Soil Processes and the Carbon Cycle

Abstract

The interaction of organic materials with mineral particles is a fundamental process in the surface horizons of most soils. Organo-mineral interactions not only influence the dynamics of soil organic matter (Oades, 1988; Amato and Ladd, 1992; Golchin et al., 1995a; Oades, 1995; Chenu et al., 1996), but also contribute to the formation and stabilization of soil aggregates (Tisdall and Oades, 1982; Oades, 1993). Interactions between small organic molecules and clay surfaces have been described and reviewed at length (Mortland, 1970; Theng, 1974) and interactions of organic polymers with clays are reasonably well understood (Theng, 1979; 1982). Interaction of particulate organic matter (POM) with mineral particles, however, has received less attention and the turnover, composition, and distribution of POM within the soil matrix are not well known. In classical fractionation schemes, SOM has been extracted from soils using alkaline solutions and the unextractable fraction or humin, which includes POM, has not been studied extensively. In this chapter we will focus on the interaction of POM with mineral particles and consider the role of POM in the formation of aggregates of different sizes. We present a conceptual model describing the involvement of POM and microbial metabolites derived from its decomposition in soil aggregation. The importance of biological processes associated with the decomposition of POM and the associated chemical changes will be identified and discussed with respect to their involvement in the proposed model of aggregation. The conceptual model, which is based on our previous works and selected literature results, may be applied generally to soils where organic matter is an important agent responsible for binding soil mineral particles together creating an aggregate hierarchy (Oades and Waters, 1991; Oades, 1993). In the model, we proposed the existence of three levels of aggregation (< 20 µm, 20-250 µm, and > 250 µm) in which the mechanisms of stabilization differ.