Abstract The Southern Ocean (south of 30°S) contributes significantly to global ocean carbon uptake through the solubility, physical and biological pumps. Many studies have estimated carbon export to the deep ocean, but very few have attempted a basin‐scale perspective, or accounted for the sea‐ice zone (SIZ). In this study, we use an extensive array of BGC‐Argo floats to improve previous estimates of carbon export across basins and frontal zones, specifically including the SIZ. Using a new method involving changes in particulate organic carbon and dissolved oxygen along the mesopelagic layer, we find that the total Southern Ocean carbon export from 2014 to 2022 is 2.69 ± 1.23 PgC y−1. The polar Antarctic zone contributes the most (41%) with 1.09 ± 0.46 PgC y−1. Conversely, the SIZ contributes the least (8%) with 0.21 ± 0.09 PgC y−1 and displays a strong shallow respiration in the upper 200 m. However, the SIZ contribution can increase up to 14% depending on the depth range investigated. We also consider vertical turbulent fluxes, which can be neglected at depth but are important near the surface. Our work provides a complementary approach to previous studies and is relevant for work that focuses on evaluating the biogeochemical impacts of changes in Antarctic sea‐ice extent. Refining estimates of carbon export and understanding its drivers ultimately impacts our comprehension of climate variability at the global ocean scale.

Abstract This study provides the longest trend analysis of Nitrogen (N) and phosphorus (P) surplus in France from 1920 to 2020, modeled with the CaSSiS model at both national and departmental levels. At the national scale, the century long average annual N surplus is about 37 ± 13 kg N per ha of utilized agricultural area (UAA) per year, while P surplus averages about 9 ± 7 kg P ha UAA−1 year−1. However, significant periods of change correspond to important agricultural and economic events such as the World Wars and major agri‐environmental reforms. Analysis of N and P use efficiency (NUE and PUE, respectively) revealed varying trends over time. NUE averaged 67%, ranging from 52% to 78%, while PUE exhibited larger fluctuations, ranging from 30% to 130%. At the departmental level, N surplus fluctuated between −15 and 140 kg N ha UAA−1, and P surplus ranged from −15 to 41 kg P ha UAA−1. Temporal trends revealed an increase in N surplus in 96% of departments from 1920 to 1990, followed by a decline in about 89% of departments from 1990 to 2020. P surplus increased in all departments until 1974, followed by a consistent decrease. Analysis of five contrasting French departments highlighted the impact of agricultural practices on nutrient surplus. These findings underscore the importance of tailored nutrient management strategies to achieve balanced inputs and outputs, promoting sustainable agriculture and minimizing environmental impacts. This study contributes valuable insights for informed decision‐making in nutrient management policies and practices.

Abstract Terrestrial dissolved inorganic carbon (DIC) and total alkalinity (TAlk) loads have contrasting effects on the pH and carbonate chemistry of the coastal ocean. While TAlk can buffer against ocean acidification, elevated exports of free CO2 can further exacerbate ocean acidification. In this study, we quantify terrestrial DIC and TAlk loads from rivers and mangrove floodplains across six bioregions and varying flow conditions to assess their impact on the buffering capacity of the Great Barrier Reef (GBR) lagoon in Australia. For a mid‐flow year, median terrestrial DIC and TAlk loads ranged from 0.72 to 0.89 Tg C yr−1 and 0.26 to 1.03 Tg C yr−1, respectively. We find that mangrove‐dominated terrestrial inputs only have a small influence on the whole GBR but contribute 12.5% (range: 1.9%–45.7%) of the DIC and 18.7% (range: 2.8%–68.2%) of the TAlk inner shelf inventory. Depending on the approach used to estimate TAlk loads, mangroves have a potential short‐term buffering effect on near‐shore coastal waters due to higher TAlk loads. However, long‐term mangrove TAlk production via pyrite formation complicates this interpretation, highlighting the need for ongoing monitoring to understand the complex interplay between terrestrial inputs and their effect on the GBR carbonate chemistry.

