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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.

Applying 15 N techniques to accurately determine the fate of fixed ammonium (NH 4f) in the strongly and weakly held pools require modifications to existing methodologies. Modifications are necessary for measurement of total NH 4f in soils by direct digestion with 5 M HF: 1 M HCl, excluding alkali pretreatment, followed by distillation and quantification of NH 4. Quantification by distillation was used as a precursor to optimize microdiffusion protocols for continuous flow-isotope ratio mass spectrometry (CF-IRMS). This paper reports on the modifications applied to these procedures since the direct 5 M HF: 1 M HCl digestion of soil samples may also dissolve some organic N fractions. Approach: Distillation followed by 15 N microdiffusion trials were conducted on soil digests amended with rice husks, manure, compost or glycine, using different molarities (2, 5, and 10 M) and volumes (5, 10, 15, 25, 32.5, and 40 mL) of KOH. Results: The distillation study identified 32.5 mL of 2 M KOH to be the optimum volume and molarity of KOH that must be combined with 10-mL aliquots of direct 5 M HF: 1 M HCl digests of each of seven soils to ensure that only NH 4 in the digest is recovered and none of the organic N is hydrolyzed during the process. Results also showed that a minimum incubation time of 192 h was needed to trap approximately 100 µg 15 N on the disks for subsequent accurate analysis by CF-IRMS, with minimal recovery of organic N. Conclusions/Recommendations: These findings support the use of a direct-digestion/distillation method to quantify total NH 4f and thereby provide opportunity to distinguish between strongly and weakly held NH 4f in soils.

Aglime (agricultural lime), commonly applied to acid soils to increase the soil pH and productivity, may lead to the release of CO2 into the atmosphere or to carbon (C) sequestration, although the processes involved are not fully understood. As large acreages of arable land are limed annually, exploring soil management practices that reduce aglime-induced CO2 emissions from acid soils while maintaining or improving the soil quality is paramount to mitigating the effects of global climate change. This study, therefore, assessed the effects of organic residues and ammonium on CO2 emissions and soil quality indicators in two limed soils. Two contrasting acid soils (Nariva series, Mollic Fluvaquents and Piarco series, Typic Kanhaplaquults) were amended with varying combinations of aglime (0% and 0.2% w/w CaCO3), organic residue (0% and 5% w/w biochar or poultry litter), and NH4-N (0% and 0.02% w/w) and were incubated in 300 mL glass jars for 31 days. The sampling for CO2 was performed on 11 occasions over the course of the incubation, while soil sampling was conducted at the end. The results indicate that aglime application significantly (p < 0.05) increased the cumulative CO2 emissions in all cases except with the addition of poultry litter. Alternatively, ammonium did not regulate the effect of aglime on CO2 emissions, which was l because of the low rate at which it was applied in comparison to aglime. The results also showed that poultry litter significantly (p < 0.05) increased the soil electrical conductivity (EC), available nitrogen (N), and pH, especially in the Piarco soil, while the hardwood biochar had little to no effect on the soil properties. Our findings indicate the potential for utilizing poultry litter to reduce the impact of aglime on CO2 emissions while improving the soil quality. Further studies utilizing 13C to trace aglime CO2 emissions are, however, required to identify the mechanism(s) that contributed to this reduction in the emissions.

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