Effect of grazing intensity on nitrogen cycle of alpine summer pasture soil

Alpine pastures are relevant agro-ecosystems classified as High Nature Value Farmland thanks to their high biodiversity. The aim of this study is to characterize the agroecological relationships among grazing animals, soil conditions, and microorganisms in an alpine summer pasture located in the Dolomites, Eastern Italian Alps, at an elevation around 2,000 m asl, grazed by dairy cows from mid-June to mid-September, with an area of 180 ha and a stocking rate of 0.84 livestock unit/ha. The grazing intensity was characterized at a fine spatial scale by combining GPS tracking of grazing animals and farmer’s interviews. To assess the impact of grazing on soil conditions and microbial functions, topsoil cores were sampled at two different depths in three pasture areas at different grazing intensity during September, the end of the grazing period. Topsoil samples were analysed for bulk density, content of ammonium, nitrate, and the N isotopic ratio (δ15N), as an index of relative rates of N inputs and losses. In addition, the functional potentials of microbial communities were assessed by quantifying with real-time PCR the abundances of target genes nirK and nosZ for denitrification, amoA Archea and Bacteria for nitrification, and nifH for nitrogen fixation. Nitrogen content and bulk density were significantly affected by both grazing intensity and soil depth and their interaction, while microbial functions was significantly affected only by grazing intensity, showing a likely and diversified impact of grazing on nitrogen cycle into topsoil. These results could be used to develop minimally invasive biophysical indicators of regulating ecosystem services in grasslands soils. These tools can inform sustainable management of grazing livestock systems in accordance with agroecology principles.