THE BIOGEOCHEMISTRY OF POTASSIUM AT HUBBARD BROOK

A synthesis of the biogeochemistry of K was conducted during 1963-1992 in the reference and human-manipulated watershed-ecosystems of the Hu bbard Brook Experimental Forest (HBEF), NH. Results showed that during the first two years of the study (1963-65), which coincided with a dr ought period, the reference watershed was a net sink for atmospheric i nputs of K. During the remaining years, this watershed has been a net source of K for downstream ecosystems. There have been long-term decli nes in volume-weighted concentration and flux of K at the HBEF; howeve r, this pattern appears to be controlled by the relatively large input s during the initial drought years. Net ecosystem loss (atmospheric de position minus stream outflow) showed an increasing trend of net loss, peaking during the mid-1970s and declining thereafter. This pattern o f net K loss coincides with trends in the drainage efflux of SO42- and NO3-, indicating that concentrations of strong acid anions may be imp ortant controls of dissolved K loss from the site. There were no long- term trends in streamwater concentration or flux of K. A distinct patt ern in pools and fluxes of K was evident based on biotic controls in t he upper ecosystem strata (canopy, boles, forest floor) and abiotic co ntrols in lower strata of the ecosystem (mineral soil, glacial till). This biological control was manifested through higher concentrations a nd fluxes of K in vegetation, aboveground litter, throughfall and fore st floor pools and soil water in the northern hardwood vegetation with in the lower reaches of the watershed-ecosystem, when compared with pa tterns in the high-elevation spruce-fir zone. Abiotic control mechanis ms were evident through longitudinal variations in soil cation exchang e capacity (related to soil organic matter) and soil/till depth, and t emporal and disturbance-related variations in inputs of strong-acid an ions. Marked differences in the K cycle were evident at the HBEF for t he periods 1964-69 and 1987-92. These changes included decreases in bi omass storage, net mineralization and throughfall fluxes and increased resorption in the latter period. These patterns seem to reflect an ec osystem response to decreasing rates of biomass accretion during the s tudy. Clearcutting disturbance resulted in large losses of K in stream water and from the removal of harvest products. Stream losses occur f rom release from slash, decomposition of soil organic matter and displ acement from cation exchange sites. Elevated concentrations of K persi st in stream water for many years after clearcutting. Of the major ele ments, K shows the slowest recovery from clearcutting disturbance.