Large differences in soil pH and available Ca in the surface soil exist among tree species growing in a mixed hardwood forest in northwestern Connecticut. The observed association between tree species and specific soil chemical properties within mixed-species stands implies that changes in the distribution and abundance of tree species alter the spatial and temporal pattern of soil acidity and Ca cycling in this forest. With continuing stress of acid atmospheric deposition, these alterations could have large effects on forest community and ecosystem dynamics. The objectives of this thesis were 1) to identify and quantify specific biogeochemical processes that are responsible for the differences in Ca availability in the surface soil (forest floor and upper 20 cm of the mineral soil) and other related soil properties under different tree species and 2) to separate tree species effects from soil effects on soil properties in the surface soil. Mineral weathering, leaching, organic mineralization, and uptake of Ca were studied beneath sugar maple (Acer saccharum), hemlock (Tsuga canadensis, Carr.), American beech (Fagus grandifolia, Ehrh.), red maple (Acer rubrum, L.), white ash (Fraxinus americana) and red oak (Quercus rubra, L.).

Tree species can modify soil weathering by changing soil pH and by producing organic acids that form metal complexes. I used soil mineral data and stable strontium isotopes as a marker for Ca to investigate tree species effects on Ca weathering in the soil. I also determined the quantity, nature and degree of neutralization of organic acids that are produced in the forest floor under different tree species and examined the role of organic acids on base cation leaching. I found no significant tree species effect on Ca weathering, which was attributed to the low Ca content of the soil parent material. Organic acids had a significant effect on base cation leaching from forest floors, with higher leaching from forest floors that contained more exchangeable base cations (sugar maple and white ash).

The immediate availability of Ca in forests growing on acid soils is largely dependent on mineralization of organic Ca. Litter quality and quantity affects the quantity and timing of calcium release in the soil, which may differ significantly among tree species. I estimated organic Ca mineralization rates in the forest floor and upper 15 cm of the mineral soil by field incubations. Net Ca mineralization rates in the upper 15 cm of the soil beneath sugar maple and white ash were significantly higher than beneath the other tree species.

By comparing Ca weathering, mineralization, and leaching rates between sugar maple and hemlock, I inferred that sugar maple trees could sustain high amounts of available Ca in the surface soil by taking up appreciable amounts of Ca from deeper soil layers. Beneath hemlock more fine roots were present in the forest floor than beneath sugar maple, increasing potential Ca uptake in the forest floor. Calcium mineralization rates that are similar to Ca uptake rates in the surface soil may have kept exchangeable Ca contents in the surface soil small beneath hemlock resulting in small Ca leaching losses to deeper soil layers. With a simple model I illustrated that even a slightly higher Ca uptake from the deep soil can substantially increase Ca availability in the surface soil within the life span of these trees.

Full text of my thesis at: www.gcw.nl/dissertations/3028/dis3028.pdf