Abstract. The biogeochemical behavior of carbon in the forested watersheds of the Hubbard Brook Experimental Forest (HBEF) was analyzed in long-term studies. The largest pools of C in the reference watershed (W6) reside in mineral soil organic matter (43% of total ecosystem C) and living biomass (40.5%), with the remainder in surface detritus (14.5%). Repeated sampling indicated that none of these pools was changing significantly in the late-1990s, although high spatial variability precluded the detection of small changes in the soil organic matter pools, which are large; hence, net ecosystem productivity (NEP) in this 2nd growth forest was near zero (± about 20 g C/m 2 -yr) and probably similar in magnitude to fluvial export of organic C. Aboveground net primary productivity (ANPP) of the forest declined by 24% between the late-1950s (462 g C/m 2 -yr) and the late-1990s (354 g C/m 2 -yr), illustrating age-related decline in forest NPP, effects of multiple stresses and unusual tree mortality, or both. Application of the simulation model PnET-II predicted 14% higher ANPP than was observed for 1996-1997, probably reflecting some unknown stresses. Fine litterfall flux (171 g C/m 2 -yr) has not changed much since the late-1960s. Because of high annual variation, C flux in woody litterfall (including tree mortality) was not tightly constrained but averaged about 90 g C/m 2 -yr. Carbon flux to soil organic matter in root turnover (128 g C/m 2 -yr) was only about half as large as aboveground detritus. Balancing the soil C budget requires that large amounts of C (80 g C/m 2 -yr) were transported from roots to rhizosphere carbon flux. Total soil respiration (TSR) ranged from 540 to 800 g C/m 2 -yr across eight stands and decreased with increasing elevation within the northern hardwood forest near W6. The watershedwide TSR was estimated as 660 g C/m 2 -yr. Empirical measurements indicated that 58% of TSR occurred in the surface organic horizons and that root respiration comprised about 40% of TSR, most of the rest being microbial. Carbon flux directly associated with other heterotrophs in the HBEF was minor; for example, we estimated respiration of soil microarthropods, rodents, birds and moose at about 3, 5, 1 and 0.8 g C/m 2 -yr, respectively, or in total less than 2% of NPP. Hence, the effects of other heterotrophs on C flux were primarily indirect, with the exception of occasional 2 -yr) were small, larger quantities of C were transported within the ecosystem and a more substantial fraction of dissolved C was transported from the soil as inorganic C and evaded from the stream as CO 2 (4.0 g C/m 2 -yr). Carbon pools and fluxes change rapidly in response to catastrophic disturbances such as forest harvest or major windthrow events. These changes are dominated by living vegetation and dead wood pools, including roots. If biomass removal does not accompany large-scale disturbance, the ecosystem is a large net source of C to the atmosphere (500-1200 g C/m 2 -yr) for about a decade following disturbance and becomes a net si...

Forest decline in the northeastern United States has been linked to the effects of acid deposition on soil nutrients. To test this link, we added a calcium silicate mineral to a paired watershed at the Hubbard Brook Experimental Forest, New Hampshire, in an amount designed to gradually replace the estimated amount of calcium lost as a result of human activity in the 20th Century (primarily because of acid deposition). The experimental restoration resulted in a recovery of tree biomass increment. The improved calcium nutrition also promoted higher aboveground net primary production and increased the photosynthetic surface area in the treated watershed relative to that in the reference watershed. These results demonstrated that soil acidification accelerated by acid deposition has contributed to the decline of forest growth and health on naturally acidic soil in the northeastern United States and that decline can be reversed by the addition of calcium.

Globally significant increases in the riverine delivery of nutrients and suspended particulate matter have occurred with deforestation. We report here significant increases in streamwater transport of dissolved silicate (DSi) following experimental forest harvesting at the Hubbard Brook Experimental Forest, NH, USA. The magnitude of the streamwater response varied with the type of disturbance with the highest DSi export fluxes occurring in the manipulations that left the most plant materials on the soil surface and disturbed the soil surface least. No measurable loss of amorphous silica (ASi) was detected from the soil profile; however, ASi was redistributed within the soil profile after forest disturbance. Mass-balance calculations demonstrate that some fraction of the DSi exported must come from dissolution of ASi and export as DSi. Land clearance and the development of agriculture may result in an enhanced flux of DSi coupled with enhanced erosion losses of ASi contained in phytoliths.

