Organo-mineral associations and size-fractionated colloidal organic carbon dynamics in a redox-controlled wetland

The formation of organo-mineral associations serves as a crucial mechanism for stabilizing soil organic matter, particularly for determining the fate of soil organic carbon (OC) in redox dynamic wetlands characterized by high C content. Despite its importance, few studies have assessed the retention, transformation, and transport of colloids (1–1000 nm) and colloidal OC (COC) in those environments. This leaves a crucial knowledge gap concerning the quantities of colloids and COC and their molecular compositions, especially considering the significant role of metabolically active depressional wetlands that may play as biogeochemical hotspots for C cycling. To address this gap, we conducted a study in a Delmarva Bay depressional wetland located in Blackbird State Forest, Delaware, USA. We established a transect encompassing upland (U), transition (T), and wetland (W) zones based on seasonal hydrologic conditions. We installed piezometers at 50 cm, 100 cm, and 200 cm depths within each zone and collected pore-water samples from 11/2017 to 05/2019. We then fractionated these samples into dissolved (<2.3 nm), natural nanoparticle (NNP, 2.3–100 nm), fine colloid (100–450 nm), and particulate (450–1000 nm) fractions via sequential centrifugation and ultrafiltration, and quantified their concentration and molecular composition. We observed variations in the concentration and molecular composition of soil COC both vertically at different soil depths and horizontally along a redox gradient from the U-T-W transect. The sum of the NNP and fine colloid fractions accounted for 47±20% of the operationally defined “dissolved” (<450 nm) fraction. Isotope ratio mass spectrometry and X-ray photoelectron spectroscopy analyses further revealed that the NNP fraction is more enriched in heavier δ13C isotope and oxidized carbonyl/carboxyl C functional groups (C=O, p<0.05) with significantly higher surficial atomic percentages of C (p<0.01), N (p<0.01), and Mg:Al ratios (p<0.05), and lower atomic percentages of Al (p<0.01) and Si (p<0.01) compared to the larger particles. The formation of cation bridges and/or hydrogen bonds between carboxylic OC and phyllosilicate minerals likely dominated the mineral-OC association in the NNP fraction. The higher surface C enrichment in the particulate fraction compared to the NNP fraction suggests a patchy distribution of more plant-derived OC through organo-organic interactions in the larger fractions. Along the transect, Zone U exhibited significant enrichment of heavier δ13C isotope and lower SUVA values than the Zones T and W indicating inefficient decomposition and dissociation of SOM from minerals due to the reductive dissolution of Fe and Al oxides under anoxic conditions. Furthermore, an increase in low molecular weight microbial metabolites with reduced aromatic OC content was observed at deeper depths in Zones U and T. Our study provides new insights into the concentration and molecular composition of size-fractionated COC, highlighting the need to separately consider the NNP and fine colloid fractions from the operationally defined “dissolved” phase. This is crucial for assessing the biogeochemical cycling of OC in redox-active wetlands, a pressing need amidst the growing concerns about the impacts of climate change.