Abstract
A number of laboratory, pot and field experiments were conducted to select the efficient phosphate solubilizing bacterial inoculants for wheat and rice cultivation in Bangladesh. Forty phosphate solubilizing bacteria (PSB) were isolated on tricalcium phosphate (TCP) based solid medium from rhizosphere of rice, wheat, lentil and chickpea from four agroecological zones (AEZS) of Bangladesh and eight PSB were selected on the basis of P solubilization in liquid culture. The selected isolates widely varied in terms of decreasing pH, increasing P solubilization and acid phosphatase activity in liquid media with five insoluble P substrates viz. tricalcium phosphate (TCP), ferric phosphate (Fe-P), aluminium phosphate (Al-P), rock phosphate (RP) and calcium phytate (Ca-phytate). The PSB isolate MRI showed the highest P solubilization (45.1%) and acid phosphatase activity (136.3 μg pNP mL' h1) from the Al-P and Ca-phytate substrates, respectively followed by the isolate MW1. Morphological, biochemical and molecular characterization indicated that all the eight PSB isolates belong to the genus Pseudomonas and were identified as P. aeruginosa (IL1, MR1, RC1, IC2 and MdR3), P. fluorescens (IW1 and MyR2) and P. putida (MW1). Combined restriction fragment length polymorphism (RFLP) analyses revealed that all the eight PSB isolates depicted in four distinct groups which varied from phenotypic and biochemical point of view but indicated that they were genetically related. Six PSB inoculants with an uninoculant (Io, MRI, IL1, IW1, IC2, RC1 and MW1) were tested with four levels of P fertilizer to investigate their performance on wheat and T. aman rice in pot culture using Mymensingh, Madhupur and Ishurdi soils. The PSB inoculant MR1 and MW1 showed better results in terms of available P, phosphatase activity and PSB population in soils resulting increased grain and straw yields of wheat (Triticum aestivum) and T. aman rice (Oryza sativa) in pot culture with all three soils. The field experiments were conducted to investigate the effects of two PSB inoculants (MR1 and MW1) and their mixture with inorganic or organic sources of P on T. aman rice and wheat at Mymensingh, Madhupur and Ishurdi. Fourteen treatments were comprised with all possible combinations with two PSB inoculants, their mixture and inorganic (50 or 100% P from triple super phosphate) or organic sources of P (50% P from cowdung) for both the crops and the experiments were conducted following RCB design with four replications for each treatment. Application of single or mixed PSB inoculants with 50% P from triple super phosphate (TSP) or cowdung (CD) increased the PSB population and phosphatases activity in rhizosphere of T. aman rice and wheat. Strong significant correlation (Po.01) was observed between these two variables resulted in higher availabile soil P due to inoculation of PSB with T. aman rice and wheat which were also comparable to the 100% P from TSP. Consequences of higher availability of soil P due to PSB inoculation attributed to increase the P uptakes and grain yields of T. aman rice (5.7 to 34.1, 2.6 to 21.4 and 0.9 to 15.2% yield increase) and wheat (35.1 to 92.4, 21.3 to 61.9 and 4.6 to 26.1% yield increase) over Po control treatment which were also comparable to 100% P from TSP (28.1, 14.5, and 12.3% yield increase for T. aman rice and 77.4, 51.4 and 18.0% yield increase for wheat) at Mymensingh, Madhupur and Ishurdi, respectively. The PSB inoculant MR1 showed better performance at Mymensingh and Ishurdi while mixed inoculants (MR1+ MW1) performed better at Madhupur in both the crops. Thus, 50% P can be saved in the cultivation of T. aman rice and wheat by the integrated use of PSB inoculants with inorganic or organic sources of P. Survival of two PSB cultures (MR1 and MW1) and its mixtures were tested for long duration in six carrier materials against two temperature viz. air condition and room temperature. Two PSB inoculants (MR1 and MW1) and their mixtures survived with desired population (≥10 cfu g1 carrier) for longer time in air condition (360 days) than that at room temperature (270 days) with all carrier materials. Peat soil was found the best carrier for survival of PSB inoculants followed by either its mixture with pond slurry or pressmud at the ratio of 1:1. The results revealed that mixture of peat soil either with pond slurry or pressmud can also be used as an alternate carrier for production of phosphatic biofertilizer than peat soil alone.