Forests of the northeastern US have experienced multiple environmental changes over the last three decades, including rising temperatures and atmospheric CO2 concentrations and declines in acidic deposition, including nitrogen (N) deposition. We used measurements over a 32-year time span (1992–2023) from the Hubbard Brook Experimental Forest, New Hampshire, USA to assess the overall impact of these environmental changes on foliar chemistry (N, carbon, and phosphorus concentrations) and isotopic composition (δ15N and δ13C) of three dominant northern hardwood species: sugar maple, American beech, and yellow birch. Intrinsic water use efficiency estimated from foliar δ13C increased for all three species in response to rising atmospheric CO2 concentrations. Foliar N concentrations showed large declines after peaks during 2001–2006. These responses are likely driven by the combination of rising atmospheric CO2 and declining N deposition, but we cannot distinguish between these drivers. Foliar δ15N and N:P ratios declined over the study period for sugar maple but not for American beech and birch. Foliar C concentrations declined for all three species, but larger relative declines in foliar N than C led to increases in foliar C:N ratios, which could further slow N mineralization rates and amplify N limitation in forests where these impacts are present.

European spongy moth (Lymantria dispar dispar, LDD) is a major outbreaking defoliator in the northeastern US whose outbreaks are expected to increase in frequency and intensity with climate change. During LDD outbreaks, substantial defoliation reduces forest productivity and tree growth and increases tree mortality. However, the patterns of LDD-induced defoliation and their environmental drivers are not well quantified at large scales, limiting our capacity to predict future outbreaks and to design forest management practices. We developed a novel, scalable method to map detailed defoliation patterns across New York state from 2020 to 2023 based on satellite estimates of the rapid decline in canopy greenness induced by LDD during the peak growing season. Spatiotemporal analysis revealed that the extent of defoliation increased with decreasing precipitation and increasing temperature anomalies during the previous growing season but not the current season. Neither mean climate nor forest type explained the spatial patterns of defoliation. Our results provide novel regional evidence that dry and hot climatic anomalies during the growing season promote the risk of LDD outbreak and canopy defoliation the following year, consistent with reported climatic controls on pathogen infection rates of LDD. This early warning signal of LDD outbreak can be leveraged to improve the modeling and management of LDD outbreaks in northeastern forests in future climatic conditions.

Analyses of a Swedish tree-ring archive suggest that increased atmospheric carbon dioxide has lowered soil-nitrogen supply, which could cap carbon storage by land ecosystems. Analyses of a Swedish tree-ring archive suggest that increased atmospheric carbon dioxide has lowered soil-nitrogen supply, which could cap carbon storage by land ecosystems.

Abstract. Nitrogen availability constrains the terrestrial carbon uptake and storage, yet large uncertainties remain in the magnitude of the effect, because the interactions of the carbon and nitrogen (N) dynamics are challenging to observe in undisturbed ecosystems at relevant timescales. Long-term experiments with 15N tracer applications allow study of the nitrogen cycle in a fairly undisturbed manner, and they are therefore a valuable data source to test the biogeochemical dynamics simulated by terrestrial biosphere models. In this study we applied the model QUINCY (QUantifying Interactions between Terrestrial Nutrient CYcles and the climate system), which includes an explicit representation of terrestrial 15N fluxes and pools. We used observations from a long-term (10-year) 15N tracer experiment in a temperate deciduous forest to evaluate the nitrogen dynamics simulated by QUINCY. Recovery in soil N dominated overall ecosystem 15N recovery in both observations and simulations over the long-term. The observed gradual movement of the 15N tracer to lower soil layers was also captured by the model. However, in the short-term modeled uptake and losses of 15N into leaves and fine roots were too fast, and recovery in litter and surface soil was too slow, indicating that the model likely overestimates plant competitiveness for newly added N relative to soil microbes. Downward vertical transport of 15N tracer in the soil was slow compared to measurements, which may be indicative either of too low bioturbation or vertical transport via leaching. Overall, the QUINCY model results showed good agreement with the observations making it a valuable tool to study long term nitrogen dynamics. Running the model for an extended period for example indicated that the ecosystem retained a very large share of the added 15N tracer (>90 %), and that this retention persisted at multi-decadal timescales. This study shows that explicit inclusion of isotopic tracers allows for a more profound evaluation of carbon-nitrogen turnover and dynamics and thereby can contribute to reduce uncertainties in modelling nitrogen flow and constrains in terrestrial ecosystems.

