Drought and nitrogen (N) additions have been shown to affect plant photosynthesis, but the effect of their interaction on CO2 diffusion conductance (i.e., stomatal conductance, gsc, mesophyll conductance, gm) and water use efficiency (WUE) at the leaf level (i.e., instantaneous water use efficiency, WUEins; intrinsic water use efficiency, WUEint) has rarely been reported. Herein, we examined the interactive effects of drought and N addition on gsc and gm and WUE (i.e., WUEins and WUEint) in soybean (Glycine max (Linn.) Merr.) and analysed the interaction mechanisms mitigating water stress based on leaf anatomy and molecular activity. Drought decreased gsc, gm and WUEins, but N addition had a positive interactive effect with water stress on leaf physiological traits. The variables gsc, gm and WUEins were significantly increased by the additions of 7.5 and 15.0 g urea m−2 under moderate water stress (MW) conditions (P < 0.05), but they decreased under the interaction effect of SW and N addition. The decreased Tcw and increased Sm and Sc under the water stress × N addition interaction positively affected gm, and the increased SS dominated the gsc increase. In addition, these positive effects were further enhanced with increased AQP, CA and Rubisco activities and a decrease in the ABA content. Accordingly, moderate soil N additions of 7.5 and 15.0 g urea m−2 greatly mitigated the negative effects of droughts, especially the MW effects, by enhancing CO2 diffusion conductance and WUE.

This study was supported by the Research Program Sponsored by the State Key Laboratory of Sustainable Dryland Agriculture (in preparation), Shanxi Agricultural University (No. 202001-9), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2021L156) and the Fundamental Research Program of Shanxi Province (No. 20210302124229). We are grateful for the support and technical assistance from the research group of Eco-climatology, Institute of Applied Ecology, CAS and from the research group on Plant Biology under Mediterranean Conditions, Department of Biology of the University of the Balearic Islands (UIB).

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