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ABSTRACT Marine sediments harbour diverse microbial populations, but with increasing depth, these microbes are thought to have low activity due to depleted electron acceptors and lack of new organic matter after burial. However, physiochemical changes in environmental parameters could impact the metabolic activity of microbes in marine sediments. We performed seasonal sampling of shallow sediments to examine changes in population and abundance in relation to physiochemical changes over the year. We used amplicon sequencing, quantitative PCR and geochemistry to assess seasonal abundance of microbial populations at 3 depths (12–14, 38–40 and 48–50 cm) in shallow coastal sediments. 16S rRNA amplicon sequencing showed the sediment microbiome consists of common sediment taxa with minor seasonal variation. However, bacterial gene counts of 16S rRNA genes were highest in summer (2.50 × 10¹² genes/g of sediment) and lowest in spring (1.64 × 10¹¹ genes/g sediment). We observed differences in sediment temperature at depth across seasons (Summer 28°C–25.5°C; Winter 8.7°C–6.3°C) and correlated changes in dissolved organic matter composition that are not typically reported for this environment. We conclude deeper microbial populations in shallow sediments may experience seasonal abundance shifts resulting in a more variable subsurface community than initially presumed in the literature. 

Abstract Many challenges remain before we can fully understand the multifaceted role that natural organic matter (NOM) plays in soil and aquatic systems. These challenges remain despite the considerable progress that has been made in understanding NOM’s properties and reactivity using the latest analytical techniques. For nearly 4 decades, the International Humic Substances Society (IHSS, which is a non-profit scientific society) has distributed standard substances that adhere to strict isolation protocols and reference materials that are collected in bulk and originate from clearly defined sites. These NOM standard and reference samples offer relatively uniform materials for designing experiments and developing new analytical methods. The protocols for isolating NOM, and humic and fulvic acid fractions of NOM utilize well-established preparative scale column chromatography and reverse osmosis methods. These standard and reference NOM samples are used by the international scientific community to study NOM across a range of disciplines from engineered to natural systems, thereby seeding the transfer of knowledge across research fields. Recently, powerful new analytical techniques used to characterize NOM have revealed complexities in its composition that transcend the “microbial” vs. “terrestrial” precursor paradigm. To continue to advance NOM research in the Anthropocene epoch, a workshop was convened to identify potential new sites for NOM samples that would encompass a range of sources and precursor materials and would be relevant for studying NOM’s role in mediating environmental and biogeochemical processes. We anticipate that expanding the portfolio of IHSS reference and standard NOM samples available to the research community will enable this diverse group of scientists and engineers to better understand the role that NOM plays globally under the influence of anthropogenic mediated changes. 

Abstract Tidal channels are biogeochemical hotspots that horizontally exchange carbon (C) with marsh platforms, but the physiochemical drivers controlling these dynamics are poorly understood. We hypothesized that C‐bearing iron (Fe) oxides precipitate and immobilize dissolved organic carbon (DOC) during ebb tide as the soils oxygenate, and dissolve into the porewater during flood tide, promoting transport to the channel. The hydraulic gradient physically controls how these solutes are horizontally exchanged across the marsh platform‐tidal channel interface; we hypothesized that this gradient alters the concentration and source of C being exchanged. We further hypothesized that trace soil gases (i.e., CO₂, CH₄, dimethyl sulfide) are pushed out of the channel bank as the groundwater rises. To test these hypotheses, we measured porewater, surface water, and soil trace gases over two 24‐hr monitoring campaigns (i.e., summer and spring) in a mesohaline tidal marsh. We found that Fe²⁺and DOC were positively related during flood tide but not during ebb tide in spring when soils were more oxidized. This finding shows evidence for the formation and dissolution of C‐bearing Fe oxides across a tidal cycle. In addition, the tidal channel contained significantly (p < 0.05) more terrestrial‐like DOC when the hydraulic gradient was driving flow toward the channel. In comparison, the channel water was saltier and contained significantly (p < 0.05) more marine‐like DOC when the hydraulic gradient reversed direction. Trace gas fluxes increased with rising groundwater levels, particularly dimethyl sulfide. These findings suggest multiple physiochemical mechanisms controlling the horizontal exchange of C at the marsh platform‐tidal channel interface. 

Abstract We characterized the dissolved organic matter (DOM) under baseflow conditions from a set of rivers in the Mohawk and Hudson River watersheds. The rivers in this study drain a range of bedrock geologies and land cover. We identify how those factors influence riverine DOM reactivity and the source, age, and composition of the biolabile DOM. We performed laboratory incubation experiments to characterize each river's reactive and non‐reactive DOM pools. Measurements of dissolved organic carbon concentration, radiocarbon, Ultraviolet‐visible spectroscopy absorbance, and Fourier‐transform ion cyclotron resonance mass spectrometry (FTICR‐MS) analysis were performed at each incubation start and end, allowing us to determine the quantity, age, and composition of the reactive and nonreactive DOM pools. We find that lithology controls bulk DOM ages, with watersheds underlain by shale/limestone having the most aged DOM and crystalline/metasedimentary watersheds having the youngest DOM. We observe that for a given lithology, bulk DOM age increases with the proportion of agricultural land in the watershed–suggesting agricultural practices mobilize aged DOM. FTICR‐MS analysis reveals that both lithology and land cover influence DOM composition. Shale/limestone watersheds showed DOM compositions distinct from other watershed lithologies, and the percentage of nitrogen‐containing DOM correlated with agricultural influence. In two of the studied rivers we find that the biolabile DOM fraction is older than the bulk DOM (upwards of 7 kyr) revealing that aged DOM may be preferentially consumed in these rivers. Our findings provide insight into how riverine carbon cycles may respond to watershed disturbances that influence DOM inputs to rivers.