Temperature and duration of extraction affect the biochemical composition of soil water-extractable organic matter

Water-extractable organic matter is regarded as readily available substrate for soil microbes. However, little is known about the influence of extraction temperature and duration on its biochemical composition. The effect of temperature (20 vs. 80 °C) and extraction duration (1-24 h) on the amounts of water-extractable organic C (WEOC), water-extractable organic N (WEON), carbohydrates, phenolics, ninhydrin-reactive organic N (NRON), and mineral N were evaluated using four agricultural soils from New Zealand and eastern Canada. More WEOC and WEON were extracted in hot than in cold water, and the same was found for all biochemical compounds tested. Biochemical components increased rapidly during the first 4 h of extraction at 80 °C, and at a slower rate thereafter. In contrast, at 20 °C, concentrations of all measured components were at or close to their maximum after 1 h and showed little change thereafter. Glucose and nitrate in the 20 °C extracts both declined substantially between 1 and 24 h, likely due to microbial activity. Moreover, the disappearance of glucose induced a decline in the ratio of carbohydrate-C to phenolic-C. The carbohydrates measured in water extracts at both temperatures were predominantly of microbial origin after 1 h. However, the proportion of microbial carbohydrates gradually declined and the proportion of plant carbohydrates increased from 1 to 24 h at 80 °C. Based on these findings, we recommend limiting extractions to 1 h or less at 20 °C and to 4 h or less at 80 °C to minimize compositional changes that may occur during longer extraction periods.