Elevational shifts in foliar-soil δ15N in the Hengduan Mountains and different potential mechanisms

The natural abundance of stable nitrogen isotopes (delta N-15) provides insights into the N dynamics of terrestrial ecosystems, the determination of which is considered an effective approach for gaining a better understanding ecosystem N cycling. However, there is currently little information available regarding the patterns and mechanisms underlying the variation in foliar-soil delta N-15 among mountain ecosystems. In this study, we examined the determinants of foliar-soil delta N-15 in association with N transportation rates along an elevational gradient in the Hengduan Mountains. Despite the relatively high levels of available N produced from high N fixation and mineralization, we detected the lowest levels of foliar delta N-15 at 3500 m a.s.l., reflecting the stronger vegetation N limitation at medium high elevations. The enhanced vegetation N limitation was driven by the combined effects of higher microbial immobilization and inherent plant dynamic (the shifts of delta N-15 in vegetation preference, including vegetation community) with changing climate along the elevational gradient. Unexpectedly, we established that soil delta N-15 was characterized by an undulating rise and uncoupled correlation with foliar delta N-15 with increasing elevation, thereby indicating that litter input might not be a prominent driver of soil delta N-15. Conversely, soil nitrification and denitrification were found to make a more pronounced contribution to the pattern of soil delta N-15 along the elevational gradient. Collectively, our results serve to highlight the importance of microbial immobilization in soil N dynamics and provide novel insights that will contribute to enhancing our understanding of N cycling as indicated by foliar-soil delta N-15 along elevational gradients.

Automated summary

This study investigated the determinants of foliar-soil delta N-15 and N transportation rates at different elevations in the Hengduan Mountains. The results showed that vegetation N limitation was stronger at medium high elevations due to higher microbial immobilization and shifts in vegetation preference. Surprisingly, soil delta N-15 was not strongly correlated with foliar delta N-15, indicating that litter input may not be the main driver of soil delta N-15. Instead, soil nitrification and denitrification played a more significant role in shaping the pattern of soil delta N-15 along the elevational gradient.