Soil organic carbon stocks and sequestration rates of inland, freshwater wetlands: Sources of variability and uncertainty

Drivers of Soil Carbon Variability in North America's Prairie Pothole Wetlands: A Review

There is an ongoing demand for region-specific soil organic carbon estimates to support sustainable land management and inform carbon credit programs. The Prairie Pothole Region is prominent agricultural area that extends through Canada and the United States, and features a significant number of wetlands commonly referred to as prairie potholes. The contribution of these wetlands to landscape-level soil organic carbon storage is complex and may not be consistent across the region as influenced by several environmental and management factors. This study reviews existing literature to identify the main factors that contribute to variability in soil organic carbon stocks in prairie pothole wetlands. Soil organic carbon stock data from 10 studies in the Prairie Pothole Region were summarized through a meta-analysis. Variable importance and regression analyses were used to assess which factors explain variability in soil organic carbon. Wetland class explained up to 26.6% of the variability in soil organic carbon. Other important factors included ecoregion as well as land management. There were significant differences in average wetland soil organic carbon stocks across the ecoregions. Data limitations restricted our ability to accurately estimate the stocks for wetland class and land management. The findings from this study highlighted the need for targeted studies in the Northern short grassland ecoregion as well as studies that consider wetland classes under various land uses. To advance wetland carbon research in the Prairie Pothole Region, recommendations were provided on landscape-level carbon modelling, soil carbon measurement and monitoring, and improved wetland classification systems.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13157-025-01898-9.

Factors Regulating the Potential for Freshwater Mineral Soil Wetlands to Function as Natural Climate Solutions

There are increasing global efforts and initiatives aiming to tackle climate change and mitigate its impacts via natural climate solutions (NCS). Wetlands have been considered effective NCS given their capacity to sequester and retain atmospheric carbon dioxide (CO2) while also providing a myriad of other ecosystem functions that can assist in mitigating the impacts of climate change. However, wetlands have a dual impact on climate, influencing the atmospheric concentrations of both CO2 and methane (CH4). The cooling effect associated with wetland CO2 sequestration can be counterbalanced by the warming effect caused by CH4 emissions from wetlands. The relative ability of wetlands to sequester CO2 versus emit CH4 is dependent on a suite of interacting physical, chemical, and biological factors, making it difficult to determine if/which wetlands are considered important NCS. The fact that wetlands are embedded in landscapes with surface and subsurface hydrological connections to other wetlands (i.e., wetlandscapes) that flow over and through geochemically active soils and sediments adds a new layer of complexity and poses further challenges to understanding wetland carbon sequestration and greenhouse gas fluxes at large spatial scales. Our review demonstrates how additional scientific advances are required to understand the driving mechanisms associated with wetland carbon cycling under different environmental conditions. It is vital to understand wetland functionality at both wetland and wetlandscape scales to effectively implement wetlands as NCS to maximize ecological, social, and economic benefits.

Wetlands function as carbon sink: Evaluation of few floodplains of middle Assam, northeast India in the perspective of climate change

Floodplain wetlands are biologically rich and productive ecosystems that can capture carbon (C) from the atmosphere through macrophytes and phytoplanktons and hold it in soil for a long time thus playing a critical role in mitigating climate change. The Assam state of India has about 1392 floodplain wetlands engulfing around 100,000 ha area in the Brahmaputra and Barak River basin. In the present study, five different wetlands in the middle Assam viz., 47-Morakolong, Jaliguti, Charan, Chatla, and Urmal were chosen for the estimation of C capture and its storage in soil. The net primary planktonic productivity (NPP) of Chatla was much higher (300mgC/m3/hr) than other wetlands where it ranged around 100-150 mgC/m3/hr. Macrophyte coverage was highest (80%) in Chatla followed by Urmal (50%) and 30% in others. Total organic carbon (TOC) content in water was also significantly higher in Chatla than in others. The C content at different depths of the soil (upper 15 and 15-30 cm) of the wetlands varied widely from 1.3 to 7% and in absolute terms, the total C accumulated in top 30 cm varied from 12.65 to 76.95 MgC/ha. The amount of C in upper 30 cm of corresponding upland sites was estimated to be 8.8-33.62 MgC/ha. Thus, wetlands are superior in terms of C accumulation and storage in their soil compared to the corresponding upland sites. If properly managed, the wetlands can be very effective in capturing and storing C and offset GHG emission and global warming to a great extent.

