Forest nutrient and carbon pools at Walker Branch Watershed: Changes during a 21-year period

A 21-yr perspective on changes in nutrient and C pools on undisturbed uplan d forest sites is provided. Plots originally representing four col er types have been sampled three times. On each plot, forest biomass, forest floor, and soil, to a depth of 60 cm, were measured, sampled, and analyzed for Ca , Mg, C, N, and P. Exchangeable soil Ca and Mg hale declined in most soils. Despite the low exchangeable Ca, cumulative sequestration in the biomass h as exceeded the soil pool, suggesting that soil supplies below 60 cm are sa tisfying the biomass demand. Extractable soil P also declined, with means r anging from 4.2 to 18.2 kg ha(-1), as a result of reductions in the mineral soil and Oi horizon, The loss of extractable soil P exceeded biomass seque stration in all but one plot, suggesting abiotic soil processes as the remo val mechanism. Soil C and N were either stable, although highly variable, o r declined, which was unexpected in these undisturbed sites. The net C bala nce of these sites was controlled by aboveground sequestration, which offse t changes in the soil and forest floor, Soil parent material and geomorphic setting strongly influenced the changes in soil properties during the 21-y r period, reflecting the importance of those factors in assessing soil nutr ient and C cycles over that of forest cover type. The variability encounter ed in the periodic soil measurements highlights the difficulty in detecting temporal changes in soil chemical properties.