Soil Organic Matter and Soil Productivity: Searching for the Missing Link

Soil-organic matter (SOM) is a complex array of components including soil fauna and flora at different stages of decomposition (Berg et al . , 1982). I ts concentration in soils can vary from 0.5% in mineral soils to almost 100% in peat soils (Brady, 1974). Organic matter (OM) in the surface mineral soil is considered a major determinant of forest ecosystem productivity because it affects water retention, soil structure, and nutrient cycling (Powers et al., 1990; Paul 199 1). Soil-organic matter is the major source of nitrogen available to plants and contains as much as 65% of the total soil phosphorus (Bauer and Black, 1994). During decomposi t ion, OM is broken down into various components including carbohydrates, amino acids, proteins, nucleic acids, lipids, tannins, lignin, and humus. Humus refers to the microbial resistant forms of SOM that remain after major portions of added plant and animal residues have decomposed. I ts chemical nature has not been completely determined but i t is bel ieved to consist of various polymeric compounds, probably aromatic and aliphatic in composition (Schnitzer and Schulten, 1992). Humus has been found to affect the physical properties of soil (Elliot, 1986; Beare et al., 1994). The nonhumus components of SOM are crucial in nutrient cycling dynamics and are the primary source of food and energy for soil microorganisms (Cambardella and Elliot, 1992; Wander et al., 1994). As such, these components are the driving force for productivity. The input into the OM pool in forest soils comes mainly from forest litter and fine root mortality, exudates, and sloughing (Vogt et al., 1986). Estimates of the net annual carbon (C) budget of a tree relegated to root growth and maintenance