Mineralisation releases nutrients in soils are tied up in

Mineralization releases nutrients in soils that are tied up in Organic matter.
Mineralization is the conversion of a nutrient from the organic (i.e. bound to carbon and hydrogen) form to the inorganic form. The process occurs when organic materials, such as soil organic matter, manure, plant residue, or biosolids, are decomposed by soil microorganisms.
Mineralization is the main organic matter conversion process, which leads not only to the preservation of organic matter in the soil but also to its sequestration.