Saturday, 15 July 2006

Impact of Subsurface Hydrology on Phosphorus Translocation in Karst Topography.

Janice Branson, Tennessee Tech Univ, 8409 Wood Road, Corryton, TN 37721-2116

Soluble forms of phosphorous, when added to soils as fertilizers, can be fixed and over time may form highly insoluble compounds. This requires more frequent additions of P fertilizers to meet plant requirements. However, soluble phosphorous can be mobile and has the potential to contribute to non-point source pollution of ground and surface water. The objective of this study was to examine the effect of subsurface hydrology on concentration of soluble phosphorous. Two low terrace landscapes with pasture management were sampled using National Cooperative Soil Survey methods. Transects were established across each landscape along a 3% gradient to the edge of a creek. One pasture was underlain with karst topography with observable sinkholes and areas with outcrops having springs. Springs or karst topography did not underlie the second site. Fertilizer (19-19-19) was applied (224.2 kg/ha) in March 2005. Transects were sampled in February 2005 and May 2005. Soil moisture was measured using a gravimetric method. Phosphorous was determined using the Mehlich I procedure. In both sites, the highest concentrations of P occurred in the upper horizons to a depth of approximately 65 cm. At depths of 170 cm or greater, concentrations decreased substantially. In the karst landscape, P content ranged from 2 to 5 mg/kg in horizons where subsurface flow was active. In pedons downslope from obvious sinkholes, phosphorus in horizons at similar lower depths increased to a range of 10 to 15 mg/kg. In the non- karst landscape, horizons at lower depth had low levels of phosphorus (~1 to 4 mg/kg). Sites adjacent to the creek had significantly higher P concentrations (10 to 15 mg/kg). However, higher concentrations were measured in horizons directly above a lithic or paralithic contact or where a buried A horizon was present in several pedons. As expected, concentrations at the surface significantly increased following the addition of fertilizer, but only slight increases were observed at depth. While P fixation did retain significant amounts of P in surface horizons, the presence of a relatively consistent flow of moisture, indicated some phosphorus remained soluble. Subsurface hydrologic movement of solutes did translocate soluble P in lower horizons with the sinkholes acting as avenues for outflow. The presence of impenetrable layers or high concentrations of organic matter acted as barriers to translocation of soluble P in pedons lacking substantial subsurface hydrologic flow. In managing soils for soluble P, both the underlying geologic strata and hydrology need to be considered in determining frequency, concentration, and timing of P fertilizer application.

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