Minimizing phosphorus loss from upland grazing systems

Runoff from pastures and crop production areas contributes phosphorus (P) to surface water. This thesis is a report on effectiveness of practices implemented to reduce P losses from pastures. Five grazing practices were implemented on bromegrass pastures in Iowa. Changes in soil and root characteristics were monitored for two years. Rainfall simulations were conducted to quantify the impact of grazing on P losses in surface runoff. When comparing all grazed treatments to ungrazed and hay for stockpile treatments, root surface area density (RSAD) in the 5-15 cm depth of grazed treatments was higher (p=0.0569), suggesting that grazing increased RSAD in the subsurface depths. In year 1, soil test (Bray-1) P and soluble P in runoff were positively (p<0.10) correlated in 5 cm and 10 cm rotational treatments. In year 2, total P in runoff was correlated with soil test P in the 5 cm rotational treatment and particulate P in the 5 cm rotational treatment. Information from this study will provide a better understanding of the effects of grazing management on soil and forage grass characteristics. In a separate study, the application of phosphonate, an active ingredient in fungicides, was evaluated for its ability to induce smooth bromegrass (Bromus inermis) to accumulate higher amounts of P than untreated control plants. In the first of two studies, smooth bromegrass was grown in a controlled-environment chamber in pots of Clarion silt loam with very high levels of Bray-1 P. Plants receiving phosphonate had significantly higher P concentration per gram dry weight of leaf tissue than the control plants. In the second study, smooth bromegrass was grown in a controlled-environment chamber in Clarion silt loam soil with optimum levels of Bray-1 P. Only plants receiving 16 kg ha−1 phosphonate had significantly higher tissue P concentrations. Although previous studies suggest increased uptake is due to increased fine root production, no differences were found in root biomass between phosphonate-treated plants and control plants in either study. With further study, phosphonate application could be used to induce hyperaccumulation of P by riparian grasses, reducing the potential for P to enter surface water.