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.

Previous studies have demonstrated changes in plant growth and reproduction in response to nutrient availability, but how investigations of such responses to multiple levels of nutrient enrichment remains unclear. In this study, we manipulated nitrogen (N) and phosphorus (P) availability to examine seed production responses to three levels each of N and P addition in a factorial experiment: no N addition (0 g N m-2 yr-1), low N addition (10 g N m-2 yr-1), high N addition (40 g N m-2 yr-1), and no P addition (0 g P m-2 yr-1), low P addition (5 g P m-2 yr-1), high P addition (10 g P m-2 yr-1). Low N addition enhanced seed production by 814%, 1371%, and 1321% under ambient, low, and high P addition levels, respectively. High N addition increased seed production by 2136%, 3560%, and 3550% under ambient, low, and high P addition levels, respectively. However, P addition did not affect seed production in the absence of N addition, but it did enhance it under N addition. Furthermore, N addition enhanced seed production mainly by increasing the tiller number and inflorescence abundance per plant, whereas P addition stimulated it by decreasing the plant density yet stimulating height of plants and their seed number per inflorescence. Our results indicate seed production is limited not by P but rather by N in the temperate steppe, whereas seed production will be increased by P addition when N availability is improved. These findings enable a better understanding of plant reproduction dynamics of steppe ecosystems under intensified nutrient enrichment and can inform their improved management in the future.