Lignin dimers as potential markers for 14C-young terrestrial dissolved organic matter in the critical zone

Abstract

The origin, molecular composition and fate of dissolved organic matter (DOM) provides essential information that links surface and subsurface processes and explores the functioning of the Critical Zone. Therefore it is important to identify specific marker compounds that provide information on the temporal and spatial linkages in the Critical Zone. Here, we used ultra-high resolution mass spectrometry and accelerator mass spectrometry in order to identify markers for fast transport of surface-derived DOM through the Critical Zone. We assessed the molecular composition and radiocarbon age of solid phase extracted DOM (SPE-DOM) from forest top soils. The14 C ages of SPE-DOM in our study were similar to that of bulk DOM. Calibrated ages ranged from 0 to 49 years and the within-site variability was larger than between sampling sites. Spearman rank correlation between the14 C ages and the DOM composition identified 129 sum formulae that were significantly correlated to14 C age. We found that molecular entities with younger14 C ages had lower molecular weight, higher unsaturation and less oxygen and heteroatoms than those associated with older14 C ages. A chemical library search suggested that phenylpropanoids and compounds, which are known to be lignin derived, are key molecular species for terrestrial DOM with young14 C ages. Among them, lignin dimers emerged as prominent surface-derived compounds that can potentially be used as markers for fast transport of water and DOM into the subsurface and groundwater.