Shifts in climate are fundamentally altering hydrologic processes in temperate forested headwater catchments, which serve as sensitive indicators of ecosystem-scale water balance. In the Northeastern United States, rising temperatures and increases in both total and extreme precipitation have been observed over recent decades. While these changes have been linked to altered soil moisture dynamics and evapotranspiration regimes at sites like Harvard Forest, the role of groundwater and its connection to streamflow generation remains underexplored. We use regression analysis and interaction models to evaluate shifts in streamflow generation processes in headwater catchments. Following a climatic shift in 2016 that was magnified by a prolonged drought, we observed a decrease in average streamflow response time, indicating more rapid water movement through these systems. Our findings suggest a transition toward more shallow-dominated flow paths, including intermittent shallow subsurface flow through preferential flow paths, particularly since 2016 in Massachusetts. Increasingly frequent extreme precipitation events appear to activate shallow subsurface pathways more often, resulting in faster hydrologic responses. However, we also find that wetland-connected streams show more stability in response time and flow dynamics, acting as buffers against these rapid shifts.

Chemical weathering in mountain critical zones controls river chemistry and regulates long‐term climate. Mountain landscapes contain diverse landforms created by geomorphic processes, including landslides, glacial moraines, and rock glaciers. These landforms generate unique flowpaths and water‐rock interactions that modify water chemistry as precipitation is transformed to streamflow. Variations in lithology and vegetation also strongly control water chemistry. Prior work has shown that landslides generate increased dissolved solute concentrations in rapidly uplifting mountains. However, there is still uncertainty regarding the magnitude which different geomorphic processes and land cover variations influence solute chemistry across tectonic and climatic regimes. We measured ion concentrations in surface water from areas that drain a variety of landforms and across land cover gradients in the East River watershed, a tributary of the Colorado River. Our results show that landslides produce higher solute concentrations than background values measured in streams draining soil‐mantled hillslopes and that elevated concentrations persist centuries to millennia after landslide occurrence. Channels with active bedrock incision also generate high solute concentrations, whereas solute concentrations in waters draining moraines and rock glaciers are comparable to background values. Solute fluxes from landslides and areas of bedrock incision are 1.6–1.8 times greater than nearby soil‐mantled hillslopes. Carbonic acid weathering dominates surface water samples from watersheds with greater vegetation coverage. Geomorphically enhanced weathering generates hotspots for net CO 2 release or sequestration, depending on lithology, that are 1.5–3.5 times greater than background values, which has implications for understanding links among surface processes, chemical weathering, and carbon cycle dynamics in alpine watersheds.

Peat wetlands at high latitudes have been a key focus for geochemical and hydrologic research due to their roles in carbon storage and greenhouse gas emissions. At the local scale, fens facilitate nutrient cycling and methane production via microbial processes. At the watershed scale, their distribution and hydrologic connectivity with surface waters can substantially influence water quality. However, the role of mid-latitude montane fens in moderating watershed functions remains understudied, despite their susceptibility to pronounced redox shifts driven by high-elevation seasonal dynamics. We conducted a comprehensive analysis of (1) base cations, (2) biologically active, redox-sensitive nutrients and metals, and (3) carbon compounds in Rocky Mountain fens and surface waters along an elevational gradient in the East River watershed (Upper Gunnison Basin, Colorado, USA). Our aim was to evaluate whether fens act as passive transporters or active transformers of these solute groups. Fen base cation chemistry was largely indistinguishable from surface water and groundwater, suggesting transport-dominated behavior controlled by bedrock lithology. In contrast, anoxic conditions in fens produced distinct redox-sensitive nutrient signatures. A quasi-linear relationship between Cl-normalized NO3 2- and PO4 3- in high-elevation fens and surface waters suggests a possible mixing signal originating from nutrient-enriched fen waters and more dilute surface or groundwater sources. Stable isotope data for dissolved methane, unlike conservative cations, indicate active carbon transformation via methanogenesis, with limited microbial methane oxidation. Collectively, our study offers an integrated assessment of the aqueous geochemistry of montane fens and their potential to modulate surface water chemistry across mid-latitude alpine watersheds.

