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 a mended with straw from Hordeum vulgare or Vicia sativa revealed that the lo ss of C during the first 56 days of incubation mainly occurred from particl es >2,000 mum, without a concomitant reduction in the size of these large p articles. Preliminary studies of POM from non-amended soil had shown that t he 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 pa rticles (>80%). Therefore we decided to compare the POM <1.4 g cm(3) with P OM >400 mum. There was a very close relationship between POM>400 mum and PO M <1.4 g cm(-3) with regard to amounts of C and N, as well as the appearanc e of these fractions under the microscope. Similarly there was a close rela tionship 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 n pt N mineralization. This indicated that the biological activity during dec omposition was actually localized in the POM. Due to the lighter workload a nd 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.