Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest

Soil–atmosphere fluxes of trace gases (especially nitrous oxide (N₂O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N₂O, methane (CH₄) and carbon dioxide (CO₂) in field chambers monthly (bi-weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO₂ and CH₄ showed a strong seasonal pattern, with low rates in winter, but N₂O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N₂O flux and decreased CH₄ uptake in both treatment years, especially during winter. Annual N₂O fluxes in sugar maple treatment plots were 207 and 99 mg N m⁻² yr⁻¹ in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N₂O or CO₂ fluxes, but CH₄ uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N₂O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N₂O fluxes from northern hardwood forests.     

Snow depth, soil freezing and nitrogen cycling in a northern hardwood forest landscape

Increases in soil freezing associated with decreases in snow cover have been identified as a significant disturbance to nitrogen (N) cycling in northern hardwood forests. We created a range of soil freezing intensity through snow manipulation experiments along an elevation gradient at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains, NH USA in order to improve understanding of the factors regulating freeze effects on nitrate (NO₃ ^?) leaching, nitrous oxide (N₂O) flux, potential and in situ net N mineralization and nitrification, microbial biomass carbon (C) and N content and respiration, and denitrification. While the snow manipulation treatment produced deep and persistent soil freezing at all sites, effects on hydrologic and gaseous losses of N were less than expected and less than values observed in previous studies at the HBEF. There was no relationship between frost depth, frost heaving and NO₃ ^? leaching, and a weak relationship between frost depth and winter N₂O flux. There was a significant positive relationship between dissolved organic carbon (DOC) and NO₃ ^? concentrations in treatment plots but not in reference plots, suggesting that the snow manipulation treatment mobilized available C, which may have stimulated retention of N and prevented treatment effects on N losses. While the results support the hypothesis that climate change resulting in less snow and more soil freezing will increase N losses from northern hardwood forests, they also suggest that ecosystem response to soil freezing disturbance is affected by multiple factors that must be reconciled in future research.     

Effects of mild winter freezing on soil nitrogen and carbon dynamics in a northern hardwood forest

Overwinter and snowmelt processes are thought to be critical to controllersof nitrogen (N) cycling and retention in northern forests. However, therehave been few measurements of basic N cycle processes (e.g.mineralization, nitrification, denitrification) during winter and littleanalysis of the influence of winter climate on growing season N dynamics.In this study, we manipulated snow cover to assess the effects of soilfreezing on in situ rates of N mineralization, nitrification and soilrespiration, denitrification (intact core, C₂H₂ – based method),microbial biomass C and N content and potential net N mineralization andnitrification in two sugar maple and two yellow birch stands with referenceand snow manipulation treatment plots over a two year period at theHubbard Brook Experimental Forest, New Hampshire, U.S.A. The snowmanipulation treatment, which simulated the late development of snowpackas may occur in a warmer climate, induced mild (temperatures >–5 °C) soil freezing that lasted until snowmelt. The treatmentcaused significant increases in soil nitrate (NO₃ ^–)concentrations in sugar maple stands, but did not affect mineralization,nitrification, denitrification or microbial biomass, and had no significanteffects in yellow birch stands. Annual N mineralization and nitrificationrates varied significantly from year to year. Net mineralization increasedfrom 12.0 g N m^–2 y^–1 in 1998 to 22 g N m^–2 y^–1 in 1999 and nitrification increased from 8 g N m^–2 y^–1 in 1998 to 13 g N m^–2 y^–1 in 1999.Denitrification rates ranged from 0 to 0.65 g N m^–2 y^–1. Ourresults suggest that mild soil freezing must increase soil NO₃ ^– levels by physical disruption of the soil ecosystem and not by direct stimulation of mineralization and nitrification. Physical disruption canincrease fine root mortality, reduce plant N uptake and reduce competitionfor inorganic N, allowing soil NO₃ ^– levels to increase evenwith no increase in net mineralization or nitrification.     

