Adsorption of DOM by soil has been implicated as a very important factor in controlling the loss of DOM Leaching of dissolved organic matter (DOM) and the associated from ecosystems in stream water (McDowell and Wood, nutrient elements can be a significant form of loss from developing 1984; Nelson et al., 1993; Qualls et al., 2002). Several ecosystems. We studied how the adsorption of dissolved organic C (DOC) and N (DON) changes during soil development and deter-authors have found positive correlations between pamined which soil characteristics control adsorption. We sampled 77, rameters derived from DOC adsorption experiments 255, 616, and about 1200ϩ yr-old andesitic soils at five depths and and soil characteristics such as clay content, oxalate did adsorption isotherm experiments fit to a modified Langmuir equaextractable Al and Fe, dithionite-citrate-bicarbonate tion. We measured DOC and DON in soil solution at the 10-to 20-, (DCB) extractable Fe, and surface area (Donald et al., 40-, and 150-cm soil depths during the snowmelt period to compare 1993; Moore et al., 1992; Nelson et al., 1993). Few studies with adsorption experiments. Ability of the soils to adsorb DOM inexist, however, on the adsorption of DON (Kaiser and creased with soil age. Regression analyses were performed between Zech, 2000b) despite N being the limiting nutrient in adsorption capacity or the null point concentration of either DOC or most terrestrial ecosystems (Schlesinger, 1997). DON and the independent variables soil organic C (SOC), N, allo-While several authors have studied the adsorption of phane, oxalate extractable Fe, crystalline Fe, and specific surface area. The best relationships were found between adsorption capacity and DOC to ferrihydrite and aluminum hydroxides, much the allophane/SOC ratio (r 2 ϭ 0.88), and between the null point less is known about the adsorption of DOC to allophane, concentration of DOC or DON and the SOC/allophane ratio (DOC: an aluminosilicate. Allophanic soils are known to accur 2 ϭ 0.85; DON: r 2 ϭ 0.77). Stepwise multiple regression indicated mulate organic matter rapidly and this has been attribthat oxalate-extractable Fe and specific surface area contributed only uted to their large surface area and positive surface small increases in the multiple R 2. High correlations between the nullcharge (Wada, 1989; Parfitt, 1990). The short-rangepoint adsorption of DOC or DON and the DOC (r 2 ϭ 0.92) or DON order aluminosilicates, allophane, and imogolite strongly (r 2 ϭ 0.86) field soil solution concentrations indicated that results adsorb DOC (Parfitt et al., 1977; Yuan et al., 2000). obtained in laboratory experiments were applicable to field condi-Dahlgren and Marrett (1991) attributed the strong adtions. The cause of the increased ability of the soils to adsorb and sorption of DOC in the Bs horizon of a volcanic Spodoretain DOM during soil development appears to be an increase in allophane concentrations.
