Abandonment of traditional livestock grazing reduces soil fertility and enzyme activity, alters soil microbial communities, and decouples microbial networks, with consequences for forage quality in Mediterranean grasslands

Abstract

Extensive livestock grazing is a global human activity. In the Iberian Peninsula, extensive grazing and seminatural grasslands and open woodlands such as dehesas have co-evolved with human use for millennia. However, social, demographic, and economic factors are now pushing this traditional activity towards both conventional intensification and land abandonment, with consequences for the biodiversity and functioning of these seminatural ecosystems. Soils can be particularly affected by grazing abandonment due to the cessation of inputs of pre-processed organic matter (dungs and urine) and of trampling, with still poorly understood consequences for the composition, network configuration, and activity of soil microbial communities and the capacity of soils to store C. In this work, we used 20 pairs of adjacent plots (40 plots in total) located in seminatural grasslands from central Spain. For each pair, one plot was extensively grazed by livestock and the other one was abandoned. We evaluated the effects of extensive grazing abandonment on soil fertility (C and N contents, and P and K bioavailability), forage quality (fibre and protein content), and soil microbial community composition (amplicon sequencing of 16 S [bacteria] and ITS [fungi]), network coupling, and activity (extracellular hydrolytic enzymes linked to the biogeochemical cycling of C, N, P, and S). Grazing resulted in higher soil fertility in terms of C, N, and P, and grassland forage quality (lower fibre). Grazing also affected soil microbial community composition, but not richness or diversity. These effects occurred primarily through changes in nutrients and soil water availability. Actinobacteria significantly increased in abandoned plots, while Acidobacteria, Verrucomicrobia, and Planctomycetes decreased. Bacterial and, particularly, fungal networks were generally less coupled in abandoned plots. Furthermore, grazing resulted in greater soil enzyme activity via direct effects. These results support the notion that extensive grazing with intermediate stocking rates provides a positive effect on grass quality, soil fertility, nutrient cycling, and microbial network configuration, and thus warn about the potential negative effects of land abandonment.