Can direct 1H NMR identify the missing fraction of dissolved organic matter due to solid-phase extraction? An intercomparison of direct and SPE-DOM 1H NMR

Abstract

Coastal blue carbon ecosystems, such as salt marshes, mangroves, and seagrasses, play a crucial role in the global carbon cycles by sequestering and releasing dissolved organic matter (DOM). Gaining deeper insights into the molecular composition of DOM in these ecosystems is important for understanding its role in a warming climate. However, structural characterisation of DOM remains challenging due to its intricate nature, extraction-based biases, and shortcomings of standard analytical methods. This study explores the capability of direct proton nuclear magnetic resonance (¹H NMR) spectroscopy, in comparison to solid-phase extracted DOM (SPE-DOM) ¹H NMR, for analysing DOM in its entirety. The main goal is to identify the missing 40 % fraction of DOM due to SPE by using direct ¹H NMR. A total of 23 samples were analysed, obtained from three different coastal ecosystems in Malaysia, Germany, and Colombia. Comparison of direct and SPE-DOM ¹H NMR spectra revealed the loss of aliphatic and carboxyl-rich alicyclic molecules (CRAM) during SPE. One sample also showed a loss of carbohydrates due to SPE. Aromatic and unsaturated structural groups were not detectable in direct ¹H NMR due to their low concentrations at natural abundance. Therefore, the missing 40 % fractions of DOM during SPE likely covers all structural classes. Additionally, direct ¹H NMR revealed significant variability among samples, reflecting ecosystem-based diversity in the molecular composition of DOM at natural abundance. In contrast, produced more uniform spectra, suggesting the retention of similar compound classes across samples.