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.