Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in delta(15)N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and delta(15)N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar delta(15)N varied widely, between 0 and -2% for nitrogen-fixing species, between 0 and -7% for deciduous non-fixing species, and between 0 and -11% for coniferous species. Relatively constant delta(15)N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant delta(15)N differences among species at individual sites. In fact, the largest variation among plant delta(15)N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low delta(15)N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar delta(15)N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T-r). Foliar and fine root nitro-gen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with delta(15)N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between delta(15)N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T-r.