Differential Effects of Nitrogen Chemical Forms on Soil Bacterial Communities and Ecosystem Multifunctionality in a Temperate Meadow Steppe

ABSTRACT Understanding the impact of nitrogen (N) enrichment on soil microbial community and its associations with ecosystem functionality is crucial, given rising global atmospheric N deposition and even greater agricultural inputs. Most previous studies simulating N deposition used single N species, yet the specific effects of different N forms are not fully understood. This limitation hinders our capacity to link changes in soil microbial communities with ecosystem multifunctionality and to assess the impact of atmospheric N deposition on ecosystem services. To bridge this knowledge gap, our study, conducted from 2016 to 2018 in a typical temperate grassland in Inner Mongolia, China, evaluated the differential effects of the addition of four N forms—ammonium bicarbonate (AC), ammonium nitrate (AN), ammonium sulfate (AS), and urea (UR)—on soil bacterial community structure and function, taxonomic interactions, and ecosystem multifunctionality. These forms were applied at five levels (0, 2, 10, 20, and 50 g N m −2 year −1 ) with five replicates as part of a long‐term experiment. Our findings reveal that N forms in combination with increased N loading differentially influence bacterial community structure and functional characteristics, and co‐occurrence networks, alongside ecosystem multifunctionality. Notably, AN/AS significantly reduced bacterial taxonomic and functional diversity and network complexity, unlike AC, which showed minimal changes. The alteration in bacterial community structure and function under AN/AS was more significantly linked to changes in ecosystem multifunctionality than those under AC/UR, with AC having minimal connections. Additionally, keystone taxa responded differently to N levels and showed varied correlations with ecosystem multifunctionality. These results underscore the distinct effects of different N forms on soil bacterial communities and their cascading influence on ecosystem multifunctionality, emphasizing the need for management practices targeted to the specific chemical species of N deposition and fertilization.