Investigating the combined application of different levels of triple superphosphate and native phosphate-solubilizing Pseudomonas bacteria on growth indices of maize

 Phosphorus (P) is one of the most necessary plant nutrients and the second key plant nutrient after nitrogen, which deeply affects the overall growth of plants. Most of the present P in calcareous soils is biologically unavailable, therefore mobile P is rare despite the abundance of both organic and inorganic forms of P in soils. It has been well-known that P use efficiency of triple superphosphate (TSP) fertilizer in alkaline soils is low. Due to the increase in the price of phosphate fertilizers and their low plant absorption (10–30%), the use of indigenous phosphate solubilizing bacteria (PSB) to increase P-fertilizer-use efficiency and reduce its application amount can be a perfect way. The combined use of bio-fertilizers and superphosphate is an important component of integrated nutrient management, which leads to sustainable agriculture. This research was carried out to investigate the contribution of native phosphate solubilizing Pseudomonas sp. along with superphosphate application to increase soil P solubility, yield and nutrient concentration of maize and improve some soil properties in a calcareous soil with low organic matter and phosphorus.

Eleven strains of Pseudomonas sp (Pseudomonas sp. Pseudomonas koreensis) bacteria were obtained from the Gene Bank of Iran Soil and Water Research Institute. The phosphate solubilizing activity of each bacterium was evaluated in Sperber medium quantitatively (liquid) and qualitatively (plates). Each bacterium was cultured in two plates with three replications and incubated at 28 °C. Then the colony and halo diameter were measured on the third, fifth, seventh, tenth, and fourteenth days after incubation. To evaluate phosphate solubility by bacteria in a liquid medium, 50 ml of Sperber medium was prepared and sterilized with three replications. Then 1 ml of fresh culture suspension of each bacterium with a population of 107 cells/ml was inoculated into each Erlenmeyer flask (treatments and controls) and incubated at 150 rpm and 150 ° C for 14 days. Soluble phosphorus (Vanadate-molybdate method) and pH on the first, third, tenth and fourteenth days of incubation were measured. To assay the interaction effects of bacteria and triple superphosphate fertilizer on quantitative and qualitative yield of maize plant and characteristics of soil cultivated in a soil with low level of organic matter and phosphorous, a factorial based on randomized completely design including 8 bacteria (B0, B1, B2, B3, B4, B5, B6, B7) and 4 triple superphosphate fertilizer levels (0, 40, 70 and 100% of the critical limit of soil phosphorus) with 3 replications was conducted in greenhouse conditions. Sterilized pots containing 7 kg of washed sands and soil (in a ratio of one to three) were prepared. A certain amount of triple superphosphate fertilizer was added to each pot based on the treatments. The K as potassium sulfate and N as urea were used based on soil test and fertilizer recommendations for maize. Five germinated seeds (704 single cross cultivar) were planted in each pot. Seeds sown were inoculated with 1 ml of the fresh bacterial suspension inoculum (1×108 CFU ml-1). Then after two weeks, three plants per pot were preserved and the rest were removed. Pots including different treatments were irrigated, up to 80% moisture content of field capacity (FC), by distilled water using a weight method. After 90 days, the dry weight of the plant was calculated (oven at 70 ° C for 48 hours). To analyze the nutrients in the plant, roots and shoots were powdered separately by mill and digested through dry burning method. The amount of phosphorus, potassium, nitrogen, iron, zinc, copper and manganese were measured in shoot and root of the plants. Besides, the soil of each pot was sampled and after air drying and passing through a 2 mm sieve, some characteristics of the cultivated soil such as pH, EC, P (ava), TNV, Fe, Mn, Cu, Zn were measured and compared with the initial soil properties (before planting). Data statistical analysis was performed using SAS software and the mean comparison of treatment was carried out using Duncan's method.

The results of phosphorus solubilizing bacteria effect on the dissolution of insoluble mineral phosphates in solid and liquid media showed the largest halo diameter (19.11 mm) and the highest dissolution of P (156.25% compared to the control) after the fourteenth day incubation was belong to strain P21-1 and strain P55-1, respectively and a decrease in pH (45.5- 47.3 %). The results of the mean comparison showed that P0B3 treatment could increase the shoot dry weight by 47.45 %. P40B1 treatment was able to increase 61.62% of root dry weight. P70B6 and B70B2 treatments were also able to increase P in shoots by 17.64%, P100B4 treatment could increase root phosphorus by 160%. Besides, the combined use of bio-fertilizer and superphosphate increased the elements of iron, zinc, manganese, P, and reduced the electrical conductivity of the soil.

 All strains used in this study were able to dissolve phosphorus in both liquid and solid media. The results showed that the co-application of biofertilizer and phosphorus fertilizer increased the growth of maize plant (shoot and root dry weight) and shoot and root phosphorus concentration compared to the control. In addition, the use of these treatments increased the available phosphorus, iron and zinc in soil and decreased the electrical conductivity of the soil. Therefore, the findings of this study indicate that the use of inoculants containing Pseudomonas sp. and Pseudomonas koreensis can be considered as a suitable supplement to triple superphosphate fertilizer in crop cultivation strategies.