Nine years of low-dose biochar amendment suppresses nitrification rate in low-yield brown soil

Nitrification is the crucial process of nitrogen (N) transformation and ammonia oxidation is the first and the speed-limited step of nitrification. The one-time amendment of large quantities of biochar has been reported to alter soil fertility. However, it remains unclear how continuous low-dose biochar application affects the abundance and community composition of ammonia oxidizers and the subsequent nitrification and crop production. In this article, we calculate the soil net nitrification rate and the abundance and communities of ammonia-oxidizers across different growth stages of peanuts at 0–20 cm soil layer under four fertilization regimes, namely control with zero fertilization (CK), biochar application (BC), chemical fertilizer application (NPK) and biochar-based fertilizer application (BBF). Results showed that continuous application of low-dose biochar can significantly increase crop yield by 7.8 % compared to unfertilized soil indirectly through regulating the microbial mechanism in the nitrification process. The highest bacterial amoA gene copy numbers were found in the NPK treatment, 2.35-fold higher than the CK treatment. Compared to the single input of NPK chemical fertilizer, biochar-based fertilizer reduced the nitrification rate by reducing the relative abundance of bacteria, especially Nitrosomonas and Nitrosospira. In summary, our findings provide evidence that: (1) sustained low-dose biochar input could also increase the ammonia-oxidizing bacterial abundance, changing soil properties and increasing the crop yield; (2) biochar-based fertilizer could significantly reduce the nitrification rate compared to NPK application by reducing the AOB-amoA gene abundance and the relative abundance of Nitrosomonas and Nitrosospira; (3) soil C/N ratio was also important for the ammonia-oxidizing community.