Investigating the Effects of Increased Soil Calcium Availability On DOC and DON Sorption.

The interactions between dissolved organic matter (DOM) and soil surfaces play an important role in terrestrial biogeochemical cycling.  Sorption of DOM can lead to retention and accumulation of carbon, nitrogen, and other nutrients in soils, increasing soil fertility and reducing export from the ecosystem.  We are studying how increased soil calcium (Ca) availability affects the sorption of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in forest soils.  Across the Northeastern U.S. acid deposition has been linked to a loss of soil base cations, largely Ca.  In 1989, calcium carbonate (lime) was added to two subcatchments within the Woods Lake Watershed, Adirondack Park, NY.  Eighteen years after the Ca application, soil pH and exchangeable Ca concentrations remain elevated in the organic horizons and upper mineral soils of the treated subcatchments, relative to the controls.  We are investigating sorption of control DOC and DON in soils from control and Ca-amended plots, as well as the sorption of limed-plot DOC and DON in limed soils.  Additionally, we are studying how sorption processes change with soil depth. 

Results indicate that the upper 0-10 cm and 10-20 cm of mineral soil are a net source of DOC and DON in this forest.  However, solution added to limed soils showed less net desorption of DOC and DON relative to control soils.  At 20-40 cm depth, all soils show a net adsorption of DOC and DON when the initial DOC concentration was greater than 240 mg DOC Kg^-1 soil.  These findings suggest that surface soils are a net source of DOC and DON in this ecosystem, while the deeper soils are a net sink.  Increasing soil exchangeable Ca availability may influence sorption by stabilizing DOC and DON, thereby reducing losses from the upper mineral soils.  These findings have important implications for DOC and DON retention and loss in forests where acid deposition has resulted in long-term depletion of soil exchangeable Ca.