Data from: Water controls the divergent responses of terrestrial plant photosynthesis under nitrogen enrichment

Quantifying leaf photosynthetic response to nitrogen (N) deposition under
contrasting water conditions is important for reliably modeling
terrestrial carbon and water cycles, a topic that has not been well
understood. Here, we analyzed 737 paired observations from 102
publications to assess the response of eleven leaf photosynthesis-related
properties to N addition under different water conditions. Our research
includes global experiments, with 19 conducted in the field and 83 in
greenhouses. Treatments without water reduction were classified as
'no water change', while those with reduced water or
precipitation causing physiological drought were categorized as
'drought'. We found that, compared to the control group, N
addition significantly increased leaf photosynthetic rate (Pn; 20.9%),
leaf transpiration (E; 8.3%), and stomatal conductance (gs; 14.1%).
However, the decrease in Pn (-11.6%), E (-24.7%), and gs (-23.9%) under
the combination of N addition and drought indicated that N addition could
not offset the negative effects of drought. Furthermore, N addition
significantly enhanced water use efficiency (WUE) by 19.8% under no water
change conditions and by 21.1% under drought conditions. Within plant
functional groups, herbaceous species exhibited greater susceptibility to
N addition than woody species, especially under drought conditions. The
observed patterns of increase in Pn with longer experimental duration and
WUE with higher N rate under drought conditions showed that plants would
gradually adapt to long-term water stress in the context of N deposition.
Furthermore, our results showed that drought could strengthen the
correlations between leaf photosynthetic properties. Lastly, our study
demonstrated that N addition and drought significantly impacted leaf
nitrogen content and SPAD, respectively, and further affected gs, Pn, and
WUE. Synthesis: Our results emphasize the crucial role of water conditions
in shaping the response of leaf photosynthesis to nitrogen (N) enrichment,
and also acknowledge the significance of leaf functional traits in
regulating the dynamics of leaf photosynthetic processes.