Soil Chemical Dynamics after Calcium Silicate Addition to a Northern Hardwood Forest

Acidic deposition has resulted in the loss of available soil Ca from base-poor soils in the northeastern United States. In 1999, wollastonite (CaSiO3) was experimentally added to a watershed at the Hubbard Brook Experimental Forest in New Hampshire in an attempt to restore the base saturation of the soil to its estimated pre-acidification level. We measured the total Ca in the O horizon and the top 10 cm of mineral soil to track the fate of the added Ca. We also measured soil pH and exchangeable cations to assess the impact of the treatment on soil acidity. In the first 11 yr after treatment, Ca was transported downward through the forest floor and upper mineral soil in a progressive fashion. By Year 11, at least 650 kg ha−1 of the 1028 kg ha−1 of Ca that was added to the watershed was no longer in the O horizon or the top 10 cm of mineral soil. Soil pH and exchangeable Ca concentrations increased significantly in organic and mineral soils after treatment. Exchangeable H and Al concentrations decreased significantly. The pool of exchangeable Ca increased significantly after treatment, peaking in the O horizons 3 yr after treatment and in the upper mineral soil 7 yr after treatment. The pools of exchangeable Al and H steadily and significantly decreased through the study period. Only about 3% of the added Ca was exported from the watershed in stream water after 11 yr. Wollastonite treatment was thus an effective means of increasing available pools of Ca in this forest ecosystem.