Characterization of rhizobacteria and their formulation into biofertilizers for potato (solanum tuberosum l.) growth promotion in Tanzania
Rhizobacteria and their plant growth-promoting (PGP activities have widely been investigated as biofertilizers for many crops but rarely for the potato (Solanum tuberosum L.). In the present study, potato-grown soils from three agro-ecological zones in Tanzania were sampled, characterized, and a total of 145 rhizobacteria isolated from them. The isolates were screened in vitro for PGP activities like nutrients solubilization, nitrogen fixation, and phytohormones production, and 52 most-promising isolates were characterized and identified by 16S-rRNA sequencing. Their effects on pot-grown potatoes were evaluated against non-inoculated plants as controls. Klebsiella grimontii MPUS7, Serratia marcescens NGAS9, and Citrobacter freundii LUTT5 were selected based on their effects on pot-grown potatoes and formulated in different carrier materials as possible biofertilizers. Their shelf lives were determined as colony-forming units (CFU) under refrigerated (8 o C) and room conditions (25 ± 2 o C) for eight months and their effects on field-grown potatoes evaluated against recommended fertilizers and non-treated plants. Data analysis involved XLSTAT (2.3). Significant differences (p ≤ 0.05) were evident for the fertility-related properties of the sampled soils but not their nutrient contents. Though slightly acidic (pH < 6.5), non-saline (Electrical conductivity < 1 dS m -1 ), and greater proportions of silt + clay (average 46.85 ± 15.18%) than gravel and sand, the soils contained low nutrient levels which may not be ideal for potato production. The rhizobacteria significantly differed in their invitro PGP activities. Except for the number of days to emergence and flowering, rhizosphere soil pH, and salinity, potato growth-related parameters like tuber numbers, sizes, weights, and nutrient contents were enhanced by the rhizobacterial treatments in the screen house by up to 90%. The PM-1 formulation supported the maximum K. grimontii MPUS7 growth (5.3 log-CFU g -1 ) under refrigeration and C. freundii LUTT5 (4.1 log-CFU g -1 ) under refrigerated and room conditions but the highest K. grimontii MPUS7 population (3.7 log-CFU g -1 ) was realized in room-stored PM-4. The formulations also showed varied activities at enhancing potato growth and most had similar or greater effects on potato growth as the artificial fertilizers and can effectively be used to replace the use of synthetic fertilizers in potato production systems in Tanzania.