DISSOLVED ORGANIC-CARBON IN OLD FIELD SOILS - COMPOSITIONAL CHANGES DURING THE BIODEGRADATION OF SOIL ORGANIC-MATTER

The quality of soil dissolved organic carbon (DOC) was examined using the Leenheer DOC fractionation scheme, which separates soluble organic compounds into well-defined functional groups that exhibit similar reactive properties. DOC fractions were measured for five previously cultivated old fields undergoing secondary succession, and an undisturbed oak savanna. Despite differences in field age (time since abandonment), plant community composition, distribution and amounts of phytomass, C and N storage, and potential amounts of CO2-C and net-N mineralization, the quality of soil DOC did not appear to differ. Nearly all DOC occurred in acid fractions (77%); hydrophilic acids alone constituted 50%. The fractionation procedure was also performed at four different times during a 210-day regulated in vitro incubation of the soils. Despite decreasing mineralization response to soil DOC concentrations, the fractional composition of the DOC remained relatively constant throughout the incubation. Although we could not evaluate DOC utilization, the results demonstrated that soil DOC was altered during the decomposition of soil organic matter; both total amounts and the relative N content of the hydrophobic acid fraction increased during the incubation period.