Bioremediation of agricultural nitrate pollution – challenges and opportunities

Agriculture is the major cause of nitrogen pollution worldwide, leading to eutrophication in the surrounding and downstream rivers, lakes, and oceans. Nitrogen runs out from the field mostly in the form of nitrate where subsurface drainage is installed, which is common in areas with poorly drained soils such as the U.S. Midwest and northern Europe. Nitrate contamination in groundwater wells can also cause human diseases, and therefore, is a serious public health concern. Agricultural drainage displays distinct characteristics from municipal wastewater and animal manure, which include high nitrate, low ammonium, and low organic carbon concentrations as well as low temperature. The remediation technologies also need to be deployable in rural settings, low cost, and have minimum impacts on agricultural production. In this review article, we first summarize the challenges associated with agricultural nitrate pollution. We also briefly summarize microbial nitrogen transforming reactions that are potentially useful for nitrate bioremediation. We then critically evaluate currently available nitrate remediation technologies. Because bioremediation is much less expensive than physical and chemical treatments, we mostly focus on bioremediation technologies, including wetlands, denitrification bioreactors, saturated riparian buffers, controlled drainage, and controlled drainage ditches. Current bioremediation technologies exhibit substantial variability in performance when implemented at field scale. This review discusses recent advances and emerging strategies to enhance nitrate removal under challenging field conditions, including bioaugmentation, biostimulation, and other novel technologies. Looking forward, the effective management of agricultural subsurface drainage will likely depend on the integration of multiple conservation practices to achieve targeted nitrate reduction goals.