Direct Evidence for Temporal Molecular Fractionation of Dissolved Organic Matter at the Iron Oxyhydroxide Interface

Abstract

While the importance of organic matter adsorption onto reactive iron-bearing mineral surfaces to carbon stabilization in soils and sediments has been wellestablished, fundamental understanding of how compounds assemble at the mineral interface remains elusive. Organic matter is thought to layer sequentially onto the mineral surface, forming molecular architecture stratified by bond strength and compound polarity. However, prominent complexation models lack experimental backing, despite the role of such architecture in fractionated, compound-dependent persistence of organic matter and modulating future perturbations in mineral stabilization capacity. Here, we use kinetic assays and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry under high temporal frequency to directly detect the molecular partitioning of organic matter onto an iron oxyhydroxide during adsorption. We observed three sequential intervals of discrete molecular composition throughout the adsorption reaction, in which rapid primary adsorption of aromatic compounds was followed by secondary lignin-like and tertiary aliphatic compounds. These findings, paired with observed differential fractionation along formulas nitrogen and oxygen content and decreasing selective sorption with reaction time, support "zonal" assembly models. This work presents direct detection of sequential molecular assembly of organic matter at the mineral interface, an important yet abstruse regulator of carbon stabilization and composition across temporal and spatial scales.

High-pH and anoxic conditions during soil organic matter extraction increases its electron-exchange capacity and ability to stimulate microbial Fe(III) reduction by electron shuttling

“…The presence of Fe(II) at the end of extraction in all anoxic extracts suggested that the higher amount of extracted OM is probably related to microbial Fe(III) mineral reduction and the release of mineralbound OM during mineral dissolution. A correlation between the dissolved organic carbon (DOC) concentration and the amount of Fe(II) in pore water was reported before for sediment samples that were incubated in the dark under anoxic conditions for 14 d (Dadi et al, 2017). In situ monitoring of the DOC flux in pore water of marine sediment or freshwater wetland also suggested an increase in DOC with increasing microbial iron(III) mineral reduction (Burdige et al, 1992(Burdige et al, , 1999Chin et al, 1998).…”

Effects of extraction conditions on the redox properties of soil organic matter (SOM) and its ability to stimulate microbial iron(III) mineral reduction by electron shuttling

“…The SOM extracted anoxically showed a higher aromaticity, suggesting that the additional organic matter mobilized by reductive dissolution of iron minerals possesses a higher degree of aromaticity. This is in line with findings described by other studies (Gu et al, 1994;Lv et al, 2016;Avneri-Katz et al, 2017;Coward et al, 2019).…”

“…This difference could be caused by the higher content of aromatic functional groups in HA than in FA and the resulting differences in sorption properties. OM with higher aromaticity and larger molecular weight was shown to have a higher absorption affinity to ferrihydrite (Lv et al, 2016;Coward et al, 2019). Since our HA extracted under all conditions https://doi.org/10.5194/bg-2019-308 Preprint.…”

Metabolic tradeoffs and heterogeneity in microbial responses to temperature determine the fate of litter carbon in simulations of a warmer world

“…Our modeled responses to warming thus suggest that one possible explanation for differences in terrestrial C pool responses to warming may be diverse temperature sensitivities of underlying decomposer communities. Nonetheless, a number of additional factors must be taken into consideration when interpreting our results in the context of global climate change, including soil mineral-mediated modulation of the substrate supply (Schimel and Schaeffer, 2012;Coward et al, 2018), plant-microbe feedbacks (Melillo et al, 2011;Sistla et al, 2014;Suseela and Tharayil, 2018), and temporal variation in temperature. Furthermore, we modeled CUE as a somewhat inflexible property of individual taxa rather than as the emergent property of allocation to different, biochemically impor-tant physiological processes that it is (Hagerty et al, 2018).…”