Higher ammonium-to-nitrate ratio shapes distinct soil nitrifying community and favors the growth of Moso bamboo in contrast to broadleaf tree species

A pot experiment was carried out to study the response of Moso bamboo (Phyllostachys edulis) and broadleaf tree species (Schima superba, Cinnamomum camphora, Cyclobalanopsis glauca), representing respective native invasive and non-invasive plants in subtropical China, to N addition with three NH4+/NO3− ratios. We used 15N isotope tracing and high-throughput sequencing to study the N cycling and soil microbial communities. With an increasing NH4+/NO3− ratio, total plant biomass increased for bamboo but decreased for broadleaf trees, coinciding with the preferential assimilation of NH4+ by bamboo and NO3− by broadleaf trees, as indicated by the activities of glutamine synthetase and nitrate reductase and the enrichment of 15N in roots. The ammonia-oxidizing archaea (AOA) rather than the ammonia-oxidizing bacteria (AOB) community responded to the addition of N with different NH4+/NO3− ratios and a distinct AOA community was formed when the NH4+/NO3− ratio was greater than one. The relationship between AOA abundance and NO3− concentration was positive for C. camphora and C. glauca but negative for P. edulis. Under addition of N with a high NH4+/NO3− ratio, the AOA composition was positively related to NO3− concentration for C. camphora but negatively for P. edulis. Addition of N with a high NH4+/NO3− ratio favored biomass production but suppressed soil nitrification for bamboo in contrast to broadleaf trees. The species-specific response to soil NH4+/NO3− ratio may be an important factor that facilitates bamboo invasion into broadleaf forests.