Soil Organic Carbon Stocks of German Salt Marshes: A Comparative Study Along Low- and High-Energy Coastlines

Blue carbon ecosystems, such as salt marshes, store comparably large amounts of organic carbon in their soils and function more effectively as carbon sinks than most other terrestrial ecosystems. Here we provide the first comprehensive study, quantifying soil organic carbon (SOC) stocks in grazed and non-grazed German salt marshes. In Germany, salt marshes are found along the low-energy, microtidal coastline of the Baltic Sea as organogenic ecosystems and along the high-energy, mesotidal coastlines of the North Sea as minerogenic ecosystems. One-meter soil cores were taken across 14 sites covering three distinct salt marsh types: Baltic Sea, North Sea mainland, and North Sea island. Baltic salt marshes held on average the greatest SOC stocks with 221 ± 56.3 (mean ± SE) Mg SOC/ha followed by North Sea mainland salt marshes with 187 ± 24.9 Mg SOC/ha and North Sea island salt marshes with 78 ± 9 Mg SOC/ha. Our findings indicate that livestock grazing resulted in a 1.5-fold increase in SOC density. The microtidal Baltic salt marshes store more SOC in their topsoil than mesotidal North Sea salt marshes, most likely due to the higher sediment deposition rates in North Sea mainland salt marshes causing SOC dilution through mineral inputs. We conclude greater aeration in high-marsh soils might counterbalance SOC accumulation under proceeding succession. Positive livestock grazing effects were relatively consistent within North Sea salt marshes, likely caused by trampling-induced changes in soil biogeochemistry. By contrast, grazing had variable effects on SOC in Baltic Sea salt marshes, with belowground plant productivity identified as the primary driver.