The 15N-Gas flux method for quantifying denitrification in soil: Current progress and future directions

Denitrification in soil is a challenging process to quantify under in situ conditions, which seriously hampers the ability to accurately close or balance the nitrogen budget of terrestrial ecosystems. The ¹⁵N Gas Flux method is one of the best-suited techniques for in situ measurement of denitrification. Using a stable ¹⁵N-NO₃ - tracer injected or applied on the surface of soil under a closed static chamber, this method enables the measurement of both N₂O and N₂ denitrification fluxes. Its main limitation is certainly the poor sensitivity towards N₂ emissions, which is a common weakness of all denitrification measurement methods. We have also identified 4 assumptions on which this technique relies to be accurate: 1) the tracer is distributed homogenously in the confined soil volume, leading to the formation of a single isotopic pool in equilibrium, 2) absence of hybrid molecule forming processes, 3) quantitative recovery of produced denitrification products in the flux chamber headspace (no diffusive losses) and 4) no stimulatory impacts of nitrate tracer addition on the dynamics of the denitrification process. In this review, we revisit the principles of the ¹⁵N Gas Flux method, explore its evolution through time as well as the different models of calculation that have been developed; before assessing the impact of the 4 above-mentioned assumptions through literature compilation and simulation. Finally, we elaborate and discuss key technical aspects of this method to help the reader in understanding and optimally applying the ¹⁵N Gas Flux method for the measurement of denitrification.

The outcome of this review shows that in order to address the main limitation of the ¹⁵N Gas Flux method (poor N₂ sensitivity), a hybrid approach using an artificial N₂-depleted atmosphere in addition to ¹⁵N isotopic tracer seems to be a promising lead, although only a few studies have used it so far (even less so in the field). In particular, we demonstrate here the existence of a threshold of 10% atmospheric N₂ concentration background, below which the sensitivity increases drastically. We also show here that the 4 assumptions mentioned above are unlikely to be fully fulfilled in the field. The non-homogenous distribution of the ¹⁵N tracer in soil has been shown by various authors to cause a 25% underestimation of the rate of denitrification at maximum. Through simulation, we show here that the presence of hybrid molecules should have a moderate impact on total fluxes (N₂ or N₂O similarly) as long as they contribute for no more than 50% of the total emissions (at which point they cause a 12.5% overestimation). The underestimation of denitrification due to subsoil diffusion, which has been reported to be as high as 37%, remains a challenge to quantify. Finally, the impact of substrate isotopic tracer and water additions on a hypothetical stimulation of the denitrification process needs further validation. A decision tree has been implemented at the end of this study to help users applying the ¹⁵N Gas Flux method optimally. Overall, our findings show that this method still holds a substantial promise for a more accurate quantification of in situ denitrification whilst considering the recommended mitigation of the methodological weaknesses in future research.