Asynchronous Declines in Ammonia and Nitrite Oxidation Caused by Soil Acidification Reduce Nitrite Accumulation and Nitrous Oxide Emissions

Nitrous oxide (N₂O), a powerful trace gas, is closely associated with nitrite (NO₂⁻) content in neutral and alkaline dryland soils, while this relationship has not been tested under soil acidification and crop rhizosphere. By examining key N transformations in maize rhizosphere and bulk soils along a pH gradient (4.2-6.8), we found that N₂O emissions significantly declined with increasing acidity, particularly in the rhizosphere. This reduction was driven by decreased NO₂⁻ accumulation, resulting from stronger suppression of potential ammonia oxidation (NH₃, PAO) than for NO₂⁻ oxidation (PNO). Acidification preferentially inhibited NH₃-oxidizing bacteria and their core taxa relative to the nxrA-encoding NO₂⁻-oxidizer, thereby lowering the PAO/PNO ratio. Both NO₂⁻ and N₂O were significantly enriched in the rhizosphere at pH above 5.2, whereas this positive effect disappeared at pH 4.2. Overall, increasing the pH may enhance rhizosphere N₂O emissions by decoupling NH₃ and NO₂⁻ oxidation, highlighting the importance of maintaining their balance when amending acidic soils.