Effect of nitrogen addition on leaf photosynthesis and water use efficiency of the dominant species Leymus chinensis (Trin.) Tzvelev in a semi-arid meadow steppe

Effective utilization of water is the cornerstone of maintaining plant biomass. Water use efficiency (WUE), defined as plant carbon assimilated as biomass per unit of water input, is significantly affected by global change, particularly by nitrogen (N) deposition. Generally, N availability promotes WUE by stimulating photosynthetic. However, the degree to which increased N availability may influence leaf WUE and photosynthesis properties (A, leaf net CO2 assimilation rate; gs, stomatal conductance, and E, transpiration rate), especially in salinized-alkalized grasslands, is not studied well. We conducted a randomized block manipulative experiment to evaluate the multilevel N addition (0, 5, 10, 20, 40 g N m− 2 year−1) on leaf photosynthesis properties and leaf WUE of the dominant species (Leymus chinensis (Trin.) Tzvelev) in the Songnen meadow steppe from 2016 to 2018. We have three key findings: (1) N availability increased photosynthetic and WUE properties, instantaneous WUE (Wi = A/E), intrinsic WUE (Wg = A/gs) and long-term WUE (WL) inferred from 13C composition, were all showed a non-linear increasing pattern in response to N availability; (2) N application decreased leaf mass per area and increased leaf total N content via enhancing soil inorganic N content, thus increased photosynthetic characteristics (e.g., A, E and gs), ultimately, promoted Wi and Wg; (3) N application enhanced WL was attributed to the N-induced improvement in Wi and Wg. Results of the present work are critical to our prediction of how meadow steppe dominated by L. chinensis will respond to severe N deposition in the future.