Sustained Livestock Grazing Enhances Soil Organic Carbon Storage in Sub‐Arctic Grassland

ABSTRACT

Grazing systems remain under‐represented in global carbon cycle models, and grazed grasslands are rarely recognised as an active carbon sink. Emerging evidence indicates that grazing can enhance plant carbon allocation below‐ground, contributing to soil organic carbon (SOC) sequestration. Yet, long‐standing grazing practices are increasingly being abandoned in sub‐arctic regions with poorly understood consequences for SOC sequestration. Here, we examined the long‐term consequences of abandonment of livestock grazing for SOC in sub‐arctic grassland and heathland across 34 sites in Iceland, by comparing paired ungrazed exclosures (20–83 years) with adjacent continuously grazed land. Our results show that the cessation of grazing was associated with 8% lower SOC stocks in the topsoil (0–10 cm), but without effect on SOC in deeper soil. Overall, SOC stocks (on average 130 Mg C ha ⁻¹ in 0–60 cm) were an order of magnitude larger than carbon in total plant biomass, including roots, shoots and litter. With the cessation of grazing, root carbon and nitrogen stocks in the topsoil, as important factors of SOC dynamics, were 29% and 9% lower, respectively. Our findings indicate that soil nitrogen limitation associated with cessation of grazing promoted vegetation shifts from grassland into heathland or birch woodland at multiple sites. Both heathland and birch woodland were associated with markedly lower SOC stocks in the upper 30 cm of soil compared to grazed grassland. Therefore, we emphasise that sustained extensive grazing is important for maintaining SOC stocks of sub‐arctic grassland.