Biodegradability and Biochemical Composition of Water-Extractable Organic Matter Recovered at Different Extraction Temperatures.

Water-extractable organic matter (WEOM) is considered to be a labile fraction of soil organic matter (SOM) that plays an important role in many key soil processes including respiration, soil aggregation, denitrification and N mineralization. Cold (20-25°C) and hot water (70-80°C) extraction methods have been used to recover WEOM. However, the influence of extraction temperature on the amount, biochemical composition and biodegradability of WEOM is largely unknown. Twenty mineral soils from New Zealand and eastern Canada, representing different land uses and cropping histories, were collected (0-15 cm) and extracted with water at temperatures ranging from 20 to 80°C. In all cases, the amounts of WEOM recovered increased exponentially as water temperature increased (> 5% of total SOM was extracted at 80°C). The WEOM was influenced by management history, with grassland soils having more WEOM than arable and forest soils at any given temperature. WEOM was highly biodegradable in all cases with 10 to 20% mineralized after one week of incubation at 20^oC, and more than 60% mineralized after 42 d. Contrary to expectation, the biodegradability of WEOM tended to increase with increasing extraction temperature, indicating that the SOM solubilized at higher temperature was more biodegradable than SOM extracted in cold water. There was no evidence that heating to 80°C changed the biodegradability of cold (20 °C) WEOM.  Preliminary analysis of the extracts showed that polysaccharides are preferentially solubilized as extraction temperature increases. The amount of hexoses, pentoses, phenols and ninhydrin-reactive N compounds all tended to increase with increasing extraction temperature.