Pile Burning Effects on Soil Water Repellency, Infiltration, and Downslope Water Chemistry in the Lake Tahoe Basin, USA

Thinning of conifers followed by pile burning has become a popular treatment to reduce fuel loads in the Lake Tahoe Basin, USA.  However, concern has been voiced about burning within or near riparian areas because of the potential effect on nutrient release and, ultimately, lake water quality.  Our objective was to quantify the effects of pile burning on soil physical and chemical properties and resulting near-stream surface and subsurface water chemistry.  Twenty-seven hand-built piles of three contrasting fuelbed types (large wood, mixed-diameter slash, small-diameter slash) were burned.  Burn sites were located throughout the basin and included both granitic and volcanic parent materials as well as glacial outwash.  We suspected that post-fire changes in soil physical and chemical properties would ensue as maximum soil surface temperatures averaged 400 ℃ for all pile types and exceeded 200 ℃ for >30 h beneath large-wood piles.  Post-fire soil water repellency was greatest for large-wood piles, yet increased for all pile types when soil moisture content fell below a threshold of 7 % to 10 % during the dry summer season.  Soil bulk density increased moderately whereas water infiltration rates decreased more than fourfold after burning of large-wood piles and mixed-slash piles.  Surface runoff and subsurface flow concentrations of nitrate, phosphate, and sulfate were measured downslope from the piles at 6 mo and 18 mo after burning.  Mean NO₃, ortho-PO₄, and SO₄ concentrations were low (<10 mg L^-1) at both sampling dates and typically declined downslope from the pile edge.  The results showed that pile burning-regardless of fuel composition-had a limited effect on downslope water quality despite associated changes in soil physicochemical properties.