The influence of fire on the radiocarbon signature and character of soil organic matter in the Siskiyou National Forest, Oregon

Background/Question/Methods

Forest fire frequency is likely to increase under forecasted conditions of global climate change. In addition to volatilizing aboveground biomass and litter layers, forest fires have a profound effect on belowground C pools and the cycling of soil organic matter as a whole. However, the influence of fire on belowground organic matter cycling is not well defined and varies widely with fire intensity. To investigate the effects of fire on soil organic matter characteristics, we examined soils from a mixed hardwood/evergreen forest across a range of burn intensities. Root size and abundance, C and N concentrations, litter layer loss and soil charcoal abundance were measured to examine the effect of burn intensity on soil organic matter characteristics. Rock fragment abundance was measured in all soils to estimate post-fire erosion of soil fines. In addition, soils were density separated into light and heavy fractions. Density fractions were radiocarbon dated, and ¹⁴C was used as a conservative tracer to address the following questions: i) When soils burn, which C is volatilized and which is transformed to charcoal? ii) How does fire change the distribution of soil organic matter among density fractions and depths? and iii) What is the source of ash input to the mineral soil following fire?

Results/Conclusions

Results indicated that increasing burn intensity led to progressive loss of forest floor mass, but not to progressive loss of C from the mineral soil. C distribution among density fractions and the ¹⁴C content of the respective fractions indicated that the most significant source of charcoal in soils is the transformation of light fraction organics in situ, not incorporation of charred litter layer organics. Though fire significantly increased the charcoal content of the soils, fire did not significantly change the distribution of soil organic matter among density fractions.  Other significant changes in soil organic matter characteristics included a progressive enrichment in N and fine roots with increasing burn severity, most likely due to encroachment of N fixing shrubs following the loss of native vegetation. Increased concentrations of rock fragments in burnt areas are suggestive of erosion in these areas, consistent with previous studies documenting varying degrees of soil erosion following fire. In addition, soils from severely burnt plots were depleted in ¹⁴C in comparison to soils from unburnt plots. This ¹⁴C depletion is most likely the combined result of erosion and preferential combustion of organics enriched in ¹⁴C relative to the bulk soil.