The Molecular Composition of Dissolved Organic Matter in Forest Soils as a Function of pH and Temperature

Abstract

We examined the molecular composition of forest soil water during three different seasons at three different sites, using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). We examined oxic soils and tested the hypothesis that pH and season correlate with the molecular composition of dissolved organic matter (DOM). We used molecular formulae and their relative intensity from ESI-FT-ICR-MS for statistical analysis. Applying unconstrained and constrained ordination methods, we observed that pH, dissolved organic carbon (DOC) concentration and season were the main factors correlating with DOM molecular composition. This result is consistent with a previous study where pH was a main driver of the molecular differences between DOM from oxic rivers and anoxic bog systems in the Yenisei River catchment. At a higher pH, the molecular formulae had a lower degree of unsaturation and oxygenation, lower molecular size and a higher abundance of nitrogen-containing compounds. These characteristics suggest a higher abundance of tannin connected to lower pH that possibly inhibited biological decomposition. Higher biological activity at a higher pH might also be related to the higher abundance of nitrogen-containing compounds. Comparing the seasons, we observed a decrease in unsaturation, molecular diversity and the number of nitrogen-containing compounds in the course of the year from March to November. Temperature possibly inhibited biological degradation during winter, which could cause the accumulation of a more diverse compound spectrum until the temperature increased again. Our findings suggest that the molecular composition of DOM in soil pore waters is dynamic and a function of ecosystem activity, pH and temperature.

Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls

“…The high molecular diversity characteristic of SOM is often attributed to the chemistry of plant-derived compounds and their decomposition byproducts464748. Whether microbial metabolites, secretions and cellular material can also produce chemically heterogeneous SOM remains unclear4849.…”

Fueling Diversity in the Subsurface: Composition and Age of Dissolved Organic Matter in the Critical Zone

“…Informative subsections of the DOM pool that potentially contain signaling information can be identified using nontargeted ultrahigh resolution mass spectrometry (HR-MS; Hertkorn et al, 2008Hertkorn et al, , 2013. HR-MS facilitates the analysis of the detailed molecular composition of DOM, generating information on individual molecular formulae and enabling the interpretation of molecular patterns across natural gradients (Jaffé et al, 2012;Ohno et al, 2014;Kellerman et al, 2015;Kothawala et al, 2015;Roth et al, 2015). Natural gradients of DOM age occur in groundwater following recharge events that lead to mixing or exchange of waters containing surface-sourced or in situ produced DOM (Sukhija et al, 2006;McMahon et al, 2011).…”

The Molecular Composition of Humic Substances Isolated From Yedoma Permafrost and Alas Cores in the Eastern Siberian Arctic as Measured by Ultrahigh Resolution Mass Spectrometry

“…The quantitative comparison of the occupation density of the six chemical class‐specific areas on the van Krevelen diagram (Figure ) enabled us to reveal the origin‐specific molecular features of the alkali‐extracted OM from the yedoma core versus the alas one. For HA representing the most abundant fraction of this OM (Table ), it was the presence of N‐containing saturated compounds usually referred to as aliphatic amines, peptides, and amino sugars (Roth et al, ), whose content increased with depth in the yedoma core, being negligibly small in the alas core. In addition, the much higher content of saturated CHO species (aliphatics) was observed in the yedoma HA fractions as compared to the alas ones.…”