Moderate NO3–-N instead of NH4+-N alleviates the effect of salt stress on walnut growth

Walnut (Juglans regia L.) is an important oilseed crop, and salt stress threatens the growth of walnut tree. In this study, NO3− or NH4+ was applied at three concentrations (4, 32, and 100 mM) to investigate the effect of NO3−-N and NH4+-N on walnut seedlings under 100 mM NaCl stress. Results showed that moderate (32 mM) NO3−-N application alleviated the effect of salt stress. Moreover, plants treated with 32 mM NO3−-N showed no significant morphological difference from those subjected to the nonstress treatment. The treatment of 32 mM NO3−-N application enhanced plant growth, increased antioxidant enzyme activities (superoxide dismutase and catalase), and restricted Na⁺ and Cl⁻ uptake and transport. Additionally, it might induce beneficial shifts in the rhizosphere microbiome. Low-concentration (4 mM) NO3⁻ or NH4⁺ treatment individually induced the minor alleviation of salt stress. By contrast, 100 mM NO3⁻ and all tested concentrations of NH₄⁺ further inhibited biomass and root growth, thereby exacerbating salt injury. Notably, 100 mM NH₄⁺ caused severe defoliation and seedling mortality. Furthermore, in contrast to its NO3− counterpart, 32 mM NH4⁺ shifted the root microbiome and impaired microbial diversity, likely contributing to increased salt sensitivity. This study demonstrates that moderate NO3⁻ application can effectively mitigate salt stress during walnut growth, offering a potential strategy for fertilizing walnut plantations under saline conditions.