Sediment aggregation and water quality in wildfire-affected river basins

Off-site transfer of nutrient-rich burnt soil has implications for downstream water quality. Coarsening of effective particle size (EPS) distributions in burnt material via aggregation of fines into composite particles modifies post-fire sediment and nutrient transport dynamics. Experiments were undertaken to establish temperature controls on wildfire-enhanced soil aggregation. Burnt and unburnt soil from a temperate eucalypt forest were analysed for EPS and settling velocity using a LISST-ST (Laser In Situ Scatter and Transmissometry with Settling Tube) particle size analyser. Next, samples were burnt (250-550 degrees C) before further analysis with the LISST-ST. Settling velocities of naturally burnt soil aggregates were greater than unburnt aggregates of the same EPS. Experimental burning indicated that dense water-stable aggregates form at relatively low temperatures (250 degrees C) probably due to distillation and carbonisation, through pyrolysis, of organic volatiles in surface litter. Under these conditions, the EPS distribution of burnt surface soil coarsens with up to 50% of the < 63-mu m fraction becoming aggregated. A positive relationship between 'plant-available' phosphorus and burn temperature was observed. Given that a large proportion of soil particulate phosphorus is associated with the < 63-mu m fraction, fire-related aggregation processes have potentially important implications for post-fire fine sediment and nutrient transport and storage dynamics.