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Dissolved organic and inorganic carbon (DOC and DIC) influence water quality, ecosystem health, and carbon cycling. Dissolved carbon species are produced by biogeochemical reactions and laterally exported to streams via distinct shallow and deep subsurface flow paths. These processes are arduous to measure and challenge the quantification of global carbon cycles. Here we ask: when, where, and how much is dissolved carbon produced in and laterally exported from the subsurface to streams? We used a catchment‐scale reactive transport model, BioRT‐HBV, with hydrometeorology and stream carbon data to illuminate the “invisible” subsurface processes at Sleepers River, a carbonate‐based catchment in Vermont, United States. Results depict a conceptual model where DOC is produced mostly in shallow soils (3.7 ± 0.6 g/m2/yr) and in summer at peak root and microbial respiration. DOC is flushed from soils to the stream (1.0 ± 0.2 g/m2/yr) especially during snowmelt and storms. A large fraction of DOC (2.5 ± 0.2 g/m2/yr) percolates to the deeper subsurface, fueling deep respiration to generate DIC. DIC is exported predominantly from the deeper subsurface (7.1 ± 0.4 g/m2/yr, compared to 1.3 ± 0.3 g/m2/yr from shallow soils). Deep respiration reduces DOC and increases DIC concentrations at depth, leading to commonly observed DOC flushing (increasing concentrations with discharge) and DIC dilution patterns (decreasing concentrations with discharge). Surprisingly, respiration processes generate more DIC than weathering in this carbonate‐based catchment. These findings underscore the importance of vertical connectivity between the shallow and deep subsurface, highlighting the overlooked role of deep carbon processing and export. Plain Language Summary Dissolved organic and inorganic carbon (DOC and DIC) are important chemical species that affect water quality, ecosystem health, and carbon dioxide emissions from streams. DOC and DIC are produced through different reactions at and below the ground surface before they are transported to streams through underground flow paths. However, it is difficult to measure and observe these reactions and transport pathways, limiting our understanding of when, where, and how much dissolved carbon species are produced and exported from distinct subsurface depths. Here we used a computational model, BioRT‐HBV, to simulate reactions and transport processes and to better understand the production and export of dissolved carbon at Sleepers River, a small catchment in Vermont, United States. Results show that DOC was primarily produced through shallow subsurface reactions and exported through shallow flow paths. DIC was produced in both the shallow and deep subsurface but primarily exported through deep flow paths. Reactions that produced DOC and DIC occurred faster under warm and wet conditions (summer and spring), while export of DOC and DIC increased under wet conditions (spring, snowmelt, storms). Results suggest that climate change may affect the production and export of dissolved carbon species through increased temperatures and intense storm events. Key Points Dissolved organic carbon (DOC) was mainly produced in warm summer and exported in wet spring from shallow subsurface Dissolved inorganic carbon (DIC) was comparably produced in the shallow and deep subsurface but exported mostly from the deep subsurface in wet spring DIC originated more from biogenic (soil respiration and deep respiration) than geogenic (carbonate weathering) sources in a carbonate‐based catchment

Winters in snow-covered regions have warmed, likely shifting the timing and magnitude of nutrient export, leading to unquantified changes in water quality. Intermittent, seasonal, and permanent snow covers more than half of the global land surface. Warming has reduced the cold conditions that limit winter runoff and nutrient transport, while cold season snowmelt, the amount of winter precipitation falling as rain, and rain-on-snow have increased. We used existing geospatial datasets (rain-on-snow frequency overlain on nitrogen and phosphorous inventories) to identify areas of the contiguous United States (US) where water quality could be threatened by this change. Next, to illustrate the potential export impacts of these events, we examined flow and turbidity data from a large regional rain-on-snow event in the United States’ largest river basin, the Mississippi River Basin. We show that rain-on-snow, a major flood-generating mechanism for large areas of the globe (Berghuijs et al 2019 Water Resour. Res. 55 4582–93; Berghuijs et al 2016 Geophys. Res. Lett. 43 4382–90), affects 53% of the contiguous US and puts 50% of US nitrogen and phosphorus pools (43% of the contiguous US) at risk of export to groundwater and surface water. Further, the 2019 rain-on-snow event in the Mississippi River Basin demonstrates that these events could have large, cascading impacts on winter nutrient transport. We suggest that the assumption of low wintertime discharge and nutrient transport in historically snow-covered regions no longer holds. Critically, however, we lack sufficient data to accurately measure and predict these episodic and potentially large wintertime nutrient export events at regional to continental scales.