Rivers and streams play an important role within the global carbon cycle, in part through emissions of CO2 to the atmosphere. However, the sources of this CO2 and their spatiotemporal variability are difficult to constrain. Recent work has highlighted the role of carbonate buffering reactions that may serve as a source of CO2 in high alkalinity systems. In this study, we seek to develop a quantitative framework for the role of carbonate buffering in the fluxes and spatiotemporal patterns of CO2 and the stable and radio- isotope composition of dissolved inorganic carbon (DIC). We incorporate DIC speciation calculations of carbon isotopologues into a stream network CO2 model and perform a series of experiments, ranging from the degassing of a groundwater seep to the simulation of 5th-order stream network. We find that carbonate buffering reactions contribute >60% of emissions in high-alkalinity, moderate GW-pCO2 environments that may characterize carbonate bedrock systems. However, atmosphere equilibration timescales of pCO2 are minimally affected, which contradicts hypotheses that carbonate buffering maintains high pCO2 across Strahler orders in high alkalinity systems. In contrast, alkalinity dramatically increases isotope equilibration timescales, which significantly complicates isotopic methods of CO2 source partitioning by decoupling pCO2 from isotopic composition even under low alkalinity. Based on similar impacts on atmospheric equilibration for stable and radio- carbon isotopologues, we develop a quantitative method for distinguishing groundwater from stream corridor CO2 sources in carbonate-dominated watersheds. Together, these results provide a framework to guide fieldwork and interpretations of stream network CO2 patterns across variable alkalinities.

Inland waters emit significant amounts of carbon dioxide (CO2) to the atmosphere; however, the global magnitude and source distribution of inland water CO2 emissions remain uncertain. These fluxes have previously been "statistically upscaled" by independently estimating dissolved CO2 concentrations and gas exchange velocities to calculate fluxes. This scaling, while robust and defensible, has known limitations in representing carbon source limitations and spatial variability. Here, we develop and calibrate a CO2 transport model for the continental United States, simulating carbon transport and transformation in >22 million hydraulically connected rivers, lakes, and reservoirs. We estimate 25% lower CO2 fluxes compared to upscaling estimates forced by the same observational calibration data. While precise CO2 source distribution estimates are limited by the resolution of model parameterizations, our model suggests that stream corridor CO2 production dominates over groundwater inputs at the continental scale. Our results further suggest that the lack of observational networks for groundwater CO2 and scalable metabolic models of aquatic CO2 production remain the most salient barriers to further coupling of our model with other Earth system components.

