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Inhibition Of Fe(III) Reduction By Nitrate: Impact Of Anoxic Chemical and Biological Fe(II) Oxidation
C. J. Matocha, M.S. Coyne, A.F. Miller
Department of Plant and Soil Sciences
Elevated levels of nitrate in water supplies are undesirable because of the potential impacts on aquatic life and human health. Nitrate-dependent, iron(II) oxidation is an important process in the removal of elevated nitrate from water and the inhibition of soil iron(III) reduction, yet, the mechanisms are poorly understood. The purpose of this study is to investigate the contribution of chemical and biological Fe(II) oxidation by nitrite and nitrate.
2011 Project Description
We have further explored the strong coupling of the nitrogen cycle with soil iron and carbon. Addition of nitrate to soil under iron(III)-reducing conditions led to a reoxidation of iron(II) coupled to nitrate reduction.
This past year, activities have included conducting laboratory and field experiments to investigate the contribution of chemical and biological iron(II) oxidation by nitrite and nitrate to the overall process of nitrate-dependent, iron(II) oxidation.
The results derived from these activities have been disseminated via three presentations at the annual American Society of Agronomy Meetings in San Antonio, TX. These results have also been incorporated into an undergraduate soil science class in the section dealing with the nitrogen and iron cycles. One Ph.D. graduate student, one M.S. graduate student, and one undergraduate research assistant have contributed to the project.
Slurries from a moderately well-drained soil (Sadler silt loam) were incubated anaerobically in the lab to reduce all indigenous soil Fe(III) to Fe(II) prior to nitrate addition which required 28 d. Acetate was the predominant water-soluble organic carbon species which emerged during the anoxic incubation, ranging from 190 to 350 micromolar. This represented only a small fraction of the total dissolved organic carbon in the anoxic soil slurries. Nitrate was added under anoxic conditions and relevant redox-active species were followed.
At early times (5 min- 6 h), water and acid-extractable fractions of iron(II) decreased with concomitant nitrate reduction and there was little change in acetate concentrations. The rate of dissolved Fe(II) oxidation was 7.26 micromolar/hr and the rate of nitrate reduction was 3.5 micromolar/hr during the first 6 h of reaction. Nitrite was not detected in the soil slurries as an intermediate product of nitrate reduction.
Nitrous oxide accumulated during the first 6 h of the reaction and decreased to negligible levels at longer time periods (>24 h). Dinitrogen was measured as the predominant product of nitrate reduction. At longer reaction times (24 h-72 h), acetate concentrations decreased drastically with a steady oxidation of iron(II) in the acid-extractable fraction as nitrate was reduced.
Fourier transform infrared analysis of the clay fraction seems to indicate that phyllosilicate iron(III) might be the source of the iron utilized during nitrate reduction.
Another interesting discovery is that reduced manganese(II) is also oxidized during nitrate reduction. We are currently utilizing 15N stable isotope analysis to tease out chemical from biological iron(II) oxidation. It is plausible that chemical and biological iron(II) oxidation coupled to nitrate reduction operate simultaneously at early time periods, whereas biological pathways may predominate at longer times.
Another interesting finding is that nitrate-dependent iron(II) oxidation also occurred in a well-drained soil, indicating that this process could be occurring on a broader scale. These results will help predict the fate of fertilizer nitrate in soils under iron(III)-reducing conditions.
Dhakal, P., and C.J. Matocha. 2011. Reactions of nitrite with goethite and surface iron(II)-goethite complexes. 103rd Annual Meetings of SSSA, San Antonio, TX. (Abstract No. 347-2).
Pyzola, S.M., and C.J. Matocha. 2011. Nitrate reduction coupled to iron(II) oxidation in an agricultural soil. 103rd Annual Meetings of SSSA, San Antonio, TX. (Abstract No. 351-4).
Matocha, C.J., P. Dhakal, and S.M. Pyzola. 2011. Transformations of nitrate and nitrite by Fe(II)-bearing minerals. 103rd Annual Meetings of SSSA, San Antonio, TX. (Abstract No. 281-13).