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

Abstract

International audienceSpatio-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

Groundwater control of biogeochemical processes causing phosphorus release from riparian wetlands

“…Our observations are consistent with previous laboratory experiments that have shown that soil rewetting after a dry period could release organic and inorganic P as a result of microbial biomass being killed by osmotic shock (Blackwell et al, 2010;Turner and Haygarth, 2001). Previous monitoring campaigns in the same watershed, but focusing on the composition of dissolved organic matter using ultraviolet spectrometry, carbon isotopes and molecular biomarkers highlighted the release of microbial-derived organic compounds in the soil solution at the same time of the year (Jeanneau et al, 2014;Lambert et al, 2013). Hence, the most probable mechanism causing P release when the water table rose at the beginning of the hydrological season was mobilization of a P pool of microbial origin, limited in size, which can migrate under water saturated conditions (McGechan et al, 2005).…”

“…This indicates that reductive dissolution of Fe (hydr)oxides is the mechanism causing the P release observed under anoxic conditions (Carlyle and Hill, 2001). Anoxic conditions at this time of year were due to low rainfall in a period of high water table, which decreased water velocity and thus water oxygenation (Jeanneau et al, 2014). As a result, Fe (hydr)oxides became solubilized as Fe 2þ (Bourri e et al, 1999), which released previously adsorbed P (Hoffmann et al, 2009;Vidon et al, 2010).…”

“…an important P sorbing compound in acidic soils (Li et al, 2012;Surridge et al, 2012). Anoxic conditions resulting from a high water table and low flow velocity can cause reductive dissolution of Fe (hydr)oxides in RW soils (Jeanneau et al, 2014;Knorr, 2013;Lambert et al, 2013;Li et al, 2012), which may cause release of previously adsorbed P (Carlyle and Hill, 2001;Hoffmann et al, 2009).…”

A comparative study on the pore-size and filter type effect on the molecular composition of soil and stream dissolved organic matter

“…1) well-known hydrologic and biogeochemical context and has already been the subject of numerous investigations of the sources and dynamics of both soil and stream DOM (Morel et al, 2009;Lambert et al, 2011Lambert et al, , 2013Lambert et al, , 2014Aubert et al, 2013;Jeanneau et al, 2014Jeanneau et al, , 2015. Table S1).…”

New molecular evidence for surface and sub-surface soil erosion controls on the composition of stream DOM during storm events

“…FAs with a high molecular weight (> C19 : 0) are from plant origin while FAs with a low molecular weight (< C19 : 0) come from microbial origin except for C16 : 0 and C18 : 0 which can be derived from both (Frostegard et al, 1993;Zelles, 1999). The proportion of microbial markers among the analysed compounds (f mic ) was calculated according to Jeanneau et al (2014).…”