Impact of a moderate/high-severity prescribed eucalypt forest fire on soil phosphorous stocks and partitioning

This study examines the direct impact of a moderate/high-severity prescribed fire on phosphorous (P) stocks and partitioning in oligotrophic soils of a dry eucalypt forest within Sydney's water supply catchments, Australia. We also quantify and characterize the P present in the ash produced in this fire, and explore its relationships with the maximum temperatures recorded in the litter layer during the burn. In these oligotrophic soils, P concentrations were already relatively low before the fire (< 130 mg kg(-1), mainly in organic forms). The fire consumed the entire litter layer and the thin Oa soil horizon, creating 6.3 +/- 3.1 t ha(-1) of ash, and resulted into direct net P losses of similar to 7 kg ha(-1). The P lost was mostly organic and there was a moderate net gain of inorganic and non-reactive P forms. Importantly, only a small proportion of the post-fire P was bioavailable (equivalent to similar to 3% of the total P lost during fire). Higher total P concentrations in ash corresponded with higher maximum temperatures (> 650 degrees C) recorded in the burning litter layer, but effects of fire temperature on ash P partitioning were not significant. Fire not only transformed P chemically, but also physically. Our results show that, immediately after fire, up to 2 kg ha(-1) of P was present in the ash layer and, therefore, highly erodible and susceptible to be transported off-site by wind-and water erosion. Even if most of this P was, initially, of low bioavailability, its transfer to depositional environments with different geochemical conditions (e.g. anoxic sediments in water reservoirs) can alter its geochemical forms and availability. Further investigation of potential P transformations off-site is therefore essential, particularly given that SE-Australian water supply catchments are subject to recurrent perturbation by prescribed fire and wildfires. The latter have already resulted in major algal blooms in water supply reservoirs. (c) 2017 Elsevier B.V. All rights reserved.