Size-density fractionation for in situ measurements of rape straw decomposition - An alternative to the litterbag approach?

Decomposition rates have been elucidated by litterbag studies or in some instances by field-scale CO2 flux monitoring, but some deficiencies are apparent in these methods. In a randomized block experiment with additions of 0, 4 or 8 t of straw material the particulate organic matter (POM > 100 mu m) was monitored for 20 months. During the first 4 months the POM in the amended treatments decreased quite rapidly, and more slowly during the remaining 16 months. By separating the total POM into light (rho < 1.4 g cm(-3)) and heavy (rho > 1.4 g cm(-3)) fractions, further resolution of the residue decomposition was possible. The heavy fraction C was completely unaffected by the application of residues, and apparently consisted mainly of ''native'' organic matter with a low rate of decay. The differences in POM between treatments could be attributed completely to differences in the light POM fraction. Analysis of lignin and cellulose in light fractions from the 8 t treatment showed that cellulose was preferentially utilized in the early stages of decomposition. The analysis also indicated that the initial lignin concentration was high in the light fraction (20%) compared with that of the rape straw (15%). Thus the ''native'' light fraction must have been rich in lignin, which may explain the slower rate of decay of light fraction in the unammended treatment. The estimates of decomposition from POM were in qualitative agreement with estimates of decomposition based on field scale CO2 fluxes, but indicated a considerably higher turnover in the initial phase of decomposition. This is of special interest, since it has been recognized that the static chamber method for estimating field scale CO2 fluxes underestimates high rates of CO2 evolution, and thus the study of POM may be complementary to this approach. Furthermore, it seems the study of POM may be an alternative to the litterbag method in some instances. In contrast to the litterbag method, POM fractionation approach allows the added residue to be completely exposed to the soil environment, and thus to the full range of faunal and other soil interactions. (C) 1997 Elsevier Science Ltd.