Surface runoff hydrology and phosphorus transport along two agricultural hillslopes with contrasting soils.

摘要

Agricultural phosphorus (P) is a major pollutant of surface waters. The majority of agricultural P loss from upland watersheds to surface waters is through surface runoff from a small percentage of the landscape. Targeting these areas requires a better understanding of the mechanisms of runoff generation operating in agroecosystems under varying management and soil conditions. The role of subsurface soil hydrologic properties in runoff generation is a poorly understood research area. The study was conducted within a 39.5-ha mixed land use watershed in east-central Pennsylvania. Runoff generation was quantified along two agricultural hillslopes, one with colluvial soils, the other with primarily residual soils. The colluvial hillslope is composed of somewhat poorly drained soils with fragipans and high clay content (30–55%) argillic horizons (Fine, mixed, semiactive, mesic Aquic Fragiudalfs); the residual hillslope has primarily well drained soils with moderate clay content (15–35%) argillic horizons (Fineloamy, mixed, semiactive, mesic Typic Hapludults). Three sites along each hillslope were monitored for surface runoff water quality (dissolved reactive P, Fe-strip P, particulate P, and sediment), hydraulic head, shallow water table depth, soil moisture, redox potential, and soil temperature from June–Dec. 2000 and April–July 2001. Rainfall during the study period was below average. Three high peak intensity (>8 cm hr−1) events were recorded during the study period with return periods of 2.5, 4, and 5 years; no large rainfall depth (>4 cm) events were recorded. Runoff production was substantially greater from the colluvial hillslope than from the residual hillslope. Runoff production in the colluvial soils was dependent on the occurrence of a saturated soil zone from the soil surface to a depth of at least 30 cm. The saturated layer was caused either by the rise of a fragipan-perched water table or the formation of a transient argillic horizon-perched water table during infiltration. The loss of all P fractions was greater from the colluvial hillslope than from the residual hillslope, despite greater soil Mehlich-3 P concentrations in the residual soils. Subsurface soil properties can greatly influence surface runoff generation and P export, and should be integrated into nutrient management planning and nutrient transport modeling.