Native phosphate-solubilizing bacteria from Al-Qadisiyah soils: screening, identification and effect on sunflower phosphorus nutrition in a calcareous soilq

Ali Mohsen Essa

Ali Mohsen Essa Department of Chemistry, College of Education, University of Al-Qadisiyah, Al-Diwaniyah, Iraq.

Abstract

Phosphorus (P) applied to the calcareous soils of central Iraq is quickly bound as calcium phosphates, so crops recover little of it. We screened phosphate-solubilizing bacteria (PSB) native to six calcareous soils of Al-Qadisiyah and tested the most active ones on sunflower (Helianthus annuus L.). Twelve isolates were compared on National Botanical Research Institute’s phosphate (NBRIP) agar and in NBRIP broth with tricalcium phosphate (TCP), and grouped by phenotype. Three isolates were then inoculated into an unsterilized calcareous soil (Olsen P 5.6 mg kg⁻¹) given 0, 15 or 30 mg P kg⁻¹, in a factorial pot trial with four blocks harvested 60 days after emergence. Net soluble P ranged from 59.7 to 332.0 mg L⁻¹ (6.0–33.2% of TCP-P) and rose as broth pH fell (r = −0.98). The plate solubilization index explained only 56% of the variation in broth P and ranked the isolates differently. Six isolates were Bacillus-like, four Pseudomonas-like and two unresolved Gram-negative rods. Inoculation increased total P uptake by 23.0% across P rates (p< 0.001), with no significant inoculant × P interaction. Relative gains were largest without added P (21–43%) and smallest at 30 mg P kg⁻¹ (12–20%). The Pseudomonas-like isolate QPSB07 also raised shoot P concentration and residual Olsen P, and its uptake at 15 mg P kg⁻¹ matched about 24.7 mg P kg⁻¹ of fertilizer alone. Broth-based selection of native PSB is worth pursuing for sunflower on calcareous soils, but molecular identification and field trials are still needed.

Keywords: Phosphate-solubilizing bacteria, Calcareous soil, Sunflower, Tricalcium phosphate, Phosphorus uptake, Olsen P, Biofertilizer, Iraq

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