Sorption of Organic Carbon Fractions by Spodosol Mineral Horizons David A. N. Ussiri and C. E. Johnson* ABSTRACT mary sources of DOM in many forest soils as reflected by large concentrations of DOC in seepage waters be- Mobility of organic matter (OM) in soils is restricted by sorption neath the forest floor (McDowell and Likens, 1988; to mineral surfaces. We investigated the sorption of hydrophilic and hydrophobic OM fractions on mineral soils of the Hubbard Brook Johnson et al., 2000). Dissolution and movement of or- Experimental Forest (HBEF), New Hampshire. Organic matter was ganic carbon (OC) in the forest floor and upper mineral extracted with 0.1 M NaOH from O- and Bh-horizons, fractionated horizons results in the transfer of OM, metals, nutrients, into hydrophilic and hydrophobic fractions, and characterized by 13 C and pollutants through the soil to surface and ground cross-polarization magic-angle spinning (CPMAS) nuclear magnetic waters (Chiou et al., 1986; Pohlman and McColl, 1988; resonance (NMR) spectroscopy. Sorption experiments were con- Berggren et al., 1990; Qualls and Haines, 1991; Liu and ducted with E, Bh, Bs1, Bs2, and C horizon soils at suspension pH Gary, 1993). Mobilization of OC, Al, and Fe in upper of 3, 4, and 5. In all horizons, we observed net release of indigenous horizons and its subsequent transport to the subsoil OM when OM-free solution was added. Net release of OM was great- plays an important role in podzolization, which forms est from Bh-horizon soils, which had the greatest OM content among Spodosols (Petersen, 1976; Dawson et al., 1978). the horizons we studied. The OM in the equilibrium solutions was primarily hydrophilic (65–97%) regardless of the fraction added. Sorp- The mobility of DOM in soils is controlled by its tion behavior was related to the content of carboxylic functional sorption to mineral surfaces (McDowell and Likens, groups in the added solution. Hydrophobic OM had greater affinity 1988). Concentrations and fluxes of DOC in soil solution to soils than hydrophilic OM, and Bh-horizon OM exhibited greater typically decrease with increasing soil depth (Guggen- sorption than O-horizon OM. Maximum adsorption occurred at pH berger and Zech, 1993; Johnson et al., 2000). The reduc- 4, and decreased with either an increase or decrease in pH. The pH- tion in DOC concentration is accompanied by a change dependence of OM sorption probably reflects a balance between in DOM composition, with a general preferential de- lower charge density of carboxyl groups at low pH and lower positive crease in hydrophobic DOM fractions (Guggenberger charge of adsorption sites at higher pH. The hydrophilic fraction and Zech, 1993; Kaiser et al., 1996). Thus the nature of contained higher organic N than the hydrophobic fraction, suggesting DOM itself affects sorption, with hydrophilic com- that dissolved organic nitrogen (DON) is more mobile than dissolved organic carbon (DOC) in these soils. pounds being less likely to sorb to mineral surfaces than hydrophobic compounds (Kaiser et al., 1996; Kaiser and Zech, 1997). Sorption of DOM to soil mineral surfaces is also influenced by soil properties such as soil OC D issolved organic matter (DOM) plays an impor- concentration, Al and Fe oxide and hydroxide content, tant role in many biogeochemical processes in soils and the mineralogy of the clay fraction (McDowell and and waters. It controls biological availability of nutrients Wood, 1984; Jardine et al., 1989; Donald et al., 1993). and trace elements by forming complexes (Stevenson, Also, DOM sorption may be influenced by adsorbed 1994), and contributes to the acid-base chemistry of soils SO 2- 4 (Zech et al., 1994) and surface area (Kaiser et and waters (Cronan and Aiken, 1985; David and Vance, al., 1996). 1991). It is utilized as a growth substrate by microbial Effects of pH on OM sorption have shown varying communities in soils and waters (Qualls and Haines, results. The adsorption of OM to iron oxides (Gu et al., 1992). Dissolved OM also influences cation-leaching 1994), Al hydroxides (Parfitt et al., 1977; Davis, 1982), processes (Cronan et al., 1978), plays an important role and bulk soils (Jardine et al., 1989; Kennedy et al., 1996) in the cycling of nutrients such as C, N, P, and S within increased with decreasing pH as a result of increasing the soil, and acts as a major mode of export of N and positive charges on the hydroxides. However, David P from soil systems to streams and groundwaters (Mc- and Zech (1990) observed decreasing DOC adsorption Dowell and Wood, 1984; Qualls and Haines, 1991). In in the B horizon of acid soils with decreasing pH, and addition, the association of major plant nutrients with in a batch adsorption experiment, Vance and David DOC, either as part of molecules, or adsorbed by DOC, (1992) saw no effect of pH between pH 3 and 6 on DOC renders otherwise immobile nutrients such as P mobile, adsorption in mineral soil samples. and therefore subject to leaching (Stevenson, 1994). Dis- The purpose of this study was to investigate the ad- solved OM also plays an important role in mineral sorption of OM fractions to Spodosol mineral horizon weathering and soil formation (Raulund-Rasmussen et soils from the HBEF in New Hampshire, USA. Our al., 1998). objectives were (i) to determine the OM sorption prop- The forest canopy and forest floor layers are the pri- erties of Spodosol mineral horizons of the HBEF; (ii) Dep. of Civil and Environmental Engineering, 220 Hinds Hall, Syra- Abbreviations: CPMAS, cross-polarization magic-angle spinning; cuse Univ., Syracuse, NY 13244. Received 30 Aug. 2002. *Correspond- DOC, dissolved organic carbon; DOM, dissolved organic matter; ing author (cejohns@mailbox.syr.edu). DON, dissolved organic nitrogen; HBEF, Hubbard Brook Experimen- tal Forest; NMR, nuclear magnetic resonance; OC, organic carbon; Published in Soil Sci. Soc. Am. J. 68:253–262 (2004).  Soil Science Society of America OM, organic matter; TOC, total organic carbon; W1–W9, Watersheds 1 through 9. 677 S. Segoe Rd., Madison, WI 53711 USA 253