Effects of monoculture regime on the soil nirK- and nosZ-denitrifying bacterial communities of Casuarina equisetifolia

The forest quality of Casuarina equisetifolia has reduced dramatically due to soil sickness under continuous monoculture. The effect of microbes on the growth of C. equisetifolia plants has been extensively investigated. In this study, quantitative PCR assays and Illumina MiSeq sequencing of targeted nirK and nosZ genes were carried out to identify the variations in abundance in the soil and community structure of denitrifying bacteria under the long-term consecutive monoculture of C. equisetifolia. The principal component analysis illustrated that the denitrifying bacterial community structure was obviously different among the soils in the treatments of no C. equisetifolia cultivation (CK), the first continuous plantation (FCP), the second continuous plantation (SCP), and the third continuous plantation (TCP). Taxonomic analysis demonstrated that in both the nirK and nosZ gene-containing denitrifying bacterial communities, Proteobacteria increased significantly in C. equisetifolia mono-cultures. Furthermore, the genera Bosea, Sinorhizobium, Streptomyces, Azospirillum, Mesorhizobium, Thiobacillus, Ochrobactrum, and Rhodoferax were enriched, while Rhodopseudomonas, Pseudomonas, Chelativorans, and Che-latococcus decreased under continuous monoculture (SCP and TCP). Redundancy analysis showed that Strepto-myces, Pseudomonas, and Rhodopseudomonas were positively correlated with soil physicochemical factors. In summary, C. equisetifolia monoculture led to dramatic shifts in the quantity and community structure of deni-trifying bacteria, contributing greatly to soil sickness in this tree species.