Spraying micronutrients improves nitrogen use efficiency and rubber tree (Hevea brasiliensis) seedling growth by enhancing the interplay of nitrogen metabolism, root development, and photosynthesis

Rubber tree is crucial for industrial development globally, yet it grows slowly at seedling stage because of low nitrogen use efficiency (NUE). Spraying foliar nitrogen fertilizers (FNF), especially combined with micronutrients is expected to improve NUE, However, the mechanisms remain unknown. Here, FNF with three levels (i.e., 0.5 %, 1 % and 2 %) are applied alone or combined with micronutrients (W: B 0.2 %, Fe 0.1 %, Zn 0.1 %, Mn 0.1 %, Mo 0.05 %) to investigate their effects on NUE and the growth of rubber seedlings. Results showed that FNF application alone followed the diminishing return law and exhibited a 15 N recovery efficiency (NUE 15 N ) threshold. Specifically, the increased N level from 0.5 % to 1 % improved nitrogen accumulation (TNC), biomass and root development by 5.8 %, 8.5 % and 10.2 %–64.5 % respectively, without altering NUE 15 N . However, with N increased to 2 %, the NUE 15 N and TNC were reduced by 24.8 % and 4.5 %, ultimately decreasing the biomass by 2.2 %. Interestingly, co-applying micronutrients improved the NUE 15 N and TNC threshold by 9.3 % and 3.7 %, with the greatest effect at 1 % N level. 15 N allocation and structural equation modeling revealed that N fertilizer primarily affected biomass synthesis via cascading effects on root development and N allocation. Excessive N addition inhibited the transpiration rate and N transport from leaves to roots, reducing root diameter and tips. Conversely, micronutrients enhanced crop growth by strengthening the interactions among N assimilation, photosynthesis and root development. Micronutrients also stimulated N metabolism, including nitrate reduction to nitrite and ammonium assimilation into glutamine. These positive effects boosted chlorophyll synthesis by 13.6–28 %, thereby enhancing photosynthesis and NUE 15 N . Random forest analysis identified key predictors of crop growth, including photosynthetic rate, transpiration rate, stem- 15 N, root- 15 N and root morphology. Collectively, this study demonstrates the effectiveness of micronutrients to optimize NUE 15 N and N metabolism, thereby informing N management for rubber cultivation. • 1 % N boosts rubber seedling growth, while 2 % N reduces 15 N recovery efficiency by diminishing returns. • Co-application of micronutrients increases thresholds for 15 N recovery efficiency, biomass and N accumulation. • N fertilizer affects biomass via root development and nitrogen allocation cascades. • Micronutrients enhance N metabolism, chlorophyll content and growth via root-photosynthesis synergy.