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.