Screening of acid-tolerant consortia of azospirillum and phosphate solubilizing bacteria from lateritic soils

Abstract

A study was undertaken on “Screening of acid-tolerant consortia of Azospirillum and phosphate solubilizing bacteria from lateritic soils”. The main objective was to develop an acid-tolerant and efficient consortium of Azospirillum and phosphate solubilizing bacteria (PSB) for acidic soils of Kerala. Lateritic soils with acidic pH were collected from ten different locations of Thrissur district for the isolation of Azospirillum and phosphate solubilizing bacteria. The pH of the soil ranged from 5.22 (Vellanikkara) to 6.51 (Madakkathara). The highest population of Azospirillum was recorded in soils of Madakkathara (MD) and Mulayam (MU) (1.1x105 MPN g-1). However, Chelakkara (CH), Elanad (EL) and Perumpilavu (PV) recorded least population (0.9x104 MPN g-1). Highest population of PSB was obtained from Mulayam (12.7x106 cfu g-1) and least PSB population was obtained from Perumpilavu (PV) (1.0x105 cfu g-1). The population of both Azospirillum and phosphate solubilizing bacteria were higher in rhizosphere soil than non-rhizosphere soil. A total of six isolates of Azospirillum and 35 isolates of PSB were obtained. The isolates AND-4 (Azospirillum from Nadavarambu) and AMU-2 (Azospirillum from Mulayam) were acid-tolerant upto pH 4.5. Similarly, PMD-7 (PSB from Madakkathara) and POL-1 (PSB from Ollur) were also tolerant upto pH 4.5. The AND-4 isolate of Azospirillum sp. fixed highest amount of nitrogen (17.94 mg of N g-1 of malate) followed by AMU-2 isolate (14.58 mg of N g-1 of malate). The isolate AMU-2 showed maximum IAA production (51.95 µg ml-1) followed by AND-4 (30.53 µg ml-1). The PMD-7 was the most efficient P-solubilizer (207.22 µg ml-1) followed by POL-1(187.78 µg ml-1). The PMD-7 isolate produced maximum IAA (33.07 µg ml-1). Sixteen combinations of Azospirillum and PSB were compatible. The three consortia selected based on acid tolerance, efficiency and compatibility were AND-4 (Azospirillum sp.) + PMD-7 (Acinetobacter sp.), AMU-2 (Azospirillum sp.) + PMD-7 (Acinetobacter sp.) and AND-4 (Azospirillum sp.) + POL-1 (Bacillus megaterium). Among the consortia, Azospirillum sp. + Bacillus megaterium (T4) was found to be the most efficient in enhancing the growth of okra under acidic pH based on biometric and yield parameters. However, the POP recommendation (T1) recorded maximum plant height, maximum number of fruits, fruit yield and minimum days for flowering. The population of Azospirillum and PSB increased at the time of harvest in all the treatments except in T7 (Absolute control). The population of Azospirillum was highest in T3 (Azospirillum sp. + Acinetobacter sp.) (23.38x103 MPN g-1) and minimum in T7 (Absolute control) (7.73x103 MPN g-1). The population of PSB was highest in T3 (Azospirillum sp. + Acinetobacter sp.) (6.43x105 cfu g-1) and minimum in T7 (Absolute control) (2.08x105 cfu g-1). The maximum nitrogen uptake was recorded in T1 (KAU POP recommendation) (0.99 g plant-1). Among the consortia, T4 (Azospirillum sp. + Bacillus megaterium) (0.81 g plant-1) and T3 (Azospirillum sp. + Acinetobacter sp.) (0.80 g plant-1) recorded highest nitrogen uptake. The maximum uptake of phosphorus was noticed in T1 (KAU POP recommendation) (0.41 g plant-1) followed by T4 (Azospirillum sp. + Bacillus megaterium) (0.36 g plant-1). The most efficient isolates AND-4 and AMU-2 were identified as Azospirillum sp. The most efficient PSB isolates POL-1 and PMD-7 were identified as Bacillus megaterium and Acinetobacter sp. respectively. The present studies clearly showed that the acid-tolerant and efficient consortium of Azospirillum + Bacillus megaterium (T4 treatment) performed better under pot culture studies, which was on par with chemical fertilizer. However, further studies are needed to evaluate its efficiency under field conditions.