Soil respiration (Rsoil) is the second largest terrestrial carbon (C) flux, and therefore, it is imperative to understand and quantify its responses to global environmental change. Rsoil consists of two component CO2 fluxes: autotrophic respiration from the metabolic activity of roots (Ra-root) and heterotrophic respiration (Rh) derived from the metabolic activity of mycorrhizal fungi and microbial decomposition of detritus, soil organic matter, and rhizodeposits. Increased nitrogen (N) availability often reduces Rsoil in forest ecosystems, but it remains unclear which contributing fluxes govern Rsoil responses and if suppression of Rsoil results from increased N availability itself or from the tendency of added N to acidify soil. Here, we address these uncertainties in a long-term, large-scale factorial N × pH experiment in six temperate forest stands in central New York, USA. We anticipated that increasing soil N availability would decrease plant belowground C allocation and related root-associated respiration and that soil acidification would suppress microbial decomposition, thereby reducing Rh. We found that both acidifying and deacidifying N additions suppressed annual Rsoil by 19% and 13%, respectively (-1.8 Mg C ha-1 year-1 overall), but acidification (from pH 4.67 to 4.22) alone did not detectably affect this flux. Annual Rsoil decreased steeply (R2 = 0.66, p < 0.001) as soil N availability increased. Nitrogen additions generally suppressed Rh, especially in the forest floor (-34%), whereas the effects of acidification alone varied by soil depth, with substantial suppression in the forest floor (-33%) partially offset by stimulation at depth. A novel partitioning of Rsoil component responses suggests that N additions suppressed root-associated respiration by ~1.1 Mg C ha-1 year-1 (62% of the Rsoil suppression), while acidification alone had no effect. Our findings demonstrate that soil N availability, not soil pH, is the predominant biogeochemical control over Rsoil in these temperate forests, with larger responses of plant-driven C fluxes than microbial-driven C fluxes.

Photosynthesis links terrestrial carbon, water and nutrient cycles. Photosynthetic least-cost theory suggests that plants optimize photosynthesis at the lowest summed investments in nutrient and water use. The theory predicts that increasing nutrient availability should increase nutrient allocation toward photosynthetic enzymes and reduce stomatal conductance, allowing similar photosynthetic rates achieved at a lower ratio of leaf intercellular to atmospheric CO2 concentration (χ) and reduced water loss. The theory suggests similar responses to increasing soil pH in acidic soils due to common correlations between soil pH and nutrient availability. However, empirical tests of the theory outside of environmental gradients are rare. To test this theory experimentally, we measured photosynthetic traits in mature Acer saccharum Marshall trees growing in a 9-year, nitrogen-by-pH manipulation in the northeastern USA. Increasing soil nitrogen availability did not affect net photosynthesis (Anet) or stomatal conductance (gs) rates, but was associated with increased area-based leaf nitrogen content (Narea), increased photosynthetic capacity (Vcmax, Jmax) and decreased χ (i.e, increased water-use efficiency). These patterns strengthened the tradeoff between nitrogen and water use, indicated by steeper slopes of Narea-χ and Vcmax-χ with increasing soil nitrogen availability. When examined across all plots, soil pH had no effect on any traits. However, in plots without nitrogen additions, increasing soil pH increased the slopes of Narea-χ and Vcmax-χ, though did not modify χ. Supporting the theory, A. saccharum maintained Anet across the soil nitrogen availability gradient by trading less efficient nitrogen use for more efficient water use. Additionally, the effects of soil pH on nitrogen-water use tradeoffs appear to occur through indirect pH effects on soil nitrogen availability. These results indicate that elevated nitrogen deposition could stimulate photosynthesis less than commonly expected and instead reduce water losses, and conversely, that reductions in photosynthesis expected from increasing nitrogen limitation in some regions could be lessened if accompanied by increased transpiration.

Fine roots and root-associated carbon (C) inputs contribute disproportionally to soil C stocks. Here, we quantified fine root dynamics in a mixed northern hardwood forest at the Woods Lake Watershed in the Adirondack Park, NY, USA, where an experimental lime application in 1989 led to the near-doubling of forest floor organic matter stocks two decades later. Prior work linked this organic matter accumulation with lower heterotrophic respiration and decreased abundances of major fungal saprotrophs and ectomycorrhizal fungi. We investigated whether liming-driven shifts in fine root dynamics and depth distribution also contributed to forest floor accumulation. Forest floor mass in limed plots again roughly doubled that in unlimed plots 32 years after liming, with persistently large accumulations in the Oa horizon. Liming decreased fine root production across all horizons measured (Oe, Oa, 0–10 cm mineral soil) and by 40% overall, with largest effects in the Oa horizon (−45%) where root turnover was also reduced (−59%). Thus, liming decreased, rather than increased, root detrital C inputs to the forest floor. These results suggest that liming must have suppressed decomposition even more than shown previously or increased some other C input to the forest floor to explain its substantial C accumulation.

