Levels of Phosphorus in Soils, Plants, Surface Water and Shallow Groundwater in Bahiagrass-Based Beef Cattle Pasture.

Relatively little information exists regarding possible magnitudes of P losses from grazed pastures. Whether or not P losses from grazed pastures are significantly greater than background losses and how these losses are affected by soil, forage management, or stocking density are not well understood. A long-term quantitative assessment of soil chemical properties may serve as an indicator of a soil’s capacity for sustainable production of crops and animals in an economically sound, socially acceptable, and environmentally friendly manner.  Soil samples were collected at 0-20, 20-40, 40-60, and 60-100 cm across the pasture’s landscape (top slope, TS; middle slope, MS; and bottom slope, BS) in the fall and spring of 2004 to 2006, respectively. Forage availability and P uptake of bahiagrass were also measured from the TS, MS, and BS. Bi-weekly (2004-2006) groundwater and surface water samples were taken from wells located at TS, MS, and BS and from the run-off/seepage area (SA). Concentrations of extractable soil P and degree phosphorus saturation (DPS) varied significantly (p≤0.001) with landscape position (LP) and soil depth (SD), but there was no interaction effect of LP and SD. Overall, there was no buildup of soil P. There had been no movement of extractable total P into the soil pedon since average DPS in the upper 20 cm was 21% while DPS at 60-100 cm was about 3%. Our livestock operations contribute negligible concentrations of P to groundwater (0.67 mg L^-1) and surface water (0.55 mg L^-1).  The greatest forage availability of 6,842 kg ha^-1 and the highest P uptake of 20.4 kg P ha^-1 were observed from the top slope in 2005. Both forage availability and P uptake of bahiagrass at the bottom slope were consistently the lowest when averaged across landscape positions and years. These results can be attributed to the grazing activities as animals tend to graze more at the bottom slope and leave more number of animal excretions and this behavior may lead to an increase in the concentration of soil P. Effective use and cycling of P is critical for pasture productivity and environmental stability. Phosphorus cycling in pastures is complex and interrelated and pasture management practices could influence the interactions and transformations occurring within the P cycle.