Phosphorus and nitrogen exports from SE Australian forests following wildfire

Summary Following a wildfire in south-eastern Australia that burnt over 1 million hectares of forested land in early 2003, two former research catchments (136 and 244 ha), in the East Kiewa valley, Victoria, were re-instrumented. Discharge, suspended sediment, bedload and nitrogen (N) and phosphorus (P) were measured for 3 years post-fire. The nutrient data consisted of 15-min estimation of particulate P and total Kjeldahl N via a regression with turbidity, and dissolved forms of P and N estimated from over 1100 water samples. The regression-based estimation allowed rigorous estimation of errors, seldom reported in water quality data. The fire appears to have increased the total P and N exports by around 5–6-fold, peaking at 1.6 kg ha−1 of P and 15.3 kg ha−1 of total combined N. Nutrients transported as particulate matter dominated the first post-fire year, with 94% of total combined P and 69% of total combined N. Although dissolved forms increased in importance, the particulate load comprised 86% of the total combined P load and 68% of the total combined N load over the three post-fire years. Concentrations and loads of P and N exhibited a rapid recovery to unburnt levels during the second post-fire year. Particulate forms declined sharply through a reduction in sediment delivery. Nitrate had the slowest relaxation time, suggesting a persistent subsurface pathway and the effect of nitrification. Notably, dissolved N fluxes were predominantly in baseflow even in the first post-fire year. The errors calculated for this study indicate the uncertainty that can exist in water quality estimation despite high resolution experimental design. Loads estimated from intermittent sampling should be treated with caution. A simple model with time as the single parameter proved to be a good predictor of mean three monthly concentrations.