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The pace and degree of nutrient limitation are among the most critical uncertainties in predicting terrestrial ecosystem responses to global change. In the northeastern United States, forest growth has recently declined along with decreased soil calcium (Ca) availability, suggesting that acid rain has depleted soil Ca to the point where it may be a limiting nutrient. However, it is unknown whether the past 60 y of changes in Ca availability are strictly anthropogenic or partly a natural consequence of long-term ecosystem development. Here, we report a high-resolution millennial-scale record of Ca and 16 other elements from the sediments of Mirror Lake, a 15-ha lake in the White Mountains of New Hampshire surrounded by northern hardwood forest. We found that sedimentary Ca concentrations had been declining steadily for 900 y before regional Euro-American settlement. This Ca decline was not a result of serial episodic disturbances but instead the gradual weathering of soils and soil Ca availability. As Ca availability was declining, nitrogen availability concurrently was increasing. These data indicate that nutrient availability on base-poor, parent materials is sensitive to acidifying processes on millennial timescales. Forest harvesting and acid rain in the postsettlement period mobilized significant amounts of Ca from watershed soils, but these effects were exacerbated by the long-term pattern. Shifting nutrient limitation can potentially occur within 10,000 y of ecosystem development, which alters our assessments of the speed and trajectory of nutrient limitation in forests, and could require reformulation of global models of forest productivity.

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.

Monitoring solutes in precipitation inputs and stream water exports at small watersheds has greatly advanced our understanding of biogeochemical cycling. Surprisingly, although inputs to and outputs from ecosystems are instrumental to understanding sources and sinks of nutrients and other elements, uncertainty in these fluxes is rarely reported in ecosystem budgets. We illustrate error propagation in input-output budgets by comparing the net hydrologic flux of Ca in a harvested and reference watershed at the Hubbard Brook Experimental Forest, New Hampshire. We identify sources of uncertainty and use a Monte Carlo approach to combine many sources of uncertainty to produce an estimate of overall uncertainty. Sources of uncertainty in precipitation inputs included in this study were: rain gage efficiency (undercatch or overcatch), gaps in measurements of precipitation volume, selection of a model for interpolating among rain gages, unusable precipitation chemistry, and chemical analysis. Sources of uncertainty in stream water outputs were: stage height-discharge relationship, watershed area, gaps in the stream flow record, chemical analysis, and the selection of a method for flux calculation. The annual net hydrologic flux of Ca in the harvested and reference watersheds was calculated from 1973 through 2009. Relative to the reference watershed, the harvested watershed showed a marked increase in Ca flux after it was cut in 1983-1984, and slowly declined toward pretreatment levels thereafter. In 2009, the last year evaluated, the 95% confidence intervals for the annual estimates approach the 95% confidence intervals of the pretreatment regression line, suggesting that the increased net loss of Ca in the harvested watershed may soon be indistinguishable from the reference. Identifying the greatest sources of uncertainty can be used to guide improvements, for example in reducing instances of unusable precipitation chemistry and gaps in stream runoff. Our results highlight the value of estimating uncertainty in watershed studies, including those in which replication is impractical. [ABSTRACT FROM AUTHOR]

The interaction between human activities and watershed geology is accelerating long-term changes in the carbon cycle of rivers. We evaluated changes in bicarbonate alkalinity, a product of chemical weathering, and tested for long-term trends at 97 sites in the eastern United States draining over 260 000 km². We observed statistically significant increasing trends in alkalinity at 62 of the 97 sites, while remaining sites exhibited no significant decreasing trends. Over 50% of study sites also had statistically significant increasing trends in concentrations of calcium (another product of chemical weathering) where data were available. River alkalinization rates were significantly related to watershed carbonate lithology, acid deposition, and topography. These three variables explained ~40% of variation in river alkalinization rates. The strongest predictor of river alkalinization rates was carbonate lithology. The most rapid rates of river alkalinization occurred at sites with highest inputs of acid deposition and highest elevation. The rise of alkalinity in many rivers throughout the Eastern U.S. suggests human-accelerated chemical weathering, in addition to previously documented impacts of mining and land use. Increased river alkalinization has major environmental implications including impacts on water hardness and salinization of drinking water, alterations of air-water exchange of CO2, coastal ocean acidification, and the influence of bicarbonate availability on primary production. [ABSTRACT FROM AUTHOR]

