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 ryegrass between size and density fractions of soil organic matter in a sandy and a loam soil. Our hypotheses were a) that the applied C-14 would be transferred from light and soluble fractions to intermediate and heavy macroorganic matter fractions (> 150 mu m) and finally become stabilized in microaggregates (< 150 mu m), and b) that the physical protection of C-14 associated with microaggregates against decomposition would decrease with increasing saturation of the microaggregates with soil organic matter. Generally, the hypotheses were confirmed. Immediately after application most of the label was present in the soluble and light macroorganic matter fractions. Newly synthesized microbial biomass fed on the labeled components of the fractions. The amounts of C-14 in the soluble and light macroorganic matter fractions decreased rapidly, while the amounts of C-14 in the intermediate and heavy macroorganic matter fractions and in microaggregates remained more or less stable. At the end of the incubation most of the residual soil C-14 was found 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-labeled material were similar in the light intermediate and heavy fractions 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 considerable amounts of C-14 must have transferred from the soluble and light macroorganic matter fractions and newly synthesized microbial biomass to the intermediate and heavy macroorganic matter fractions and the microaggregates, and that C-14 was not yet physically protected against microbial degradation during the whole incubation period. The degree of physical protection of C-14 against decomposition in the microaggregate fraction < 20 mu m was negatively correlated with the degree of saturation of this particle size fraction with soil organic matter.