Nitrogen cycle and exposure pathways

When nitrogen input owed to mineralization and atmospheric deposition exceeds the demand of both the vegetation and the microbes in undisturbed maturated forest 

Fixation of molecular nitrogen, N2, to ammonia in forest ecosystems can occur on and/or in a variety of forest substrates including plant canopy and stems, epiphitic plants compartments, wood, litter, soil and roots. A recent review of the magnitude of N inputs to forest ecosystems indicates that non-symbiotic fixation ranges from 1 to 5 kg/ha/yr and symbiotic fixation ranges from about 10 to 160 kg/ha/yr in early successional ecosystems where N2-fixing species are present. 

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. 

from the viewpoint of environmental risk assessment (critical loads) the most important exposure pathways are nitrate leaching and denitrification, which are both very sensitive to anthropogenic pollution. These links of biogeochemical nitrogen cycle should be firstly quantitatively parameterized to assessing environmental risk. 

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