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1. Disentangling the responses of dissolved organic carbon and nitrogen concentrations to overlapping drivers in a northeastern United States forested watershed

   https://doi.org/10.3389/frwa.2023.1065300  

   Ruckhaus, Manya ; Seybold, Erin C. ; Underwood, Kristen L. ; Stewart, Bryn ; Kincaid, Dustin W. ; Shanley, James B. ; Li, Li ; Perdrial, Julia N. ( March 2023 , Frontiers in Water)

   The concurrent reduction in acid deposition and increase in precipitation impact stream solute dynamics in complex ways that make predictions of future water quality difficult. To understand how changes in acid deposition and precipitation have influenced dissolved organic carbon (DOC) and nitrogen (N) loading to streams, we investigated trends from 1991 to 2018 in stream concentrations (DOC, ~3,800 measurements), dissolved organic nitrogen (DON, ~1,160 measurements), and dissolved inorganic N (DIN, ~2,130 measurements) in a forested watershed in Vermont, USA. Our analysis included concentration-discharge (C-Q) relationships and Seasonal Mann-Kendall tests on long-term, flow-adjusted concentrations. To understand whether hydrologic flushing and changes in acid deposition influenced long-term patterns by liberating DOC and dissolved N from watershed soils, we measured their concentrations in the leachate of 108 topsoil cores of 5 cm diameter that we flushed with solutions simulating high and low acid deposition during four different seasons. Our results indicate that DOC and DON often co-varied in both the long-term stream dataset and the soil core experiment. Additionally, leachate from winter soil cores produced especially high concentrations of all three solutes. This seasonal signal was consistent with C-Q relation showing that organic materials (e.g., DOC and DON), which accumulate during winter, aremore »flushed into streams during spring snowmelt. Acid deposition had opposite effects on DOC and DON compared to DIN in the soil core experiment. Low acid deposition solutions, which mimic present day precipitation, produced the highest DOC and DON leachate concentrations. Conversely, high acid deposition solutions generally produced the highest DIN leachate concentrations. These results are consistent with the increasing trend in stream DOC concentrations and generally decreasing trend in stream DIN we observed in the long-term data. These results suggest that the impact of acid deposition on the liberation of soil carbon (C) and N differed for DOC and DON vs. DIN, and these impacts were reflected in long-term stream chemistry patterns. As watersheds continue to recover from acid deposition, stream C:N ratios will likely continue to increase, with important consequences for stream metabolism and biogeochemical processes.« less
2. The predominant control of hydroclimatic conditions on carbon and weathering fluxes at the hillslope scale

   Wen, Hang ; Sullivan, Pamela ; Billings, Sharon ; Ajami, Hoori ; Cueva, Alejandro ; Flores, Alejandro ; Hirmas, Daniel ; Koop, Aaron ; Murenbeeld, Kathryn ; Zhang, Xi ; et al ( December 2021 , American Geophysical Union annual conference)

   Soil biota generate CO2 that can vertically export to the atmosphere, and dissolved organic and inorganic carbon (DOC and DIC) that can laterally export to streams and accelerate weathering. These processes are regulated by external hydroclimate forcing and internal structures (permeability distribution), the relative influences of which are rarely studied. Understanding these interactions is essential a hydrological extremes intensify in the future. Here we explore the question: How and to what extent do hydrological and permeability distribution conditions regulate soil carbon transformations and chemical weathering? We address the questions using a hillslope reactive transport model constrained by data from the Fitch Forest (Kansas, United States). Numerical experiments were used to mimic hydrological extremes and variable shallow-versus-deep permeability contrasts. Results demonstrate that under dry conditions (0.08 mm/day), long water transit times led to more mineralization of organic carbon (OC) into inorganic carbon (IC) form (>98\%). Of the IC produced, ~ 75\% was emitted upward as CO2 gas and ~ 25\% was exported laterally as DIC into the stream. Wet conditions (8.0 mm/day) resulted in less mineralization (~88\%), more DOC production (~12\%), and more lateral fluxes of IC (~50\% of produced IC). Carbonate precipitated under dry conditions and dissolved under wet conditionsmore »as the fast flow rapidly droves the reaction to disequilibrium. The results depict a conceptual hillslope model that prompts four hypotheses for our community to test. H1: Droughts enhance carbon mineralization and vertical upward carbon fluxes, whereas large hydrological events such as storms and flooding enhance subsurface vertical connectivity, reduce transit times, and promote lateral export. H2: The role of weathering as a net carbon sink or source to the atmosphere depends on the interaction between hydrologic flows and lithology: transition from droughts to storms can shift carbonate from a carbon sink (mineral precipitation) to carbon source (dissolution). H3: Permeability contrasts regulate the lateral flow partitioning via shallow flow paths versus deeper groundwater though this alter reaction rates negligibly. H4: Stream chemistry reflect flow paths and can potentially quantify water transit times: solutes enriched in shallow soils have a younger water signature; solutes abundant at depth carry older water signature.« less
3. Geologic Controls on Source Water Drive Baseflow Generation and Carbon Geochemistry: Evidence of Nonstationary Baseflow Sources Across Multiple Subwatersheds

   https://doi.org/10.1029/2019WR026577  

   Richardson, C. M. ; Zimmer, M. A. ; Fackrell, J. K. ; Paytan, A. ( July 2020 , Water Resources Research)

