Recovering decomposing plant residues from the particulate soil organic matter fraction:: size versus density separation

A detailed size separation of particulate organic matter (POM) from soils amended with straw from Hordeum vulgare or Vicia sativa revealed that the loss of C during the first 56 days of incubation mainly occurred from particles >2,000 mum, without a concomitant reduction in the size of these large particles. Preliminary studies of POM from non-amended soil had shown that the stable heavy (>1.4 g cm(-3)) POM fraction was mainly (>80%) composed of particles <400 <mu>m, whereas the light fraction was dominated by larger particles (>80%). Therefore we decided to compare the POM <1.4 g cm(3) with POM >400 mum. There was a very close relationship between POM>400 mum and POM <1.4 g cm(-3) with regard to amounts of C and N, as well as the appearance of these fractions under the microscope. Similarly there was a close relationship between changes in the C content of the POM fractions and the CO2 respired, and this was also the case when comparing changes in POM-N with npt N mineralization. This indicated that the biological activity during decomposition was actually localized in the POM. Due to the lighter workload and lower expenditure for reagents in connection with size separation of POM, we recommend the size separation procedure in connection with studies of residue decomposition in arable systems.