Low Climate Benefit of Nordic Coastal Marshes: Site Conditions Outweigh Grazing Effects and Shape Trade‐Offs Between Carbon Storage and Its Stability

Abstract

Coastal marshes, recognized as effective organic carbon (OC) sinks, have gained attention for their potential contribution to climate mitigation through protection and restoration. However, the climate mitigation potential of Nordic coastal marshes remains understudied, likely due to their heterogeneous and often non‐tidal nature. To fill this gap, we examined soil OC storage and accumulation rates, and the effects of grazing, a common management practice, across eight Nordic coastal marsh areas spanning broad climate and environmental gradients. We also assessed soil methane emissions in selected areas. The Nordic marshes studied store a median of 7 kg OC m −2 (interquartile range, IQR: 6–8) in the top 15–35 cm of soil and accumulate 41 g OC m −2 yr. −1 (IQR: 32–47). Considering only the additional OC, attributed to the presence of the marsh habitat, these values drop to 4 kg OC m −2 (IQR: 2–6) and 21 g OC m −2 yr. −1 (IQR: 11–33). Globally, both rates are comparatively low. OC stocks and accumulation rates increased with marsh age, root: shoot ratio (stress adaptation), and δ 15 N (fast N cycling), but declined with soil δ 13 C (related to faster decomposition under warmer conditions and sandier soils). Danish marshes had the highest but also most vulnerable OC stocks due to faster turnover, labile compounds, and coarser soil grain sizes. Although grazing only weakly increased soil OC stocks and had no effect on OC accumulation rates, it significantly reduced methane fluxes compared to ungrazed marshes. In ungrazed areas, methane emissions weakened the carbon sink by 32% in Finland and 68% in Denmark. However, estimated greenhouse gas emissions from on‐site cattle, even at low grazing intensity, largely outweighed the coastal marsh climate benefits. A comprehensive Nordic marsh management strategy is needed, extending beyond the focus on their limited, yet relevant, role in climate mitigation, and considering biodiversity, coastal protection and nutrient retention.

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