Threats to inland wetlands and uncertainty around global soil carbon stocks and sequestration rates

Wetlands are vital in climate change mitigation, carbon and nutrient cycling, water quality improvement, flood regulation, wildlife habitat provision, and biodiversity maintenance. However, various threats undermine their integrity and capacity to provide these ecosystem services. In this review and quantitative analysis, we identified the most significant threats to inland wetlands, estimated soil organic carbon (SOC) storage and sequestration rates across regions and climate zones, and pinpointed the sources of uncertainties in global estimates. We found that natural system modification, pollution, biological resource use, agricultural and aquacultural activities, and water regulation are the top five threats, impacting 61 %, 59 %, 59 %, 52 %, and 46 % of Ramsar sites in inland wetlands, respectively. Management plans are lacking in 47-76 % of Ramsar sites globally and over 66 % in Least Developed Countries. Literature data show median SOC stocks of 118.7 Mg ha-1 in temperate and 150.3 Mg ha-1 in the 0-50 cm soil depth of tropical inland wetlands. Our analysis of data from Ramsar sites indicates lower median stocks of 87 Mg ha-1 in temperate and 105 Mg ha-1in tropical wetlands for the 0-100 cm depth, highlighting significant uncertainties. These uncertainties stem from inconsistent definitions of wetlands, inaccurate wetland area data, measurement errors, and variability in sampling and modelling. Addressing these issues requires improved mapping, monitoring, and standardized protocols. Additionally, raising awareness among policymakers and the public about the threats to wetlands, which can destabilize local livelihoods and global carbon and nutrient cycles, is crucial.

Revealing the hidden carbon in forested wetland soils

Inland wetlands are critical carbon reservoirs storing 30% of global soil organic carbon (SOC) within 6% of the land surface. However, forested regions contain SOC-rich wetlands that are not included in current maps, which we refer to as 'cryptic carbon'. Here, to demonstrate the magnitude and distribution of cryptic carbon, we measure and map SOC stocks as a function of a continuous, upland-to-wetland gradient across the Hoh River Watershed (HRW) in the Pacific Northwest of the U.S., comprising 68,145 ha. Total catchment SOC at 30 cm depth (5.0 TgC) is between estimates from global SOC maps (GSOC: 3.9 TgC; SoilGrids: 7.8 TgC). For wetland SOC, our 1 m stock estimates are substantially higher (Mean: 259 MgC ha-1; Total: 1.7 TgC) compared to current wetland-specific SOC maps derived from a combination of U.S. national datasets (Mean: 184 MgC ha-1; Total: 0.3 TgC). We show that total unmapped or cryptic carbon is 1.5 TgC and when added to current estimates, increases the estimated wetland SOC stock to 1.8 TgC or by 482%, which highlights the vast stores of SOC that are not mapped and contained in unprotected and vulnerable wetlands.

