Effects of tree species on soil properties in a forest of the Northeastern United States

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 (Chapter 2). 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 (Chapter 3). 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.

Organic acids forming metal complexes influence the distribution and mobilization of aluminum and iron in the soil and enhance the podzolization process. Aluminum affects the Ca availability in the soil by displacing Ca on the exchange complex. I compared concentrations of exchangeable and organic-bound solid Al and of aqueous Al species at different soil depths to study the effect of tree species on the Al solubility and mobility in the soil (Chapter 4). In spite of pronounced differences in dissolved organic Al between tree species, no significant differences were found in the concentrations of organic-bound solid Al between tree species at different depths. The intensity and duration that tree species exert on Al migration was probably too small to have caused pronounced differences in Al redistribution in the soil.

Litter quality and quantity affects the quantity and timing of calcium release in the forest floor. As a result it can influence the availability of Ca in the soil profile. I estimated organic Ca mineralization rates in the forest floor and upper 15 cm of the mineral soil by field incubations (Chapter 5). Per unit area no significant differences in net Ca mineralization rates were observed in the forest floor among tree species. However, net Ca mineralization rates in the mineral soil were substantial beneath sugar maple and white ash and 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 (Chapter 6). 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.