Canopy exchange processes enhance acid-neutralizing capacity in subtropical evergreen broad-leaved forest

Forest canopy serves as the first interface for atmospheric deposition into ecosystems, thereby playing a crucial role in regulating hydrological processes and associated water chemical cycles by means of canopy exchange processes. However, it remains poorly understood how canopy exchange processes alter chemical processing of rainwater, such as the acid-neutralizing capacity (ANC) in acidic areas. This study examined the hydrological partitioning and associated chemical composition in a typical subtropical evergreen broad-leaved forest in southern China. Hydrological components, including rainfall, throughfall, and stemflow, were measured and sampled from January 2020 to December 2021, and the canopy exchange process was then estimated using a canopy budget model. Results showed that pH and ANC increased in throughfall but decreased in stemflow. During the dry season, throughfall exhibited significantly higher neutralization capacity (ΔpH = 1.11 ± 0.89; ΔANC = 0.11 ± 0.13 meq L^-1), whereas in the wet season, the neutralization capacity of throughfall was substantially weaker (ΔpH = 0.07 ± 0.85; ΔANC = 0.03 ± 0.09 meq L^-1). In contrast, stemflow showed persistent acidification across both seasons (dry-season ΔpH: -1.29 ± 1.01; wet-season ΔpH: -1.18 ± 0.69), with its ΔANC significantly below zero in the wet season (-0.07 ± 0.09 meq L^-1, p = 0.009). The canopy budget model indicated that most solutes (except F^-) were leached from the forest canopy, with K⁺ exhibiting the most pronounced canopy leaching effect. In addition, canopy exchange contributed positively 83% to acid-neutralizing capacity (ANC_NTF) of net throughfall flux. Among ions mediating canopy processes, Ca²⁺, K⁺, Mg²⁺, and F^- all made positive contributions to the acid neutralizing capacity of canopy exchange (ANCCE) in both dry and wet seasons, with K⁺ contributing the most (31.43% in the dry season and 25.52% in the wet season). In contrast, NO₃^- and Cl^- exhibited negative contributions: the negative effect of NO₃^- was stronger in the wet season (-20.69%) than in the dry season (-0.63%), whereas Cl^- showed the opposite pattern (dry: -12.03%-; wet: -12.03%). Our results suggest that the forest canopy may play a significant role in mitigating the effects of atmospheric acid deposition. This finding contributes to a better understanding of forest eco-hydrological processes and could have important implications for forest management in similar subtropical regions globally.