Characteristics of rhizosphere soil from natural and agricultural environments

ABSTRACT In the first part of this chapter, we present an overview of the methodologies adopted to study the rhizosphere soil, focusing on the protocols devised for its separation. For these methodologies and protocols, we also discuss the advantages and disadvantages inherent in their use. The sections of the chapter are dedicated to reports on three case studies, where the bulk and rhizosphere soil of three arboreal species are compared and contrasted. The first case study deals with the strategy used by Genista aetnensis Biv. to colonize the inhospitable volcanic soils on the flanks of Mount Etna (Sicily, Italy). In this environment, Genista are able to overcome the low availability of nutrients through forcing the roots to excrete oxalic acid, and to preserve P by the hosting of a microbial population in the rhizosphere soil that is responsible for the biological cycling of P. As a by-product of the weathering promoted by the roots, the yellowish-coloured collar around them, which is due to the presence of amorphous Fe-oxyhydroxides, reveals the thickness of the rhizosphere soil. The second case presented deals with the ability of Erica arborea L. to colonize a soil derived from alkaline marine deposits in Central Italy. The Erica plants, which are established in this environment due to the formation of superficial acid horizons, have been able to modify the upper 60 cm of soil through root excretion of organic acids until the differences between bulk and rhizosphere are removed. The roots of Erica are now colonizing the horizon underneath, where the rhizosphere soil is more acidic than the bulk. At deeper levels, carbonates persist and roots of Erica are rare. The final case study reports on the chemical fractionation of lanthanides in bulk and rhizosphere soil of adult vines (Vitis vinifera L.) from two vineyards, one in Tuscany (Italy) and the other in Galicia (Spain). In these soils, the presence of lanthanides has been ascribed mostly to the long-lasting practices of cultivation and, in particular, to the use of fertilizers and the deep mechanical working of the soil, which have greatly affected the soil characteristics over the centuries. The chemical fractions more involved in the binding of lanthanides have resulted in the organic matter and the Fe-oxyhydroxides. In both soils, root activity since the planting of the vineyard (some decades ago) has been able to modify the chemical fractionation of lanthanides within the horizons, with a small effect on the redistribution throughout the profile.