The Influence of Azotobacter on the Growth and Mercury Content of Water Spinach Grown in Mine Tailings

Abstract

Growing food crops in gold mine tailings is limited by low nitrogen and mercury contamination. Little is known about the responses of water spinach (Ipomoea aquatica L.) to nitrogen-fixing bacteria biofertilizer. This study aimed to analyze changes in growth media properties, growth, biomass of water spinach, and mercury in both tailings-based growth media and intact plants following the application of the nitrogen-fixing Azotobacter. A liquid inoculum of Azotobacter was analyzed before the experiment. A greenhouse experiment was arranged in a randomized block design to evaluate three inoculant concentrations. Acidity and electrical conductivity of the inoculant were 7.95 and 1.74 mS/cm, respectively, while the Azotobacter count was 9.18 on a log scale. Introducing 5% and 10% inoculants increased microbial counts, total nitrogen, and acidity of the growth media, as well as shoot growth and biomass, but did not affect root length. Azotobacter did not affect mercury levels in the soil but increased mercury accumulation in intact plants. Mercury levels in soil and plants remained higher than the maximum threshold value. While soil pH and nitrogen levels showed a positive correlation with plant growth, mercury concentration in the soil exhibited a significant negative correlation. Because of high mercury accumulation, the water spinach was not safe for cultivation.