Despite widespread support for the role of organic nitrogen (N) as a significant plant N source, its impact on interpretations of foliar δ15N signatures is rarely considered. Global scale analyses infer that foliar δ15N signatures increase with increasing rates of N cycling, and that ectomycorrizal (ECM) and ericoid-mycorrhizal (EM) plants are more δ15N depleted than arbuscular mycorrhizal (AM) and non-mycorrhizal plants. Despite these global patterns, neither of these mechanisms is supported within northeastern US temperate forests. Here AM plants are more δ15N depleted than ECM plants. N cycling rates are also lower in ECM than AM tree stands, in contradiction with the globally observed positive correlation between rates of N cycling and foliar δ15N signatures. These patterns, as well as other recent data suggest that there may be a need to reconsider which mechanisms drive variation in foliar δ15N.
We compiled net mineralization, gross nitrification, and denitrification rates, as well as fractionation factors for these processes to calibrate a Rayleigh isotope fractionation model. This allowed us to predict the δ15N signatures of specific soil N forms in tropical, temperate forest, and arctic ecosystems. We then check to see if variation in pool δ15N signatures can quantitatively explain variation in foliar d15N, explicitly including the possibility that plants take up significant amounts of organic N.
Results/Conclusions
Rayleigh model estimations of organic, ammonium, and nitrate δ15N signatures suggested that variation in the δ15N signatures of these soils N forms are enough to explain variation in foliar δ15N across tropical, temperate, and arctic systems without an ECM/EM fractionation mechanism. Sensitivity analysis shows the model is sensitive to the magnitude of the fractionation factor chosen, which varies considerably for nitrification and denitrification. Despite this sensitivity we were able to place bounds on the percent contribution of individual N forms to total plant N nutrition, demonstrating that foliar δ<15N signatures of the three systems we modeled can A) be explained by δ15N signatures of available N forms alone and B) supports a dominant role of organic N in total plant N nutrition. We believe this provides important quantitative support for the role of organic N in plant N nutrition and highlights the necessity of considering organic N sources when interpreting variation in foliar δ15N signatures.