Bone biochar and humic acid improved soil quality and promoted Olea europaea growth in coastal saline soil by enhancing the stoichiometric homeostasis of nutrient elements

Abstract Soil salinisation is a major challenge to agricultural development and ecological security, which can be alleviated by the application of soil amendments. Combined application of biochar and organic fertilisers can effectively improve soil properties and mitigate the negative effect of saline soil on plant growth by increasing carbon (C), nitrogen (N), and phosphorus (P) content; however, their effects on the stoichiometric characteristics of these elements both and further impacting on plant growth are unclear. The current study compared the effects of two application rates (2.5% and 5.0%) of bone biochar (BC), humic acid (HA) and their combination (1:1) (BH) on olive seedlings ( Olea europaea L.) grown in coastal saline soil. The results showed that the combination of BC and HA at the application rate of 5% (BH5.0) significantly ( P < 0.05) improved soil properties with increased soil porosity and decreased soil sodium (Na + ) content compared with the control, and promoted plant growth and biomass of Olea europaea by enhancing leaf photosynthesis and the selective absorption of K + and Ca 2+ . Furthermore, these positive effects of combined application were better than those of BC and HA alone. Structural equation model showed that the amendments had a total positive effect on plant growth ( β = 0.834), and the stoichiometric dynamic balance of C, N and P in plants had a direct positive effect on the growth of Olea europaea ( β = 0.36). In short, BH treatment can promote plant growth in coastal saline soil by improving soil quality and C-N-P stoichiometric homeostasis of plants, and the effect is better than that of BC and HA single application. The results deepen our understanding of the mechanism of bone biochar and humic acid jointly improving the plant growth in coastal saline environments and provide a guide for the cultivation of olive which can be used as both coastal shelter-forest and cash crop in large areas of saline-alkali coastal land. Graphical Abstract