LONG-TERM SOIL CHEMISTRY CHANGES IN AGGRADING FOREST ECOSYSTEMS

Assessing potential long-term forest productivity requires identificat ion of the processes regulating chemical changes in forest soils. We r esampled the litter layer and upper two mineral soil horizons, A and A B/BA, in two aggrading southern Appalachian watersheds 20 yr after an earlier sampling. Soils from a mixed-hardwood watershed exhibited a sm all but significant decrease in soil pH. Extractable base cation conte nt declined substantially in both mineral horizons. For example, Ca2levels in the A horizon fell from 236 kg ha-1 in 1970 to 80 kg ha-1 in 1990. Proportionally, the decline was greatest for Me2+, which droppe d from 111 to 20 kg ha-1. A white pine (Pinus strobus L.) plantation w as planted in 1956, after clear-felling hardwoods and recutting sprout s for 15 yr. Soil pH and base cation concentrations declined in the A horizon from 1970 to 1990. Soil pH declined from 5.9 to 5.0 and Ca2+ l evels from 534 to 288 kg ha-1. Cation content did not change significa ntly in the AB/BA soil horizon. Nutrient budgets were constructed usin g these soil and litter data plus existing data on weathering rates, f orest productivity, and hydrologic fluxes and associated chemistry. De creases in soil base cations and soil pH are attributed to leaching an d to the sequestration of nutrients in biomass.