The role of manganese (Mn) in ecosystem carbon (C) biogeochemical cycling is gaining increasing attention. While soil Mn is mainly derived from bedrock, atmospheric deposition could be a major source of Mn to surface soils, with implications for soil C cycling. However, quantification of the atmospheric Mn cycle, which comprises emissions from natural (desert dust, sea salts, volcanoes, primary biogenic particles, and wildfires) and anthropogenic sources (e.g., industrialization and land-use change due to agriculture), transport, and deposition, remains uncertain. Here, we use compiled emission data sets for each identified source to model and quantify the atmospheric Mn cycle by combining an atmospheric model and in situ atmospheric concentration measurements. We estimated global emissions of atmospheric Mn in aerosols (<10 mu m in aerodynamic diameter) to be 1,400 Gg Mn year(-1). Approximately 31% of the emissions come from anthropogenic sources. Deposition of the anthropogenic Mn shortened Mn "pseudo" turnover times in 1-m-thick surface soils (ranging from 1,000 to over 10,000,000 years) by 1-2 orders of magnitude in industrialized regions. Such anthropogenic Mn inputs boosted the Mn-to-N ratio of the atmospheric deposition in non-desert dominated regions (between 5 x 10(-5) and 0.02) across industrialized areas, but that was still lower than soil Mn-to-N ratio by 1-3 orders of magnitude. Correlation analysis revealed a negative relationship between Mn deposition and topsoil C density across temperate and (sub)tropical forests, consisting with atmospheric Mn deposition enhancing carbon respiration as seen in in situ biogeochemical studies.

Historic harvesting and mortality from air pollution drastically reduced the abundance of red spruce (Picea rubens), a late-successional dominant of cool-temperate forests of the northeastern U.S. and southeastern Canada, leaving few opportunities to understand the natural growth and disturbance responses of this species. Timbers salvaged from the Moosilauke Ravine Lodge, a structure built from trees harvested in the late 1930s, provided an opportunity to reconstruct radial growth patterns and dynamics of a former old-growth red spruce stand located in Jobildunc Ravine on Mount Moosilauke in the White Mountains of New Hampshire. Ravine Lodge tree-ring series were compared with data from a 255-year-old red spruce found living in Jobildunc Ravine, from the Nancy Brook site in the White Mountains, and from other dendroecological studies across the region. Ring counts provide minimum tree ages of 187-286 years for timbers from Jobildunc Ravine, suggesting they established between the mid-Seventeenth and mid-Eighteenth Centuries. Dendroecological analyses identified early decades of suppression in the understory followed by 2-5 growth releases and 2-4 growth declines for each sample, indicating occasional, small-scale disturbances of the canopy before the 1930s. A growth decline in 1834-1835 coincides with an outbreak of spruce budworm (Choristoneura fumiferana) in eastern Canada, perhaps reflecting a regional defoliation event that occurred as far south as Mount Moosilauke. This study illustrates the insights that can be gained from wood from historic structures on the dynamics of now-scarce old-growth red spruce forests.

Manganese (Mn) is a key cofactor in enzymes responsible for lignin decay (mainly Mn peroxidase), regulating the rate of litter degradation and carbon (C) turnover in temperate and boreal forest biomes.While soil Mn is mainly derived from bedrock, atmospheric Mn could also contribute to soil Mn cycling, especially within the surficial horizon, with implications for soil C cycling. However, quantification of the atmospheric Mn cycle, which comprises emissions from natural (desert dust, sea salts, volcanoes, primary biogenic particles, and wildfires) and anthropogenic sources (e.g. industrialization and land-use change due to agriculture) transport, and deposition into the terrestrial and marine ecosystem, remains uncertain. Here, we use compiled emission datasets for each identified source to model and quantify the atmospheric Mn cycle with observational constraints. We estimated global emissions of atmospheric Mn in aerosols (<10 µm in aerodynamic diameter) to be 1500 Gg Mn yr-1. Approximately 32% of the emissions come from anthropogenic sources. Deposition of the anthropogenic Mn shortened soil Mn “pseudo” turnover times in surficial soils about 1-m depth (ranging from 1,000 to over 10,000,000 years) by 1-2 orders of magnitude in industrialized regions. Such anthropogenic Mn inputs boosted the Mn-to-N ratio of the atmospheric deposition in non-desert dominated regions (between 5×10-5 and 0.02) across industrialized areas, but still lower than soil Mn-to-N ratio by 1-3 orders of magnitude. Correlation analysis revealed a negative relationship between Mn deposition and topsoil C density across temperate and (sub)tropical forests, illuminating the role of Mn deposition in these ecosystems.

