Carbon, nitrogen and microbial gradients induced by plant residues decomposing in soil

Understanding the role of the soil matrix in the decomposition of soil orga nic matter requires an assessment of how the soil matrix controls the avail ability of substrate to microorganisms. We aimed to identify the sites of m icrobial assimilation of a decomposing substrate the initial location of wh ich in the soil matrix was known. We incubated wheat straw doubly labelled with C-13 and N-15 as a single layer in the middle of a core of soil and we separated, after different times of incubation, soil layers situated at di fferent distances from the straw. We analysed them for their C-13 and N-15 contents and dehydrogenase activity. The presence of the straw induced stee p gradients of dehydrogenase activity in the core; the activity increased a t the contact with the straw and decreased to reach the reference value bet ween 3 and 4 mm from the straw. This stimulated microbial activity was link ed with an incorporation of C-13 derived from the straw in the core to appr oximately 4 mm from it. Nearly half of this C-13 was present in the microbi al biomass. Low temperature scanning electron microscopy showed that the mi crobial biomass was largest near the straw. The straw induced strong spatia l heterogeneity of microbial activity at the millimetric scale, presumably because of diffusion of soluble compounds from the straw. This heterogeneit y allowed us to identify the sites of microbial assimilation as being the s oil nearest to the straw.