Short-term Response of Soil Iron to Nitrate Addition
- C. J. Matocha * and
- M. S. Coyne
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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