Sorption of Organic Carbon Fractions by Spodosol Mineral Horizons

Mobility of organic matter (OM) in soils is restricted by sorption to mineral surfaces. We investigated the sorption of hydrophilic and Hydrophobic OM fractions on mineral soils of the Hubbard Brook Experimental Forest (HBEF), New Hampshire. Organic matter was extracted with 0.1 M NaOH from O- and Bh-horizons, fractionated into hydrophilic and hydrophobic fractions, and characterized by ¹³C cross-polarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy. Sorption experiments were conducted with E, Bh, Bs1, Bs2, and C horizon soils at suspension pH of 3, 4, and 5. In all horizons, we observed net release of indigenous OM when OM-free solution was added. Net release of OM was greatest from Bh-horizon soils, which had the greatest OM content among the horizons we studied. The OM in the equilibrium solutions was primarily hydrophilic (65-97%) regardless of the fraction added. Sorption behavior was related to the content of carboxylic functional groups in the added solution. Hydrophobic OM had greater affinity to soils than hydrophilic OM, and Bh-horizon OM exhibited greater sorption than O-horizon OM. Maximum adsorption occurred at pH 4, and decreased with either an increase or decrease in pH. The pH-dependence of OM sorption probably reflects a balance between lower charge density of carboxyl groups at low pH and lower positive charge of adsorption sites at higher pH. The hydrophilic fraction contained higher organic N than the hydrophobic fraction, suggesting that dissolved organic nitrogen (DON) is more mobile than dissolved organic carbon (DOC) in these soils.