Effect of Fe (III) addition on ammonium loss and associated microbial gene expression in soils

The combined effects of temperature, Fe (III) contents, NH4+-N contents, and soil-liquid ratio were evaluated relative to the loss of NH4+-N in soils using a response surface methodology (RSM). The microbial mechanisms were explored for nitrogen transformation by quantifying functional genes related to nitrification and denitrification. According to parameter optimization analysis for prediction equation, the maximum NH4+-N loss was 86.1% under the conditions of 17.0 °C, 0.772 g·kg-1 Fe (III), 21.9 mg·kg−1 NH4+-N, and soil: liquid ratio of 1:1. The prediction result was similar to experimental data in the current study, which the NH4+-N loss was 83.2% under the condition of 25 °C, 0.723 g·kg-1 Fe (III), 20 mg·kg−1 NH4+-N, and soil-liquid ratio of 1:1. While the N2O flux reached its minimum value of 8.35 μg·m⁻²·h–¹ under the experimental conditions, correlating with gene copy numbers for ammonia-oxidizing bacteria ammonia monooxygenase subunit A gene (AOB-amoA), and nitrite reductase genes (nirK) were maximum values of 4.5×105 and 4.8×105 copies·g-1, respectively. NH₄⁺-N loss resulted from multiple interacting processes beyond ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) mediated oxidation. The research findings can provide insights for reducing nitrogen application to avoid NH4+ toxicity and increasing soil planting suitability.