Soil bacterial community structure and functions but not assembly processes are affected by the conversion from monospecific Cunninghamia lanceolata plantations to mixed plantations

Abstract

Forest conversion from monoculture plantations to mixed plantations has profound effects on ecological functions as well as soil bacterial community structure, and the creation of mixed plantations may substantially increase soil bacterial diversity. However, the potential function of soil nitrogen (N) cycling and the soil bacterial community assembly altered by forest conversion remain poorly understood. At Meicheng Forest Farm in 2018, we investigated the impacts of forest conversion on soil bacterial compositions and functions related to N cycling, bacterial networks, and assembly processes. The results showed that (1) soil bacterial composition and β diversity, but not α diversity, were significantly influenced by forest management practices; (2) forest conversion had profound effects on N cycling processes, especially N fixation, including the abundance of the N-fixing genera Bradyrhizobium, Burkholderia, and Geobacter, which increased in the mixed plantations; (3) bacterial network analysis showed that keystone taxa (i.e., Gemmatimonas in the mixed plantations and Gp2 in the pure plantations) might play key roles in bacterial networks, and mixed plantations seemed to have more complex network structures; and (4) stochastic processes, especially ecological drift, were predominant in bacterial community assembly processes, which remained unchanged under different management practices. Soil pH may affect bacterial community assembly processes. Overall, our study revealed distinct patterns of soil microbial compositions, especially among N cycling-related bacteria, between different forest management practices and further enhanced our understanding of how forest conversion affects potential microbial functioning and assembly processes.