Thermochemical Transformation\nof Calcium during Biomass\nBurning and the Effects on Postfire Aqueous Dissolution of Macronutrients

Calcium is commonly the most abundant element in fire\nresidues\nand its speciation largely determines the geochemical properties of\nfire residues and their effects on postfire soil chemistry. To explore\nthe effects of biomass composition and fire conditions on ash Ca speciation,\nthis study characterizes the speciation of Ca in charcoal and ash\nsamples that were derived from different plant compartments and thermal\nconditions, using Ca K-edge X-ray absorption near edge spectroscopy.\nResults showed that biomass contains abundant organic Ca complexes,\nwhich were mineralized into fairchildite and calcite after heating\nat 450 to 600 °C and then CaO, as temperature increased to 750\n°C. Apatite could be an abundant Ca species in fire residues\nif the Ca/P molar ratio of the biomass is small (<2). The mineralization\nof organic Ca to the identified Ca minerals during burning was negligibly\naffected by the oxygen level. Calcium speciation in prescribed fire\nresidues resembled that of biomass ash burned at 550 °C with\nsimilar Ca/P molar ratios. Batch experiments showed that macronutrients\n(Ca, Mg, K, and P) were differentially released, as a result of different\nsolubility of minerals in ashes and reprecipitation of minerals. The\naqueous solubility of Ca, Mg, and P decreased as pH increased from\n5 to 9, while K showed no pH dependency and was almost completely\nsoluble. Results from this study improve our understanding of the\nchemistry of fire residues and their geochemical behaviors, which\ncan help evaluate the impact of fire on postfire soil properties and\nmacronutrient cycling.