Plants strongly influence soil properties through rhizodeposition, in which exudates diffuse from roots, additional secretions are actively released, and root cells are sloughed into the soil. This contribution by plants of carbon compounds belowground is at the core of soil health, water holding capacity, and the soil carbon storage that pulls carbon dioxide out of the atmosphere. Once in soil, organic matter can bind with minerals such as iron hydroxides, where it can be protected from microbial attack for millenia, preserving very large terrestrial soil carbon pools. However, those same compounds contributed by roots to soil may also destabilize the long-term protective associations of SOM with minerals, making that soil organic matter (SOM) more vulnerable to microbial attack and decomposition. Plant roots thus influence both the buildup and breakdown of soil carbon pools. DOE’s E3SM Land Model (ELM) includes a representation of soil carbon storage on minerals, but the potential vulnerability of SOM–mineral associations to effects of rhizodeposition is not yet represented in ELM. To begin testing for this effect of rhizodeposition on soil carbon storage and decomposition, we worked to develop a novel approach during this TES Exploratory project DE-SC0019142 – we harnessed the power of plant viral infection. We examined whether plant virus infection can serve as a tool to intensify rhizodeposition at the root surface, and therefore possibly intensify mobilization of SOM from minerals making it visible to our analytical techniques. Viral infection is widespread in terrestrial ecosystems; 25-70% of plants have virus infection, yet the influence of such infection on root traits and terrestrial soil carbon dynamics remains largely unexplored. We used two plant hosts: the annual Avena sativa (oats) and the genetically tractable, model grass Brachypodium distachyon. These grasses were infected with the broad host range virus Barley Yellow Dwarf Virus (BYDV) via aphids (Rhopalosiphum padi). BYDV infects at least 150 grass species in agricultural and natural ecosystems, and in previous experiments, oats infected with BYDV had roots that were very sticky to the touch, strongly suggesting that infection altered rhizodeposition. We developed this new experimental approach mostly in a one virus (Barley Yellow Dwarf Virus)–one plant (Avena sativa) system. (Several effects of infection in a Brachypodium-BYDV system were similar in nature to effects on Avena sativa, but were more variable.) In the BYDV-Avena system, we developed protocols for consistently infecting target plants (and avoiding infection of control plants) using aphid caging on leaves. We measured that infected plants exhibited reduced photosynthesis, plant (including root) biomass, and root:shoot ratio, as well as simplified root system architecture. We established procedures for sampling the organic compounds carried specifically in phloem (vascular tissue) of leaves and roots, using aphid stylectomy. We used FTICR-MS, Orbitrap GC-MS, and LC-MS/MS to analyze organic compounds in phloem, liquid around roots of plants grown hydroponically, and pore water around roots in soil, and found differences in the compounds in solution bathing roots when infected and uninfected plants were grown hydroponically. Finally, we synthesized isotopically-labeled mineral–organic matter (MAOM) associations in the lab and developed assays using them in solution and in soil. Assays quantified the extent and rate of mineralization of labeled MAOM that was mobilized by functionally distinct rhizodeposits and then attacked by microbes. Two mechanisms for MAOM mobilization emerged, with distinct dynamics. During “direct” mobilization, rhizodeposits such as the strong ligand oxalic acid could drive rapid dissolution of minerals, mobilizing MAOM. During “indirect” mobilization, rhizodeposits such as the simple sugar glucose did not attack minerals directly but instead intensified microbial activity, which led to mobilization via changes in e.g. pH, Eh, and microbial metabolite production (Li et al. 2021). Mechanistic understanding derived from these data and our ongoing experiments using these techniques will inform future development of ELM. Plant roots not only contribute newly fixed organic compounds to soils, but also root activities can drive mineralization of the carbon and nutrients mobilized off minerals via “indirect” or “direct” mechanisms. Using viral infection as a new tool, ongoing combined experimentation and modeling will explore the strength and larger-scale significance of the cascade of processes from rhizodeposition to MAOM mobilization for soil carbon storage and nutrient cycling in terrestrial ecosystems. And if viral infection leads quite generally to “sticky roots”, our perception of the potential importance of prevalent virus infection in terrestrial landscapes will be transformed.

Speleothem oxygen isotope records (d(18)O) of tropical South American rainfall in the late Quaternary show a zonal "South American Precipitation Dipole" (SAPD). The dipole is characterized by opposing east-west precipitation anomalies compared to the present-wetter in the east and drier in the west at the mid-Holocene (similar to 7 ka), and drier in the east and wetter in the west at the Last Glacial Maximum (similar to 21 ka). However, the SAPD remains enigmatic because it is expressed differently in western versus eastern d(18)O records and isotope-enabled climate model simulations usually misrepresent the magnitude and/or spatial pattern of d(18)O change. Here, we address the SAPD enigma in two parts. First, we re-interpret the d(18)O data to account for upwind rainout effects that are known to be pervasive in tropical South America, but are not always considered in Quaternary paleoclimate studies. Our revised interpretation reconciles the d(18)O data with cave infiltration and other proxy records, and indicates that the centroid of tropical South American rainfall has migrated zonally over time. Second, using an energy balance model of tropical atmospheric circulation, we hypothesize that zonal migration of the precipitation centroid can be explained by regional energy budget shifts, such as changing Saharan albedo associated with the African Humid Period, that have not been modeled in previous SAPD studies. This hypothesis of a migrating precipitation centroid presents a new framework for interpreting d(18)O records from tropical South America and may help explain the zonal rainfall anomalies that predate the late Quaternary.

