Blue nitrogen follows the fate of tidal wetlands

Blue nitrogen follows the fate of tidal wetlands | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Blue nitrogen follows the fate of tidal wetlands Adam Langley, Samantha Chapman, Le Wang, Tania Maxwell, Philip Rivera, and 37 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5522814/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Tidal wetlands are hotspots of soil accumulation due to high sedimentation rates and low soil oxygen concentrations that inhibit organic matter decomposition1. Accordingly, tidal wetlands can sequester “blue carbon” at much higher rates than other ecosystems2,3 helping to offset human emissions. Organic carbon burial is tightly linked to the cycling of nitrogen, which is a key pollutant and limiting nutrient for many ecosystems4–6. Yet, the current global burial rate of “blue nitrogen” and how it may respond to future change remain uncertain. We assembled a global database of 8012 soil nitrogen measurements from 255 tidal wetland sites and found that the relationship between soil carbon and nitrogen concentrations was strikingly consistent but differed between the two dominant types of emergent tidal wetlands – marshes and mangroves. Leveraging extensive knowledge of blue carbon accumulation, we estimated that tidal wetlands, which cover less than 0.1% of the Earth’s surface, bury 3.2 Tg N yr-1, representing 13-15% of marine nitrogen burial. This rate could more than triple globally by 2100 if wetland elevation increases with accelerating sea-level rise. Alternatively, if wetlands become submerged and shorelines erode, nitrogen sinks could reverse and become sources, exacerbating coastal pollution. Earth and environmental sciences/Climate sciences/Climate change/Climate-change impacts/Environmental health Biological sciences/Ecology/Biogeochemistry/Element cycles Figures Figure 1 Figure 2 Figure 3 Figure 4 Main The accrual of “blue carbon” captured by tidal marshes, mangroves, seagrasses, and tidal freshwater wetlands has made preserving and restoring tidal wetlands a focal natural climate change solution 1 . Recent eutrophication crises, such as the extensive Sargassum bloom in the tropical Atlantic and persistent red tides along the Florida Gulf coast, are linked to escalating nutrient runoff 6,7,12 , even while N loads may be declining in some uplands 13 . Despite the importance of N in ecosystem functioning and water quality 8,14,15 , the magnitude of tidal wetland N soil burial has yet to be explicitly estimated. Inferred estimates of global marine N burial range from 22-25 Tg N year -1 , but the current contribution of tidal wetlands is unclear 16–18 . Moreover, the burial rate is likely to change with ongoing physical changes. For instance, sea level rise (SLR) causes increased tidal inundation, which can increase sediment deposition, allowing for an expanding soil volume, faster burial and preservation of organic matter 5,19 . We estimate current and future N burial in marsh and mangrove soils considering trends in wetland area change and wetland elevation gain 20,21 . The N cycle is intimately linked to the C cycle, so stoichiometric relationships to C can help constrain N budgets. Valuation of the C sink capacity of tidal wetlands as a natural climate solution has led to an increasing demand for tidal wetland conservation and restoration worldwide 21–23 . Specifically, blue C syntheses have estimated the magnitudes of C stocks in mangrove and tidal marsh biomass and soil C sequestration potential 2,24–26 . Studies of C storage and flux in coastal wetlands exceed those on N many times because of growing scientific and economic interest in blue C. In addition to controlling production of potent greenhouse gases like nitrous oxide, N availability is a key regulator of C fluxes. For example, N availability is a limiting nutrient for aquatic and terrestrial primary productivity. Thus, N uptake and storage are co-benefits to wetland conservation and restoration that have gained recognition within international sustainability efforts 27 . If soil N stoichiometry is predictable by commonly measured parameters and relatively simple techniques, such as C storage using loss-on-ignition methods, then we can leverage the extensive knowledge about present and future C storage in wetlands to estimate the current status and future capacity for the critical co-benefit of N storage. To estimate global blue N stocks and accumulation rates we (1) assembled a database of soil N concentration in mangroves and tidal marsh soils, (2) identified the best predictors of soil N concentrations, (3) estimated current N stocks and accumulation rates, and (4) projected how those stocks and rates may change in the future. Our database of tidal wetland soil profiles included 8012 samples from 910 cores across 255 tidal wetland sites (Fig. S1) in which core segments were analyzed for both organic C and N concentrations. We developed models to predict soil C:N and used extensive databases of soil C accumulation rates to estimate N accumulation rates in tidal wetlands worldwide 22,24 . Finally, because tidal wetlands can adjust to the acceleration of relative sea level rise (rSLR) by increasing rates of surface elevation gain 2,5 , we extrapolated the potential for global N accumulation in, or release from, tidal wetland soils according to projections for future sea level and change in tidal wetland extent. Results And Discussion Predictors of soil N The model using only soil [C], habitat type, and their interaction yielded accurate prediction of tidal wetland soil [N] (leave-one-out validation, r = 0.94, Model 1 in Table S1, Fig. S2). The relationship between soil [C] and soil [N] differed between marshes and mangroves (soil[C] x habitat, t 5775 = 28.5, p<0.0001), but was remarkably consistent within each habitat type (Fig. 1, Table S1). In soils with higher [C], which also have lower bulk density, mangroves tended to have higher C:N ratios than marshes (habitat marsh, t 4581 =-4.81, p<0.0001, Model 2 in Table S2, Fig. S3). Soils with lower bulk density have higher [C], reflecting a stronger plant influence due to the relatively low mineral content. Mangroves produce woody structures with higher C:N than other plant tissues 3 with higher portions of compounds like lignin that slow decomposition 28 . Soil C:N tended to increase with depth overall when accounting for bulk density and habitat (depth, t 7327 = 13.18, p<0.0001, Model 3 in Table S2, Fig. S4A), but the direction and strength of the relationship between C:N and depth varied greatly across studies (Fig. S4B). The strong correlation between soil [C] and [N] and ample data on wetland soil C stocks and accumulation rates, allow for the prediction of soil N stocks and accumulation rates globally. Tidal wetland N accumulation