Bioavailable P (BAP) in agricultural runoff represents P potentially available for algal uptake and consists of soluble P (SP) and a variable portion of participate P (PP). Evaluation of the impact of agricultural management on BAP in runoff will aid assessment of the resultant biological productivity of receiving water bodies. Soluble P, PP, and bioavailable PP (BPP) (estimated by NaOH extraction) were determined over a 5‐yr period in runoff from 20 unfertilized and fertilized, grassed, and cropped watersheds in the Southern Plains. Soluble P, BPP, and BAP loss in runoff was reduced by practices minimizing erosion and runoff, with respective mean annual amounts ranging from 237 to 122, 1559 to 54, and 1796 to 176 g P ha−1 yr−1 (for peanut‐sorghum [Arachis hypogaea L.‐Sorghum bicolor (L.) Moench] and native grass watersheds, respectively). However, as vegetative cover improved, BAP (SP plus BPP) comprised a larger portion of total P (TP) loss (29% for peanut‐sorghum and 88% for native grass). This results from an increasing contribution to BAP of SP (13% for peanut‐sorghum and 69% for native grass watersheds) and BPP to PP (26% for peanut‐sorghum and 69% for native grass watersheds). Clearly, P bioavailability is a dynamic function of physiochemical processes controlling erosion, particle size enrichment, P desorption‐dissolution reactions, and plant residue breakdown, in addition to soil and fertilizer P management. Hence, the change in trophic state of a water body may not be adequately reflected by TP inputs only. To more reliably evaluate the biological response of a water body to agricultural P inputs, particularly from conservation tillage practices, it may be necessary to determine BAP in runoff.

The effect of livestock grazing on organic C and N in range-Land soils is not well defmed. In this study on sandy rangehmd in western Oklahoma, we sampled 8 pastures moderately grazed by cattle and 8 adjacent exclosures ungrazed by livestock for 50 years. The sagebrush was largely controlled by herbicide in the study areas. The C and N concentrations in the surface 5 cm of soil, total herbage production, and total N uptake by vegetation were similar (P > 0.05) in grazed and nongrazed areas. CiU-bOU and N concentrations in soils sampled to a constant mass to a depth of 5 cm or less were not (P > 0.05) different from concentrations determined on soil sampled to a constant depth of 5 cm. When calculated on a content basis, grazing increased (P < 0.001) the bulk density (1.35 g cm") compared to nongrazed pastures (1.19 g cm-3 and had a significant (P < 0.01) effect on C and N in the surface 5 cm of soil. Litter and total N in litter were greater (P < 0.01) on nongrazed areas. Little bluestem (Schizachyrfum scoparium (Michx.) Nash) and sand bluestem (Andropogon haZlii Hack.) produced more herbage and had greater frequency on nongrazed areas, whereas blue grama [Bouteloua gracilis (H.B.K.)Lag. ex Griffiths], sand dropseed [Sporobolus cryptandnrs (Torr.)Gray], and western ragweed (Ambrosia pdostachya DC.) increased in frequency on grazed areas. Thus, 50 years of moderate grazing by cattle had no measurable effect on C and N concentrations in the surface 5 cm of the sandy soil or on total N uptake by plants as compared with nongrazed areas; however, significant differences occurred in species composition which may alter mechanisms of C and N balance.

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Plant available nitrogen limits production of native wvarm-season grasses on marginal farmland in the Southern Plains. In this western Oklahoma study, N was applied at 0,355 70, or 105 kg N ha" yr-' to a mixed stand of blue grama (Bouteloua gracilis (H.B.K.) Lag. es Griffiths), sideoats grama (B. curfipendula (Michs.) Torr.), little bluestem (Schizachyrium scoparium (M&s.) Nash), sand bluestem (Andropogon h&ii Hack.), switchgrass (Panicum virgatum L.) and indian grass (Sorghastrum nutans (L.) Nash). The grass was established on sandy loam soil farmed an estimated 90 years. With near-normal precipitation the fwt year, production of perennial grasses increased linearly with 26 kg herbage produced kg1 N applied. In drouth conditions, the second and third years, production averaged 10 kg herbage kg1 N applied. The fourth and fifth year the stand was not fertilized and residual effects measured. Herbage production increased 10 kg for each kg N applied over the previous 3 years. Blue Grama made up much of this increased herbage production along with warm-season annuals (Panicum capillare L. and Amaranthus retrof7exus L.). With increasing N rates the residual N effect increased the proportion of blue grama and decreased the proportion of taller perennial grasses. Thus, N fertilization of mised native warm-season grass stands established on marginal farmland, typical of stands established on sandier soils under the USDA Conservation Reserve Program, can result in substantial herbage yield increases, however, some of the increased yield may be from weedy species.

Knowledge of rates and amounts of N mineralization in geologic strata is needed to predict N availability to plants when these materials are disturbed by mining or construction and left as surface or subsurface plant‐growth media. Laboratory incubations were conducted to determine ammonification and nitrification rates in a Cretaceous shale, two strip‐mine coal spoils, and a soil from a strip mine in northwestern Colorado. The total N concentration in the soil (1193 ppm), a vegetated spoil (1,083 ppm), and the shale (1112 ppm) were similar, while that of the fresh spoil was lower (730 ppm). No net N mineralization was measured in the shale, and less than 6 ppm NO3‐‐N accumulated in the fresh spoil during a 168‐day incubation. However, net mineralization of 49‐ppm NO3‐‐N was found in the vegetated spoil and 75‐ppm NO3‐‐N in the soil during the same incubation period.Nitrification of added NH4+ (60 ppm NH4+‐N) was measured in the shale, spoils, and soil. Accumulated levels of NO3‐‐N were 100 ppm in the vegetated spoil and 117 ppm in the soil after a 168‐day incubation. However, only 36‐ppm NO3‐‐N accumulated in the fresh spoil and no NO3‐‐N accumulated in the shale during the same incubation period.Rates and total amounts of CO2 evolution from the geologic materials resembled that from the soil, indicating considerable heterotropic microbial activity in these geologic materials. The addition of NH4+ did not significantly affect the rate or amount of CO2 evolution from the geologic or soil materials, indicating that heterotropic microorganisms were capable of using the forms of N present in the geologic materials.