Mangroves are carbon dense ecosystems. Their root exudates could remobilise buried soil organic matter in the form of CO2 emission, notably by stimulating organic matter decay indirectly via an exudate sugar-driven and microbially mediated pathway, or directly by the breakage of organo-mineral bonds. Here, we used a manipulative laboratory incubation to test the effect of root exudate type on CO2 emission in two contrasting mangrove soils: a peat soil with mostly particulate organic matter (Dumbea, New Caledonia, France) and a mineral soil dominated by organo-mineral associations (Can Gio, Vietnam). Using a custom-made 20 cm long needle with a side-port near the tip,we spiked two exudates types, oxalic acid and glucose, into the mineral and organic mangrove soils. The soil CO2 emission was quantified with a gas analyser over time. Iron and pH were mapped at high spatial resolution using two-dimensional Diffusive Equilibrium Thin-films (2D-DET) gels. The root exudate inputs significantly increased the CO2 emission in both mangroves (by an order of magnitude; p< 0.01). The organic rich and mineral mangrove soil CO2 emission responded similarly to both root exudate types. There was no difference in soil CO2 emission between glucose and oxalic acid treatment. Oxalic acid reduced the soil pH consistently across the vertical soil profile in the mineral mangrove soil, while in the peat soil there was a sharp pH decrease in the few top millimetres of soil. For both soil types, the iron concentration was multiplied by an order of magnitude under oxalic acid treatment with a peak in the soil surface, and was slightly increased under glucose treatment. Our results reveal that root exudation could be a major driver of carbon, pH, and iron dynamics in mangrove soils. These findings highlight the importance of understanding root-soil interaction to constrain mangrove carbon budgets.

Coastal wetlands store high amounts of organic carbon (OC) in their sediments, but long-term preservation of this carbon depends on habitat type, sediment depth, and the molecular characteristics of organic matter (OM). This study explores the dynamics of OC deposition and preservation across vertical profiles (0-30 cm) in two adjacent coastal habitats-mudflat, and salt-marsh-within the macrotidal system of the Aiguillon Bay (France). A multi-tracer approach was applied, combining stable isotopes δ13C, C/N ratios, lignin phenols, and fatty acids. Sediment OC content ranged from 13.4 to 23.2 mgC g-1, with the highest concentrations found in the salt-marsh. δ13C and C/N signatures revealed dominant marine source in the mudflat, with a secondary contribution from microphytobenthos, and mixed marine-C₃ plant inputs in the salt-marsh. Fatty acids and lignin compositions supported this partitioning, with surface mudflat layers enriched in labile microbial and algal-derived compounds, whereas deeper salt-marsh sediments retained more resistant, C3 plant-derived signatures resembling those of terrestrial OM source. OM degradation rates were closely linked to source composition and depth. Degradation was concentrated within the top 5 cm of salt-marsh and the top 10 cm of mudflat. Below these depths, biomarker profiles changed minimally, delineating a transition to longer-term preservation. First-order degradation constants were three times higher in mudflat (0.53 yr-1) than in salt-marsh (0.17 yr-1), despite similarly high sedimentation rates (1.8 and 2.2 cm yr-1, respectively). This reflects differences in OM lability, with even minor contributions from microphytobenthos enhancing reactivity in mudflats. Salt-marshes, with their intermediate OM reactivity and high sedimentation rates, emerged as hotspots of carbon accumulation (366 gC m-2 yr -1), while mudflats also contributed substantially to coastal carbon sequestration (239 gC m-2 yr -1). These results highlight the value of depth-resolved, biomarker-based approaches to identify habitat-specific degradation dynamics; ultimately better understanding carbon accumulation in coastal ecosystems.

Nature-based solutions to climate change must incorporate mitigation strategies that sustain and enhance forest carbon sequestration, requiring comprehensive accounting of forest carbon budgets, including carbon stored in roots and soils. Forests' capacity to remain as carbon sinks under elevated CO2 (eCO(2)) may depend on tree root systems adjusting to overcome nutrient and water limitation. It remains uncertain whether and how root systems can change across depth under eCO(2) in mature forests. We assessed fine root biomass, morphology, depth distribution and C:N ratio, using 1-m-deep soil cores from Years 5 and 7 of the Birmingham Institute of Forest Research Free-Air CO2 Enrichment experiment (BIFoR FACE), a mature, deciduous forest subject to eCO(2) (+150 mu l/L, i.e., mid-21st century projected atmospheric CO2 concentration). Fine root biomass was similar to 40% greater under eCO(2), concentrated in the top 50 cm and equivalent to similar to 36% more root carbon standing stock. Contrary to previous results, the distribution of fine root biomass did not shift to greater depths. Changes in morphology were variable, but, on average, there was greater fine root length under eCO(2) and, with depth, higher specific root length. Under eCO(2), greater fine root biomass and changes in morphology result in higher fine root surface area and thereby a greater potential for resource acquisition across the soil profile. Better characterisation of fine roots under eCO(2) can benefit belowground carbon modelling, improving predictions of forest carbon sinks and refining estimates of forests as natural climate solutions for climate policy.

