Fire regimes in southwestern ponderosa pine forests have shifted in recent years from frequent, low-intensity surface fires to infrequent, stand-replacing fires; yet, few studies have investigated the long-term effects of this change on ecosystem processes. The purpose of our research was to identify temporal patterns in nitrogen (N) cycling dynamics following stand-replacing wildfires. We hypothesized that net N nitrification and mineralization rates would increase immediately following fire, but then decline over time to eventually resemble those of unburned forest. We also expected to find higher values of d¹⁵N in soil from recently burned forests signifying a more open N cycle. We employed a chronosequence of 7 sites that had experienced a stand-replacing wildfire between 1973 and 2005. Each site was paired with a nearby unburned control. We used modified resin cores to measure in situ rates of net N mineralization and nitrification in the mineral soil and organic horizon at two time periods over 12-months.

Results/Conclusions Net nitrification rates were consistently higher across the burned sites then the unburned controls, but no temporal trend was observed. Regression analyses demonstrated that rates of net ammonification and total mineralization began to converge with comparable unburned forest at approximately 30 years post-fire. Delta ¹⁵N levels were variable and do not support the post-fire patterns previously observed in this ecosystem. Our results suggest that stand-replacing wildfires have profound, long-term influences on N cycling in these ecosystems, which may have implications for post-fire regeneration.