Earthworms, litter and soil carbon in a northern hardwood forest

The important role of soil carbon (C) in the global C cycle has stimulated interest in better understanding the mechanisms regulating soil C storage and its stabilization. Exotic earthworm invasion of northern forest soils in North America can affect soil C pools, and we examined their effects on these mechanisms by adding ¹³C labeled leaf litter to adjacent northern hardwood forests with and without earthworms. Two types of labeled litter were produced, one with the ¹³C more concentrated in structural (S) components and the other in non-structural (NS) components, to evaluate the role of biochemical differences in soil C stabilization. Earthworm invasions have reduced soil C storage in the upper 20 cm of the soil profile by 37 %, mostly by eliminating surface organic horizons. Despite rapid mixing of litter into mineral soil and its incorporation into aggregates, mineral soil C has not increased in the presence of earthworms. Incorporation of litter C into soil and microbial biomass was not affected by biochemical differences between S versus NS labeled litter although NS litter C was assimilated more readily into earthworm biomass and S litter C into fungal hyphae. Apparently, the net effect of earthworm mixing of litter and forest floor C into mineral soil, plus stabilization of that C in aggregates, is counterbalanced by earthworm bioturbation and possible priming effects. Our results support recent arguments that biochemical recalcitrance is not a major contributor to the stabilization of soil C.     

Effects of soil frost on soil respiration and its radiocarbon signature in a Norway spruce forest soil

Apart from a general increase of mean annual air temperature, climate models predict a regional increase of the frequency and intensity of soil frost with possibly strong effects on C cycling of soils. In this study, we induced mild soil frost (up to −5 °C in a depth of 5 cm below surface) in a Norway spruce forest soil by removing the natural snow cover in the winter of 2005/2006. Soil frost lasted from January to April 2006 and was detected down to 15 cm depth. Soil frost effectively reduced soil respiration in the snow removal plots in comparison to undisturbed control plots. On an annual basis 6.2 t C ha⁻¹ a⁻¹ were emitted in the control plots compared with 5.1 t C ha⁻¹ a⁻¹ in the snow removal plots. Only 14% of this difference was attributed to reduced soil respiration during the soil frost period itself, whereas 63% of this difference originated from differences during the summer of 2006. Radiocarbon (Δ¹⁴C) signature of CO₂ revealed a considerable reduction of heterotrophic respiration on the snow removal plots, only partly compensated for by a slight increase of rhizosphere respiration. Similar CO₂ concentrations in the uppermost mineral horizons of both treatments indicate that differences between the treatments originated from the organic horizons. Extremely low water contents between June and October of 2006 may have inhibited the recovery of the heterotrophic organisms from the frost period, thereby enhancing the differences between the control and snow removal plots. We conclude that soil frost triggered a change in the composition of the microbial community, leading to an increased sensitivity of heterotrophic respiration to summer drought. A CO₂ pulse during thawing, such as described for arable soils several times throughout the literature, with the potential to partly compensate for reduced soil respiration during soil frost, appears to be lacking for this soil. Our results from this experiment indicate that soil frost reduces C emission from forest soils, whereas mild winters may enhance C losses from forest soils.     

Environmental control of fine root dynamics in a northern hardwood forest

Understanding how exogenous and endogenous factors control the distribution, production and mortality of fine roots is fundamental to assessing the implications of global change, yet our knowledge of control over fine root dynamics remains rudimentary. To improve understanding of these processes, the present study developed regression relationships between environmental variables and fine root dynamics within a northern hardwood forest in New Hampshire, USA, which was experimentally manipulated with a snow removal treatment. Fine roots (< 1 mm diameter) were observed using minirhizotrons for 2 years in sugar maple and yellow birch stands and analyzed in relation to temperature, water and nutrient availability. Fine root dynamics at this site fluctuated seasonally, with growth and mortality peaking during warmer months. Monthly fine root production was strongly associated with mean monthly air temperature and neither soil moisture nor nutrient availability added additional predictive power to this relationship. This relationship exhibited a seasonal temperature hysteresis, which was altered by snow removal treatment. These results suggest that both exogenous and endogenous cues may be important in controlling fine root growth in this system. Proportional fine root mortality was directly associated with mean monthly soil temperature, and proportional fine root mortality during the over‐winter interval was strongly related to whether the soil froze. The strong relationship between fine root production and air temperature reported herein contrasts with findings from some hardwood forest sites and indicates that controls on fine root dynamics vary geographically. Future research must more clearly distinguish between endogenous and exogenous control over fine root dynamics in various ecosystems.     

