Peer Review #1 of "Changes in nutrients and decay rate of Ginkgo biloba leaf litter exposed to elevated O3 concentration in urban area (v0.1)"

Abstract

Ground-level ozone (O 3) pollution has been widely concerned in the world, particularly in the cities of Asia, including China. Elevated O 3 concentrations have potentially influenced growth and nutrient cycling of trees in urban forest. The decomposition characteristics of urban tree litters under O 3 exposure are still poorly known. Ginkgo biloba is commonly planted in the cities of northern China and is one of the main tree species in the urban forest of Shenyang, where concentrations of ground-level O 3 are very high in summer. Here, we hypothesized that O 3 exposure at high concentrations would alter the decomposition rate of urban tree litter. In open-top cambers (OTCs), 5-year-old G. biloba saplings were planted to investigate the impact of elevated O 3 concentration (120 ppb) on changes in nutrient contents and decomposition rate of leaf litters. The results showed that elevated O 3 concentration significantly increased K content (6.31 ± 0.29 vs 17.93 ± 0.40, P＜0.01) in leaves of G. biloba, significantly decreased the contents of total phenols (2.82 ± 0.93 vs 1.60 ± 0.44, P＜0.05) and soluble sugars (86.51 ± 19.57 vs 53.76 ± 2.40, P＜0.05), but did not significantly alter the contents of C, N, P, lignin and condensed tannins, compared with that in ambient air. Furthermore, percent mass remaining in litterbags after 150 days under ambient air and elevated O 3 concentration was 56.0% and 52.8%, respectively. No significant difference between treatments was observed in mass remaining at any sampling date during decomposition. The losses of the nutrients in leaf litters of G. biloba showed significant seasonal differences regardless of O 3 treatment. However, we found that elevated O 3 concentration slowed down the leaf litter decomposition only at the early decomposition stage, but slightly accelerated the litter decomposition at the late stage (after 120 days). This study provides our understanding of the ecological processes regulating biogeochemical cycles from deciduous tree species in high-O 3 urban area.