Fire severity effects on nutrient dynamics and microbial activities In a Siberian larch forest

Abstract

High-latitude ecosystems store large amounts of carbon in soil organic matter and are among the most vulnerable to climate change. In particular, fire severity and frequency are increasing in boreal ecosystems, and these events are likely to have direct and indirect effects on climate feedbacks via increased emission of carbon (C) from soil and changes in vegetation composition, respectively. In this study we created experimental burns of three severities in the northeastern Siberian arctic, near Cherskiy, RU, and quantified dissolved C, nitrogen (N), and phosphorus (P), and microbial respiration and extracellular enzyme activities at 1-day, 8-days, and 1-year post-fire. Our objective was to determine how fire affects C, N, and P pools, soil microbial processes, and how these effects scale across severity and time since fire. We found labile C and nutrients increased immediately post-fire, but appeared similar to unburned controls within a week. Phosphorus alone remained elevated through 1-year post-fire. Leucine aminopeptidase activities initially increased with fire severity, but by 1-year, activities decreased with fire severity at a rate an order of magnitude faster. Fire severity suppressed phosphatase and β-glucosidase activities at all time points. Soil respiration was reduced by half in high severity plots 1-year post-fire, while net rates of N mineralization increased by an order of magnitude. We found that changes in soil C and nutrient pools, soil respiration, and net N mineralization rates responded in a threshold-fashion to fire severity, although P was uncoupled from C and N by changing at a distinct severity threshold. Extracellular enzyme activities and edaphic variables scaled linearly with fire severity. The interaction of threshold and linear response curves to fire severity may help explain the variability across studies in soil microbial community responses to fire. Microbial communities recovering from more severe fires have the possibility to decrease future ecosystem C losses through reduced respiration. The changing fire regime in permafrost ecosystems has the potential to alter soil microbial community dynamics, the retention of nutrients, and the stoichiometry of C, N, and P availability.