Xylanase, invertase and protease at the soil–litter interface of a loamy sand

Organic residues are commonly added to soils, but little is known about C and N dynamics at the soil–litter interface (detritusphere). We investigated soil microbial processes in the detritusphere at the microscale by placing maize litter bags between two soil cores (each tube: 3.0 cm long, 5.6 cm diameter) and incubating at 9°C for 27 days. Subsequently, the soil cores were frozen with liquid nitrogen and cut with a microtome to yield samples at 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00, 3.00, 4.00, 5.00, 6.00 and 10.0 mm from the litter. Microbial biomass N, protease, xylanase and invertase activities for the maize straw were two orders of magnitude higher than the corresponding values of control soil. Microscale investigations of controls (no litter addition) showed low spatial heterogeneity of protease and higher heterogeneity for xylanase, invertase and dissolved organic substances within the first 10 mm of the soil cores. The detritusphere was characterised by high turnover of organic material visible as gradients in xylanase, invertase and protease activities and the depletion of DOC at the soil–litter interface. The scale of the soil–litter interface ranged from 1.1–1.3 mm, in which the gradients of the enzyme activities followed an exponential function (y=c+exp (b0+b1 x1+b2 x2)). The high local release of substrates seems to be the major mechanism driving C and N turnover within the 1–2 mm from the surface of litter. The transport (mass flow and diffusion) of dissolved organic compounds which provides energy for soil microorganisms is the cause of higher enzyme activities within the close vicinity of the litter surface.