CO2 evolution and denaturing gradient gel electrophoresis bacterial community profiles in soil following addition of low molecular weight substrates to simulate root exudation

Simulating the evolution of both C-14 and C-12-CO2 in the rhizoplane was monitored during the diffusion of C-14-labelled glucose, oxalic acid, or glutamic acid into soil from a filter placed on the surface of a sandy loam. After 3 and 7 d, soil was sampled from four layers (0-2, 2-4, 4-6, and 6-14 mm) to determine residual C-14 in each layer. The mineralisation pattern of oxalic acid was characterised by a lag phase probably due to the presence, in the early stages of exposure, of a few microorganisms able to mineralise this substrate. Glucose and glutamic acid showed a positive priming effect with a CO2 flush from native organic matter. Oxalic and glutamic acids changed the denaturing gradient gel electrophoresis profiles of soil bacterial communities with the appearance of a few extra-bands in the 0-2 mm soil layer. The addition of the substrates onto the soil surface formed a gradient due to their diffusion in soil. That of oxalic acid was specific probably because almost all of this compound reacted with CaCO3, and was localised in the 0-2 mm soil layer.