Abstract Methane (CH 4 ) is a potent greenhouse gas whose global budget remains uncertain due to incomplete and uncertain constraints on several natural and anthropogenic sources. While livestock dominate animal‐derived methane emissions, emissions from wild animals—particularly ruminants—represent a persistent but under‐quantified component of the global methane cycle. In this study, we present a global, spatially explicit gridded estimate of methane emissions from wild ruminants. The data set provides yearly estimated of methane emissions for the 2000–2024 timeframe. We derive gridded methane emissions by combining species‐level population and global distribution inventories with body‐mass‐based emission estimates through an allometric relationship, allowing for a consistent representation of wild ruminant methane emissions at the global scale. Our results indicate that global methane emissions from wild ruminants sum up to around 2.95 Tg yr −1 , which is substantially lower than estimates derived from IPCC Tier 1 (∼15 Tg yr −1 ) methodologies but overlaps with previous allometric‐based assessments. Differences among estimates are primarily attributable to contrasting population data sets and methodological approaches. Although subject to uncertainties related to population data, seasonal variability, and the exclusion of non‐ruminant herbivores, this study provides a transparent and reproducible baseline estimate of methane emissions from wild animals. The resulting gridded product is intended to support global methane budgeting efforts, atmospheric modeling, and future studies aiming to refine estimates of natural methane sources.

Abstract Lake‐groundwater systems are characterized by pronounced seasonal recharge dynamics and nutrient accumulation within low‐permeability aquifers. Microbial communities mediate nitrogen transformations in these heterogeneous systems, yet their spatiotemporal dynamics and responses to the groundwater flow regimes remain largely unresolved. In this study, the evolution of groundwater microbial communities was investigated across contrasting recharge zones in the Poyang Lake area, one of China's largest lake‐groundwater systems, during dry and wet seasons. The discharge area with the low‐permeability aquifer was enriched with NH 4 + ‐N, Fe 2+ , and total organic carbon (TOC), exhibited lower oxidation‐reduction potential (ORP), and showed limited seasonal variability across seasons compared with other aquifers experiencing stronger hydrological seasonality. Despite the hydrochemical stability, microbial communities in the low‐permeability aquifer displayed greater dry‐to‐wet seasonal shifts, suggesting a heightened biological sensitivity to even subtle hydrological variations. From dry to wet season, denitrification potential decreased, whereas nitrogen fixation potential increased, with the strongest shifts occurring in the low‐permeability aquifer. Although microbial communities in this aquifer were associated with Fe 2+ , NH 4 + ‐N, and ORP, these variables independently explained only a small proportion of the total variance (<8%). Intriguingly, community assembly was primarily stochastic, with stronger drift in the low‐permeability aquifer, indicating environmental‐microbial decoupling. Microbial source tracking analysis suggested that microbial communities in the discharge area were largely associated with upstream groundwater with additional potential contributions from lake‐groundwater exchange, which may reflect delayed recharge signatures. These findings suggest that hydrological dynamics, more than static chemistry, modulate variability of microbial community assembly and nitrogen‐cycling in groundwater.

Abstract Nutrient fertilization of freshwater systems can influence greenhouse gases (GHGs) by altering carbon and nutrient cycling. Increases in productivity and organic matter availability may drive CO 2 and CH 4 , while nitrogen (N) availability may impact N 2 O concentrations and emissions. While the impact of nutrient concentrations on GHGs individually in lakes and ponds has been well described, we have less information on how increases in nutrient concentration impact the relative influence of CO 2 , CH 4 , and N 2 O on total GHG concentrations and emissions and the total impact on global warming potential (GWP) from these systems. We used experimental mesocosms (limnocorrals) to establish replicated treatments of increasing N:phosphorus (P) across a wide gradient of N loads and measured the concentration and emissions of CO 2 , CH 4 , and N 2 O as well as GWP in total CO 2 equivalents. With increasing TN concentrations, we observed declines in emissions and concentrations of CO 2 (−17.01 vs. 11.31 mm m −2 d −1 with greater N load treatment) and increases in N 2 O (mean of 56.77 vs. −0.36 μmol m −2 d −1 with greater N load treatment). We did not observe any systematic pattern between nutrient concentrations and CH 4 . Interestingly, nutrient loads did not alter GWP due to greater CO 2 uptake offsetting increased N 2 O under high TN concentrations and variable CH 4 across the treatments. Our study suggests that increases in N 2 O from even extreme N loading may not necessarily increase the total warming impact from these systems, as higher productivity and greater CO 2 influx may override increased N 2 O emissions as a result of fertilization.

