The southwestern Tibetan Plateau (TP) is characterized by cold and dry alpine grasslands, yet how plants and soil nitrogen cycles adapt to conditions that are drier and colder than other parts of the TP remain unclear. We measured δ13C and δ15N in Astragalus, a widespread N-fixing legume, and soils along a 3500–5000 m transect in the southwestern TP. Plant δ13C and inferred intrinsic water-use efficiency (iWUE, 85.6–114.2 μmol CO2 mol−1 H2O) were primarily controlled by growing-season temperature (GST), with SEM showing a significant direct effect of GST on iWUE (standardized path coefficient = 0.45, p < 0.01). Conversely, soil δ15N (2.8‰ to 11.3‰) was negatively associated with mean annual precipitation (MAP; R2 = 0.26, p < 0.01), while plant–soil Δ15N was mainly driven by mean annual precipitation (MAP, R2 = 0.24, p < 0.01). These Δ15N patterns may reflect shifts in the relative contribution of atmospheric N2 fixation and soil N uptake by Astragalus, as inferred from isotopic evidence. These results suggest potential responses of plant carbon–water coupling and isotope-inferred N acquisition under future warming and drying in high-elevation ecosystems. • Plant δ13C-derived iWUE was mainly associated with growing-season temperature. • Soil δ15N decreased significantly with increasing mean annual precipitation. • Temperature and precipitation exerted divergent controls on carbon–water and nitrogen isotope signals.
Qu et al. (Tue,) studied this question.