Biological treatment of nitrate-contaminated groundwater by <i>Bacillus subtilis</i>: The potential use of treated groundwater as agricultural irrigation water

Abstract

High levels of nitrate (NO₃^-) pollution in groundwater can cause potential health problems. In this study, a novel concept for groundwater treatment was designed and implemented for: i) the biological treatment of real nitrate-contaminated groundwater and ii) the use of treated groundwater to enhance plant growth. Plant growth-promoting rhizobacteria (PGPR; <i>Bacillus subtilis</i>) were used to investigate the effect of nitrate on bacterial growth in nitrate-contaminated groundwater. No significant effect of cations on bacterial growth was observed, whereas the presence of nitrate in the growth medium significantly increased the growth rate after 40 h-incubation. The OD₆₀₀ value of high concentrations of nitrate (100 mg/L) was almost 1.4-fold smaller than that of low concentrations of nitrate (i.e., 10 mg/L) owing to its toxic effects at high concentrations. Plant growth experiments showed that the dried biomass weight of kidney beans watered with <i>B. subtilis</i>-treated groundwater was almost 1.7- to 2.0-fold higher than that of other treated samples, whereas germination failure was observed when plants were directly watered with nitrate-contaminated groundwater. The novel findings of this study highlight the application of <i>B. subtilis</i> for the aerobic denitrification of nitrate-contaminated groundwater and its potential for use in treating nitrate-contaminated groundwater for enhanced plant growth.