Seven plants δ2Halk values in Naqu from 2015 to 2019

The δ2Halk values of seven co-occurring herbaceous plants (angiosperms) collected from 2015 to 2019 located in Naqu, Central Tibet. They were grown annually from seed within a circular sample area in climate chambers under controlled environmental conditions (Bai et al., 2019). These species were Poa pratensis (Pl, Gramineae), Kobresia parva (Kp, Cyperales), Potentilla bifurca (Pb, Rosales), Potentilla saundersiana (Ps, Rosales), Lagotis gaertn (Lg, Tubiflorae), Stipa purpurea (Sp, Gramineae), and Kobresia humilis (Kh, Cyperales). The mature leaves of 26 herbaceous plant species in the ambient condition experiments were collected from 10–20 September in 2015, 2016, 2017, 2018 and 2019. Leaf-wax n-alkane analysis was conducted by desiccating leaf samples by placing them in paper envelopes for seven days at 40℃, following which they were ground using a centrifugal mill (Retsch ZM 200) and stored in self-sealing low-density polyethylene (LDPE) bags until lipid extraction could be performed. 60 KHZ frequency ultrasonic waves were applied for approximately for 10 min to 0.5 g of the dried plant leaf material. This was done three times. The dry extracts were re-dissolved in 1 ml of CH2Cl2 before column chromatography using a column of neutral alumina over activated silica gel (100–200 mesh) was employed to fractionate the solution into aliphatic, aromatic and polar, and acidic fractions. The saturated, aromatic and polar, and acidic fractions were eluted with hexane (50 ml), CH2Cl2 (50 ml) and CH3OH (20 ml), respectively. The instrument used was a GC-thermal conversion isotope ratio mass spectrometer [GC-thermal conversion-isotope-ratio mass spectrometry (GC/TC/IRMS)]. A HP5890 gas chromatograph equipped with a HP-1 MS column (60 m × 0.32 mm i.d., 0.5 μm film thickness, Agilent), along with a high temperature pyrolysis unit connected on-line to a Thermo Delta V Advantage isotope ratio mass spectrometer, were used for plant leaf analyses