Practical Guide to Measuring Wetland Carbon Pools and Fluxes

Wetlands cover a small portion of the world, but have disproportionate influence on global carbon (C) sequestration, carbon dioxide and methane emissions, and aquatic C fluxes. However, the underlying biogeochemical processes that affect wetland C pools and fluxes are complex and dynamic, making measurements of wetland C challenging. Over decades of research, many observational, experimental, and analytical approaches have been developed to understand and quantify pools and fluxes of wetland C. Sampling approaches range in their representation of wetland C from short to long timeframes and local to landscape spatial scales. This review summarizes common and cutting-edge methodological approaches for quantifying wetland C pools and fluxes. We first define each of the major C pools and fluxes and provide rationale for their importance to wetland C dynamics. For each approach, we clarify what component of wetland C is measured and its spatial and temporal representativeness and constraints. We describe practical considerations for each approach, such as where and when an approach is typically used, who can conduct the measurements (expertise, training requirements), and how approaches are conducted, including considerations on equipment complexity and costs. Finally, we review key covariates and ancillary measurements that enhance the interpretation of findings and facilitate model development. The protocols that we describe to measure soil, water, vegetation, and gases are also relevant for related disciplines such as ecology. Improved quality and consistency of data collection and reporting across studies will help reduce global uncertainties and develop management strategies to use wetlands as nature-based climate solutions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13157-023-01722-2.

Large increases in methane emissions expected from North America's largest wetland complex

Natural methane (CH₄) emissions from aquatic ecosystems may rise because of human-induced climate warming, although the magnitude of increase is highly uncertain. Using an exceptionally large CH₄ flux dataset (~19,000 chamber measurements) and remotely sensed information, we modeled plot- and landscape-scale wetland CH₄ emissions from the Prairie Pothole Region (PPR), North America's largest wetland complex. Plot-scale CH₄ emissions were driven by hydrology, temperature, vegetation, and wetland size. Historically, landscape-scale PPR wetland CH₄ emissions were largely dependent on total wetland extent. However, regardless of future wetland extent, PPR CH₄ emissions are predicted to increase by two- or threefold by 2100 under moderate or severe warming scenarios, respectively. Our findings suggest that international efforts to decrease atmospheric CH₄ concentrations should jointly account for anthropogenic and natural emissions to maintain climate mitigation targets to the end of the century.

Variability of carbon stored in inland freshwater wetland in Northeast India

Inland freshwater wetland ecosystems are among the largest sink of carbon (C) in the biosphere. However, improved scientific understanding of the C stability and sequestration potential is required to predict response of C pool under environmental change and to identify priorities for lacustrine C sink management. This study analyses the concentration of organic C fractions based on their stability and estimates C stock along with depth and eco-zones of the Rudrasagar lake in Northeast India. Sediment samples up to 100 cm depth were collected from littoral, sub-littoral and deep layers, and analysed for organic C concentrations. Results showed that C concentration decreases with depth in the littoral layer but increases with depth in sub-littoral and deep layers. Two-way analysis of variance showed that concentrations of soil organic C (SOC) fractions were significantly different among the eco-zones but not between the soil depth. Average SOC stock was significantly higher in the deep layer (334.9 Mg C ha^-1) followed by sub-littoral (248.4 Mg C ha^-1) and littoral layer (106.1 Mg C ha^-1). Overall, we show that substantial spatial variability in SOC stock exists among the eco-zones and depth that may be driven by water inundation in deep layer and fluctuating hydrological conditions at the edges of the lacustrine ecosystem. Our study demonstrates that inland freshwater wetland is a major sink of organic C and if disturbed it can act as a carbon dioxide source.

Effect of Seawall Embankment Reclamation on the Distribution of Cr, Cu, Pb and Zn Pollution in Invasive Spartina alterniflora and Native Phragmites australis Coastal Saltmarshes of East China

