• Contamination of denitrogen gas bottles. Bottles 15N2 are nearly always sold at a purity of around 99% (and >97%  isotopic enrichment). However, that single remaining percent for foreign substance can turn out to be detrimental since it turned out that a significant fraction of it is in the form of 15N-labelled ammonia and nitrate \cite{Dabundo_2014}.
  • Typical SIP experiments involve using high substrate concentrations to achieve maximum labelling. Since 15N2 is also non-toxic, there is no limitation of supplying the incubation vials with atmospheric or even super-atmospheric concentrations of 15N2 gas (e.g. in anoxic incubations). However, this might not be necessary since even in very active systems only a small fraction of the denitrogen gas eventually gets fixed. To save up on costs, some of the gas can be replaced with another inert gas such helium or argon. We have incubted several types of soil under an atmosphere of 40:40:20 (15N2, He, O2) and noticed no difference in labelling compared to incbating the samples under 80:20 (15N2, O2; data not shown), but this should pobably be confirmed for every type of sample.