Distribution and Diffusion of Root Exudates of Zea mays in Soil

Distribution and diffusion of root exudates of Zea mays were studied in a loamy Haplic Luvisol by means of 14CO2 pulse labelling of shoots in two-compartment pots, in which the roots were separated from sterile or nonsterile soil by a screen. Root hairs but not roots could penetrate the screen into the soil. Root-free soil in the bottom pot compartment of one treatment was sterilized with cycloheximide and streptomycin to inhibit microbial decomposition of exudates. The soil from the bottom part of the pots was frozen and sliced by a microtome into 15 layers, each 1 mm thick. Four zones of exudate concentrations were found according to the distribution of the 14C activity in the rhizosphere profile: 1) 1…2 (3) mm: maximal concentration of exudates around root hairs; 2) 3…5 mm: presence of exudates is caused by their diffusion from the root hairs; 3) 6… 10 mm: insignificant amounts of exudates diffused from the previous zones; 4) > 10 mm: complete lack of exudates. The amount of 14C exudates was higher in the first 1-mm layer from the roots of nonsterile soil compared to the sterile soil due to stimulation of exudation by microorganisms. The coefficient of vertical exudate diffusion in the soil was 1.9 × 10•7 cm2/Vs.