Nitrate reduction soils

Soil Nutrient. Molybdenum has been widely used to increase crop productivity in many soils woddwide (see Fertilizers). It is the heaviest element needed for plant productivity and stimulates both nitrogen fixation and nitrate reduction (51,52). The effects are particularly significant in leguminous crops, where symbiotic bacteria responsible for nitrogen fixation provide the principal nitrogen input to the plant. Molybdenum deficiency is usually more prominent in acidic soils, where Mo(VI) is less soluble and more easily reduced to insoluble, and hence unavailable, forms. Above pH 7, the soluble anionic, and hence available, molybdate ion is the principal species. 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

Denitrification, a dissimilatory pathway of nitrate reduction (see Section 3.3 also) into nitrogen oxides, N2O, and dinitrogen, N2, is performed by a wide variety of microorganisms in the forest ecosystems. Measurable rates of N20 production have been observed in many forest soils. The values from 2.1 to 4.0 kg/ha/yr are typical for forest soils in various places of Boreal and Sub-Boreal Forest ecosystems. All in situ studies (field monitoring) of denitrification in forest soils have shown large spatial and temporal variability in response to varying soils characteristics such as acidity, temperature, moisture, oxygen, ambient nitrate and available carbon. 

Buresh RJ, Patrick WH, Jr. 1981. Nitrate reduction to ammonium and organic nitrogen in an estuarine sediment. Soil Biology and Biochemistry 13 279-283. 

Soil In both soils and water, chemical and biological mediated reactions can transform thiram to compounds containing the mercaptan group (Alexander, 1981). Decomposes in soils to carbon disulfide and dimethylamine (Sisler and Cox, 1954 Kaars Sijpesteijn et al., 1977). When a spodosol (pH 3.8) pretreated with thiram was incubated for 24 d at 30 °C and relative humidity of 60 to 90%, dimethylamine formed as the major product. Minor degradative products included nitrite ions (nitration reduction) and dimethylnitrosamine (Ayanaba et al., 1973). 

Biodegradation of fuel oils in sediments is inhibited under anaerobic conditions (Bartha and Atlas 1977). Under anaerobic conditions, some soil microorganisms are capable of nitrate reduction using fuel oils as the carbon source, although nitrite may be an unwanted by-product. However, the addition of a small amount of oxygen (0.2 volume percent oxygen) to the medium can accelerate the degradation of the oil without the concomitant production of nitrite (Riss and Schweisfurth 1987). 

Similar mechanisms operate in the action of nitrate reductase and nitrite reductase. Both of these substances are produced from ammonia by oxidation. Plants and soil bacteria can reduce these compounds to provide ammonia for metabolism. The common agricultural fertilizer ammonium nitrate, NH4NO3, provides reduced nitrogen for plant growth directly, and by providing a substrate for nitrate reduction. NADH or NADPH is the electron donor for nitrate reductase, depending on the organism. 

Primary denitrifying genera in soils are Pseudomonas and Alkaligens. Simultaneous with denitrification is organic matter oxidation. The utilization of glucose through nitrate reduction by Pseudomonas denitrificans can be written as... 

Fazzolari E., Nicolardot B., and Germon J. C. (1998) Simultaneous effects of increasing levels of glucose and oxygen partial pressures on denitrification and dissimilatory nitrate reduction to ammonium in repacked soil cores. Euro. J. Soil Biol. 34, 47-52. 

Silver W. F., Hennan D. J., and Firestone M. K. (2001) Dissimilatory nitrate reduction to ammonium in upland tropical forest soils. Ecology 82, 2410—2416. 

Yin S. X., Chen D., Chen L. M., and Edis R. (2002) Dissimilatory nitrate reduction to ammonium and responsible microorganisms in two Chinese and Australian paddy soils. Soil Biol. Biochem. 34, 1131 — 1137. 

Unknown intermediate steps exist between the nitrite (NOJ), N2, and NH3. forms. The fraction denitrified and the N2O/N2 ratio increase with increasing amounts of initial NOJ. Nitrate reduction is the main pathway of denitrification, N2 and N2O production, in soils. 

Assimilatory nitrate reduction is simultaneous reduction of nitrate and uptake of N into the biomass of any organism. This process might be dominant when nitrogen is in low supply, which is typical in aerobic soil and water column conditions. We can consider this as a primary N input to many microorganisms and plants. 

Nitrate reduction occurs at Eh values in the range of +200 to +300 mV. Nitrate can poise soil Eh for several days as observed in conditions when drained organic soils (usually containing high levels of nitrate) are flooded (Figure 4.18). In an organic soil, maintenance of nitrate levels between 25 and 50 mg L resulted in steady Eh levels of soil slurry (Figure 4.19). 

In wetlands, nitrate is used by microbes and plants as a nitrogen source or as an electron acceptor to support catabolic activities of select heterotrophic bacteria. As early as 1882, Maquenne reported the following observations on nitrate reduction in soils ... 

Nitrate reduction occurs in arable soil that contains high organic matter. 

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