The effects of atmospheric nitrogen (N) deposition on forests ecosystems depend on the fate of N in the ecosystem, and the role of tree canopy in intercepting, transforming, and assimilating atmospheric N inputs. In this study, increased in N deposition was simulated with the application of N fertilizer with two contrasting strategies: above and below the canopy, in a sessile oak (Quercus petraea (Matt.) Liebl.) stand. Three weeks after the labeled fertilization, more than twice as much fertilizer was recovered in plants in the above-canopy application (12 +/- 5 %), than in the below-canopy application (5 +/- 2 %). By contrast, the litter layer on the forest floor retained more fertilizer in the below-canopy (37 +/- 8%) than in the above-canopy (7 +/- 5 %) application. Seven months later, more fertilizer was recovered in wood and fine roots in below-canopy (8 +/- 1 %) than in above-canopy (4 +/- 1 %) fertilization. Wood recovery was higher in the below-canopy (6.7 +/- 2.7 % vs 3.6 +/- 0.6 % in the above-canopy). This experiment shows that fertilization approach alters the short-term fate of the added N in a broadleaved forest, highlighting the importance of the forest canopy in intercepting atmospheric N deposition also in these widely represented forests.

Earth System Models (ESMs) have implemented nitrogen (N) cycles to account for N limitation on terrestrial carbon uptake. However, representing inputs, losses, and recycling of N in ESMs is challenging. Here, we use global rates and ratios of key soil N fluxes, including nitrification, denitrification, mineralization, leaching, immobilization, and plant uptake (both NH4 + and NO3 - ), from the literature to evaluate the N cycles in the land model components of two ESMs. The two land models evaluated here, E3SM Land Model version 1 (ELMv1)-ECA and CLM5.0, originated from a common model but have diverged in their representation of plant-microbe competition for soil N. The models predict similar global rates of gross primary productivity (GPP) but have approximately two-fold to three-fold differences in their underlying global mineralization, immobilization, plant N uptake, nitrification, and denitrification fluxes. Both models dramatically underestimate the immobilization of NO3 - by soil bacteria compared with literature values and predict dominance of plant uptake by a single form of mineral nitrogen (NO3 - for ELM, with regional exceptions, and NH4 + for CLM5.0). CLM5.0 strongly underestimates the global ratio of gross nitrification:gross mineralization and both models are likely to substantially underestimate the ratio of nitrification:denitrification. Few experimental data exist to evaluate this last ratio, in part because nitrification and denitrification are quantified using different techniques and because denitrification fluxes are difficult to measure at all. More observational constraints on soil nitrogen fluxes such as nitrification and denitrification, as well as greater scrutiny of the functional impact of introducing separate NH4 + and NO3 - pools into ESMs, could help to improve confidence in present and future simulations of N limitation on the carbon cycle.

Microbial community structure and function regularly covary with soil pH, yet effects of these interactions on soil carbon are rarely tested experimentally within natural ecosystems. We investigated the enduring (25 year) impacts of liming on microbial community structure and decomposition at an acidic northern hardwood forest, where experimental liming increased pH one unit and surprisingly doubled the organic carbon stocks of the forest floor. We show that this increase in carbon storage corresponded with restructuring of the bacterial and fungal communities that drive decomposition. In the Oe horizon, liming reduced the activities of five extracellular enzymes that mediate decomposition, while the Oa horizon showed an especially large (64%) reduction in the activity of a sixth, peroxidase, which is an oxidative enzyme central to lignocellulose degradation. Decreased enzyme activities corresponded with loss of microbial taxa important for lignocellulose decay, including large reductions in the dominant ectomycorrhizal genera Russula and Cenococcum, saprotrophic and wood decaying fungi, and Actinobacteria (Thermomonosporaceae). These results demonstrate the importance of pH as a dominant regulator of microbial community structure and illustrate how changes to this structure can produce large, otherwise unexpected increases in carbon storage in forest soils.

