Organic matter and enzyme activity in particle-size fractions of soils obtained after low-energy sonication

We have developed a method to separate different size fractions of soil particles which allows sensitive assays of enzymes in the moist fractions. Four soils of different chemical and physical properties were dispersed using low-energy sonication and separated into size fractions (> 200 μm, 200-63 μm, 63-2 μm, 2-0.1 μm and <0.1 μm) by a combination of wet sieving and centrifugation in water. The separation procedure enabled adequate reproducibility; the recovery of the size fractions was about 98–100%. The applied sonication energy of 170 J g−1 did not disrupt microaggregates completely and about 37% of the clay remained in silt-sized particles. Organic C, total N and the soil enzymes invertase and xylanase were analysed within these fractions and compared with the values in the bulk soil. The mean recovery of C and N was about 101 and 99%, respectively. Based on dry matter, the highest contents of C and N were generally found in the clay-size fractions, whereas the silt-size fraction showed highest absolute amounts (about 56% of total C and 54% of total N). The C-to-N ratio of the fractions typically decreased from coarser to finer sizes, reflecting the mineralisation and humification status of the soil organic matter (SOM). Highest invertase activities were measured in silt and clay and were related to organic C; the mean recovery was about 90%. In contrast the summed activities of xylanase surpassed the activity of the bulk soil considerably (plus 31%). Based on organic C the xylanase exhibited highest efficiency in the coarse and fine sand fractions. This soil enzyme seemed to be mainly adsorbed to the less mineralized particulate organic matter of the coarser fractions.