ABSTRACT
Biomolecules produced by microorganisms or by plants as well as humic and fulvic acids are involved in the weathering of clay minerals, in the release of aluminum and iron, and in the formation of short-range ordered minerals and organomineral complexes. Because the amount of natural organic compounds in the rhizosphere is much higher than in bulk soil, these reactions are particularly important in rhizospheric soils (Fig. 1).Natural organic compounds present in organomineral complexes or adsorbed on variable charge minerals may strongly influence the adsorption of nutrients and their availability for plants. Furthermore, organic ligands may compete with nutrients for common adsorption sites on variable charge minerals and soils and may remove nutrients, previously adsorbed on soil colloids, rendering them more available for plants. Before the last 15 years, studies on the competition in adsorption between organic compounds and nutrients were rare and scattered in the literature of the environmental sciences, whereas in the last decade research on this topic has increased significantly. 207
Figure 1 Interactions among root exudates and biomolecules released by microorganisms, clay minerals, and nutrients. Biological compounds promote weathering of clay minerals by facilitating the formation of short-range-ordered minerals (SROM) (mainly Al and/or Fe precipitation products) and organomineral complexes. In the rhizosphere biomolecules (including biopolymers as enzymes, E) may be adsorbed on SROM and may compete with nutrients influencing their mobility.
