Iron (Fe) affects soil nitrogen (N) cycling processes both in anoxic and oxic environments. The role of Fe in soil N transformations including nitrification, mineralization, and immobilization, is influenced by redox activity, which is regulated by soil pH. The effect of Fe minerals, particularly oxides, on soil N transformation processes depends on soil pH, with Fe oxide often stimulating nitrification activity in the soil with low pH. We conducted lab incubations to investigate the effect of Fe oxide on N transformation rates in two subtropical agricultural soils with low pH (pH 5.1) and high pH (pH 7.8). ¹⁵N-labeled ammonium and nitrate were used separately to determine N transformation rates combined with Fe oxide (ferrihydrite) addition. Iron oxide stimulated net nitrification in low-pH soil (pH 5.1), while the opposite occurred in high-pH soil (pH 7.8). Compared to the control, Fe oxide decreased microbial immobilization of inorganic N by 50 % in low-pH soil but increased it by 45 % in high-pH soil. A likely explanation for the effects at low pH is that Fe oxide increased NH₃-N availability by stimulating N mineralization and inhibiting N immobilization. These results indicate that Fe oxide plays an important role in soil N transformation processes and the magnitude of the effect of Fe oxide is dependent significantly on soil pH.

Subtropical ecosystems are often characterized by high N cycling rates, but net nitrification rates are often low in subtropical acid soils. NO₃⁻-N immobilization into organic N may be a contributing factor to understand the observed low net nitrification rates in these acid soils. The effects of Fe oxide and organic matter on soil N transformations were evaluated using a ¹⁵N tracing study. Soil net nitrification was low for highly acidic yellow soil (Ferralsols), but gross ammonia oxidation was 7 times higher than net nitrification. In weakly acidic purple soil (Cambisols), net nitrification was 8 times higher than in Ferralsols. The addition of 5% Fe oxide to Cambisols, reduced the net nitrification rate to a negative rate, while NO₃⁻-N immobilization rate increased 8 fold. NO₃⁻-N immobilization was also observed in Ferralsols which contained high Fe oxides levels. A possible mechanism for these reactions could be stimulation of NO₃⁻-N immobilization by Fe oxide which promoted the abiotic formation of nitrogenous polymers, suggesting that the absence of net nitrification in some highly acid soils may be due to high rates of NO₃⁻-N immobilization caused by high Fe oxide content rather than a low pH.