Models of terrestrial system dynamics often include nitrogen (N) cycles to better represent N limitations on terrestrial carbon (C) uptake, but simulating the fate of N in ecosystems has proven challenging. Here, key soil N fluxes and flux ratios from the Community Land Model version 5.0 (CLM5.0) are compared with an extensive set of observations from the Hubbard Brook Forest Long-Term Ecological Research site in New Hampshire. Simulated fluxes include microbial immobilization and plant uptake, which compete with nitrification and denitrification, respectively, for available soil ammonium (NH4 + ) and nitrate (NO3 - ). In its default configuration, CLM5.0 predicts that both plant uptake and immobilization are strongly dominated by NH4 + over NO3 - , and that the model ratio of nitrification:denitrification is ~1:1. In contrast, Hubbard Brook observations suggest that NO3 - plays a more significant role in plant uptake and that nitrification could exceed denitrification by an order of magnitude. Modifications to the standard CLM5.0 at Hubbard Brook indicate that a simultaneous increase in the competitiveness of nitrifying microbes for NH4 + and reduction in the competitiveness of denitrifying bacteria for NO3 - are needed to bring soil N flux ratios into better agreement with observations. Such adjustments, combined with evaluation against observations, may help to improve confidence in present and future simulations of N limitation on the C cycle, although C fluxes, such as gross primary productivity and net primary productivity, are less sensitive to the model modifications than soil N fluxes.

Soil microbial community composition routinely correlates with pH, reflecting both direct pH effects on microbial physiology and long-term biogeochemical feedbacks. We used two watershed-scale liming experiments to identify short- (2 years) and long-term (25 years) changes in the structure and function of bacterial and fungal communities in organic horizons (Oe and Oa ) of acid forest soils. Liming increased soil pH, extractable calcium, and soil carbon stocks, reduced biomass-specific respiration, and caused major changes in the soil microbiome in the short and long term. More taxa responded to liming in the short term (70%) than in the long term (30%), with most showing consistent directional responses at both sites. The ratio of change in relative abundance between limed and reference sites was twofold higher at the long than the short-term site, indicating that the effects of liming grew over time. Liming impacts were most pronounced in fungi, as steep declines of dominant ectomycorrhizal fungi (Cenococcum and Russula) occurred at both sites. Liming favoured neutrophilic bacteria over acidophilic populations according to estimated environmental pH optima. Collectively, these results demonstrate that a liming-induced change of one pH unit has an immediate and persistent effect on the structure and function of microbial communities in acid forest soils. The corresponding suppression of respiration indicates that anthropogenic alterations of soil pH, as driven by acid deposition or liming, can affect forest floor C stocks due to pH-driven shifts in community structure.

Climate warming and management will likely affect carbon (C) fluxes of montane grassland ecosystems. In this study, we assessed the effect of simultaneous warming (+2°C) and decreased precipitation (−25%) on carbon exchange of montane grasslands in S‐Germany by translocating large intact plant‐soil cores from a high altitude to a low altitude site. Cores received two common grassland management regimes: intensive (4–5 cuts and slurry application) and extensive (1–2 cuts and slurry application). Diurnal patterns of net ecosystem exchange (NEE) and total ecosystem respiration (Reco) were measured over 1.5 years in 2–3 weeks intervals during the snow free period. Additional data on environmental controls, that is, photosynthetic active radiation, grass height and soil moisture and temperature, were used to develop empirical models to estimate daily and annual fluxes of gross primary production (GPP) and Reco. Considering the 2 years period (2014 and 2015), we found that, under warmer and slightly drier conditions, both GPP and Reco significantly ( p < 0.01) increased (up to 20%) but with a higher temperature sensitivity of Reco, particularly in intensive managed grassland. The higher temperature sensitivity of Reco reduced the NEE by 0.7 t C ha −1 yr −1 for both extensive and intensive management, respectively. Considering additional carbon inputs via slurry and exports via harvest (i.e., annual net ecosystem carbon budget), our results showed that managed grasslands are already a source of C under current climate conditions (1.7–1.8 t ha −1 yr −1 ) which significantly ( p < 0.05) increased under climate warming (2.3–2.9 t ha −1 yr −1 ).