Abstract Northern peatlands are important sources of methane (CH 4 ) in the atmosphere. However, the magnitude of CH 4 emissions and their response to environmental factors are poorly constrained within the Hudson Bay Lowlands (HBL), the largest contiguous peatland complex in North America. This study investigated seasonal (April–November) eddy covariance‐derived ecosystem scale CH 4 emissions and their predictors from 16 site‐years over four different HBL peatlands. Average seasonal emissions were greatest at a permafrost‐free treed fen over 7 years (6.0 g CH 4 m −2 ; wettest and warmest peatland) and about 40% lower at a co‐located bog over 6 years (3.8 g CH 4 m −2 ). Emissions were least at a permafrost peat plateau 250 km to the north near the Hudson Bay coast over 1 year (2.6 g CH 4 m −2 ; driest and coolest peatland) and about 60% higher at a co‐located thawed peatland over 2 years (4.1 g CH 4 m −2 ). The combined temporal and spatial variability was not well explained by average air temperature or water table depth but instead was related to measures of soil temperature, soil moisture, and gross primary productivity, which were also significantly and positively correlated. At the daily scale, hysteresis was observed in the exponential CH 4 flux–soil temperature relationship. Water table depth was an important predictor of day‐to‐day variations in CH 4 flux. Results from these paired peatlands suggest that warming will generally increase CH 4 emissions in the HBL, which may be moderated by peat drying or exacerbated by peat plateau collapse and wetting in permafrost‐affected peatlands.

Abstract Peatlands influence the global carbon cycle by storing carbon and releasing greenhouse gases such as methane (CH 4 ) and carbon dioxide (CO 2 ). Imaging hot spots for gas accumulation in peat remains challenging due to spatial and temporal heterogeneity and the invasive nature of traditional techniques. Minimally invasive geophysical methods such as ground‐penetrating radar (GPR) have been used to image gas distribution in peat, but the need for direct ground contact limits its applicability in isolated environments. To address these issues, this study evaluated the feasibility of laboratory‐based air‐coupled GPR, in which the antenna is suspended above the surface, to image hot spots for biogenic gas accumulation in peat. Air‐coupled and ground‐based GPR measurements were applied to a peat monolith (0.75 × 0.31 × 0.25 m) from the Everglades (FL, USA) and constrained by flux measurements from gas traps fitted with time‐lapse cameras and analyzed via gas chromatography. Air‐coupled GPR imaged hot spots for gas accumulation with lateral dimensions of 0.05 × 0.03 m to 0.15 × 0.20 m, with gas content up to 25%, fluxes up to 171.9 mg CH 4 m −2 day −1 , and CH 4 contents exceeding 70%. Hot spots were associated with slightly higher porosity and distinct peat structure, suggesting that physical properties of peat may influence gas storage and release behavior. These results highlight the role of peat physical properties in CH 4 emissions, demonstrate the potential of air‐coupled GPR for non‐invasive monitoring of biogenic gas dynamics under controlled conditions, and support future evaluation of drone‐based GPR surveys in peatlands.

