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For other uses, see Mineralization (disambiguation).

Mineralization in soil science is the overall transformation of an element in an organic form into an inorganic (mineral) form. It is the result of the decomposition of organic matter (the oxidation of organic compounds in the soil) by soil fauna. Mineralization is the opposite of immobilization, and the two processes occur simultaneously; the term mineralization is typically used to imply ‘net mineralization’, or the sum of these two opposing processes over a designated time. The process of mineralization is primarily concerned with the plant nutrient nitrogen (N), but also applies to phosphorus (P) and sulfur (S). It is an important process in the nitrogen cycle because the resulting mineral nutrients are the dominant plant-available forms in the soil solution.

Factors controlling mineralization
Given that mineralization is a result of decomposition (a largely biological process) any factor that influences soil biological activity also affects the rate of mineralization. This includes soil environmental conditions (pH, temperature, moisture, aeration, etc.) as well as organic matter characteristics (physical size, nutrient concentration and ratios, presence or absence of slowly decomposing compounds such as lignin or cellulose, etc.). Additionally, in agriculture, many soil management practices affect mineralization rate, primarily via their influence on soil fauna activity, including tillage, irrigation, liming, etc.

Substrate nutrient ratios
One key variable controlling mineralization is the nutrient-to-carbon ratio in the organic compound. Specifically, the ratio of carbon (C) to the element of interest determines if mineralization or immobilization will dominate. This is because the soil microbes primarily responsible for mineralization require nutrients (i.e. N, P, and S) from their food sources in a characteristic ratio relative to C. If the nutrient-to-carbon ratio in the organic compound exceeds the ratio required by the decomposer, then the excess nutrient will be mineralized and released into the soil solution, becoming available to plants.

The carbon-to-nitrogen ratio (C:N), carbon-to-phosphorus ratio (C:P), and carbon-to-sulfur ratio (C:S) values of 25:1, 250:1, and 400:1, respectively, are general biological thresholds, with values higher than these resulting in immobilization and lower values resulting in mineralization. When microbes encounter organic compounds with nutrient ratios above these thresholds, they may consume mineral nutrients from the soil solution to meet their needs, and these nutrients are said to be immobilized in the microbial biomass. This can reduce the concentration of inorganic nutrients in the soil, and thus the nutrients available to plants.

Nitrogen mineralization
The dominant forms of plant-available nitrogen in the soil are ammonium (NH4+) and nitrate (NO3-), and soil N mineralization (resulting in the release of these ions) encompasses two processes. The first is ammonification, an enzymatic process where NH4+ is released from an organic N-containing monomeric compound. The second is nitrification, where NH4+ is oxidized first to nitrite (NO2-) and then to NO3-. In these ion forms (NH4+ and NO3-), N can either be taken up by plants, or undergo other transformations as part of the broader nitrogen cycle. The general process of mineralization from a nitrogen-containing amino compound (R-NH2) to NO3- is shown in the reaction below.

Role of soil fauna in nitrogen mineralization in agriculture
To varying degrees, soil micro, meso and macro fauna are all involved in the mineralization of organic soil N. The soil fauna dominant in this process depends on several factors, including land-use type, i.e. agriculture or forestry. In agriculture, N mineralization is primarily a result of micro-fauna activity, with the greatest contribution from bacteria, and to a lesser extent fungi and archaea. These micro-organisms cause immobilization when they assimilate NH4+ or NO3- directly from the soil solution, and it is incorporated into their biomass. In contrast, mineralization results when these micro-organisms absorb N-containing monomeric compounds (such as amino acids, amines, amino sugars, and urea), and excrete any N in excess of the microbe’s requirements as NH4+ back into the soil solution. In general, bacterial-dominated soil food webs result in much quicker N mineralization than those that are fungal-dominated.

Soil mesofauna, such as nematodes, mites, collembola, and protists, as well as macro-fauna, such as earthworms, regulate soil N mineralization through two main pathways. First, through predation of micro-organisms, and second, via physical changes to both the organic substrates subject to degradation, and to the micro-fauna habitat. Effects of predation on N mineralization are generally due to the release of excess NH4+ when N-rich bacteria are consumed. However, predation also results in changes in to the microbial community structure which indirectly regulates N mineralization. Meso and macrofauna enhance N mineralization through physical effects by shredding organic matter and mixing it with the mineral soil. Additionally, the macro-pores created as they move through the soil, and the soil aggregates created by their faecal pellets and casts, indirectly contribute to N mineralization by creating favourable environments for microbial activity.