MICROBIAL CHANGES IN A MODEL RHIZOSPHERE

Apparatus was developed to study the changes in the biomass size and distribution and the carbon dynamics within a model rhizosphere. Glucose was allowed to diffuse into a soil column from a source solution of effectively constant concentration via a millipore membrane (approximating a root-soil interface). This arrangement allowed for details of changes in biomass and nutrient utilization to be measured over distances from the source-soil interface in the order of 50 μm. Determination of soluble C showed the formation of steep concentration gradients from an early stage and that the bulk of the extractable C was made up from compounds of low soil mobility, other than glucose, that appeared to have been released by the local biomass. The presence of soluble C, near to the interface, at concentrations higher than in the source also suggested compounds other than glucose to be present. The biomass data indicated that after 5 days, over 90% of the new biomass C was present within 2 mm of the interface between the soil and nutrient solution. The biomass concentration profiles were shown to be much less steep than those for the soluble C and had shown the greatest rate of penetration between days 3 and 5. Between 1.5 and 2.5 times more C was found to be released by chloroform fumigation of the biomass, within 0.5 mm of the interface, than was estimated as being biomass C by the use of the ninhydrin method of determining biomass. Of the total amounts of 14C carbon detected, the fraction mineralized showed the greatest increase between days 3 and 5. © 1991