rates and stocks Extrapolating from a database of C accumulation rates across 103 studies representing a broad geographical distribution 24 , we found that the median N accumulation rate is 7.52 (IQR: 4.12-13.61) g N m -2 yr -1 in mangroves and 8.19 (IQR: 5.37-12.44) g N m -2 yr -1 in marshes (Fig. 2A). These areal rates are roughly twenty times greater than implied N accumulation rates based on upland terrestrial C sink estimates 29 , assuming C:N of buried organic matter = 15 (ref 3). Our estimate falls within previous N accumulation rate estimates from regional marsh 30 and mangrove 31 compilations. Scaling to the area of marshes and mangroves that the dataset represents (Table S5), this areal rate yields a global N burial rate of 3.2±0.1 Tg N yr -1 (Fig. 2B). We applied our C:N relationships to estimates of C mass in wetlands 22 to determine that the global mass of N in tidal wetland soils is approximately 278 Tg N to 1 m depth. Our estimate of N accumulation in tidal wetlands represents 13-15% of the global burial of N estimated for the ocean 16,18 . As with blue C, the N accumulation rate depends on the soil accumulation rate, which is strongly influenced by the rate of rSLR and availability of sediments, among other factors 2,19,25,32 . How the global N accumulation rate may change in the future depends on the fate of tidal wetland extent and how much vertical accommodation space is afforded by increasing rSLR rates 2 . Wetland N burial could represent a strong N sink from surrounding coastal waters, but the impact depends on the sources of buried N. Tidal wetland N demand can be satisfied with inputs by N delivered during tidal flooding, N-fixation, groundwater delivery, and atmospheric deposition (Fig. 3) 33,34 . If the N buried in wetlands derives largely from runoff and surrounding waters, then the effect of accreting wetlands on estuarine N could be large. However, if N derives from fixation, wetland soil N gain would constitute less of a sink for estuarine N loads. There is little consensus on the contribution of different N sources that support tidal wetland N burial globally 34 , and sources are likely to vary at the scale of catchments 35 . However, here we have quantified global rates of tidal wetland N accumulation in soil, which represents a large and relatively certain N flux. Future potential for global N accumulation in tidal wetland soils We estimate future global N burial resulting from changing wetland area and sea level rise (SLR) scenarios (Table S4). Our compilation of N stocks and accumulation rates, along with and increasing accuracy of tidal wetland area assessment from remote sensing techniques, allowed us to estimate how tidal wetland N budgets may change in the future. We used recently reported projections of changes in tidal wetland area and SLR to project possible changes in N accumulation and release (Fig. 4, Table S4). Projections of future wetland area differ widely, owing to uncertainty in factors such as accommodation space, sediment loads, and the maximal rSLR rates that tidal wetlands can tolerate 32 . We used six scenarios of SLR and change in tidal wetland extent to provide estimates of future change in coastal N burial. Though local isostatic rSLR can deviate greatly from global eustatic SLR, for tractability we assumed that rising SLR increases rSLR, and we assumed that the rate of elevation gain in surviving wetlands would match rSLR (see methods). To estimate a low tidal wetland elevation gain, we assumed that the wetland elevation gain rate matches the pace of rSLR for a best-case scenario in which mean global warming remains at 1.5°C by 2100 36 . To estimate a high-end for global mean elevation gain, we assume that tidal wetlands will gain elevation following rSLR rates up to a rate of 8 mm yr -1 by 2100, a mid-range probability of SLR for scenario of 4°C warming by 2100 36 . Though actual sea level rise could surpass this rate, tidal wetlands elevations are unlikely to gain elevation beyond 8 mm yr -1 32 . We found that global N accumulation in wetlands has likely increased since the beginning of the Industrial Revolution despite the loss of marsh area, owing to accelerated elevation gain in response to rSLR, similar to observations for C uptake 26 . If the current global mean rate of rSLR remains constant and the wetland area does not change, tidal wetlands would bury an additional 109 Tg N by 2100 (Fig 4, Table S4). However, future rSLR and wetland area projections vary substantially based on policy, social, and ecological uncertainties, as well as global variations in isostatic adjustment in land elevation that influence rSLR which is a significant control of tidal wetland organic matter accumulation 2 . The six N futures we generated resulted in vastly different outcomes for global N accumulation (Table S4). Considering future scenarios of wetland area change and rSLR, total net N accumulation ranges from -138 to +274 Tg N by 2100. High-SLR scenarios (2, 4, and 6 in Fig. 4) resulted in greater N burial than the corresponding low-SLR scenarios (1, 3, 5 in Fig. 4; Table S4) owing to more rapid N uptake in surviving wetlands. To isolate the influences of area change and elevation gain, rSLR and area change varied independently herein (Table S4); however, higher rates of rSLR (scenarios 2, 4, and 6) are likely to lead to greater wetland losses 37 . If high SLR occurs with gains in wetland area 38 , then global wetland soil N stocks could nearly double by 2100. At the other extreme, in scenario 1, high wetland losses and low rates of elevation gain could result in a net global release of half of existing tidal wetland soil N stocks. Summing gross N fluxes to global net rates masks important variation across the local scale where N fluxes hold ecological relevance. Importantly, unlike CO 2 and its effects on climate, the impacts of N eutrophication are heterogeneous and relatively local, such that N trends could have opposing influences in different places. For instance, N may be released from a collapsing salt marsh in one estuary, while topography may allow lateral transgression of a mangrove and thus high N uptake elsewhere. To capture the possibility of these effects, we estimated gross N uptake as the rate of soil N accumulation in surviving wetlands (Fig. 4c) and gross N loss rates assuming the erosional loss of 50 cm of soil when wetlands are lost (Fig. 4d). Even if accelerating wetland elevation gain yields an increase in global net N burial, erosion of collapsing wetlands could result in a gross release of 150 Tg of N, which could have strong effects on local N fluxes (Fig. 4d). Will wetland elevation gain be N limited? Tidal wetlands are broadly expected to respond to accelerating rSLR with increased rates of elevation gain up until a threshold. While it is known that N