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1. Water‐Soluble Organic Matter From Soils at the Terrestrial‐Aquatic Interface in Wetland‐Dominated Landscapes

   https://doi.org/10.1029/2022JG006994  

   Wardinski, Katherine M. ; Hotchkiss, Erin R. ; Jones, C. Nathan ; McLaughlin, Daniel L. ; Strahm, Brian D. ; Scott, Durelle T. ( September 2022 , Journal of Geophysical Research: Biogeosciences)

   Hydrologic controls on carbon processing and export are a critical feature of wetland ecosystems. Hydrologic response to climate variability has important implications for carbon‐climate feedbacks, aquatic metabolism, and water quality. Little is known about how hydrologic processes along the terrestrial‐aquatic interface in low‐relief, depressional wetland catchments influence carbon dynamics, particularly regarding soil‐derived dissolved organic matter (DOM) transport and transformation. To understand the role of different soil horizons as potential sources of DOM to wetland systems, we measured water‐soluble organic matter (WSOM) concentration and composition in soils collected from upland to wetland transects at four Delmarva Bay wetlands in the eastern United States. Spectral metrics indicated that WSOM in shallow organic horizons had increased aromaticity, higher molecular weight, and plant‐like signatures. In contrast, WSOM from deeper, mineral horizons had lower aromaticity, lower molecular weights, and microbial‐like signatures. Organic soil horizons had the highest concentrations of WSOM, and WSOM decreased with increasing soil depth. WSOM concentrations also decreased from the upland to the wetland, suggesting that continuous soil saturation reduces WSOM concentrations. Despite wetland soils having lower WSOM, these horizons are thicker and continuously hydrologically connected to wetland surface and groundwater, leading to wetland soils representing the largest potential source of soil‐derived DOM to the Delmarva Bay wetland system. Knowledge of which soil horizons are most biogeochemically significant for DOM transport in wetland ecosystems will become increasingly important as climate change is expected to alter hydrologic regimes of wetland soils and their resulting carbon contributions from the landscape.

    

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2. Gallionellaceae in rice root plaque: metabolic roles in iron oxidation, nutrient cycling, and plant interactions

   https://doi.org/10.1128/aem.00570-23  

   Chan, Clara S. ; Dykes, Gretchen E. ; Hoover, Rene L. ; Limmer, Matt A. ; Seyfferth, Angelia L. ( December 2023 , Applied and Environmental Microbiology)

   Glass, Jennifer B. (Ed.)

   On the roots of wetland plants such as rice, Fe(II) oxidation forms Fe(III) oxyhydroxide-rich plaques that modulate plant nutrient and metal uptake. The microbial roles in catalyzing this oxidation have been debated and it is unclear if these iron-oxidizers mediate other important biogeochemical and plant interactions. To investigate this, we studied the microbial communities, metagenomes, and geochemistry of iron plaque on field-grown rice, plus the surrounding rhizosphere and bulk soil. Plaque iron content (per mass root) increased over the growing season, showing continuous deposition. Analysis of 16S rRNA genes showed abundant Fe(II)-oxidizing and Fe(III)-reducing bacteria (FeOB and FeRB) in plaque, rhizosphere, and bulk soil. FeOB were enriched in relative abundance in plaque, suggesting FeOB affinity for the root surface. Gallionellaceae FeOBSideroxydanswere enriched during vegetative and early reproductive rice growth stages, while aGallionellawas enriched during reproduction through grain maturity, suggesting distinct FeOB niches over the rice life cycle. FeRBAnaeromyxobacterandGeobacterincreased in plaque later, during reproduction and grain ripening, corresponding to increased plaque iron. Metagenome-assembled genomes revealed that Gallionellaceae may grow mixotrophically using both Fe(II) and organics. TheSideroxydansare facultative, able to use non-Fe substrates, which may allow colonization of rice roots early in the season. FeOB genomes suggest adaptations for interacting with plants, including colonization, plant immunity defense, utilization of plant organics, and nitrogen fixation. Taken together, our results strongly suggest that rhizoplane and rhizosphere FeOB can specifically associate with rice roots, catalyzing iron plaque formation, with the potential to contribute to plant growth.

   In waterlogged soils, iron plaque forms a reactive barrier between the root and soil, collecting phosphate and metals such as arsenic and cadmium. It is well established that iron-reducing bacteria solubilize iron, releasing these associated elements. In contrast, microbial roles in plaque formation have not been clear. Here, we show that there is a substantial population of iron oxidizers in plaque, and furthermore, that these organisms (SideroxydansandGallionella) are distinguished by genes for plant colonization and nutrient fixation. Our results suggest that iron-oxidizing and iron-reducing bacteria form and remodel iron plaque, making it a dynamic system that represents both a temporary sink for elements (P, As, Cd, C, etc.) as well as a source. In contrast to abiotic iron oxidation, microbial iron oxidation results in coupled Fe-C-N cycling, as well as microbe-microbe and microbe-plant ecological interactions that need to be considered in soil biogeochemistry, ecosystem dynamics, and crop management.

