The ¹⁵N gas-flux method to determine N₂ flux: a comparison of different tracer addition approaches

The ¹⁵N gas-flux method allows for the quantification of N₂ flux and tracing soil N transformations. An important requirement for this method is a homogeneous distribution of the ¹⁵N tracer added to soil. This is usually achieved through soil homogenization and admixture of the ¹⁵N tracer solution or multipoint injection of tracer solution to intact soil. Both methods may create artefacts. We aimed at comparing the N₂ flux determined by the gas-flux method using both tracer distribution approaches. Soil incubation experiments with silt loam soil using (i) intact soil cores injected with ¹⁵N label solution, (ii) homogenized soil with injected label solution, and(iii) homogenized soil with admixture of label solution were performed. Intact soil cores with injected ¹⁵N tracer solution show a larger variability of the results. Homogenized soil shows better agreement between repetitions, but significant differences in ¹⁵N enrichment measured in soil nitrate and in emitted gases were observed. For intact soil, the larger variability of measured values results rather from natural diversity of non-homogenized soil cores than from inhomogeneous label distribution. Generally, comparison of the results of intact cores and homogenized soil did not reveal statistically significant differences in N₂ flux determination. In both cases, a pronounced dominance of N₂ flux over N₂O flux was noted. It can be concluded that both methods showed close agreement, and homogenized soil is not necessarily characterized by more homogenous ¹⁵N label distribution.