Changes in soil structure and hydraulic properties in a wooded-shrubland ecosystem following a prescribed fire

Pre- and post-fire measurements were made for a low-intensity prescribed fire in a semiarid, shrub-woodland transition zone, and objectives were to: (i) determine changes in near-saturated hydraulic conductivity (K_f measured with a tension infiltrometer), air permeability (k_a measured with an air permeameter), and soil physical properties at shrub undercanopy and interspace microsites immediately before and after a fall burn and for a 13-mo period; and (ii) quantify the importance and effect of post-fire soil structure on hydraulic properties using pre- and post-fire measurements. At undercanopy microsites, structure deteriorated from a moderate to a weak subangular blocky structure after the fire that broke down to a structureless soil 10 mo later. At interspace microsites, post-fire soil structure deteriorated from a moderate-strong subangular blocky structure with hard dry consistency to a weak subangular blocky structure with soft dry consistency. After 10 mo, the intercanopy maintained a weak-moderate soil structure that became structureless-weak after 13 mo. Immediately after the fire, at both microsites, there was incomplete organic combustion, a decrease in bulk density, and an increase in k_a; however, at undercanopy microsites, there was no significant change in K_f even though there was a slight to moderate hydrophobicity, whereas at interspace microsites where no water repellency existed, K_f increased. These changes may be a result of expansion of vaporized water through soil pores that broke up aggregates, deteriorating soil structure. Thus, mechanisms that contributed to changes immediately and after the first year post-fire were different for low-intensity burns than for higher intensity burns.