The Role of Microbial Communities in the Formation and Decomposition of Soil Organic Matter

Organic matter is mainly present in the top 20–30 cm of most soil profiles and is essentially an array of organic macromolecules consisting principally of combinations of carbon, oxygen, hydrogen, nitrogen, phosphorus and sulphur. Soil organic matter is commonly measured as the quantity of organic carbon. The global pool of organic carbon in soil to a depth of 1 m has been estimated at 1,200–1,550 Pg (2 m: 2,370–2,450 Pg), and as such is significantly greater than either the biological-biota (560 Pg) or atmospheric (760 Pg) carbon pools (Baldock 2007). Almost all organic matter in soil is directly and indirectly derived from plants via photosynthesis. Thus atmospheric carbon dioxide is transformed by reduction into simple and complex organic carbon compounds, which in combination with key nutrients enable the plant to function and grow. Carbon dioxide is released directly from plants by respiration, but most of the fixed carbon is retained and ultimately transferred to the soil ecosystem via a combination of spatially distinct pathways over a variety of timescales. The most important pathways are the direct addition of senes -cent material as