Controls on the Extractability of Soil Organic Matter in Water over the 20 to 80°C Temperature Range

Cold (room temperature, 20 degrees C) and hot water (70-80 degrees C) methods have been used to extract soil organic matter (SOM), but the controls on organic matter solubility within the 20 to 80 degrees C range are not well understood. We measured dissolved organic matter (DOM) as well as cations (Ca, Mg, K, Na, NH4, Mn, Al), inorganic anions (Cl, NO3, PO4, SO4, HCO3) and Si in water extracts of three soils (pasture soil, pasture soil that had been chemically fallowed for 6 yr, and a forest soil) at temperatures ranging from 20 to 80 degrees C (16-h incubation in temperature-controlled water baths). Dissolved organic C (DOC) and N increased exponentially in response to temperature increase. Cation solubility increased substantially with temperature (total soluble cations averaged 2.6 mmol(c) kg(-1) of soil at 20 degrees C, increasing to 8.6 mmol(c) kg(-1) at 80 degrees C). Effects of temperature on solubility of inorganic anions were relatively small (total soluble anions averaged 0.7 mmol(c) kg(-1) at 20 degrees C vs. 1.1 mmol(c) kg(-1) at 80 degrees C). The cation surplus (cations minus inorganic anions), which became greater as temperature increased, was balanced by the negative charge carried by the DOM (estimated charge density of DOM was 600-1900 cmol(c) kg(-1) of C). Temperature-induced increases in DOM and cations were related to NH4+ and OH- production by N mineralization at low (20-50 degrees C) temperatures. The mineralized NH4+ displaced indigenous cations (Ca, Mg, K, Na) while the OH- caused desorption of organic matter to balance the cations. At higher temperature (> 50 degrees C), increases in soluble cations and organic matter were at least partly attributable to the release of cations and OH- by hydrolysis of silicate minerals.