This case study of Kongsfjorden, western coastal Svalbard, provides insights on how freshwater runoff from marine- and land-terminating glaciers influences the biogeochemical cycles and distribution patterns of carbon, nutrients, and trace elements in an Arctic fjord system. We collected samples from the water column at stations along the fjord axis and proglacial river catchments, and analyzed concentrations of dissolved trace elements, together with dissolved nutrients, as well as alkalinity and dissolved inorganic carbon. Statistical tools were applied to identify and quantify biogeochemical processes within the fjord that govern the constituent distributions. Our results suggest that the glacier type affects nutrient availability and, therefore, primary production. Glacial discharge from both marine-terminating glaciers and riverine discharge from land-terminating glaciers are important sources of dissolved trace elements (dAl, dMn, dCo, dNi, dCu, and dPb) that are involved in biological and scavenging processes within marine systems. We identified benthic fluxes across the sediment-water interface to supply fjord waters with silicate, dFe, dCu, and dZn. Our data show that intensive carbonate weathering in proglacial catchments supplies fjord waters with additional dissolved carbonates and, therefore, attenuates reduced buffering capacities caused by glacial runoff. Our study provides valuable insight into biogeochemical processes and carbon cycling within a climate-sensitive, high-latitude fjord region, which may help predict Arctic ecosystem changes in the future.

Abstract Rice cultivation produces methane (CH4) due to anaerobic conditions induced by flood irrigation, significantly contributing to global warming. While most studies use national emission factors (EFs), our study synthesized 726 published measurements across India (the second largest methane emitter after China) to develop district‐level water regime‐specific EFs for estimating district‐scale emissions and warming potential. CH4 emissions from Indian rice fields increased from 3.7 (3.4–4.1) Tg to 4.8 (4.4–5.3) Tg during 1966–2017, driven by rice area and water‐regime variations. Meanwhile, district‐level emissions increased by ∼930%, influenced by management practices such as animal manure, fertilizer application, and water input, accurately reflecting regional variations compared to previous estimates. Employing a novel muti‐output random forest mitigation model (R2 ∼ 0.9), we found that a 25% warming reduction at the district‐level requires curtailing animal manure, nitrogen fertilizer, and water input by 8.5%, 12.9%, and 10.9%, respectively. These curtailments nearly double for a 50% mitigation scenario. Comparing our emissions with previous bottom‐up studies (used as inputs in global climate models) revealed discrepancies in prior national figures. With top‐down estimates, our emissions correlated positively, suggesting higher reliability. Including our new regionally validated data in global climate models may provide more accurate climate projections at the Indian and global scales.

Abstract Afforestation is widely believed to sequester carbon (C) in soil. However, the effect of afforestation on soil organic C (SOC) accumulation is still debated due to the contrasting features of particulate and mineral‐associated organic C (POC and MAOC). We conducted a field investigation of 144 paired sampling sites by comparing afforested and non‐afforested lands to investigate the POC and MAOC dynamics after afforestation across the Danjiangkou basin in subtropical China, where forests are dominated by Platycladus orientalis, Quercus variabilis and Pinus massoniana. The average contents of SOC, POC, and MAOC were significantly increased by afforestation; however, POC and MAOC responded differently to afforestation type. All afforestation types promoted the POC content, and MAOC also showed positive responses to afforestation except that afforestation with P. massoniana from shrubland significantly reduced the MAOC content. With increasing SOC content, the POC grew at a faster rate than MAOC at high SOC levels. Afforestation hindered the growth rate of POC, while it promoted the growth rate of MAOC as SOC accrued, which potentially obscured the distinct patterns of C accumulation triggered by afforestation. The variation partitioning suggests that, under afforestation, microbial traits had a higher contribution to both POC and MAOM variations compared with non‐afforested land. These results suggest that the robust buildup of microbial biomass due to increased plant C input following afforestation could contribute to soil C accumulation by promoting microbial necromass.