Abstract. Coastal wetlands are among the most carbon-dense ecosystems on Earth, yet belowground carbon cycling remains poorly understood compared to aboveground processes. Root exudation, the release of labile organic compounds from live roots, represents a critical pathway for transferring plant-derived carbon to soils. This flux is poorly quantified because existing measurement techniques fail under flooded and tidal conditions, leaving a knowledge gap in carbon budgets. Here, we adapted and applied a sealed-cuvette system to quantify root exudation in situ across the two most common and widespread mangrove genera worldwide (Rhizophora and Avicennia) and across contrasting wet and dry seasons in a deltaic mangrove (Cần Giờ, Vietnam). The sealed cuvette method was successful in measuring root exudation, improving current methods for coastal wetlands. Mean root exudation rates were 126 ± 172 μg C g−1 h−1 for Avicennia and 68.5 ± 96.1 μg C g−1 h−1 for Rhizophora, with seasonal rates of 52.4 ± 67.2 μg C g−1 h−1 for the wet season and 135 ± 168 μg C g−1 h−1 for the dry season. Root exudation did not differ significantly across genera and seasons and was similar in magnitude to those of terrestrial forests. When upscaled at the ecosystem level, the root exudation is estimated to represent a considerable portion of the mangrove GPP (6.1–11.9 %). We conclude that root exudation is a non-negligible and previously unaccounted-for component of mangrove carbon budgets and highlight the need for quantification of this ‘missing carbon flux’.

Tropical ecosystems contain the world's largest biodiversity of vascular plants. Yet, our understanding of tropical functional diversity and its contribution to global diversity patterns is constrained by data availability. This discrepancy underscores an urgent need to bridge data gaps by incorporating comprehensive tropical root data into global datasets. Here, we provide a database of tropical root characteristics. This new database, TropiRoot 1.0, will be instrumental in evaluating an array of hypotheses pertaining to root functional ecology and plant biogeography, both within the tropics and relative to other global biomes. The data compilation was conducted by the TropiRoot Initiative, in partnership with the Fine-Root Ecology Database (FRED) and the Global Root Trait (GRooT) database, Colorado State University (CSU) and the Smithsonian Tropical Research Institute (STRI). Literature search and data extraction were conducted between 2020 and 2024. Literature was identified using Web of Science, Scopus, and complemented using the expert knowledge of members of TropiRoot. To provide broad environmental and geographical distributions, literature searches included root characteristics (traits) across global change drivers, natural gradients, and from different continents. We adopted FRED standardized data columns and streamlined the format to enhance accessibility for data extraction across various user groups. This optimized framework resulted in a smaller, yet comprehensive datasheet. To make the database compatible with other global root trait initiatives, column identification was standardized following the codes provided by FRED. These efforts culminated in data extracted from 104 new sources, resulting in more than 8000 rows of data (either species or community data). Most of the data in TropiRoot 1.0 include root characteristics such as root biomass, morphology, root dynamics, mass fraction, architecture, anatomy, physiology, and root chemistry. This initiative represents a 30% increase in the currently available data for tropical roots in FRED. TropiRoot 1.0 contains root characteristics from 25 different countries, where seven are located in Asia, six in South America, five in Central America and the Caribbean, four in Africa, two in North America, and 1 in Oceania. Due to the volume of data, when ancillary data were available, including soil data, these data were either extracted and included in the database or its availability was recorded in an additional column. Multiple contributors checked the entries for outliers during the collation process to ensure data quality. For text-based observations, we examined all cells to ensure that their content relates to their specific categories. For numerical observations, we ordered each numerical value from least to greatest and plotted the values, checking apparent outliers against the data in their respective sources and correcting or removing incorrect or impossible values. Some data (soil and aboveground) have different columns for the same variable presented in different units, including originally published units, but root characteristics data had units converted to match those reported in FRED. By filling a gap from global databases, TropiRoot 1.0 expands our knowledge of otherwise so far underrepresented regions and our ability to assess global trends. This advancement can be used to improve tropical forest representation in vegetation models. The data are freely available and should be cited when used.

