Plant growth-promoting rhizobacteria able to improve NPK availability: selection, identification and effects on tomato growth

Altagracia Reyes-Castillo1, 3, Macarena Gerding2*, Pía Oyarzúa2, Erick Zagal2, Javiera Gerding4, and Susana Fischer2
Nitrogen, P and K are essential macronutrients that are not readily available to plants. Rhizobacteria are able to convert these unavailable forms for subsequent uptake by the plant, diverse species have been characterized as N2 fixers, P solubilizers and capable to solubilize mineral K from unavailable forms. The objective of this study was to select rhizobacteria capable of improving NPK availability and promoting tomato (Solanum lycopersicum L.) growth. Fifteen strains were studied. Four strains were selected for their capacity to fix N2, three for their ability to solubilize P, and six for their capacity to solubilize biotite and K-feldspar, isolated from tomato, lentil, chili pepper, faba bean and lettuce crops in Andisol and Alfisol soils. Through 16SrRNA sequencing, selected strains were identified as Pseudomonas gessardi, P. koreensis, P. brassicacearum, P. marginalis, Acinetobacter calcoaceticus and Rahnella aquatica. Phosphorus solubilizing strains did not show a positive effect on plant growth or an increase in available soil P. The N2 fixing bacteria Tmt-16 strain increased root growth in 23.57%; maintained the highest N content in plant tissue, 2.60%, higher amount of N available in the soil, 2.95 mg kg-1, and a higher content of N-NH4+ 1.95 mg kg-1. The K solubilizing strains Ls-C21, Ltj-62 and LsC-58 reached 17.0 to 19.0 mg kg-1 available K and 0.04 to 0.05 mg kg-1 exchangeable K (p ≤ 0.05.). These four endemic rhizobacteria can be potentially used as biofertilizers, allowing a reduction in the use of chemical fertilizers and a more sustainable production of tomatoes.
Keywords: Acinetobacter, N2 fixation, phosphorus and potassium solubilization, Pseudomonas, Rahnella, Solanum lycopersicum.
1Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Campo Experimental Tecomán, Colima, CP. 28100 México.
2Universidad de Concepción, Facultad de Agronomía, Av. Vicente Méndez 595, Chillán, Chile.
*Corresponding author (mgerding@udec.cl).
3Universidad de Concepción, Facultad de Agronomía, Programa de Doctorado en Ciencias de la Agronomía, Av. Vicente Méndez 595, Chillán, Chile.
4Centro de Estudios Mineralógicos, Geoatacama, Río Lauca 3194, Coquimbo, Chile.