Carbon sequestration and biogeochemical cycling in a saltmarsh subject to coastal managed realignment

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Abstract

Globally, wetlands provide the largest terrestrial carbon (C) store, and restoration of degraded wetlands provides a potentially important mechanism for climate change mitigation. We examined the potential for restored saltmarshes to sequester carbon, and found that they can provide a modest, but sustained, sink for atmospheric CO2. Rates of C and nutrient cycling were measured and compared between a natural saltmarsh (high- and low-shore locations), claimed arable land on former high-shore saltmarsh and a managed realignment restoration site (high- and low-shore) in transition from agricultural land to saltmarsh 15 years after realignment, at Tollesbury, Essex, UK. We measured pools and turnover of C and nitrogen (N) in soil and vegetation at each site using a range of methods, including gas flux measurement and isotopic labelling. The natural high-shore site had the highest soil organic matter concentrations, topsoil C stock and below-ground biomass, whereas the agricultural site had the highest total extractable N concentration and lowest soil C/N ratio. Ecosystem respiration rates were similar across all three high-shore sites, but much higher in both low-shore sites, which receive regular inputs of organic matter and nutrients from the estuary. Total evolution of 14C-isotopically labelled substrate as CO2 was highest at the agricultural site, suggesting that low observed respiration rates here were due to low substrate supply (following a recent harvest) rather than to inherently low microbial activity. The results suggest that, after 15 years, the managed realignment site is not fully equivalent to the natural saltmarsh in terms of biological and chemical function. While above ground biomass, extractable N and substrate mineralisation rates in the high-shore site were all quite similar to the natural site, less dynamic ecosystem properties including soil C stock, C/N ratio and below-ground biomass all remained more similar to the agricultural site. These results suggest that reversion to natural biogeochemical functioning will occur following restoration, but is likely to be slow; we estimate that it will take approximately 100 years for the restored site to accumulate the amount of C currently stored in the natural site, at a rate of 0.92 t C ha−1 yr−1.

Introduction

Globally, wetlands provide the largest terrestrial carbon stores, and restoration of degraded wetlands provides a potentially important mechanism for climate change mitigation. To date, much research has focused on restoring degraded peatlands, for example through re-wetting. However, this research has highlighted uncertainties regarding its overall impact on C and greenhouse gas balances, due to the potential for enhanced release of CH4 following re-wetting (Strack et al., 2004; Baird et al., 2009). There is now growing interest in the potential for restored coastal wetland systems to sequester large amounts of carbon (Craft et al., 2003; Shepherd et al., 2007; Santín et al., 2009; Livesley and Andrusiak, 2012). Additionally, restoring coastal wetlands may avoid the offsetting effects of enhanced methane production associated with peat re-wetting, due to the presence of sulphates which allows sulphate-reducing bacteria to outcompete methanogens for energy sources (Bartlett et al., 1987; Andrews et al., 2006; Poffenbarger et al., 2011). Therefore, per unit area, restoration of coastal wetlands such as saltmarshes may contribute more to C sequestration, and therefore to climate regulation, than peatlands. However, at present, evidence is sparse.

As well as carbon sequestration, saltmarshes provide a range of other ecosystem services. These include immobilisation of pollutants (e.g. retention of diffuse nutrient and faecal pollutants into accumulating sediments), flood defence and shore line erosion control and they are a significant reservoir of wild species diversity (Jones et al., 2011). However historically, human activity has focused on the land-claim (‘reclamation’) of saltmarsh for agriculture, and more recently for port development leading to an estimate by French (1997) that 25% of the world's intertidal estuarine habitat had been lost due to land claim. Accelerated sea-level rise also poses a threat to existing saltmarsh through coastal squeeze, as sea defences restrict their natural landward migration to higher elevation (Blackwell et al., 2004). Globally, efforts are now being made to restore and create saltmarshes to mitigate historic losses and on-going development. Since the early 1990s, the driving force for restoration was the unsustainable increasing cost of maintaining and upgrading existing sea defences (Andrews et al., 2006). However, managed realignment is also undertaken for purposes of habitat or biodiversity enhancement or restoration, for example in Europe, salt-marsh restoration allows government compliance with the European Union Habitats Directive (C.E.G., 1992) which states there should be ‘no further net loss of coastal marsh’ (UK Biodiversity Group, 1999). UK targets aim to create 2240 ha of saltmarsh between 1999 and 2015, primarily via a process known as ‘managed realignment’; the landward retreat of coastal defences and subsequent tidal inundation of previously-claimed agricultural land (Garbutt et al., 2006).

