Transformations of Nitrate and Nitrite by Iron(II)-Bearing Minerals.

The biogeochemistry of iron is complex and is coupled with the cycling of nutrients and contaminants.  Microbial iron(III) reduction produces iron(II), which can either reprecipitate or resorb to mineral surfaces.  Iron(II)-bearing minerals and surface iron(II) species can serve as  reductants of nitrate and nitrite under specified conditions.  We have demonstrated nitrate-dependent iron(II) oxidation in an agricultural soil (Sadler silt loam, Glossic Fragiudalf) under anoxic conditions.  Nitrite and nitrous oxide were produced as reaction intermediates during the first six hours of reduction and disappeared at longer times.  One unanswered question is the nature of the iron(II)-bearing minerals involved.  In this study, we have characterized the clay fraction from the Sadler soil using x-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy.  X-ray diffraction analysis of the unreacted clay fraction indicated the predominant mineralogy to be hydroxy-interlayered vermiculite, mica, and kaolinite.  The OH-bending region revealed the presence of phyllosilicate Fe(III) at 882 cm^-1 based on spectral fitting.  This feature is assigned as an AlFe(III)-OH band.  Additional experiments are investigating changes in this band with progressive anoxia and following addition of nitrate and nitrite.  These results will help better explain the role of soil iron in nitrate and nitrite transformations.  The coupled iron-nitrogen cycle could represent a significant path of natural N fertilizer loss through the production of nitrous oxide.