Water Stability of Aggregates as Affected by Heating Temperature and Duration of Exposure in a Water Repellent Eucalyptus Surface Soil

Abstract

Water repellency (WR) in soils is a phenomenon caused by organic compounds that prevents the spontaneous penetration of water into soils. It is common under plant species such as Eucalyptus, Casuarina, and Pine that are containing high amounts of waxes and resins in their litter materials. These species are also known to be highly prone to wildfires. Depending on heating dynamics, fire-generated heat may alter soil properties. WR and aggregate stability (AS) are two of the properties that are reported as affected by heat. Furthermore, WR in soils and AS are reported mostly as positively related. This study aimed to examine the effects of different heating temperatures (TH) and exposure durations (DE) on WR, AS, and their interrelation, using a water-repellent Eucalyptus grandis forest soil. Water-repellent aggregates (diameter: 3-5 mm) collected from the surface (0-5 cm) soil were exposed to four temperatures (150, 200, 250, 300°C) separately for three durations (30, 60, and 120 min). The WR was determined using the molarity of an ethanol droplet test up to its minimum measurement of 90° contact angle, and water drop penetration time test for samples with contact angles <90°. The percentage of waterstable aggregates (%WSA) was determined using wet sieving apparatus. The WR decreased with increasing TH and DE up to 200°C. Samples became non-repellent at temperatures ≥250°C under all DE levels. The %WSA increased up to TH of 250°C, while decreased at 300°C under all DE levels. The change in %WSA under shortest DE (30 min) was lower than that at longer DE (60, 120 min). With increasing TH, the relation between SWR and the % change in WSA increased up to 200°C, then slightly decreased up to 250°C, and became negative beyond that for all DE levels. Results revealed that although WR of aggregates decreased with heating, AS did not always decreased, where the relation was not essentially positive as claimed in previous reports. The DE did not show significant impact on WR, AS, or their interrelation. Further studies are necessary with more DE and TH levels to verify these impacts and exact interrelations.