ARA and 15N2 tracer assays are both

ARA and 15N2 tracer assays are both classical and well-established techniques for quantifying N2 fixation in environmental samples. While 15N2-tracer assays are commonly used in marine science , its application in terrestrial systems lags behind that of the ARA. Here, we assessed the applicability of the 15N2-tracer assay on low-activity soil samples in comparison to the ARA to determine the optimal (more sensitive) method for quantification of diazotrophic activity. For this comparison, 15N2 tracer assays and ARAs were performed on the same batch of samples, the latter after 2 or 5 days of incubation with 15N2. Ethylene was not detected in soil samples supplemented with fructose or artificial root exudate (Fig. 1A), yet about 200 ppm ethylene was detected in the control (liquid culture of Anabaena torulosa; Fig. 1B). Ethylene concentrations in controls remained constant throughout the incubation; hence illustrating that ethylene was not consumed (Fig. 1A). In contrast, N2 fixation activity was detected in these samples by analysis of 15N incorporation in bulk soil samples using IRMS (15N2-tracer assay) showing enrichments ranging on average from 14& to 18& (day 2) and 20& to 27& (day 5) compared to natural abundance controls (0.5& on average) (Fig. 1C).