Prefire And Postfire Erosion Of Soil Nutrients Within A Chaparral Watershed

Prescribed burns are an effective and increasingly popular strategy for inhibiting wildfires. The goal of this study was to characterize soil nutrient loss after a prescribed fire within a chaparral watershed in southern California. The study compared hillslope sediments for approximately 1 year before the fire with those during the year after the fire. Samples were collected in 11 sediment traps located randomly along fall line transects throughout a steep, 0.65-ha watershed (∼65% average slope). Concentrations of soluble ammonium nitrogen, nitrate nitrogen, phosphate phosphorus, Ca, Mg, K, and Na, exchangeable ammonium nitrogen, and total C and N were analyzed. Sediment production averaged approximately 1600 g per sediment trap for the entire year before the fire but increased to approximately 10,700 g for the year after the fire. The average sedimentation rates for the less than 2-mm fraction doubled from a prefire rate of 2.7 to 7.5 g d -1 after the fire and increased to 86 g d -1 after the first major rain event after the fire. The greatest concentrations of soluble and exchangeable base cations (Ca, Mg, K, Na) in the eroded sediment were measured at the first sampling date after the fire. Input and microbial mineralization of fresh organic matter approximately 4 months after the fire is believed to be the source of increases in the concentrations of soluble nitrate nitrogen and phosphate phosphorus and exchangeable ammonium nitrogen. Our findings indicate that prescribed fire may cause less loss of N and P than wildfires.