Grazing Intensity Has More Effect on the Potential Nitrification Activity Than the Potential Denitrification Activity in An Alpine Meadow

Abstract

On the Qinghai–Tibet Plateau, nitrogen (N) cycling, such as nitrification and denitrification, in the alpine meadow soils have been considerably affected by grazing, with possible consequences for nitrous oxide (N2O) emissions. However, there is a lack of understanding about how the potential nitrification activity (PNA) and the potential denitrification activity (PDA) might be affected by the grazing intensity. We collected the soil samples in alpine meadow in the east of the Qinghai–Tibet Plateau that was grazed at different intensities from 2015 in peak growing season 2021. We determined the soil physical and chemical properties, the functional gene abundances of nitrifiers and denitrifiers, and the soil PNA and PDA to explore the relationships between a range of abiotic and biotic factors and the PNA and PDA. We found that the PNA and the nitrifiers were significantly affected by the grazing intensity but that the PDA and the denitrifiers were not. The ammonia-oxidizing archaea (AOA) abundance was highest but the ammonia-oxidizing bacteria (AOB)abundance was lower than the control significantly at the highest grazing intensity. The AOA abundance and the soil NH4+-N explained most of the variation in the PNA. The pH was the main predictor of the PDA and controlled the nirS abundance but not the nirK and nosZ abundances. Overall, the PNA was more responsive to the grazing intensity than the PDA. These findings can improve estimations of the nitrification and denitrification process and N2O emissions in alpine meadow.