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 dinitrogen gas eventually gets fixed. To save up on costs, some of the gas can be replaced with another inert gas such as helium or argon. We have incubated several types of soil under an atmosphere of 40:40:20 (15N2, He, O2) and noticed no difference in labelling compared to incubating the samples under 80:20 (15N2, O2; data not shown), although this should probably be best confirmed for every type of sample.