Prolonged benefits following a single application of compost in organic dryland wheat (Triticum aestivum L.) in the western United States have been documented, including the long-term bioavailability of phosphorus (P). Information on the bioavailability and forms of P following a one-time compost application in marginal organic agroecosystems with low and infrequent use of fertilizers is required to promote sustainability. The goal of this study was to evaluate changes in Hedley P pools following a one-time application of manure compost and its potential influence on short-term (3-yr) P bioavailability. Steer manure compost at 0, 25, and 50 Mg dry weight ha(-1) was applied to two organic dryland sites, Snowville and Blue Creek, with contrasting soil characteristics in northern Utah in a randomized complete block design. Potential phosphatase assays, a laboratory aerobic incubation study, and soil P fractionations were carried out to assess the transformation of P 1 and 3 yr after a one-time compost application. At 0-10 cm, compost application resulted in increased acid phosphatase at Snowville (67.3 vs. 42.7 mu g p-nitrophenol g(-1) soil, p = .003), while alkaline phosphatase increased at Blue Creek (124 vs. 49 mu g p-nitrophenol g(-1) soil, p = .0185). The path coefficient between compost P and Hedley labile inorganic P was significant at Snowville, but not at Blue Creek, suggesting that the pathways of P differed between the sites. The Hedley stable P pool was a sink of compost P at Snowville but a source at Blue Creek. In conclusion, our work shows that a one-time compost application had a differential effect on the variation in P dynamics in organic dryland winter wheat-fallow soils. The potential long-term benefits may be greater in semiarid soils with lower available P levels, higher carbonate levels and pH, and drier conditions.