availability commonly limits productivity in most tidal marshes 4,8 and some mangroves 39,40 , it is not yet known to what extent plant N limitation translates into N limitation of elevation gain 41 . The N burial rate of a tidal wetland will relate to the rate of elevation gain given that correlations between rates of rSLR and soil C sequestration are robust, and variation in soil C:N ratios are constrained (Fig. 1). If the elevation gain rate accelerates in response to rSLR, N accumulation in tidal wetlands will also increase. Based on the stoichiometry of wetland soil sinks and rates of C accumulation, we estimate that organogenic wetlands (those in which soil derives mostly from endogenous organic matter) in areas with low inputs of nutrients may have lower potential elevation gain due to N scarcity, particularly where rates of rSLR are high. For example, a brackish marsh on the Chesapeake Bay, USA, historically gained elevation at roughly 2 mm yr ‑1 , near the millennial rate of rSLR, requiring a N accumulation of 4 g N m -2 yr -1 . However, from 2000 to 2020, the rSLR rate rose to 7 mm yr -1 42 , which would require 14 g N m -2 yr -1 if the marsh were to gain elevation at this rate, acknowledging this demand could be lower with adjustments in C:N through changes in N and biomass allocation or plant species turnover. Given that depositional inputs are negligible 43 and the soil C:N ratio is stable down the profile at this site 44 , a large increase in N inputs through fixation and tidal input or a large decrease in N losses from denitrification or tidal export will be required to meet the increased demand (Fig. 3). This adjustment is unlikely considering recent evidence that N limitation of plant productivity has strengthened in this marsh 43 . We predict that organogenic tidal wetlands with low terrigenous inputs are the tidal wetland settings most likely to experience increasing N limitation of both plant productivity and elevation gain 45 . Soil accumulation approaching future rates of rSLR will likely either be limited by N availability or necessitate a substantial increase in ecosystem net N uptake rates in ogranogenic tidal wetlands. Fates of released N Regardless of its source, once N enters the soil organic N pool, especially below the rooting zone, it may remain nearly inert for millennia depending on biochemical complexity of organic matter, redox status, soil type, and mineral associations 46 . Alternatively, if the wetland is disturbed or the soil erodes, soil N can be released abruptly into surrounding waters 47 and subjected to mineralization, desorption, or re-burial; however, the ultimate fate of eroded soil N remains highly uncertain 11 . If only a fraction of the tidal wetland pool is eroded and recirculated, it could contribute significantly to local coastal N pollution. Chesapeake Bay, for example, has 160,000 ha of coastal wetlands that hold roughly 2.4 Tg of soil N to 1 m depth and are currently burying 6.4 Gg N yr -1 given the soil N densities reported here. As rSLR accelerates, surviving wetlands are likely to increase N burial up to 22 Gg N yr -1 , possibly mitigating historically high riverine N input. However, wetland area loss is expected to accelerate as well. If wetland area is lost at a rate of 1% per year, the N loss from erosion could negate N buried by surviving wetlands. Given the large and potentially dynamic pool of N in tidal wetland soils, we highlight an urgent need to better understand the fates of N released from wetlands that are eroding or transforming to other ecosystem states. Implications Wetland soil accumulation represents a large and historically consistent N sink that has reduced N loads in estuarine and marine waters. The future influence of tidal wetlands on the river-estuary-ocean continuum hinges on future rSLR, wetland response to rSLR, and hydrobiogeomorphic drivers of wetland area change. If wetlands can survive future rSLR or are restored on a large scale, high rates of N uptake to support accelerating wetland elevation gain could improve water quality for seagrass beds and coral reefs. In less polluted regions, accelerating N burial could strengthen N limitation and oligotrophication 13 . However, rSLR could lead to the loss of tidal wetland area and erosion of an uncertain fraction of the soil therein. Wetland erosion could have two impacts on estuarine N budgets: diminishing N uptake and release of currently stored N, exacerbating eutrophication that leads to harmful algal blooms, dead zones, and fish kills 6 . Changes in tidal wetland N accumulation rates represent relatively certain and high-leverage control over local and global N cycling that merits consideration from land managers and global policymakers. Blue C sequestration in tidal wetlands has been well documented, with estimates ranging from 44 to 75 Tg yr -1 of C 24,48,49 , representing about 1% of total C sequestration in natural sinks 29 . We estimate that 3.2 Tg N yr -1 is sequestered by mangroves and marshes, representing 13-15% of global N burial 16–18 . Including other blue N ecosystems, such as seagrass beds, and mud flats, will increase this estimate. The global monetary value of N burial is difficult to estimate owing to small-scale variation in sources and additivity. Yet, our global N burial rates suggest that the monetary value of blue N burial ($22-106 kg -1 (ref 10) x 3.2 x10 9 kg yr -1 = $70-339 billion USD) could exceed that of blue C ($191 billion 50 ). Future research examining both the sources of N, as well as the alternative fates of N when wetlands are lost, will help refine the value of blue N sequestration and its contribution to coastal and estuarine water quality. Declarations Acknowledgments We thank K Wieder for helpful advice. LH Pérez-Bernal carried out C and N analyses from Mexican wetlands. Funding: The work of JAL was supported by National Science Foundation, Grant/Award Number: DEB- 0950080, DEB-1457100, DEB-1557009 and DEB-2051343. The work of JAL and SKC was supported by the National Science Foundation DEB-1655659 and DEB-2224999. The work by ACRF and JASC; KWK was supported by projects UNAM-PAPIIT IN102821 and IN110624.the U.S. Geological Survey Climate R&D Program; etc. