Finnigan delta 5 advantage isotope ratio mass spectrometer

The roots, stems, and leaves of ‘M9-T337’ seedlings labelled with ¹⁵N were washed three times with distilled water, fixed at 105°C for 30 min, and dried at 70°C to a constant weight before being ground into a powder. Then, 0.3 g of powdered material from each tissue type was digested using H₂SO₄-H₂O₂ and fixed in distilled water to a final volume of 50 mL. Five milliliters of the clear supernatant were removed to determine the total P content using the vanadium molybdate yellow colorimetric method (Zhang et al., 2020 (link)). The remaining 0.1 g of powdered material from each tissue type was used to determine the total N content and ¹⁵N abundance using a Flash 2000HT elemental analyzer coupled to a Finnigan DELTA V Advantage isotope ratio mass spectrometer (Thermo Fisher Scientific, Germany). The total N contents or ¹⁵N contents of the leaf, stem, and root were calculated by multiplying the N concentration or ¹⁵N concentration by the dry weight, respectively. ¹⁵N utilization efficiency (¹⁵NUE) was calculated as the ratio of the total ¹⁵N content in the seedling to the total ¹⁵N in the fertilizer.

C and N isotope analysis was carried out on a Finnigan Delta V Advantage Isotope Ratio Mass Spectrometer (IRMS, Thermo Fisher Scientific, Inc., Waltham, MA, USA) and a Flash 2000 HT Elemental Analyzer (Thermo Fisher Scientific, Inc., Waltham, MA, USA). The δ¹³C range represents the broadness of resources used in a community, and the δ¹⁵N range represents the trophic levels (i.e., food chain length) within a community. The δ¹³C range and δ¹⁵N range of native species in each river were first calculated as the differences between maximal and minimal values for δ¹³C and δ¹⁵N to compare the difference in resource use and food chain length between the invaded and reference rivers [8 (link),20 (link)]. Then, trophic niche size was calculated using two complementary methods. The total isotopic niche size (TA) was quantified as the total area within the convex hull shaping the community in the two-dimensional δ¹³C-δ¹⁵N space [20 (link)]. The core isotopic niche (SEA) was calculated as the standard ellipse area based on Bayesian statistics, which reflected the core of the trophic niche. SEA is less sensitive to extreme values than TA and provides complementary information about the isotopic niche of a community [15 (link)].