The effect of sheep grazing abandonment on soil bacterial communities in productive mountain grasslands

Livestock grazing abandonment entails important shifts on the overall ecosystem function, but the effects of this land -use change on specific bacterial taxa remain poorly understood in mountain grasslands. Moreover, we currently lack knowledge about the feedbacks between changes in ecosystem functions affected by livestock abandonment in moun-tain grasslands and the soil bacterial communities. Here, we evaluated the behavior of bacterial communities' structure and composition at taxa level as a function of short (1-year) and long-term (15-years) grazing abandonment in a moun-tain grassland. We also linked the observed responses in the bacterial communities to changes in several ecosystem functions (i.e. primary production, plant species biodiversity, carbon stocks and soil fertility). The alpha diversity of the bacterial communities did not show a significant response as a consequence of grazing abandonment. However, we identified significant changes on the overall composition of soil bacterial communities between the long-term abandoned grassland areas and grazed or abandoned areas in the short term. We also evidenced a balance between the number of operational taxonomic units (OTUs) whose relative abundance is favored by livestock grazing (19.51 %) and those with higher relative abundances in long-term grazing exclusion areas (20.23 %) that could behave as indicators of grazing abandonment. Structural Equation Modeling analyses proved that several bacterial taxa asso-ciated with relevant ecosystem functions, such as Rhodospirillales order within Alphaproteobacteria phylum, featured significant changes in their relative abundance between grazing treatments. The direct and indirect effects of grazing exclusion on woody species encroachment and soil organic carbon were strongly linked to the changes in the abun-dance of bacterial taxa indicators. The assessment of the bacterial community response to livestock abandonment in mountain grasslands may thus provide early warning signs before subtle changes in ecosystem functions occur.

Automated summary

Livestock grazing abandonment has important effects on the overall ecosystem function, including on specific bacterial taxa in mountain grasslands. This study evaluated the behavior of bacterial communities in response to short and long-term grazing abandonment, and linked these responses to changes in ecosystem functions such as primary production, biodiversity, carbon stocks, and soil fertility. The study found that the overall composition of soil bacterial communities changed significantly between long-term abandoned grassland areas and grazed or short-term abandoned areas. It also identified specific bacterial taxa that could serve as indicators of grazing abandonment. The study highlights the importance of assessing bacterial community responses to livestock abandonment in mountain grasslands as early warning signs of changes in ecosystem functions.