Adsorption of Dissolved Organic Carbon and Nitrogen in Soils of a Weathering Chronosequence.

Leaching of dissolved organic matter (DOM) and the associated nutrient elements can be a significant form of loss from developing ecosystems. We studied how the adsorption of dissolved organic C (DOC) and N (DON) changes during soil development and determined which soil characteristics control adsorption. We sampled 77, 255, 616, and about 1200+ yr-old andesitic soils at five depths and did adsorption isotherm experiments fit to a modified Langmuir equation. We measured DOC and DON in soil solution at the 10- to 20-, 40-, and 150-cm soil depths during the snowmelt period to compare with adsorption experiments. Ability of the soils to adsorb DOM increased with soil age. Regression analyses were performed between adsorption capacity or the null point concentration of either DOC or DON and the independent variables soil organic C (SOC), N, allophane, oxalate extractable Fe, crystalline Fe, and specific surface area. The best relationships were found between adsorption capacity and the allophane/SOC ratio (r² = 0.88), and between the null point concentration of DOC or DON and the SOC/a!lophane ratio (DOC: r² = 0.85; DON: r² = 0.77). Stepwise multiple regression indicated that oxalate-extractable Fe and specific surface area contributed only small increases in the multiple R². High correlations between the nullpoint adsorption of DOC or DON and the DOC (r² = 0.92) or DON (r² = 0.86) field soil solution concentrations indicated that results obtained in laboratory experiments were applicable to field conditions. The cause of the increased ability of the soils to adsorb and retain DOM during soil development appears to be an increase in allophane concentrations. [ABSTRACT FROM AUTHOR]