Effect of long-term chemical fertilizer application on the rhizospheric microorganisms of the rice variety Bw 372 grown in the low country wet zone of Sri Lanka

Abstract

The use of chemical fertilizers in rice cultivation is prevalent in Sri Lanka, yet limited research has been conducted to assess their impact on soil microorganisms. Therefore, this study aimed to investigate the long-term effects of chemical fertilizer application on microorganisms in the rhizosphere of the rice variety Bw 372, grown in a paddy field located in the Low Country Wet Zone of Sri Lanka. During the 2017/2018 maha season, soil samples were collected from the rhizosphere of two-month old rice plants grown in a plot where chemical fertilizer was applied and a plot without chemical fertilizer for ten consecutive seasons. Eight samples were collected from each plot, with a plot size of 18 m2. The collected soil samples were analyzed for pH, available phosphorus content, exchangeable potassium content, and organic matter content. Isolation and enumeration of rhizospheric bacteria and fungi were carried out by serial dilution and spread plate techniques. Phosphate solubilization ability and nitrogen fixation ability of isolated bacteria and fungi were evaluated on Pikovskya’s agar medium and Burk’s nitrogen free medium, respectively. Data was analyzed by using SAS statistical software package. Soil pH of the chemical fertilizer applied plot and plot without chemical fertilizer were 4.98 and 5.01, respectively. The results showed that although soil pH was not affected by chemical fertilization, available P content, exchangeable K content and organic matter content of paddy soil were increased by long-term chemical fertilization. Organic matter content of the chemical fertilizer applied plot and plot without chemical fertilizer were 15.67% and 11.84%, respectively. According to the results, long-term chemical fertilization significantly increased (p≥0.05) the total culturable bacterial population, fungal population in chemical fertilizer applied plot compared to the plot without chemical fertilizer application. Further, significantly higher number of free-living nitrogen-fixing bacterial isolates were recorded in the plot without chemical fertilizer compared to the chemical fertilizer applied plot. However, number of phosphate-solubilizing bacterial isolates and phosphate-solubilizing fungal isolates in each plot were not affected by long-term chemical fertilization. A total of 10 phosphate solubilizing fungal isolates and 06 nitrogen-fixing bacterial isolates were recorded from rhizospheric soils in both plots.