FORMS OF PHOSPHORUS IN SEDIMENTS ERODED FROM BURNT SOILS

Wildland fires can increase erosion and nutrient loss in runoff by los s of vegetative cover and changes in soil structure and chemical compo sition. Thus, we investigated the effect of vegetation burning intensi ty on the loss of particulate P forms in eroded sediments by using a s equential fractionation method that distinguishes organic and inorgani c P forms of different lability. Moderate burns, while not affecting P form distribution in the soil, led to high levels of inorganic P (lar gely Bic-Pi, NaOH-Pi, and HCl-P) in eroded sediments, with phosphorus enrichment ratios ranging from 3.36 to 5.04. Severe burns modified P f orm distribution in the soil, and led to the almost complete disappear ance of extractable organic P from the soil. Phosphorus form distribut ion in sediments eroded from the severely burnt plot was similar to th at in the surface layer of the soil of that plot (0-2.5 cm). Within 10 mo of burning, and independently of burn intensity, sediments eroded from both the moderately and severely burnt plots had the same P form content and distribution as sediments eroded from control plots. As a consequence of burning, the annual total P losses due to removal of pa rticulate matter in runoff increased from 1.42 kg ha-1 (control plot) to 4.35 kg ha-1 (moderately burnt plot) and 9.10 kg ha-1 (severely bur nt plot). The results of the P fractionation reveal that burning leads to a 2- to 11-fold increase in rapid-turnover inorganic and organic P contents in eroded sediments. This implies greatly increased risks of eutrophication of receiving waters.