The new excess term of the triple oxygen isotope composition in meteoric water, expressed as Delta ' 17O, is conceived to track the component of kinetic fractionation in the water cycle, much like the traditional deuterium excess (dexcess) based on dual hydrogen and oxygen isotope compositions. Here, we use theoretical models to investigate the common and distinct features of variations in these two parameters that result from isotopic fractionation in each step of the water cycle. The objective is to demonstrate their different responses and sensitivities to hydroclimate processes and to explore an interpretive framework based on paired precipitation d-excess and Delta ' 17O data.For oceanic evaporation as the first step of the water cycle, both models and observations suggest that the dexcess and Delta ' 17O of oceanic evaporation fluxes commonly respond to relative humidity as well as boundary-layer aerodynamics and isotopic gradients at the site of evaporation, reflecting similar moisture source information on short timescales. As the isotopic signal of oceanic vapor is transmitted to precipitation, d-excess and Delta ' 17O show distinct non-conservative behaviors in condensation and Rayleigh distillation, which result in decoupling between d-excess and Delta ' 17O in precipitation. This decoupling is particularly pronounced when the degree of distillation is small. Additionally, we develop a new model to show that the interaction between vapor mixing and distillation causes lower Delta ' 17O in precipitation than in the case of Rayleigh distillation without the presence of external vapor sources, whereas only minimal effects are observed in d-excess. Furthermore, we find that precipitation Delta ' 17O has a relatively higher sensitivity than d-excess to terrestrial evaporation-dominated moisture recycling and re-evaporation of raindrops in light rains. Finally, we develop stochastic model simulations to show that the idealized Rayleigh distillation model modified to incorporate the raindrop re-evaporation/ equilibration and vapor mixing is sufficient to reproduce the observed patterns in the relationships among delta 18O, d-excess, and Delta ' 17O in precipitation.This forward-looking review built on the analysis of theoretical models highlights new opportunities in leveraging the joint information from precipitation d-excess and Delta ' 17O data to fingerprint water cycle processes at a range of spatiotemporal scales. However, we also emphasize the challenges associated with these complex and opaque tracers, which aggregate multiple fractionation steps within the water cycle. We suggest that sitespecific, multiple-year 12 monthly means of paired precipitation d-excess and Delta ' 17O data are most useful to disentangle their complex controls. This effort will provide a mechanistic basis for future applications of triple oxygen isotope techniques in geological records for paleo-reconstructions.

The deuterium excess (d‐excess) of precipitation varies seasonally at sites across the globe, an observation that has often been linked to seasonal changes in oceanic evaporation conditions, continental moisture recycling, and subcloud raindrop re‐evaporation. However, there have been very few studies to quantify and evaluate the relative importance of these processes. Here, we revisit the mechanisms of precipitation d‐excess seasonality in low‐latitudes and mid‐latitudes through a new analysis of precipitation isotope databases along with climate reanalysis products and moisture tracking models. In low‐latitudes, the raindrop re‐evaporation effect, indicated by local relative humidity, exerts a strong and prevalent control on observed d‐excess seasonality and overprints the effect of oceanic evaporation conditions. In mid‐latitudes, the effect of oceanic evaporation conditions becomes stronger and seems dominant in the observed d‐excess seasonality. However, the ultimate d‐excess signals are produced after complex modulations by several reinforcing or competing processes, including prior distillations, moisture recycling, supersaturation in snow formation, and raindrop re‐evaporation. Among these processes, substantial increases in the proportion of recycled moisture during the warm and dry season do not produce higher precipitation d‐excess in mid‐latitude continental interiors. We develop a simple seasonal water storage model to show that contributions of previously evaporated residual water storage and higher transpiration fractions may lead to relatively low d‐excess in evapotranspiration fluxes during periods of enhanced continental moisture recycling. This study underscores the ubiquitous nonconservative behavior in d‐excess throughout the water cycle, as opposed to using d‐excess as a simple tracer for remote conditions at oceanic moisture sources.