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We conducted a resurvey of the O horizon in 2001 in watersheds previously sampled in 1984 under the Direct/Delayed Response Program (DDRP) to evaluate the effects of reductions in acidic deposition in the northeastern United States. In this 17‐yr interval, median base saturation in the Oa horizon decreased from 56.2% in 1984 to 33.0% in 2001. Effective cation exchange capacity (CECe), normalized to soil C concentration, showed no significant change between 1984 and 2001. The change in base saturation was the result of almost equivalent changes in C‐normalized exchangeable Ca (CaN) and exchangeable Al (AlN). The median CaN declined by more than 50%, from 23.5 to 10.6 cmolc kg−1 C, while median AlN more than doubled, from 8.8 to 21.3 cmolc kg−1 C. We observed the greatest change in soil acid–base properties in the montane regions of Central New England (CNE) and Maine, where base saturation decreased by more than 50% and median soil pH in 0.01 M CaCl2 (pHs) decreased from 3.19 to 2.97. Changes in median concentrations of other exchangeable cations were either statistically insignificant (MgN, KN) or very small (NaN). We observed no significant change in the median values of either total soil C content (%C) or total soil N content (%N) over the 17‐yr interval. The acidification of the Oa horizon between 1984 and 2001 occurred despite substantial reductions in atmospheric acidic deposition. Our results may help to explain the surprisingly slow rate of recovery of surface waters.

We quantified the dynamics of the tree stratum at Hubbard Brook Experimental Forest (HBEF), New Hampshire, to examine why live biomass reached a plateau in about 1980. Total aboveground biomass increased from 209 Mg·ha–1 in 1981 to 216 Mg·ha–1 in 2001. From 1991 to 2001, in-growth of ≥10 cm diameter at breast height (DBH) trees averaged 4.7 trees·ha–1·year –1 with a corresponding in-growth biomass of 0.29 Mg·ha–1·year–1. Mortality of trees ≥10 cm DBH averaged 5.3 trees·ha–1·year–1 (1.12% of trees·year–1). Dying trees represented 2.24 Mg·ha–1·year–1 of aboveground biomass from 1991 to 2001. The biomass pools of standing dead, snags, and coarse woody debris in this forest currently are near steady state with residence times of 7.5, 15, and 6.2 years, respectively. The plateau in live biomass was mostly associated with lower wood production. Aboveground net primary productivity was estimated at 6.53 Mg·ha–1·year–1 (3.28 Mg·ha–1·year–1 for aboveground woody tissues and 3.25 Mg·ha–1·year–1 for leaf production), considerably lower than published estimates for the 1956–1965 period at the HBEF. Net ecosystem productivity in this young, second-growth forest is near zero, indicating that it may not be a sink for carbon.

Changes in lake water chemistry between 1984 and 2001 at 130 stratified random sites across the northeastern United States were studied to evaluate the population-level effects of decreases in acidic deposition. Surface-water SO4 2- concentrations decreased across the region at a median rate of −1.53 μequiv L-1 year-1. Calcium concentrations also decreased, with a median rate of −1.73 μequiv L-1 year-1. This decrease in Ca2+ retarded the recovery of surface water acid neutralizing capacity (Gran ANC), which increased at a median rate of 0.66 μequiv L-1 year-1. There were small increases in pH in all subregions except central New England and Maine, where the changes were not statistically significant. Median NO3 - trends were not significant except in the Adirondacks, where NO3 - concentrations increased at a rate of 0.53 μequiv L-1 year-1. A regionwide decrease in the concentration of total Al, especially in ponds with low ANC values (ANC < 25 μequiv L-1), was observed in the Adirondack subregion. These changes in Al were consistent with the general pattern of increasing pH and ANC. Despite the general pattern of chemical recovery, many ponds remain chronically acidic or are susceptible to episodic acidification. The continued chemical and biological recovery at sites in the northeastern United States will depend on further controls on S and N emissions.

Liming has been used to mitigate effects of acidic deposition in forest ecosystems. This study was designed to examine the effects of calcium (Ca) supply on the spatial patterns and the relations between soil and soil solution chemistry in a base-poor forest watershed. Watershed 1 at the Hubbard Brook Experimental Forest in New Hampshire, USA was experimentally treated with wollastonite (CaSiO 3 ) in October, 1999. Exchangeable Ca (Ex-Ca), soil pH s (in 0.01M CaCl 2 ), effective cation exchange capacity (CEC e ), and effective base saturation (BS e ) increased, while exchangeable acidity (Ex-Acid) decreased in organic soil horizons in 2000 and 2002.Mineral soils experienced either small increases in Ex-Ca, pH s , CEC e , BS e , small decreases in Ex-Acid or no changes. Thus, most of the added Ca remained in the forest floor during the study period. Prior to the treatment the BS e decreased with increasing elevation in organic and mineral soil horizons. This spatial pattern changed significantly in the forest floor after the treatment, suggesting that soils at higher elevations were more responsive to the chemical addition than at lower elevations. Soil solutions draining the forest floor responded to the treatment by increases in concentrations of Ca, dissolved silica, pH, and acid neutralizing capacity (ANC), and a decrease in inorganic monomeric Al (Al i ). Treatment effects diminished with increasing soil depth and decreasing elevation. Positive correlations between Ca/Al m in soil solution and ExCa/Ex-Al ratios in soil indicated that changes in the chemistry of soils significantly influenced the chemistry of soil water, and that Ca derived from the dissolution of wollastonite mitigated the mobilization of Al within the experimental watershed.