Reduction of arsenate [As(V)] to arsenite[As(III)] influences the mobility and toxicity of arsenic (As), yet the mechanisms controlling the rate of reduction in soils and natural waters are poorly understood. The goal of this study was to determine processes affecting reduction rates of both aqueous and sorbed phase As(V). Reduction experiments were conducted anaerobically in serum bottles with a range of glucose and As(V) concentrations. Serum bottles were inoculated with microorganisms extracted directly from an agricultural soil having naturally elevated concentrations of As (unenriched population), or with a pure culture isolate obtained from the same soil after enrichment for As(V) reduction. At As(V) concentrations ranging from 6 to 600 μM, the rate of As(V) reduction by the soil isolate was first order with respect to both As(V) concentration and microbial biomass. Reduction rates of As(V) with the soil isolate were 2 to10 fold greater than in the unenriched population, suggesting As(V) reducers represented only a subset of the unenriched population. Compiled data indicated that the pure culture isolate was fermenting glucose, and potentially reducing As(V) as a detoxification mechanism. In a parallel study, reduction rates of As(V) with the unenriched population were evaluated in the presence of goethite or ferrihydrite. When redox potential decreased from 500 to near 0 mV, aqueous As concentrations decreased by approximately 30% in a goethite suspension with a high As surface coverage, yet increased by seven fold in a goethite suspension with a low As surface coverage. In a ferrihydrite suspension, aqueous As concentrations during reduction increased approximately 100 fold faster than in a goethite suspension at similar initial aqueous As(V) concentrations, corresponding to differences in Fe oxide surface areas and reductive dissolution rates. The results indicate that rates of As mobilization during reduction in soils are highly dependent on oxide surface area and As surface coverage.

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Pea (Pisum sativum L.) is increasingly being rotated with wheat (Triticum aestivum L.) in Montana. Our objective was to compare economic net returns among wheat-only and pea-wheat systems during an established 4-yr crop rotation. e experimental design included three wheat-only (tilled fallow-wheat, no-till fallow-wheat, no-till continuous wheat) and three no-till peawheat (pea-wheat, pea brown manure-wheat, and pea forage-wheat) systems as main plots, and high and low available N rates as subplots. Net returns were calculated as the di erence between market revenues and operation and input costs associated with machinery, seed and seed treatment, fertilizer, and pesticides. Gross returns for wheat were adjusted to re ect grain protein at " at" and "sharp" discount/premium schedules based on historical Montana elevator schedules. Cumulative net returns were calculated for four scenarios including high and low available N rates and at and sharp protein discount/premium schedules. Pea-wheat consistently had the greatest net returns among the six systems studied. Pea fallow-wheat systems exhibited greater economic stability across scenarios but had greater 4-yr returns (US$287 ha -1 ) than fallow-wheat systems only under the low N rate and sharp protein discount schedule scenario. We concluded that pea-wheat systems can reduce net return uncertainties relative to wheat-only systems under contrasting N fertility regimes, and variable wheat protein discount schedules in southwestern Montana. is implies that pea-wheat rotations, which protected wheat yield and/or protein levels under varying N fertility management, can reduce farmers' exposure to annual economic variability.

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