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.

Separation of total nitrogen from sediments into organic and inorganic forms for isotopic analysis

“…32.8%) as a result of partial dissolution of clay edges (Paramasivam and Breitenbeck, 2000). A method avoiding KOBr pretreatment has been applied to soil samples, whereby the soil is digested with HF-HCl and the ammonium in the acid solution is recovered for isotopic analysis using steam distillation under alkaline conditions (Eudoxie and Gouveia, 2001). However, steam distillation with strong alkali was found to partially recover acid soluble ON (Bremner, 1959;Paramasivam and Breitenbeck, 2000).…”

Distillation and Microdiffusion Modifications for total Nh4f Quantification and 15N Recovery

“…Microdiffusion study of organically amended direct soil-acid digests: Eudoxie, G.D. and G.A. Gouveia [6] observed non-significant effects of N quantity on diffusion period, which could have been masked (in their experiments) by the stronger effect of volume. In these incubations, volume was constant, which might have resulted in a recognizable quantity effect.…”

“…Microdiffusion study of organically amended direct soil-acid digests: Nitrogen recovered as total NH 4 from amended soil-acid digests increased steadily with time for all treatments ( Table 4). The sampling period was selected to span 192 h [6] , allowing for the slower diffusion rate expected due to the larger volume [16] and lower molarity of alkali. Total NH 4 values after 192 h were similar to recovery via distillation.…”

“…This component of the total NH 4f was removed with the KOBr washing along with organic N and extractable ammonium (NH 4e ) and could only be determined by calculating the difference in the amounts obtained quantitatively from the two methods. A proposed sub-classification [6] of total NH 4f into strongly and weakly held fractions was suggested and predicted expected differences in fraction behavior, which would have some influence on N availability and management in soils. Significant quantities of recently NH 4f could be weakly held [4] with a possible shorter existence, which may have tremendous implications for N management particularly on highactivity clayey soils.…”

“…Since strongly held NH 4f in soil-acid digests [5] were not concentrated enough to allow direct injection into automated 15 N analyzers, a microdiffusion protocol [6] was developed imitating in many aspects the original specimen container [7] . Though somewhat small for the required volume for microdiffusion of the extractable N fraction, the specimen container is suitable for soil-acid digest since soils usually contain higher levels of NH 4f and the concentration of N in the digest would be greater.…”