Mineralization of Organic Nitrogen

Because of this microbial control, ammonification is generally accompanied by immobilization the two processes tend to counteract each other so far as the production of inorganic N is concerned. 

FIGURE 8.7 Processes involved in the transformation and conversion of ammonia in soils. 

Let us assume that 100 units (dry weight basis) of plant detritus is undergoing decomposition in a wetland. The carbon content of detrital matter is 40 units assuming 40% carbon content (dry weight basis). Carbon and nitrogen use efficiency is set at approximately 40% for aerobes and 10% for anaerobes. Based on the above assumptions, 16 units of carbon are assimilated by aerobes and 4 units of carbon by anaerobes. To maintain C N ratio of 10 in their biomass, aerobic microbes and anaerobes would require 1.6 units and 0.4 units of nitrogen, respectively. 

FIGURE 8.8 Relationship of decomposition of organic matter and detrital C N ratio. 

Repeated cycles of ammoniflcation bring the C N ratio of detritus below critical level, where net ammoniflcation occurs. Now the material is at a stage when the net mineralization of nitrogen can be realized. As the reactions of mineralization and immobilization are simultaneous and opposite in direction, it is the net mineralization and immobilization that controls the amount of available inorganic nitrogen. This net effect depends on the type of carbonaceous material and its nitrogen content. 

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Reddy, K. R., Khaleel, R., Overcash, M, R., and Westerman, P. W. (1979a). A nonpoint source model for land areas receiving animal wastes. I. Mineralization of organic nitrogen. Trans. ASAE 22, 863-872. 

Another factor responsible for the observed variability in positive results may be adsorption of ammonium and other nitrogen compounds onto humic substances (the main photoreactive fraction of DOM). Hence, detection of photoammonification may to some extent reflect photochemicaUy induced desorption of ammonium from large organic molecules, rather than the mineralization of organic nitrogen. This phenomenon, however, would be limited to fresh and low salinity waters, since at higher salinities ammonium ions are replaced by cationic macroconstituents of seawater. 

FIGURE 8.9 Mineralization of organic nitrogen plant residues with different C N ratios. (Data from Tusneem and Patrick, 1971.)... 

FIGURE 8.19 Mineralization of organic nitrogen as influenced by soil redox potential. (Redrawn from McLatchy and Reddy, 1998.)... 

FIGURE 8.24 Relationship between carbon-to-nitrogen ratio of plant litter and mineralization of organic nitrogen (Ponnamperuma, 1981). 

Nitrification-denitrification reactions are the major pathway for N loss from wetland ecosystems to the atmosphere. The process involves a series of sequential microbial processes that include mineralization of organic nitrogen to ammonium, oxidation of ammonium to nitrate, and denitrification of nitrate to nitrous oxide or dinitrogen gas. Nitrification-denitrification in wetlands occurs primarily in two zones the aerobic-anaerobic interface at the surface of the flooded soil or sediment, and the oxidized rhizosphere of wetland plants (Figure 16.6). 

Harmsen GW, Van Schreven DA (1955) Mineralization of organic nitrogen in soU. Adv Agron... 

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