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

• Published in: The Science of the total environment Vol. 621; pp. 1103 - 1114
• Main Authors: Santín, Cristina, Otero, Xose L., Doerr, Stefan H., Chafer, Chris J.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 15.04.2018
• Subjects: 31P NMR; Algal blooms; Ash; Australia; bioavailability; Bioavailable phosphorus; Eucalyptus; forest fires; forests; Fuel reduction burn; Organic phosphorus; phosphorus; prescribed burning; sediments; soil horizons; temperature; water erosion; water reservoirs; water supply; watersheds; Australia; Organic phosphorus; Bioavailable phosphorus; Algal blooms; Fuel reduction burn; Ash; 31P NMR; P NMR
• DOI: 10.1016/j.scitotenv.2017.10.116
• ISSN: 0048-9697; 1879-1026

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 (<130mgkg−1, mainly in organic forms). The fire consumed the entire litter layer and the thin Oa soil horizon, creating 6.3±3.1tha−1 of ash, and resulted into direct net P losses of ~7kgha−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 2kgha−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. [Display omitted] •A moderate/high-severity prescribed fire in dry eucalypt forest was investigated.•Fire led to net phosphorous losses of ~7kgha−1 from litter and surface soil.•Fire increased inorganic P stocks, but only a minor proportion was bioavailable.•~2kg total Pha−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