    

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3. Distinct microbial communities alter litter decomposition rates in a fertilized coastal plain wetland

   https://doi.org/10.1002/ecs2.3619  

   Koceja, Megan E. ; Bledsoe, Regina B. ; Goodwillie, Carol ; Peralta, Ariane L. ( June 2021 , Ecosphere)

   Human activities have led to increased deposition of nitrogen (N) and phosphorus (P) into soils. Nutrient enrichment of soils is known to increase plant biomass and rates of microbial litter decomposition. However, interacting effects of hydrologic position and associated changes to soil moisture can constrain microbial activity and lead to unexpected nutrient feedbacks on microbial community structure–function relationships. Examining feedbacks of nutrient enrichment on decomposition rates is essential for predicting microbial contributions to carbon (C) cycling as atmospheric deposition of nutrients persists. This study explored how long‐term nutrient addition and contrasting litter chemical composition influenced soil bacterial community structure and function. We hypothesized that long‐term nutrient enrichment of low fertility soils alters bacterial community structure and leads to higher rates of litter decomposition especially for low C:N litter, but low‐nutrient and dry conditions limit microbial decomposition of high C:N ratio litter. We leveraged a long‐term fertilization experiment to test how nutrient enrichment and hydrologic manipulation (due to ditches) affected decomposition and soil bacterial community structure in a nutrient‐poor coastal plain wetland. We conducted a litter bag experiment and characterized litter‐associated and bulk soil microbiomes using 16S rRNA bacterial sequencing and quantified litter mass losses and soil physicochemical properties. Results revealed that distinct bacterial communities were involved in decomposing higher C:N ratio litter more quickly in fertilized compared to unfertilized soils especially under drier soil conditions, while decomposition rates of lower C:N ratio litter were similar between fertilized and unfertilized plots. Bacterial community structure in part explained litter decomposition rates, and long‐term fertilization and drier hydrologic status affected bacterial diversity and increased decomposition rates. However, community composition associated with high C:N litter was similar in wetter plots with available nitrate detected, regardless of fertilization treatment. This study provides insight into long‐term fertilization effects on soil bacterial diversity and composition, decomposition, and the increased potential for soil C loss as nutrient enrichment and hydrology interact to affect historically low‐nutrient ecosystems.

    

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4. Trees in the Stream: Determining Patterns of Terrestrial Dissolved Organic Matter Contributions to the Northeast Pacific Coastal Temperate Rainforest

   https://doi.org/10.1029/2022JG007027  

   Behnke, M. I. ; Fellman, J. B. ; D’Amore, D. V. ; Spencer, R. G. M. ( April 2023 , Journal of Geophysical Research: Biogeosciences)

   Dissolved organic matter (DOM) composition in small watersheds depends on complex antecedent conditions that ultimately influence DOM generation, processing, and stability downstream. Here, we used ultrahigh resolution Fourier‐transform ion cyclotron resonance mass spectrometry and total dissolved nitrogen and dissolved organic carbon concentrations to investigate how DOM is produced in distinct sub‐catchment types (poor fen, forested wetland, and upland forest) and transported through a watershed in the northeast Pacific coastal temperate rainforest (NPCTR). We traced a suite of previously identified source‐specific marker formulae from vegetation and soil downstream and used them to test models of terrestrial DOM inputs. Marker formulae escaped microbial degradation and were exported from the watershed, demonstrating strong land‐to‐ocean connectivity through the transfer of unmodified tree DOM from specific tree species into the marine environment. Simple hydrologic and temperature variables were better able to predict inputs of soil‐sourced DOM into the stream network than tree‐sourced DOM, highlighting the role of antecedent conditions (e.g., plant growth stage) in DOM source availability and hydrologic flow connectivity, particularly for plant‐derived material. Forested wetland pore waters featured thousands of nitrogen‐containing molecular formulae that potentially provide a path of direct organic nitrogen uptake to organisms. The modified aromaticity index peaked in midsummer (up to 0.55 for fen headwaters) suggesting DOM inputs from freshly produced vegetation provide a strong summertime terrestrial signal. As the climate changes, new watershed‐scale conditions may further complicate predictions of DOM source availability, flow connectivity, and downstream fate in NPCTR watersheds.

    

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5. Organic matter cycling in a model restored wetland receiving complex effluent

   https://doi.org/10.1007/s10533-022-01002-x  

   Zhou, Xingzi ; Johnston, Sarah Ellen ; Bogard, Matthew J. ( December 2022 , Biogeochemistry)

   Wetlands have been used to treat anthropogenic effluents for decades due to their intense biogeochemical processes that transform and uptake nutrients, organic matter, and toxins. Despite these known functions, we lack generalizable knowledge of effluent-derived dissolved organic matter (DOM) cycling in wetlands. Here, we quantify the cycling of DOM in one of Canada’s more economically important wetland complexes (Frank Lake, Alberta), restored to hydrologic permanence in the 1980s using urban and agro-industrial effluents. Optical analyses and PARAFAC (parallel factor analysis) modelling showed a clear compositional change from more bioavailable and protein-like DOM at effluent input sites to more aromatic and humic-like at the wetland outflow, likely due to DOM processing and inputs from marsh plants and wetland soils. Microbial incubations showed that effluent DOM was rapidly consumed, with the half-life of DOM increasing from as low as 35 days for effluent, to 462 days at the outflow, as a function of compositional shifts toward aromatic, humic-like material. Long-term averaged dissolved organic carbon (DOC) export was low compared to many wetlands (10.3 ± 2.0 g C m⁻² yr⁻¹). Consistent with predictions based on water residence time, our mass balance showed Frank Lake was a net source of DOM across all measured years, but shifted from a source to sink among wet and drought years that respectively shortened or lengthened the water residence and DOM processing times. Overall, Frank Lake processes and transforms effluent DOM, despite being a longer-term net source of DOM to downstream environments.

    

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