DOI: 10.5061/dryad.rbnzs7hdk
title: Elevational shifts in foliar-soil δ15N in the Hengduan Mountains and different potential mechanisms

description: 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.
description: Plant leaf and fine root
 samples were washed with deionized water to remove dust particles and then
 dried at 60℃ for more than 48 h prior to further analysis. Soil samples
 were sieved through a 2-mm sieve, and having removed the residual
 vegetation, then dried at 60℃ to a constant weight. Both vegetation (i.e.,
 leaves, litter, and roots)
 and soil samples were finely ground using a GT300 ball mill (PowteQ Inc.,
 China) prior to isotope measurements. All N isotope and
 concentration analyses were carried out using an isotope ratio mass
 spectrometer.  5-g samples of fresh soil were placed into
 20-mL glass tubes, the headspace of which was replaced with a
 synthetic gas comprising
 O<sub>2</sub> and <sup>15</sup>N<sub>2</sub> , Control samples were received unlabeled N<sub>2</sub> gas. The tubes were incubated at 20℃ in the dark for 9 days, and thereafter, the air and soil samples were collected and measured to calculate biological nitrogen fixation rate. Four sub-samples of fresh soil were labeled with <sup>15</sup>N, to which 2 mL of either (<sup>15</sup>NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> or K<sup>15</sup>NO<sub>3</sub> solution was evenly added at an amount equivalent to 20 mg N kg<sup>-1</sup>. Thereafter, one sub-sample of each <sup>15</sup>N labeled portion was extracted with 2 M KCl (T0), and the other was incubated in the dark at 20℃ for 1 day (T1). The soil extract was used for determing the content of NH4<sup>+</sup> and NO<sub>3</sub><sup>-</sup> concentration and <sup>15</sup>N abundance to calculate the gross mineralization, nitrification and microbial assimulation rate.  The copy numbers of denitrifying genes were determined based on quantitative the real-time PCR analysis. The detail information was in method.