Microdiffusion of Strongly Fixed Ammonium in Soil‐Acid Digests Prior to Automated Nitrogen‐15 Analysis

The concentration of strongly fixed‐ammonium (NH 4f ) in soil is not high enough to allow direct injection of soil digests into automated 15 N analyzers. Investigations were therefore conducted to develop microdiffusion protocols primarily as a preconcentration step in NH 4f ‐ 15 N analysis of soils using continuous flow–isotope ratio mass spectrometry (CF–IRMS). The 5 M HF:1 M HCl soil digest containing NH 4f was treated with 10 M KOH in a 140‐mL polypropylene specimen container, modified to collect NH 3 ‐N in a petri dish containing H 3 BO 3 indicator solution which was then quantified by acidimetric titration. For 15 N analysis, however, acidified Whatman GF‐D filter paper discs substituted for the H 3 BO 3 indicator solution trap, which were subsequently processed for direct analysis via CF–IRMS. With the modifications described, analyses (quantitative and 15 N) were performed on 5‐ to 20‐mL samples of 5 M HF:1 M HCl soil digests. Recovery was quantitative in 48 to 96 h with up to ∼525 μg NH 4 ‐N and was more dependent on digest volume than on its N content. Fractionation of 15 N was effectively nonexistent within the diffusion periods used except only for immediately (a few hours) into the diffusion; suggesting that the need to optimize diffusion is not that critical to the accuracy of the 15 N results provided a sufficient quantity of N is obtained for the CF‐IRMS. Additionally, diffusion times were slightly affected by soil type, indicating a mild matrix effect probably because of mineralogical variation among soils. These diffusion methods therefore proved to be very accurate and reliable for practical use as a preconcentration step prior to 15 N analysis of the NH 4f fraction in soils.