Global mycorrhizal status drives leaf δ¹⁵N patterns

Nitrogen (N) availability, which can be represented by the natural abundance of the stable N isotope (δ¹⁵N value), 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 modelling have not included mycorrhizal types. Here, we produced a global map of foliar δ¹⁵N based on machine learning driven by climatic, edaphic, vegetation and dominant mycorrhizal factors. The predicted global average foliar δ¹⁵N value was 0.48 ± 0.27‰. Plants in tropical areas were predicted to have significantly higher foliar δ¹⁵N values than plants from subtropical, temperate and boreal areas. The mean annual temperature was identified as the primary driver of the spatial pattern of foliar δ¹⁵N. These results provide isotopic evidence for greater N limitations in temperate and boreal regions than in 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 and then ericoid mycorrhizae. Synthesis. 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 better understanding of N cycling dynamics between plant types and global ecosystems.