Differences in nutrient release and decay rate of poplar leaf litter and fine roots and their relationship with substrate quality and decomposition environment under ozone pollution

Ground-level ozone (O₃) pollution affects both the biochemical traits of plants and the decomposition of litter, consequently altering nutrient cycling in ecosystems. However, current research on litter decomposition primarily focuses on leaves, paying less attention to fine roots in the soil. O₃ pollution can lead to differences in the decomposition rates and nutrient release of poplar leaves and the quality of fine root litter. This study conducted a one-year decomposition experiment using five different qualities of leaf and fine root litters placed in a free-air O₃ concentration elevation system (O₃-FACE) under two treatments (ambient air and elevated O₃). The results showed that elevated O₃ significantly reduced the decomposition rate of the fast-decomposition pool while accelerated the decomposition rate of the slow-decomposition pool, and the impact on leaves was more pronounced than fine roots. The decomposition rate of the fast-decomposition pool gradually decreased with decreasing substrate quality, with exhibiting a maximum difference in residual mass of up to 21.7 % and 7.3 % on leaf litters and fine roots, respectively. Elevated O₃ had a more pronounced effect on C, N and P release in leaves compared to fine roots, markedly slowed down its release in leaves by reducing the quality of the litter. The C:N and lignin:N ratios of leaf litter, as well as the lignin:N ratio of fine roots, linearly increased with decreasing substrate quality. Substrates of lower quality decomposed at slower rates, with a corresponding decrease in the nutrient release rate. Nutrient release was more strongly affected by the quality of substrates than the decomposition environment. The findings provide deeper insight into the ecological processes of these two litter sources regulating nutrient cycles in poplar plantations under an O₃-polluted environment and could help improve terrestrial biogeochemical process models.