Some factors influencing denitrification in soils include oxygen and redox potential, moisture content, concentration of nitrogen oxide, organic carbon, pH, temperature, and soil chemicals such as pesticides, etc. In general, the rate of denitrification is inversely related to the oxygen concentration. Though denitrification can occur in very dry soils, generally it proceeds only in those soils in which the moisture content exceeds 6O per cent. Since the majority of the denitrifiers are heterotrophs, their denitrifying activity is strongly affected by the availability of electrons or reducing equivalents in organic carbon compounds. Denitrification occurs best at pH range of 7 to 8.
At lower pH, N2O predominates and sometimes both N2O and NO are the major products. Denitrification is temperature- dependent within the range 1O-35°C with a Q1O nearly equal to 2.O.
In some organisms, e.g., Azospirillum brasilense, both denitrification and nitrogen fixation can rarely occur simultaneously though they often occur independently of each other in a mutually exclusive manner (Knowles, 1981). The plant rhizosphere contains many anaerobic niches supplied with electron donors from root exudates and in these sites, denitrification occurs. This phenomenon accounts for substantial loss (up to 35 per cent) of applied fertilizer nitrogen from grassland soils (Woldendorp, 1963).
Some estimates of N abundance of various naturally occurring substances suggest that on a global basis, denitrification in terrestrial habitats amounts to nearly twice of that occurring in the sea (Wada et al., 1975). In some conditions, nitrogen losses occur but cannot be attributed to true denitrification. At low pH values, HNO2 decomposes abiotically, resulting in the release of NO and NO2. In waterlogged rice field soils, NO2 is reduced to NO at low pH, and the same occurs in forest humus at pH 4-5 under anaerobic conditions. According to Knowles (1981), there is a very high (2-10 times) uncertainty factor involved in estimates of global fluxes in the nitrogen cycle.
In many of the published nitrogen budgets the values assigned to denitrification are quite arbitrary, being assigned to equal discrepancy in all other inputs and outputs, with a view to balancing the budget. Nitrous oxide (N2O) is a critical link in the return of nitrogen to the atmosphere by denitrification. Upon its diffusion from the troposphere to stratosphere, N2O changes into NO. This NO reacts with ozone to form NO2 plus O2. The NO2 in turn again changes into NO. The nitrogen oxides are slowly converted into HNO2 which returns to the ground in precipitation. One risk inherent in the increasing usage of larger amounts of combined nitrogen inputs from either biological or industrial nitrogen fixation, into our agriculture, is that the consequent increase in denitrification may deplete the ozone shield, allowing passage of more of the short wavelength ultraviolet radiation to the earth.
This UV can cause mutations and skin cancer. Some or all of the phases of the nitrogen cycle can become affected by excessive or indiscriminate application of chemical fertilizers by farmers. Thus, denitrification is harmful to agriculture but is important from the viewpoint of environmental protection. When no more nitrogen can be recuperated for agricultural purposes, denitrification may be encouraged with a view to reducing nitrate contamination of water.
Certain climax vegetation types are now known to inhibit nitrification by soil microorganisms (see Moore, 1975). This probably is the result of toxin production in the leaf litter, a process that decreases the rate of nitrate production and loss. Although nitrate is probably the most preferred form of nitrogen for higher plants, this form of nitrogen is equally susceptible to be lost or leached off from the ecosystems. Microbial suppression of nitrification thus constitutes an important mechanism of nitrogen retention and conservation. Studies on old field succession especially measurements of ammonium and nitrate levels in soils at various stages of succession have clearly revealed that the early successional stages have higher nitrate contents but lower ammonium levels than later (climax) stages. The density of nitrifying bacteria (Nitrosomonas and Nitrobacter) in the soil also falls with the progress of succession.
