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

Simulating the evolution of both 14C and 12C-CO2 in the rhizoplane was monitored during the diffusion of 14C-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 14C 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.