Identifying response groups of soil nitrifiers and denitrifiers to grazing and associated soil environmental drivers in Tibetan alpine meadows

Defining response groups within N-related microbial communities is needed to predict land management effect on soil N dynamics, but information on such response groups and associated environmental drivers is scarce. We investigated the abundance and major populations of ammonia-oxidizing archaea (AOA) and bacteria (AOB), and nirS- and nirK-harboring denitrifiers under different grazing managements in Tibetan alpine meadow soils. Grazing increased AOB and AOA abundances up to 42 fold and 3.7 fold, respectively, and increased the percentage of AOB within total ammonia oxidizers from 3.1% to 10.8%. The abundance of nirK-like denitrifiers increased with grazing intensity, while the abundance of nirS-like denitrifiers tended to decrease. However, sub-groups within each of these broad groups of (de)nitrifiers responded differently to grazing. Soil nitrate was the main driver of the abundance of denitrifier sub-groups (nirK or nirS) positively responding to grazing, while soil moisture and carbon concentration were the main drivers of the abundance of denitrifier sub-groups negatively responding to grazing. AOB and nirK-harboring denitrifiers thus generally responded more positively to grazing than AOA and nirS-harboring denitrifiers, but significant functional diversity existed within each group. Our approach demonstrates the usefulness of the concept of response groups to better characterize and understand (de)nitrifier response to grazing.