Impact of livestock pasture-use intensity and soil geochemical properties on microbial functional potential of nutrient cycles in alpine pastures

Alpine pastures deliver provisioning (forage) and non-provisioning ecosystem services, including the regulation of N cycle. We report here the results of two studies conducted in three summer farms in the Italian eastern Alps during the summers of 2020 and 2023 to investigate the effects of pasture-use intensity by dairy cows on soil geochemical properties, microbial communities and N cycle. We monitored livestock pasture-use intensity with GPS tracking and categorized it into three levels (Low, Medium, and High) in 2020 and two levels (Low and High) in 2023, based on the 2020 results. We assessed geochemical properties (pH, total C, organic C, total N, and C/N ratio) with chemical analyses and used real-time PCR to measure the total microbial abundance (16S gene) and the potentials of nitrogen fixation (nifH gene), nitrification (amoA genes of archaea and bacteria), intermediate denitrification (nirK gene) and final denitrification (nosZ gene) before, during and after the grazing season. Geochemical properties differed between summer farms, reflecting permanent soil features. Pasture-use intensity did not influence pH, while total C, organic C, total N, and C/N ratio were higher at the Low than at the other intensity classes. Higher pasture-use intensities increased the total microbial abundance and the various steps of the N cycle, except for terminal denitrification that was not affected. This result might suggest a possible mismatch, at high pasture-use intensities, between the intermediate and terminal steps of nitrification that might lead to N2O volatilization and should be further investigated. The effects of pasture-use intensity on soil geochemical properties and microbial genes were consistent across years, apart from few differences in statistical significance. The abundances of the genes related to general microbial populations, nitrogen fixation and denitrification increased from the beginning to the end of the summer in 2020. These results were only partially confirmed in 2023, possibly because of climatic trends, warranting further investigation. These studies indicate that pasture-use intensity may influence soil microbial communities and N cycle both directly, likely through nutrients deposition, and indirectly through changes in the soil geochemistry. They also provide gene-based proxies for future studies addressing the interface between grazing management, soil microbial communities and nitrogen dynamics in alpine pastures. Research supported by the Agritech National Research Center Task 7.1.2 and the consortium iNEST (Interconnected North-East Innovation Ecosystem) Task 1.1, funding from the European Union Next-GenerationEU (Piano Nazionale di Ripresa e Resilienza (PNRR) Missione 4 Componente 2, Investimento 1.4 – D.D. 1032 17/06/2022, CN00000022 and Investimento 1.5 D.D. 1058 23/06/2022, ECS_00000043, respectively).