The Effect of Phosphate-Solubilizing Microorganisms on Wheat Yield, Reduction of Phosphate Fertilizer Consumption and Water Use Efficiency in Dry Region

Phosphorus (P) is the second key nutrient for plants after nitrogen and affects plant growth. Due to the complex behavior of P in acidic and alkaline soils, less than 20 % of the applied phosphorus fertilize can be utilized by plants and the remainder is fixed in the soil or changes to the forms that are not available for plant absorption (Vance et al., 2003). Phosphate-solubilizing bacteria (PSB) can convert insoluble phosphate into available forms for plants through the processes of acidification, exchange reactions and production of organic acids (Pereira & Castro, 2014). Many lands in Iran's Sistan Plain contain limited available phosphorous (an average of 6.7 mg/kg), leading to reduced crop growth and yield. On the other hand, pH levels are high in the region (an average of 8.2), which has a significant impact on the reduced solubility and availability of P in the soil. Also, the region is facing an acute drought and water shortage. Hence, the research was conducted to investigate the effects of different phosphate-solubilizing microorganisms and various phosphorus fertilizer levels on wheat (Triticum aestivum) yield and water use efficiency in the Sistan region.
A factorial experiment based on completely randomized block design was conducted with three replications at Agricultural Research Station of Zahak located in Zahak county in the Sistan region during the growing season of 2012- 2013. The experimental site lies at latitude 30º 54´ and longitude 61º 41´ and at 483 m above sea level and has a mean annual rainfall of 55 mm. Treatments consisted of three levels of phosphate fertilizer in the form of triple superphosphate (application at rate of 125 kg/ha based on the soil test result, application at 65% of the soil test recommendation (81 kg/ha) and control without fertilizer use) and four Pseudomonas fluorescens strains (PSM1, PSM2, PSM3 and PSM4 inoculants) along with control where no inoculation was done. The wheat cultivar used in the study was Hamoon. P fertilizer was applied at sowing time. For each treatment, 12 plant lines were established, measuring 4 m in length and a spacing of 20 cm. The distance between the treatments was 1.5 m and the replications were spaced 3.5 m apart. The effects of treatments were evaluated on wheat yield and yield components as well as water use efficiency.
The results showed that the effect of application of different phosphorus fertilizer rates and phosphate solubilizing microorganisms as well as their interactions on plant height, 1000-grain weight, yield and water use efficiency was significant (p≤ 0.01).The greatest wheat yield (4.71 ton/ha) was obtained by the application of 65% of phosphate fertilizer based on the soil test, which was, respectively, 10 and 17% higher than the yield produced under 100% phosphate fertilizer and control treatment. PSM3 inoculant exhibited the greatest impact on plant height and was the second most effective treatment in increasing 1000-grain weight of wheat after PSM1. All phosphate-solubilizing bacteria treatments increased the wheat grain yield. PSM3 inoculant gave the highest grain yield (4.72 ton/ha), which was 26.6% more than the control. This is in agreement with the results of Babana et al., 2016 who found that PSM increased grain yield in wheat. Phosphate-solubilizing microorganisms improved water use efficiency and the highest water use efficiency was associated with PSM3 inoculant that exceeded the control by 23%. An analysis of the interactive effects between phosphorus fertilization and phosphate-solubilizing bacteria application indicated that the highest grain yield of wheat (5.33 ton/ha) was related to the treatment T9 (65% phosphorus fertilizer × PSM3), which led to a 35% decrease in the use of triple superphosphate fertilizer and an increase of 28 % in the wheat grain yield as compared to the control.
The application of Pseudomonas fluorescens strain (PSM3 inoculant) together with 65% of phosphate fertilizer based on the soil test was considered to be the best treatment to obtain optimum wheat grain yield, and water use efficiency, which would result in a decrease of phosphorus fertilizer use and consequently the reduced environmental costs for wheat production in the Sistan region.