Method to label soil cores with 15NH3 gas as a prerequisite for 15N isotopic dilution and measurement of gross N mineralization

Application of 15N isotopic dilution techniques to measure gross N mineralization require the soil 14NH4+ pool to be uniformly enriched with 15N for accurate results. Solutions containing 15N labelled salts have typically been added to soil which may also be mixed to ensure uniform distribution. This approach is not ideal for course-textured soils where the addition of solution can result in enhanced microbial activity and increased N mineralization in many circumstances. A technique and equipment were developed to enrich the soil 14NH4+ pool by injecting 15N enriched NH3 gas into intact soil cores. This technique was tested in two course-textured Western Australian agricultural soils with low organic matter contents. Injection of NH3 enabled gross N mineralization to be measured without major soil disturbance or change in the soil water content and proved suitable for use under field conditions. The gas injector distributed a mixture of 15N-labelled NH3 and air throughout the surface 0–12 cm of soil cores. The soil 14NH4+ pool was enriched with 15N as the 15NH3 dissolved to form 15NH4+. A series of independent tests were made to determine that the injector delivered a constant amount of 15N to assess uniformity in the vertical and radial distribution of 15NH4+ throughout soil cores and to examine the effect of NH3 concentration on the rate of gross N mineralization and gross NH4+ consumption. The injector was found to apply a constant amount of 15NH4+ to replicate soil cores. Distribution of the 15NH3 within intact soil cores was more uniform in the loamy sand compared with sandy-clay loam. An increase in NH3 concentration up to four times the proposed working concentration did not significantly alter the rate of gross N mineralization or NH4+ consumption; differences in gross N mineralization rates with respect to NH3 concentration in the injected gas typically reflected the variability in the amount of microbial biomass-N present within undisturbed soil cores. The injection of small quantities of NH3 in this technique appeared to minimize the effect of NH3 fixation and subsequent release on the measurement of gross N mineralization. Nevertheless, it is recommended that users of this technique examine whether the release of fixed NH3 is likely to cause an error in the estimate of gross N mineralization for other soils and rates of injection of NH3.