Changes in the biogeochemistry of potassium following a whole-tree harvest

A whole-tree harvest (WTH) was conducted at the Hubbard Brook Experimental Forest, New Hampshire, in 1983-1984 to evaluate changes in the loss and supply of nutrients such as K in response to clear-cutting. The WTH removed 554 mmol m^-2 associated with forest biomass. During the first 2 yr following the WTH, there were marked increases in concentrations and fluxes of K in drainage waters (stream efflux 24 mmol m^-2 yr^-1), coinciding with elevated losses of NO₃/^-. While these losses decreased within 3 yr, concentrations and fluxes of K remained high in Bs2 horizon soil water and streamwater (10.5 mmol m^-2 yr^-1) for 10 yr relative to the reference watershed (4.8 mmol m^-2 yr^-1). Soil exchangeable K pools increased within 3 yr after the WTH. In the high-elevation zone of the watershed, soil pools returned to near-preharvest values after 8 yr. In the remainder of the watershed, exchangeable K pools decreased after 8 yr but remained about 20% above preharvest pools. The increase in exchangeable K in the mineral soil probably contributes to the long-term pattern of elevated K concentrations in streamwater after the WTH. Although stream and WTH removal of K (8.5 mmol m^-2 yr^-1) was large in comparison to preharvest exchangeable K pools (284 mmol(c) m^-2), the increased release of K from mineralization of soil organic matter and weathering resulted in an increase in K on the soil exchange complex (43 mmol m^-2 yr^-1 in the first 8 yr). The increase in exchangeable K provided an available pool of K for the regrowing vegetation.