Chemical changes in atmospheric base cations deposition induced by forest canopies significantly influence the buffer action against soil acidification and the consequent nutrient cycles. However, the hydrochemical characteristics of canopy-intercepted base cations via throughfall ( TF) to the forest floor remain poorly understood. Here, we investigated the concentrations and fluxes of TF base cations (potassium (K +), calcium (Ca 2+), magnesium (Mg 2+) and sodium (Na +)) as well as their spatio-temporal patterns on an event and within event basis in a Chinese pine plantation ( Pinus tabuliformis Carr.), Northern China. A neutralization factor was applied to distinguish the contributions of base cations to acid neutralizing capacity (ANC) in bulk precipitation ( BP) and TF. Results showed that the increased concentrations and fluxes of TF base cations were highest for K +, followed by Mg 2+, Ca 2+, and Na +. Canopy leaching was the dominant contribution of net throughfall K + and Mg 2+ fluxes, while net throughfall Ca 2+ flux was dominated by dry deposition. The spatial TF Ca 2+ and Mg 2+ concentrations were significantly related to the canopy cover and the TF water amount. Intra-event variations in TF base cation concentrations depended on the antecedent dry period and rainfall intensity. In addition, TF ANC was significantly enhanced compared to BP. Ca 2+ was the dominant neutralization factor for ANC, followed by K + and Mg 2+ in both BP and TF. These findings improve our understanding of the spatio-temporal variabilities of TF base cations, and provide insights into the underlying mechanisms that influence the varying magnitudes and directions of TF ANC in forest ecosystems.

11institutetext: Knowledge-based Systems and Document Processing Research Group Faculty of Computer Science Otto-von-Guericke-University Magdeburg 11email: katrin.krieger@ovgu.de Chemical changes of base cations in bulk precipitation ( BP) induced by canopy alteration significantly influence the buffer action against soil acidification and the consequent biogeochemical processes in forest ecosystems. However, the underlying mechanisms that influence the varying magnitudes and directions of acid neutralizing capacities (ANC) in BP during canopy passage remain less understood. Here, we investigated the chemical compositions of base cations (potassium (K +), calcium (Ca 2+), magnesium (Mg 2+) and sodium (Na +)), the capacity of individual alkaline ions to neutralize acidity, in addition to their spatio-temporal variabilities outside and beneath the canopies of Chinese pine plantation ( Pinus tabuliformis Carr.) throughout 2019 growing season with a large proportion of annual rainfall. Results showed that volume-weighted mean concentrations of base cations in throughfall ( TF) were significantly increased with the highest enrichment ratio by K +, followed by Mg 2+, Ca 2+ and Na +. Canopy leaching mainly occurred for K + (59.06%) and Mg 2+ (54.20%) whereas dry deposition of Ca 2+ (57.27%) were the dominant net throughfall flux component. TF ANC was significantly higher than BP, indicating that rainwater beneath the pine canopy became less acidic. Ca 2+ was the predominant acid neutralizing component, followed by K + and Mg 2+ in both BP and TF. However, the capacity of Ca 2+ for neutralizing acidity was weakened whereas K + and Mg 2+ were enhanced in TF compared to BP. Additionally, the concentrations of TF base cations were significantly negatively or positively correlated to canopy cover and TF amount, with exception of TF K +. Antecedent dry period and rainfall intensity were the most influential meteorological parameters affecting the intra-event variation trend and magnitude of TF base cations. Our findings contribute to understanding the controlling processes and underlying mechanisms that drive the complex interactions between base cations nutrients and forest ecosystem.