Soil bacterial communities are pivotal in regulating terrestrial biogeochemical cycles and ecosystem functions. The increase in global nitrogen (N) deposition has impacted various aspects of terrestrial ecosystems, but we still have a rudimentary understanding on whether there is a threshold for N input level beyond which soil bacterial communities will experience critical transitions. Using high-throughput sequencing of the 16S rRNA gene, we examined soil bacterial responses to a long-term (13 years) multi-level N addition experiment in a temperate steppe of northern China. We also examined plant community diversity and environmental parameters in field. Together, we aimed to ask: (ⅰ) Does soil bacterial diversity respond non-linearly to N addition? Specifically, we hypothesized that when N addition level reaches a certain level, soil bacterial diversity will suddenly and substantially decrease. (ⅱ) Among soil environmental conditions, plant productivity and plant communities, which factors play more important roles in driving changes in soil bacterial diversity and community composition under N enrichment?
Results/Conclusions
We found that plant diversity decreased linearly along N input levels. However, bacterial diversity responded nonlinearly to N input, such that it was unaffected by N input below 16, but decreased substantially when N input exceeded 32 g N m-2 y-1. A meta-analysis across Inner Mongolia further confirmed this nonlinear response of bacterial diversity to N inputs. Substantial changes in soil bacterial community structure also occurred between N input of 16 to 32 g N m-2 y-1. Further analysis revealed that loss of soil bacterial diversity was primarily driven by soil pH, not by plant diversity. However, bacterial communities were significantly affected by plant communities, even after controlling for changes in soil environmental factors caused by N addition. Overall, our study identified a tipping point of N input level for bacterial diversity and composition and demonstrated differential contribution of plant communities to bacterial diversity and communities. The nonlinear response of bacterial diversity and linear response of plant diversity to N input observed in our study indicated that although bacteria communities lags behind plant communities in response to N deposition, further increase in N input could tigger a critical transitation in bacterial communities and lead to the collapse of bacterial diversity.