Mangrove forests store significant amounts of blue carbon, mainly as soil organic matter. Insights into mangrove organic matter are limited, despite its importance for understanding blue carbon accumulation and vulnerability to global change. Here, we quantified soil organic matter preserved through chemical recalcitrance and association with the mineral phase, as key factors influencing blue carbon persistence. We found that the nature of the soil organic matter varied with mangrove geomorphic settings. Delta settings were dominated by presumably the most persistent soil organic matter associated with minerals, while open coast karstic settings contained mostly particulate soil organic matter, likely preserved due to low microbial activity. Across mangrove soil depths, there was little difference in soil organic matter pools. The soil organic matter pool across mangroves' geomorphic settings exhibited greater variation than that observed across all terrestrial biomes. These findings underscore the need to tailor mangrove conservation and restoration to geomorphic settings.

Leaf, stem, and fine root morphological and chemical traits serve as key indicators of plant performances in restoration projects. However, traits and their major dimensions of variations (closely integrated trait groups) at the whole-plant level during mangrove restoration remain poorly understood. We evaluated the intraspecific variations and covariations of 22 leaf, stem, and fine root traits for individuals of the mangrove species Bruguiera sexangula at young and mature stages in Hainan, China using a ‘space-for-time’ approach. During B. sexangula tree development, the intraspecific trait variation was mainly characterized by the coordination of fine root, stem, and leaf traits (e.g., decreases in leaf area and root diameter (RD); increases in stem and root tissue density (RTD)). Additionally, root traits varied along a ‘collaboration’ gradient (a spectrum reflecting root nutrient acquisition strategies) with trade-offs between RD and specific root length (SRL), reflecting strategies ranging from ‘outsourcing’ (thick RD) in young individuals to ‘do-it-yourself’ (large SRL) in mature individuals. However, the intraspecific trait covariations of leaf and fine root traits deviated from expectation of the ‘conservation’ gradient, evidenced by the decoupling between leaf nitrogen content and specific leaf area (SLA), and the lack of trade-offs between root nitrogen content and RTD. Our findings highlight significant shifts in mangrove traits and ecological strategies, both above- and belowground, during the restoration of B. sexangula, underscoring the importance of adopting a whole-plant perspective in future trait-based mangrove research.

This article is a Commentary on Milligan et al . (2026), 249 : 777–791 .

Salt marshes are blue carbon (C) ecosystems characterized by intense atmospheric CO2 uptake and C sequestration but also by organic and inorganic C exports through the tide. However, uncertainties about the main biotic factors controlling these vertical and horizontal C fluxes imply studying terrestrial and aquatic metabolisms simultaneously at small timescales (diurnal and tidal) to distinguish their contributions to net ecosystem CO2 exchange (NEE). In a temperate salt marsh, four sampling 24 h cycles were performed to measure all water C biogeochemical parameters (including CO2 partial pressures, pCO(2)), nutrients, and aquatic metabolism simultaneously to NEE from high tide during marsh immersion (imported coastal waters influenced by the continental shelf) to low tide during marsh emersion (exported channel waters influenced by the marsh drainage). At high tide, water CO2 oversaturation (water pCO(2) > air pCO(2)) due to marsh aquatic heterotrophy and CO2-concentrated water inputs from the coastal end-member induced water-air CO2 emissions during marsh immersion. At low tide, water pCO(2) in the channel were also mainly controlled by the marsh aquatic metabolism, inducing a water CO2 oversaturation in winter due to dominant heterotrophy and a water CO2 undersaturation in spring and summer due to dominant autotrophy. In winter, the greatest increases in dissolved inorganic carbon (DIC; from 2354 to 3963 mu mol kg(-1)), total alkalinity (TA; from 2508 to 4016 mu mol kg(-1)) and dissolved inorganic nitrogen (DIN; from 27.7 to 68.4 mu M) were measured simultaneously during low tide at night, probably due to intense aerobic/anaerobic microbial respiration of organic matter in channel waters and/or sediments resulting in the greatest water pCO(2) increase (from 533 to 1461 ppmv). On the contrary, in spring and summer, large water pCO(2) decreases (down to 83 ppmv) and dissolved organic carbon (DOC) increases (up to 1040 mu M) from high to low tide could be related to intense autochthonous and allochthonous marsh primary production, including benthic microalgae, phytoplankton and macroalgae. This study suggests that the horizontal exchanges of coastal waters with the salt marsh significantly modify water C dynamics and associated water CO2 sink/source state in the channel due to an intense marsh metabolism (production and respiration). At the daily scale, plant and phytoplankton metabolism rates played a major and a minor role, respectively, in the marsh CO2 sink measured by atmospheric eddy covariance at the ecosystem scale (NEE), even during immersion where emerged plants located on the highest marsh levels can maintain a low CO2 uptake, despite aquatic heterotrophy and associated water-air CO2 emissions.

