Potential of Dutch saltmarshes as deposits of blue carbon

Abstract

Vegetated coastal areas such as tidal saltmarshes, mangroves and saltmarshes provide benefits such as coastal protection, wave action reduction, and trapping of sediment. Additionally, they provide storage for carbon from the atmosphere and oceans in their living biomass aboveground, and living- and non-living biomass underground. This storage has been termed “blue carbon”. This study aimed to determine the soil carbon pool of the island-, mainland-, and estuarine saltmarshes of Texel, the Eastern and Western Scheldt, and Groningen in the Netherlands. This was done to estimate the blue carbon potential for these areas, and to make estimations for the whole Dutch salt marsh area to make a global comparison. The objective was reached by collecting sedimentary soil samples, analysing the organic carbon content, and compiling and interpreting existing data within the context of the wider literature. Results showed that there is a significant correlation between %LOI and %Corg of the soil samples (p<0.01) and a significant negative correlation between soil carbon content and soil depth (p<0.01). Furthermore, no significant correlation between organic carbon content and salt marsh type (p>0.05) and no correlation between organic carbon content and gradient zones was shown (p>0.05). The island salt marsh has the highest carbon stock (302 Mg C/ha) and an annual carbon accumulation of 4 Mg C/ha/yr. Followed by the estuarine salt marsh with a carbon stock of 287 Mg C/ha and an accumulation of 5.3 Mg C/ha/yr, and mainland salt marsh with a carbon stock of 215 Mg C/ha and accumulation of 5.8 Mg C/ha/yr. The observed differences are suggested to be related to differences in salt marsh elevation, accretion and erosion rates, and soil grain size. When compared to average global estimations, the national carbon stock (276 Mg C/ha) and annual carbon accumulation (5.08 Mg C/ha/yr) were higher than global averages (244 Mg C/ha and 2.45 Mg C/ha/yr) and European accumulation rates (3.12 Mg C/ha/yr). However, European average carbon stock estimates were higher than estimations of this study (312 Mg C/ha). It is suggested that the higher carbon stock and accumulation rates are related to the Netherlands latitude being situated in a carbon accumulation peak between 48.5 – 58.5◦ N. Furthermore, it is also suggested that lower temperatures than other European countries resulted in a lower than average carbon stock. Even though the higher marsh gradient zone had fewer measurements, the results are still considered reliable due to the inclusion of pioneer marshes when higher marshes were inaccessible. It is recommended that data collection during further research is done outside of coastal bird breeding seasons, that research includes marsh elevation and soil grain size, and includes δ13C measurements. Following these recommendations would provide a detailed description of the carbon content in higher marshes, the effect of elevation and grain size on soil carbon content, and the origin of the organic carbon.