Global chronic nitrogen (N) deposition to forests can alleviate ecosystem N limitation, with potentially wide ranging consequences for biodiversity, carbon sequestration, soil and surface water quality, and greenhouse gas emissions. However, the ability to predict these consequences requires improved quantification of hard-to-measure N fluxes, particularly N gas loss and soil N retention. Here we combine a unique set of long-term catchment N budgets in the central Europe with ecosystem 15 N data to reveal fundamental controls over dissolved and gaseous N fluxes in temperate forests. Stream leaching losses of dissolved N corresponded with nutrient stoichiometry of the forest floor, with stream N losses increasing as ecosystems progress towards phosphorus limitation, while soil N storage increased with oxalate extractable iron and aluminium content. Our estimates of soil gaseous losses based on 15 N stocks averaged 2.5 ± 2.2 kg N ha −1 yr −1 and comprised 20% ± 14% of total N deposition. Gaseous N losses increased with forest floor N:P ratio and with dissolved N losses. Our relationship between gaseous and dissolved N losses was also able to explain previous 15 N-based N loss rates measured in tropical and subtropical catchments, suggesting a generalisable response driven by nitrate (NO 3 − ) abundance and in which the relative importance of dissolved N over gaseous N losses tended to increase with increasing NO 3 − export. Applying this relationship globally, we extrapolated current gaseous N loss flux from forests to be 8.9 Tg N yr −1 , which represent 39% of current N deposition to forests worldwide.

Elevated inputs of reactive nitrogen (Nr) have large and often detrimental effects on the environment and the magnitude and type of effects depend on the fate of Nr, such as water (NO3) and air (N2O) pollution. Research has shown that soils are the primary sink of Nr in temperate forest ecosystems. Redox conditions, in conjunction with soil composition, stand to have an important but poorly constrained impact on the fate of this reactive N. In this study, we used tracer-level additions to contrast the fate of N-15-NO3- in organic (Oa) and in iron oxide-rich mineral (B) horizons under oxic and suboxic conditions. We performed stirred-jar laboratory incubations (up to 26 days) using soils from an acidic mixed-hardwood forest. We followed the fate of the tracer N into soil and solution pools (dissolved organic matter, NO3-, and NH4+), and also monitored solution concentrations of redoxsensitive species (NO2-, Fe2+, Fe3+, Mn). At the end of the incubations, similar to 3-7 times more tracer was recovered in soil material from the highly organic Oa horizon (57 +/- 3% for oxic conditions and 73 +/- 3% for suboxic) than in soil material from the organic-poor mineral B horizon (20 +/- 5% for oxic, and 9 +/- 2% for suboxic). For the B horizon, some N-15-NO3- remained in solution (29 +/- 13%) under oxic conditions, while none remained under suboxic conditions (0.2 +/- 0.02%); the main fate of N-15-NO3- went unrecovered, presumably due to gaseous losses, with more lost under suboxic (86 +/- 3%) than oxic (47 +/- 7%) conditions. Apparent gaseous losses were substantial in the Oa horizons as well, amounting to 35 +/- 3% (oxic) and 12 +/- 4% (suboxic) of tracer additions. Transformation rates of N-15-NO3- were greater under suboxic conditions for both horizon soil materials. Under both redox conditions retention was reaction rate limited in Oa horizon material and capacity limited in B horizon material. Our results indicate that in these acidic soils organic matter (OM) drives retention of nitrate in soil under both oxic and suboxic conditions, likely also accompanied by microbial denitrification; while lack of OM in iron oxide-rich mineral soils leaves nitrate available for other fates, with especially large gaseous loss in suboxic conditions. Mechanistically, it is likely that reduction of NO3- to NO2- is largely biotically driven, while subsequent reactions of NO2- are competitive between retention (nitrosation and microbial immobilization) and gasification (denitrification, chemodenitrification, self-decomposition) reactions. Both soil characteristics and redox conditions are critical determinants of NO3- fate in temperate forest ecosystems.

Forest soils are important for retaining nitrogen (N), especially in areas where anthropogenic activities have led to historically high inputs of N. As forests age and their N demands for biomass accumulation decline, the capacity for N retention of soils may change as well, although little work has been done to further our understanding of this process. We conducted a mineral soil reciprocal transplant study in three northern hardwood forests of different ages (young, recently mature, and old growth) in New Hampshire, USA to determine how the retention of isotopically labeled nitrogen from leaf litter would differ depending on characteristics of the incubated soil’s origin and destination. After 18 months of incubating the soil bags below the 15N-labeled litter, we did not find retention of litter-derived N to be related to the age of the incubation site forest, but rather that it differed based on the origin of the incubated soil. We found that the soil C content was the strongest predictor of how much of the tracer was recovered in the transplanted soil bags. Furthermore, the C content of soils changed during incubation and tended to change in the direction of equilibrating with the soil C concentration of the incubation site. This finding suggests that site characteristics are important in determining soil C concentrations and consequently N retention capacities.