Quantifying phosphorus levels in soils, plants, surface water and shallow groundwater associated with bahiagrass-based pastures

Authors: Sigua, Gilbert; Hubbard, Robert; Coleman, Samuel

Submitted to: Environmental Science and Pollution Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2009
Publication Date: 7/23/2009
Citation: Sigua, G.C., Hubbard, R.K., Coleman, S.W. 2009. Quantifying phosphorus levels in soils, plants, surface water and shallow groundwater associated with bahiagrass-based pastures. ESPR-Environmental Science and Pollution Research Journal. 17:210-219.

Interpretive Summary: 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 (0.3 mg/L) 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. Critical to determining environmental balance and accountability is an understanding of P excreted, P removal by plants, and acceptable losses of soil P within the manure management and crop production systems and export of P off-farm. Further research effort on optimizing forage-based cow-calf operations to improve pasture sustainability and water quality protection therefore is still warranted. We hypothesized that properly managed cow-calf operations would not be major contributors to excess loads of extractable P in surface and ground water. To verify our hypothesis, we examined the comparative concentrations of total P among soils, forage, surface water and groundwater beneath bahiagrass-based pastures with cow-calf operations. Our results indicate that current pasture management including cattle rotation in terms of grazing days and current fertilizer application (inorganic + manures + urine) for bahiagrass pastures offer little potential for negatively impacting the environment. Properly managed livestock operations contribute negligible loads of P to shallow groundwater and surface water. Overall, there was no buildup of soil TP in bahiagrass-based pasture. Therefore, results of this study may help to renew the focus on improving fertilizer efficiency in subtropical beef cattle systems, and maintaining a balance of P removed to P added to ensure healthy forage growth and minimize P runoff.

Technical Abstract: Recent assessments of water quality status have identified eutrophication as one of the major causes of water quality “impairment” not only in the United States, but also around the world. In most cases, eutrophication has accelerated by increased inputs of phosphorus (P) due to intensification of crop and animal production systems since the early 1990’s. 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.