Redox Transformations of Nitrogen

Pathway 4 shows denitrification. This process is linked to microbial respiration where electrons are added to nitrate or nitrite, resulting in the production of nitrous oxide or nitrogen gas. The 

FIGURE 8.5 Oxidation and reduction reactions of nitrogen in wetlands. Numbers 1-7 refer to pathways of nitrogen reactions. 1 = ammonification 2 = immobilization 3 = nitrification 4 = denitrification 5 = dissimilatory nitrate reduction to ammonia 6 = dinitrogen fixation and 7 = ammonia volatilization. 

Pathway 6 shows dinitrogen fixation, which is the reduction of atmospheric, inert nitrogen to ammonia that plants and microbes can then use. It occurs in the water column, soil-floodwater interface, root zone, and anaerobic soil in wetlands. It is carried out by free-living bacteria (Clostridium), bacteria living in symbiotic association with plants (Rhizobium), cyanobacteria (Anabaena), and periphyton. 

Pathway 7 shows ammonia volatilization, which is a physicochemical process controlled by the pH of the environment. There are many biotic processes that can alter the pH of a wetland including photosynthesis and denitrification. An alkaline pH favors the presence of unionized ammonia, whereas acidic or neutral pH favors that of ionized ammonia. Loss of nitrogen due to volatilization is insignificant at pH 7.5, but it dramatically increases at pH 7.5. 

FIGURE 8.6 Nitrogen budget in the biosphere. (From Galloway et al., 2004.) 

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