Changes in water extractable organic matter during incubation of forest ﬂoor material in the presence of quartz, goethite and gibbsite surfaces Katherine Heckman a,⇑ , Angelica Vazquez-Ortega b,1 , Xiaodong Gao b,1 , Jon Chorover b,1 , Craig Rasmussen b,1 a USDA Forest Service, Center for AMS, L-397, 7000 East Ave., Livermore, CA 94550, USA b Dept. of Soil, Water and Environmental Science, University of Arizona, P.O. Box 210038, Tucson, AZ 85721-0038, USA Received 11 January 2011; accepted in revised form 9 May 2011; available online 18 May 2011 Abstract The release of dissolved organic matter (DOM) from forest ﬂoor material constitutes a signiﬁcant ﬂux of C to the mineral soil in temperate forest ecosystems, with estimates on the order of 120–500 kg C ha 1 year 1 . Interaction of DOM with min- erals and metals results in sorptive fractionation and stabilization of OM within the soil proﬁle. Iron and aluminum oxides, in particular, have a signiﬁcant eﬀect on the quantity and quality of DOM transported through forest soils due to their high sur- face area and the toxic eﬀects of dissolved aluminum on microbial communities. We directly examined these interactions by incubating forest ﬂoor material, including native microbiota, for 154 days in the presence of (1) goethite (a-FeOOH), (2) gibb- site (c-Al(OH) 3 ), and (3) quartz (a-SiO 2 ) sand (as a control). Changes in molecular and thermal properties of water extractable organic matter (WEOM, as a proxy for DOM) were evaluated. WEOM was harvested on days 5, 10, 20, 30, 60, 90, and 154, and examined by thermogravimetry/diﬀerential thermal analysis (TG/DTA) and diﬀuse reﬂectance Fourier transform infrared (DRIFT) spectroscopy. Results indicated signiﬁcant diﬀerences in WEOM quality among treatments, though the way in which oxide surfaces inﬂuenced WEOM properties did not seem to change signiﬁcantly with increasing incubation time. Dissolved organic C concentrations were signiﬁcantly lower in WEOM from the oxide treatments in comparison to the control treat- ment. Incubation with goethite produced WEOM with mid-to-high-range thermal lability that was depleted in both protein and fatty acids relative to the control. The average enthalpy of WEOM from the goethite treatment was signiﬁcantly higher than either the gibbsite or control treatment, suggesting that interaction with goethite surfaces increases the energy content of WEOM. Incubation with gibbsite produced WEOM rich in thermally recalcitrant and carboxyl-rich compounds in compar- ison to the control treatment. These data indicate that interaction of WEOM with oxide surfaces signiﬁcantly inﬂuences the composition of WEOM and that oxides play an important role in determining the biogeochemistry of forest soil DOM. Published by Elsevier Ltd. 1. INTRODUCTION The importance of soil organic C to the global C cycle is well-recognized (e.g., Amundson, 2001). Yet despite dec- ades of research, many questions remain as to how physi- cal, chemical and biological soil processes interact to control soil organic C stabilization (cf. von Lu ¨tzow et al., 2006). In this context, the present study focuses speciﬁcally on the biogeochemistry of dissolved soil organic matter (DOM) and how biodegradation of organic materials in the presence of oxide surfaces alters DOM chemical and physical properties. While DOM comprises only 1% of the total soil organic carbon pool, it is the most mobile 0016-7037/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.gca.2011.05.009 ⇑ Corresponding author. Tel.: +1 925 422 9556; fax: +1 925 423 7884. E-mail address: kaheckman@fs.fed.us (K. Heckman). 1 Tel.: +1 520 621 1646; fax: +1 520 621 1647. www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 4295–4309