Soil freezing alters fine root dynamics in a northern hardwood forest

The retention of nutrients within an ecosystem depends on temporal andspatial synchrony between nutrient availability and nutrient uptake, anddisruption of fine root processes can have dramatic impacts on nutrientretention within forest ecosystems. There is increasing evidence thatoverwinter climate can influence biogeochemical cycling belowground,perhaps by disrupting this synchrony. In this study, we experimentallyreduced snow accumulation in northern hardwood forest plots to examinethe effects of soil freezing on the dynamics of fine roots (< 1 mm diameter)measured using minirhizotrons. Snow removal treatment during therelatively mild winters of 1997–1998 and 1998–1999 induced mild freezingtemperatures (to –4 °C) lasting approximately three months atshallow soil depths (to –30 cm) in sugar maple and yellow birch stands.This treatment resulted in elevated overwinter fine root mortality in treatedcompared to reference plots of both species, and led to an earlier peak infine root production during the subsequent growing season. These shiftsin fine root dynamics increased fine root turnover but were not largeenough to significantly alter fine root biomass. No differences inmorality response were found between species. Laboratory tests on pottedtree seedlings exposed to controlled freezing regimes confirmed that mildfreezing temperatures (to –5 °C) were insufficient to directlyinjure winter-hardened fine roots of these species, suggesting that themarked response recorded in our forest plots was caused indirectly bymechanical damage to roots in frozen soil. Elevated fine root necromass intreated plots decomposed quickly, and may have contributed an excess fluxof about 0.5 g N/m²·yr, which is substantial relative tomeasurements of N fluxes from these plots. Our results suggest elevatedoverwinter mortality temporarily reduced fine root length in treatmentplots and reduced plant uptake, thereby disrupting the temporalsynchrony between nutrient availability and uptake and enhancing ratesof nitrification. Increased frequency of soil freezing events, as may occurwith global change, could alter fine root dynamics within the northernhardwood forest disrupting the normally tight coupling between nutrientmineralization and uptake.     

Winter soil frost conditions in boreal forests control growing season soil CO2 concentration and its atmospheric exchange

The impact of changes in winter soil frost regime on soil CO₂ concentration and its atmospheric exchange in a boreal Norway spruce forest was investigated using a field‐scale soil frost manipulation experiment. The experiment comprised three treatments: deep soil frost, shallow soil frost and control plots (n= 3). Winter soil temperatures and soil frost distribution were significantly altered by the different treatments. The average soil CO₂ concentrations during the growing season were significantly lower in plots with deep soil frost than in plots with shallow soil frost. The average CO₂ soil–atmosphere exchange rate exhibited the same pattern, and differences in soil respiration rates among the treatments were statistically significant. Both the variation in soil CO₂ concentration and the CO₂ soil–atmosphere exchange rate could statistically be explained by the differences in the maximum soil frost depth during the previous winter. A response model for growing season soil respiration rates suggests that every 1 cm change in winter soil frost depth will change the emission rates by ca. 0.01 g CO₂ m^?2 day^?1, corresponding to 0.2–0.5% of the estimated net ecosystem productivity (NEP). This suggests that the soil frost regime has a significant influence on the C balance of the system, because interannual variations in soil frost up to 60 cm have been recorded at the site. We conclude that winter climate conditions can be important in controlling C balances in northern terrestrial ecosystems, and also that indirect effects of the winter season must be taken into account, because these can affect the prevailing conditions during the growing season.     

Chemical changes in soil and soil solution after calcium silicate addition to a northern hardwood forest

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 (CaSiO₃) in October, 1999. Exchangeable Ca (Ex-Ca), soil pH_s (in 0.01 M CaCl₂), 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/total monomeric Al (Al_m) in soil solution and Ex-Ca/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.     

