Persistence of dissolved organic matter explained by molecular changes during its passage through soil

Dissolved organic matter affects fundamental biogeochemical processes in the soil such as nutrient cycling and organic matter storage. The current paradigm is that processing of dissolved organic matter converges to recalcitrant molecules (those that resist degradation) of low molecular mass and high molecular diversity through biotic and abiotic processes. Here we demonstrate that the molecular composition and properties of dissolved organic matter continuously change during soil passage and propose that this reflects a continual shifting of its sources. Using ultrahigh-resolution mass spectrometry and nuclear magnetic resonance spectroscopy, we studied the molecular changes of dissolved organic matter from the soil surface to 60 cm depth in 20 temperate grassland communities in soil type Eutric Fluvisol. Applying a semi-quantitative approach, we observed that plant-derived molecules were first broken down into molecules containing a large proportion of low-molecular-mass compounds. These low-molecular-mass compounds became less abundant during soil passage, whereas larger molecules, depleted in plant-related ligno-cellulosic structures, became more abundant. These findings indicate that the small plant-derived molecules were preferentially consumed by microorganisms and transformed into larger microbial-derived molecules. This suggests that dissolved organic matter is not intrinsically recalcitrant but instead persists in soil as a result of simultaneous consumption, transformation and formation

Character and environmental lability of cyanobacteria-derived dissolved organic matter

Autotrophic dissolved organic matter (DOM) is central to the carbon biogeochemistry of aquatic systems, and the full complexity of autotrophic DOM has not been extensively studied, particularly by high-resolution mass spectrometry (HRMS). Terrestrial DOM tends to dominate HRMS studies in freshwaters due to the propensity of such compounds to ionize by negative mode electrospray, and possibly also because ionizable DOM produced by autotrophy is decreased to low steady-state concentrations by heterotrophic bacteria. In this study, we investigated the character of DOM produced by the widespread cyanobacteriaMicrocystis aeruginosausing high-pressure liquid chromatography-electrospray ionization-high-resolution mass spectrometry.M. aeruginosaproduced thousands of detectable compounds in axenic culture. These compounds were chromatographically resolved and the majority were assigned to aliphatic formulas with a broad polarity range. We found that the DOM produced byM. aeruginosawas highly susceptible to removal by heterotrophic freshwater bacteria, supporting the hypothesis that this autotroph-derived organic material is highly labile and accordingly only seen at low concentrations in natural settings

Photochemical degradation of natural organic sulfur compounds (CHOS) from iron-rich mine pit lake pore waters - an initial understanding from evaluation of single-elemental formulae using ultra-high-resolution mass spectrometry.

In order to better understand the chemical diversity of dissolved organic matter (DOM) in iron-rich mine waters, a variety of sediment pore waters was analysed by means of ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). A considerable number of the DOM elemental formulae were found to contain sulfur. In a rather simplified experiment, DOM was exposed to sunlight in the presence of dissolved ferric iron, which is common in the oxygenated acidified epilimnetic waters of mine pit lakes. The photochemical alteration of the CHOS (carbon-, hydrogen-, oxygen- and sulfur-containing) compounds was then categorised by following the changes in signal intensity of mass peaks. Nearly 20,000 elemental compositions were identified and sorted into the following categories: totally degraded, partially degraded, not significantly degraded, minor new photoproducts, and newly formed photoproducts. A large proportion of the CHOS compounds were found to be entirely degraded; the degradation ratios exceeded those of the CHO compounds. The pools of totally degraded compounds and those of newly formed products were contrasted with respect to photochemically relevant mass differences. These results indicate that photochemical loss of sulfur-containing low molecular weight compounds can be considered likely. One feasible explanation is the photodegradation of sulfonic acids within the CHOS pool eventually leading to the release of sulfate