Soil water regulates plant diversity response to gradual and step nitrogen addition

Nitrogen enrichment often increases plant aboveground biomass but reduces biodiversity. The mechanisms through which increased nitrogen can lead to the loss of plant species are still highly controversial. Furthermore, atmospheric nitrogen increases gradually over years, while our current understanding of the effects of nitrogen deposition largely relies on step nitrogen addition experiments.

In this study, we conducted a step versus gradual nitrogen addition field experiment in a semiarid grassland during 3-years, focusing on the potential mechanisms underlying species loss.

Our findings revealed that both gradual and step nitrogen addition significantly increased plant aboveground biomass by 150% and 221%, respectively. However, step nitrogen addition resulted in a significant reduction in plant species richness by 18%, while gradual nitrogen addition did not significantly alter species richness. Our structure equation model indicated that reduction in soil water crucially regulated the extent of species loss under step versus gradual nitrogen additions. The regulation of soil water on plant diversity was further supported by our meta-analysis of water and nitrogen addition experiments conducted across arid and semiarid grasslands worldwide.

Collectively, soil water content is the dominant regulator of plant species loss after nitrogen enrichment in water-limiting grasslands. Our findings suggested that 3-years total nitrogen amount rather than the nitrogen input in the final year of experiment determined decline of plant diveristy, i.e., nitrogen addition had a legacy effect on grassland community.