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.