Nitrogen cycling forest ecosystems

Under low-dose conditions, forest ecosystems act as sinks for atmospheric pollutants and in some instances as sources. As indicated in Chapter 7, the atmosphere, lithosphere, and oceans are involved in cycling carbon, nitrogen, oxygen, sulfur, and other elements through each subsystem with different time scales. Under low-dose conditions, forest and other biomass systems have been utilizing chemical compounds present in the atmosphere and releasing others to the atmosphere for thousands of years. Industrialization has increased the concentrations of NO2, SO2, and CO2 in the "clean background" atmosphere, and certain types of interactions with forest systems can be defined. 

Since nitrogen is a nutrient, which limits the productivity of almost all Boreal and Sub-Boreal Forest ecosystems, its biogeochemical cycling is relatively well understood at present. The major N transformations and fluxes are shown in Figure 3. 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

Schulze, E-D. (2000). Carbon and nitrogen cycling in European forest ecosystems. Berlin Springer, 500 p. 

Including food-web processes in quantitative models of carbon and nitrogen cycling of forest ecosystems. The competition between sapro-trophic and mycorrhizal fungi for resources is a key aspect of belowground functioning (Lindahl et al., 2002). Quite detailed computer models... 

Wallenda, T., Stober, C., Hogbom, L. et al. (2000). Nitrogen uptake processes in roots and mycorrhizas. In Carbon and Nitrogen Cycling in European Forest Ecosystems, Ecological Studies 142, ed. E. D. Schulze. Berlin Springer-Verlag, pp. 122-43. 

Aber, J. D. (1992). Nitrogen cycling and nitrogen saturation in temperate forest ecosystems. Trends in Ecology and Evolution, 1, 220-3. 

The nutrient cycles of the forest catchment ecosystems are to a large extent determined by biota, especially by the primary production of plants and by microbial decomposition. Severe losses from the ecosystem of important nutrients, e.g., Ca, Mg, K and P, are expected to lower the productivity when occurring in the root zone. Most nutrients available for circulation in the temperate forested ecosystems are found in the tree layer or in the accumulated organic mater of soil layer. This is especially true for the most important macronutrients (C, N, P, K, Ca, Mg and S). Nitrogen is... 

From these results we can conclude that nitrogen is relatively available in soils of Tropical Rain Forest ecosystems and that forest soils mineralize and nitrify large amounts of nitrogen. P. Vitousek andR. Sanford have shown similar results earlier in 1986 studying nitrogen cycling in moist tropical forests. 

The leaching of nitrogen from Forest ecosystems due to disruption of the natural biogeochemical cycle is shown in Table 2. 

Table J. Possible factors influencing the risk of N saturation ami nitrate leaching after disturbance of natural biogeochem-ical cycle of nitrogen in Forest ecosystems.

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