Abstract Wetlands play a crucial role as natural climate solutions (NCS) by sequestering atmospheric carbon dioxide (CO 2 ) in the form of organic carbon (OC) in soils. However, spatial heterogeneity and temporal variability in OC sequestration rates introduce uncertainties that must be addressed to inform climate policy and meet national climate targets. This study integrates expert knowledge with statistical learning techniques to develop localized estimates of OC sequestration rates in wetlands within agricultural landscapes. Experts identified direct process controls—including carbon quantity and quality, cation exchange capacity, aggregate reactivity, redox potential, and air temperature—along with human activities that influence these controls. Using geospatial proxies for these variables, alongside field‐based OC sequestration measurements, we compared and trained statistical learning models that achieved high predictive accuracy (adjusted R 2 ≥ 0.70, MAE ≤ 0.15 Mg C ha −1 yr −1 , RMSE ≤ 0.19 Mg C ha −1 yr −1 ). Variable importance analysis identified wetland inundation probability (a proxy for wetland redox potential), Human Impact Index (a proxy for wetland carbon quantity and quality), and landscape soil properties (proxies for wetland soil texture, cation exchange capacity, and aggregate reactivity) as the most influential predictors explaining the spatiotemporal variation in OC sequestration rates. This study demonstrates that statistical learning models, informed by expert knowledge of process controls, can estimate OC sequestration rates within wetlands, providing potentially critical data to guide policy development and wetland management as effective NCS strategies.

Abstract The increasing frequency of toxic cyanobacterial blooms (TCBs) in freshwater lakes in recent years has been problematic, both in terms of our understanding of basic drivers, and attempts to manage them. While phosphorus has been determined to limit the growth of blooms, some cyanophytes manufacture toxins with a high nitrogen (N) demand, so toxin production may be N‐limited. Skaneateles Lake, an oligotrophic lake in New York, has experienced TCBs in recent years, despite its status as one of the cleanest lakes in the United States, and strict watershed regulations regarding agricultural and other nutrient sources. This study investigated whether gaseous ammonia (NH 3 ) deposition contributes to the nitrogen (N) load in the lake, potentially exacerbating the toxicity of these blooms. Using a network of NH 3 air samplers, we estimated both direct and indirect NH 3 deposition to the lake, and compared these fluxes to other sources of N to the lake and its watershed. Follow up sampling campaigns were conducted to extend the deposition estimates to Skaneateles into a second and third year, and to compare estimates of NH 3 deposition to nearby Owasco Lake. Our findings indicate that, not including potential contributions from reduced N deposition to the watershed, dry gaseous NH 3 deposition directly to Skaneateles Lake alone represents 11%–12% of the watershed N load to the lake with ammonium (NH 4 + ) deposition contributing another 7%–10% suggesting that strategies to manage atmospheric reduced N emissions from nearby agricultural sources could reduce the N loading to Skaneateles.

Abstract Peatlands store a substantial amount of carbon in the terrestrial ecosystem. They are both long‐term sinks of organic carbon and a major natural source of atmospheric methane. The accumulation of carbon is a result of net primary production surpassing decomposition rates over millennia, whereas methane production is intricately linked to the anaerobic decomposition of carbon mass. Warming‐induced alterations in net primary productivity and decomposition rates are impacting net emissions, thereby jeopardizing the carbon sink capacity of these carbon‐rich ecosystems and potentially turning them into sources of carbon dioxide and methane. In this study, we modeled the past and future trends of peatland carbon and methane fluxes and their influence on the climate system. We found that peatlands >25°N will remain a carbon sink and methane source under a low‐warming scenario (RCP2.6), but they would shift from being not only a source of methane but also a source of carbon dioxide under a high‐warming scenario (RCP8.5) by the mid‐21st century leading to a strong radiative forcing (0.25 W m −2 ) by the end of the 23rd century. This underlines the potential warming feedback in which peatland radiative forcing on the climate system would shift from negative to positive in the future.

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 −1 for TA and 10.4–18.1 mmol m −2 d −1 for 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.