Impact of Karst Rocky Desertification on N-Fixing Tree Species Root System and Bacteria

The study aimed to investigate the effect of soil textural or soil mineral fraction substrates (loam and gravel) from karst desertification areas on the rhizospheric and root-associated bacterial community structure of Dalbergia odorifera (an N-fixing tree), using high-throughput sequencing techniques, based on treatment methods of whole-root and two-chambered split-root systems. Further, this study determined the relative importance of the plant and substrate properties on the rhizospheric, non-rhizospheric and endospheric bacteria composition. The type of substrate exerts a significant influence on both rhizospheric and non-rhizospheric bacterial communities, whereas endophytic communities within the root system are predominantly determined by plant species rather than substrate type. The analysis revealed that endospheric bacterial diversity was considerably lower than that of rhizospheric and non-rhizospheric communities. Cluster analysis indicated that endospheric bacterial samples formed a distinct cluster, while rhizospheric and non-rhizospheric bacteria in the soil substrate grouped into one branch, and those in the gravel substrate formed another branch. In comparison to the gravel treatments, a reduced bacterial abundance was observed in the rhizosphere and non-rhizosphere of nitrogen-fixing plants in soil, potentially due to the interplay of lower nutrient availability and increased porosity in gravel treatments. Proteobacteria, which are involved in the nitrogen cycle, exhibited the highest abundance. In contrast, Acidobacteria, Firmicutes and other bacterial phyla involved in nutrient cycling demonstrated higher abundance, with their presence being more pronounced in extreme environments, such as gravel treatments, compared to soil substrates. These results suggest that nitrogen-fixing plants can respond to extreme environments by increasing bacterial abundance. The findings of this study provide a theoretical basis for the use of D. odorifera for ecosystem recovery and vegetation restoration.