Short-term effects of experimental fire for a soil under eucalyptus forest (SE Australia)

The effects of different fire intensities on physiochemical soil properties have been studied in this research. The experiment was conducted in a eucalyptus forested area near the Namadgi National Park (ACT, Australia), and four fire intensities were achieved by adding different amounts of fuel load: 0kgm⁻² (control), 2kgm⁻² (low fire intensity), 4kgm⁻² (moderate fire intensity) and 8kgm⁻² (high fire intensity). Soil surface peak temperatures reached at each plot were 14, 142, 317 and 525°C, respectively. Immediate changes in soil properties under different fuel loads were studied and monitored monthly during a 7-month period. Average pH increased significantly with fuel load immediately after fire and decreased progressively to initial values. Changes in soil pH from burnt plots seem to be caused mainly by an ephemeral ash layer, and the recovery time has been relatively short. The complete oxidation of soil organic matter and the release of substantial amounts of cations after high fire intensity also contributed to increased pH. Only small variations in soil texture have been observed in this study even after high intensity fire, although the clay content decreased slightly during the experiment. This can be due to relatively low pre-burn clay content, but also to spatial variability of texture. In this case, post-fire erosion processes might have contributed to decreased clay content and are probably more important than other processes associated to temperature. Aggregate stability indices showed good correlation coefficients with the organic matter content. The role of organic matter as cementing agent of soil aggregates is extremely important after fire, since aggregate stability indices showed good correlation coefficients with the organic matter content. Low intensity fire enhanced soil water repellency. In this case, fire-enhanced soil water repellency persisted or increased progressively, but decreased below initial levels at the end of the experiment, but soil water repellency was completely destroyed after moderate or high fuel load treatments. The persistence of wettable conditions after destruction of soil water repellency can be considered as an index of fire severity, since it is a direct consequence of strong reduction of organic matter content.