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

Understanding the role of the soil matrix in the decomposition of soil organic matter requires an assessment of how the soil matrix controls the availability of substrate to microorganisms. We aimed to identify the sites of microbial assimilation of a decomposing substrate the initial location of which 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 different distances from the straw. We analysed them for their C-13 and N-15 contents and dehydrogenase activity. The presence of the straw induced steep gradients of dehydrogenase activity in the core; the activity increased at the contact with the straw and decreased to reach the reference value between 3 and 4 mm from the straw. This stimulated microbial activity was linked with an incorporation of C-13 derived from the straw in the core to approximately 4 mm from it. Nearly half of this C-13 was present in the microbial biomass. Low temperature scanning electron microscopy showed that the microbial biomass was largest near the straw. The straw induced strong spatial heterogeneity of microbial activity at the millimetric scale, presumably because of diffusion of soluble compounds from the straw. This heterogeneity allowed us to identify the sites of microbial assimilation as being the soil nearest to the straw.