Structural characteristics and behavior of fire‐modified soil aggregates

The transport dynamics of burnt material in fluvial systems require attention since the off‐site transfer of soil material from wildfire‐affected slopes includes ash and nutrients which may have negative impacts on downstream water quality. The aim of this work is to explore the presence, structural characteristics, and fluvial behavior of fire‐modified soil aggregates in burnt soil material sampled from a water supply catchment near Sydney, Australia, and to compare their fluvial behavior to that of composite particles from unburnt soils. Samples of burnt and unburnt soil material were analyzed for effective particle size, settling velocity, composite particle density, and porosity using established microscope‐based image analysis approaches. Burnt soil aggregates exhibit significantly higher settling velocities (mean 11.47 ± 1.11 mm s −1 ) than unburnt particles of similar diameter (3.36 ± 0.91 mm s −1 ) reflecting an increase in density because of reduction in organic content and a reduction in pore space linked to shrinkage. Soil aggregates in severely burnt soil are dense and inorganic and behave differently to their unburnt counterparts and discrete grains of the same size. Burnt composite particles are robust and readily transported within fluvial systems. Their presence in burnt soil and mobilized material is likely to have important implications for postfire fine (<63 μm) sediment and nutrient budgets.