Soil Structural Degradation during Low-severity Burns

Jian, Mathew and Berli, Markus and Ghezzehei, Teamrat A.

Geophysical Research Letters, vol. 45(5553-5561) , 2018.

Abstract

Low-severity wildfires and prescribed burns have been steadily increasing for over three decades, currently accounting for more than half of total burned area in the Southwestern United States. Most observations immediately after low-severity burns report little adverse impacts on soil properties and processes. In a few studies, however, significant deterioration of soil structure has been observed several months after such fires. Here we show that rapid vaporization of pore water during low-severity burns raises pneumatic gas pressure inside large aggregates (20-30 mm) to damaging levels; on the order of aggregate tensile strength and high-enough to cause visco-plastic deformation. However, the impact on soil structure was not immediately perceptible. This suggests that other natural forces, such as wetting-drying and thermal cycles, are required to disrupt the weakened aggregates. Thus, adverse consequences of the suggested mechanism on soil processes and services (e.g., infiltration, erodibility, and organic matter protection) are likely overlooked. ## Plain Language Summary Fire researchers and the public are often concerned about the increasing threat of fires that are considered moderate to high in intensity—a measure fuel energy—and in severity—a measure of impact on ecosystems. What does not get much attention is, however, that the area impacted by low‐severity and low‐intensity fires is also on the rise. Currently, more than half of the burned areas in the Southwestern United States are low‐severity wildfires and controlled burns. These fires last only for a few minutes and the soil surface temperature rarely exceeds 200o C and does not heat more than a few centimeters of soil. Because of this low energy input, the effect of low‐severity burns on soil quality has been generally assumed negligible. This study was motivated by long‐term monitoring studies in Spain and United States that showed significant disaggregation of surface soil occurred several months after low‐severity burns. Here we show that soil water is vaporized very rapidly when moist soil aggregates are subjected to rapid heating. The escaping vapor momentarily creates disruptive stress that may exceed the bonds that hold soil aggregates together, causing disaggregation and loss of soil functions such infiltrability, resistance to erosion, and protection of organic matter.