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# Field methods and soil characterization A Trimble-Yuma rugged outdoor tablet and GPS unit were used to locate the sample sites in the field to sub-meter accuracy. At each of the 20 sample sites soil pits were dug to depth of refusal (saprolite/saprock interface). Each pit was described and sampled according to genetic horizon (Soil Survey Staff, 2010). In general, 5 horizons per pedon were described resulting in a total of 100 collected samples. Soil morphological and physical properties including color, structure, consistence, root abundance, and rock fragment content were described in the field (Soil Survey Staff, 2010). Soil bulk density measurements were taken at each pedon if possible, using a simple core and hammer method (reference – Blake and Hartge, 19XX). Assuming parent material homogeneity within the soil profile, a representative sample of the ‘parent’ bedrock was collected at the saprolite/saprock boundary at each pedon for analysis. All samples were air dried and sieved to isolate the fine earth, <2 mm, fraction and all soil characterization analysis conducted on the fine earth fraction unless otherwise stated. Soil pH was measured on all samples at weight to volume ratios of 1:2 (soil: water) solution, 1:2 (soil: 1 M KCl) solution and 1:4 (soil: 0.02 M CaCl 2 ) solution (Soil Survey Staff, 2004). Soil electrical conductivity (μS/cm) was determined for all samples using a 1:2 (soil: water) extract (Soil Survey Staff, 2004). Soil organic matter content was determined by loss on ignition (Konen et al., 2002) for all collected samples. Samples (20g) were placed in a furnace at 105°C for 24 hrs, and 550°C for 5 hrs, using the change in weight as a proxy for organic matter. Total carbon and nitrogen content (g kg -1 ) and stable isotope signature (δ 15 N and δ 13 C) were measured for all samples on a continuous- flow gas-ratio mass spectrometer (Finnigan Delta PlusXL) coupled to an elemental analyzer (Costech) at the University of Arizona, Environmental Isotope Laboratory. The samples did not contain carbonates and it was assumed that total carbon was equivalent to total organic carbon. Particle size was determined by laser diffraction using a Beckman Coulter LS 13 320 Laser Diffraction Particle Size Analyzer at the University of Arizona, Center for Environmental Physics and Mineralogy. Following pretreatment to remove organics using NaOCl adjusted to pH 9.5, roughly 0.2 and 0.1 g of sample were weighed into tubes and mixed for 24 hours with 5 ml of deionized water using a Thermo Scientific: Labquake® shaker/rotator, followed by the addition of 5 ml of 5% sodium hexametaphosphate solution for an additional 24 hours to ensure dispersion of soil particles prior to particle size analysis. # Soil elemental and mineralogical characterization Elemental concentrations for major and trace cations were determined for all soil and rock samples by x-ray fluorescence (XRF). Soil and rock samples were pressed into pellets at a pressure of 25 tons for 120 seconds bound with a layer of cellulose wax (3642 Cellulose binder – SPEX SamplePrep PrepAid TM ) and analyzed using a Polarized Energy-Dispersive X-ray Fluorescence spectrometer (EDXRF – SPECTRO XEPOS,Kleve – Germany) at the University of Arizona, Arizona Laboratory for Emerging Contaminants. Prior to mineralogical analysis samples were pretreated to remove organic matter (Jackson, 1969). Samples were treated with 100 mL solution of 6% NaOCl, adjusted pH to 9.5 with HCl, rinsed with DI water, centrifuged, dried and mixed. Mineral phases for soil and rock samples were identified by quantitative X-ray diffraction (XRD). A known amount of internal standard (corundum) was added to each sample, to allow quantitative interpretation of XRD peaks. Samples were ground using a McCrone Micronizing Mill and prepped for XRD analysis according to protocol (Eberl, 2003). All sample preparation steps are intended to maximize random orientation, to increase the exposed surface area of the included minerals. Samples were run as random powder mounts, measured from 5 to 65° two-theta, with a step size of 0.02 degrees 2-theta at the University of Arizona, Center for Environmental Physics and Mineralogy using a PANalytical X'Pert PRO-MPD X-ray diffraction system (PANalytical, Almelo, AA, The Netherlands) generating Cu–Kα radiation and accelerating potential of 45 kV and current of 40 mA. The resulting diffractograms were analyzed using Rietveld mineralogical analysis software to identify present mineral phases and to quantify mineral abundances.