   The contributions and composition of baseflow sources across an extended recession period were quantified for six subwatersheds of varying size in a structurally complex watershed in coastal California using endmember mixing analysis and related to catchment characteristics (e.g., topography, geology, land use, and soil characteristics). Both shallow subsurface and deep groundwater reservoirs were important contributors for streamflow during low flow periods, and the composition of baseflow sources across subwatersheds was directly related to geologic indices. A binary classification of underlying bedrock permeability (e.g., low vs. high) best explained the changes in shallow subsurface water and deeper groundwater inputs through the seasonal recession. Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and specific UV absorbance at 254 nm (SUVA₂₅₄) were used to provide additional insight into endmember characteristics and their contributions to baseflow. Stream water DIC concentrations were broadly controlled by mixing of groundwater and shallow subsurface water endmembers with relatively constant DIC concentrations, while stream water DOC concentrations reflected both spatial and temporal changes in shallow subsurface water DOC. Results from this study show (1) the importance of considering baseflow as a dynamic mixture of water from multiple sources, (2) the effect of geology on source composition at the subwatershedmore »scale during low flow conditions, and (3) the impact of shifting baseflow sources on stream water dissolved carbon concentrations and the utility of using dissolved carbon concentrations to obtain additional insight into temporal variability in baseflow sources.

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4. Soil CO ₂ Controls Short‐Term Variation but Climate Regulates Long‐Term Mean of Riverine Inorganic Carbon

   https://doi.org/10.1029/2022GB007351  

   Stewart, Bryn ; Zhi, Wei ; Sadayappan, Kayalvizhi ; Sterle, Gary ; Harpold, Adrian ; Li, Li ( August 2022 , Global Biogeochemical Cycles)

   The evasion of CO₂from inland waters, a major carbon source to the atmosphere, depends on dissolved inorganic carbon (DIC) concentrations. Our understanding of DIC dynamics across gradients of climate, geology, and vegetation conditions however have remained elusive. To understand its large‐scale patterns and drivers, we collated instantaneous and mean (multiyear average) DIC concentrations from about 100 rivers draining minimally‐impacted watersheds in the contiguous United States. Within individual sites, instantaneous concentrations (C) measured at daily to seasonal scales exhibit a near‐universal response to changes in river discharge (Q) in a negative power law form. High concentrations occur at low discharge when DIC‐enriched groundwater dominates river discharge; low concentrations occur under high flow when relatively DIC‐poor shallow soil water predominates river discharge. Such patterns echo the widely observed increase of soil CO₂and DIC with depth and the shallow‐and‐deep hypothesis that emphasizes the importance of flow paths and source water chemistry. Across sites, mean concentrations (C_m) decrease with increasing mean discharge (Q_m), a long‐term climate measure, and reachs maxima at around 200 mm/yr. A parsimonious model reveals that high mean DIC arises from soil CO₂accumulation when rates of DIC‐generating reactions are relatively high compared to its export fluxes in arid climates. Although instantaneous and meanmore »DIC concentrations similarly decrease with increasing discharge, results here highlight their distinct drivers: daily to seasonal‐scale instantaneous concentration variations (C) are controlled by subsurface CO₂distribution over depth (from below), whereas long‐term mean concentrations (C_m) are regulated by climate (from above). The results emphasize the significance of land‐river connectivity via subsurface flow paths. They also underscore the importance of characterizing subsurface CO₂distribution to illuminate belowground processes in order to project the future of water and carbon cycles in a warming climate.

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5. BioRT-Flux-PIHM v1.0: a biogeochemical reactive transport model at the watershed scale

   https://doi.org/10.5194/gmd-15-315-2022  

   Zhi, Wei ; Shi, Yuning ; Wen, Hang ; Saberi, Leila ; Ng, Gene-Hua Crystal ; Sadayappan, Kayalvizhi ; Kerins, Devon ; Stewart, Bryn ; Li, Li ( January 2022 , Geoscientific Model Development)

   Abstract. Watersheds are the fundamental Earth surface functioning units that connect the land to aquatic systems. Many watershed-scale models represent hydrological processes but not biogeochemical reactive transport processes. This has limited our capability to understand and predict solute export, water chemistry and quality, and Earth system response to changing climate and anthropogenic conditions. Here we present a recently developed BioRT-Flux-PIHM (BioRT hereafter) v1.0, a watershed-scale biogeochemical reactive transport model. The model augments the previously developed RT-Flux-PIHM that integrates land-surface interactions, surface hydrology, and abiotic geochemical reactions. It enables the simulation of (1) shallow and deep-water partitioning to represent surface runoff, shallow soil water, and deeper groundwater and of (2) biotic processes including plant uptake, soil respiration, and nutrient transformation. The reactive transport part of the code has been verified against the widely used reactive transport code CrunchTope. BioRT-Flux-PIHM v1.0 has recently been applied in multiple watersheds under diverse climate, vegetation, and geological conditions. This paper briefly introduces the governing equations and model structure with a focus on new aspects of the model. It also showcases one hydrology example that simulates shallow and deep-water interactions and two biogeochemical examples relevant to nitrate and dissolved organic carbon (DOC). These examples are illustrated in twomore »simulation modes of complexity. One is the spatially lumped mode (i.e., two land cells connected by one river segment) that focuses on processes and average behavior of a watershed. Another is the spatially distributed mode (i.e., hundreds of cells) that includes details of topography, land cover, and soil properties. Whereas the spatially lumped mode represents averaged properties and processes and temporal variations, the spatially distributed mode can be used to understand the impacts of spatial structure and identify hot spots of biogeochemical reactions. The model can be used to mechanistically understand coupled hydrological and biogeochemical processes under gradients of climate, vegetation, geology, and land use conditions.« less