Coordinated soil–plant–microbial responses shaped by moderate grazing intensity in subalpine mountain grasslands

Mountain grasslands of the Central Pyrenees are highly sensitive socio-ecological systems traditionally managed under extensive grazing, where soil properties, vegetation dynamics and microbial processes are tightly coupled. However, the effects of grazing are often assessed separately for either soil or forage, which constrains integrated evaluations of overall ecosystem functioning. In this study, we compared two subalpine grassland plots subjected to contrasting grazing intensities (moderate-high vs. moderate-low), integrating soil physical, chemical and biological indicators with forage nutritional quality. Grazing intensity was characterized using GPS-derived cattle movement data from five adult animals during the grazing season. Soils under lower grazing intensity retained more water, accumulated larger organic carbon pools and exhibited longer carbon residence times together with higher microbial functional diversity. By contrast, higher grazing intensity favored labile carbon dynamics, with more dissolved organic carbon and higher microbial respiration. No significant differences were observed in mesofauna diversity, total glomalin content or β-glucosidase activity. Although forage composition was dominated by mixed grasses and legumes, nutritional quality was low across all plots, due to high fiber content and deficiencies in Zn, Cu and Se. Multivariate analyses showed that soil carbon and hydrological status drive ecosystem variability through coordinated soil–microbial–forage responses. Our results suggest that even subtle variations in grazing intensity generate integrated functional shifts across ecosystem compartments. Under ongoing climate change, such tightly coupled systems may be particularly vulnerable to grazing mismanagement, emphasizing the need for adaptive strategies that integrate soil health, microbial functioning and forage quality to sustain mountain grassland resilience.