High ammonia oxidizers persist seven years after cessation of nutrient addition

Ammonia oxidizers are the primary agents for nitrification. The most significant change in an ecosystem’s nitrogen (N) cycle is the stimulation of ammonia oxidizer activity caused by N enrichment and the subsequent development of a highly nitrifying soil environment. However, few studies have examined the duration of N-induced impacts on nitrification after the cessation of N inputs. We conducted a 15 years nutrient addition experiment in a Tibetan alpine meadow, and then nutrient addition was ceased for 7 years, and the ammonia-oxidizer communities were examined. Ammonia oxidizers, especially ammonia-oxidizing bacteria (AOB), exhibited a continuous hysteretic response to N enrichment and subsequent cessation. Soil ammonia availability has been proposed as a primary driver controlling AOB abundances under nutrient addition. Although plant N and soil inorganic N concentrations showed rapid recovery in plots where nutrient additions were discontinued, soil microbial N mainly controlled changes in N mineralization and availability of soil N to ammonia oxidizers. Pyrosequencing data revealed that nutrient cessation significantly impacts AOB, and AOB composition established connections than AOA and comammox Nitrospira, indicating the greater ecological importance of AOB in the overall network. Our findings suggest that a high nitrification environment induced by N enrichment is not possible simply by reducing N input, and it requires a long recuperation time.