Spatial heterogeneity of soil water around single roots: use of CT-scanning to predict fungal growth in the rhizosphere.

Abstract

Whole-body computed tomography X-ray scanning (X-ray CT) of wheat, cotton and radish growing in pots containing 350 g of a white sand: clay mixture produced images from which digital data of the volumetric water content were obtained. The volumetric water contents were spatially heterogeneous around the root and altered with the depth at which the image was taken. Data represent a 'snapshot in time' of the soil around single roots in a living soil-plant system. The volumetric water contents of soil around seedlings were assessed for the growth of two soil-borne fungi, Gaeumannomyces graminis var. tritici and Rhizoctonia solani. The heterogeneity of water content was biologically significant: water contents around the roots of wheat, radish and cotton were of a great enough range to suggest the existence of heterogeneous sites in the rhizosphere which might favour or restrict fungal growth. In soil surrounding wheat roots, the most favourable regions for the growth of G. graminis were nearest the root, suggesting that the inner rhizosphere might be more amenable to the growth of this pathogenic fungus than the outer region of the rhizosphere, and the bulk soil. The technique of X-ray CT enables spatial examination of the response of soil water to environmental treatments of the soil-root system, and the non-destructive observation of the effect of the spatial heterogeneity of volumetric water content upon fungal growth in the soil, over time and at a range of depths.