Dinitrogen from soil

The biological reaction that counterbalances the loss of nitrogen from soils or agroecosystems is biological nitrogen fixation (BNF), which is the enzymatic reductimi of the atmospheric dinitrogen (N2) to ammonia, catalyzed by the nitrogenase... 

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. 

Soil-borne bacteria of the family Rhizobiaceae and leguminous plants form a symbiotic relationship during which a new organ, the root nodule, is developed. Within these root nodules the bacteria fix atmospheric dinitrogen and the product of nitrogen fixation, ammonia, is exported to the plant [69,70]. Root nodules develop from primordia which are established at specific sites in the root cortex shortly after Rhizobium infection. The peptide enod40 is believed to play a critical role in inducing the de-differentiation and the mitotic division of root cortical cells, i.e. the initial steps in nodule development. This however, is not entirely undisputed [3,4,69-72]. 

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). 

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