WHOLE-TREE CLEAR-CUTTING EFFECTS ON EXCHANGEABLE CATIONS AND SOIL ACIDITY

It has been hypothesized that logging reduces soil fertility and site productivity. The objective of this study was to determine short-term (3-yr) effects of logging on soil pH, exchangeable cations, and cation-exchange capacity (CEC). We intensively sampled soils before and after the whole-tree harvesting of a northern hardwood forest at the Hubbard Brook Experimental Forest, New Hampshire. Cation-exchange capacity decreased by 23% in the Oa horizon and 24% in the E horizon but increased by 67% in the Bh horizon and 34% in the Bs1 horizon. Overall, the number of exchange sites in the solum did not change appreciably with harvesting (202 vs. 206 kmol(c) ha-1). In the Oa, E, and Bh horizons, there was a decrease in the ratio of exchangeable base cations (Ca, Mg, and K) to exchangeable Al and H. As a result, base saturation decreased from 49 to 39% in the Oa, from 22 to 17% in the E, and from 14 to 11% in the Bh horizon. Soil pH decreased by 0.11, 0.32, and 0.24 pH units in the Oa, E, and Bh horizons, respectively. The acidification of the E and Bh horizons was probably the result of increased production of H through nitrification and mobilization of Al from the forest floor and mineral soil, while mixing of mineral soil into the forest floor largely explains the changes in the chemistry of the Oa horizon. At Hubbard Brook, accelerated leaching losses of nutrient cations following clear-cutting were not the result of depletion of exchangeable cation pools.