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5522814","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":411618289,"identity":"e04e6ea6-4500-45d2-a9ee-ad31d19a3b0c","order_by":0,"name":"Adam 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University","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"","lastName":"Friess","suffix":""},{"id":411618306,"identity":"8a29cce1-f51d-455b-9a63-6fb4726cb0f5","order_by":17,"name":"Glenn Guntenspergen","email":"","orcid":"https://orcid.org/0000-0002-8593-0244","institution":"US Geological Survey","correspondingAuthor":false,"prefix":"","firstName":"Glenn","middleName":"","lastName":"Guntenspergen","suffix":""},{"id":411618307,"identity":"87d78218-b64e-47c7-b6e9-2e59c28b9ef9","order_by":18,"name":"James Holmquist","email":"","orcid":"https://orcid.org/0000-0003-2546-6766","institution":"Smithsonian Environmental Research Center","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Holmquist","suffix":""},{"id":411618308,"identity":"dd33f580-4175-4c84-bb9f-bd81bb58438e","order_by":19,"name":"Scott Jones","email":"","orcid":"https://orcid.org/0000-0002-1056-3785","institution":"University of Northern Florida","correspondingAuthor":false,"prefix":"","firstName":"Scott","middleName":"","lastName":"Jones","suffix":""},{"id":411618309,"identity":"468bf9bb-31a6-4b35-8b4f-be1bf331f433","order_by":20,"name":"Johanna Jupin","email":"","orcid":"https://orcid.org/0000-0002-3695-9366","institution":"Institut de recherche pour le développement","correspondingAuthor":false,"prefix":"","firstName":"Johanna","middleName":"","lastName":"Jupin","suffix":""},{"id":411618310,"identity":"a9936748-0bf2-4cdc-89eb-b332258aa144","order_by":21,"name":"Brian Kelleher","email":"","orcid":"","institution":"Dublin City University","correspondingAuthor":false,"prefix":"","firstName":"Brian","middleName":"","lastName":"Kelleher","suffix":""},{"id":411618311,"identity":"eda9e3ae-aaf4-4fb5-81c0-688f017eed4a","order_by":22,"name":"Anthony Grey","email":"","orcid":"","institution":"Dublin City 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Survey","correspondingAuthor":false,"prefix":"","firstName":"Ken","middleName":"","lastName":"Krauss","suffix":""},{"id":411618315,"identity":"d63931b2-eae1-4428-aa18-b4d19f2db253","order_by":26,"name":"Catherine Lovelock","email":"","orcid":"https://orcid.org/0000-0002-2219-6855","institution":"University of Queensland","correspondingAuthor":false,"prefix":"","firstName":"Catherine","middleName":"","lastName":"Lovelock","suffix":""},{"id":411618316,"identity":"48e433ed-0746-41cd-816c-344195e15422","order_by":27,"name":"S. Alex McClellan","email":"","orcid":"https://orcid.org/0000-0001-8636-7687","institution":"LSU","correspondingAuthor":false,"prefix":"","firstName":"S.","middleName":"Alex","lastName":"McClellan","suffix":""},{"id":411618317,"identity":"fae8caa0-7916-40fd-a63f-6f589aa13c32","order_by":28,"name":"James Megonigal","email":"","orcid":"https://orcid.org/0000-0002-2018-7883","institution":"Smithsonian Environmental Research Center","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Megonigal","suffix":""},{"id":411618318,"identity":"094d83c0-9345-4c7b-9b56-1f5d54a3da39","order_by":29,"name":"James Morris","email":"","orcid":"https://orcid.org/0000-0002-0511-642X","institution":"Univ South Carolina","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Morris","suffix":""},{"id":411618319,"identity":"cfe0737f-5a89-42a5-a919-96c5d7bc1c29","order_by":30,"name":"Scott 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Research and Development Center","correspondingAuthor":false,"prefix":"","firstName":"Andre","middleName":"","lastName":"Rovai","suffix":""},{"id":411618323,"identity":"e5669b4b-eb37-42aa-8bea-d9bff13e48bb","order_by":34,"name":"Ana Carolina Ruiz-Fernández","email":"","orcid":"https://orcid.org/0000-0002-2515-1249","institution":"Unidad Académica Mazatlán, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México","correspondingAuthor":false,"prefix":"","firstName":"Ana","middleName":"Carolina","lastName":"Ruiz-Fernández","suffix":""},{"id":411618324,"identity":"07588b92-f984-4053-aa0d-c955802d39f8","order_by":35,"name":"Neil Saintilan","email":"","orcid":"https://orcid.org/0000-0001-9226-2005","institution":"Macquarie University","correspondingAuthor":false,"prefix":"","firstName":"Neil","middleName":"","lastName":"Saintilan","suffix":""},{"id":411618325,"identity":"6385eff7-0dae-4ca4-8e3b-41556e3d3828","order_by":36,"name":"Joan Sanchez-Cabeza","email":"","orcid":"","institution":"Universidad Nacional Autónoma de México","correspondingAuthor":false,"prefix":"","firstName":"Joan","middleName":"","lastName":"Sanchez-Cabeza","suffix":""},{"id":411618326,"identity":"4c68be53-746a-4813-a368-295b03648df0","order_by":37,"name":"Lorae Simpson","email":"","orcid":"","institution":"St. Johns River Water Management District","correspondingAuthor":false,"prefix":"","firstName":"Lorae","middleName":"","lastName":"Simpson","suffix":""},{"id":411618327,"identity":"815c1ba1-f899-4f25-b308-bc1f859eccc1","order_by":38,"name":"Craig Smeaton","email":"","orcid":"https://orcid.org/0000-0003-4535-2555","institution":"University of St Andrews","correspondingAuthor":false,"prefix":"","firstName":"Craig","middleName":"","lastName":"Smeaton","suffix":""},{"id":411618328,"identity":"7960c7a4-e080-442f-b77e-5144aa6495dd","order_by":39,"name":"Amanda Spivak","email":"","orcid":"https://orcid.org/0000-0001-6743-0783","institution":"University of Georgia","correspondingAuthor":false,"prefix":"","firstName":"Amanda","middleName":"","lastName":"Spivak","suffix":""},{"id":411618329,"identity":"5f82adb8-b674-4de2-be60-d9688e577b1c","order_by":40,"name":"Havalend Steinmuller","email":"","orcid":"","institution":"Louisiana Universities Marine Consortium","correspondingAuthor":false,"prefix":"","firstName":"Havalend","middleName":"","lastName":"Steinmuller","suffix":""},{"id":411618330,"identity":"9012f03f-d442-4afe-9513-f1e15e8dbe81","order_by":41,"name":"Nathaniel Weston","email":"","orcid":"https://orcid.org/0000-0002-6837-360X","institution":"Villanova University","correspondingAuthor":false,"prefix":"","firstName":"Nathaniel","middleName":"","lastName":"Weston","suffix":""}],"badges":[],"createdAt":"2024-11-25 20:00:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5522814/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5522814/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":75577913,"identity":"306c4024-a373-4477-bc8a-0fe5d8a08c57","added_by":"auto","created_at":"2025-02-06 05:08:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":151705,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe relationship between soil [N] and soil organic [C] across all depths \u0026lt;1 m for mangroves and tidal marshes\u003c/strong\u003e. Lines represent the best linear fit. Histograms show the distribution of data along each axis. Number of samples for mangrove = 2911 and marsh = 5101.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5522814/v1/fb75607905b132239ef99872.png"},{"id":75577914,"identity":"336f8a92-b492-4dca-8ab0-7b18ef066ee3","added_by":"auto","created_at":"2025-02-06 05:08:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":54704,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDistribution of area-based and scaled continental N accumulation rates.