Stemflow nutrient inputs to soil in a successional hardwood forest

Stemflow and throughfall from a regenerating (8-year-old) southern Appalachian hardwood forest were collected to examine the relative importance of tree bole nutrient leaching in response to acid deposition. Samples from nine (2 m²) stemflow collection plots were analyzed for four dormant season and 11 growing season rainstorm events. Results showed that, relative to throughfall fluxes, stemflow accounted, on average, for approximately 8.5% of total water reaching the forest floor during both dormant and growing season storms. Relative to foliar leaching, K-, SO₄-, and PO₄ ions appear to be the most easily leached ions from young tree stems. Proportional nitrate and base cation stemflow fluxes increased significantly (p<0.05) with growing-season storm-event duration, suggesting that the stemsurface nutrient pool is depleted by precipitation more slowly than the foliar pool. On average, proportional stemflow fluxes of SO₄ (12%) and K (14%) were consistently higher than reported maximum values for more mature forest stands, which indicates that small-scale stemflow inputs of ions such as these to the forest floor may be important in early successional ecosystems.     

Colder soils in a warmer world: A snow manipulation study in a northern hardwood forest ecosystem

In this special section of Biogeochemistry, we present results from asnow manipulation experiment in the northernhardwood forest ecosystem at the Hubbard BrookExperimental Forest in the White Mountains ofNew Hampshire, U.S.A. Snow is important as aninsulator of forest soils. Later developmentof snowpacks, as may occur in a warmer climate,may result in increases in soil freezing (i.e.colder soils in a warmer world) and could causechanges in fine root and microbial mortality,hydrologic and gaseous losses of nitrogen (N),and the acid-base status of drainage water. Inour study, we kept soils snow free by shovelinguntil early February during the mild winters of1997/1998 and 1998/1999. The treatment producedmild, but persistent soil freezing and inducedsurprisingly significant effects on rootmortality, soil nitrate (NO₃ ^–) levelsand hydrologic fluxes of C, N and P. In thisspecial section we present four papersaddressing, (1) soil temperature and moistureresponse to our snow manipulation treatment(Hardy et al.), (2) theresponse of fine root dynamics to treatment(Tierney et al.), (3) theresponse of soil inorganic N levels, insitu N mineralization and nitrification,denitrification and microbial biomass to thetreatment (Groffman et al.)and (4) soil solution concentrations and fluxesof C, N and P (Fitzhugh et al.). In this introductory paper we: (1)review the literature on snow effects on forestbiogeochemistry, (2) introduce our manipulationexperiment and (3) summarize the resultspresented in the other papers in this issue.     

Organic matter chemistry and dynamics in clear-cut and unmanaged hardwood forest ecosystems

Forest harvesting alters the organic matter cycle by changing litter inputs and the decomposition regime. We hypothesized that these changes would result in differences in organic matter chemistry between clear-cut and uncut watershed ecosystems. We studied the chemistry of soil organic matter (SOM), and dissolved organic carbon (DOC) in soil solutions and stream samples in clear-cut and uncut sites at the Hubbard Brook Experimental Forest in New Hampshire using DOC fractionation techniques and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy.Alkyl-C (aliphatic compounds) and O-alkyl-C (carbohydrates) were the largest C fractions in soil and dissolved organic matter at Hubbard Brook. Alkyl-C ranged from 29–48% of soil C, 25–42% of soil solution C, and 22–42% of streamwater DOC. Carbohydrates comprised 32–49%, 36–43%, and 29–60% of C in soils, solutions, and streamwater, respectively. In both soils and soil solutions, the carbohydrate fraction decreased with increasing soil depth, while the aromaticity of organic matter increased with depth. There were no significant differences in the structural chemistry of SOM between clear-cut and uncut watersheds.The aromatic-C fractions in soil solutions at the clear-cut site ranged from 12–16%, approximately 40% greater than at the uncut site (8.5–11%). Thus, clear-cutting has resulted in the leaching of more highly decomposed organic matter, and depletion of more aliphatic compounds in the soluble organic pool. Because DOC fluxes are small compared to the SOM pool, large differences in soil solution chemistry do not substantially alter the overall composition of SOM. While the organic chemistry of stream DOC varied greatly among 3 sampling dates, there were no obvious clear-cutting effects. Thus, temporal variations in flowpaths and/or in-stream processes appear to be more important than disturbance in regulating the organic carbon chemistry of these streams.     