Disturbance of forest ecosystems, such as that caused by harvesting or acid deposition, is thought to alter the ability of the ecosystem to retain nutrients. Although many watershed studies have suggested depletion of available calcium (Ca) pools, interpretation of ecosystem Ca mass balance has been limited by the difficulty in obtaining mineral weathering flux estimates. While many studies have suggested that weathering flux is insufficient to maintain available soil pools, measurements of concomitant changes in available soil pools are rare. Here, we critically examined application of the Ca:Na ratio method in interpreting the long-term Ca budget of six northern hardwood watershed ecosystems at the Hubbard Brook Experimental Forest, Woodstock, New Hampshire, USA. Storage of sodium (Na) in biomass and secondary minerals and on cation exchange sites was low enough so that net ecosystem Na loss was essentially equivalent to mineral weathering flux. Mineral chemistry and mass balance considerations constrained the Ca:Na ratio of weathering products to a sufficiently narrow range that spatial and temporal changes in the ecosystemanipulated watersheds with aggrading biomass compared to three reference watersheds with relatively mature forest conditions. Although accelerated loss of Ca in the first few years following disturbance has been documented by prior studies, this study suggests that excess Ca loss continues for at least three decades after treatment, with no evident trend toward the conditions shown in the reference watershed. It is not likely that changes in previously quantified Ca pools account for this sustained loss, suggesting that a previously unstudied Ca pool or release mechanism may be important in ecosystem response to disturbance. Possible sources include Ca oxalate, which is known to accumulate in forest soils, but has not been considered in the context of an ecosystem mass balance. [ABSTRACT FROM AUTHOR]

Examines the biogeochemical responses of forest streams to a 2-month calcium addition in Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Decline of the upstream concentration of aluminum; Increase of hydrogen-ion concentration in the buffered stream; Positive relation of uptakes rates to acidity.

The Hubbard Brook Ecosystem Study officially began on 1 June 1963. This archive contains the results of 50 yr of collection and analysis of (at least) weekly stream water and precipitation samples obtained during the period 1963-2014 (from 1 June 1963 to 30 May 2013). Stream chemistry for the nine gauged watersheds and precipitation chemistry for precipitation gauges distributed throughout the Hubbard Brook Experimental Forest are reported as concentrations in (mg/L). [ABSTRACT FROM AUTHOR]

Anion fluxes from a forest soil are usually correlated with those ofbase cations (BC). Declines in base cation deposition or long-term depletion from the soil may change these relationships. We used multiple regression to identify biogeochemical variables predicting annual volume-weighted concentrations of BC in streamwater draining a forested watershed, and analysis of variance to compare the effects of Ca and Cl inputs on BC fluxes out of soil horizons in irrigated plots. For the watershed, anion concentrations in streamwater predicted BC export most precisely (R2 = 0.84). The best two-variable model (adjusted R2 = 0.91) also included BC concentration in bulk deposition. Consistent with predictions from equations governing exchange chemistry, the proportion of charge contributed by Ca2+ increased with increasing total anion concentration, while that of Na+ decreased. At the plot scale, Cl- concentrations in treatment solutions had a stronger effect (p = 0.06) onBC concentration in Oa-horizon solutions than did Ca2+ concentrations (p = 0.33). In individual horizons of individual plots, BC and total ion concentrations were correlated, but cation composition was not consistent within horizons from different plots. This study detected no evidence of long-term cation depletion in the soils controlling streamwater, but did detect extremely base-poor plots. Because acid deposition affects surface horizons first, streamwater chemistry may not be an adequate way to assess nutrient supply of forest soils. [ABSTRACT FROM AUTHOR]