Taking the heat: Soil microbial communities and soil biogeochemistry after wildfire and postfire logging in central Oregon

Background/Question/Methods

Fire helps reduce dead and accumulated vegetation and enriches the soil by releasing nutrients bound in litter.  But when fuel loads are high and wildfires burn hot, problems may arise.  It is well established that severe wildfire negatively impacts soil nutrient pools; however, the effect of postfire timber removal on soil productivity is not well understood and its application remains highly controversial among land managers, scientists, and the interested public.  Postfire logging recoups the economic value of timber killed by wildfire, but whether such forest management activity supports or impedes forest recovery in stands differing in structure from historic conditions remains unclear.  The aim of this study was to determine the impact of mechanical logging after wildfire on soil bacterial and fungal communities and other measures influencing soil productivity.  We compared soil bacterial and fungal communities and biogeochemical responses of 1) soils compacted, and 2) soils compacted and then subsoiled, to 3) soils receiving no mechanical disturbance, across seven stands in a mixed-conifer forest in central Oregon, 1-3 years after postfire logging.

Results/Conclusions

Soil strength of the sandy loam volcanic soils was greatest in the compacted treatment and least in the subsoiled treatment.  Compaction decreased plant-available nitrogen (N) on average by 27% compared to no mechanical disturbance, while subsoiling decreased plant-available (Bray) phosphorus (P) on average by 26% compared to the compacted and non-mechanically disturbed treatments.  Neither bacteria nor fungal richness significantly differed among treatments.  A shift in bacterial communities corresponding with an increase in plant available N and P suggests that soil microbes in these postfire landscapes are resilient to mechanical disturbance.  Results suggest that nutrients critical to soil productivity were reduced by mechanical applications used in timber harvesting, yet soil bacteria and fungi, essential to mediating decomposition and nutrient cycling, appeared resilient to mechanical disturbance.  Clearly, effects of postfire timber harvesting on soil microbes, nutrients, and processes warrant longer term investigation.  Management decisions about whether or not to harvest fire-killed trees should be balanced with the recovery potential of a site, and the potential for high densities of fire-killed trees to increase the area of severely burned soil in the event of future fire.