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

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.