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

Highlights: • A moderate/high-severity prescribed fire in dry eucalypt forest was investigated. • Fire led to net phosphorous losses of ~ 7 kg ha⁻¹ from litter and surface soil. • Fire increased inorganic P stocks, but only a minor proportion was bioavailable. • ~ 2 kg total P ha⁻¹ was transferred from litter and soil to the highly-erodible ash. • T > 650 °C in the burning litter layer related to higher ash total P concentrations 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 ~ 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 ~ 3% of the total P lost during fire). Higher total P concentrations in ash corresponded with higher maximum temperatures (> 650 °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.