Manure amendments to rangelands may alter soil functions related to nutrient recycling. We investigated the influence of grazing and cattle manure on soil carbon, nitrogen, Mehlich 3 phosphorus, and activities of alkaline phosphatase and dehydrogenase. Fertilizer treatments (unamended, manure, or urea potassium dihydrogen phosphate [KH2PO4] fertilizer) were imposed under grazed and nongrazed conditions in a short-grass native rangeland. Manure was applied at rates of ∼ 125 kg N ha−1 and ∼ 42 kg P ha−1, and urea KH2PO4 was applied at 75 kg N ha−1 and 20 kg P ha−1, respectively. Total aboveground biomass and soil samples at 4 depths (0–200 mm) were collected throughout 2 growing seasons. A controlled environment study also evaluated fertilizer source effects on enzymatic activities at 5 P rates (0–120 mg·kg−1 P as manure or urea KH2PO4). Amendments significantly (P < 0.05) increased extractable P following the second application for the 3 uppermost depth increments. Extractable P was greatest on manure-amended plots, increasing 44% from February 1999 to July 2000 at the surface. However, increases in P extractability as a proportion of total P applied were similar for manure and KH2PO4. Enzymatic activities were significantly (P < 0.001) influenced by sampling date and soil depth. There were no consistent grazing effects on enzyme activities. Amendments did not influence dehydrogenase activities in the field; however, in the controlled environment, activities averaged 16% greater across all rates for manure-amended soil as compared with urea KH2PO4–amended soil (P = 0.025). Phosphatase activities increased significantly following manure applications under both field (P = 0.007) and controlled environment (P = 0.003) conditions. Elevated phosphatase activities following manure applications probably led to enhanced P mineralization and similar P extractabilities as a proportion of total P applied for manure- and KH2PO4–amended soils. Therefore, when determining applications rates, total manure P should be considered bioavailable.
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Vol. 59 • No. 3