Quantification and molecular characterization of organo-mineral associations as influenced by redox oscillations: relevance in carbon cycling and stabilization
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Redox-induced biogeochemical transformations are the key processes that control the fate and transport of soil organic carbon (SOC) via association with metal oxides in redox-sensitive wetlands. 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. Lack of such understanding limits our ability to predict the role that soil colloids play in C cycling in wetland soils and how colloidal interactions/processes may respond to climate change. Further, the question of why some soil organic matter (SOM) persists for millennia whereas others decompose readily has motivated concerted efforts to improve our understanding of SOM stability. It is therefore important to investigate the fluxes and distribution of organo-mineral complexes in wetlands to better understand their role in the biogeochemical cycling of organic C and associated elements. My dissertation includes results from laboratory-based soil column and microcosm studies as well as field observations, focusing on (1) quantification of dissolved organic C and colloidal organic C (COC) loads, (2) identification of the molecular composition of colloids and associated OC in different size fractions, and (3) characterization of the influence of wetland hydrology on the concentration and composition of the organo-mineral complexes in a depressional wetland. ☐ In Chapter 2, I present the results that addressed the dynamics of size-fractionated organo-mineral complexes by conducting redox oscillations in soil microcosms. Molecular composition of natural nanoparticle (NNP, 2.3-100 nm), fine colloid (100-450 nm), and particulate (450-1000 nm) fractions were measured using isotope ratio mass spectrometry (IRMS) and x-ray photoelectron spectroscopy (XPS). My findings clearly demonstrate an increase in colloid and OC concentrations and the presence of more microbial-derived C in larger size fractions. It indicates that redox oscillations promote the formation of molecularly diverse organo-mineral associations within the colloidal size range. In Chapter 3, I present the results of an investigation that studied the influence of redox oscillations on the mobilization of OC in soil columns. Results showed that bulk OC concentration in the mobile phase was increased by 425-1018% (121.55 ± 10.53 mg L-1) and 311-830% (97.50 ± 5.36 mg L-1) after respective 1st and 2nd reducing half-cycles (RHCs) in comparison to the oxic (control) soil column leachate (14.24 ± 5.43 mg L-1) samples. Organic C in the leachate samples after the 1st RHC had greater contributions from NNP and dissolved fractions, higher aromatic C content, and was more biologically reactive than in the after the 2nd RHC. The observed OC release as influenced by their molecular composition and redox fluctuations provides a baseline for the size continuum of soil OC and its potential ecological and environmental roles. Chapter 4 shows the concentration and molecular composition of the dissolved, NNP, fine colloid, and particulate fractions in pore-waters collected from a Delmarva bay depressional wetland (divided into three zones: upland, transition, and wetland) located at Blackbird State Forest, Delaware, the USA from Feb. 21, 2018 to May 01, 2019. Results reveal that (1) dissolved, NNP, fine colloid, and particulate fractions comprise 45 ± 4%, 38 ± 4%, 8 ± 3% and 7 ± 3% of the bulk OC (< 1000 nm) concentration, respectively, (2) organic C in the upland was more enriched in δ13C stable isotopes than the transition and wetland zones, (3) NNP was more enriched in δ13C than the fine colloid and particulate fractions, (4) and NNP fraction had a higher proportion of mostly oxidized OC (p < 0.05), while the particulate fraction has more aliphatic/aromatic OC functional groups. In Chapter 5, I discuss the concentrations of the major and trace elements in the pore water samples measured by the inductively coupled plasma mass spectrometry (ICP-MS). The samples were collected during water-table rising (Dec. 14, 2018) and declining (May 01, 2019) phases from the depressional wetland. My results revealed that the metal:C ratios were significantly higher in larger size fractions of the organo-mineral complexes, following the order: dissolved < NNP < fine colloid< particulate. ☐ Overall, my findings clearly demonstrate significant new insights into the differences in the concentration and molecular composition of size-fractionated COC, which imply the importance of taking into consideration of the NNP and fine colloid fractions separately (as opposed to combining them into the “dissolved” fraction following the conventional definition of 450 nm) when assessing the cycling and transport of various elements and associated organic C in depressional wetlands.
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Keywords
Colloids, Molecular composition, Organic matter, Organo-mineral associations, Redox oscillations, Wetland