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 so me 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 mat ter (POM > 100 mu m) was monitored for 20 months. During the first 4 m onths the POM in the amended treatments decreased quite rapidly, and m ore 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)) fracti ons, further resolution of the residue decomposition was possible. The heavy fraction C was completely unaffected by the application of resi dues, and apparently consisted mainly of ''native'' organic matter wit h a low rate of decay. The differences in POM between treatments could be attributed completely to differences in the light POM fraction. An alysis of lignin and cellulose in light fractions from the 8 t treatme nt showed that cellulose was preferentially utilized in the early stag es of decomposition. The analysis also indicated that the initial lign in concentration was high in the light fraction (20%) compared with th at of the rape straw (15%). Thus the ''native'' light fraction must ha ve been rich in lignin, which may explain the slower rate of decay of light fraction in the unammended treatment. The estimates of decomposi tion from POM were in qualitative agreement with estimates of decompos ition based on field scale CO2 fluxes, but indicated a considerably hi gher turnover in the initial phase of decomposition. This is of specia l interest, since it has been recognized that the static chamber metho d for estimating field scale CO2 fluxes underestimates high rates of C O2 evolution, and thus the study of POM may be complementary to this a pproach. Furthermore, it seems the study of POM may be an alternative to the litterbag method in some instances. In contrast to the litterba g method, POM fractionation approach allows the added residue to be co mpletely exposed to the soil environment, and thus to the full range o f faunal and other soil interactions. (C) 1997 Elsevier Science Ltd.