pH threshold in controlling dominant nitrification pathway in acidic soils

Autotrophic (AN) and heterotrophic (HN) nitrification pathways regulate soil nitrogen availability and are responsible for nitrate losses to the environment. It is often assumed that HN plays a more important role than AN in acidic soils. However, so far, no detailed study has attempted to identify how pH affects the relative importance of gross rates of AN (GAN) and HN (GHN) in acidic soils. Combining ¹⁵N dilution technique with acetylene inhibition along a natural soil pH gradient of 3.7–6.9, we revealed a negative exponential relationship between GHN/GAN ratio and soil pH, with a threshold of pH=4.5 and 4.7 with and without the addition of ammonium, respectively. Variations of fungal and ammonia-oxidizing archaea (AOA) abundances along the pH gradient further confirmed the existence of this threshold. Soil nitrification was predominantly fungi-driven HN below the threshold and AOA-driven AN above the threshold. Overall, we provide evidence that a pH threshold controls the dominant nitrification pathway in acidic soils by affecting specific microbes, which could be important for predicting soil nitrification patterns and developing effective mitigation techniques to promote nitrogen availability and decrease nitrogen loss risk.