Nitrogen Mineralization, Immobilization, and Nitrification

TL;DR: The biogeochemical cycling of N in ecosystems can be divided into an external and an internal N cycle as mentioned in this paper, and the internal cycle consists of those processes that convert N from one chemical form to another or transfer N between ecosystem pools.

Abstract: 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).