Organic matter composition and origin differ between restored and natural UK saltmarshes up to 100 years after the breach

Abstract. Atmospheric carbon dioxide uptake by restored saltmarshes can be converted to carbon credits that are sold to offset emissions and provide funding for restoration. While carbon stocks in restored marshes were found to reach equivalence with natural sites, the shares of autochthonous and allochthonous carbon sources entering these marshes are less well known. This question has consequences for carbon crediting because only the former can be attributed to the restoration and legitimately included. To determine if differences do exist, the conventional tools for determining organic matter source – elemental, stable isotope and thermogravimetric analyses – were applied to sediments from four restored saltmarshes of different ages. Comparisons were then made with existing data from nearby natural marshes. Breach depth horizons in cores were successfully identified using foraminifera, unicellular marine protists. The C/N-ratio was persistently lower in restored saltmarshes compared with natural counterparts even one hundred years after the creation of a breach. Additionally, δ¹³C signals indicated a mixture of organic matter sources to the restored sites and there was often a higher proportion, based upon thermal sensitivity, of recalcitrant material in the organic matter of the restored marshes. Together, these results indicated that a higher proportion of allochthonous organic matter that had undergone the processes of decomposition in an external setting, was entering these restored marshes. Understanding the composition of the organic matter in restored marsh sediment will allow more useful and constructive climate change mitigation methods to be developed.