Short‐term Response of Soil Iron to Nitrate Addition

The inhibition of soil Fe(III) reduction by fertilizer NO 3 − 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 NO 3 − to a moderately well‐drained agricultural soil (Sadler silt loam) under Fe(III)‐reducing (anoxic) conditions. Stirred‐batch experiments were conducted where NO 3 − 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 NO 3 − were monitored as a function of time. Addition of 1 mM NO 3 − 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 NO 3 − , suggesting preferential use of NO 3 − by nitrate reductase enzyme. Active nitrate reductase was confirmed by the fivefold decline in NO 3 − reduction rates in the presence of tungstate (WO 4 2− ), a well‐known inhibitor of nitrate reductase. In addition, NO 3 − –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 NO 2 − – and NO 3 − –dependent Fe(II) oxidation by autotrophic bacteria. These two processes became more important at a greater initial oxalate‐Fe(II)/NO 3 − concentration ratio. The inhibitory effects in Fe(II) production were short‐term in the sense that once NO 3 − was depleted, Fe(II) production resumed. These results underscore the complexity of the coupled N–Fe redox system in soils.