Chemical and Biological Characteristics of Physically Uncomplexed Organic Matter

Physical fractionation methods are based on the premise that soil organic matter (SOM) can be divided into pools of functional relevance. Physically uncomplexed organic matter (OM) is isolated on the basis of particle size and/or density. Our objective here is to review research on the biological and chemical characteristics of physically uncomplexed OM that demonstrates its value (or otherwise) as a meaningful pool of SOM. Chemical characterization indicates that fractions isolated by size are not identical to those separated by density; even materials separated using variations of a particular fractionation method (i.e., different sizes or different densities) have different chemical and biological properties. Physically uncomplexed OM often contains a substantial portion of whole soil carbon (C) and nitrogen (N) and, compared with the whole soil or heavy fraction, has a wide C/N ratio and high O‐alkyl (i.e., carbohydrates) and low carbonyl (i.e., proteins) C contents. The response of physically uncomplexed OM to changes in land use and management practices is greater than that of other labile OM fractions or the whole soil C and N. Studies to elucidate the nutrient availability of physically uncomplexed OM suggest that it is not an immediate source of nutrients. That the quantity of physically uncomplexed OM is not always related to the amount of plant residue inputs suggests that other factors may control its accumulation in soil. Thus the quantity and the biological and chemical properties of physically uncomplexed OM are affected by the amount, composition, and accessibility of plant residues entering the soil; environmental conditions that may enhance or constrain decomposition; and the fractionation technique used.