Tillage changes microbial biomass and enzyme activities in particle-size fractions of a Haplic Chernozem

An understanding of the distribution of soil microorganisms and enzyme activities in particle-size fractions of a fine-sandy loamy Haplic Chernozem is important in order to determine how tillage systems (conventional, minimum and reduced) can be better managed to increase C and N sequestered into SOM and thus improve the long-term productivity of soil. The physical fractionation procedure enabled us to completely recover Corg, Nt, microbial biomass and several enzyme activities. The recoveries of alkaline phosphatase, xylanase, protease and invertase were 85.0, 91.5, 93.2% and 102%, respectively. Microbial biomass and soil enzyme activities in particle-size fractions were affected mostly by the type of tillage and to a lesser extent by the date of soil sampling. Reduced and minimum tillage increased the Corg content of the bulk soil in the 0–10 cm layer from 15.8 to 17.6 mg g−1 Corg mainly due to a higher Corg content of the larger particle-size fractions (>63 μm). In contrast, conventional tillage caused a lower Corg contents of the coarse sand fraction in the 20–30 cm layer than minimum tillage treatment. Independent of tillage treatment, 80–95% of the biomass N was located in the fractions smaller than 63 μm. Soils in the 0–10 cm layer showed similar Nmic-to-Corg ratios in the single particle-size fractions ranging from 0.90 to 4.18 mg Nmic g−1 Corg. Xylanase activity was mainly located in the coarse sand fraction (15.8–40.5%), invertase activity in the silt fraction (48.9–59.6%), and alkaline phosphatase activity in the silt (37.9–43.0%) and clay fractions (48.9–54.0%). Protease activity was bimodally distributed, showing high values in the coarse sand and in the clay fraction of the reduced and minimum tillage treatments. The type of tillage influenced the spatial distribution of soil enzymes within the soil profile as well as the amounts of soil enzyme activities.