Soil microbial and organic matter responses to short-interval reburns in the Pacific Northwest, USA

The area burned by high severity wildfire is increasing in many regions on the planet as a product of fuel accumulation, fire suppression, and climate change. As more forested land is impacted by high severity wildfires, there is greater potential for short-interval reburns, where the same area is burned by two or more wildfires within 20 years. Though reburn effects on vegetation are gaining significant research attention, it remains unclear how soil biogeochemical processes will respond to short-interval reburns. Studies in some forests have shown that short-interval reburns drive compounding losses of soil carbon (C) and nitrogen (N), but results vary based on ecosystem type, age, and fire dynamics. A key uncertainty is how reburn history influences dissolved organic carbon (DOC) quantity and composition and microbial respiration, which together influence C processing and organic matter (OM) cycling in soils.

To address this knowledge gap, we quantified differences in soil biogeochemistry across soils from forest stands that experienced zero, one (in 2023), or three (in 2003, 2017, and 2023) wildfires within a 20-year period, and classified those stands as unburned, long-interval reburn, and short-interval reburn, respectively. These fires occurred in the Pacific Northwest, USA, in wet conifer forests with higher productivity than most previously studied reburns. We collected five replicate soil samples from 0–5 cm mineral soil depths and quantified microbial biomass C and N, soil organic C and N, and OM concentrations, pH, and DOC and total dissolved N concentrations. Additionally, we carried out a 35-day lab incubation to quantify microbial CO₂ respiration and net inorganic N fluxes. Finally, we characterized the chemical quality of DOC using excitation-emission indices and parallel factor analysis.

While wildfire decreased soil C and microbial biomass C and N in both short-interval and long-interval reburns, we observed no effect of fire nor short-interval reburn on soil N, pH, or OM. However, soil from short-interval reburn sites had lower DOC concentrations (F_2,12 = 14.5, p < 0.001) and CO₂ fluxes (F_2,10 = 26.6, p < 0.001) than both long-interval reburn and unburned stands. Chemical quality analyses indicated that “fresh” DOC comprised a larger proportion of overall DOC contents after short-interval reburn (F_2,10 = 4.2, p = 0.048) compared to long-interval reburn, with similar “freshness” between unburned and short-interval reburn soils.

Taken together, our preliminary results suggest that the short-interval reburn soils exhibited lower DOC concentrations and suppressed microbial respiration. Interestingly, these lower CO₂ fluxes were not fully explained by microbial biomass C and N, which appeared to be buffered, possibly due to less fuel consumption during the third fire. Instead, we hypothesize that reduced DOC quantity, rather than DOC composition (“freshness”), was the primary constraint on microbial processing under our experimental conditions. As such, carbon quantity appears to exert stronger control than DOC composition. These results suggest that slower decomposition may facilitate soil C retention following short-interval reburns. Our findings have implications for soil recovery trajectories, as decreased microbial processing may contribute to rebuilding soil OM over time after short-interval reburns.