Patterns and controls of foliar nitrogen isotope composition on the Qinghai-Tibet Plateau, China

Abstract

Determine if relationships between temperature, precipitation and foliar nitrogen isotopes (δ15N) observed in various ecosystems also operate in the high-cold Qinghai-Tibet Plateau; measure variation of foliar δ15N with altitude, and examine relationships differences among plant functional groups (PFGs). Foliar δ15N at 82 grassland sites were determined and grouped for PFGs to determine relationships with temperature, precipitation and altitude. Residual foliar δ15N were used in regressions to eliminate effects of non-target variables. For relationships with potential breakpoints, piecewise regressions were used. Relationships of foliar δ15N with temperature and precipitation differed among PFGs. Foliar δ15N of C3 species respectively increased and decreased with August temperature (TEM8) and precipitation (PRE8) across sites with TEM8 ≥ 9.95 °C and PRE8 < 145.7 mm, whereas it was invariant across sites with TEM8 < 9.95 °C and PRE8 ≥ 145.7 mm. Foliar δ15N decreased with altitude. Complex relationships of foliar δ15N with climate factors suggest caution in its use to monitor nitrogen cycling. Although the altitude pattern of foliar δ15N appeared to be mainly induced by temperature and precipitation, there are other altitude related factors in Qinghai-Tibet high elevations that influence foliar δ15N. Determining the functions of these factors provides foci for future research.