Long - Term Soil Chemistry Changes in Aggrading Forest Ecosystems Jennifer

Assessing potential long-term forest productivity requires identification of the processes regulating chemical changes in forest soils. We resampled the litter layer and upper two mineral soil horizons, A and AB/BA, in two aggrading southern Appalachian watersheds 20 yr after an earlier sampling. Soils from a mixed-hardwood watershed exhibited a small but significant decrease in soil pH. Extractable base cation content declined substantially in both mineral horizons. For example, Ca levels in the A horizon fell from 236 kg ha" in 1970 to 80 kg ha" in 1990. Proportionally, the decline was greatest for Mg"", which dropped from 111 to 20 kg ha". A white pine (Pinus strobus L.) plantation was planted in 1956, after clear-felling hardwoods and recutting sprouts 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 Ca levels from 534 to 288 kg ha". Cation content did not change significantly in the AB/BA soil horizon. Nutrient budgets were constructed using these soil and litter data plus existing data on weathering rates, forest productivity, and hydrologic fluxes and associated chemistry. Decreases in soil base cations and soil pH are attributed to leaching and to the sequestration of nutrients in biomass. LNG-TERM SITE PRODUCTIVITY may depend on both longand short-term changes in soil chemical composition. In soils that have been depleted of nutrients by years of exploitative agricultural practices, reforestation increases nutrient and organic matter content (Fisher, 1990). Planting forest species on normal production agriculture soil can decrease soil pH and base cation concentrations (Binkley et al., 1989). Only a few studies in the USA have examined long-term changes in soils that remained forested (Johnson et al., 1991). Decreases in soil pH and base cation concentrations appear common. These changes have been attributed to cation leaching and biomass accumulation. Cation leaching can result from natural processes or acidic deposition. Leaching as a result of natural processes was reported in two N2-fixing red alder (Alnus rubra Bong.) stands in western Washington by Van Miegroet and Cole (1984, 1985) and Binkley and Sollins (1990). Large accumulations of N during a 55-yr period under alder resulted in high rates of N mineralization and nitrification. High levels of soil solution NC^" increased cation leaching and decreased soil pH in surface horizons. This resulted in the displacement of cations to lower soil horizons on one site but cation removal from another site. In a long-term study, Johnson et al. (1988) attributed soil cation losses to biomass accumulation and leaching. Calcium was translocated from subsoil horizons to aboveground biomass. Decreases in soil Mg content resulted USDA Forest Service, Coweeta Hydrologic Lab, 999 Coweeta Lab Road, Otto, NC 28763. Received 10 Feb. 1993. *Corresponding author. Published in Soil Sci. Soc. Am. J. 58:325-331 (1994). Abbreviations: HDWD, mixed-hardwood watershed: WP, white pine watershed; AAS, atomic absorption spectrophotometer: CEC, cationexchange capacity; BS, base saturation; cmol H*, exchangeable acidity; ANOVA, Analysis of Variance; SE, Standard error. 326 SOIL SCI. SOC. AM. J.. VOL. 58, MARCH-APRIL 1994 from leaching losses possibly caused by SOi deposition (Johnson and Todd, 1990). We studied long-term changes in forest soil nutrient content in two aggrading forest ecosystems. Our study objectives were to (i) examine the 20-yr changes in surface soil and forest floor chemistry in an undisturbed mixedoak forest and a white pine plantation and (ii) quantify biogeochemical processes underlying any chemical