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As climate mitigation efforts lag, dependence on anthropogenic CO₂removal increases. Enhanced rock weathering (ERW) is a rapidly growing CO₂removal approach. In terrestrial ERW, crushed rocks are spread on land where they react with CO₂and water, forming dissolved inorganic carbon (DIC) and alkalinity. For long-term sequestration, these products must travel through rivers to oceans, where carbon remains stored for over 10,000 years. Carbon and alkalinity can be lost during river transport, reducing ERW efficacy. However, the ability of biological processes, such as aquatic photosynthesis, to affect the fate of DIC and alkalinity within rivers has been overlooked. Our analysis indicates that within a stream-order segment, aquatic photosynthesis uptakes 1%–30% of DIC delivered by flow for most locations. The effect of this uptake on ERW efficacy, however, depends on the cell-membrane transport mechanism and the fate of photosynthetic carbon. Different pathways can decrease just DIC, DIC and alkalinity, or just alkalinity, and the relative importance of each is unknown. Further, data show that expected river chemistry changes from ERW may stimulate photosynthesis, amplifying the importance of these biological processes. We argue that estimating ERW’s carbon sequestration potential requires consideration and better understanding of biological processes in rivers. 

Methylmercury (MeHg) and, to a lesser extent, inorganic mercury (IHg) contamination of rice is a global public health concern, but little is known about how soil and grain Hg concentrations respond to elevated CO2 (ECO2), or how ECO2 alters movement of Hg through the soil-plant-grain system. To advance knowledge of how Hg contamination of rice will change in the future, this study explored the effect of elevated CO2 (ECO2, c. 800 ppm) on soil, iron plaque, root, stem/leaf, and grain concentrations of MeHg and IHg. We observed evidence that ECO2 increased accumulation of MeHg, but not IHg, in rice grain. For IHg, ECO2 did not alter its uptake from the soil, translocation through the plant, or concentration in rice grain. However, ECO2 did reduce uptake of IHg from the air into leaf tissues, likely as a result of the reduced stomatal conductivity and thus more limited direct uptake from the air. Methylmercury concentrations in the grain of plants grown at ECO2 were significantly higher than those of plants grown at ambient CO2. Moreover, MeHg concentrations were also elevated in stem/leaf (82 %) and root tissue (37 %) for ECO2 plants, although the root-tissue results were not statistically significant. In contrast, soil MeHg concentrations were virtually indistinguishable between treatments, indicating that higher rice grain MeHg concentrations were not likely due to higher microbial IHg methylation in soil. Plant uptake of MeHg into stem/leaves and grain from the soil was significantly greater in the ECO2 treatment; however, translocation patterns of MeHg within the plant itself did not differ between treatments. Notably, these patterns existed despite consistently lower transpiration in the ECO2 treatment, and thus less mass flow of solute towards and through the plant. Our results indicate that as CO2 concentrations rise, the human health risks related to MeHg in grain will likely increase. 

Abstract Northern high-latitudes are projected to get warmer and wetter, which will affect rates of permafrost thaw and mechanisms by which thaw occurs. To better understand the impact of rain, as well as other factors such as snow depth, canopy cover, and microtopography, we instrumented a degrading permafrost plateau in south-central Alaska with high-resolution soil temperature sensors. The site contains ecosystem-protected permafrost, which persists in unfavorable climates due to favorable ecologic conditions. Our study (2020–2022) captured three of the snowiest years and three of the four wettest years since the site was first studied in 2015. Average thaw rates along an across-site transect increased nine-fold from 6 ± 5 cm yr⁻¹(2015–2020) to 56 ± 12 cm yr⁻¹(2020–2022). This thaw was not uniform. Hummock locations, residing on topographic high points with relatively dense canopy, experienced only 8 ± 9 cm yr⁻¹of thaw, on average. Hollows, topographic low points with low canopy cover, and transition locations, which had canopy cover and elevation between hummocks and hollows, thawed 44 ± 6 cm yr⁻¹and 39 ± 13 cm yr⁻¹, respectively. Mechanisms of thaw differed between these locations. Hollows had high warm-season soil moisture, which increased thermal conductivity, and deep cold-season snow coverage, which insulated soil. Transition locations thawed primarily due to thermal energy transported through subsurface taliks during individual rain events. Most increases in depth to permafrost occurred below the ∼45 cm thickness seasonally frozen layer, and therefore, expanded existing site taliks. Results highlight the importance of canopy cover and microtopography in controlling soil thermal inputs, the ability of subsurface runoff from individual rain events to trigger warming and thaw, and the acceleration of thaw caused by consecutive wet and snowy years. As northern high-latitudes become warmer and wetter, and weather events become more extreme, the importance of these controls on soil warming and thaw is likely to increase. 

Abstract Northern post-glacial lakes are significant, increasing sources of atmospheric carbon through ebullition (bubbling) of microbially-produced methane (CH₄) from sediments. Ebullitive CH₄flux correlates strongly with temperature, reflecting that solar radiation drives emissions. However, here we show that the slope of the temperature-CH₄flux relationship differs spatially across two post-glacial lakes in Sweden. We compared these CH₄emission patterns with sediment microbial (metagenomic and amplicon), isotopic, and geochemical data. The temperature-associated increase in CH₄emissions was greater in lake middles—where methanogens were more abundant—than edges, and sediment communities were distinct between edges and middles. Microbial abundances, including those of CH₄-cycling microorganisms and syntrophs, were predictive of porewater CH₄concentrations. Results suggest that deeper lake regions, which currently emit less CH₄than shallower edges, could add substantially to CH₄emissions in a warmer Arctic and that CH₄emission predictions may be improved by accounting for spatial variations in sediment microbiota.