Phosphorus Chemistry in Plant Ash: Examining the Variation across Plant Species and Compartments

Natural terrestrial ecosystems are often phosphorus (P)-limited and rely heavily upon the internal cycling of P in plant litter. Fires thoroughly transform the aboveground P pools by turning litter (and living biomass) into ash and charcoal, changing the forms and fluxes of P entering soil. Therefore, it is important to understand the chemistry and availability of P in the burned residues to predict the impacts of fire on P cycling. This study characterizes the speciation and availability of P in fire ash produced from different plant species and compartments to explore P thermochemistry during biomass burning and geochemical behavior of fire ash. Great variations in P availability (measured by bicarbonate extractable P) were observed for ashes derived from compartments of a range of coniferous and deciduous trees, with labile P ranging from 9 to 78% of total P. Combining sequential extraction and P X-ray absorption near edge structure spectroscopy, we identified the main P species in ash, with apatite being the dominant P species (which contributes to the HCl-extractable P pool). A correlation between elemental stoichiometry and P availability indicates that relative abundances of the main P-complexing cations (i.e., calcium, potassium, and magnesium) affect P chemistry and mobility in ash, while the relationships depend upon plant species and compartments. Results from this study reveal how initial biomass composition (because of different physiological roles of macronutrients in plant compartments) and complete burning affect ash P chemistry and improve our understanding of the disturbance of vegetative fire to ecosystem characteristics and processes.