Salt marshes are among the most efficient blue carbon (C) sinks in the world, partly due to the slow decomposition of their plant-derived organic matter (OM) in the soil. The fate of this C sink under sea-level rise is still uncertain due to limited knowledge about the processes controlling OM decomposition under different inundation levels. In an in-situ manipulative experiment, we compared salt marsh OM decomposition and quality across simulated sea-level scenarios and litter types (absorptive root, fine transportive root, leave, and rhizome of the shrubby C3 halophyte Halimione Portulacoide) for 170 days. The OM decomposition rate varied only between the longest and shortest inundation treatments, that was lower than the mean inundation of our site. The OM decomposition and C loss rates varied strongly across litter types. Fine absorptive was the slowest to decay, releasing up to 40% less C than the other litter types. Changes in lignin composition varied across litter types, but were unaffected by sea-level rise scenarios. Our study suggests that 1) the assessment of soil C dynamics in salt marshes based on aboveground litter or bulk belowground litter patterns is inadequate because of a marked difference in OM decomposition across litter types; 2) belowground litter lignin quality could be a good proxy for OM decomposition in salt marshes; and 3) sea-level rise is unlikely to decrease OM decomposition under current sea-level rise projections.

Fine roots are a major source of the stabilised carbon in soils. However, the response of fine root production to an increase in atmospheric CO2 and its impact on carbon dynamics in terrestrial forests remain poorly understood. Minirhizotrons can help to quantify fine root production and associated carbon dynamics in long-term, in-situ experiments such as Free Air CO2 Enrichment experiments. Yet, using minirhizotrons requires the manual annotation of thousands of images. Artificial Intelligence (AI) technology for image processing is fast developing and has proven to be successful in simple systems, such as agronomous crops. Here, we quantified how AI (RootPainter) annotation compares with humans, and determined the implications in terms of root production and carbon dynamics in a mature deciduous forest (BIFOR-FACE). Firstly, we quantified the variation in outputs of 30 annotated minirhizotron images using AI and human analysts of varying levels of expertise, comparing them to a gold standard established through expert consensus. We find that root annotation varied substantially among humans, with novices and AI over-annotating root length by 244% and 206% respectively, compared to our gold standard. Secondly, we quantified root length for five minirhizotron tubes in March and June (n = 1060 images) using AI and then a trained human analyst. AI over-estimated root length by more than an order of magnitude compared to a trained human user, and there was a poor linear relationship between annotated images with AI and humans (r² < 0.22 for both months). This over-annotation by AI resulted in inaccurate quantification of root production and mortality, and thus erroneous carbon budget.

Evidence supporting a carbon fertilisation effect, where increasing levels of carbon dioxide (CO2) in the atmosphere lead to photosynthetic enhancement in trees, suggests that forests can sequester more carbon under elevated CO2 (eCO2). However, it remains largely unclear where and for how long this carbon is stored within the forest ecosystem. To sustain photosynthetic enhancement under eCO2 concentrations, trees are likely to require higher intake of nutrients from the soil, which should stimulate root growth. This ongoing study (2022-2026) investigates the hypothesis that fine root biomass and turnover rates will increase, and proliferation will be higher at greater depths, because of eCO2. It is vital that the consequences of increased atmospheric CO2 on plant carbon allocation are understood to improve the accuracy of models projecting the future of forests as global carbon sinks. This study is carried out at the Birmingham Institute of Forest Research Free Air Carbon Enrichment (BIFoR FACE) experiment, the only FACE experiment in a mature, temperate forest simulating atmospheric CO2 concentrations to those predicted to be the mid-century planetary norm. For ambient and elevated CO2 treatments, dry biomass of fine roots, specific root length (SRL) and depth distribution were assessed. 1m soil cores, deeper than the standard 30cm, were used to investigate changes in fine root depth distribution. Changes in fine root growth rates are calculated from minirhizotron images taken at monthly intervals over a 2-year period, with 15 replicates per treatment. Average fine root biomass was >30% higher under eCO2 in all depths down to 70cm. As expected, fine root biomass declined approximately exponentially with depth under both elevated and ambient CO2 conditions, but this slope of decline was lower under eCO2. Other than in the O horizon, average SRL was also higher under eCO2 with depth, meaning roots were on average longer per unit biomass. This implies that trees adapt root proliferation and morphology to increase the volume of soil exploited under eCO2, particularly at greater depths.

