Influence of Temperature on Water-Extractable Organic Matter and Ammonium Production in Mineral Soils

Cold (room temperature) and hot water (70-80 degrees C) extraction Methods have been used to assess the availability of organic matter and N in soil; however, the influence of temperature on their extractability is largely unknown. Twenty-one mineral soils from New Zealand and eastern Canada were incubated for 16 h in water (1:6 soil/water ratio) at temperatures ranging from 20 to 80 degrees C. After centrifugation and removal of the supernatant solution, the soil residue was extracted with 2 mol L(-1) KCl to recover NH(4)-N released during incubation. In all cases, water-extractable organic C (WEOC) and N (WEON) increased exponentially with temperature. The temperature dependence of organic matter solubility differed considerably among the soils (estimated increase in WEOC and WEON: 1.1-5.3% degrees C(-1)). These differences were not explained by management history or soil physicochemical properties, although there was evidence that organic matter extractability in some clay soils was le ss sensitive to temperature. Ammonium N was released in significant amounts at all temperatures. Peak NH(4)-N release typically occurred at 50 degrees C, where NH(4)-N comprised an average of 49% of the total recovered N (WEON plus NH(4)-N). Factors that may account for NH(4)-N production during the 16-h incubation include mineralization at the lower temperatures and abiotic processes (e.g., release of clay-fixed NH(4) and thermal degradation of organic N) at higher temperatures. The possibility that mineralization made a significant contribution to NH(4)-N production at temperatures as high as 50 degrees C warrants further investigation.