During the past century, systematic wildfire suppression has decreased fire frequency and increased fire severity in the western United States of America. While this has resulted in large ecological changes aboveground such as altered tree species composition and increased forest density, little is known about the long-term, belowground implications of altered, ecologically novel fire regimes, especially on soil biological processes. To better understand the long-term implications of ecologically novel, high-severity fire, we used a 44-y high-severity fire chronosequence in the Sierra Nevada where forests were historically adapted to frequent, low-severity fire, but were fire suppressed for at least 70 years. High-severity fire in the Sierra Nevada resulted in a long-term (44+ y) decrease (>50%, p < 0.05) in soil extracellular enzyme activities, basal microbial respiration (56-72%, p < 0.05), and organic carbon (>50%, p < 0.05) in the upper 5 cm compared to sites that had not been burned for at least 115 y. However, nitrogen (N) processes were only affected in the most-recent fire site (4 y post-fire). Net nitrification increased by over 600% in the most recent fire site (p < 0.001), but returned to similar levels as the unburned control in the 13-y site. Contrary to previous studies, we did not find a consistent effect of plant cover type on soil biogeochemical processes in mid-successional (10-50 y) forest soils. Rather, the 44-y reduction in soil organic carbon (C) quantity correlated positively with dampened C cycling processes. Our results show the drastic and long-term implication of ecologically novel, high-severity fire on soil biogeochemistry and underscore the need for long-term fire ecological experiments.

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