\u003c/strong\u003e (A) Histogram of area-based N accumulation rates. Note that\u0026nbsp;the y-axis is on log scale. Total observations = 213 for mangrove and 400 for marsh. \u0026nbsp;(B) The global distribution of scaled N accumulation rates, which sum to 3.2 Tg yr\u003csup\u003e-1\u003c/sup\u003e. Error bars represent the scaled standard error from our modeled C:N estimates.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5522814/v1/2a219d30af0ea749452b76d3.png"},{"id":75577912,"identity":"7a7d4464-5c97-40c2-907b-2de355e55456","added_by":"auto","created_at":"2025-02-06 05:08:14","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":136258,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic of global N fluxes in contemporary marshes and mangroves.\u003c/strong\u003e Soil N stock reflects the upper meter of soil. Error represents scaled standard error from modeled C:N estimates.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5522814/v1/f3923ee84069078cef9b8e73.png"},{"id":75577915,"identity":"8f0b0df5-c364-4e8a-99fc-96db547df2b8","added_by":"auto","created_at":"2025-02-06 05:08:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":382409,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHistorical reconstructions and hypothetical future N burial and release.\u003c/strong\u003e (\u003cstrong\u003eA\u003c/strong\u003e) Past (dotted line) and hypothetical future (dashed and solid lines, the upper end of each line represents the year 2100) global net N burial in tidal wetlands. Colors indicate the global rate of N burial as it varies with the elevation gain rate of surviving wetlands and gains or losses in wetland area. Six N futures (numbered 1-6) represent combinations of high and low projections for tidal wetland area change and wetland elevation gain (Table 5, Methods). Mean wetland elevation in 2020 is defined as zero. (\u003cstrong\u003eB\u003c/strong\u003e) Global net N burial is the sum of (\u003cstrong\u003eC\u003c/strong\u003e) gross N burial by changes in elevation gain and (\u003cstrong\u003eD\u003c/strong\u003e) gross N loss by wetland area loss for futures 1-6. Colors in B-D represent the six N futures.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5522814/v1/df2d0b5b925d0e68ec491cfd.png"},{"id":82917088,"identity":"8de3bf92-fb50-4aef-a6d1-03f42b3ee0e4","added_by":"auto","created_at":"2025-05-16 16:28:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1685042,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5522814/v1/5876973c-7a39-49e8-8d49-63f2af43d160.pdf"},{"id":75577916,"identity":"49c2cfcc-029f-481a-b284-afe53a84b809","added_by":"auto","created_at":"2025-02-06 05:08:16","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2603876,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-5522814/v1/a7c44975e881f2aaa3e03516.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Blue nitrogen follows the fate of tidal wetlands","fulltext":[{"header":"Main","content":"\u003cp\u003eThe accrual of \u0026ldquo;blue carbon\u0026rdquo; captured by tidal marshes, mangroves, seagrasses, and tidal freshwater wetlands has made preserving and restoring tidal wetlands a focal natural climate change solution\u003csup\u003e1\u003c/sup\u003e. Recent eutrophication crises, such as the extensive \u003cem\u003eSargassum\u003c/em\u003e bloom in the tropical Atlantic and persistent red tides along the Florida Gulf coast, are linked to escalating nutrient runoff\u003csup\u003e6,7,12\u003c/sup\u003e, even while N loads may be declining in some uplands\u003csup\u003e13\u003c/sup\u003e. Despite the importance of N in ecosystem functioning and water quality\u003csup\u003e8,14,15\u003c/sup\u003e, the magnitude of tidal wetland N soil burial has yet to be explicitly estimated. Inferred estimates of global marine N burial range from 22-25 Tg N year\u003csup\u003e-1\u003c/sup\u003e, but the current contribution of tidal wetlands is unclear\u003csup\u003e16\u0026ndash;18\u003c/sup\u003e. Moreover, the burial rate is likely to change with ongoing physical changes. For instance, sea level rise (SLR) causes increased tidal inundation, which can increase sediment deposition, allowing for an expanding soil volume, faster burial and preservation of organic matter\u003csup\u003e5,19\u003c/sup\u003e. We estimate current and future N burial in marsh and mangrove soils considering trends in wetland area change and wetland elevation gain\u003csup\u003e20,21\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe N cycle is intimately linked to the C cycle, so stoichiometric relationships to C can help constrain N budgets. Valuation of the C sink capacity of tidal wetlands as a natural climate solution has led to an increasing demand for tidal wetland conservation and restoration worldwide\u003csup\u003e21\u0026ndash;23\u003c/sup\u003e. Specifically, blue C syntheses have estimated the magnitudes of C stocks in mangrove and tidal marsh biomass and soil C sequestration potential\u003csup\u003e2,24\u0026ndash;26\u003c/sup\u003e. Studies of C storage and flux in coastal wetlands exceed those on N many times because of growing scientific and economic interest in blue C. In addition to controlling production of potent greenhouse gases like nitrous oxide, N availability is a key regulator of C fluxes. For example, N availability is a limiting nutrient for aquatic and terrestrial primary productivity. Thus, N uptake and storage are co-benefits to wetland conservation and restoration that have gained recognition within international sustainability efforts\u003csup\u003e27\u003c/sup\u003e. If soil N stoichiometry is predictable by commonly measured parameters and relatively simple techniques, such as C storage using loss-on-ignition methods, then we can leverage the extensive knowledge about present and future C storage in wetlands to estimate the current status and future capacity for the critical co-benefit of N storage.\u003c/p\u003e\n\u003cp\u003eTo estimate global blue N stocks and accumulation rates we (1) assembled a database of soil N concentration in mangroves and tidal marsh soils, (2) identified the best predictors of soil N concentrations, (3) estimated current N stocks and accumulation rates, and (4) projected how those stocks and rates may change in the future. Our database of tidal wetland soil profiles included 8012 samples from 910 cores across 255 tidal wetland sites (Fig. S1) in which core segments were analyzed for both organic C and N concentrations. We developed models to predict soil C:N and used extensive databases of soil C accumulation rates to estimate N accumulation rates in tidal wetlands worldwide\u003csup\u003e22,24\u003c/sup\u003e. Finally, because tidal wetlands can adjust to the\u0026nbsp;acceleration of relative sea level rise (rSLR) by increasing rates of surface elevation gain\u003csup\u003e2,5\u003c/sup\u003e, we extrapolated the potential for global N accumulation in, or release from, tidal wetland soils according to projections for future sea level and change in tidal wetland extent.