Soil organic matter (SOM) consists of various functional pools that are stabilized by specific mechanisms and have certain turnover rates. For the development of mechanistic models that predict changes in SOM storage, these pools have to be quantified and characterized. In the past, numerous fractionation schemes have been developed to separate and analyse such SOM fractions. In this review, the SOM fractions obtained with such operational fractionation procedures are described in terms of their pool sizes, chemical properties, and turnover rates. The main objective of this review is to evaluate these operationally defined fractions with respect to their suitability to describe functional SOM pools that could be used to parameterize SOM turnover models. Fractionation procedures include (1) physical separation of SOM into aggregate, particle size, and density fractions and fractions according to their magnetic susceptibility, and (2) various wet chemical procedures that fractionate SOM according to solubility, hydrolysability, and resistance to oxidation or by destruction of the mineral phase. Furthermore, combinations of fractionation methods are evaluated. The active SOM pool with turnover rates

SOM fractionation methods: Relevance to functional pools and to stabilization mechanisms

Differentiating with fluorescence spectroscopy the sources of dissolved organic matter in soils subjected to drying

Controls of bioavailability and biodegradability of dissolved organic matter in soils

Biodegradation of soil-derived dissolved organic matter as related to its properties

Dissolved and water-extractable organic matter in soils: a review on the influence of land use and management practices

Publisher Summary
Humus is defined as being synonymous with soil organic matter, which in turn is the total organic matter in a soil exclusive of the biomass and undecayed material Dissolved humus consists of the dissolved organic material (DOM) in soil, which influences the overall ecology and geochemistry of soil This chapter focuses on the DOM's ecological role in the unsaturated zone, with the saturated zone and the surface aquatic ecosystems being peripherally considered The relation of the DOM to other ecological components in soil is influenced by two types of fluxes The first one is physical diffusion The DOM diffuses over a concentration gradient The second physical flux is convection, which is dependent on a gradient in the soil's water content as a result of precipitation The dissolved humus is mobile and so it can be fully available theoretically This mobility and availability is not constant; it can vary both over space and time The potential ecological function and in situ availability of the dissolved humus are considered in the chapter so that one is able to understand and predict its environmental role

A. Zsolnay

Papers

Dissolved Humus in Soil Waters