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SOIL CHEMISTRY

Short-term Response of Soil Iron to Nitrate Addition

Department of Plant and Soil Science, Univ. of Kentucky, N-122 Agricultural Science Center-North, Lexington, KY 40546-0091

M. S. Coyne

Department of Plant and Soil Science, Univ. of Kentucky, N-122 Agricultural Science Center-North, Lexington, KY 40546-009

^* Corresponding author (cjmato{at}uky.edu).

The inhibition of soil Fe(III) reduction by fertilizer NO₃^– applications is complex and not completely understood. This inhibition is important to study because of the potential impact on soil physicochemical properties. We investigated the effect of adding NO₃^– to a moderately well-drained agricultural soil (Sadler silt loam) under Fe(III)-reducing (anoxic) conditions. Stirred-batch experiments were conducted where NO₃^– was added (0.05 and 1 mM) to anoxic slurries and changes in dissolved Fe(II) and Fe(III), oxalate-extractable Fe(II), and dissolved NO₃^– were monitored as a function of time. Addition of 1 mM NO₃^– inhibited Fe(II) production sharply with reaction time, from 10% after 1 h to 85% after 24 h. The duration of inhibition in Fe(II) production was closely related to the presence of available NO₃^–, suggesting preferential use of NO₃^– by nitrate reductase enzyme. Active nitrate reductase was confirmed by the fivefold decline in NO₃^– reduction rates in the presence of tungstate (WO₄^2–), a well-known inhibitor of nitrate reductase. In addition, NO₃^––dependent Fe(II) oxidation was observed to contribute to the inhibition in Fe(II) production. This finding was attributed to a combination of chemical reoxidation of Fe(II) by NO₂^–– and NO₃^––dependent Fe(II) oxidation by autotrophic bacteria. These two processes became more important at a greater initial oxalate-Fe(II)/NO₃^– concentration ratio. The inhibitory effects in Fe(II) production were short-term in the sense that once NO₃^– was depleted, Fe(II) production resumed. These results underscore the complexity of the coupled N–Fe redox system in soils.

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				S. Rakshit, C. J. Matocha, and M. S. Coyne Nitrite Reduction by Siderite Soil Sci. Soc. Am. J., June 18, 2008; 72(4): 1070 - 1077. [Abstract] [Full Text] [PDF]