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

Restoring saltmarshes to enhance atmospheric carbon dioxide uptake is thought to represent a means of climate change mitigation. 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 issue is important because allochthonous carbon is usually excluded from stock calculations. 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 individual cores were successfully identified using foraminifera, unicellular marine protists. The OC/N-ratio was persistently lower in restored saltmarshes compared with natural counterparts even 100 years after the creation of a breach. Additionally, δ 13 C 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 was entering these restored marshes. Understanding the composition of the organic matter in restored marsh sediment will allow the development of robust and constructive climate change mitigation methods.