Low-Intensity, Fire-Induced Changes of Soil Structure, Physical and Hydraulic Properties.

Few studies have quantified pre- and post-fire-induced soil conditions and causes of reduced infiltration capacity or hydraulic conductivity (e.g., increased hydrophobicity or soil structure deterioration), particularly with pre- and post-fire measurements at the same site. Such measurements were made in August 2009 for a low intensity controlled burn in the Upper Gleason Creek Watershed in the eastern Great Basin in Nevada, USA. Soil hydraulic and physical property measurements included Ks made with a mini-disc-tension infiltrometer, ka using an air permeameter, hydrophobicity, bulk density, percent organic matter, and soil moisture. The objectives were to (1) measure and compare Ks and in situ ka and other soil properties at interspace and undercanopy microsites immediately before and after the fire; and, (2) use these comparisons to quantify the importance of post-fire soil structure and hydrophobicity changes and its affect on hydraulic properties. The burn was conducted in a transition zone between an Artemisia spp. shrub community and pinyon-juniper (Pinus monophylla-Juniperus osteosperma) woodlands. Before the burn, the soil structure exhibited moderate to strong coarse subangular blocky structure with hard dry consistency down to 7 cm depth. The structure deteriorated after the fire to a weak medium subangular blocky structure that was soft in dry consistency. Results at undercanopy sites show a slight hydrophobicity and incomplete combustion of organic matter, but no significant changes in soil properties. However, results at interspace microsites show a decrease in bulk density and an increase in Ks and ka. These changes could be attributed to the expansion of vaporized water through the soil, which could break up aggregates and deteriorate the soil structure. Thus, the mechanisms that contributed to these changes—immediately after the fire—differ from those that lead to soil alteration 6 to 12 months later. We hypothesize that seasonal changes (precipitation, soil loading from snowpacks, and temperature) will reverse these soil property changes with time, such that Ks, ka, and porosity will decrease, and bulk density will increase. More research is needed that details the causes and effects of these two phases and at what critical point the mechanisms change.