Simulated temperatures of forest fires affect water solubility in soil and litter

Abstract

As wildfires are of increasing concern in a warming world, there is a need to understand how fire temperatures affect solute concentrations of forest litter and soils in drinking water catchments. In addition, the concentrations are expected to be affected by time since the previous fire. We sampled soil and litter from recently (2 months) and less recently (4.5 years) burnt sites from jarrah forest in SW Australia. The samples were heated at 250°C, 350°C, and 500°C for 30min followed by leaching to determine solute compositions at these temperatures and in unburnt samples. At 250°C–350°C, we found increased concentrations of manganese (Mn), arsenic (As), total phosphorus (TP), phosphate (PO43-), ammonia (NH4+), potassium (K), calcium (Ca), mangesium (Mg), cobalt (Co), barium (Ba), sulphate (SO42-), alkalinity and dissolved organic carbon in soils, as well as of zinc (Zn), As, Ca, Ba, alkalinity, aluminium (Al) and chromium (Cr) in litter. At 350°C–500°C, divalent cations and organic carbon declined, while soils generated very high Al and Cr concentrations. The time following the fire was important, with the more recent fire generating higher concentrations. The elevated concentrations in 250°C–350°C were attributed to a decomposition of organic matter and mineral transformations, including CaCO3 formation. Based on thermodynamics, we propose a couple of burn severity indicators: activities of calcium and carbonates that are calculated from pH, alkalinity and Ca concentration. The indicators do not only show the degree of post-fire transformations, but they also inform on CaCO3 formation. Further studies include: (1) application to field data, (2) association with organic contaminants, and (3) validation in other geographical locations.