Effects of soil frost on nitrogen net mineralization, soil solution chemistry and seepage losses in a temperate forest soil

Freezing and thawing may alter element turnover and solute fluxes in soils by changing physical and biological soil properties. We simulated soil frost in replicated snow removal plots in a mountainous Norway spruce stand in the Fichtelgebirge area, Germany, and investigated N net mineralization, solute concentrations and fluxes of dissolved organic carbon (DOC) and of mineral ions (NH₄⁺, NO₃⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺). At the snow removal plots the minimum soil temperature was −5 °C at 5 cm depth, while the control plots were covered by snow and experienced no soil frost. The soil frost lasted for about 3 months and penetrated the soil to about 15 cm depth. In the 3 months after thawing, the in situ N net mineralization in the forest floor and upper mineral soil was not affected by soil frost. In late summer, NO₃⁻ concentrations increased in forest floor percolates and soil solutions at 20 cm soil depth in the snow removal plots relative to the control. The increase lasted for about 2–4 months at a time of low seepage water fluxes. Soil frost did not affect DOC concentrations and radiocarbon signatures of DOC. No specific frost effect was observed for K⁺, Ca²⁺ and Mg²⁺ in soil solutions, however, the Na⁺ concentrations in the upper mineral soil increased. In the 12 months following snowmelt, the solute fluxes of N, DOC, and mineral ions were not influenced by the previous soil frost at any depth. Our experiment did not support the hypothesis that moderate soil frost triggers solute losses of N, DOC, and mineral ions from temperate forest soils.     

Dynamics of oxidized and reduced iron in a northern hardwood forest

Iron (Fe) is ubiquitous in forest ecosystems and its cycle is thought to influence the development of soil, particularly Spodosols (podsolization), and the biogeochemistry of macronutrients such as carbon (C), nitrogen (N), and phosphorus (P), as well as many trace metals. The cycle of Fe in northern hardwood forests remains poorly understood. To address some of these uncertainties, we constructed a biogeochemical budget of Fe for a small catchment at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, USA. Horizonal, temporal, and elevational patterns of concentrations and fluxes of oxidized and reduced Fe species were assessed in leaf litter, soil, soil solution, and stream water. The chemistry of dissolved Fe was evaluated in the context of its relationship with dissolved organic carbon, pH, and dissolved oxygen. Soil solution fluxes of Fe were highest in the organic (Oa, 52.5 mol ha^?1 year^?1) horizon and decreased with depth in the mineral (Bh, 50.5 mol ha^?1 year^?1, and Bs, 19.7 mol ha^?1 year^?1) horizons, consistent with podsolization theories predicting immobilization of Fe following downward transport to mineral soils. The export of Fe in stream water (1.8 mol ha^?1 year^?1) was lower than precipitation input (3.5 mol ha^?1 year^?1). The low stream flux indicates most Fe in drainage waters was immobilized in the soil and retained in the watershed. The portion of total Fe as Fe(II) was ~10?C60% in soil solutions, seemingly high for soils that are considered to be well-drained, oxidizing environments. Organic complexes likely stabilized Fe(II) in solution under oxidizing conditions that would otherwise promote considerably higher Fe(III)-to-Fe(II) ratios. Our study indicates that there are organic matter-derived sources of dissolved Fe(II) as well as substantial mobilization of Fe(II), possibly the result of the reduction of Fe-bearing soil minerals.     

