Temperature effects on release and ecologically relevant properties of dissolved organic carbon in sterilised and biologically active soil samples

To evaluate the effects of temperature and biological activity on the release and quality of dissolved organic carbon (DOC), incubation experiments were conducted with soil from the Ap-horizon of a Gleyic Fluvisol (sandy clay loam, pH 7.2; 6.4% Corg). Samples were either non-sterile, sterilised by γ-irradiation or reinoculated after sterilisation. Incubations were performed at three temperatures: 5, 20 and 35 °C for 12 days with continuous CO2-monitoring. DOC was obtained by percolating the samples before and after incubation with 1 mM CaCl2 solution. DOC quality was assessed by ultra-filtration, UV-absorbance, Cu-complexation capacity and biodegradability. In the sterile samples, DOC-concentrations were hardly affected by incubation temperature and among the quality parameters, only the UV-absorbance of DOC decreased with increasing incubation temperature in all size fractions. In the reinoculated and non-sterile samples, DOC-concentrations decreased strongly with increasing incubation temperature. This was closely related to respiration activity and accompanied by increases in Cu-complexation capacity and biodegradability. In the larger DOC size fractions separated by ultra-filtration (MWCO>1000), UV-absorbance increased after incubation at higher temperatures. The results show that DOC is readily utilised as substrate by microorganisms and at the same time modified in its composition by selective degradation of UV-inactive compounds and by the release of low molecular, easily degradable compounds with high Cu-complexation capacity.