Constraints on the Sources and Production Mechanisms of Dissolved Organic Matter in Soils from Molecular Biomarkers

Abstract

Spatio‐temporal modifications of the composition of dissolved organic matter (DOM) from a wetland in an agricultural catchment were investigated using thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide coupled to gas chromatography and mass spectrometry (THM‐GC–MS). The results were compared with previous data acquired on the same samples using ultraviolet spectroscopy and the stable C isotope ratio. The correlation between molecular markers and bulk‐scale descriptors validated the use of THM‐GC–MS to investigate the fate of DOM in soils. Molecular proxies, including lignin markers, tannin markers, carbohydrates, and fatty acids, were classified into plant‐derived and microbial markers. At the beginning of the hydrologic year, associated with the recharge period, the DOM composition was similar along the depth profile and >70% of the analyzed markers were from plant‐derived origins. The rise in the water table was associated with a shift from vertical to horizontal water flow due to water saturation and resulted in a clear vertical heterogeneity, i.e., a varying composition throughout the profile. In the surface horizons, the proportion of plant‐derived markers remained >70%, while in the deep horizon this proportion was <30%. Last, the lowering of the water table resulted in the homogenization of the DOM composition along the depth profile.