Analysis of Rhizobial and Non-Rhizobial Endophytes Associated with Cowpea Root Nodules at Different Phosphorus Levels Using 16S rRNA High-throughput Sequencing

Abstract

The coexistence of rhizobia and non-rhizobial endophytes (NRE) within cowpea (Vigna unguiculata [L.] Walp.) root nodules has been reported in several regions worldwide; however, such information remains limited for Tanzania. This study assessed the diversity and community composition of rhizobial and non-rhizobial endophytic bacteria in cowpea root nodules collected from soils receiving different phosphorus (P) application levels using high-throughput 16S rRNA gene sequencing. The results revealed the consistent coexistence of rhizobial and NRE communities across all phosphorus treatments. Seven bacterial phyla were identified, namely Proteobacteria,
Bacteroidetes, Firmicutes, Actinobacteria, TM7, Verrucomicrobia, and Cyanobacteria. Proteobacteria dominated nodules from P-enriched soils (P20N, P40N, and P60N), while a lower relative abundance was observed in P-deficient soil (P0N). Alpha diversity indices indicated
higher richness and evenness in nodules from P-enriched soils, with the highest diversity observed at P60N. Members of the genus Bradyrhizobium were abundant across all treatments, with peak relative abundance at P20N, and Bradyrhizobium kavangense  nominated nodules from P20N, P40N, and P60N soils. Diverse NRE taxa were also detected, with their relative abundance varying across phosphorus levels. Overall, the results demonstrate that soil phosphorus availability is strongly associated with shifts in the structure and diversity of cowpea nodule-associated bacterial communities. These findings provide new insights into the role of phosphorus enrichment in shaping legume-microbe interactions under cowpea cultivation in Tanzania..