Over the past two decades, headwater streams of the northern hemisphere have shown increased amounts of dissolved organic carbon (DOC), coinciding with decreased acid deposition. The exact nature of the mechanistic link between precipitation composition and stream water DOC is still widely debated in the literature. We hypothesize that soil aggregates are the main source of stream water DOC and that DOC release is greater in organic rich, riparian soils versus hillslope soils. To test these hypotheses, we collected soils from two main landscape positions (hillslope and riparian zones) from the acid-impacted Sleepers River Research Watershed in northeastern Vermont. We performed aqueous soil extracts with solutions of different ionic strength (IS) and composition to simulate changes in soil solution. We monitored dynamic changes in soil particle size, aggregate architecture and composition, leachate DOC concentrations, dissolved organic matter (DOM) characteristics with fluorescence spectroscopy and trends in bioavailability. In low IS solutions, extractable DOC concentrations were significantly higher, particle size (by laser diffraction) was significantly smaller and organic material was separated from mineral particles in scanning electron microscope observations. Furthermore, higher DOC concentrations were found in Na+ compared to Ca2+ solutions of the same IS. These effects are attributed to aggregate dispersion due to expanding diffuse double layers in decreased IS solutions and to decreased bridging by divalent cations. Landscape position impacted quality but not quantity of released DOC. Overall, these results indicate that soil aggregates might be one important link between Critical Zone inputs (i.e. precipitation) and exports in streams.

Terrestrial production and export of dissolved organic and inorganic carbon (DOC and DIC) to streams depends on water flow and biogeochemical processes in and beneath soils. Yet, understanding of these processes in a rapidly changing climate is limited. Using the watershed‐scale reactive‐transport model BioRT‐HBV and stream data from a snow‐dominated catchment in the Rockies, we show deeper groundwater flow averaged about 20% of annual discharge, rising to ∼35% in drier years. DOC and DIC production and export peaked during snowmelt and wet years, driven more by hydrology than temperature. DOC was primarily produced in shallow soils (1.94 ± 1.45 gC/m2/year), stored via sorption, and flushed out during snowmelt. Some DOC was recharged to and further consumed in the deeper subsurface via respiration (−0.27 ± 0.02 gC/m2/year), therefore reducing concentrations in deeper groundwater and stream DOC concentrations at low discharge. Consequently, DOC was primarily exported from the shallow zone (1.62 ± 0.96 gC/m2/year, compared to 0.12 ± 0.02 gC/m2/year from the deeper zone). DIC was produced in both zones but at higher rates in shallow soils (1.34 ± 1.00 gC/m2/year) than in the deep subsurface (0.36 ± 0.02 gC/m2/year). Deep respiration elevated DIC concentrations in the deep zone and stream DIC concentrations at low discharge. In other words, deep respiration is responsible for the commonly‐observed increasing DOC concentrations (flushing) and decreasing DIC concentrations (dilution) with increasing discharge.  DIC export from the shallow zone was ~66% of annual export but can drop to ∼53% in drier years. Numerical experiments suggest lower carbon production and export in a warmer, drier future, and a higher proportion from deeper flow and respiration processes. These results underscore the often‐overlooked but growing importance of deeper processes in a warming climate. Key Points The timing, duration, and size of snowmelt is more influential than temperature in regulating the production and export of dissolved carbon The shallow soil zone produces and exports most of the dissolved carbon, primarily driven by snowmelt hydrology rather than temperature The deep zone, on average, produces 14 ± 8% and exports 27 ± 8% of dissolved carbon (DOC & DIC) and becomes more important (36 ± 2% of export) in warmer, drier years

A changing climate has the potential to mobilize soil carbon, shifting seasonally snow-covered, forested ecosystems from carbon sinks to sources. To determine the sensitivity of soil carbon fluxes to changes in temperature and moisture, we quantified seasonal and spatial variability of soil carbon dioxide (CO₂) fluxes (N = 746) and dissolved organic carbon (DOC) in leachate (N = 260) in high-elevation, mixed conifer forests in Arizona and New Mexico. All sites have cold winters, warm summers, and bimodal soil moisture patterns associated with snowmelt and summer monsoon rainfall. We employed a state factor approach, quantifying how distal controls (parent material, regional climate, topography) interacted with proximal variability in soil temperature (−3 to 26 °C) and moisture (2–76 %) to influence carbon effluxes. Carbon loss was dominated by CO₂flux (250–1220 g C m⁻² year⁻¹) rather than leached DOC (7.0–9.4 g C m⁻² year⁻¹). Significant differences in mean growing season CO₂flux were associated with parent material and aspect; differences appear to be mediated by how these distal controls influence primarily moisture and secondarily temperature. Across all sites, a multiple linear regression model (MLR) relying on moisture and temperature best described growing season CO₂fluxes (r² = 0.63, p < 0.001). During winter, the MLR describing soil CO₂flux (r² = 0.98, p < 0.001) relied on distal factors including snow cover, clay content, and bulk carbon, all factors that influence liquid water content. Our findings highlight the importance of state factors in controlling soil respiration primarily through influencing spatial and temporal heterogeneity in soil moisture.

