Invertase and xylanase activity of bulk soil and particle-size fractions during maize straw decomposition

The activity of invertase and xylanase during maize straw decomposition was investigated in an incubation experiment (14°C, 52 weeks) with two treatments: (i) straw mixed with soil (mixed) and (ii) straw placed on the soil surface (surface). A soil without straw was included as reference (ref. soil). The particle-size fractions (2000–200 μm, 200–63 μm, 63–2 μm, 2–0.1 μm and <0.1 μm) were obtained by a combination of wet sieving and centrifugation using low-energy ultrasonication (0.2 kJ g−1) to disrupt macroaggregates (>250 μm). Invertase and xylanase in the bulk soils with straw applied were mainly influenced by a highly variable activity in the >200 μm fraction, which contained most of the added organic matter during the incubation. Enzyme activity associated with particles <63 μm was less affected by maize straw. Invertase activity clearly differed between the mixed and surface treatment, particularly in the early phase of incubation. This difference seemed to be linked to different microbial communities. Xylanase activity was mainly located in the coarser fractions throughout the incubation and did not differ between treatments. After an initial lag phase, xylanase activity increased steadily, showing highest values when 30–40% of the maize straw was released as CO2. In conclusion, the separation of differently sized organo-mineral particles yielded enzyme pools which differ significantly in size and stability, both factors strongly depending on enzyme type and substrate localisation.