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 NO 3 − . 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.