Carbon management practices regulate soil bacterial communities in response to nitrogen addition in a pine forest

Background: Carbon (C) and nitrogen (N) represent the commonest limiting nutrients for microbial growth in terrestrial ecosystems. However, most of our understanding of how C:N ratios modulate microbial growth comes from short-term growth assays under controlled conditions. Methods: Four levels of N additions including control (CK, 0 kg·hm^{− 2}·a^{− 1}), low N (LN, 50 kg·hm^{− 2}·a^{− 1}), medium N (MN, 100 kg·hm^{− 2}·a^{− 1}), and high N (HN, 150 kg·hm^{− 2}·a^{− 1}) inputs were applied monthly in a pine forest (Pinus tabulaeformis). Three C management practices were further conducted in each N additions plot, namely aboveground litter and belowground root removal (LRR), aboveground litter removal (LR), and intact soils (non-removal, NR). Soil bacterial richness, diversity, community composition and soil properties were measured. Results: MN and HN significantly increased the relative abundance of copiotrophic taxa, but decreased that of oligotrophic taxa. Bacterial richness and diversity were not altered by N enrichment. LRR significantly increased the relative abundance of Gemmatimonadetes while decreased that of Actinobacteria. C management did not affect bacterial richness and community structure but LR significantly decreased bacterial diversity. Structural equation modeling showed that N addition induced the decrease in soil pH was responsible for the changes in the bacterial community structure. C management exhibited a direct negative effect on bacterial diversity and had an indirect positive effect on it via increasing soil moisture and microbial biomass C:N ratio. Conclusions: The findings highlight the contrasting impacts of N addition and C management on soil bacterial communities and emphasize the regulation of above- and below-ground C substrate supplies on the N responses of soil bacterial communities.