Progress 08/01/02 to 07/31/05
Outputs
In the past year, research was aimed to characterize nitrate and nitrate reduction using naturally occurring iron minerals and field soil. Thus far, two peer-review journal articles on this research have been published (see publications below), one accepted pending revision (Soil Science Society of America Journal), and another is in review (Soil Science Society of America Journal). Siderite, a major secondary product of microbial Fe(III) reduction, was able to reduce nitrite to nitrous oxide. The reaction rate increased with a decrease in pH and was dependent on initial nitrite and siderite concentrations. X-ray diffraction indicated lepidocrocite and goethite as major oxidation products of siderite. Nitrate reduction experiments were performed using a surface soil taken from a field in western Kentucky. Addition of nitrate to simulate fertilizer addition under Fe(III)-reducing conditions triggered the onset of autotrophic nitrate-dependent Fe(II) oxidation. Another
important process that occurred was concomitant Fe(III) and nitrate reduction linked to Fe(II) oxidation by nitrite.
Impacts
Our results indicate that nitrous oxide, an important green house gas, can be generated abiotically in environments where siderite precipitation is favored and nitrite accumulates. Transformation of nitrogen by alternate pathways such as the nitrogen-iron linkages discovered in this project may impact fertilizer use efficiency and water quality.
Publications
- Matocha, C.J., Karathanasis, A.D., Rakshit, S. and Wagner, K. M. 2005. Reduction of dissolved Cu(II) by Fe(II). J. Environ. Qual.34:1539-1546.
- Rakshit, S, Matocha, C.J. and Haszler, G.R. 2005. Nitrate reduction in the presence of wustite. J. Environ. Qual. 34:1286-1292.
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Progress 01/01/04 to 12/31/04
Outputs
Field and laboratory studies have shown that the iron and nitrogen cycles are closely coupled. Research was aimed to (1) characterize nitrate reduction using naturally occurring iron minerals, metallic iron, and field soil, and (2) investigate the reduction of copper by iron. One peer-review journal publication on this research has been accepted (J. Environ. Qual.), another is in review (J. Environ. Qual.), and a third is in preparation for submission (Soil Sci. Soc. Amer. J.). Wustite, a secondary product of metallic iron corrosion, was able to reduce nitrate to ammonium with nitrite as a transient intermediate. X-ray diffraction and scanning electron microscopy indicated magnetite as the reaction product. Kinetic data revealed a complex rate equation with fractional order dependence on proton and wustite concentration and nearly first order dependence on nitrate concentration. The reaction was strongly dependent on temperature and calculated activation energies
indicate a surface chemical controlled reaction. Metallic iron rapidly reduced nitrate to ammonium and the activation energy value indicated a diffusion controlled reaction. Nitrate reduction experiments were performed using a surface soil taken from a field in western Kentucky that contained extractable ferrous iron. When adding nitrate to simulate fertilizer addition, a linear decrease in nitrate concentration was observed with concomitant changes in iron speciation under anoxic conditions. Sterilized runs showed a lag in nitrate reduction, suggesting a possible abiotic pathway of nitrate reduction. The chemical nature of the reactive ferrous iron species is currently being investigated. We previously discovered that addition of dissolved copper to simulate poultry litter addition increased the oxidation rate of iron in the absence of oxygen. It was found that cupric copper was reduced by ferrous iron rapidly and the dissolved cuprous form was stabilized in the presence of chloride.
Structural characterization indicated cuprite and ferric hydroxide as the solid reaction products.
Impacts
It is known that field conditions at the surface and subsoil conducive for biological denitrification of nitrate are also environments where ferrous iron can accumulate and potentially react. This research shows that nitrate reduction is accompanied by changes in naturally occurring iron in soil and corrosion products of metallic iron. Furthermore, cupric copper can function as an oxidant of ferrous iron and may help explain the often cited interaction of soil iron and copper in plants.
Publications
- Matocha, C.J., Scheckel, K.G., and Sparks, D.L. 2004. Kinetics and Mechanism of Soil Biogeochemical Processes. Chapter 6. In Chemical Processes in Soils. SSSA Special Publication, Madison, WI. (In press).
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Progress 01/01/03 to 12/31/03
Outputs
Nitrate is weakly held in soil and is prone to leaching into water supplies. In addition, dissolved organic nitrogen, an unrecognized nutrient form of nitrogen until recently, may be formed during nitrate reduction. To better understand the role of iron in mediating the fate of nitrogen in soils, several naturally occurring iron minerals have been screened for their ability to reduce nitrate. One of the minerals, wustite, was able to reduce nitrate to ammonium with nitrite as an intermediate. This is significant for three reasons. The concentrations of nitrite produced exceeded the mean contaminant level for drinking water. The nitrite produced could react to form dissolved organic nitrogen. Thirdly, this could be a pathway of natural attenuation of nitrate. Rate coefficients describing nitrate reduction by wustite decreased with pH. Another system has involved using metallic iron as an amendment for poultry litter to minimize nitrate leaching into water supplies.
Metallic iron was more efficient than wustite at reducing nitrate to ammonium. It was also found that addition of dissolved copper to simulate poultry litter addition increased the oxidation rate of iron because of direct reduction of divalent copper to form solid iron copper precipitates. Nitrate reduction by reduced iron could represent a significant pathway of natural attenuation. Knowing potential sinks would make animal manure application and fertilization more efficient and might answer some long-term questions about loss of nitrogen from soil, which has always been attributed to biological denitrification or leaching as default pathways.
Impacts
This research will be significant to U.S. Agriculture because it will provide kinetic data for natural attenuation of nitrate. Better understanding of these processes will improve predictions of nitrogen fate that could impact crop production, global warming, and overall environmental quality.
Publications
- Matocha, C.J., Karathanasis, A.D., and Coyne, M.S. 2004. Reduction of dissolved Cu(II) by Fe(II): Characterization of Products. Environ. Sci. Technol. In review.
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Progress 01/01/02 to 12/31/02
Outputs
Nitrate (NO3-) loss from soil is typically attributed to leaching or biological denitrification as default pathways. Field conditions at the surface and subsoil conducive for biological denitrification are also environments where other potential reductants are known to accumulate such as ferrous (Fe(II)) iron. Oxidation of dissolved Fe(II) by NO3- in aqueous solution is thermodynamically favorable at all pH. Stirred-batch experiments conducted under deoxygenated conditions indicated that NO3- reduction by dissolved Fe(II) was rate-limited. Dissolved Cu(II) used to simulate animal manure addition catalyzed the reduction of NO3- to N2O and ammonium (NH4+)with Fe(II) serving as the reductant when in large excess. At lower levels of Fe(II), NO3- was not reduced significantly. Further experiments are being conducted with solid Fe(II) compounds and soils.
Impacts
This research will be significant to U.S. Agriculture when it documents kinetic data for nitrate reduction by Fe(II). This information can be used in environmental fate models.
Publications
- No publications reported this period
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