Coupled iron reduction-ammonium oxidation (Feammox) in alkaline soils polluted with nitrogen

Abstract

Although nitrogen fertilizers help stimulate plant and microbial growth in N-limited soils, the use of excess N fertilizers to improve agricultural yield in Canada can cause adverse side effects of groundwater and surface water pollution, greenhouse gas production, soil acidification, and human health issues. Two N-removal pathways currently used to treat N-polluted wastewater and groundwater include denitrification and anaerobic ammonium oxidation (anammox). This study explored a novel anaerobic N-removal pathway that converts ammonium (NH4+) to inert nitrogen gas (N2) or nitrite (NO2-) while reducing Fe(III) to Fe(II), a.k.a. iron ammonium oxidation (Feammox) via a 118-day anaerobic incubation which included four sequential biostimulation experiments. The goal of the incubation was to identify Feammox in neutral-alkaline soil samples from a N-polluted site in Alberta by stimulating the bioremediation of NH4+. This was done amending the soils with vitamins and sources of NH4+ and Fe(III). The treatments for the anaerobic controls and soil slurries included one or more of the following: ammonium chloride (A), 2-line ferrihydrite (FH), and ferric citrate (FC). Amendments were added to these treatments in four sequential Feammox biostimulation experiments: 1) FC, FH, and A, 2) FC and FH, 3) FC and A, and 4) vitamin and molybdate solutions. The soil slurry with ferric citrate and NH4Cl amendments (S-FCA) had the most notable dissolved NH4+-N loss during the 118-day incubation, particularly when FC and A were added concurrently, i.e. a decrease of 14 ± 1.7 mg L-1 dissolved NH4+-N in the first experiment and a decrease of 13 ± 6.5 mg L-1 dissolved NH4+-N in the third experiment. S-FCA also exhibited signs of Fe(III) reduction throughout the incubation. In the incubation all samples generated minimal dissolved NO2- (0-2 mg L-1). Following the 118-day incubation the S-FCA treatment was subcultured to reproduce results; however, the subcultures did not show notable NH4+ loss, possibly due to dilution or N mineralization. Overall, this study showed a correlation between concurrent ferric citrate and NH4Cl amendments and dissolved NH4+-N loss in near-neutral anaerobic conditions; however, it did not provide clear evidence of Feammox. Additional experiments are necessary to isolate Feammox.