Coastal reclamation by seawall embankments and the spread of invasive C4 perennial grass Spartina alterniflora have recently become more prevalent in eastern China's coastal wetlands. While trace metals (TMs), carbon, and nitrogen dynamics concerning reclamation have extensively been explored across China's coastal wetlands, to date, the impact of reclamation by coastal embankment and exotic plant invasion on TMs' pollution dynamics in coastal marshes remains largely unexplored. We compared TMs Cr, Cu, Pb, and Zn cumulation in coastal embankment-reclaimed versus unreclaimed S. alterniflora and Phragmites australis saltmarshes in eastern China coastal wetlands. In both S. alterniflora and P. australis marshes, coastal embankment reclamation spurred an increase in Cr, Cu, Pb, and Zn concentrations by 31.66%, 53.85%, 32.14%, 33.96% and by 59.18%, 87.50%, 55.55%, 36.84%, respectively, in both marsh types. Reclamation also reduced plant biomass, soil moisture, and soil salinity in both plants' marshes. Our findings suggest that the impact of coastal embankment reclamation and replacement of native saltmarshes by invasive S. alterniflora had a synergistic effect on TM accumulation in the P. australis marshes, as corroborated by bioaccumulation and translocation factors. Reclamation by coastal embankments and invasive alien plants could significantly impair the physico-chemical properties of native plant saltmarsh and essentially weaken the accumulation of Cr, Cu, Pb, and Zn potential of the coastal saltmarshes. Our findings provide policymakers with an enhanced knowledge of the relationship between reclamation, plant invasiveness, and TM pollution dynamics in coastal wetlands, providing a baseline for attaining future goals and strategies related to the tradeoffs of various wetland reclamation types.

Natural infrastructure in dryland streams (NIDS) can establish regenerative wetland sinks that reverse desertification and strengthen climate resilience

In this article we describe the natural hydrogeomorphological and biogeochemical cycles of dryland fluvial ecosystems that make them unique, yet vulnerable to land use activities and climate change. We introduce Natural Infrastructure in Dryland Streams (NIDS), which are structures naturally or anthropogenically created from earth, wood, debris, or rock that can restore implicit function of these systems. This manuscript further discusses the capability of and functional similarities between beaver dams and anthropogenic NIDS, documented by decades of scientific study. In addition, we present the novel, evidence-based finding that NIDS can create wetlands in water-scarce riparian zones, with soil organic carbon stock as much as 200 to 1400 Mg C/ha in the top meter of soil. We identify the key restorative action of NIDS, which is to slow the drainage of water from the landscape such that more of it can infiltrate and be used to facilitate natural physical, chemical, and biological processes in fluvial environments. Specifically, we assert that the rapid drainage of water from such environments can be reversed through the restoration of natural infrastructure that once existed. We then explore how NIDS can be used to restore the natural biogeochemical feedback loops in these systems. We provide examples of how NIDS have been used to restore such feedback loops, the lessons learned from installation of NIDS in the dryland streams of the southwestern United States, how such efforts might be scaled up, and what the implications are for mitigating climate change effects. Our synthesis portrays how restoration using NIDS can support adaptation to and protection from climate-related disturbances and stressors such as drought, water shortages, flooding, heatwaves, dust storms, wildfire, biodiversity losses, and food insecurity.

Storage, patterns and influencing factors for soil organic carbon in coastal wetlands of China

Soil organic carbon (SOC) in coastal wetlands, also known as "blue C," is an essential component of the global C cycles. To gain a detailed insight into blue C storage and controlling factors, we studied 142 sites across ca. 5000 km of coastal wetlands, covering temperate, subtropical, and tropical climates in China. The wetlands represented six vegetation types (Phragmites australis, mixed of P. australis and Suaeda, single Suaeda, Spartina alterniflora, mangrove [Kandelia obovata and Avicennia marina], tidal flat) and three vegetation types invaded by S. alterniflora (P. australis, K. obovata, A. marina). Our results revealed large spatial heterogeneity in SOC density of the top 1-m ranging 40-200 Mg C ha^-1 , with higher values in mid-latitude regions (25-30° N) compared with those in both low- (20°N) and high-latitude (38-40°N) regions. Vegetation type influenced SOC density, with P. australis and S. alterniflora having the largest SOC density, followed by mangrove, mixed P. australis and Suaeda, single Suaeda and tidal flat. SOC density increased by 6.25 Mg ha^-1 following S. alterniflora invasion into P. australis community but decreased by 28.56 and 8.17 Mg ha^-1 following invasion into K. obovata and A. marina communities. Based on field measurements and published literature, we calculated a total inventory of 57 × 10⁶  Mg C in the top 1-m soil across China's coastal wetlands. Edaphic variables controlled SOC content, with soil chemical properties explaining the largest variance in SOC content. Climate did not control SOC content but had a strong interactive effect with edaphic variables. Plant biomass and quality traits were a minor contributor in regulating SOC content, highlighting the importance of quantity and quality of OC inputs and the balance between production and degradation within the coastal wetlands. These findings provide new insights into blue C stabilization mechanisms and sequestration capacity in coastal wetlands.

