Low climate benefits of Nordic coastal marshes

Saltmarshes, recognized as effective organic carbon (OC) sinks, have gained attention for their potential contribution to climate mitigation through protection and restoration. However, the heterogeneous and geologically young nature of Nordic coastal marshes likely explains the limited research on their climate mitigation potential. To fill this gap, we examined soil OC storage, long-term OC accumulation rates, and soil methane emissions across four Nordic coastal marshes spanning broad climate and environmental gradients. Additionally, we evaluated the effects of grazing, a common management practice. The four Nordic saltmarshes assessed store a median of 7 kg OC m⁻² (interquartile range, IQR: 8–6) in the top 15-35 cm of soil and accumulate 41 g OC m⁻²yr⁻¹ (IQR: 47–32). Considering only the saltmarsh's additional OC, most relevant to climate mitigation, these values drop to 4 kg OC m⁻² (IQR: 6–2) and 21 g OC m⁻² yr⁻¹ (IQR: 33–11). Globally, both rates are comparatively low. Higher saltmarsh age and root:shoot ratio strongly and positively correlated with OC stocks and accumulation rates. The elevated root:shoot ratios seemed a morphological adaption to stressful conditions (higher soil salinity, slightly alkaline soils, warmer temperatures, and low water and nutrient availability) reflected in the saltmarsh plant composition. Soil methane emissions reduced the climate benefit of OC accumulation by 0.15–7.3% in Danish saltmarshes, which remained strong CO2eq sinks, but by 70% in Finnish saltmarshes, leaving them as much weaker sinks. Grazing slightly increased soil OC stocks but did not affect OC accumulation rates or methane fluxes. However, greenhouse gas emissions from livestock farming, even at low grazing intensity, largely outweighed saltmarsh climate benefits. A comprehensive Nordic saltmarsh management strategy is needed, extending beyond the current focus on biodiversity to include coastal protection, nutrient retention, and other ecosystem services, including their limited, yet relevant, role in climate mitigation.