Molecular composition and dynamics of colloidal organic carbon in a seasonally-flooded depressional wetland

Understanding the mechanisms governing the composition of organo-mineral associations are critical to predicting the dynamics of soil organic matter and associated elements. In wetland, redox induced biogeochemical transformations are considered as the key processes that control the stabilization of soil organic C through the dissolution of associated metal oxides. Given the particularly significant role of wetland soils in C storage (20-30% of terrestrial C) and cycling, the dynamics of soil colloids (1-1000 nm) and colloidal OC and their molecular composition remain a critical knowledge gap. Here we present size distribution, concentration, and composition of organo-mineral associations in pore water samples from a depressional wetland located in Delaware, USA, as influenced by seasonal fluctuations in water table level. The samples were collected from piezometers installed at multiple depths (50 cm, 100 cm, and 200 cm) and in three zones (upland, transitional, and wetland), respectively. Four size fractions were analyzed: dissolved (<2.3 nm), natural nanoparticle (2.3-100 nm, NNP), fine colloid (100-450 nm), and particulate (450-100 nm). Our results revealed that dissolved, NNP, fine colloid, and particulate fractions comprised 47 4.1%, 37 4.0%, 8 2.8% and 8 2.7% of the bulk OC (<1000 nm) concentration, respectively. Isotope ratio mass spectrometry analyses demonstrate enrichment of heavier 13C stable isotopes in the upland in comparison to the transition and wetland zones. NNP was more enriched in 13C than the larger size fractions. X-ray photoelectron spectroscopy further confirmed that the NNP fraction had a greater proportion of mostly oxidized OC, while the particulate fraction had more aliphatic/aromatic OC functional groups. Our findings clearly demonstrated that organo-mineral associations have significant differences in their elemental composition within colloidal size range. It also provides a broad overview of the importance of taking into consideration of the NNP and fine colloid fractions in assessing the terrestrial cycling and transport of organic C and other associated elements particularly in redox sensitive wetlands.