Fluorescence spectroscopy is a common tool to assess optical dissolved organic matter (DOM) and a number of characteristics, including DOM biodegradability, have been inferred from these analyses. However, recent findings on soil and DOM dynamics emphasize the importance of ecosystem-scale factors, such as physical separation of substrate from soil microbial communities and soil physiochemical cycles driving DOM stability. We apply this principle to soil derived DOM and hypothesize that optical properties can only supply information on biodegradability when evaluated in the larger ecosystem because substrate composition and the activity/abundance of the microbial community ultimately drive DOM degradation. To evaluate biodegradability in this context, we assessed aqueous soil extracts for water extractable organic carbon (WEOC) content, biodegradability, microbial biomass and DOM characteristics using fluorescence spectroscopy across a range of environmental conditions (covariant with season and land use) in northern Vermont, USA. Our results indicate that changes in environmental conditions affect composition, quantity, and biodegradability of DOM. WEOC concentrations were highest in the fall and lowest in the summer, while no significant differences were found between land covers; however, DOM biodegradability was significantly higher in the agricultural site across seasons. Despite a shift in utilized substrate from less aromatic DOM in summer to more aromatic DOM in winter, biodegradability was similar for all seasons. The only exception was cold temperature incubations where microbial activity was depressed, and processing was slowed. These results provide examples on how fluorescence based metrics can be combined with context relevant environmental parameters to evaluate bioavailability in the context of the larger ecosystem.

Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r² = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59–71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, δ¹³CDIC increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, [Formula: see text] increased up to 12 times atmospheric values leading to substantial degassing.

Determination of subsurface solute fluxes is central in critical zone (CZ) science because key processes such as bio‐geochemical weathering, nutrient dynamics, and contaminant transport can be determined. With passive capillary fiberglass wick samplers (PCaps), solute, and water fluxes can be assessed; however, the presence of fiberglass can impact soil solution chemistry. To determine which solutes are suitable for sampling by fiberglass wick PCaps, flow‐through experiments were performed where aqueous soil extracts were percolated through the wicks and changes in effluent solution pH, dissolved inorganic carbon (DIC), anions, major cations, and trace metals including rare earth elements (REE) were monitored. Results indicated dissolution of wicks releasing the glass constituents B, Na, Si, Ca, Mg as well as F− and DIC. Barium, K, and Sr were retained, likely due to exchange reactions with either glass constituents or interlayer cations of clay colloids. Stop‐flow was included to mimic precipitation events revealing increased pulse‐like release of glass constituents. Results of the full‐scale experiment indicate substantial contribution from wick material (59 ± 20 for Si, 92 ± 7 for Na, 29 ± 19 for Mg, and −26 ± 32 for Ca, all values in percentage of total effluent concentrations) that cannot be corrected for, hence the use of PCaps for these solutes is not recommended. A great number of other solutes were however not impacted by the presence of wicks such as most anions (Cl−, NO3−, SO42−) and many trace metals (Al, Ti, Mn, V, Fe, Co, As, Y, Mo, Sn, Pb, U, and all REE).

Quantitative characterization of dissolved organic matter (DOM) in soil and vadose zone solution is needed to interpret mechanisms of nutrient and C cycling as well as bio-weathering processes. Passive capillary wick samplers (PCaps) are useful for soil solution sampling because they can provide measures of water and associated DOM-constituent flux in the unsaturated zone, however potential impacts of the wick material on DOM chemical properties has not been investigated yet. We therefore conducted experiments where aqueous soil extracts were transported along PCap fiberglass wicks in flow-through experiments. Results indicated limited dissolved organic carbon (DOC) sorption and DOM fractionation, and related parameters (total dissolved nitrogen [TDN], DOM fluorescence components) also remained largely unaffected. We note that this experiment does not account for the extent to which soil hydrologic processes may be affected by PCap field installations. However, given that the wicks did not fractionate significantly DOM, we compared field-based PCap DOM solution collected in situ with laboratory-based aqueous soil extraction (ASE) of DOM from the same soils to assess differences in DOM quality. Spectroscopic analysis of DOM in ASE solutions showed lower O-H stretch/carboxlyate band intensity ratios, more pronounced aliphatic C-H stretching (Fourier Transform Infrared analysis), higher specific ultraviolet-absorbance (SUVA254) values as well as greater abundance of fluorescence components in the region attributed to fulvic acids. We conclude that difference in molecular properties of DOM derived from laboratory ASE vs. PCap field collection of the same soils is attributable to differential disturbance effects of the two methods of soil solution collection.

River water quality is crucial to ecosystem health and water security, yet its deterioration under climate change is often overlooked in climate risk assessments. Here we review how climate change influences river water quality via persistent, gradual shifts and episodic, intense extreme events. Although distinct in magnitude, intensity and duration, these changes modulate the structure and hydro-biogeochemical processes on land and in rivers, hence reshaping land–river connectivity and the quality of river waters. To advance understanding of and forecasting capabilities for water quality in future climates, it is essential to perceive land and rivers as interconnected systems. It is also vital to prioritize research under climate extremes, where the dynamics of water quality often challenge existing theories and models and call for shifts in conceptual paradigms. River water quality affects water security and is expected to degrade under climate change—an issue that has garnered limited attention. Here the authors review the impacts of climate change and climate extremes on water quality, highlighting the pivotal role of land–river connectivity.