The Rhizosphere and Soil Formation
This chapter discusses rhizospheres and some of their broad biological, physical, and chemical effects on soil formation. By most accounts, the rhizosphere is narrowly conceived in space and time. Plant roots that are rhizospheres are networks within the bulk soil. They are biological hotspots where respiration, gas exchange, nutrient and moisture use, and localized supplies of organic matter are most concentrated. Soil scientists and ecologists have long divided the soil profile into an upper "solum" and the lower "parent material," in part because of the physical and chemical effects of rooting. Growing roots and their mycorrhizal hyphae follow pores and channels that are generally not less than their own diameters. Rhizospheres not only alter soil minerals physically from individual grains to whole horizons, but chemically interact with soils in a wide range of spatial scales as well. A major source of soil acidity is derived from the uptake of nutrients by vegetation. Organic acids play significant and varied roles in rhizosphere acidification and mineral weathering, contributing protons and serving as ligands that complex metals. Organic acids are weak acids with pKa values that range widely from as low as 3 (carboxylic) to as high as 9 (phenolic). Redox cycling in rhizospheres of relatively well-aerated soils is a little-studied process with considerable potential impact on soil acidity. Over millennial time scales, the upper soil system is mechanically mixed by bioturbation, a mixing that is broadly affected by the rhizosphere. The concept of the rhizosphere has been significant to ecological, biological, agronomic, and forestry sciences in the first 100 years of its use. © 2007 Elsevier Inc. All rights reserved.
