Short-term impacts of forest clear-cut on P accessibility in soil microaggregates: An oxygen isotope study

Forest clear-cuts may have severe effects on the soil structure and related nutrient cycling, though with yet unknown consequences for nutrient pools such as phosphorus (P) within microaggregates. We sampled the bulk mineral topsoil prior to clear cut as well as 1 and 2 years thereafter from the experimental forest site Wüstebach, Germany, and we assessed the degree of oxygen isotope exchange in HCl-extractable soil phosphate of two microaggregate size fractions (< 20 μm, 20-250 μm) after incubating soil with 18O-labeled water. We found that after the clear-cut, microaggregate phosphates exchanged significantly more oxygen with the incubation water than before clear cut. One and two years after clear cut, the respective δ18O values of soil phosphates (δ18OP,HCl) were elevated by 16 and 38% (< 20 μm) and by 43 and 53% (20-250 μm) than before the clear-cut, respectively, indicating that additional microaggregate P had been made available to biological P cycling. The degree of oxygen exchange after the clear-cut was significantly greater in larger soil microaggregates than in the smaller sized ones, reflecting that also at microaggregate level size controlled the increase in the bioavailability of P with changes in land management.