Increasing precipitation exacerbates the negative effect of nitrogen addition on diversity of semi-arid steppe early in the growing season

Abstract The general effects of global change on plant diversity have been well documented. However, how these effects vary across seasons remains poorly understood, posing challenges for a better understanding of the seasonal dynamics of plant communities in a changing world. This study examines the seasonal responses of plant diversity and community composition to simulated nitrogen deposition and increased precipitation to address these challenges. From 2018 to 2023, a field manipulative experiment with nitrogen and water addition treatments was conducted in a semi-arid steppe of Inner Mongolia, China. Plant taxonomic, functional, and phylogenetic diversity, along with community composition, were measured in different months (June, July, and August) during the growing season over six years. Nitrogen addition consistently reduced all three dimensions of diversity throughout the growing season. Water addition alone had no effect on taxonomic or phylogenetic diversity but increased functional diversity in June, and decreased it in July and August. Notably, water addition amplified the negative effect of nitrogen addition on diversity in the early growing season. This interaction weakened as the season progressed, disappearing entirely by late season. These findings demonstrate that water availability critically regulates nitrogen impacts early in the growing season, highlighting the importance of precipitation changes early in the growing season for maintaining plant diversity in the semi-arid grassland. Under conditions of increased precipitation in the future, the inhibitory effect of nitrogen enrichment on plant diversity will occur earlier compared with later in the growing season.