Changes in soil organic matter quality during sea-influenced marsh soil development at the North Sea coast

Salt marsh soils sequester large amounts of organic matter (OM). The question we address in this study is how OM quality changes during initial soil development in salt marshes. To answer this question, we studied soils at six sites at the German North Sea coast. At each site, three zones – low, mid, and high marsh – that differ in inundation frequency were analyzed. We found that organic carbon (OC) and nitrogen (N) contents increased significantly with decreasing inundation frequency at all sites, while inorganic carbon contents decreased. δ13C signatures of the OC strongly decreased at all sites from low to high marsh (from − 15.3 to − 21.5‰), indicating a decrease in the proportion of marine-derived OC. The decrease in sea-derived OC was associated with an increase in C/N ratio, which can be attributed to the difference in the C/N ratios between sea- and land-derived OM inputs. Increases in OC and N contents in the bulk soils during soil development resulted from increases of the OC content in the coarse size fraction (> 200 μm), and were associated with increases in the content of hot water extractable C and N (Chwe and Nhwe). The proportion of OM found in the fraction < 2 μm decreased with soil development. The δ13C signature of the OC sequestered in this fraction decreased from − 20.6 to − 24.0‰. The smaller decrease in the δ13C signatures of the OC in the fraction < 2 μm compared to the δ13C signatures of the OC in the bulk soils indicates that the OC is relatively rigidly bound to the minerals in this fraction. The low δ13C signatures of OC stored in the fraction < 2 μm indicates that OM in this fraction is mainly of terrestrial origin. In conclusion, this study shows that OM contents and the proportion of relatively labile OM increases during initial marsh soil development due to inputs of terrestrial OC.