Abstract: Aims Nitrogen (N) addition significantly affected the aboveground net primary productivity (ANPP) and belowground net primary productivity (BNPP) of grassland ecosystems. However, the different responses of plant ANPP and BNPP to N addition and their saturation response thresholds at different levels of N addition are not clear. Methods Therefore, eight N addition levels (0, 1, 2, 4, 8, 16, 24, 32 g N m-2 year-1) were set up in the Leymus secalinus grassland in northern Shanxi Province, and the changes of plant ANPP, BNPP and total net primary productivity (NPP) were monitored for five years from 2017 to 2021. The temporal stability, N saturation threshold and N response efficiency of plant productivity were also calculated. Important findings The results showed that: (1) ANPP and NPP were increased nonlinearly with the increase of N addition level. The saturation response thresholds of ANPP and NPP across five years (2017-2021) were 25.7 g N m-2year-1 and 21.3 g N m-2year-1, respectively. (2) ANPP and BNPP had different sensitivities to N addition, and the sensitivity of ANPP was higher than that of BNPP. The change in NPP was mainly caused by changes in ANPP, suggesting that plant productivity distribution to the ground increased as the level of N addition increased. (3) NREANPP showed an exponential downward trend with the increase of N addition levels, while the NREBNPP and NRENPP showed a linear downward trend with the increase of N addition level. (4) Structural equation model analysis showed that soil NO3--N and pH regulated the effects of N addition and years on plant ANPP and BNPP. There was different regulatory mechanisms of ANPP and BNPP under low N and high N addition treatments. Both ANPP and BNPP were directly affected by N addition under low N addition treatments, while the soil inorganic N indirectly regulated the response of ANPP and BNPP to N addition under high N addition treatments. This study showed that plant productivity in grassland ecosystem responded nonlinearly with N addition level, which provided data support for improving the ecosystem service function of Leymus secalinus grassland in northern Shanxi Province under the background of N deposition.

Key words: Nitrogen deposition, above-ground productivity, underground productivity, Nitrogen response efficiency, Response threshold