This study examines how greenhouse gas (GHG) production and organic matter (OM) transformations in coastal wetland soils vary with the availability of oxygen and other terminal electron acceptors. We also evaluated how OM and redox-sensitive species varied across different size fractions: particulates (0.45-1μm), fine colloids (0.1-0.45μm), and nano particulates plus truly soluble (<0.1μm; NP+S) during 21-day aerobic and anaerobic slurry incubations. Soils were collected from the center of a freshwater coastal wetland (FW-C) in Lake Erie, the upland-wetland edge of the same wetland (FW-E), and the center of a saline coastal wetland (SW-C) in Washington state. Anaerobic methane production for FW-E soils were 47 and 27,537 times greater than FW-C and SW-C soils, respectively. High particulate Fe2+ and dissolved sulfate concentrations in FW-C and SW-C soils suggest that iron and/or sulfate reduction inhibited methanogenesis. Aerobic CO2 production was highest for both freshwater soils, which had a higher proportion of OM in the NP+S fraction (64±28% and 70±10% for FW-C and FW-E, respectively) and C:N ratios reflective of microbial detritus (1.7±0.2 and 1.4±0.3 for FW-E and FW-C, respectively) compared to SW-C, which had a higher fraction of particulate (58±9%) and fine colloidal (19±7%) OM and C:N ratios reflective of vegetation detritus (11.2 ± 0.5). The variability in GHG production and shifts in OM size fractionation and composition observed across freshwater and saline soils collected within individual and across different sites reinforce the high spatial variability in the processes controlling OM stability, mobility, and bioavailability in coastal wetland soils.

The hyporheic zone is a hotspot for biogeochemical cycling where interactions with mineral metals preserve the release and biodegradation of organic matter (OM). A small fraction of OM can still be exchanged between localized sediments and the overlying water column, and recent evidence suggest there exists a longitudinal structuring in sediment dissolved OM (DOM) chemistry across the continental United States (CONUS). In this study, we tested a hypothesis that water extractable sediment DOM chemistry could be explained by sediment metal contents and integrative watershed scale features at the CONUS scale. Crowdsourced samples were characterized for high resolution mass spectrometry and coupled with sediment metals determined via x-ray fluorescence as well as with land cover and soil elemental information obtained from national databases. Our results highlight weak relationships between DOM chemistry and elemental composition at the CONUS scale indicating limited transferability of organo-metal linkages into multi-scale hydrobiogeochemical models.

To address some challenging biological and environmental problems, scientists at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL) have been pairing scientific capabilities at their home institution with the bright, tunable energy x-rays and high-resolution instruments at several national synchrotron user facilities. Synchrotron techniques can help answer a variety questions, and two examples featuring synchrotron-based structural and chemical imaging in bio/geochemical systems will be presented. In the first example, x-ray fluorescence spectromicroscopy and microtomography were used to visualize and better understand phosphorous uptake in poplar trees. Endophyte-promoted phosphorous uptake was seen inside poplar roots, where the fixated phosphorous appeared to be in the form of an organic phosphate. Analysis of the tomography data showing increased root mass for the plants inoculated with the endophytes supported the picture of increased nutrient uptake in those plants. These results along with in-house proteomics characterization point to the biological relevance of the symbiosis between endophytes and the host plant. In the second example, scanning transmission x-ray microscopy (STXM) combined with x-ray absorption near edge structure (XANES) helped us investigate soil mineral – soil organic matter (SOM) interactions in an alkaline soil from Washington state. Ca mineral–organic associations were found to be predominant which may play a critical role in the stabilization/degradation of SOM and mineral. Micro- and nanoscale characterization of the chemical state of both Ca from the mineral and C from the organic matter are crucial for understanding such stabilization mechanisms as well as soil nutrient dynamics.