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

Abstract

The concentration of strongly fixed‐ammonium (NH4f) in soil is not high enough to allow direct injection of soil digests into automated 15N analyzers. Investigations were therefore conducted to develop microdiffusion protocols primarily as a preconcentration step in NH4f‐15N analysis of soils using continuous flow–isotope ratio mass spectrometry (CF–IRMS). The 5 M HF:1 M HCl soil digest containing NH4f was treated with 10 M KOH in a 140‐mL polypropylene specimen container, modified to collect NH3‐N in a petri dish containing H3BO3 indicator solution which was then quantified by acidimetric titration. For 15N analysis, however, acidified Whatman GF‐D filter paper discs substituted for the H3BO3 indicator solution trap, which were subsequently processed for direct analysis via CF–IRMS. With the modifications described, analyses (quantitative and 15N) 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 NH4‐N and was more dependent on digest volume than on its N content. Fractionation of 15N 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 15N 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 15N analysis of the NH4f fraction in soils.