Biodegradation of dissolved organic carbon in soil extracts and leachates from a temperate forest stand and its relationship to ultraviolet absorbance

The amount and biodegradability of dissolved organic carbon (DOC) in forest floors can contribute to carbon sequestration in soils and the release of CO₂-C from soil to the atmosphere. There is only limited knowledge about the biodegradation of DOC in soil extracts and leachates due to the limitations inherent in degradation experiments. Differences in the biodegradation of DOC were studied in forest soil extracts using cold and hot water and 4 mmol/L CaCl₂ solution and in soil leachates sampled under different conditions over a wide range of DOC concentrations. From these results, we developed a simple and rapid method for determining the biodegradable organic C in forest floors. The hot water extracts and CaCl₂ extracts after CH3Cl fumigation contained higher concentrations of biodegradable organic C than the cold water extracts and CaCl₂ extracts before fumigation, with rapid DOC degradation occurring 24–48 h after incubation with an inoculum, followed by slow DOC degradation till 120–168 h into the incubation. During a 7-d incubation with an inoculum, the variation in DOC degradation in the different soil extracts was consistent with the change in special UV absorbance at 254 nm. Relatively higher levels of biodegradable organic C were detected in soil leachates from the forest canopy than in forest gaps between April and October 2008 (P <0.05). Relatively lower concentrations of DOC and biodegradable organic C were observed in soil leachates from N-fertilized plots during the growing season compared with the control, with the exception of the plot treated with KNO₃ at a rate of 45 kg N ha⁻¹ a⁻¹. Around 77.4% to 96.3% of the variability in the biodegradable organic C concentrations in the forest floors could be accounted for by the initial DOC concentration and UV absorbance at 254 nm. Compared with the conventional inoculum incubation method, the method of analyzing UV absorbance at 254 nm is less time consuming and requires a much smaller sample volume. The results suggest that the regression models obtained using the initial DOC concentration and UV absorbance can provide a rapid, simple and reliable method for determining the biodegradable organic C content, especially in field studies involving relatively large numbers of samples.