Canopy modification of base cations deposition in a subtropical broadleaved forest: Spatial characteristics, canopy budgets and acid neutralizing capacity

Abstract Atmospheric deposition of base cations in contact with the canopy alters their chemical characteristics and plays a vital role in offsetting soil acidification and nutrient imbalance by replenishing the base cationic pool in forest ecosystems. However, the effects of site characteristics on deliveries of base cations fluxes via throughfall (TF) and stemflow (SF) across topographic gradients within a watershed remain less understood. Here, we examined the concentrations and fluxes of base cations (potassium (K+), calcium (Ca2+), magnesium (Mg2+) and sodium (Na+)), acid neutralizing capacities (ANC) and alkalinity (ALK) in bulk precipitation (BP), TF and SF in an upper slope plot (UP) and a lower slope plot (LP) in a subtropical broadleaved montane forest, Southeast China. In addition, a canopy budget model was applied to distinguish the relative contributions of dry deposition (DD) and canopy exchange (CE) to net TF + SF (NTF) flux. Results showed that annual volume weighted mean concentrations of base cations in TF and SF were significantly higher than those in BP in both UP and LP. Annual fluxes of TF or SF base cations had no significant differences between UP and LP, resulting in a comparable of the NTF fluxes. However, the relative contributions of DD and CE to NTF were different at the two slope plots. NTF K+ flux was dominated by CE regardless of slope position. Unlike K+, NTF Ca2+ and Mg2+ fluxes were dominated by DD in UP, while CE in LP. In addition, ANC and ALK of TF and SF were significantly higher compared to BP, suggesting rainwater beneath the canopy became less acidic and may mitigate in part the risk of soil acidification, especially in the dormant season. This study can provide insights into the spatial variability of chemical characteristics of base cations, and improve our understanding of the quantities and controls of external and internal nutrient inputs occurring within the canopy in forest watershed.