Isolation and Characterization of Indigenous Phosphate-Solubilizing Bacteria from Paddy Soils in Eastern Guilan, Iran: A Sustainable Approach to Enhancing Phosphorus Bioavailability

Phosphorus (P) fixation in acidic paddy soils presents a significant agronomic and environmental challenge, particularly in Guilan Province, northern Iran. Intensive rice cultivation on low-pH soils in this region leads to excessive chemical fertilizer application and subsequent eutrophication of downstream wetlands. This study aimed to evaluate the population density and phosphate-solubilizing potential of indigenous bacteria inhabiting the rice rhizosphere in five representative paddy fields across eastern Guilan (Lahijan, Siahkal, and Langarud districts). Soil samples underwent physicochemical analyses, including pH, electrical conductivity, organic carbon, and available phosphorus. Bacterial isolates were recovered using Pikovskaya's (PVK) agar medium. The phosphate solubilization index (SI) was quantified, and a subset of high-performing isolates was further assessed qualitatively for organic acid production and phosphatase enzyme activity. Results revealed considerable variability in phosphate-solubilizing bacteria (PSB) populations, ranging from ۲.۸ × ۱۰⁶ to ۹.۴ × ۱۰⁶ CFU g⁻¹ dry soil, with the highest densities recorded in the Lahijan fields, which exhibited elevated organic carbon content. Notably, the solubilization index showed a strong negative correlation with soil pH (r = -۰.۸۲, p < ۰.۰۱), indicating that more acidic conditions promoted greater halo-zone formation. Seventeen isolates demonstrated strong SI values exceeding ۲.۵, accompanied by the production of gluconic acid and alkaline phosphatases. These findings establish, for the first time, a baseline inventory of phosphate-mobilizing microbiota in the paddy rhizospheres of eastern Guilan and highlight the potential for developing a customized, low-cost biofertilizer consortium tailored to the region's unique soil constraints. The study advocates for a paradigm shift from indiscriminate chemical fertilizer application toward ecologically intelligent, microbiome-driven strategies that enhance phosphorus use efficiency while protecting the fragile aquatic ecosystems of the Caspian lowlands.

کلیدواژه ها:

Phosphate-solubilizing bacteria ، Paddy soils ، Guilan ، Biofertilizer ، Phosphorus use efficiency ، Bacillus