Forest soil Ca depletion through leaching and vegetation uptake may threaten long‐term sustainability of forest productivity in the southeastern USA. This study was conducted to assess Ca pools and fluxes in a representative southern Piedmont forest to determine the soil Ca depletion rate. Soil Ca storage, Ca inputs in atmospheric deposition, and outputs in soil leaching and vegetation uptake were investigated at the Panola Mountain Research Watershed (PMRW) near Atlanta, GA. Average annual outputs of 12.3 kg ha−1 yr−1 in uptake into merchantable wood and 2.71 kg ha−1 yr−1 soil leaching exceeded inputs in atmospheric deposition of 2.24 kg ha−1 yr−1 The annual rate of Ca uptake into merchantable wood exceeds soil leaching losses by a factor of more than five. The potential for primary mineral weathering to provide a substantial amount of Ca inputs is low. Estimates of Ca replenishment through mineral weathering in the surface 1 m of soil and saprolite was estimated to be 0.12 kg ha−1 yr−1 The weathering rate in saprolite and partially weathered bedrock below the surface 1 m is similarly quite low because mineral Ca is largely depleted. The soil Ca depletion rate at PMRW is estimated to be 12.7 kg ha−1 yr−1 At PMRW and similar hardwood‐dominated forests in the Piedmont physiographic province, Ca depletion will probably reduce soil reserves to less than the requirement for a merchantable forest stand in ≈80 yr. This assessment and comparable analyses at other southeastern USA forest sites suggests that there is a strong potential for a regional problem in forest nutrition in the long term.

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The effects of acidic atmospheric deposition on leaching of base cations from the canopy and the origin of the major ions in throughfall and stemflow were evaluated in a 2-year study of adjacent deciduous and coniferous forests at Panola Mountain Research Watershed in the Georgia Piedmont. In each forest, the NO3− and SO42− in throughfall and stemflow were derived primarily from atmospheric deposition, whereas the base cations Ca2+, Mg2+, and K+ were derived primarily from canopy leaching. Acidic atmospheric deposition was partially neutralized in each forest. Exchange of H+ with base cations appeared to be the major mechanism for the neutralization of atmospheric acidity by the deciduous canopy. Major neutralization mechanisms could not be differentiated in the coniferous canopy. Base-cation leaching accounted for 86% of the base cations in throughfall and stemflow in the deciduous forest and 69% in the coniferous forest. Exchange with H+ accounted for about 30% of base cations in throughfall in the deciduous forest, whereas it could not be clearly estimated in coniferous throughfall. The current level of acidic atmospheric deposition is hypothesized to have caused an increased leaching of base cations of the deciduous canopy, but methods were insufficient to determine its effect on the coniferous canopy.

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