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.Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 32130069Funding provided by: Strategic Priority Research Program A of the Chinese Academy of Sciences*Crossref Funder Registry ID: Award Number: XDA26010102Funding provided by: EU H2020 Marie Skłodowska-Curie Actions*Crossref Funder Registry ID: Award Number: 839806Funding provided by: Aarhus University Research FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100002739Award Number: E-2019-7-1Funding provided by: Danish Independent Research Foundation*Crossref Funder Registry ID: Award Number: 1127-00015BFunding provided by: Nordic Committee of Agriculture and Food Research*Crossref Funder Registry ID: Award Number: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 O2 and 15N2 , Control samples were received unlabeled N2 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 15N, to which 2 mL of either (15NH4)2SO4 or K15NO3 solution was evenly added at an amount equivalent to 20 mg N kg-1. Thereafter, one sub-sample of each 15N 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+ and NO3- concentration and 15N 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