Over the last centuries, human activities have exerted increasing pressures on the environment, leading to drastic alterations in the functioning of freshwater bodies (e.g., eutrophication). Global biogeochemical models have proven crucial to investigate interactions between humans, hydrology, and water quality of surface fresh waters. However, most do not account for high-resolution spatial and temporal variability within watersheds, and they typically lack any representation of benthic dynamics that can drive pollution legacy effects. We present here the Integrated Model to Assess the Global Environment-Dynamic Global Nutrient Model (IMAGE-DGNM), which couples global, spatially explicit hydrology and integrated assessment models with process-based biogeochemistry in surface fresh waters. The new Dynamic In-Stream Chemistry (DISC) module calculates advective transport from headwaters to estuaries, processes in the water column and in bed sediments, as well as the exchanges between these two compartments. As application examples of IMAGE-DGNM, we simulate sediment dynamics and nitrogen cycling in two large river basins. We assess in-stream concentration time series at specific locations, and identify governing processes in transfers along the aquatic continuum. Results highlight the importance of benthic dynamics in watersheds highly perturbed by damming. The implementation of such dynamics within IMAGE-DGNM allows for including the temporal effect of pollution legacies in large scale water quality studies and shifts in pollutant speciation along river continua. This new framework therefore incorporates new features for large basin to global scale studies that are crucial to better predict the effects on receiving ecosystems and evaluate future environmental management pathways. Plain Language Summary Humans have strongly modified the functioning of the Earth's surface fresh waters, through pollution emissions and infrastructure, which has led to widespread ecological deterioration. Over the past decades, our understanding of the interactions between humans and the environment has been translated into models to investigate future sustainable pathways. However, large-scale water quality models are usually too coarse to identify spatiotemporal pollution hotspots within river networks. Most of all, they lack any representation of pollution remobilization from bed sediments, which can delay the response to mitigation measures. To bridge these gaps, we developed a new tool simulating in-stream pollutant transfer and transformation processes, allowing for the assessment of changes in the water quality over time within whole river networks. This tool is applicable globally and is applied here to two large watersheds to simulate sediment and nitrogen dynamics. Our results show that including processes in bed sediments is crucial to correctly assess water quality in heavily dammed river networks, such as the Mississippi. The new tool is of great importance for future projections and policy development. It will allow for i...

Dissolved carbon (C) leaching in and from soils plays an important role in C transport along the terrestrial-aquatic continuum. However, a global overview and analysis of dissolved carbon in soil solutions, covering a wide range of vegetation types and climates, is lacking. We compiled a global database on annual average dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in soil solutions, including potential governing factors, with 762 entries from 351 different sites covering a range of climate zones, land cover types and soil classes. Using this database we develop regression models to calculate topsoil concentrations, and concentrations versus depth in the subsoil at the global scale. For DIC, the lack of a proportional globally distributed cover inhibits analysis on a global scale. For DOC, annual average concentrations range from 1.7 to 88.3 (median = 25.27) mg C/L for topsoils (n = 255) and from 0.42 to 372.1 (median = 5.50) mg C/L for subsoils (n = 285, excluding lab incubations). Highest topsoil values occur in forests of cooler, humid zones. In topsoils, multiple regression showed that precipitation is the most significant factor. Our global topsoil DOC model ($${\mathrm{R}}^{2}=0.36$$ R 2 = 0.36 ) uses precipitation, soil class, climate zone and land cover type as model factors. Our global subsoil model describes DOC concentrations vs. depth for different USDA soil classes (overall ($${\mathrm{R}}^{2}=0.45$$ R 2 = 0.45 ). Highest subsoil DOC concentrations are calculated for Histosols.

Rivers play an important role in the global carbon (C) cycle. However, it remains unknown how long-term river C fluxes change because of climate, land-use, and other environmental changes. Here, we investigated the spatiotemporal variations in global freshwater C cycling in the 20th century using the mechanistic IMAGE-Dynamic Global Nutrient Model extended with the Dynamic In-Stream Chemistry Carbon module (DISC-CARBON) that couples river basin hydrology, environmental conditions, and C delivery with C flows from headwaters to mouths. The results show heterogeneous spatial distribution of dissolved inorganic carbon (DIC) concentrations in global inland waters with the lowest concentrations in the tropics and highest concentrations in the Arctic and semiarid and arid regions. Dissolved organic carbon (DOC) concentrations are less than 10 mg C/L in most global inland waters and are generally high in high-latitude basins. Increasing global C inputs, burial, and CO 2 emissions reported in the literature are confirmed by DISC-CARBON. Global river C export to oceans has been stable around 0.9 Pg yr –1 . The long-term changes and spatial patterns of concentrations and fluxes of different C forms in the global river network unfold the combined influence of the lithology, climate, and hydrology of river basins, terrestrial and biological C sources, in-stream C transformations, and human interferences such as damming.

This paper presents the spatially explicit (0.5° spatial resolution) Dynamic InStream Chemistry (DISC)-SILICON module, which is part of the Integrated Model to Assess the Global Environment-Dynamic Global Nutrient Model global nutrient cycling framework. This new model, for the first time, enables to integrate the combined impact of long-term changes in land use, climate, and hydrology on Si sources (weathering, sewage, and soil loss) and sinks (uptake by diatoms, sedimentation, and burial) along the river continuum. Comparison of discharge and dissolved silica results with observations shows good agreement both in the Rhine and Yangtze. The simulated total Si export for the Rhine is stable during the period 1900–2000. The total Si export for the Yangtze decreased (155–51 Gmol yr –1 ) because of damming and transformation of 40% of the natural vegetation to cropland. As a result of dam construction in the Yangtze, diatom primary production (from 24 to 48 Gmol yr –1 ) and burial (15 to 32 Gmol yr –1 ) increased and the DSi export decreased (139–46 Gmol yr –1 ) from the 1950s to 1990s. The Three Gorges Reservoir has a large contribution to diatom primary production (11%) and burial (12%) in the Yangtze basin. DISC-SILICON reproduces a flooding-induced increase in Si inputs and burial and the legacy of this temporary storage in subsequent dry years.

<p><strong>Abstract.</strong> Abstract. Dissolved carbon leaching in and from soils plays an important role in C transport along the terrestrial-aquatic continuum. However, a global overview and analysis of dissolved carbon in soil solutions, covering a wide range of vegetation types and climates, is lacking. We compiled a global database on annual average dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in soil solutions, including potential governing factors, with 762 entries from 351 different sites covering a range of climate zones, land cover types and soil classes. Using this database we develop regression models to calculate topsoil concentrations, and concentrations vs. depth in the subsoil at the global scale. For DIC, the lack of a proportional globally distributed cover inhibits analysis on a global scale. For DOC, annual average concentrations range from 1.7 to 88.3 (median = 25.27) mg C/L for topsoils and from 0.42 to 372.1 (median = 5.50) mg C/L for subsoils (excluding lab incubations). Highest topsoil values occur in forests of cooler, humid zones. In topsoils, multiple regression showed that precipitation is the most significant factor. Our global topsoil DOC model (R<sup>2</sup> = 0.36) uses precipitation, soil class, climate zone and land cover type as model factors. Our global subsoil model describes DOC concentrations vs. depth for different USDA soil classes (overall R<sup>2</sup> = 0.45). Highest subsoil concentrations are calculated for Histosols.</p>