Speciation and Aqueous\nDissolution of Macronutrients\nin Fire Ash: Variation across Ecosystems and the Effects on Nutrient\nCycling

This study investigated the speciation and aqueous dissolution\nof macronutrients in fire ash from diverse ecosystems and speciation\nof ash and smoke from laboratory burning, exploring the variations\nand their causes. The speciation of phosphorus (P), calcium (Ca),\nand potassium (K) in fire ash from five globally distributed ecosystems\nwas characterized by using X-ray absorption spectroscopy and sequential\nfractionation. Aqueous dissolution of the macronutrients was measured\nby batch experiments at acidic and alkaline pHs. The results showed\nthat P existed mainly as Ca phosphates, Ca as double carbonates, calcite,\nand sulfates, and most K was associated with Ca carbonates. Mineralogy\nand the relative abundance of the species were primarily controlled\nby elemental stoichiometry and fire temperature. Differences in Ca\nand P speciation existed between ash and smoke from laboratory burning,\npossibly caused by the temperature difference and/or mass fractionation\nduring burning. The rates, extents, and pH dependencies of macronutrient\ndissolution differed among macronutrients and depended on their speciation,\nwith K being highly soluble and the P and Ca regulated by solution\npH. The variability in ash macronutrient chemistry and ecosystem-specific\nfire ash loads resulted in varying loads and availability of individual\nmacronutrient from fire among ecosystems. This study provides a mechanistic\nunderstanding of how fires transform the chemistry of macronutrients\nand affect macronutrient returns to soils across different ecosystems,\nwhich is essential for evaluating the disturbance to ecosystem nutrient\ncycling by fires.