Abstract. A synthesis of the biogeochemistry of K was conducted during in the reference and human-manipulated watershed-ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. Results showed that during the first two years of the study , which coincided with a drought period, the reference watershed was a net sink for atmospheric inputs of K. During the remaining years, this watershed has been a net source of K for downstream ecosystems. There have been long-term declines in volume-weighted concentration and flux of K at the HBEF; however, this pattern appears to be controlled by the relatively large inputs during the initial drought years. Net ecosystem loss (atmospheric deposition minus stream outflow) showed an increasing trend of net loss, peaking during the mid-1970s and declining thereafter. This pattern of net K loss coincides with trends in the drainage efflux of SO: and NO;, indicating that concentrations of strong acid anions may be important controls of dissolved K loss from the site. There were no long-term trends in streamwater concentration or flux of K. A distinct pattern in pools and fluxes of K was evident based on biotic controls in the upper ecosystem strata (canopy, boles, forest floor) and abiotic controls in lower strata of the ecosystem (mineral soil, glacial till). This biological control was manifested through higher concentrations and fluxes of K in vegetation, aboveground litter, throughfall and forest floor pools and soil water in the northern hardwood vegetation within the lower reaches of the watershedecosystem, when compared with patterns in the high-elevation spruce-fir zone. Abiotic control mechanisms were evident through longitudinal variations in soil cation exchange capacity (related to soil organic matter) and soil/till depth, and temporal and disturbance-related variations in inputs of strong-acid anions. Marked differences in the K cycle were evident at the HBEF for the periods 1964-69 and 1987-92. These changes included decreases in biomass storage, net mineralization and throughfall fluxes and increased resorption in the latter period. These patterns seem to reflect an ecosystem response to decreasing rates of biomass accretion during the study. Clearcutting disturbance resulted in large losses of K in stream water and from the removal of harvest products. Stream losses occur from release from slash, decomposition of soil organic matter and displacement from cation exchange sites. Elevated concentrations of K persist in stream water for many years after clearcutting. Of the major elements, K shows the slowest recovery from clearcutting disturbance.

The depletion of calcium in forest ecosystems of the northeastern USA is thought to be a consequence of acidic deposition and to be at present restricting the recovery of forest and aquatic systems now that acidic deposition itself is declining. This depletion of calcium has been inferred from studies showing that sources of calcium in forest ecosystems namely, atmospheric deposition and mineral weathering of silicate rocks such as plagioclase, a calcium-sodium silicate do not match calcium outputs observed in forest streams. It is therefore thought that calcium is being lost from exchangeable and organically bound calcium in forest soils. Here we investigate the sources of calcium in the Hubbard Brook experimental forest, through analysis of calcium and strontium abundances and strontium isotope ratios within various soil, vegetation and hydrological pools. We show that the dissolution of apatite (calcium phosphate) represents a source of calcium that is comparable in size to known inputs from atmospheric sources and silicate weathering. Moreover, apatite-derived calcium was utilized largely by ectomycorrhizal tree species, suggesting that mycorrhizae may weather apatite and absorb the released ions directly, without the ions entering the exchangeable soil pool. Therefore, it seems that apatite weathering can compensate for some of the calcium lost from base-poor ecosystems, and should be considered when estimating soil acidification impacts and calcium cycling.

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In the 1970s, the U.S. Congress passed legislation restricting the sale of gasoline with alkyl‐lead additives. In the intervening years, the amount of lead (Pb) consumed in gasoline has declined sharply, resulting in lower rates of atmospheric Pb deposition. At the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, the input of Pb in precipitation has declined by 97% between 1976 and 1989. The purpose of this paper is to evaluate the long‐term response of forest and associated aquatic ecosystems to these declining inputs of Pb. Lead continues to accumulate in the forest ecosystem of the HBEF, in spite of lower inputs, due to extremely low losses in drainage water. However, between 1977 and 1987 the Pb content in the forest floor declined by 29%. Lead now appears to be accumulating in the mineral soil. The stratigraphy of Pb in sediment from a nearby lake shows continued Pb deposition, though at a declining rate. Much of our understanding of the long‐term patterns of lead accumulation in ecosystems has been shaped by paleoecological studies, in which inputs of lead are assumed to be irreversibly retained. Using a regression model and historical information concerning Pb consumption in gasoline, we estimated Pb inputs to the HBEF ecosystem during the period 1926‐1989. Based on this analysis and our field observations, it is clear that Pb mobility at the HBEF is greater than previously reported: °30% of the total atmospheric Pb input between 1926 and 1987 was not retained in organic soils or pond sediments. The potential for Pb pollution in drainage water may therefore be greater than suspected in regions with continued high rates of atmospheric Pb deposition.

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Summary Negative correlations between soil pH and cation exchange capacity (CEC) or base saturation in soils of the northeastern USA and Scandinavia have raised questions regarding the nature of cation exchange in acid forest soils. Using data from three small‐catchment studies and an extensive regional survey of soils in the northeastern USA, I examined relationships among total carbon, effective CEC (CECe), soil pHs (in 0.01 m CaCl2) and base saturation. Organic matter is the predominant source of soil surface charge in these coarse‐grained, glacially derived soils. Correlation coefficients (r) between total carbon and CECe ranged from 0.43 to 0.74 in organic horizons and from 0.46 to 0.83 in mineral horizons. In all cases, the intercepts of functional relations between CECe and total C were near zero. In O horizons, the CECe per unit mass of organic carbon (CECe:C) was positively correlated with pHs in three of the four data sets, consistent with the weak‐acid behaviour of the organic matter. However, CECe:C was negatively correlated with pHs in mineral soils in two data sets, and uncorrelated in the other two. The CECe in mineral soils represents the portion of total CEC not occupied by organically bound Al. The negative correlations between CECe:C and pHs can therefore be explained by increased Al binding at higher pHs. Aluminium behaves like a base cation in these soils. When Al was considered a base cation, the relation between base saturation and pHs could be effectively modelled by the extended Henderson–Hasselbalch equation. When modelled without Al as a base cation, however, there were no consistent relationships between pHs and base saturation across sites or soil horizons. Because of the non‐acidic behaviour of Al, it is difficult to predict the effect of ongoing reductions in acid deposition on the base status of soils in the northeastern USA.