Discussion
This study comprehensively evaluates the nitrification determinants focusing on ammonia oxidizers between N addition and N cessation. The results demonstrated that N enrichment increased AOB abundance, even 7-year after cessation of N addition. Soil N availability appeared to be adequate in soils often anthropogenically fertilized (Fig. 6b). Once N addition stopped, soil N accessibility relied primarily on mineralization. Increases in soil microbial biomass N under N cessation would maintain a high nitrification environment by enhancing soil mineralization (Fig. 6b). Therefore, the soil nitrogen cycle pattern shifted from the interactions between different soil nitrogen pools (e.g., ammonium and nitrate) under N addition towards microbes-driven nitrogen cycling under N cessation (Fig. 6). Our findings suggest that a high nitrification environment induced by N enrichment will not be declined by reducing N input but require a long time.