Since nitrogen (N) typically limits plant growth in boreal and cold temperate forests, shifts in plant N availability and acquisition as forests age can alter ecosystem dynamics and functioning. In a series of wildfire chronosequence studies (14 sites, ranging from 1 to 72 y), we observed a pattern of declining natural abundance of the stable isotope 15N in jack pine (Pinus banksiana) foliage with stand age. We tested three hypotheses as potential mechanisms for this pattern: 1) wildfire consumes the 15N-depleted organic horizons leaving more enriched mineral soil N pools in the youngest sites; 2) higher nitrification rates in the younger sites drive an enrichment of the plant-available ammonium (NH4+) pool; or lastly 3) reduced ectomycorrhizal root colonization in the younger sites allows greater 15N transfer to plant hosts. To test these hypotheses, we measured natural abundance 15N in organic (Oa) and mineral (0-15 cm) soil samples across the chronosequences; foliar 15N in jack pine seedlings located on organic vs. mineral soil patches within a 1 y-old burn site; potential nitrification rates in incubated (25C, 28 d) organic and mineral soil samples and the resulting 15NH4+ pool; and percent ectomycorrhizal root colonization in seven of the chronosequence sites (3 to 57 y) and on the 1 y-old seedlings.
We found strong support for our first hypothesis, but little evidence to support the latter two. There was no difference in delta 15N values as a function of time in mineral or organic soil samples. Mineral soil samples were more enriched than the organic soils with mean delta values of 2.79 and -1.76 per mil, respectively. Likewise, jack pine seedlings growing on mineral soil patches had significantly higher foliar 15N values than those on organic horizons (P = 0.0067). In contrast, though more enriched 15NH4+ values corresponded to higher rates of potential nitrification, there was little net nitrification in the samples from the youngest sites. We found no relationship between percent ectomycorrhizal root colonization and foliar 15N in the 1 y-old seedlings. Overall, our results suggest a change in N source pools as these jack pine forests recover over time following fire, with a shift away from mineral soil to organic horizon N as stands age. Our findings further emphasize the importance of organic horizons in the functioning of these nutrient-poor systems.