Shrub expansion increases nitrification and denitrification in alpine meadow soils

Shrubification is a common phenomenon in grasslands worldwide, but the effects of such a plant community shift on soil nitrification and denitrification are rarely studied. We investigated the effects of shrub species (Dasiphora fruticosa, Hippophae tibetana, Salix oritrepha, Spiraea alpina) on soil nitrification and denitrification and abundances of related microbial functional groups in an alpine meadow. All four shrub species generally increased fungal gene abundance but decreased archaeal gene abundance compared with grasses. This led to increased gene abundance ratios of bacteria to archaea, fungi to archaea, and fungi to bacteria in soils under shrubs relative to grasses. Shrub species of D. fruticosa, S. oritrepha and S. alpina generally increased soil potential nitrification rate and upregulated abundances of ammonia-oxidizing bacteria (AOB) and comammox bacteria (CAOB) compared with grasses, whereas H. tibetana did not affect nitrification or AOB and CAOB abundances. These indicated that the effects of shrubs on soil nitrification rate and related microbial function groups changed with shrub species. The increased soil nitrification potentials under shrubs relative to grasses were not related to ammonia-oxidizing archaea, because their abundances decreased under all shrubs. All four shrub species increased soil potential denitrification rates by 50 %-70 % compared with grasses, associated with increases of nirK gene abundance under D. fruticosa, H. tibetana and S. alpina and with an increase of nirS gene abundance under S. oritrepha. Changed abundances of soil microbial functional groups under shrubs were mainly ascribed to changes in soil pH and increased soil organic matter content and nutrient availability. It is concluded that shrubification increases potentials of nitrification and denitrification and related functional gene abundances in alpine meadows.