DECOMPOSITION AND TRANSFER OF PLANT RESIDUE C-14 BETWEEN SIZE AND DENSITY FRACTIONS IN SOIL

The aim of our study was to follow the transfer of C-14-labeled ryegra ss between size and density fractions of soil organic matter in a sand y and a loam soil. Our hypotheses were a) that the applied C-14 would be transferred from light and soluble fractions to intermediate and he avy macroorganic matter fractions (> 150 mu m) and finally become stab ilized in microaggregates (< 150 mu m), and b) that the physical prote ction of C-14 associated with microaggregates against decomposition wo uld decrease with increasing saturation of the microaggregates with so il organic matter. Generally, the hypotheses were confirmed. Immediate ly after application most of the label was present in the soluble and light macroorganic matter fractions. Newly synthesized microbial bioma ss fed on the labeled components of the fractions. The amounts of C-14 in the soluble and light macroorganic matter fractions decreased rapi dly, while the amounts of C-14 in the intermediate and heavy macroorga nic matter fractions and in microaggregates remained more or less stab le. At the end of the incubation most of the residual soil C-14 was fo und in the microaggregates. In the sandy soil C-14 was concentrated in the 20-150 mu m fraction, whereas in the loam a larger proportion was present in the < 20 mu m fraction. The mineralization rates of C-14-l abeled material were similar in the light intermediate and heavy fract ions of macroorganic matter and in the microaggregates 0 and 180 days after the application of C-14-labeled ryegrass. In all fractions, C-14 mineralized more rapidly than total C. The results indicate that cons iderable amounts of C-14 must have transferred from the soluble and li ght macroorganic matter fractions and newly synthesized microbial biom ass to the intermediate and heavy macroorganic matter fractions and th e microaggregates, and that C-14 was not yet physically protected agai nst microbial degradation during the whole incubation period. The degr ee of physical protection of C-14 against decomposition in the microag gregate fraction < 20 mu m was negatively correlated with the degree o f saturation of this particle size fraction with soil organic matter.