Rhizosphere engineering by plants: Quantifying soil-root interactions

The interface between roots and soils, the rhizosphere, is often ill-defined, complex, and spatially heterogeneous. It varies temporally because the dynamic interaction between roots and soils affects the growth rate of the roots themselves and can be modified by roots to improve growth conditions. Analytical techniques to study the rhizosphere have lagged behind those of other aspects of plant-soil interactions so that quantitative models describing the processes involved are rare. A recent surge in research on rhizosphere development has been driven by concerns about the need to increase crop production to underpin food security. Advances in genetics and phenotyping have facilitated studies of physical and chemical changes, which have been complemented by new analytical techniques. This article reviews this research and identifies major gaps in understanding. Root mucilage generally increases soil wetness and hydraulic conductivity adjacent to the root and, together with the release of root cap cells, facilitates root elongation by reducing the frictional resistance of the soil. It also affects water and nutrient availability in the rhizosphere and contributes to the development of aggregates and soil structure. Soil deformation during root elongation results in the radial displacement of particles and increases in bulk density of up to 30% adjacent to growing root tips. Together with microbial activity, these movements, added to the wetting and drying induced by root activity, result in dispersion and aggregation of soil particles and the generation of soil physical structure. Besides mucilage, roots also release many other compounds including enzymes such as phosphatases. The interaction of these enzymes in soils is complex depending on the speed with which they are adsorbed by surfaces and the availability of their specific substrates. Nevertheless, in particular circumstances, uptake of up to 70% more P has been measured in plants exuding phytase and supplied with inorganic P fertilizer, and larger values have been found for plants supplied with monogastric animal manure. The physical, chemical, and biological modification of the soil surrounding roots and the ways in which this can be managed represent a considerable challenge for future research efforts with far-reaching consequences for crop production and other ecosystem services.