Abstract The abrupt warming events punctuating the Termination 1 (about 11.7–18 ka Before Present, BP) were marked by sharp rises in the concentration of atmospheric methane (CH4). The role of permafrost organic carbon (OC) in these rises is still debated, with studies based on top‐down measurements of radiocarbon (14C) content of CH4 trapped in ice cores suggesting minimum contributions from old and strongly 14C‐depleted permafrost OC. However, organic matter from permafrost can exhibit a continuum of 14C ages (contemporaneous to >50 ky). Here, we investigate the large‐scale permafrost remobilization at the Younger Dryas‐Preboreal transition (ca. 11.6 ka BP) using the sedimentary record deposited at the Lena River paleo‐outlet (Arctic Ocean) to reflect permafrost destabilization in this vast drainage basin. Terrestrial OC was isolated from sediments and characterized geochemically measuring δ13C, Δ14C, and lignin phenol molecular fossils. Results indicate massive remobilization of relatively young (about 2,600 years) permafrost OC from inland Siberia after abrupt warming triggered severe active layer deepening. Methane emissions from this young fraction of permafrost OC contributed to the deglacial CH4 rise. This study stresses that underestimating permafrost complexities may affect our comprehension of the deglacial permafrost OC‐climate feedback and helps understand how modern permafrost systems may react to rapid warming events, including enhanced CH4 emissions that would amplify anthropogenic climate change.

Abstract Marine mesozooplankton play an important role for marine ecosystem functioning and global biogeochemical cycles. Their size structure, varying spatially and temporally, heavily impacts biogeochemical processes and ecosystem services. Mesozooplankton exhibit size changes throughout their life cycle, affecting metabolic rates and functional traits. Despite this variability, many models oversimplify mesozooplankton as a single, unchanging size class, potentially biasing carbon flux estimates. Here, we include mesozooplankton ontogenetic growth and reproduction into a 3‐dimensional global ocean biogeochemical model, PISCES‐MOG, and investigate the subsequent effects on simulated mesozooplankton phenology, plankton distribution, and organic carbon export. Utilizing an ensemble of statistical predictive models calibrated with a global set of observations, we generated monthly climatologies of mesozooplankton biomass to evaluate the simulations of PISCES‐MOG. Our analyses reveal that the model and observation‐based biomass distributions are consistent ( = 0.40, total epipelagic biomass: 137 TgC from observations vs. 232 TgC in the model), with similar seasonality (later bloom as latitude increases poleward). Including ontogenetic growth in the model induced cohort dynamics and variable seasonal dynamics across mesozooplankton size classes and altered the relative contribution of carbon cycling pathways. Younger and smaller mesozooplankton transitioned to microzooplankton in PISCES‐MOG, resulting in a change in particle size distribution, characterized by a decrease in large particulate organic carbon (POC) and an increase in small POC generation. Consequently, carbon export from the surface was reduced by 10%. This study underscores the importance of accounting for ontogenetic growth and reproduction in models, highlighting the interconnectedness between mesozooplankton size, phenology, and their effects on marine carbon cycling.

Abstract In this work, we investigated the seasonal cycle of atmospheric potential oxygen (APO), a unique tracer of air‐sea gas exchanges of molecular oxygen (O2) and carbon dioxide (CO2), expressed as APO = O2 + 1.1 × CO2. APO data were obtained from flask air samples collected since the late 1990s at three Japanese ground stations and on commercial cargo ships sailing between Japan and Australia/New Zealand, North America, and Southeast Asia. We also analyzed the APO spatial distribution and seasonal cycles with simulations from an atmospheric transport model using climatological oceanic O2 fluxes from an empirical product that relate O2 flux to ocean heat as input. Model simulations reproduced the observed APO seasonal cycles generally well, but with larger amplitudes and earlier occurrence of seasonal minima and maxima than in the observations. Moreover, the observed seasonal cycles exhibited larger APO enhancements than the simulations in autumn and early winter, especially in the North Pacific at 20°N–60°N. These enhancements remained when refining the comparison by adjusting the simulated APO peak‐to‐peak amplitudes and seasonal phases to the observations. This suggests additional O2 emissions in the North Pacific, not well expressed in the air‐sea O2 fluxes used as input for our model simulations. The average autumn enhancement at 40°N–60°N was approximately twice that measured at 20°N–40°N. Confirming previous studies, our results indicate two distinct mechanisms possibly contributing to the additional oceanic O2 emissions: outgassing from a subsurface shallow oxygen maximum at 20°N–40°N and autumn phytoplankton bloom at 40°N–60°N.