Nutrient release from combustion residues of two contrasting herbaceous vegetation types

Fire is a critical regulator of biogeochemical cycles in approximately 40% of the earth's land surface. However, little is known about nutrient release from combustion residues (ash and char) from herbaceous or grassland fires of varying intensity. Much of our knowledge in this area is derived from muffle furnace temperature gradient experiments. Therefore, we used two approaches (muffle and flame burning) to combust herbaceous biomass from contrasting nutrient level sites to estimate the forms and availability of nutrients after fire. Clear differences were measured in total and extractable nutrient concentrations in combustion residues of different plant types, with most carbon (C) and nitrogen (N) being volatilized (>99%), while P remained in high concentrations in the residues. Different combustion methods yielded contrasting results, where temperatures greatly affected nutrient quantity and form in muffle furnace residues, while relatively similar residues resulted from flame combustion at varying intensities. It was also found that only 5% of N and 50% of P remaining in flame combustion residues were extractable. Flame residues appeared to be composed of mixtures of materials (ash and char) created at low (<350 °C) muffle temperatures (extractable P forms), and high (>450 °C) muffle temperatures (pH, extractable potassium (K), and extractable NH(4)-N). We attribute dissimilar results of the combustion methods to heterogeneity of combustion (zones of low oxygen availability) and short duration (<300 s) of combustion characterizing natural fires in herbaceous, grassland systems. These results can be adapted to ecosystem level models to better predict nutrient changes that may occur after a fire event.