Molecular tagging: a novel method for molecular-level characterisation of near-natural, drained and rewetted peatlands

Abstract

It is difficult to overstate the ecological and environmental importance of peatlands; they represent the largest worldwide terrestrial carbon store, a natural filter for our drinking water, and a home to unique flora and fauna.

Damage to these ecosystems, primarily occurring due to human activity, threatens the release of carbon stocks into the atmosphere and natural waters, as well as the destruction of habitats. In order to better understand the mechanisms underlying these changes, indicators of peat status and quality need to be defined. Only then can we successfully direct the restoration of drained peatlands, and continue the protection of those in near-natural condition. The logical indicator of peat quality is the molecular composition of the natural organic matter (NOM) it consists of, but to date such characterisation remains elusive due to its nature as a complex mixture.

Progress towards structural elucidation of NOM is highly desirable. The most viable analytical tool to achieve this endeavour is NMR spectroscopy, which is capable of assigning a resonance to each individual compound. However, using NMR experiments currently at our disposal, characterisation of individual compounds is hindered by the presence of thousands of similar molecules.

Carbon and hydrogen represent the most abundant elements in NOM with NMR-active nuclei. However, the low abundance of 13C and narrow chemical shift range of 1H prevent interpretation of standard NMR spectra. Herein we utilise a combination of molecular tagging procedures such as methylation (using 13C-enriched methyl groups) and uorination, applying these to peat samples in order to probe the molecular structure by inserting NMR-active nuclei and applying advanced NMR techniques.

The use of 19F in particular is highly desirable due to its excellent sensitivity, wide chemical shift range and long-range J coupling constants. These techniques allow us to filter out signals from untagged molecules, and achieve drastically improved sensitivity using a range of NMR experiments. By applying these to samples from sites assessed as drained, under restoration and near natural, we can identify individual molecular structures and track the concentration differences of these molecules between the different sites.

Methylation reactions have been successfully applied to a range of samples and the most abundant compounds identified, as well as those which exhibit notable trends with respect to depth or site. This data is used to suggest the structures of compounds which may be of importance within the degradation process and point to potential areas for future research. Also, several potential candidates for reactions incorporating uorine are investigated and discussed, along with the development of a number of novel NMR techniques which could be used to probe molecular structures of 19F-tagged molecules. In addition, a number of more traditional methods for characterisation of natural organic matter, including solid state NMR, are applied to the same samples for comparison and assessment of the information they provide.