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

BACKGROUND

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.

RESULTS

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.

CONCLUSIONS

These results suggest potential responses of plant carbon-water coupling and isotope-inferred N acquisition under future warming and drying in high-elevation ecosystems.