Indictors of wetland health improve following small-scale ecological restoration on private land

Globally wetlands are imperilled and restoring these highly productive and biodiverse ecosystems is key to regaining their lost function and health. Much of the fertile, low-lying land that was historically wetland is now farmed, so privately-owned locations play critical roles in regaining space for wetlands. However, wetland restoration on private property is often small-scale and supported by minimal funding and expertise. Little is known about what these efforts achieve, and what contexts facilitate the greatest gains in wetland health. Using a paired plot design for 18 restored and 18 unrestored wetlands, we aimed to understand changes in wetland health following restoration on private property. We characterised plant and microbial communities and soil characteristics following wetland restoration and explored how environmental settings of restored wetlands related to the clustering of wetland health indicators. We found that all indicators of wetland health significantly increased following restoration except for the ratio of Gram negative to Gram positive bacteria. Restoration enhanced plant alpha and beta diversity, adding ~13 native plant species per plot. Soils in restored wetlands contained 20% more organic matter, and 25% more microbial biomass, which was driven by an increased abundance of fungi. Restoration reduced soil bulk density by 0.19 g^-1 cm³ and Olsen Phosphorus by 23%. These effects on soil physical characteristics and microbial communities were strongest in the wettest locations. Restored wetlands clustered into three main groups based on indicators of wetland health. Hydrological flow explained the clustering of wetlands, with riverine wetlands exhibiting greater indicators of recovery than depressional wetlands, suggesting that hydrological flow may influence post-restoration recovery. Overall, this study shows that small-scale wetland restoration on private land improved wetland health, providing evidence that it can be an effective use of marginal agricultural land.

Can Restoration of Freshwater Mineral Soil Wetlands Deliver Nature-Based Climate Solutions to Agricultural Landscapes?

This study advances scientific understanding of the magnitude of carbon sequestration that could be achieved through conservation (securing existing carbon stocks) and restoration (creating new carbon stocks) of freshwater mineral soil wetlands on agricultural landscapes. Within an agricultural landscape in southern Ontario (Canada), 65,261 wetlands comprising 63,135 ha were lost. Of these, 6,899 wetlands comprising 5,198 ha were “easy-to-restore” wetlands, defined as wetlands that were small (<0.5 ha), with no hydrological inflow or outflow, and that were drained by a drainage ditch and could be restored by plugging the drainage ditch. Within these easy-to-restore wetlands, a chronosequence of wetlands that covered a range of restoration ages [i.e., drained (0 years), 15 years, 25 years, 40 years, and intact marshes] was established to capture potential changes in rates of sedimentation and organic carbon (OC) sequestration with restoration age. Three sediment cores were collected at the center of the open-water portion of the wetland and segmented in the field. In the lab, each individual segment from each core was dried, sieved through a 2-mm mesh, weighed and analyzed for 137Cs and 210Pb radioisotopes and OC. OC stocks (35.60 Mg ha–1) and OC sequestration rates (0.89 Mg C ha–2 yr–1) in wetlands restored for 40 years were comparable to if not marginally larger than intact wetlands, suggesting that restoration promotes OC sequestration but that an initial recovery phase of up to 25 years or more is needed before returning to a pre-drainage equilibrium. An economic analysis to compare the costs and benefits of wetland conservation and restoration was then conducted. The benefit-cost analysis revealed that the financial benefits of carbon sequestration are greater than the financial costs over a 30-year time horizon for retaining wetlands but not for restoring wetlands. The breakeven costs such that wetland restoration is economically feasible based on current carbon price projections is estimated to be $17,173 CAD ha–1 over the 30-year time horizon; any wetland restoration project that costs this amount or less could be justified on economic grounds based solely on the carbon benefits. This study’s findings indicate that wetlands are important nature-based climate solutions, but that incentivizing their use through a carbon market will require either scientific innovations to reduce restoration costs or increase carbon sequestration rates, or stacking carbon benefits with other ecosystem service benefits into a comprehensive market for nature-based climate solutions.

