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

Abstract

Recent assessments of water quality status have identified eutrophication as one of the major causes of water quality 'impairment' not only in the USA but also around the world. In most cases, eutrophication has accelerated by increased inputs of phosphorus due to intensification of crop and animal production systems since the early 1990 s. Despite substantial measurements using both laboratory and field techniques, little is known about the spatial and temporal variability of phosphorus dynamics across landscapes, especially in agricultural landscapes with cow-calf operations. Critical to determining environmental balance and accountability is an understanding of phosphorus excreted by animals, phosphorus removal by plants, acceptable losses of phosphorus within the manure management and crop production systems into soil and waters, and export of phosphorus off-farm. Further research effort on optimizing forage-based cow-calf operations to improve pasture sustainability and protect water quality is therefore warranted. We hypothesized that properly managed cow-calf operations in subtropical agroecosystem would not be major contributors to excess loads of phosphorus in surface and ground water. To verify our hypothesis, we examined the comparative concentrations of total phosphorus among soils, forage, surface water, and groundwater beneath bahiagrass-based pastures with cow-calf operations in central Florida, USA. Soil samples were collected at 0-20; 20-40, 40-60, and 60-100 cm across the landscape (top slope, middle slope, and bottom slope) of 8 ha pasture in the fall and spring of 2004 to 2006. Forage availability and phosphorus uptake of bahiagrass were also measured from the top slope, middle slope, and bottom slope. Bi-weekly (2004-2006) groundwater and surface water samples were taken from wells located at top slope, middle slope, and bottom slope, and from the runoff/seepage area. Concentrations of phosphorus in soils, forage, surface water, and shallow groundwater beneath a bahiagrass-based pasture and forage availability at four different landscape positions and soil depth (for soil samples only) in 2004, 2005, and 2006 were analyzed statistically following a two-way analysis of variance using the SAS PROC general linear models model. Where the F-test indicated a significant (p <or= 0.05) effect, means were separated following the method of Duncan multiple range test using the appropriate error mean squares. Concentrations of soil total phosphorus and degree of phosphorus saturation varied significantly (p <or= 0.001) with landscape position and sampling depth, but there was no interaction effect of landscape position and sampling depth. Overall, there was slight buildup of soil total phosphorus. There was no movement of total phosphorus into the soil pedon since average degree of phosphorus saturation in the upper 20 cm was 21% while degree of phosphorus saturation at 60-100 cm was about 3%. Our livestock operations contributed negligible concentrations of phosphorus to groundwater (0.67 mg L(-1)) and surface water (0.55 mg L(-1)). The greatest forage mass of 6,842 kg ha(-1) and the greatest phosphorus uptake of 20.4 kg P ha(-1) were observed at the top slope in 2005. Both forage availability and phosphorus uptake of bahiagrass at the bottom slope were consistently the lowest when averaged across landscape position and years. These results can be attributed to the grazing patterns as animals tend to graze more and leave more excretions at the bottom slope. This behavior may lead to an increase in the concentration of soil phosphorus. Effective use and cycling of phosphorus is critical for pasture productivity and environmental stability. Phosphorus cycling in pastures is complex and interrelated, and pasture management practices can influence the interactions and transformations occurring within the phosphorus cycle. Our results indicate that current pasture management systems which include cattle rotation in terms of grazing days and current fertilizer application (inorganic + manures + urine) for bahiagrass pastures in subtropical climates on loamy sand soils offer little potential for negatively impacting the environment. Properly managed livestock operations contribute negligible loads of phosphorus to shallow groundwater and surface water. Overall, there was no buildup of soil total phosphorus in bahiagrass-based pasture. Therefore, results of this study may help to renew the focus on improving inorganic fertilizer efficiency in subtropical beef cattle systems and maintaining a balance of phosphorus removed to phosphorus added to ensure healthy forage growth and minimize phosphorus runoff. Research on the pathways and rates of movement of phosphorus deposited in urine and dung through various pools and back to the plants will be the focal point of our future investigations. Further studies are needed to determine whether the environmental and ecological implications of grazing and haying in forage-based pastures are satisfied over the longer term. New knowledge based on the whole-farm approach is desirable to identify pastureland at risk of degradation and to prescribe treatments or management practices needed to protect the natural resources while maintaining an economically and environmentally viable operation.