\u0026nbsp;\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cp\u003e\u003cem\u003ePredictors of soil N\u003c/em\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe model using only soil [C], habitat type, and their interaction yielded accurate prediction of tidal wetland soil [N] (leave-one-out validation, \u0026nbsp;r = 0.94, Model 1 in Table S1, Fig. S2). The relationship between soil [C] and soil [N] differed between marshes and mangroves (soil[C] x habitat, t\u003csub\u003e5775\u003c/sub\u003e = 28.5, p\u0026lt;0.0001), but was remarkably consistent within each habitat type (Fig. 1, Table S1). In soils with higher [C], which also have lower bulk density, mangroves tended to have higher C:N ratios than marshes (habitat marsh, t\u003csub\u003e4581\u003c/sub\u003e=-4.81, p\u0026lt;0.0001, Model 2 in Table S2, Fig. S3). Soils with lower bulk density have higher [C], reflecting a stronger plant influence due to the relatively low mineral content. Mangroves produce woody structures with higher C:N than other plant tissues\u003csup\u003e3\u003c/sup\u003e with higher portions of compounds like lignin that slow decomposition\u003csup\u003e28\u003c/sup\u003e. Soil C:N tended to increase with depth overall when accounting for bulk density and habitat (depth, t\u003csub\u003e7327\u003c/sub\u003e = 13.18, p\u0026lt;0.0001, Model 3 in Table S2, Fig. S4A), but the direction and strength of the relationship between C:N and depth varied greatly across studies (Fig. S4B). The strong correlation between soil [C] and [N] and ample data on wetland soil C stocks and accumulation rates, allow for the prediction of soil N stocks and accumulation rates globally.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eTidal wetland N accumulation rates and stocks\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eExtrapolating from a database of C accumulation rates across 103 studies representing a broad geographical distribution\u003csup\u003e24\u003c/sup\u003e, we found that the median N accumulation rate is 7.52 (IQR: 4.12-13.61) g N m\u003csup\u003e-2\u003c/sup\u003e yr\u003csup\u003e-1\u003c/sup\u003e in mangroves and 8.19 (IQR: 5.37-12.44) g N m\u003csup\u003e-2\u003c/sup\u003e yr\u003csup\u003e-1\u003c/sup\u003e in marshes (Fig. 2A). These areal rates are roughly twenty times greater than implied N accumulation rates based on upland terrestrial C sink estimates\u003csup\u003e29\u003c/sup\u003e, assuming C:N of buried organic matter = 15 (ref 3). Our estimate falls within previous N accumulation rate estimates from regional marsh\u003csup\u003e30\u003c/sup\u003e and mangrove\u003csup\u003e31\u003c/sup\u003e compilations. Scaling to the area of marshes and mangroves that the dataset represents (Table S5), this areal rate yields a global N burial rate of 3.2\u0026plusmn;0.1 Tg N yr \u003csup\u003e-1\u003c/sup\u003e (Fig. 2B). We applied our C:N relationships to estimates of C mass in wetlands\u003csup\u003e22\u003c/sup\u003e to determine that the global mass of N in tidal wetland soils is approximately 278 Tg N to 1 m depth. Our estimate of N accumulation in tidal wetlands represents 13-15% of the global burial of N estimated for the ocean\u003csup\u003e16,18\u003c/sup\u003e. As with blue C, the N accumulation rate depends on the soil accumulation rate, which is strongly influenced by the rate of rSLR and availability of sediments, among other factors\u003csup\u003e2,19,25,32\u003c/sup\u003e. How the global N accumulation rate may change in the future depends on the fate of tidal wetland extent and how much vertical accommodation space is afforded by increasing rSLR rates\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eWetland N burial could represent a strong N sink from surrounding coastal waters, but the impact depends on the sources of buried N.\u0026nbsp;Tidal wetland N demand can be satisfied with inputs by N delivered during tidal flooding, N-fixation, groundwater delivery, and atmospheric deposition (Fig. 3)\u003csup\u003e33,34\u003c/sup\u003e.\u0026nbsp;If the N buried in wetlands derives largely from runoff and surrounding waters, then the effect of accreting wetlands on estuarine N could be large. However, if N derives from fixation, wetland soil N gain would constitute less of a sink for estuarine N loads. There is little consensus on the contribution of different N sources that support tidal wetland N burial globally\u003csup\u003e34\u003c/sup\u003e, and sources are likely to vary at the scale of catchments\u003csup\u003e35\u003c/sup\u003e. However, here we have quantified global rates of tidal wetland N accumulation in soil, which represents a large and relatively certain N flux.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eFuture potential for global N accumulation in tidal wetland soils\u003c/em\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe estimate future global N burial resulting from changing wetland area and sea level rise (SLR) scenarios (Table S4). Our compilation of N stocks and accumulation rates, along with and increasing accuracy of tidal wetland area assessment from remote sensing techniques, allowed us to estimate how tidal wetland N budgets may change in the future. We used recently reported projections of changes in tidal wetland area and SLR to project possible changes in N accumulation and release (Fig. 4, Table S4). Projections of future wetland area differ widely, owing to uncertainty in factors such as accommodation space, sediment loads, and the maximal rSLR rates that tidal wetlands can tolerate\u003csup\u003e32\u003c/sup\u003e. We used six scenarios of SLR and change in tidal wetland extent to provide estimates of future change in coastal N burial. Though local isostatic rSLR can deviate greatly from global eustatic SLR, for tractability we assumed that rising SLR increases rSLR, and we assumed that the rate of elevation gain in surviving wetlands would match rSLR (see methods). To estimate a low tidal wetland elevation gain, we assumed that the\u0026nbsp;wetland elevation gain rate matches the pace of rSLR for a best-case scenario in which mean global warming remains at 1.5\u0026deg;C by 2100\u003csup\u003e36\u003c/sup\u003e. To estimate a high-end for global mean elevation gain, we assume that tidal wetlands will gain elevation following rSLR rates up to a rate of 8 mm yr\u003csup\u003e-1\u003c/sup\u003e by 2100, a mid-range probability of SLR for scenario of 4\u0026deg;C warming by 2100\u003csup\u003e36\u003c/sup\u003e. Though actual sea level rise could surpass this rate, tidal wetlands elevations are unlikely to gain elevation beyond 8 mm yr\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003e\u003csup\u003e32\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe found that global N accumulation