『Abstract
Background, aim, and scope 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 1990s. 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.
Materials and methods 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≦0.05)
effect, means were separated following the method of Duncan multiple
range test using the appropriate error mean squares.
Results and discussion Concentrations of soil total phosphorus
and degree of phosphorus saturation varied significantly (p≦0.001)
with landscape position and sampling depth, but there was no interaction
effect of landscape position and sample 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.
Conclusions 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.
Recommendations and perspectives 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.
Keywords: Bahiagrass; Cow-calf; Nutrient cycling; Phosphorus;
Plant uptake; Shallow groundwater; Surface water; Water quality』
1. Background, aim, and scope
2. Materials and methods
2.1. Site description
2.2. Pasture management: fertilization and grazing days' intervals
2.3. Instrumentation and water sample collection
2.4. Water sample handling and analyses
2.5. Soil sampling and soil analyses
2.6. Plant sampling and phosphorus analysis
2.7. Data reduction and statistical analysis
3. Results
3.1. Concentration of total phosphorus in surface water and
shallow groundwater
3.2. Concentration of total phosphorus and degree of phosphorus
saturation in soils
3.3. Herbage mass and total phosphorus uptake
3.4. Input-output estimates of phosphorus
4. Discussion
5. Conclusion
6. Recommendations and perspectives
References