Distinct responses of soil bacterial communities to N and P fertilization in a Tibetan alpine meadow of China

Increased nitrogen (N) and phosphorus (P) availability, due to fertilization and deposition, is expected to influence soil microbial community structure and function. However, little is known about the sensitivity and dependency of soil microbial structure on the levels of N and P, and on the integration of N and P inputs in alpine meadow ecosystems. Here, we implemented a 3-year manipulative fertilization study with four levels (0, 10, 20, and 30 g m(-2)) of N fertilization, P fertilization, and NP fertilization (N and P fertilization at a ratio of 1:1) in a Tibetan alpine meadow. We examined how greater N and P availability affected bacterial community diversity, community structure in the top- (0-10 cm) and sub-top (10-20 cm) soil, network topological properties, and the potential mechanisms behind them. Although the & alpha; and & beta; diversities of bacteria within top- and sub-topsoil were not significantly different from those in the control across N, P, and NP fertilization levels, the integration of NP fertilization significantly changed the relative abundances of more bacterial phyla than the N and P fertilization did separately. Furthermore, the N10 and N20 fertilization treatments strengthened bacterial interactions, whereas P and NP fertilization did not affect the complexity and connectivity of the soil bacterial co-occurrence network. In addition, the mechanism of structuring the bacterial community for separate N and P fertilization differed from that for NP fertilization. The separate application of N and P fertilization affected the bacterial community composition mainly through changes in the total and available P levels in the soil, respectively, whereas NP fertilization shifted the bacterial community composition through decreasing plant species richness. These findings provide a quantification of the response of soil bacterial diversity and composition to different levels of N, P, and NP fertilization. It also suggests that in the Tibetan alpine meadow, soil bacterial communities may alter their fitness according to the relative abundances of oligotrophic/copiotrophic bacterial taxa, rather than through changing their diversity.