Are managed realignment sites net carbon sources or sinks? Insights from the Humber estuary, UK. Natural saltmarsh sites store more carbon at depth than managed realignment sites and agricultural land.

Mudflats and saltmarshes are increasingly recognised as having the potential to sequester and store blue carbon. There is interest in whether Managed Realignment (MR), the breaching of coastal defences to allow (re)inundation of land primarily aims to offset habitat losses, may also have additional benefits including the sequestration and storage of blue carbon. However, sequestration rates are expected to be highly variable because of variations in tidal range, suspended matter concentrations, vegetation assemblages and other environmental factors. In addition, to date there is limited consensus on sampling strategy, including maximum coring depth and spatio-temporal sampling interval. For example, maximum coring depths reported in the literature range from 0.1m to 1.0m. This study aims to critically assess the sampling strategy required to quantify carbon stocks in three spatially adjacent but environmentally distinct settings on the North bank of the macrotidal, hyperconcentrated, Humber Estuary, UK: an agricultural field that is a candidate for managed realignment, a recently breached managed realignment site, and a natural saltmarsh.

Cores were sampled to a depth of up to 3m at 59 locations spaced 250m apart in a 4km-long × 1km-wide area. Material extruded from each distinct horizon was grain-sized using a Malvern Mastersizer 3000 laser-diffraction particle analyser and water content and organic and inorganic carbon fractions were quantified using ThermoGravimetric Analysis (TGA; Leco, TGA701). Both organic and inorganic carbon contents were highly variable in the upper 1m, with differing trends between the natural saltmarsh and MR sites in comparison to the agricultural site: in general, organic carbon content was ~6.5% in the upper 1m and decreased to ~3-4% at a depth of 1.5m and 1.8m in the marshes, whereas organic carbon content was ~4.5% in the upper 0.3m and decreased to ~3% at and below a depth of 0.6m in the agricultural field. Inorganic carbon content was ~2% in the upper 0.3m at all sites, but whilst the marshes exhibited minimal variation with depth (varying between 1.8 and 3.8%), the agricultural field exhibited a decline to 0% between 0.4m and 0.8m, before increasing non-linearly to ~6% at a depth of 2m. However, these general trends mask large inter- and intra-site variability, with organic carbon ranging from 1.8% to 10.3% and inorganic carbon ranging from 0% to 8.6%.

The impact of our new empirical results on carbon sequestration estimates was explored using a 100,000-run Monte Carlo simulation framework in which organic and inorganic carbon contents were randomly selected from best-fit pdfs of data including and excluding cores from below 1m and 1.5m. Our results imply that carbon stocks estimated using cores extruded from only the upper 0.3m may significantly overestimate the total carbon sequestered and stored in saltmarshes and managed realignment sites. Cores to quantify carbon stocks in saltmarshes should extend to a depth of at least 1m and ideally to a depth of 1.5m.