Avoiding catastrophic climate change requires rapid decarbonization and improved ecosystem stewardship. To achieve the latter, ecosystems should be prioritized by responsiveness to direct, localized action and the magnitude and recoverability of their carbon stores. Here, we show that a range of ecosystems contain ‘irrecoverable carbon’ that is vulnerable to release upon land use conversion and, once lost, is not recoverable on timescales relevant to avoiding dangerous climate impacts. Globally, ecosystems highly affected by human land-use decisions contain at least 260 Gt of irrecoverable carbon, with particularly high densities in peatlands, mangroves, old-growth forests and marshes. To achieve climate goals, we must safeguard these irrecoverable carbon pools through an expanded set of policy and finance strategies. In order to limit warming and the most severe consequences of climate change, net global carbon emissions must reach zero by 2050. Many ecosystems contain carbon that would be irrecoverable on this timescale if lost and must be protected to meet climate goals.

Protecting irrecoverable carbon in Earth’s ecosystems

Climate change and ensuring food security for an exponentially growing global human population are the greatest challenges for future agriculture. Improved soil management practices are crucial to tackle these problems by enhancing agro-ecosystem productivity, soil fertility, and carbon sequestration. To meet Paris climate treaty pledges, soil management must address validated approaches for carbon sequestration and stabilization. The present synthesis assesses a range of current and potential future agricultural management practices (AMP) that have an effect on soil organic carbon (SOC) storage and sequestration. Through two strategies—increasing carbon inputs (e.g., enhanced primary production, organic fertilizers) and reducing SOC losses (e.g., reducing soil erosion, managing soil respiration)—AMP can either sequester, up to 714 ± 404 (compost) kg C ha−1 y−1, having no distinct impact (mineral fertilization), or even reduce SOC stocks in the topsoil (bare fallow). Overall, the carbon sequestration potential of the subsoil (>40 cm) requires further investigation. Moreover, climate change, permanent soil sealing, consumer behavior in dietary habits and waste production, as well as the socio-economic constraints of farmers (e.g., information exchange, long-term economic profitability) are important factors for implementing new AMPs. This calls for life-cycle assessments of those practices.

https://www.mdpi.com/2073-4395/11/5/882/pdf

Optimizing Carbon Sequestration in Croplands: A Synthesis

Dikes are the conventional means of flood defence along rivers and estuaries. However, dikes gradually lead to the superelevation of waterbodies compared to the subsiding embanked areas, resulting in a rapidly increasing unstable situation under sea-level rise. Therefore, future flood management requires new, sustainable strategies that not only minimise flood risk, but also steer land-level rise. An example is a transitional polder, where a dike-protected area is temporarily reopened to the tide to capture sediment until it has risen well above mean sea-level, after which it could be returned to its original function. This study explores how the sequence of opening transitional polders affects sediment capture and large-scale estuary dynamics through 2D modelling in Delft3D. To this end, different opening sequences were tested along a large estuary, using the Western Scheldt (NL) as an example. Findings show land-level rise in all permutations. However, polders opened later in an opening sequence temporarily experience a lag in muddy sediment capture, most likely due to a deficit in fines. Opening more upstream located polders alone or at the start of an opening sequence generally causes a stronger reduction in mean tidal range than opening more downstream located polders. This is explained by increased friction due to (1) locally added intertidal width and (2) shallowing of the main channels because of increased flood dominance. An upstream-to-downstream opening sequence caused the greatest reduction in mean tidal range, but this is negligible compared to the increase in tidal range due to historic dredging within the estuary for navigational purposes. Further work is needed to determine how dredging, closure of transitional polders and storm surges may negate this benefit to flood safety. 

Transitional polders along estuaries: driving land-level rise and reducing flood propagation

Soils and sediments of coastal ecology: A global carbon sink

A Threshold‐like Effect on the Interaction Between Hydrological Connectivity and Dominant Plant Population in Tidal Marsh Wetlands

Tidal marshes are coastal and estuarine ecosystems that store large amounts of sedimentary organic carbon (OC). Despite the valuable ecosystem services they deliver, tidal marshes have been converted to other land‐use types over the past centuries. Although previous studies have reported large decreases in soil organic carbon (SOC) stocks after tidal marsh embankment, knowledge on the magnitude and rate of OC losses is still limited. Here, we studied the effect of stepwise embankments of brackish and salt marshes and subsequent marsh progradation on SOC stocks in the Scheldt estuary (the Netherlands). We collected samples from soil profiles along tidal marsh–reclaimed tidal marsh chronosequences and determined total OC stocks and the stable carbon signature of the OC. Our results showed that large losses of previously sequestered SOC occur on a decadal timescale with the embankment of brackish (−8.7 ± 0.7 kg OC m⁻²) and salt marshes (−6.7 ± 0.8 kg OC m⁻²). The (incomplete) replacement of tidal marsh OC by agricultural OC is substantially faster in topsoils (ca. a century) compared with subsoils (multiple centuries). Simulations with a coupled land use–SOC model showed that large rates of marsh progradation following embankment construction resulted in a substantial increase in landscape‐scale SOC storage, whereas large SOC losses occurred in landscapes dominated by embanked tidal marshes. The findings of our study might help to assess how these management practices affect regional SOC stocks. HIGHLIGHTS: We studied how OC stocks in tidal marsh sediments are affected by embankment construction OC stocks of both brackish and salt marsh sediments were reduced by ca. 60% with embankment Gains in landscape‐scale SOC storage occurred with marsh progradation after embankment If no marsh progradation occurs, embankments always lead to SOC loss at the landscape scale

Soil organic carbon stocks in a tidal marsh landscape are dominated by human marsh embankment and subsequent marsh progradation

Antea Group and KULeuven were awarded a project in Flanders to identify the regions exporting high sediment loads to unnavigable watercourses and the sedimentation zones within them. Two types of models are applied: hydrological sediment export models (SEM) and hydraulic sediment transport models (STM). The influence of erosion control measures on sediment export as well as river engineering measures needs to be taken into account. A concept will be developed to connect the SEM and STM, enabling the sediment to be routed from upstream to the sedimentation zones. Results of the study will be used by the Flemish government to plan erosion control measures, estimate future sedimentation volumes, steer sedimentation and optimize river engineering and dredging works. Finally, model results could also be used to obtain better insights to the re-suspension risks of contaminated sediment in watercourses.

/pdf/erosion-modelling-towards-and-sediment-transport-modelling-l3fbsq6iwb.pdf

M. Van de Broek

Papers

Soil organic carbon stocks in a tidal marsh landscape are dominated by human marsh embankment and subsequent marsh progradation

Erosion modelling towards, and sediment transport modelling in unnavigable watercourses in Flanders, Belgium