1.   Increasing deposition of atmospheric nitrogen (N) due to accelerated human activities is a threat to various ecosystems. However, there is a lack of long-term experimental evidence demonstrating the seasonal dynamics of plant species turnover that ultimately determines species richness in natural ecosystems under N enrichment. Moreover, the frequency of N addition also may affect species turnover in a community, but it is rarely studied.

2.   To assess the responses of a plant community to N addition, we manipulated the amounts (0–50 g N m^-2 year^-1) and frequency (2 vs. 12 times year^-1) of N addition in an Inner Mongolian typical steppe ecosystem in northern China for 12 consecutive years (2008–2020). We measured species richness and density of plant in the growing seasons (May–September) from 2018 to 2020, starting 10 years after the initial N addition treatment.

3.   Both species gain and species loss decreased with increasing amounts of N addition, resulting in a lower plant species turnover rate and greater similarity in the community between two adjacent months throughout the growing season. Species loss and species gain increased modestly under high N addition frequency. Species gain was more important than species loss in determining species richness after a decade of N application. In addition, plant density increased at high N amounts, mainly driven by enhanced clonal growth of the dominant species, Leymus chinensis.

4.   Synthesis. Together, these results suggest that high levels of N deposition may suppress species richness due to aggravated soil chemical properties and may favour growth of a limited number of N-tolerant species compared to systems that experience low levels of N deposition. To conserve biodiversity and to facilitate restoration of degraded grassland ecosystems exposed to long-term N deposition, amelioration of the acidified soils induced by N deposition may be an important strategy to use.