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The timescales over which soil carbon responds to global change are a major uncertainty in the terrestrial carbon cycle. Radiocarbon measurements on archived soil samples are an important tool for addressing this uncertainty. We present time series (1969–2023) of radiocarbon measurements for litter (Oi/Oe and Oa/A) and mineral (0–10 cm) soils from the Hubbard Brook Experimental Forest, a predominantly hardwood forest in the northeastern USA. To estimate soil carbon cycling rates, we built different autonomous linear compartmental models. We found that soil litter carbon cycles on decadal timescales (Oi/Oe: ~7 years), whereas carbon at the organic‐mineral interface (Oa/A), and mineral soil (0–10 cm) carbon cycles on centennial timescales (~104 and 302 years, respectively). At the watershed‐level, the soil system appears to be at steady‐state, with no observed changes in carbon stocks or cycling rates over the study period, despite increases in precipitation, temperature, and soil pH. However, at the site‐level, the Oi/Oe is losing carbon (−15 g C m⁻² year⁻¹since 1998). The observed decline in carbon stocks can be detected when the Oi and Oe layers are modeled separately. This pattern suggests that the rapidly cycling litter layer at the smaller scale is responding to recent environmental changes. Our results highlight the importance of litter carbon as an “early‐warning system” for soil responses to environmental change, as well as the challenges of detecting gradual environmental change across spatial scales in natural forest ecosystems. 

Abstract This study examines the seasonal and spatial variabilities of total alkalinity (TA) and dissolved inorganic carbon (DIC) concentrations and fluxes in the Parker River, part of the Plum Island estuary, the largest macro tidal saltmarsh system in northeastern US. Our investigation reveals that the Parker River estuary is a net source of TA and DIC, with outwelling fluxes ranging 8.9–13.2 mmol m⁻² d⁻¹for TA and 10.4–18.1 mmol m⁻² d⁻¹for DIC. Porewater fluxes from marshes contributed to 71% (55%–96%) and 59% (51%–78%) of the TA and DIC outwelling flux, respectively. Spatial heterogeneity is pronounced, with high‐salinity section porewater fluxes nearly three times higher than in low‐salinity section. TA fluxes peak in summer and are lowest in the spring, with the seasonal changes controlled by both hydrology and marsh phenology. The interplay of primary production and aerobic and anaerobic respiration governs carbon transformations in the estuary, leading to net DIC export and contributing to the acidification of adjacent coastal waters. Overall, approximately 20% saltmarsh net primary production is laterally exported to the coastal ocean as TA during 2023, providing potential long‐term carbon sinks comparable to about 40% of sediment organic carbon burial. Our research underscores the spatial and temporal variabilities in TA and DIC fluxes driven by biotic processes and the importance of considering both TA and DIC fluxes to fully understand the carbon sequestration potential of saltmarshes. 

Abstract There is growing evidence that the composition of river microbial communities gradually transitions from terrestrial taxa in headwaters to unique planktonic and biofilm taxa downstream. Yet, little is known about fundamental controls on this community transition across scales in river networks. We hypothesized that community composition is controlled by flow‐weighted travel time of water, in combination with temperature and dissolved organic matter (DOM), via similar mechanisms postulated in the Pulse‐Shunt Concept for DOM. Bacterioplankton and biofilm samples were collected at least quarterly for 2 yr at 30 sites throughout the Connecticut River watershed. Among hydrologic variables, travel time was a better predictor of both bacterioplankton and biofilm community structure than watershed area, dendritic distance, or discharge. Among all variables, both bacterioplankton and biofilm composition correlated with travel time, temperature, and DOM composition. Bacterioplankton beta‐diversity was highest at shorter travel times (< 1 d) and decreased with increasing travel time, showing progressive homogenization as water flows downstream. Bacterioplankton and biofilm communities were similar at short travel times, but diverged as travel time increased. Bacterioplankton composition at downstream sites more closely resembled headwater communities when temperatures were cooler and travel times shorter. These findings suggest that the pace and trajectory of riverine bacterioplankton community succession may be controlled by temperature‐regulated growth rate and time for communities to grow and change. Moreover, bacterioplankton, and to a lesser extent biofilm, may experience the same hydrologic forcing hypothesized in the Pulse‐Shunt Concept for DOM, suggesting that hydrology controls the dispersal of microbial communities in river networks. 

