Global mycorrhizal status drives leaf δ15N patterns

Nitrogen (N) availability, which can be represented by the natural
abundance of the stable N isotope δ15N, is crucial to understanding
ecosystem-level N dynamics. Specific ecosystems are dominated by different
types of mycorrhizae, which can relate to biogeochemistry and affect
ecosystem functioning. However, few studies have addressed the impacts of
different mycorrhizal associations on variations in foliar δ15N due to
climatic and soil physicochemical factors; prior instances of foliar δ15N
modeling have not included mycorrhizal types. Here, we used
machine learning to produce a global map of foliar δ15N based on climatic,
edaphic, vegetation, and dominant mycorrhizal factors. The
predicted global average foliar δ15N value was 0.69‰. Plants in tropical
areas were predicted to have significantly larger foliar δ15N values than
plants from subtropical, temperate, and boreal areas. The mean annual
temperature was identified as the primary driver of spatial foliar δ15N
patterns. These results provide isotopic evidence of greater N limitations
in temperate and boreal regions than tropical or subtropical regions.
Furthermore, non-mycorrhizal plant species had the highest foliar δ15N
values, followed by plants associated with arbuscular mycorrhizae, orchid
mycorrhizae, ectomycorrhiza, then ericoid mycorrhizae.  Overall,
changes in foliar δ15N were predicted to be closely associated with the
type of mycorrhizal association. This study highlights the importance of
incorporating mycorrhizal data to accurately assess patterns of foliar
δ15N on a global scale. Ultimately, our findings contribute to a greater
understanding of N cycling dynamics across plant types and global
ecosystems.