Improving Cotton Productivity Through Combined Application of Zinc and Phosphorus Solubilizing Rhizobacteria

Abstract

Mineral nutrition of crop plants is a major issue rising in modern agriculture. In alkaline calcareous soils, the availability of phosphorus and zinc is critically less due to their fixation and precipitation as complexes. Chemical fertilizers are indispensable for modern agriculture however these are costly, and their efficiency is an issue under different climatic conditions. Plant growth-promoting rhizobacteria (PGPR) can improve the availability of crop nutrients through solubilization of insoluble compounds in soil. In the present study, a total of 40 rhizobacterial isolates were isolated from cotton rhizosphere and screened for solubilization of phosphorus and zinc. Fourteen isolates with ability to efficiently solubilize insoluble rock phosphate (7 isolates) and zinc oxide (7 isolates) under qualitative and quantitative assays were selected. These isolates were tested for their compatibility followed by evaluation in jar trial to improve the growth of cotton seedlings under axenic conditions. Results were quite promising where all the inoculated treatments showed improvement in growth parameters in comparison with control. Best results were shown by the combined application of IA6 and IA16, followed by the combination of strains IA7 and IA20. Based on the jar trial, the selected isolates were further tested for biochemical, morphological and plant growth-promoting characters such as catalase, urease, ammonia production, chitinase, siderophores and exopolysaccharides production, oxidase, and protease activity, and HCN and IAA production. These strains were identified as Bacillus subtilis IA6, Paenibacillus polymyxa IA7, Bacillus sp. IA16, and Bacillus aryabhattai IA20 through 16S rRNA sequencing. These identified strains were evaluated separately and in possible combinations along with two controls (with and without recommended fertilizers) under pot and field trials. The application of these strains alone and in combination enhanced cotton growth and yield under pot and field trials as compared to control. Maximum improvement in cotton growth was provided by co-inoculation treatment IA6+IA16 followed by IA6+IA20. The inoculation and co-inoculation also enhanced macro and micronutrient (N, P, K, Zn, and Fe) status of soil and plant chlorophyll contents (SPAD value) as compared to control. It is, hence, concluded that the integrated use of Bacillus subtilis IA6 and Bacillus sp. IA16 can be recommended as potential inoculant and an effective technique for cotton growth enhancement in nutrient deficient semi-arid soil conditions.