Mangrove ecosystems are one of the most carbon dense ecosystems worldwide. Yet, the stabilization and recalcitrance of carbon (C) and organic matter (OM) are little understood in mangroves, especially across eco-geomorphological settings and depths. Here, we characterized the sediment C and OM of Indo-Pacific mangroves, located in four distinct eco-geomorphological settings (i.e., delta, estuary, non-carbonated open coast, carbonated open coast) and at two different depths (i.e., 0-20 cm and 80-100 cm). We quantified the fraction of C within (i) mineralized associated organic matter (MAOM), and (ii) within particulate organic matter (POM). We coupled these analyses with lignin quantity and composition, as well as stable C isotopes analysis in mangrove sediments.We found significant variation in the quantity of MAOM and POM across mangrove eco-geomorphological settings, but not across mangrove sediment depths. The terrigenous deltaic mangrove exhibited up to three times more MAOM than the carbonate open coast mangrove, which was dominated by POM. Mangroves of the carbonate coast type had higher C content than other eco-geomorphic types. The was not different across mangrove eco-geomorphologies, but was different across mangrove sediment depths. Regarding OM recalcitrance, the lignin content displayed strong variations across the different eco-geomorphologies, however, there was no clear pattern of lignin degradation stage across depths. Finally, an inverse correlation between sediment C recalcitrance (i.e., lignin content) and stabilization (MAOM) processes were determined across mangroves.Our findings suggest that the processes leading to OM preservation differ among mangroves in various eco-geomorphological settings. Those results have important implications to guide mangrove restoration for carbon persistence and to model carbon pools across mangrove areas.

Salt marshes are among the most efficient blue carbon sinks worldwide. The fate of this carbon is uncertain due to limited knowledge about organic matter (OM) decomposition processes under sea-level rise. In an in-situ manipulative experiment, we compared salt marsh OM decomposition and quality across simulated sea-level scenarios (by modifying the inundation) and litter types (absorptive root, fine transportive root, leaves, and rhizomes of Halimione portulacoide) for 170 days. The litter decomposition varied only between the inundation treatments with the longest and shortest durations, while the decomposition differed significantly across litter types, with absorptive roots releasing up to 40% less carbon than other litters. Changes in lignin composition were minimal for absorptive roots and were unaffected by sea-level rise scenarios. Our study suggests that (i) current projections of sea-level rise are unlikely to decrease litter decomposition; (ii) separating litter types might lead to better assessments of salt marshes' OM dynamics. Global estimates of salt marsh organic matter decomposition dynamics can be improved by taking into account the differences in decomposition rates among litter types, according to an in situ manipulative experiment in a French temperate salt marsh

Root trait variation may reflect the ecological and evolutionary processes shaping biodiversity, but remains poorly quantified in the (sub)tropics. Here, we aim to further complete our knowledge of belowground functional strategies by assessing the contributions of subtropical and tropical species to global root trait diversity. We gathered root data for 1618 temperate, 341 subtropical, and 775 tropical species. We compared functional diversity among biomes and calculated the unique contribution of each biome to the global root economics space. Further, we determined if the within-variation of subtropical and tropical biomes is shaped by species niches and/or differences in evolutionary history. Root trait expressions differed among biomes, but root functional diversity did not. Furthermore, subtropical and tropical biomes accounted for 40% of the unique root functional space within the global traits space. Species climate niches and phylogenetic turnover explained variation in root traits (e.g., denser root tissue was associated with drier sites) among subtropical but not tropical species. Through their unique root traits, sub(tropical) species strongly expand the current global root trait space. This work underwrites their importance in conceptual models for more complete insights into how various belowground strategies drive plant functional biogeography and biodiversity globally. ### Competing Interest Statement The authors have declared no competing interest.

