Objectives Beijing boasts a vast mountainous area with significant potential for sludge product assimilation. We investigated the effects of sludge products on nutrient content, osmoregulation substances, and heavy metal concentrations in the leaves of landscape ecological forest trees, namely Koelreuteria paniculata and Fraxinus chinensis (Golden Leaf Ash), as well as the appropriate application rates. This study aims to provide a theoretical basis for enhancing the growth quality of landscape ecological forests and mitigating potential ecological risks.

Methods Field experiments were carried out in a newly afforested areas of stony mountains in Beijing in July 2023. Five sludge product application amounts were set up: 0, 16, 32, 48, 64 kg/plant, which were denoted as CK, T1, T2, T3 and T4 in sequence. Leaf samples were collected in May 2024, and the contents of nutrients, osmoregulatory substances and heavy metals were analyzed. The appropriate sludge products application amounts for the two colored-leaf trees were comprehensively calculated using the membership function-entropy weight method.

Results The leaf N content of both Koelreuteria paniculata and Fraxinus chinensis treated with the four sludge products was significantly higher than that of the CK. With the increase in the application amount of sludge products, the N content showed a trend of first increasing and then decreasing, and both of them reached the highest under T3 treatment, which were 24.253 g/kg and 28.810 g/kg respectively. However, there was no obvious variation pattern in the contents of leaf P and K. The contents of soluble sugar and soluble protein in the leaves of Koelreuteria paniculata and Fraxinus chinensis generally showed a trend of first decreasing and then increasing with the increase in the application amount of sludge products, but they were all lower than CK or had no significant difference from CK. The proline contents in the leaves of Koelreuteria paniculata under the T3 treatment and Fraxinus chinensis under the T2 treatment reached the highest, which were 2.50 times and 4.62 times that of CK respectively. With the increase in the application amount of sludge products, the Zn, Pb and Ni contents in the leaves of Koelreuteria paniculata all showed an increasing trend, but were not significantly different to those in CK, while Cu content was significantly higher than CK under T1, T2 and T4 treatments; the contents of Cu, Zn, Pb and Ni in the leaves of Fraxinus chinensis were the lowest under T2 treatment, while the Hg content was the highest. Among them, the contents of Cu and Ni showed a trend of first increasing, then decreasing and then increasing again, the change trends of Pb and Hg contents were opposite to them, and the Zn content first decreasing and then increasing. Through comprehensive evaluation, it was concluded that the comprehensive score value of the leaves of Koelreuteria paniculata was the largest (0.553) under the T3 treatment, and the values of various monitored trait indexes were relatively balanced; the comprehensive score value of the leaves of Fraxinus chinensis was the largest (0.620) under the T2 treatment.

Conclusions Application of sludge products did not result in the accumulation of heavy metals in the leaves of Koelreuteria paniculata and Fraxinus chinensis, nor did it inhibit their growth. On the contrary, it improved the nutritional status of the leaves of both tree species to some extent. Comprehensively considering above indicators, applying sludge products 48 kg/plant and 32 kg/plant in Koelreuteria paniculata and Fraxinus chinensis gained the highest comprehensive scores, indicating that these sludge products application rates are the most suitable for the growth of both tree species while posing the lowest risk of heavy metal contamination.