Nitrogen Mineralization, Immobilization, and Nitrification
The biogeochemical cycling of N in ecosystems can be divided into an external and an internal N cycle. The external cycle includes those processes that add or remove N from ecosystems, such as: dinitrogen (N2) fixation, dry and wet N deposition, N fertilization, N leaching, runoff erosion, denitrification, and ammonia volatilization. The internal N cycle consists of those processes that convert N from one chemical form to another or transfer N between ecosystem pools. Processes of the internal N cycle include: plant assimilation of N, return of N to soil in plant litterfall and root turnover, N mineralization (the conversion of organic N to inorganic N) microbial immobilization of N (the uptake of inorganic N by microorganisms) and nitrification (the production of nitrite {N02 and nitrate {N03 from ammonium {NH/ or organic N) (Fig. 42 1) The significance of internal N cycling processes can be illustrated by comparing the rates of these processes relative to external N cycling rates. For example, Paul and Clark (1989) estimate that the sum of all output fluxes of the external N cycle globally is about 0.25 x 1015 g N yr1, while net N mineralization in soils is more than 14 times this amount (about 3.5 x 1015 g N ye1) However, because net N mineralization is the difference between actual N mineralization and microbial immobilization of N, gross N mineralization rates may be over two orders of magnitude greater than all output fluxes of N combined (see below)