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DIVISION S-8 - NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

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

Dep. of Food Production, Faculty of Agriculture and Natural Sciences, The Univ. of the West Indies, St. Augustine Campus, Trinidad

* Corresponding author (gouv{at}carib-link.net)

The concentration of strongly fixed-ammonium (NH_4f) 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 NH_4f-¹⁵N 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 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 NH_4f fraction in soils.

Abbreviations: ANCA, automated N and C analyzer • BC, Bejucal clay • CEC, cation-exchange capacity • CF-IRMS, continuous flow–isotope ratio mass spectrometry • CV, coefficient of variation • NH_4e, exchangeable ammonium • NH_4f, fixed ammonium • PFS, Piarco fine sand • REL, River Estate loam