natural phenomenon in regions with medium to high rainfall, and agricultural production systems can accelerate soil acidification processes through perturbation of the natural cycles of nitrogen (N), phosphorus (P) and sulfur (S) in soil, through removal of agricultural produce from the land, and through addition of fertilizers and soil amendments that can either acidify soil or make it more alkaline (Kennedy 1986).Nitrogen can be added to soils in many forms, but the predominant forms of fertilizer N used are urea (CO(NH₂)₂), monoammonium phosphate (NH₄H₂PO₄), diammonium phosphate ((NH₄)₂HPO₄), ammonium nitrate (NH₄NO₃), calcium ammonium nitrate (CaCO₃+NH₄(NO₃)) ammonium sulfate ((NH₄)₂SO₄), urea ammonium nitrate (a mixture of urea and ammonium nitrate) and ammonium polyphosphate ([NH₄PO₃]n).The conversion of N from one form to the other involves the generation or consumption of acidity, , and the uptake of urea, ammonium or nitrate by plants will also affect acidity of soil (Figure 1).It can be seen in Figure 1 that ammonium-based fertilizers will acidify soil as they generate two H⁺ ions for each ammonium molecule nitrified to nitrate.The form of P fertilizer added to soil can affect soil acidity, principally through the release or gain of H⁺ ions by the phosphate molecule depending on soil pH (Figure 2).The form of P in diammonium phosphate (DAP) is HPO₄²⁻ which can make acidic soils ( pH <7.2) more alkaline but has no effect on soil with a pH >7.2.Soil acidity and P fertilizers. .
The Role of Soil pH in Plant Nutrition and Soil Remediation
Firstly, the effects of soil pH on substance availability, mobility, and soil biological processes are discussed followed by the biogenic regulation of soil pH.This functional role of soil pH in soil biogeochemistry has been exploited for the remediation of contaminated soils and the control of pollutant translocation and transformation in the environment.This paper seeks to explore the importance of pH as an indicator of soil biogeochemical processes in environmental research by discussing the biogeochemical processes that are influenced by soil pH, the biogeochemical processes that also control soil pH, and the relevance of the relationship for future research, planning, and development.Biogeochemical Processes Influenced by Soil pH.Additionally, the quantity of dissolved organic carbon, which also influences the availability of trace elements, is controlled by soil pH.Research has established that with increasing soil pH, the solubility of most trace elements will decrease, leading to low concentrations in soil solution .In contrast, Förster  found that a decrease in soil pH by one unit resulted in a ten-fold increase in metal solubility.Soil pH increases the solubility of soil organic matter by increasing the dissociation of acid functional groups  and reduces the bonds between the organic constituents and clays .Thus, the content of dissolved organic matter increases with soil pH and consequently mineralizable C and N .This explains the strong effects of alkaline soil pH conditions on the leaching of dissolved organic carbon and dissolved organic nitrogen observed in many soils containing substantial amounts of organic matter [19, 21].The former is found to be more pronounced than the latter .The metabolic quotient was found to be two-and-a-half times higher in low pH soils compared to neutral pH soils .It is observed from the literature that soil pH conditions required for microbial activity range from 5.5–8.8 [26, 31, 32].This also correlates with microbial biomass C and N contents, which are often higher above pH 7 .Soil Enzyme Activities.Soil pH is essential for the proper functioning of enzyme activity in the soil [34, 35], and may indirectly regulate enzymes through its effect on the microbes that produce them .Thus, the pH at which they reach their optimum activity (pH optima) is likely to differ .These were: (a) enzymes with acidic optima that appeared consistent among soils, (b) enzymes with acidic pH optima that varied among the soils, and (c) enzymes with optima in both acid and alkaline soil pH.Shifts in microbial community composition could potentially influence enzyme production if different microbial groups require lower nutrient concentrations to construct biomass, or have enzymes which differ in affinity for nutrients .Like many soil biological processes, soil pH influences biodegradation through its effect on microbial activity, microbial community and diversity, enzymes that aid in the degradation processes as well as the properties of the substances to be degraded.Soil pH controls mineralization in soils because of its direct effect on the microbial population and their activities.Nitrification and denitrification are important nitrogen transformation