Livestock grazing, plant community and abiotic factors shape blue carbon stocks in Nordic coastal marshes

Abstract. Coastal marshes are key habitats contributing to organic carbon (OC) storage but remain understudied in Nordic regions regarding Blue Carbon processes. This study quantified OC stocks in above- and below-ground biomass and in the top 50 cm-soil across 12 grazed and ungrazed marshes, spanning major environmental gradients, and assessed how biotic (plant communities, livestock grazing) and abiotic (soil properties, environmental conditions) drivers shape OC storage. Grazing and environmental gradients strongly structured plant communities, partly by regulating the reed Phragmites australis, prevalent in ungrazed sites. Aboveground OC stocks were reduced by grazing, both directly through biomass removal and indirectly by reducing reed dominance. Belowground OC stocks were driven by plant community composition and indirectly by grazing effects on vegetation. Root biomass was concentrated in the top 15 cm in grazed sites and deeper (15–50 cm) in ungrazed sites, reflecting contrasting plant strategies. Abiotic soil properties were major large-scale drivers of soil OC storage, while grazing affected soil OC storage indirectly through plant composition. Soil OC stocks accounted for ∼73 % of total OC in grazed sites and ∼63 % in ungrazed ones and was higher in grazed sites (99.7 ± 57.9 Mg ha⁻¹) than in ungrazed sites (78.2 ± 44.2 Mg ha⁻¹). Soil OC increased with finer textures, whereas vegetation and grazing effects were variable and locally expressed. Overall, soil OC stocks in Nordic coastal marshes fall within the lower range of global estimates. These findings highlight the need to consider soil processes, grazing and environmental gradients tin the sustainable management of Nordic coastal marshes and their carbon storage potential.