Soil hydrologic changes and nutrient flux following long-term burning of a longleaf pine-bluestem association

Typescript (photocopy).A study was conducted on the Palustris Experimental Forest near Alexandria, located in the southern mixed forest region of Louisiana, to determine the effects of 22 years of prescribed burning on soil hydrologic, forest floor, and water quality characteristics of a major forest type and to monitor the flux in soil nutrient pools 1 year after burning. Soil hydrologic parameters were determined with a mobile spray-nozzle type rainfall simulator (1260 mm/h for 45 minutes) with soils initially wet. Vertical transport of nutrients into soil was monitored using ceramic cup suction lysimeters. Treatments represented biennially applied winter, spring, or summer burns on an upland sandy loam site; and annual winter or spring, and biennial winter or spring burns on a bottomland silt loam site with associated unburned controls. Prescribed burning for 22 years has shifted plant community composition toward dominance by longleaf pine (Pinus palustris Mill.) and fire-tolerant grasses and forbs. Immediate effects of burning were to remove soil surface cover, exposing soil to raindrop impact and the potential for surface sealing. Burning the sandy loam site produced no immediate changes in infiltration or time to runoff, regardless of season. Rapid recovery of the herbaceous understory balanced litter cover removed by burning. Stabilization of sediment production occurred with reduction of exposed bare soil. Nitrogen (N) and phosphorus (P) concentrations in runoff from burned plots were not different from unburned plots. Runoff concentrations of calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na), were slightly greater from ash-covered plots relative to unburned plots. Little vertical transport of nutrients from the burned forest floor occurred from sandy loam plots during the first growing season after burning. However, significant nitrate plus nitrite (NO3 + NO2) was transported downward from unburned plots during winter. Vertical transport of Ca, Mg, K, and Na from burned plots was not significantly different from unburned. Elevated pH was not observed after burning. Long-term prescribed burning of the silt loam site did not persistently reduce infiltration capacity or time to runoff. Sediment production and nutrient concentrations in plot runoff were not significantly greater from burned compared to unburned plots 2 years after treatment