Microbial genes highlight different trends in short term for N cycling in historical alpine pastures

Context Alpine pastures are seminatural grasslands which play a crucial role in biodiversity conservation, service provisioning, and mountain livestock systems. The soil microbial communities of pasture are fundamental in ecosystem nutrient cycles, but they are relatively underexplored in European Alpine pastures. Aims We explored the many soil microbial genes encoding key functions in the nitrogen cycle in three historical alpine pastures grazed by dairy cattle, considering different soils, temporal dynamics, and exclusion of cattle grazing for one summer. Methods 216 samples were collected across four sampling times. The abundance of genetic determinants involved in nitrogen fixation (nifH), nitrification (amoA bacterial and archaeal), and denitrification (nirK and nosZ) were quantified using real-time polymerase chain reaction. Key results The terminal denitrification nosZ gene was the most sensitive indicator and responded significantly to soil chemical composition and animal grazing. Sampling time affected nitrogen fixation nifH and intermediate denitrification nirK in relation to rainfall cumulation dynamics. The amoA nitrification genes showed high variability but no significant effects from the tested factors. Conclusions In spite of a general homeostatic trend occurring in these habitats and of the short term analysis, some genes acted as sensitive reporters of soil compositional differences, intraseasonal climatic variations, and grazing disturbance. Implications A stocking rate of >0.6 livestock units per hectare can be recommended, to combine animal production with conditions that favour complete denitrification, thus potentially reducing the nitrous oxide greenhouse gas emissions. Higher livestock grazing intensity can be withstood by the ecosystem without denitrification-related drawbacks when the preceding 10 days display a cumulated rainfall lower than 22 mm.