processes of environmental concern.Like many of the biogeochemical processes, the processes, to a large extent, are controlled by soil pH.It generally increases with increasing soil pH but reaches an optimum pH [45–47].In a four-year study, Kyveryga et al.  observed that soil pH range of 6 to 8 strongly influenced the nitrification rates of fertilizer N. Generally, the nitrification rate decreases at lower soil pH values.Furthermore, maximum denitrification of between 68% and 85% occurred in a sandy and a loamy soil with pH 5.2 and 5.9, respectively .However, the degree will also depend on the specific fertilizer and its effect on soil pH.This can either occur through the direct effect of biochemical processes occurring in the living organisms in the soil system, mostly through rhizosphere processes or through the direct and indirect effects of applied organic residues, whether in unburnt, burnt, or charred forms as well as their decomposition.Rhizosphere Processes.Therefore, rhizosphere pH could increase or decrease depending on the prevailing process and types of ions released.Plant root-induced soil pH change in the rhizosphere is controlled by specific processes and factors such as (i) ion uptake coupled with the release of inorganic ions that maintain electroneutrality, (ii) the excretion of organic acid anions, (iii) root exudation and respiration, (iv) redox-coupled processes, (v) microbial production of acids after the assimilation of released root carbon, and (vi) plant genotype [58, 59].It has been revealed that 15, 6, and 0%, respectively, of the N from the total N present in the soil is required to decrease rhizosphere pH decrease by 1.2 units, maintain it, or increase it by 0.4 pH unit .The extent of effects of the processes and factors controlling rhizosphere pH change depends on plant species and growth stages .However, the rhizosphere pH changes with time as a result of variable uptake of nitrogen ions, plant species, and their growth stages of the plants .The concentrations of in the rhizosphere of the plants was in the order, lettuce = buckwheat > pine > apple > kaffir > cowpeas > corn > wheat.When unburnt organic materials or raw plant residues are applied to the soil, the pH increases to a peak and decrease afterwards.According to Xu et al.
; direct chemical reactions and oxidation of the organic anions during residue decomposition are the main mechanisms involved in organic anion-induced soil pH increase.The increase in soil pH after residue application also depends on the type of residue (either from monocots or dicots), which is related to the amount of alkalinity present, residue quality (C/N ratio), the rate of residue application and decomposition, the initial pH, and buffering capacity of the soil [70, 71].Different residues have different chemical and biochemical compositions, which determine the processes responsible for soil pH change.They observed that 40–62% of soluble alkalinity in canola and chickpea residues were responsible for the pH increases.The pH increase after residue addition often reaches a peak and declines thereafter as a result of nitrification.Residues with low carbon-nitrogen (C/N) ratios are often associated with sharp pH decline after a certain period and the extent varies with soil type and soil buffering capacity [70, 71, 74], whereas those with high C/N ratios produce smaller pH increase, or none at all .Thereafter, the pH increased by about 3.3 units with lucerne in the Wodjil soil (3.87), 1.6 with chickpea, 1.5 with medic, and 0.5 with high-N wheat, and no increase with low-N wheat.However, in a field study on the same soils , the application of chickpea residue increased soil pH by 1.3 units in both soils and reached a maximum at 3 months, whereas canola residue increased pH by 0.82 and 1.02 units in the Podzol and Cambisol, respectively, and reached a maximum pH at 9 months.Similar to unburnt organic materials, burnt or charred plant residues contain a larger amount of alkalinity due to the volatilization of organic constituents under thermal conditions leading to the concentration of alkaline constituents.The inorganic alkalinity increased with increasing pyrolysis temperature and with increasing divalent cation contents  because the organic constituents volatilize during pyrolysis.This alkalinity of biochar neutralizes acidity and increases soil pH depending on the amount of alkalinity and soil buffering capacity .Depending on the alkalify and buffering capacity of the soil receiving the biomass ash, soil pH increase can be high or low.This pH change is mostly short-lived due to other biogeochemical processes.The content of this paper highlights the role of soil pH as a master soil variable that has a bidirectional relationship with soil biogeochemical processes. .