in wetlands has likely increased since the beginning of the Industrial Revolution despite the loss of marsh area, owing to accelerated elevation gain in response to rSLR, similar to observations for C uptake\u003csup\u003e26\u003c/sup\u003e. If the current global mean rate of rSLR remains constant and the wetland area does not change, tidal wetlands would bury an additional 109 Tg N by 2100 (Fig 4, Table S4). However, future rSLR and wetland area projections vary substantially based on policy, social, and ecological uncertainties, as well as global variations in isostatic adjustment in land elevation that influence rSLR which is a significant control of tidal wetland organic matter accumulation\u003csup\u003e2\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe six N futures we generated resulted in vastly different outcomes for global N accumulation (Table S4). Considering future scenarios of wetland area change and rSLR, total net N accumulation ranges from -138 to +274 Tg N by 2100. High-SLR scenarios (2, 4, and 6 in Fig. 4) resulted in greater N burial than the corresponding low-SLR scenarios (1, 3, 5 in Fig. 4; Table S4) owing to more rapid N uptake in surviving wetlands. To isolate the influences of area change and elevation gain, rSLR and area change varied independently herein (Table S4); however, higher rates of rSLR (scenarios 2, 4, and 6) are likely to lead to greater wetland losses\u003csup\u003e37\u003c/sup\u003e. If high SLR occurs with gains in wetland area\u003csup\u003e38\u003c/sup\u003e, then global wetland soil N stocks could nearly double by 2100. At the other extreme, in scenario 1, high wetland losses and low rates of elevation gain could result in a net global release of half of existing tidal wetland soil N stocks.\u003c/p\u003e\n\u003cp\u003eSumming gross N fluxes to global net rates masks important variation across the local scale where N fluxes hold ecological relevance. Importantly, unlike CO\u003csub\u003e2\u003c/sub\u003e and its effects on climate, the impacts of N eutrophication are heterogeneous and relatively local, such that N trends could have opposing influences in different places. For instance, N may be released from a collapsing salt marsh in one estuary, while topography may allow lateral transgression of a mangrove and thus high N uptake elsewhere. To capture the possibility of these effects, we estimated gross N uptake as the rate of soil N accumulation in surviving wetlands (Fig. 4c) and gross N loss rates assuming the erosional loss of 50 cm of soil when wetlands are lost (Fig. 4d). Even if accelerating wetland elevation gain yields an increase in global \u003cem\u003enet\u003c/em\u003e N burial, erosion of collapsing wetlands could result in a \u003cem\u003egross\u003c/em\u003e release of 150 Tg of N, which could have strong effects on local N fluxes (Fig. 4d).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eWill wetland elevation gain be N limited?\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eTidal wetlands are broadly expected to respond to accelerating rSLR with increased rates of elevation gain up until a threshold. While it is known that N availability commonly limits productivity in most tidal marshes\u003csup\u003e4,8\u003c/sup\u003e and some mangroves\u003csup\u003e39,40\u003c/sup\u003e, it is not yet known to what extent plant N limitation translates into N limitation of elevation gain\u003csup\u003e41\u003c/sup\u003e. The N burial rate of a tidal wetland will relate to the rate of elevation gain given that correlations between rates of rSLR and soil C sequestration are robust, and variation in soil C:N ratios are constrained (Fig. 1). If the elevation gain rate accelerates in response to rSLR, N accumulation in tidal wetlands will also increase. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBased on the stoichiometry of wetland soil sinks and rates of C accumulation, we estimate that organogenic wetlands (those in which soil derives mostly from endogenous organic matter) in areas with low inputs of nutrients may have lower potential elevation gain due to N scarcity, particularly where rates of rSLR are high. For example, a brackish marsh on the Chesapeake Bay, USA, historically gained elevation at roughly 2\u0026nbsp;mm\u0026nbsp;yr\u003csup\u003e‑1\u003c/sup\u003e, near the millennial rate of rSLR, requiring a N accumulation of 4 g N m\u003csup\u003e-2\u003c/sup\u003e yr\u003csup\u003e-1\u003c/sup\u003e. However, from 2000 to 2020, the rSLR rate rose to 7 mm yr\u003csup\u003e-1\u003c/sup\u003e \u003csup\u003e42\u003c/sup\u003e, which would require 14 g N m\u003csup\u003e-2\u003c/sup\u003e yr\u003csup\u003e-1\u003c/sup\u003e if the marsh were to gain elevation at this rate, acknowledging this demand could be lower with adjustments in C:N through changes in N and biomass allocation or plant species turnover. Given that depositional inputs are negligible \u003csup\u003e43\u003c/sup\u003e and the soil C:N ratio is stable down the profile at this site\u003csup\u003e44\u003c/sup\u003e, a large increase in N inputs through fixation and tidal input or a large decrease in N losses from denitrification or tidal export will be required to meet the increased demand (Fig. 3). This adjustment is unlikely considering recent evidence that N limitation of plant productivity has strengthened in this marsh\u003csup\u003e43\u003c/sup\u003e. We predict that organogenic tidal wetlands with low terrigenous inputs are the tidal wetland settings most likely to experience increasing N limitation of both plant productivity and elevation gain\u003csup\u003e45\u003c/sup\u003e. Soil accumulation approaching future rates of rSLR will likely either be limited by N availability or necessitate a substantial increase in ecosystem net N uptake rates in ogranogenic tidal wetlands.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eFates of released N\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eRegardless of its source, once N enters the soil organic N pool, especially below the rooting zone, it may remain nearly inert for millennia depending on biochemical complexity of organic matter, redox status, soil type, and mineral associations\u003csup\u003e46\u003c/sup\u003e. Alternatively, if the wetland is disturbed or the soil erodes, soil N can be released abruptly into surrounding waters\u003csup\u003e47\u003c/sup\u003e and subjected to mineralization, desorption, or re-burial; however, the ultimate fate of eroded soil N remains highly uncertain\u003csup\u003e11\u003c/sup\u003e. If only a fraction of the tidal wetland pool is eroded and recirculated, it could contribute significantly to local coastal N pollution. Chesapeake Bay, for example, has 160,000 ha of coastal wetlands that hold roughly 2.4 Tg of soil N to 1 m depth and are currently burying 6.4 Gg N yr\u003csup\u003e-1\u003c/sup\u003e given the soil N densities reported here. As rSLR accelerates, surviving wetlands are likely to increase N burial up to 22 Gg N yr\u003csup\u003e-1\u003c/sup\u003e, possibly mitigating historically high riverine N input. However, wetland area loss is expected to accelerate as well. If wetland area is lost at a rate of 1% per year, the N loss from erosion could negate N buried by surviving wetlands. Given the large and potentially dynamic pool of N in tidal wetland soils, we highlight an urgent need to better understand the fates of N released from wetlands that are eroding or transforming to other ecosystem states.