Data was acquired from an isotope ratio mass spectrometer using Isodat 3.0 software.

Particulate Nitrogen calculations:
The equation describing the linear regression of the standards' observed delta 15N PN and Assigned delta 15N (obs = x and assigned = y) was used to calculate delta 15N PN (y) of the samples from the uncorrected delta 15N (x).

Refer to the following publication for more information:
Dabundo, R., Lehmann, M.F., Treibergs, L., Tobias, C.R., Altabet, M.A., Moisander, P.H., and Granger, J. 2014. The Contamination of Commercial 15N2 Gas Stocks with 15N–Labeled Nitrate and Ammonium and Consequences for Nitrogen Fixation Measurements. PLoS ONE, 9(10): e110335. doi:10.1371/journal.pone.0110335

Summary of methods from Dabundo et al. 2014:
Reagents:
Four lecture bottles of 98+ at% 15N-labeled N2 gas were purchased from Sigma-Aldrich, three from lot # SZ1670V, and one from lot # MBBB0968V. Two 1L lecture bottles of 98+ at% 15N2 were purchased from Cambridge Isotopes from lot #’s I1-11785A and I-16727. One 1L lecture bottle of 98+ at% 15N2 was purchased from Campro Scientific from lot # EB1169V. Ammonium and nitrate solutions were prepared with salts or with solutions obtained from different distributors: sodium nitrate (NaNO3), potassium nitrate (KNO3), and ammonium chloride (NH4Cl) from Fisher Scientific; analytical-grade potassium nitrate from Fluka Analytical and a gravimetric solution of ammonium chloride from SPEX CertiPrep.

Dunaliella tertiolecta cultures:
The marine green alga Dunaliella tertiolecta was cultured in growth media equilibrated with 15N2 gas in order to ascertain the susceptibility of 15N-labeled gas contaminants to assimilation by non-N2-fixing organisms. Culture medium was prepared from filtered Long Island Sound sea water supplemented with 50 umol/L NaNO3, 36.3 umol/L NaH2PO4*H2O, and 107 umol/L Na2SiO3*9H2O, as well as f/2 trace metals and f/2 vitamins, added from filter sterilized stock solutions. Medium (200 mL) was dispensed in 250 mL stoppered glass bottles. Experimental treatment bottles were equilibrated overnight with 0.2 mL 15N2 gas from either a Cambridge Isotopes or Sigma-Aldrich lecture bottle. Following inoculation, cultures were left loosely capped and placed on a windowsill with exposure to natural light. Nitrate concentrations were monitored daily. Upon the complete depletion of nitrate, 8 days after inoculation, the cultures were harvested on pre-combusted GF/F filters. Filters were dried at 60 degrees C for 18 hours pending N isotopic analysis of the particulate nitrogen.

Particulate nitrogen isotope ratio analyses:
The d15N of particulate nitrogen (PN) was analyzed using a Costech Instruments elemental combustion system (model 4010) coupled to a Thermo Scientific Delta V Advantage IRMS. Analyses were standardized using L-glutamic acid reference materials, USGS-40, and USGS-41.

particulate_N.csv