Mechanism of Exposing Previously Physically Protected Soil Organic Matter By Breakdown of Soil Aggregates during Low-Intensity Burns.

The number of natural and controlled low-intensity fires is on the rise in many arid and semi-arid parts of the Southwest United States. Yet, the ecological ramifications of these burns are not well understood since the effects of low-intensity fires on soil physical properties, in particular, soil structural and aggregate stability, are often considered benign. However, we recently reported significant deterioration of soil aggregates subjected to rapid low-intensity heating (<200°C) when compared to aggregates heated to the same temperature but at a slow heating rate. This was the result of high internal gas pressure rise within the aggregates due to rapid vaporization of pore water that exerted mechanical stress on the aggregates. Previous studies have also indicated that breakdown of soil aggregates are known to expose physically protected soil organic matter. Therefore, we hypothesized that physical breakdown of soil aggregates by such mechanism will expose previously physically occluded organic matter to leaching and microbial breakdown. Here, we present total organic carbon content of soil leachate from soil aggregates subjected to low-intensity burns. We compared a wide range of heating rates and peak temperatures. In addition, we report a novel approach in measuring soil microbial respiration rate of these soils in the laboratory using a low-cost CO₂ flux sensor. Our observation to date indicates that rapid heating rate causes more leaching of soil organic matter and an increase in microbial respiration rate when compared to slow heating rate. These observations show that soil organic matter from weakly aggregated soils from arid and semi-arid regions are very vulnerable to breakdown after low-intensity burns.