Hydrogen Isotopes in New Zealand Soil: A method comparison

Abstract

The spatial characteristic of stable hydrogen and oxygen isotope ratios of global precipitation has been widely investigated. This study extends that investigation to tracing the flow of hydrogen isotope ratio patterns from precipitation through soil to groundwater, plants and food products. Along with these environmental components, the stable hydrogen isotope ratio of soil remains poorly studied. This is because of the nature of the soil matrix presenting with various living organisms, which gives rise to analytical challenges.

Three different experimental methods have been used in this study to study the stable hydrogen isotope ratio of soil, and to establish the correlation with local precipitation hydrogen isotope ratio. The methods are 1) SOM isotope analysis: demineralisation of soil with HF (HF method), 2) treatment of soil with hydroiodic acid: conversion of soil methoxyl groups to CH3I (HI method), and 3) compound-specific isotope analysis of n-alkane (n-alkane method). Compound-specific isotope analysis by extracting soil n-alkanes, has been one of the most common methods for studying soil nature; along with soil organic matter extraction. However, soil organic matter extraction has often involved using dangerous concentrated hydrofluoric acid. In this study 20.0 vol.% HF and 0.1 M diluted HCl were used for the demineralisation. Recently, the study of stable hydrogen isotope ratio of methoxyl groups has attracted great attention. This is because methoxyl hydrogen atoms are carbon bound so they are stable and non-exchangeable with ambient hydrogen.

The aim of this study was to 1) compare the accuracy between HF, HI and the n-alkane method, and 2) to choose the optimal one for establishing the hydrogen isoscapes of soil. The result of the comparison showed that the HF method and n-alkane method had a similar accuracy. The HF method gave the smallest standard deviation of repeatability of 1.2 ‰ and reproducibility of 1.1 ‰and the n-alkane method gave the smallest standard deviation of reproducibility of 1.1 ‰ and repeatability of 1.2 ‰. Taking the chemical safety into consideration; the HF method involved more procedural steps which increased the risk of acid spill. Moreover, hydrofluoric acid is one of the most dangerous chemicals and its use should be minimised to avoid any health and safety risk. Therefore, the n-alkane method with the use of a safer solvent, n-hexane, was the optimal method to prepare the soil for hydrogen isotope ratio study.

Soil δ2Hn-alkanes did not show a clear correlation with the δ2Hprecipitation throughout New Zealand. Furthermore, none of the available meteorological parameters measured by NIWA showed a relationship with the δ2Hn-alkanes. The main reason was the complexity of soil nature and the possibility that environmental factors might have had effects on the isotopic signature. Therefore, this study concluded that none of these methods were suitable for soil hydrogen isotope isotope study. A further method modification is required for preparing soil and perhaps the chemical, physical and biological nature of soil needs to be further studied beforehand.