Stability and erodibility of model aggregates as affected by water repellency and heating duration

Abstract

Soil water repellency is commonly observed with some plant species (ex: pines, casuarina, eucalyptus, etc.) that are highly vulnerable to wildfires. The heat generated by these fires can alter the physical properties of soil aggregates influencing the erodibility of soil. These alterations can be highly dependent on the temperature as well as the duration of heating. This experiment aimed to investigate the effects of heating durations and the water repellency (WR) level on water stability of aggregate (WSA%) and the floating time (FT) when immersed in water, for water repellent model aggregates. In this experiment, non-repellent soil (Rhodudults) was hydrophobized using powdered dried Casuarina equisetifolia leaf-litter (CE) at two different rates (T1:10% CE; T2: 25% CE) to obtain two WR levels, with a control (T0: 0% CE). Model aggregates, prepared using acrylic cylindrical blocks, were subjected to three heating durations (30, 60, 120 min) at 150°C temperature with at 400°C/h heating rate. The WSA%, FT, and WR, of heated and non-heated aggregates were measured. Data were analyzed considering complete randomized design (ANOVA; p≤0.05). Control aggregates were non-repellent and not floated before and after heating. Before heating, T1 aggregates were slightly repellent and not floated, while T2 aggregates were slightly repellent and floated (~15s). After heating WR, as measured by water drop penetration time (WDPT), increased from ~2 s to ~15 s in T1, and from ~10 s to ~70 s in T2. After heating, FT increased up to 1-2 s and 70-170 s, respectively, in T1 and T2, compared to 0 s in control. However, both repellency and floating time of those aggregates decreased with the increasing heating duration. The WSA% of both repellent and non-repellent aggregates increased after heating and the percentage increase in WSA% decreased with the increasing heating duration. The percentage increase of WSA% at 30, 60, and 120 min durations, respectively, were 5.07, 3.48, and 1.7% for T0, 5.91, 4.91, and 3.14% for T1, and 9.84, 9.24, and 8.1% for T2. Strong positive relationships were observed between WSA% and WR for both T1 (R2 = 0.90–0.99) and T2 (R2 = 0.88–0.97) samples under all three heating durations. Aggregates of T2 showed the highest WR, FT, and percentage increase of WSA%, of all aggregates. Higher WR, FT, and percentage increase in WSA% was observed at 30 min heating duration than at 60 and 120 min. Water-repellent aggregates heated around 30 minutes seemed to have more potential to be eroded via surface run-off than non-repellent aggregates. Further experiments are required to determine the effects of different heating temperatures.