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

The natural abundance of stable nitrogen isotopes (δ15N) 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 δ15N among
mountain ecosystems. In this study, we examined the
determinants of foliar-soil δ15N 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 δ15N 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 δ15N in vegetation preference,
including vegetation community) with
changing climate along the elevational
gradient. Unexpectedly, we established that soil
δ15N was characterized by an undulating rise and
uncoupled correlation with foliar δ15N with increasing
elevation, thereby indicating that litter input might
not be a prominent driver of soil δ15N. Conversely, soil
nitrification and denitrification were found to make a
more pronounced contribution to the pattern of soil
δ15N 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 δ15N along elevational gradients.