Litter is decomposed in a sequential process. In a concerted action animals and microorganisms break down complex organic matter to mineral products. Higher animals fragment and partially solubilize plant material. Subsequently, microorganisms (protozoa, fungi and bacteria) further degrade the organic matter to end products that cannot be metabolized further under the prevailing environmental conditions. During the process of decomposition some parts of the organic substrate and the excess energy are used to form new biomass. Some free organic intermediates may interact chemically to form relatively recalcitrant organic matter, such as humic substances. The degree of mineralization depends strongly on the type of organic matter in the litter and the physical and chemical conditions of the environment.