Lithium isotopes have emerged as a powerful tool to probe the response of global weathering to changes in climate. Due to the preferential incorporation of 6Li into clay minerals during chemical weathering, the isotope ratio δ7Li may be used to interrogate the balance of primary mineral dissolution and clay precipitation. This balance has been linked to relative rates of chemical and physical denudation, such that dissolved δ7Li (δ7Lidiss) is highest at moderate weathering intensities when chemical and physical denudation are comparable. However, we argue that current theory linking δ7Li to weathering regimes through fluid travel times are unable to explain observations of low δ7Li and high Li concentrations in rapidly eroding settings. In this study, we re-examine the relationships between δ7Li, Li concentration, and weathering regime by incorporating Li isotopes into simulations of weathering profiles using a reactive transport model (CrunchFlow) that includes advective fluxes of regolith to simulate variable erosion rates in response to uplift. In these simulations, fractionation is implemented through a kinetic fractionation factor during clay precipitation, which allows the δ7Li of dissolved and suspended loads in the model to vary as a function of Li/Al ratios in primary and secondary minerals. When the model is run over a range of infiltration and erosion rates, simulations reproduce observed global patterns of δ7Lidiss and suspended load δ7Li as a function of weathering intensity, controlled primarily by water travel times and mineral residence times in weathered bedrock. We find that reduced water travel times at low weathering intensity, however, are inconsistent with observations of high Li concentrations. As an alternative, we demonstrate how the rapid weathering of soluble, Li-rich minerals such as chlorite under low weathering intensities may resolve this apparent discrepancy between data and theory. We also suggest that observed patterns are consistent with geothermal Li sources under low weathering intensities. This work offers a foundation guiding future studies in testing potential mechanisms underlying global riverine δ7Lidiss.

The shift from denser forests to open, grass‐dominated vegetation in west‐central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat transition (OHT) is considered by some to be evidence for drier summers, more seasonal precipitation, or a cooler climate, but others have proposed that wetter conditions and/or warming initiated the OHT. Here, we use published ( n = 2,065) and new ( n = 173) oxygen isotope measurements ( δ 18 O ) in authigenic clays and soil carbonates to test the hypothesis that the OHT is linked to increasing wintertime aridity. Oxygen isotope ratios in meteoric water ( δ 18 O p ) vary seasonally, and clays and carbonates often form at different times of the year. Therefore, a change in precipitation seasonality can be recorded differently in each mineral. We find that oxygen isotope ratios of clay minerals increase across the OHT while carbonate oxygen isotope ratios show no change or decrease. This result cannot be explained solely by changes in global temperature or a shift to drier summers. Instead, it is consistent with a decrease in winter precipitation that increases annual mean δ 18 O p (and clay δ 18 O ) but has a smaller or negligible effect on soil carbonates that primarily form in warmer months. We suggest that forest communities in west‐central North America were adapted to a wet‐winter precipitation regime for most of the Cenozoic, and they subsequently struggled to meet water demands when winters became drier, resulting in the observed open habitat expansion.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]A Process Based Stream Network Model for Predicting CO2 Concentrations and Fluxes at High ResolutionAuthorsBrianSaccardiiDMatthewWinnickiDSee all authors Brian SaccardiiDCorresponding Author• Submitting AuthorUniversity of Massachusetts AmherstiDhttps://orcid.org/0000-0003-1099-4996view email addressThe email was not providedcopy email addressMatthew WinnickiDUniversity of Massachusetts AmherstiDhttps://orcid.org/0000-0003-4237-9402view email addressThe email was not providedcopy email address

