Driving mechanisms of soil bacterial α and β diversity under long-term nitrogen addition: Subtractive heterogenization based on the environment selection

Soil bacterial α and β diversity patterns under nitrogen (N) addition have been intensively examined, but γ diversity patterns remain largely unknown, especially, the mechanisms that concurrently control changes in α, β and γ diversity remain elusive. Therefore, we formulated a conceptual framework that simultaneously considers candidate drivers including ubiquitous species, rare species, and community assembly processes to elucidate the driving mechanisms of α, β and γ diversity under N addition. The conceptual framework was tested by compiling the sequence data of seven studies published from January 1997 to May 2022 and following the same analysis as our own two long-term multilevel N addition experiments. We demonstrate that subtractive heterogenization based on environment selection simultaneously predicts the changes in α, β and γ diversity under long-term N addition. That is, long-term N addition led to the decline of ubiquitous species (subtractive processes) through low soil pH, and promoted the strength of heterogeneous selection (heterogeneous processes) via enhancing environmental heterogeneity, subsequently causing lower α diversity and γ diversity but higher β diversity. These results mean that N addition may lead to a significant loss of soil bacterial diversity around the world. Together, these findings offer a way to simultaneously predict soil bacterial α, β and γ diversity responses to the ongoing atmospheric nitrogen deposition.