Remote Sensing of Surface and Subsurface Soil Organic Carbon in Tidal Wetlands: A Review and Ideas for Future Research

Tidal wetlands, widely considered the most extensive reservoir of soil organic carbon (SOC), can benefit from remote sensing studies enabling spatiotemporal estimation and mapping of SOC stock. We found that a majority of the remote-sensing-based SOC mapping efforts have been focused on upland ecosystems, not on tidal wetlands. We present a comprehensive review detailing the types of remote sensing models and methods used, standard input variables, results, and limitations for the handful of studies on tidal wetland SOC. Based on that synthesis, we pose several unexplored research questions and methods that are critical for moving tidal wetland SOC science forward. Among these, the applicability of machine learning and deep learning models for predicting surface SOC and the modeling requirements for SOC in subsurface soils (soils without a remote sensing signal, i.e., a soil depth greater than 5 cm) are the most important. We did not find any remote sensing study aimed at modeling subsurface SOC in tidal wetlands. Since tidal wetlands store a significant amount of SOC at greater depths, we hypothesized that surface SOC could be an important covariable along with other biophysical and climate variables for predicting subsurface SOC. Preliminary results using field data from tidal wetlands in the southeastern United States and machine learning model output from mangrove ecosystems in India revealed a strong nonlinear but significant relationship (r2 = 0.68 and 0.20, respectively, p < 2.2 × 10−16 for both) between surface and subsurface SOC at different depths. We investigated the applicability of the Soil Survey Geographic Database (SSURGO) for tidal wetlands by comparing the data with SOC data from the Smithsonian’s Coastal Blue Carbon Network collected during the same decade and found that the SSURGO data consistently over-reported SOC stock in tidal wetlands. We concluded that a novel machine learning framework that utilizes remote sensing data and derived products, the standard covariables reported in the limited literature, and more importantly, other new and potentially informative covariables specific to tidal wetlands such as tidal inundation frequency and height, vegetation species, and soil algal biomass could improve remote-sensing-based tidal wetland SOC studies.

Assessing conservation and management actions with ecosystem services better communicates conservation value to the public

Fish and wildlife populations are under unprecedented threats from changes in land use and climate. With increasing threats comes a need for an expanded constituency that can contribute to the public support and financial capital needed for habitat conservation and management. Using an ecosystem services approach can provide a framework for a more holistic accounting of conservation benefits. Our objective here is to provide a greater understanding of the role that taking an ecosystem services approach can have in expanding the public constituency that supports the use of financial capital required to conserve and manage the Nation’s natural capital. To demonstrate a methodology and the usefulness of taking an ecosystem services approach when communicating the value of conserving and managing fish and wildlife habitats, we performed an evaluation of U.S. Fish and Wildlife Service owned Waterfowl Production Areas, National Wildlife Refuges, and easement lands (both wetland and grassland) in Stutsman County, North Dakota, USA. We quantified amphibian habitat, grassland-bird habitat, floral resources for pollinators, and carbon storage services under various scenarios of conservation. While we did not include all possible ecosystem services in our model, our case study shows how this process can provide a more complete picture of the collateral benefits of conservation directed primarily towards waterfowl. Using this ecosystem services approach we documented marked losses in all services modeled if current conservations lands were developed for the production of agricultural crops. By having access to a more complete picture of benefits provided by conservation lands, decision makers can better communicate their value. By garnering greater public support through a more accurate accounting of societal benefits, conservation and management of dwindling natural capital may someday attain the same level of thought and consideration that is put into the conservation and management of the Nation’s financial capital.