Water limitation drives species loss in grassland communities after nitrogen addition and warming

Nutrient addition, particularly nitrogen, often increases plant
aboveground biomass but causes species loss. Asymmetric competition for
light is frequently assumed to explain the biomass-driven species loss.
It, however, remains unclear whether other factors such as water can also
play a role. Increased aboveground leaf area following nitrogen addition
and warming may increase transpiration and lead to water limitation,
leading to a decline in diversity. To test this, we conducted field
measurements from a grassland community exposed to nitrogen and water
addition, and warming. We found that warming and/or nitrogen addition
significantly increased aboveground biomass but reduced species richness.
Water addition prevented species loss in either nitrogen-enriched or
warmed treatments, while it partially mitigated species loss in the
treatment exposed to increases in both temperature and nitrogen. These
findings thus strongly suggest that water limitation can be an important
driver of species loss as biomass increases after nitrogen addition and
warming when soil moisture is limiting. This result is further supported
by a meta-analysis of published studies across grasslands worldwide. Our
study indicates that loss of grassland species richness in the future may
be greatest under a scenario of increasing temperature and nitrogen
deposition, but decreasing precipitation.