Assessing Decadal Change in Mineral Soil Cation Chemistry at the Turkey Lakes Watershed

Mass balance studies and direct measurement of changes in soil chemistry have provided evidence of depletion of base cations in soils across decadal time scales. Losses of base cations have been attributed to leaching as a consequence of elevated levels of S and N deposition. Chemical properties of mineral soil horizons were determined at seven plots in a tolerant hardwood forest at the Turkey Lakes Watershed in 1986 and in 2003 and 2005. Archived soils from the original sampling were used to assess the impact of storage and changing methodology on results. Exchangeable cation concentrations were compared between the two sampling times, as were concentrations normalized for organic C. The effects of air-dry storage were significant for some cations, but in absolute terms the changes were small when contrasted with the spatial variation in exchangeable cation concentrations for the samples collected in 2003 and 2005. There were no statistically significant declines in pH, exchangeable Ca, Mg, or K concentrations, or Ca, Mg or K concentrations normalized for organic C concentration during the 17- to 19-yr sampling period across the watershed. Exchangeable Na and normalized Na concentrations decreased in deeper horizons of the soil profile. There were various patterns of cation change at the plot level, but these changes were unable to be attributed to the range of soil profile base status at the site. Mineral weathering inputs appeared to provide stability to the exchangeable base cation pool in spite of large leaching losses at the Turkey Lakes site.