Conservation of exchangeable cations after clear-cutting of a northern hardwood forest

Clear-cut logging of northern hardwoods disrupts forest nutrient cycling, often enhancing export of mineral nutrients (Ca, Mg, K) in drainage waters. When these leaching losses are added to mineral nutrient export in harvested wood, the total can be a significant fraction of the standing stocks of some ecosystems. Thus, changes in soil chemical properties after logging are important in determining the long-term implications of harvesting on nutrient availability and site fertility. Results of extensive analyses of soils on a clear-cut northern hardwood stand at the Hubbard Brook Experimental Forest, New Hampshire, indicate that pools of exchangeable Ca, Mg, and K (1 M NH4Cl) were unchanged in the first 8 years after clear-cutting. Decreases in exchangeable cation concentrations in upper soil horizons (Oa and E horizons) were offset by large increases in the spodic (Bh and Bs1) horizons. Effective cation exchange capacity (CECe) increased by 30-60% in spodic horizons 3 years after cutting and remained elevated after 8 years. Since soil organic matter (OM) is the principal source of CECe in these Spodosols, it appears that clear-cutting has altered the charge properties of OM. The CECe/OM ratio increased by approximately 25% in the first 8 years after clear-cutting. Some of this change may have been due to pH changes, but much of it appears to be the result of changes in the character of OM. Because of its importance in regulating CECe, the preservation and alteration of OM in spodic horizons are important mechanisms of mineral nutrient retention after clear-cutting.