Fate of biosolids Cu and Zn in a semi-arid grassland J.A. Ippolito a, *, K.A. Barbarick b , R.B. Brobst c a USDA-ARS-NWISRL, 3793 North 3600 East, Kimberly, ID 83341, United States b Department of Soil and Crop Sciences, C127 Plant Sciences Building, Colorado State University, Fort Collins, CO 80523-1170, United States c U.S. Environmental Protection Agency, Denver, CO 80202, United States 1. Introduction Biosolids land application is a major method of disposal in the US, with approximately 50% land applied (US EPA, 2007a); in US Environmental Protection Agency (US EPA) Region 8, which encompasses Colorado, Montana, North Dakota, South Dakota, Utah, and Wyoming, 85% is land applied (US EPA, 2007b (http:// www.epa.gov/unix0008/water/)). This recycling method can greatly benefit municipalities by recycling plant nutrients in an environmentally sound manner when applied at agronomic rates (Barbarick et al., 1992). Our 12-year biosolids study with the city of Fort Collins, Colorado has provided valuable information on the effects of biosolids application to a semi-arid grassland soil. For example, Harris-Pierce (1994) studied surface biosolids applica- tion with no incorporation of rates up to 30 Mg ha 1 to a city of Fort Collins owned semi-arid grassland. The author noted increasing concentrations of total Cd, Cu, Mo, and Zn in the soil 0–8-cm depth associated with increasing biosolids application rates. There was some evidence that NO 3 –N and salts were leaching to the lowest depth sampled (15–30 cm), with increased leaching associated with increasing biosolids application. Sullivan et al. (2005) revisited the research site, noting that ammonium bicarbonate- diethylenetriaminepentaacetic acid (AB-DTPA) extractable Cu and Mo increased with biosolids application rates up to of 30 Mg ha 1 in the 0–15-cm depth. Lacking from these studies, however, was a more detailed analysis pertaining to the fate and transport of biosolids-borne metals within this long-term biosolids-amended semi-arid grassland soil. The limited studies pertaining to biosolids application in semi- arid settings have focused primarily on total and plant-available soil metal concentrations. Fresquez et al. (1990) studied the effects of increasing biosolids surface application with no incorporation (22.5, 45.0, and 90.0 Mg ha 1 ) on the 0–15-cm soil depth of degraded semi-arid grassland. The authors observed that DTPA- extractable Cu and Zn concentrations increased with increasing biosolids application in each of four years following biosolids Agriculture, Ecosystems and Environment 131 (2009) 325–332 ARTICLE INFO Article history: Received 9 September 2008 Received in revised form 11 February 2009 Accepted 17 February 2009 Keywords: 4 M HNO 3 extraction Biosolids-borne metals Mobility Semi-arid grassland Sequential extraction ABSTRACT Biosolids land application applies varying trace metal amounts to soils. Measuring total soil metals is typically performed to ensure environmental protection, yet this technique does not quantify which soil phases play important metal release or attenuation roles. We assessed the distribution of biosolids- borne Cu and Zn associated with soluble/exchangeable, specifically adsorbed/carbonate-bound, amorphous and crystalline Mn/Fe oxyhydroxide-bound, residual organic, and residual inorganic phases. Biosolids were surface-applied (no incorporation) to experimental plots, at the Meadow Springs Ranch (40 53 0 46 00 N, 104 52 0 28 00 W) which is owned by the city of Fort Collins, CO, USA, in 1991 at rates of 0, 2.5, 5, 10, 21, and 30 Mg ha 1 . Plots were split in half in 2002, with one-half receiving biosolids at rates identical to 1991 rates. In 2003, 0–8, 8–15, and 15–30-cm soil depths were collected and subjected to 4 M HNO 3 digestion and sequential fractionation. The 4 M HNO 3 extraction suggested downward Cu transport, while Zn was immobilized in the soil surface. The sequential extraction procedure, more sensitive to changes in soil metal pools, suggested that repeated biosolids application did not affect vertical Zn movement, but did increase the downward transport potential of organically complexed Cu. In the given time, organically complexed Cu was likely mineralized and subsequently associated with soil mineral oxide phases. Because bioavailability of Cu is associated with dissolved phases, and soluble/ exchangeable Cu concentrations were below detection limits in the subsoil, a reduction in environmental quality should be minimal. Still, we advocate that on coarse-textured semi-arid soils, biosolids application rates should match the plant N needs to avoid potential downward trace metal transport. Published by Elsevier B.V. * Corresponding author. Tel.: +1 208 423 6524; fax: +1 208 423 6555. E-mail address: jim.ippolito@ars.usda.gov (J.A. Ippolito). Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee 0167-8809/$ – see front matter . Published by Elsevier B.V. doi:10.1016/j.agee.2009.02.013