Carbon storage in river and floodplain systems: A review of evidence to update and inform policy development for riverine nature based solutions

Abstract

The threat of climate change is increasingly motivating goals that seek to achieve net zero emissions in the next few decades (Rutter and Sasse, 2022). In the UK, net zero is a statutory requirement that must be met by 2050 (Gregg et al., 2021). An important element of this strategy is determining how nature can contribute to achieving Net zero – largely via carbon sequestration and storage (Gregg et al., 2021). The degradation of many natural systems has impacted natural carbon stores and so the role of nature-based solutions is increasingly being implemented with the beneficial aims of both increasing biodiversity as well as supporting climate change mitigation (Gregg et al., 2021).

Decomposition and combustion of organic material releases CO2 to the atmosphere, while accumulation of biomass and soil organic carbon (SOC) sequesters CO2 (Hoffmann 2021). Wetlands such as peatlands, swamps, marshes, estuaries and floodplains provide optimal conditions for the sequestration and long-term storage of carbon although the precise timing of storage will depend on erosional (turnover) time of the specific habitat/system. Low oxygen concentrations support anaerobic conditions that reduce decomposition, whilst overbank sedimentation buries organic matter protecting it from further decomposition. On floodplains, the high clay content of deposits provide sites for chemical bonding with organic matter further reducing loss of carbon through decomposition and gaseous emission (Hoffman 2021).

Research to date all points towards a substantial role for rivers and floodplains in the global carbon cycle (Whol and Knox 2022, Hoffman, 2021). An increasingly expanding literature consistently demonstrates that riparian ecosystems and floodplains can store a significantly larger amount of carbon per area compared to surrounding land (Suftin et al., 2016; Whol and Knox 2022). Floodplains cover 0.5–1% of the global land area but have been suggested to account for a range of 0.5–8% of global SOC storage. River networks contain significant portions of terrestrial C with greatest retention occurring in floodplain riparian ecosystems D’Elia et al (2017).

Although, there is a large range of estimated values of OC in watersheds (0.5 to 1.5 Pg (Aufdenkampe et al., 2011) and 0.9 Pg (Regnier et al., 2013)), some estimates in mountainous headwater streams in the USA, indicate that riparian areas including floodplains may store about 25% of the total OC while occupying less than 1% of watershed area (Wohl et al., 2012). Sutfin et al., (2016) reported 22% of carbon entering headwater streams is unaccounted for after quantifying delivery to oceans or losses to outgassing as carbon dioxide (CO2), suggesting there is a substantial reservoir of carbon in riparian systems derived from sediment deposition.

The role of rivers in carbon sequestration has often been interpreted as a conduit between terrestrial and marine carbon stores (Gregg et al., 2021). Carbon can be stored in the floodplain in many forms including above ground vegetation (Dyabla et al., 2019), and soil (Wohl et al., 2017), as well as within the river channel as large, drowned wood and vegetation (Hinshaw & Wohl, 2021).

Much of the evidence remains focussed on above ground biomass and the first metre of soil (D’Elia et al., 2017). However, it is argued in Young et al., (2019) that recent carbon accumulation rates in surface peat can be misinterpreted in relation to carbon storage. It suggests that surface/topsoil peat measurements do not account for the future ability to be decomposed/lost in comparison to deeper long-term stores. This suggests that although there may be peat/organic matter present in topsoil this may not necessarily translate into long term carbon sequestration. A need for deeper sediment cores and paleoenvironmental analysis to present the natural state of UK rivers is identified across the literature (D’Elia et al., 2017; Quine et al., 2022), however, is not yet widely implemented, although the current Natural England led project to develop a national peat map is aiming to rectify this omission.
The quantification of carbon stored in floodplains and the potential for restoration to increase this remains poorly understood (Hinshaw & Wohl, 2021; Hofmann 2021). To be able to quantify carbon storage it requires understanding how much is buried (storage quantity), over what timescales (storage period) and what processes are associated with carbon burial and storage. These factors are addressed in this report to better understand carbon storage in UK floodplains and whether current restoration is effective at increasing this.

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ORCID for David Sear: orcid.org/0000-0003-0191-6179

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