The role of roots, fungi and bacteria on clay particle organization. An experimental approach

Abstract

Experimental studies were conducted to investigate the role of different living organisms (roots, fungi, bacteria) on clay microfabric. For this purpose various microorganisms (bacteria, fungi) and roots were allowed to grow on kaolinite or montmorillonite pastes under controlled hydric conditions (−0.01 to −10 2 MPa). Microstructures were studied with a cold-stage SEM, that allowed the preservation of the organizations that characterized the wet states. With fungi, three main effects were observed in moist conditions: orientation of clay particles around the cells; secretion of extracellular polysaccharides that induced local binding of clay particles, and a general packing effect by hyphae. These effects lead to a new microstructure, in the immediate surrounding of the cell, designated as a microenvironment. A modified microstructure was recognized with all species, the size depending on the size of organism. With bacteria, polysaccharide-mediated aggregation was predominant. With grass roots, modifications of the microstructure were more complex than with fungi and occurred at a larger scale. The present results thereby confirm and complete the existing models on biologically mediated aggregation in soils.