Nitrogen Mineralization-Immobilization

Nitrogen undergoes a complicated series of cyclic pathways in the ecosystem (Fig. 8.2). The atmospheric form of free nitrogen must be fixed —incorporated into chemical compounds (e.g., NH3) which can be utilized by plants. This nitrogen fixation can be accomplished by bacterial action of both free-living soil bacteria such as azotobacter and chlostridium and symbiotic bacteria such as rhizobium. It can also be 

The rate constants and k refer to mineralization (m) and immobilization (i), respectively. Mineralization refers to production of NH3 and/or NH4 through microbial decomposition, whereas immobilization refers to incorporating N into bacterial cells (e.g., making up protein). 

The factors that affect the ability of microorganisms to decompose organic material include type of organic material, temperature, pH, and redox potential (Eh). Fungi and actinomycetes are primarily responsible for the initial decomposition of organic waste. After that, bacteria are able to produce protease, a proteolytic enzyme which breaks protein down into simple compounds such as amino acids. The amino acids are absorbed by the microorganisms and ammonia is released by the following reactions  

ORGANIC MATTER, NITROGEN, PHOSPHORUS, SYNTHETIC ORGANICS 

From this point on, NH3 undergoes various reactions in soils, including assimilation, soil adsorption, nitrification, and volatilization. 

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Davidson, E., Hart, S.C., Shanks, C.A. and Firestone, M.K. 1991 Measuring gross nitrogen mineralization, immobilization, and nitrification by isotopic pool dilution in intact soil cores. Journal of Soil Science 42 335—349. 

E. S. Jensen, Mineralization-immobilization of nitrogen in. soil amended with low C N ratio plant residues with different particle sizes. Soil Biol. Biochem. 26 519 (1994). 

Dissolved Organic Nitrogen through Immobilization and Mineralization 56... 

B. Loss of Soil Solution Dissolved Organic Carbon and Dissolved Organic Nitrogen through Immobilization and Mineralization... 

Mary, B. and Recous, S. (1994) Measurement of nitrogen mineralization and immobilization fluxes in soil as a means of predicting net mineralization. European Journal of Agronomy 3, 291-300. 

During decomposition most of the nitrogen is immobilized initially, at least, in microbial cells, which in turn are broken down. In the process, a considerable portion of the nitrogen reacts with phenols, quinones, and other ring compounds derived from lignin, or synthesized by microorganisms, to form humic substances, some of which are heterocyclic. Amino acids and ammonia are subject to such chemical reactions, that may be catalyzed by microbial enzymes. In soils with 2 1 clay minerals any ammonia that is formed is also subject to fixation in the clay lattice (see Chapter 8 and 11). 

A rather complete picture of what happens to fertilizer nitrogen added to cropped soil in greenhouse pots is shown in Table 23.11, taken from the data of Legg and Allison (1959, 1960). Immobilization of added tracer nitrogen increased 4—5 times as the nitrogen addition increased 16-fold. Soil nitrogen mineralization (column 9) was essentially a constant as fertilizer nitrogen additions were increased. 

The C N ratio reflects the mineralization-immobilization pattern of nitrogen. A wider C N ratio promotes immobilization, whereas the narrower ratio favors mineralization. 

Bruun, S., J. Luxhpi, J. Magid, A. de Neergaard, and L. S. Jensen, 2006. A nitrogen mineralization model based on relationships for gross mineralization and immobilization. Soil Biol. Biochem. 38 2712-2721. 

JANSSON S.L. 1958. Tracer studies on nitrogen transformations in soil with special attention to mineralization-immobilization relationships. Annals of the Royal Agricultural College, Sweden, 24, 101-361. 

In this chapter we review our current understanding with respect to carbon (C) and nitrogen (N) dynamics, the chemistry and biology of the rhizosphere, and aspects of immobilization and mineralization specific to the rhizosphere. Carbon... 

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