Abstract: Aims Studying the response of leaf carbon content(LCC) and nitrogen content(LNC), as well as photosynthetic characteristics, to varying levels of nitrogen addition in Leymus secalinus grasslands in northern Shanxi Province holds significant importance for the sustainable utilization of grasslands in the agro-pastoral ecotone under the backdrop of nitrogen deposition. However, whether the saturation threshold of LCC and LNC to N addition is the same as leaf photosynthetic characteristics and their potential mechanism of the interaction is not clear. Methods This study focuses on the Leymus secalinus grassland in northern Shanxi Province, across set up the eight N addition levels (0, 1, 2, 4, 8, 16, 24, 32 g·m-2·year-1), the morphological characteristics, LCC and LNC, the pigment content and the photosynthetic characteristics of Leymus secalinus leaves were measured. The N saturation threshold and N response efficiency of LCC and LNC, as well as net photosynthetic rate were also calculated. Important finding The results showed that: (1) LNC and N content per unit leaf area (LNCarea) of Leymus secalinus showed a logistic growth curve with the increase of N addition level. The N saturation response thresholds of LNC and LNCarea were 11.41 g·m-2·year-1 and 7.20 g·m-2·year-1. The C:N ratio (C:N) and C:N ratio of per unit leaf area (L Carea:LNCarea) of Leymus secalinus decreased in power function with the increase of N addition level. (2) Morphological characteristics (leaf length (LL), leaf thickness (LT), leaf width (LW), leaf area (LA)), Pigment content (Chlorophyll a content (Ca), Chlorophyll b content (Cb), total Chlorophyll content (Chl), carotenoids (Car)), Photosynthetic characteristics (net photosynthetic rate (Pn), transpiration rate (Tr)) of Leymus secalinus showed the piecewise linear mode of first increasing and then decreasing with the increase of N addition level, and the N saturation response threshold of Pn was 8.36 g·m-2·year-1. However, the stomatal limit value (Ls) and water use efficiency (WUE) changed in the opposite pattern. (3) N response efficiencies of LCC, LNC and Pn (NRELCC, NRELNC, NREPn) decreased exponentially with the increase of N addition level. (4) Structural equation modeling results showed that Pn was indirectly regulated by LCC and LNC, and the regulation mechanism was different under low N and high N addition treatments. LCC and LNC mediate the regulation of Pn by regulating leaf morphological characteristics at low N addition level; the regulation of Pn was achieved by regulating leaf area and chlorophyll content by LNC at high N addition level. This study showed that LCC and LNC showed nonlinear response, while the photosynthetic characteristics showed linear response characteristics under different levels of N addition, and the regulatory mechanism of LCC and LNC on photosynthesis provided data support for the response and change of plant organs under the background of N deposition.

Key words: Key words Nitrogen deposition, leaf carbon and nitrogen content, net photosynthetic rate, nitrogen response efficiency, response threshold