Climatic controls on water-use efficiency and nitrogen acquisition of Astragalus on southwestern Tibetan Plateau

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.