Abstract Methane (CH₄) is a potent greenhouse gas and its concentrations have tripled in the atmosphere since the industrial revolution. There is evidence that global warming has increased CH₄emissions from freshwater ecosystems^1,2, providing positive feedback to the global climate. Yet for rivers and streams, the controls and the magnitude of CH₄emissions remain highly uncertain^3,4. Here we report a spatially explicit global estimate of CH₄emissions from running waters, accounting for 27.9 (16.7–39.7) Tg CH₄ per year and roughly equal in magnitude to those of other freshwater systems^5,6. Riverine CH₄emissions are not strongly temperature dependent, with low average activation energy (E_M = 0.14 eV) compared with that of lakes and wetlands (E_M = 0.96 eV)¹. By contrast, global patterns of emissions are characterized by large fluxes in high- and low-latitude settings as well as in human-dominated environments. These patterns are explained by edaphic and climate features that are linked to anoxia in and near fluvial habitats, including a high supply of organic matter and water saturation in hydrologically connected soils. Our results highlight the importance of land–water connections in regulating CH₄supply to running waters, which is vulnerable not only to direct human modifications but also to several climate change responses on land. 

Abstract Dissolved organic matter (DOM) impacts the structure and function of aquatic ecosystems. DOM absorbs light in the UV and visible (UV–Vis) wavelengths, thus impacting light attenuation. Because absorption by DOM depends on its composition, UV–Vis absorbance is used to constrain DOM composition, source, and amount. Ferric iron, Fe(III), also absorbs in the UV–Vis; when Fe(III) is present, DOM-attributed absorbance is overestimated. Here, we explore how differing behavior of DOM and Fe(III) at the catchment scale impacts UV–Vis absorbance and evaluate how system-specific variability impacts the effectiveness of existing Fe(III) correction factors in a temperate watershed. We sampled five sites in the Connecticut River mainstem bi-weekly for ~ 1.5 years, and seven sites in the Connecticut River watershed once during the summer 2019. We utilized size fractionation to isolate the impact of DOM and Fe(III) on absorbance and show that variable contributions of Fe(III) to absorbance at 254 nm (a 254 ) and 412 nm (a 412 ) by size fraction complicates correction for Fe(III). We demonstrate that the overestimation of DOM-attributed absorbance by Fe(III) is correlated to the Fe(III):dissolved organic carbon concentration ratio; thus, overestimation can be high even when Fe(III) is low. a 254 overestimation is highly variable even within a single system, but can be as high as 53%. Finally, we illustrate that UV-Vis overestimation might impart bias to seasonal, discharge, and land-use trends in DOM quality. Together, these findings argue that Fe(III) should be measured in tandem with UV–Vis absorbance for estimates of CDOM composition or amount. 

Radiocarbon (¹⁴C) is a critical tool for understanding the global carbon cycle. During the Anthropocene, two new processes influenced¹⁴C in atmospheric, land and ocean carbon reservoirs. First,¹⁴C-free carbon derived from fossil fuel burning has diluted¹⁴C, at rates that have accelerated with time. Second, ‘bomb’¹⁴C produced by atmospheric nuclear weapon tests in the mid-twentieth century provided a global isotope tracer that is used to constrain rates of air–sea gas exchange, carbon turnover, large-scale atmospheric and ocean transport, and other key C cycle processes. As we write, the¹⁴C/¹²C ratio of atmospheric CO₂is dropping below pre-industrial levels, and the rate of decline in the future will depend on global fossil fuel use and net exchange of bomb¹⁴C between the atmosphere, ocean and land. This milestone coincides with a rapid increase in¹⁴C measurement capacity worldwide. Leveraging future¹⁴C measurements to understand processes and test models requires coordinated international effort—a ‘decade of radiocarbon’ with multiple goals: (i) filling observational gaps using archives, (ii) building and sustaining observation networks to increase measurement density across carbon reservoirs, (iii) developing databases, synthesis and modelling tools and (iv) establishing metrics for identifying and verifying changes in carbon sources and sinks. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.