Temperature Dependence of Organic Matter Solubility: Influence of Biodegradation during Soil‐Water Extraction

Water‐extractable organic matter has been shown to increase as temperature increases (from 20 to 80°C), with the rate of increase being soil dependent. We examined whether biodegradation during overnight soil‐water extraction may influence the temperature response of extractable C and N. Dissolved organic N (DON) and C (DOC), and NH 4 –N were determined after 16 h of soil‐water extraction at either 80 or 50°C (previous work in our laboratory suggested that biodegradation in soil‐water suspensions peaks at ∼50°C). For both DOC and DON, there were large differences among soils in their temperature responses (e.g., the increase in DON between 50 and 80°C ranged from 29 to 148 mg kg −1 ). More NH 4 –N was generated at 50 than at 80°C. Ammonium N produced at 50°C was largely attributable to mineralization (it was almost eliminated when microbial activity was suppressed by extracting with 2 mol L −1 KCl at 50°C). The small amounts of NH 4 –N found at 80°C were probably of abiotic origin (e.g., thermal degradation of soil organic N). Our results suggested that dissolved organic matter (DOM) was mineralized during the 50°C extraction. The release of DOM was thus underestimated at 50°C and, as a consequence, the temperature response of DOM between 50 and 80°C was overestimated (mineralization at 50°C accounted for most of the variability in the temperature response of DOM). We conclude that the temperature response of DOM can be affected by biodegradation during extraction and that an extraction at 80°C has the important merit that biodegradation during extraction should be negligible.