Phosphorus movement in a boreal plain soil (Gray Luvisolic) after forest harvest

Understanding the effect of forest harvest on phosphorus export from the hillslope requires an appreciation of the soil profile's influence on processes that control phosphorus movement. Water movement patterns and phosphorus concentration trends were studied in Gray Luvisolic soil-dominated hillslopes to infer the effect of forest harvest on phosphorus movement in typical Boreal Plain uplands. I hypothesized that phosphorus concentrations would decrease with soil depth, and that interflow would occur through upper horizons. Phosphorus movement was expected to increase after forest harvest due to higher phosphorus concentrations and greater interflow. Hydraulic properties were determined by measurements of interflow, bromide movement, soil water content, infiltration, and hydraulic conductivity. Phosphorus concentration and retention were determined with in situ soil water and air-dried samples of soil horizons. The probable solid phases controlling orthophosphate solubility were identified after modeling phosphorus complex formation. The minimal interflow observed was partially due to precipitation of just 50% of normal and subsequent dry soils. However, reduced infiltration capacity and equal vertical hydraulic conductivity of Ae and Bt horizons at harvested sites suggested that harvest also diminished interflow. Despite these hydraulic changes, most snowmelt infiltrated while soils remained at 0°C. Most applied bromide remained above 60 cm depth, and more was recovered at forested than harvested sites 13 months after application. Loss of bromide reflected vegetation uptake and lateral convective-diffusive movement. Mean soluble reactive phosphorus concentration decreased with depth, ranging from 64 mg L-1 in the LFH horizon to 0.01 mg L-1 in the groundwater zone; orthophosphate was the dominant phosphorus complex in the soil solution. The solid-phase phosphate minerals in equilibrium with orthophosphate ranged from secondary Fe-, and Ca-phosphates in the LFH to fluoroapatite in the lower Ae. Adsorption reactions probably controlled orthophosphate activity in subsoil horizons. Both extractable and water soluble phosphorus decreased while pH increased after harvest in the LFH horizon only. Downslope phosphorus movement is potentially greatest when soils are moist and frozen. However, hydraulic changes and reduced phosphorus concentration that resulted from harvest reduced the potential for downslope phosphorus movement under the conditions of this study.