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eImplications\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eWetland soil accumulation represents a large and historically consistent N sink that has reduced N loads in estuarine and marine waters. The future influence of tidal wetlands on the river-estuary-ocean continuum hinges on future rSLR, wetland response to rSLR, and hydrobiogeomorphic drivers of wetland area change. If wetlands can survive future rSLR or are restored on a large scale, high rates of N uptake to support accelerating wetland elevation gain could improve water quality for seagrass beds and coral reefs. In less polluted regions, accelerating N burial could strengthen N limitation and oligotrophication\u003csup\u003e13\u003c/sup\u003e. However, rSLR could lead to the loss of tidal wetland area and erosion of an uncertain fraction of the soil therein. Wetland erosion could have two impacts on estuarine N budgets: diminishing N uptake and release of currently stored N, exacerbating eutrophication that leads to harmful algal blooms, dead zones, and fish kills\u003csup\u003e6\u003c/sup\u003e. Changes in tidal wetland N accumulation rates represent relatively certain and high-leverage control over local and global N cycling that merits consideration from land managers and global policymakers.\u003c/p\u003e\n\u003cp\u003eBlue C sequestration in tidal wetlands has been well documented, with estimates ranging from 44 to 75 Tg yr\u003csup\u003e-1\u003c/sup\u003e of C\u003csup\u003e24,48,49\u003c/sup\u003e, representing about 1% of total C sequestration in natural sinks\u003csup\u003e29\u003c/sup\u003e. We estimate that 3.2 Tg N yr\u003csup\u003e-1\u003c/sup\u003e is sequestered by mangroves and marshes, representing 13-15% of global N burial\u003csup\u003e16\u0026ndash;18\u003c/sup\u003e. Including other blue N ecosystems, such as seagrass beds, and mud flats, will increase this estimate. The global monetary value of N burial is difficult to estimate owing to small-scale variation in sources and additivity. Yet, our global N burial rates suggest that the monetary value of blue N burial ($22-106 kg\u003csup\u003e-1\u003c/sup\u003e (ref 10) x 3.2 x10\u003csup\u003e9\u003c/sup\u003e kg yr\u003csup\u003e-1\u003c/sup\u003e = $70-339 billion USD) could exceed that of blue C ($191 billion\u003csup\u003e50\u003c/sup\u003e). Future research examining both the sources of N, as well as the alternative fates of N when wetlands are lost, will help refine the value of blue N sequestration and its contribution to coastal and estuarine water quality.\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank K Wieder for helpful advice. LH P\u0026eacute;rez-Bernal carried out C and N analyses from Mexican wetlands.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u0026nbsp;\u003c/strong\u003eThe work of JAL was supported by National Science Foundation, Grant/Award Number: DEB- 0950080, DEB-1457100, DEB-1557009 and DEB-2051343. The work of JAL and SKC was supported by the National Science Foundation DEB-1655659 and DEB-2224999. The work by ACRF and JASC; KWK was supported by projects UNAM-PAPIIT IN102821 and IN110624.the U.S. Geological Survey Climate R\u0026amp;D Program; etc. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. The findings and conclusions in this publication are those of the authors and should not be construed to represent any official USDA or U.S. Government determination or policy.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions:\u003c/strong\u003e Conceptualization: JAL, SKC; Methodology: JAL, LW; Visualization: JAL; Writing \u0026ndash; original draft: All authors; Writing \u0026ndash; review \u0026amp; editing: All authors.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability:\u0026nbsp;\u003c/strong\u003eAll data are available at github.com/phillipriv9/Blue-Nitrogen-Review.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode availability:\u003c/strong\u003e The code used R version 4.3.3 and is available at github.com/phillipriv9/Blue-Nitrogen-Review.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAdame, M. 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Accordingly, tidal wetlands can sequester “blue carbon” at much higher rates than other ecosystems2,3 helping to offset human emissions. Organic carbon burial is tightly linked to the cycling of nitrogen, which is a key pollutant and limiting nutrient for many ecosystems4–6. Yet, the current global burial rate of “blue nitrogen” and how it may respond to future change remain uncertain. We assembled a global database of 8012 soil nitrogen measurements from 255 tidal wetland sites and found that the relationship between soil carbon and nitrogen concentrations was strikingly consistent but differed between the two dominant types of emergent tidal wetlands – marshes and mangroves. Leveraging extensive knowledge of blue carbon accumulation, we estimated that tidal wetlands, which cover less than 0.1% of the Earth’s surface, bury 3.2 Tg N yr-1, representing 13-15% of marine nitrogen burial. This rate could more than triple globally by 2100 if wetland elevation increases with accelerating sea-level rise. Alternatively, if wetlands become submerged and shorelines erode, nitrogen sinks could reverse and become sources, exacerbating coastal pollution.","manuscriptTitle":"Blue nitrogen follows the fate of tidal wetlands","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-06 05:08:08","doi":"10.21203/rs.3.rs-5522814/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2ff03a82-fb13-47db-9095-55e08b0816a7","owner":[],"postedDate":"February 6th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":43891028,"name":"Earth and environmental sciences/Climate sciences/Climate change/Climate-change impacts/Environmental health"},{"id":43891029,"name":"Biological sciences/Ecology/Biogeochemistry/Element cycles"}],"tags":[],"updatedAt":"2025-05-19T12:45:36+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-06 05:08:08","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5522814","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5522814","identity":"rs-5522814","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}