Nitrous oxide and methane fluxes vs. carbon, nitrogen and phosphorous burial in new intertidal and saltmarsh sediments

Carbon (C), nitrogen (N) and phosphorous (P) burial rates were determined within natural saltmarsh (NSM) and 'managed realignment' (MR) sediments of the Blackwater estuary, UK. Methane (CH4) and nitrous oxide (N2O) fluxes were measured along with their ability to offset a portion of the C burial to give net C sequestration. C and N densities (Cρ and Nρ) of NSM sediments (0.022 and 0.0019 g cm−3) are comparable to other UK NSM sediments. Less vegetationally developed MR sediments have lower Cρ and Nρ (0.012 and 0.0011 g cm−3) while the more vegetationally developed sites possess higher Cρ and Nρ (0.023 and 0.0030 g cm−3) than NSM. Both NSM and MR areas were small CH4 (0.10–0.40 g m−2 yr−1) and N2O (0.03–0.37 g m−2 yr−1) sources. Due to their large Global Warming Potentials, even these relatively small greenhouse gas (GHG) fluxes reduced the net C sequestration within MR marshes by as much as 49%, but by only 2% from NSM. Potential MR areas within the Blackwater estuary (29.5 km2 saltmarsh and 23.7 km2 intertidal mudflat) could bury 5478 t C yr−1 and 695.5 t N yr−1, with a further 476 t N yr−1 denitrified. The saltmarsh MR would also sequester 139.4 t P yr−1. GHG fluxes would reduce the C burial benefit by 24% giving a C sequestration rate of 4174 t C yr−1. Similar areas within the Humber estuary (74.95 km2) could bury 3597 t C yr−1 and 180 t N yr−1, with a further 442 t N yr−1 denitrified. GHG fluxes would reduce the C burial benefit by 31% giving a C sequestration rate of 2492 t C yr−1. Overall, MR sites provide sustainable coastal defence options with significant biogeochemical value and, despite being net sources of CH4 and N2O, can sequester C and reduce estuarine nutrient loads. -- Highlights: ► We investigated C, N, P, CH4 and N2O fluxes from natural and restored intertidal wetlands. ► Estuary N loadings were reduced through N burial and denitrification. ► Both wetland types are sequestering large amounts of C despite producing some CH4 and N2O. ► Restored marshes can, in the short term, sequester more C and N than natural marshes.