Abstract Background The manual study of root dynamics using images requires huge investments of time and resources and is prone to previously poorly quantified annotator bias. Artificial intelligence (AI) image-processing tools have been successful in overcoming limitations of manual annotation in homogeneous soils, but their efficiency and accuracy is yet to be widely tested on less homogenous, non-agricultural soil profiles, e.g., that of forests, from which data on root dynamics are key to understanding the carbon cycle. Here, we quantify variance in root length measured by human annotators with varying experience levels. We evaluate the application of a convolutional neural network (CNN) model, trained on a software accessible to researchers without a machine learning background, on a heterogeneous minirhizotron image dataset taken in a multispecies, mature, deciduous temperate forest. Results Less experienced annotators consistently identified more root length than experienced annotators. Root length annotation also varied between experienced annotators. The CNN root length results were neither precise nor accurate, taking ~ 10% of the time but significantly overestimating root length compared to expert manual annotation (p = 0.01). The CNN net root length change results were closer to manual (p = 0.08) but there remained substantial variation. Conclusions Manual root length annotation is contingent on the individual annotator. The only accessible CNN model cannot yet produce root data of sufficient accuracy and precision for ecological applications when applied to a complex, heterogeneous forest image dataset. A continuing evaluation and development of accessible CNNs for natural ecosystems is required.

Tropical forest root characteristics and resource acquisition strategies are underrepresented in vegetation and global models, hampering the prediction of forest-climate feedbacks for these carbon-rich ecosystems. Lowland tropical forests often have globally unique combinations of high taxonomic and functional biodiversity, rainfall seasonality, and strongly weathered infertile soils, giving rise to distinct patterns in root traits and functions compared with higher latitude ecosystems. We provide a roadmap for integrating recent advances in our understanding of tropical forest belowground function into vegetation models, focusing on water and nutrient acquisition. We offer comparisons of recent advances in empirical and model understanding of root characteristics that represent important functional processes in tropical forests. We focus on: (1) fine-root strategies for soil resource exploration, (2) coupling and trade-offs in fine-root water vs nutrient acquisition, and (3) aboveground-belowground linkages in plant resource acquisition and use. We suggest avenues for representing these extremely diverse plant communities in computationally manageable and ecologically meaningful groups in models for linked aboveground-belowground hydro-nutrient functions. Tropical forests are undergoing warming, shifting rainfall regimes, and exacerbation of soil nutrient scarcity caused by elevated atmospheric CO2. The accurate model representation of tropical forest functions is crucial for understanding the interactions of this biome with the climate.

Within the coastal zone, salt marshes are atmospheric CO2 sinks and represent an essential component of biological carbon (C) stored on earth due to a strong primary production. Significant amounts of C are processed within these tidal systems which requires a better understanding of the temporal CO2 flux dynamics, the metabolic processes involved and the controlling factors. Within a temperate salt marsh (French Atlantic coast), continuous CO2 fluxes measurements were performed by the atmospheric eddy covariance technique to assess the net ecosystem exchange (NEE) at diurnal, tidal and seasonal scales as well as the associated relevant biophysical drivers. To study marsh metabolic processes, measured NEE was partitioned into gross primary production (GPP) and ecosystem respiration (Reco) during marsh emersion allowing to estimate NEE at the marsh–atmosphere interface (NEEmarsh = GPP − Reco). During the year 2020, the net C balance from measured NEE was −483 g C m−2 yr−1 while GPP and Reco absorbed and emitted 1019 and 533 g C m−2 yr−1, respectively. The highest CO2 uptake was recorded in spring during the growing season for halophyte plants in relationships with favourable environmental conditions for photosynthesis, whereas in summer, higher temperatures and lower humidity rates increased ecosystem respiration. At the diurnal scale, the salt marsh was a CO2 sink during daytime, mainly driven by light, and a CO2 source during night-time, mainly driven by temperature, irrespective of emersion or immersion periods. However, daytime immersion strongly affected NEE fluxes by reducing marsh CO2 uptake up to 90 %. During night-time immersion, marsh CO2 emissions could be completely suppressed, even causing a change in metabolic status from source to sink under certain situations, especially in winter when Reco rates were lowest. At the annual scale, tidal immersion did not significantly affect the net C uptake of the studied salt marsh since similar annual balances of measured NEE (with tidal immersion) and estimated NEEmarsh (without tidal immersion) were recorded.