In general, managed realignment schemes in the UK and elsewhere have shown that, with relatively minimal pre-treatment and/or management of the area, allowing tidal ingress through a breach of the existing seawall onto low-lying agricultural land will quickly produce intertidal mudflats that are colonised by saltmarsh plants (French et al., 2000; Wolters et al., 2005). Managed realignment sites are sinks of sediment and, given time, representative saltmarsh plant, invertebrate and bird communities can become established (Garbutt et al., 2006). Newly created saltmarsh also acts as a natural sea defence by attenuating tidal amplitude (Pethick, 2002). Self-sustaining plant communities are often the primary goal of restoration efforts as they perform some of the desirable functions of wetland ecosystems (van Andel, 1998; Möller et al., 1999; Craft et al., 2002). However, many physical and functional processes such as nutrient cycling in these sites are poorly understood, and it has yet to be shown that restored saltmarshes are functionally equivalent to referenced systems and therefore whether they do effectively compensate for the loss of habitat as intended. In particular, the capacity of managed realignment schemes to accumulate carbon following conversion from agricultural land to saltmarsh has not been fully quantified.

This study measures and compares biogeochemical functioning between a 15 year old managed realignment site in a state of transition from agricultural land to saltmarsh, relative to adjacent areas of natural saltmarsh and arable land on former saltmarsh. Our three main objectives were: 1) to compare general soil characteristics between the restored saltmarsh, natural saltmarsh and agricultural sites; 2) to quantify and compare the organic matter, carbon and nitrogen pools at all sites, and estimate how far soil carbon stocks at the restored site have progressed along a trajectory between its former agricultural condition and the natural saltmarsh; 3) to investigate potential differences in the dynamics of organic matter cycling by measuring in situ ecosystem respiration and carbon mineralisation rates.

Section snippets

Site description

This study was undertaken at the Tollesbury managed realignment site, adjacent natural marshes and arable land of the Blackwater Estuary, south-east England (51°46ʹN, 0°51ʹE, Fig. 1) in July 2010. The 21-ha restoration site had originally been a saltmarsh, but was claimed for agriculture in the late 18th century (Boorman et al., 1997). The sea defences were breached in August 1995, leaving a 50-m wide opening and allowing tidal ingress to the site for the first time in over 150 years. The

General soil characteristics

There were significant differences (p < 0.05) between sites in all of the soil properties measured. Soil conductivity was highest at the natural high shore marsh, lowest at the agricultural site, and intermediate at the restored high shore site (averages of 12.08, 0.14 and 4.40 mS cm−1 respectively, Table 1). These large differences highlight the influence of seawater on both the natural and restored sites, and of freshwater on the agricultural site. There was no significant difference between

Conclusions

In the UK, managed realignment is primarily undertaken for purposes of habitat and biodiversity enhancement or restoration, or for coastal defence. Carbon and nutrient cycling are rarely considered when these schemes are developed and monitored, let alone used as success criteria (which currently only consider vegetation development). However, with a growing policy emphasis on the wider ecosystem service implications of land-management, it is clear that enhanced carbon sequestration could

Acknowledgements

The authors would like to thank Sarah Hodgson for soil quality assessment analysis, Stephanie Ellis for the mineralisation analysis, Jonathan Roberts for CN analysis and Marc Brouard for LOI, biomass and bulk density analysis. Also our thanks go to Philip Stickler and David Watson (Cartographic Unit, Department of Geography, University of Cambridge) who assisted with the production of Fig. 1.

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