Jack pine foliar δ15N indicates shifts in plant nitrogen acquisition after severe wildfire and through forest stand development

Abstract

Natural abundance of the stable nitrogen (N) isotope 15N can elucidate shifts in plant N acquisition and ecosystem N cycling following disturbance events. This study examined the potential relationship between foliar δ15N and depth of plant N acquisition (surface organic vs. mineral soil) and nitrification as conifer stands develop following stand-replacing wildfire. We measured foliar δ15N along an 18-site chronosequence of jack pine (Pinus banksiana) stands, 1 to 72 years in age post-wildfire. Foliar δ15N was compared to total δ15N of the organic (Oe + Oa) and mineral (0–15 cm) soil horizons, and organic horizon N mineralization and nitrification as functions of total mineralization. Foliar δ15N declined with stand age, yet wildfire effects were heterogeneous. Jack pine seedlings on burned, mineral soil patches in the youngest stand were significantly more enriched than those on unburned, organic patches (P = 0.007). High foliar values in the youngest stands relative to mineral-horizon δ15N indicate that nitrification also likely contributed to seedling enrichment. Our results suggest jack pine seedlings on burned patches obtain N from the mineral soil with potentially high nitrification rates, whereas seedlings on unburned patches and increasingly N-limited, mature jack pine acquire relatively more N from organic horizons.