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.

Methods
We systematically searched the journal articles via Web of Science, Google Scholar, and China National Knowledge Infrastructure, using the following keywords: “drought” or “water stress” or “water deficit” or “reduced precipitation” or “reduced rainfall” and “nitrogen addition” or “N addition” and “leaf photosynthesis” or “plant photosynthesis”. The preliminary screened articles were then refined based on the following criteria:

1) Studies that did not investigate both the individual and combined effects of drought and N addition on leaf photosynthesis were excluded.

2) Experiments that manipulated only water or N availability or warming and eCO2 simultaneously were not considered.

3) Control and experimental plots should be established in the same location or soil condition to ensure consistency in microclimate and soil nutrient conditions.

4) The studies we selected contained at least one of the following target variables: leaf photosynthetic rate (Pn), stomatal conductance (gs), water use efficiency (WUE), or optimal photochemical efficiency of PSII (Fv/Fm).