The deuterium excess (d-excess) of precipitation, which tracks kinetic fractionations during water phase changes, has been used to trace the regions and conditions of oceanic moisture sources, in particular from polar ice-core records. Still, many observations suggest that precipitation d-excess varies significantly across terrestrial environments, both above and below the global average value 10. These variations are often interpreted to reflect either moisture recycling via terrestrial evapotranspiration or sub-cloud raindrop re-evaporation, respectively. Despite being frequently mentioned in literature, however, little work has been carried out to quantify these two competing effects on the widespread variations of d-excess. Here, we use a one-dimensional model of water vapor transport to interrogate the relative controls on d-excess of continental precipitation. We show that when the water vapor gradient is coupled with decreasing temperature, d-excess increases with net rainout and delta O-18 depletion along the model transect, while the magnitude of increase is controlled by the water balance, evaporation/transpiration ratio, and transport type. Raindrop re-evaporation functions as an additional flux of recycled moisture and further increases the d-excess downwind. Alternatively, when the water vapor gradient is coupled with decreasing relative humidity, d-excess may decrease along the model transect wherein upwind evapotranspiration is overwhelmed by local raindrop re-evaporation effects. This local effect becomes even stronger under a regime of turbulent eddy transport with high transpiration fractions, resulting in a pronounced decrease of d-excess without notable changes in delta O-18. Finally, we demonstrate that model processes capture the isotopic variations in precipitation across the altitudinal gradient of the Andes as well as the South American low-level jet zone. Broadly, this study presents a novel framework for understanding the dynamical controls of precipitation d-excess and for linking spatial isotopic variations with ecohydrological fluxes and processes in both modern and paleo-environments. (C) 2021 Elsevier B.V. All rights reserved.

Oxidative weathering of sedimentary rocks plays an important role in the global carbon cycle. Rhenium (Re) has been proposed as a tracer of rock organic carbon (OC petro ) oxidation. However, the sources of Re and its mobilization by hydrological processes remain poorly constrained. Here, we examine dissolved Re as a function of water discharge, using samples collected from three alpine catchments that drain sedimentary rocks in Switzerland (Erlenbach and Vogelbach) and Colorado, USA (East River). The Swiss catchments reveal a higher dissolved Re flux in the catchment with higher erosion rates, but have similar [Re]/[Na + ] and [Re]/[SO 4 2− ] ratios, which indicate a dominance of Re from OC petro . Despite differences in rock type and hydro‐climatic setting, the three catchments have a positive correlation between river water [Re]/[Na + ] and [Re]/[SO 4 2− ] and water discharge. We propose that this reflects preferential routing of Re from a near‐surface, oxidative weathering zone. The observations support the use of Re as a proxy to trace rock‐organic carbon oxidation, and suggest it may be a hydrological tracer of vadose zone processes. We apply the Re proxy and estimate CO 2 release by OC petro oxidation of 5.7 +6.6 / −2.0 tC km −2 yr −1 for the Erlenbach. The overall weathering intensity was ∼40%, meaning that the corresponding export of unweathered OC petro in river sediments is large, and the findings call for more measurements of OC petro oxidation in mountains and rivers as they cross floodplains.

The Amazon forest is increasingly vulnerable to dieback and encroachment of grasslands and agricultural fields. Threats to these forested ecosystems include drying, deforestation, and fire, but feedbacks among these make it difficult to determine their relative importance. Here, we reconstruct the central and western Amazon tree cover response to aridity and fire in the mid-Holocene—a time of less intensive human land use and markedly drier conditions than today—to assess the resilience of tree cover to drying and the strength of vegetation-climate feedbacks. We use pollen, charcoal, and speleothem oxygen isotope proxy data to show that Amazon tree cover in the mid-Holocene was resilient to drying in excess of the driest bias-corrected future precipitation projections. Experiments with a dynamic global vegetation model (LPJ-GUESS) suggest tree cover resilience may be owed to weak feedbacks that act to amplify tree cover loss with drying. We also compare these results to observational data and find that, under limited human interference, modern tree cover is likely similarly resilient to mid-Holocene levels of aridification. Our results suggest human-driven fire and deforestation likely pose a greater threat to the future of Amazon ecosystems than drying alone.