Climax soils often are rich in tannins and it is thought that these tannins, as well as certain other substances, e.g., phenolic acids and phenolic glycosides, are responsible for the suppression of nitrification. In some soils, low pH and phosphate deficiency have also been implicated in nitrification suppression. When ammonia is added to soils with differing degrees of phosphate deficiency, the resulting nitrification is observed to be closely related to the available phosphorus content in the soil. Recent researches (see Moore, 1975) have also demonstrated that, contrary to popular belief, many higher plants are able to absorb and utilize ammonium ions as the major source of nitrogen. This mechanism bypasses the microbial nitrification phase and reduces the risk of nitrate loss from the soil. Since, unlike nitrate, ammonium is already in the reduced form, the plant is further saved the energy that it would otherwise spend in the reduction of nitrate to ammonium level.
Henderson and Harris (1975) have studied the cycling of nitrogen in a deciduous forest watershed in eastern Tennessee (USA). They have shown that nitrogen losses are controlled by internal cycling mechanisms within forest ecosystems and that mature deciduous and coniferous forests are more conservative than younger forests. In oak-hickory and Douglas fir dominated forests organic N losses predominate over inorganic N losses, and the transport of organic N from forested catchments appears to be strongly influenced by precipitation. In the Tennessee forest (about 100 ha catchment) actually studied, it was found that the total N in the ecosystem (about 6OOO kg/ha) was distributed among vegetation, forest floor and mineral soil components in 8, 5, and 87 per cent proportions respectively. Quantification of annual transfers within the forest indicated that root mortality was the most important cycling mechanism for return of N from vegetation to soil. Nitrogen transfers due to leaf fall, foliar leaching and tree mortality also contributed to cyling. This watershed was observed to be accumulating N at a rate of about 10 kg/ha/year. The deciduous forest seems to conserve its N supply quite efficiently, resulting in a small loss of about 2.
5 per cent in streamflow from the N circulating within the watershed (Henderson and Harris, 1975). In general, deserts have higher rates of nitrogen fixation than forests because of the abundance of surface-incrusting lichens and blue-green algae in deserts. However, most of the fixed nitrogen, whether fixed biologically or otherwise, is lost as volatilized ammonia in the alkaline soils common in deserts and also through denitrification. In fact, almost two-thirds of the fixed nitrogen may be short circulated back to the atmosphere.
The nitrogen cycles of deserts are therefore, mostly open and differ profoundly from those in deciduous forests which effectively ‘internalize’ their nitrogen cycles. In these forest ecosystems, rates of nitrification as well as denitrification tend to be low as compared to those in deserts but atmospheric N inputs tend to be retained by the plentiful supply of organic detritus through microbial nitrogen fixation, etc. Although the forest and desert ecosystems differ strikingly in respect of their nitrogen cycling, one interesting property shared by both ecosystems is that in both cases the N cycles are regulated by the availability of organic carbon. Termites and their intestinal microbiota play an important role in the cycling of nitrogen and other materials. The diet of termites is generally rich in cellulose, hemicelluloses, and lignins, but is poor in nitrogen.
Termites and their intestinal microbes are important decomposers of plant litter. Earthworms are other important decomposers of litter. The intestinal microbes of termites include several bacteria and protozoa. Most of these protozoa are cellulolytic anaerobes that effectively decompose cellulose.
Examples are Trichomitopsis, Trichnympha spp., isolated from the hindgut of Zootermopsis spp. The termite gut bacteria are exemplified by Streptococcus, Bacteroides, Staphylococcus, Bacillus, and Spirochaetes.
Some of the bacteria may be nitrogen fixers in the anaerobic environment prevailing in the termite gut, and live termites have been demonstrated to reduce acetylene to ethylene, which is an indirect evidence of nitrogenase activity or nitrogen fixation (Breznak, 1982). Termites recycle nitrogen in three ways, viz., (1) storage/recycling of nitrogenous metabolic wastes; (2) recycling of termite tissues such as ex- oskeleton, and (3) digestion and assimilation of gut microbes or their lytic or secretory products. Of these, the recycling of nitrogenous wastes is rather important and is mediated by bacteria.