Evaluating the direct and indirect effects of wildfires on soil biological activity and ecosystem regeneration

There is substantial uncertainty regarding the effects of wildfire on forest ecosystem regeneration. Some studies suggest that fires can prime ecosystems for regeneration whereas others suggest that the loss of organic matter as a result of wildfires limit regeneration. Which reaction occurs likely depends on the severity of the fire and, as we suggest here, the climate of the region. We studied a forest in central Texas, where two recent fires have occurred (2011 and 2015) and the hot, dry summers represent an analog for the future climate of many forest regions. Due to the role of the soil biological community in nutrient breakdown and cycling with an ecosystem, soil biological activity is an important indicator of recovery after wildfires. Two methods of measuring soil respiration were used as proxies for biological activity: field-based CO₂ flux measurements and lab-based microcosm incubations. Soil temperature, water content, total C, N, δ¹³C, and pH were measured to determine the impact of wildfires on these variables and the impact of changes in these variables on biological activity. All samples for lab analysis were taken from the top 5 cm of the soil. Decreases in total soil organic matter (SOM) were observed in burned soils, however, lab-based respiration measurements (which controlled for temperature and water content) suggest that decomposition rates in soils burned in the 2011 fire are similar to unburned soils and decomposition rates in soils burned in the 2015 fire are only slightly reduced. On the other hand, field measurements indicate respiration rates in burned soils were much lower than they were in unburned soils during hot and dry months due to differences in soil temperature and water content between burned and unburned soils. Increased temperatures in burned soils as a result of the removal of canopy cover, the removal of organic matter insulation and the deposition of black ash on the soil surface cause more severe water limitation in burned soils. We conclude that, although the composition of the soils was not impacted enough to reduced microbial activity where the burns occurred, the surrounding environment was disturbed enough to have severe indirect effects on the soil, most importantly increased heat absorption which led to lower water contents and ultimately lower respiration rates. Because this forest represents an analog for future climates, the data presented here suggest that soil ecosystem regeneration will be slowed after wildfires due to extreme temperature and water limitation.