Project Number: 1902-13000-006-00D
Researchers: Sharpley, A. N.; Gburek, W. J.; Pionke, H. B.
Start Date: 01-Jan-96
End Date: 31-Dec-98

Research Objectives: 1) Determine effects of P management in pasture systems on the availability and transfer of soil P to runoff; 2) delineate soil management factors that define critical sources of P in a watershed; 3) define interactions between P sources, runoff and erosion, along with channel processes that determine P export from selected watersheds; and 4) develop conceptual and user-oriented models that delineate P export potential.

Approach: A multiscale watershed-based study will be conducted to chemically and hydrologically link soil P, runoff, and erosion across scale for predicting and managing P source areas, sinks, and storages within as well as export from watersheds. Using simulated runoff we will relate runoff P to soil P as a function of soil type and applied manure or fertilizer P. Critical source areas combining high soil P, runoff and erosion potentials will be instrumented to determine key processes controlling P transfer from soil to runoff and to develop a defensible basis for delineating critical P source areas. Surface runoff and groundwater recharge will be quantified and linked at watershed scales to hydrologically delineate critical P and N source areas. Process-based modeling methodologies will be incorporated into user orientated tools for predicting runoff, erosion, and nutrient export at field, farm, and watershed scales. From all these we will deveP export from watersheds.

Progress: Research was conducted at laboratory, plot, and watershed scales to investigate hydrologic and chemical variables controlling P export from agricultural watersheds. Phosphorus loss from the mixed land use watershed FD-36 in the Mahantango Creek Watershed, PA, was only 0.3 kg P/ha/yr, most of which (95%) occurred during the two largest storms of the year. Although P export was small compared to amounts of P applied as fertilizer and manure to the cropped areas of FD-36 (from 85 to 100 kg P/ha/yr), most of that exported was in forms readily available for algal uptake (70%). Combining this with hydrological analysis of stream flow, which shows runoff to occur an area extending up to 60 m from the stream channel, it is suggested that this near stream area determines P export from FD-36. This is consistent with the fact that even though over 60% of the watershed has soil P above crop response levels (>100 mg P/kg), P export was relatively low.

Undisturbed soil blocks ranging Mehlich-3 soil P content (180 to 650 mg/kg) were collected from hydrologically active areas of FD-36 and simulated rainfall applied (5 cm/hr for 30 min). Dissolved P concentrations of runoff (0.22 to 2.33 mg/L) were related (r=0.71) to Mehlich-3 P content of surface soil (0 to 5 cm). However, dissolved P was more closely related to Fe-oxide strip P content of the soil(r=0.83). This suggests that the Mehlich soil test may be used as a temporary surrogate for potential enrichment of runoff P, but extraction procedures designed to more closely simulate soil P release to runoff would provide more reliable indicators of the potential for loss.

When different sources of P were applied at the same rate (200 kg P/ha) and subjected to simulated rainfall (5 cm/hr for 30 min), P loss in surface runoff from several PA and VT soils decreased in the order; mineral fertilizer (29.7 mg/L), poultry manure (6.5 mg/L), dairy manure (3.8 mg/L), poultry compost (2.6 mg/L), and dairy compost (2.0 mg/L). This was in part due to the fact that water solubility of P in the organic materials decreased in the same order: poultry manure (75%), dairy manure (62%), poultry compost (55%), and dairy compost (45%).

In total, results show that delineation of runoff-producing areas, and recognition of the similarity between patterns of P concentration in stream flow and P concentration of near-stream soils, suggests that P management goals should focus on the near-stream areas rather than the whole watershed. With this approach, where soil P, land use, and hydrologic characteristics of the watershed are considered, we can target remedial programs to the hydrologically active areas of the watershed, which have the potential of greatly increasing the efficacy of control while reducing its cost.


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