季节性雪被对高山生态系统土壤氮转化的影响

在高山生态系统中,季节性雪被对土壤氮含量及转化有着重大影响.降雪是氮沉降的一种重要形式,直接影响着土壤中的有效氮含量;降雪形成不同厚度和持续期的雪被后,造成环境因子(土壤温度和含水量)和生物因子(土壤微生物、高山植物和高山动物)的异质性,进而对土壤中氮素矿化和微生物固持过程产生复杂的影响.本文重点介绍了持续性雪被消融期冻融交替影响土壤氮素矿化和流失的机制,并针对高山地区未来季节性雪被可能发生的变化,综述了野外原位模拟实验的主要研究成果,最后提出了开展季节性雪被对土壤氮影响研究的一些建议.     

Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest

Variation in soil temperature can account for most of the seasonal and diel variation in soil CO₂ efflux, but the temperature effect is not always consistent, and other factors such as soil water content are known to influence soil respiration. The objectives of this research were to study the spatial and temporal variation in soil respiration in a temperate forested landscape and to evaluate temperature and soil water functions as predictors of soil respiration. Soil CO₂ fluxes were measured with chambers throughout an annual cycle in six study areas at the Harvard Forest in central Massachusetts that include soil drainage classes from well drained to very poorly drained. The mean annual estimate of soil CO₂ efflux was 7.2 Mg ha^–1, but ranged from 5.3 in the swamp site to 8.5 in a well-drained site, indicating that landscape heterogeneity is related to soil drainage class. An exponential function relating CO₂ fluxes to soil temperature accounted for 80% of the seasonal variation in fluxes across all sites (Q₁₀ = 3.9), but the Q₁₀ ranged from 3.4 to 5.6 for the individual study sites. A significant drought in 1995 caused rapid declines in soil respiration rates in August and September in five of the six sites (a swamp site was the exception). This decline in CO₂ fluxes correlated exponentially with decreasing soil matric potential, indicating a mechanistic effect of drought stress. At moderate to high water contents, however, soil water content was negatively correlated with soil temperature, which precluded distinguishing between the effects of these two confounded factors on CO₂ flux. Occurrence of high Q₁₀ values and variation in Q₁₀ values among sites may be related to: (i) confounding effects of high soil water content; (ii) seasonal and diel patterns in root respiration and turnover of fine roots that are linked to above ground phenology and metabolism; and (iii) variation in the depth where CO₂ is produced. The Q₁₀ function can yield reasonably good predictions of annual fluxes of CO₂, but it is a simplification that masks responses of root and microbial processes to variation in temperature and water content throughout the soil.     

Factors controlling long-term changes in soil pools of exchangeable basic cations and stream acid neutralizing capacity in a northern hardwood forest ecosystem

Understanding the factors regulating the concentrations of basic cations in soils and surface waters is critical if rates of recovery are to be predicted in response to decreases in acidic deposition. Using a dynamic simulation model (PnET-BGC), we evaluated the extent to which atmospheric deposition of strong acids and associated leaching by strong anions, atmospheric deposition of basic cations through changes in emissions of particulate matter, and historical forest cutting have influenced soil pools of exchangeable basic cations and the acid-base status of stream water at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Historical deposition of basic cations was reconstructed from regression relationships with particulate matter emissions. Simulation results indicate that the combination of these factors has resulted in changes in the percent soil base saturation, and stream pH and acid neutralizing capacity (ANC) from pre-industrial estimates of 20%, 6.3 and 45 eq L^–1, respectively, to current values of 10%, 5.0 and –5 eq L^–1, respectively. These current values fall within the critical thresholds at which forest vegetation and aquatic biotic are at risk from soil and surface water acidification due to acidic deposition. While the deposition of strong acid anions had the largest impact on the acid-base status of soil and stream water, the reduction in deposition of basic cations associated with reductions in particulate emissions was estimated to have contributed about 27% of the depletion in soil Ca²⁺ exchange pool and 15% of the decreases in stream water concentrations of basic cations. Decline in stream water concentrations of basic cation occurred under both increasing and decreasing exchangeable pools, depending on the process controlling the acid base status of the ecosystem. Model calculations suggest that historical forest cutting has resulted in only slight decreases in soil pools of exchangeable basic cations, and has had a limited effect on stream ANC over the long-term.