Effect of NaCL and CaSO4 on Activty of Rhizobium Bacteria and Testing its Efficincy of Produceing IAA, Enzymes in Gypsiferous Soil

Rhizobium bacteria are of significant environmental and agricultural importance. However, the impact of salts on the growth of these bacteria is a crucial topic that warrants research, particularly in areas with high soil salinity levels. Therefore, this study was conducted to investigate the effects of sodium chloride (NaCl) and calcium sulfate (CaSO4) on the activity of Rhizobium bacteria and to evaluate their efficiency in producing amino acids and enzymes in gypsum soils. A laboratory experiment was conducted at the Soil Science and Water Resources Department, College of Agriculture, Tikrit University, during the academic year 2021-2022. Rhizobium bacteria were isolated from gypsum soil, and the best isolates were selected based on their activity and spread. These isolates were then morphologically characterized. A total of 10 isolates were obtained and identified according to their agricultural and microscopic characteristics. The growth rates of the isolated bacteria on YEMA medium varied from one isolate to another. Six isolates (BR1, BR3, BR5, BR6, BR9, BR10) grew after being incubated for 7 days at 28°C, and were classified as slow-growing Bradyrhizobium bacteria. Meanwhile, four isolates (BR2, BR4, BR7, BR8) grew on the same medium and were classified as fast-growing Rhizobium bacteria. All isolates exhibited the ability to produce indole-3-acetic acid (IAA) at varying levels, with the highest production recorded in isolate BR5 (13.9 µg·mL-1), followed by isolate BR4 (12.6 µg·mL-1). The lowest IAA production was observed in isolate BR2 (6.5 µg·mL-1). All isolates were able to produce catalase, except for isolate BR10. Six isolates successfully produced the oxidase enzyme, while four isolates did not. Seven isolates produced urease, while three isolates failed to do so. Additionally, five isolates produced nitrate reductase enzyme. The best isolate of Rhizobium was selected for further testing using four levels of calcium sulfate (0, 0.5, 1.0, 2.0 g·L-1) and four levels of sodium chloride (0, 0.2, 0.4, 0.6 g·L-1) to determine the optimal level of each salt that the bacteria could tolerate and grow in. The results showed that the bacterial colony growth exceeded 60% at all levels above 0.5 g·L-1 of CaSO4, with continued bacterial growth at higher levels of CaSO4. The control treatment (0 g·L-1) exhibited only 10% growth, as the bacteria require gypsum for growth. As for the sodium chloride levels, the 0.2 g·L-1 level showed the best bacterial growth (50%), while growth decreased to 10% at the 0.6 g·L-1 level due to the bacteria's inability to tolerate high levels of sodium chloride. The 0 g·L-1 level exhibited the lowest growth, indicating the bacteria's requirement for elemental sodium and chloride during growth, but only at a specific threshold.