What can the interaction between grazing regime and environment tell us about SOM stabilization mechanisms? Results from an ecological field laboratory in Spain

            Large extensions of grasslands in Europe are managed with ungulate presence for food production, rural livelihoods, and (more recently) soil carbon sequestration goals. Grazing regimes hold grasslands in ecological balance, and it is not clear how ungulate management and succession dynamics impact SOC stabilization. While there is increasing recognition that integration of large herbivores into productive systems can be key for restoring soil carbon and soil quality, research is ongoing on under what conditions this may occur, and why.

            Over three years, our research team has been monitoring changes in critical soil parameters in pasture and forest ecosystems with a wide gradient of domesticated and wild ungulate grazing pressure, in order to understand the ecological and practical repercussions of human management of the landscape. Through this monitoring, we have firstly identified that SOC accumulation diverges significantly depending on the ecological context. The mechanisms by which SOC stabilization occurs is understood to follow different paths, modulated by grazers in contrasting ways depending on this context. Secondly, our establishment of a grazing gradient by using GPS tracking collars has allowed the quantification of relationships between grazing pressure and soil processes. Most notably for SOC accumulation, we have seen that the grazing pressure of domesticated transhumant ungulates (sheep) is linearly and positively associated with both soil microbial processes and SOC accumulation. Importantly, contributions to the SOC pool are seen to depend on vegetation structure and plant composition (Figure).

            Overall, the studies have elucidated relationships between vegetational characteristics and litter quality, herbivore action, and soil microbiology, aiding to unravel the main mechanisms driving SOC accumulation in ecosystems with large mammal ungulate grazers.