Rivers and streams act as globally significant sources of nitrous oxide (N2O) to the atmosphere, in part through denitrification reactions that will increase in response to ongoing anthropogenic nitrogen loading. While many factors that contribute to the release of N2O relative to inert dinitrogen (N-2) are well described, the ability to predict N2O yields from streams remains a fundamental challenge. Here, I revisit results from the second Lotic Intersite Nitrogen eXperiments (LINX II) in the context of turbulent hyporheic exchange. Denitrification efficiency, or the fraction of nitrate delivered to the streambed by stream turbulence that is chemically reduced, emerges as the single best predictor of N2O yields and underpins the first statistically significant models of inter-site N2O yields. This mechanistic connection is supported by reactive transport modeling of hyporheic zone denitrification representing advective flowpaths, flowpath mixing, and diffusion-dominated anoxic microzones. Simulated N2O yields are inversely correlated with denitrification efficiency; however, advective models are unable to capture low LINX II N2O yields at low denitrification efficiencies. Hyporheic zone mixing exacerbates this inability to capture observed N2O yields via the promotion of N2O release from fast, oxic flowpaths. Instead, anoxic microzones are required to account for LINX II observations through consistently low N2O yields and the consumption of upstream-produced N2O. Together, these results provide a framework for controls on stream N2O yields and suggest that stream corridor restoration designs aimed at increasing the capacity of hyporheic zones to remediate nitrate loading, as opposed to increasing hyporheic exchange, will also reduce proportional N2O emissions.

Most mineral-associated organic matter (MAOM) is protected against microbial attack, thereby contributing to long-term carbon storage in soils. However, the extent to which reactive compounds released by plants and microbes may destabilize MAOM and so enhance microbial access, as well as the underlying mechanisms, remain unclear. Here, we tested the ability of functionally distinct model exudates-ligands, reductants, and simple sugars-to promote microbial utilization of monomeric MAOM, bound via outer-sphere complexes to common iron and aluminum (hydr)oxide minerals. The strong ligand oxalic acid induced rapid MAOM mineralization, coinciding with greater sorption to and dissolution of minerals, suggestive of direct MAOM mobilization mechanisms. In contrast, the simple sugar glucose caused slower MAOM mineralization, but stimulated microbial activity and metabolite production, indicating an indirect microbially-mediated mechanism. Catechol, acting as reductant, promoted both mechanisms. While MAOM on ferrihydrite showed the greatest vulnerability to both direct and indirect mechanisms, MAOM on other (hydr)oxides was more susceptible to direct mechanisms. These findings suggest that MAOM persistence, and thus long-term carbon storage within a given soil, is not just a function of mineral reactivity but also depends on the capacity of plant roots and associated microbes to produce reactive compounds capable of triggering specific destabilization mechanisms.

Inland waters are an important component of the global carbon budget. However, our ability to predict carbon fluxes from stream systems remains uncertain, as pCO2 varies within streams at scales of 1–100 m. This makes direct monitoring of large‐scale CO2 fluxes impractical. We incorporate CO2 input and output fluxes into a stream network advection‐reaction model, representing the first process‐based representation of stream CO2 dynamics at watershed scales. This model includes groundwater (GW) CO2 inputs, water column (WC), benthic hyporheic zone (BHZ) respiration, downstream advection, and atmospheric exchange. We evaluate this model against existing statistical methods including upscaling and multiple linear regressions through comparisons to high‐resolution stream pCO2 data collected across the East River Watershed in the Colorado Rocky Mountains (USA). The stream network model accurately captures GW, evasion, and respiration‐driven pCO2 variability and significantly outperforms multiple linear regressions for predicting pCO2. Further, the model provides estimates of CO2 contributions from internal versus external sources suggesting that streams transition from GW‐ to BHZ‐dominated sources between 3rd and 4th Strahler orders, with GW, BHZ, and WC accounting for 49.3%, 50.6%, and 0.1% of CO2 fluxes from the watershed, respectively. Lastly, stream network model atmospheric CO2 fluxes are 4‐12x times smaller than upscaling technique predictions, largely due to relationships between stream pCO2 and gas exchange velocities. Taken together, this stream network model improves our ability to predict stream CO2 dynamics and efflux. Furthermore, future applications to regional and global scales may result in a significant downward revision of global flux estimates.