Short-term response of soil iron to nitrate addition

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 NO ₃^- 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 NO^3-, suggesting preferential use of NO ₃^- 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)/NO ₃^- 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.