Inhibition of Fe(III) reduction by nitrate: Impact of anoxic chemical and biological Fe(II) oxidation.

Nitrate-dependent, iron(H) oxidation is an important process in the inhibition of soil iron(lII) reduction, yet, the mechanisms are poorly understood. Two proposed pathways include chemical reoxidation of Fe(I!) by nitrite and biological Fe(II) oxidation coupled to nitrate reduction by lithotrophic microorganisms. Accordingly, the overall goal of this proposed project is to investigate the contribution of chemical and lithotrophic Fe(H) oxidation by nitrite and nitrate. This goal will be achieved through two specific objectives: I) Investigate adsorbed Fe(H) surface complexes on reference minerals (gibbsite, kaolinite, and synthetic goethite) and natural materials (clay fraction from the Sadler silt loam soil) and their reactivity towards nitrite using cutting-edge spectroscopic techniques; and 2) Assess the extent to which lithotrophic activity competes with abiotic processes of nitrate and nitrite reduction and characterize these lithotrophic populations by molecular biology methods. This proposed work is both creative and original because it combines spectroscopic tools with wet chemical experiments and microbiology experiments to unravel the complexities of nitrate-dependent, iron(II) oxidation. An expected outcome is fundamental rate data of adsorbed Fe(II)-nitrite interactions that can be incorporated in transport models and an assessment of chemical versus biological nitrate-dependent Fe(II) oxidation. This corresponds to USDA's research priority to study interrelationships between soil chemical, biological, and physical characteristics and processes controlling nutrient fate in soils. This research is a timely pursuit given the high costs of nitrate fertilizer and the desire to protect water resources from elevated nitrate levels and minimize nitrous oxide release to the atmosphere.