Fungi-dominant heterotrophic nitrification in a subtropical forest soil of China

Abstract

Microorganisms play important ecological roles in NO3 − pool production, e.g., through autotrophic nitrification (the oxidation of NH4 + to NO3 −) and heterotrophic nitrification (the oxidation of organic N to NO3 −). Previous studies have mainly focused on nitrifying microorganisms capable of autotrophic nitrification; however, the contribution of microorganisms to heterotrophic nitrification has not been well investigated. The aim of this study was to identify the contribution of fungi and bacteria to autotrophic nitrification and heterotrophic nitrification in a subtropical coniferous forest soil of China. A 15N tracing experiment was conducted in a subtropical forest soil, which was treated with CK (no antibiotics), cycloheximide (fungal inhibitor), and streptomycin (bacterial inhibitor), respectively. Soil was incubated at 25 °C with 60 % water holding capacity (WHC) for 144 h after being labeled with 15NH4NO3 or NH4 15NO3 at a 20 atom% 15N excess. Autotrophic and heterotrophic nitrification rates were calculated using a full process-based N cycle model. The net nitrification rates were negative in all treatments, ranging from −0.346 mg N kg−1 day−1 in the cycloheximide treatment to −0.179 mg N kg−1 day−1 in the streptomycin treatment. The autotrophic nitrification was almost negligible (only 0.001 mg N kg−1 day−1) in the soil from subtropical coniferous forest, while heterotrophic nitrification rate was 0.993 mg N kg−1 day−1. Streptomycin reduced heterotrophic nitrification rate by 22.1 %, while cycloheximide nearly completely inhibited this process. Laboratory results showed that heterotrophic nitrification is a pivotal process producing NO3 − and that fungi rather than bacteria may dominate heterotrophic nitrification in the subtropical coniferous forest soil.