Temperature and Duration of Extraction Influence the Nature of Water Extractable Soil Organic Matter.

Cold and hot water are often used to extract readily degradable soil organic matter. However, the influence of temperature and duration of extraction on the nature of the water-extractable material is not well understood. Grassland and arable soils from New Zealand and Eastern Canada were extracted in water at 20 and 80°C for 1, 4, 8, 12 and 24 h. The water extracts were analyzed for dissolved organic C (DOC) and organic N (DON), mineral N, ninhydrin-reactive N (Nnin), phenolics, and neutral carbohydrates (hexoses and pentoses). At 20°C, most parameters increased for the first 4 h and then stabilized or declined. There was a net decrease in glucose and mineral N over time, indicating that glucose was used by the microflora and caused net N immobilization at extraction times longer than 4 h. At 80°C, all measured parameters increased rapidly for the first 1 to 4 h followed by a lower rate of increase. Dissolved carbohydrates and Nnin were much higher at 80°C than at 20°C, suggesting a higher quality of the organic matter extracted with hot water. However, the proportion of DOC accounted for by carbohydrates was similar at both temperatures for the first 12 h, and the ratio of carbohydrate-C to phenolics-C was lower at 80°C. In contrast, the proportion of DON accounted for by Nnin at 80°C was 2- to 10-fold the proportion found at 20°C. The ratio of carbohydrate-C to phenolic-C progressively declined as extraction time increased, especially at 80°C. The ratio of microbial- to plant-derived carbohydrates was > 1.0 after 1 h at both temperatures, but this ratio gradually declined thereafter, especially at 80°C, because of greater release of plant-derived pentoses than of microbial-derived hexoses. Our results indicate that (i) the nature of organic matter extracted in water changes with temperature and extraction time; (ii), though hot water extracted more organic C, its biochemical quality did not appear to be higher than C extracted at room temperature; (iii), selective removal of readily degradable organic matter is best achieved using a short-duration (< 4 h) water extraction.