The counteractive effects of nitrogen addition and watering on soil bacterial communities in a steppe ecosystem

In many grassland ecosystems, mowing, nitrogen (N) addition, and watering are carried out to improve plant productivity for the development of livestock husbandry. However, as a result of human activities, N deposition rate is predicted to increase and precipitation is expected to change. These environmental changes affect biodiversity and ecosystem functioning, and most of the previous studies have primarily focused on their effects on macro-organisms, neglecting the responses of soil microbial communities. In this study, we examined the changes in three community attributes (abundance, richness, and composition) of the entire bacterial kingdom and 16 dominant bacterial phyla/classes in response to mowing, N addition, watering, and their combinations, by conducting a 5-year experiment in a steppe ecosystem in Inner Mongolia, China. Overall, various bacterial groups responded differentially to these experimental treatments, with Acidobacteria, Proteobacteria, Alphaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria being the most sensitive. Furthermore, the treatments produced different effects, with N addition and watering having much greater effects than mowing. Addition of N primarily affected these bacterial groups by reducing soil pH, while watering affected the bacterial groups by elevating both soil water content and soil pH. As N addition and watering had contrasting effects on soil pH, they also produced counteractive effects on many bacterial groups. The counteractive effects of N addition and watering suggest that the combination of these two treatments or the concurrence of N deposition and increased precipitation may be helpful for the maintenance of soil bacterial diversity and their ecosystem functions.