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Increasing rates of atmospheric deposition of nitrogen (N) present a novel threat to the biodiversity of terrestrial ecosystems. Many forests are particularly susceptible to excess N given their proximity to sources of anthropogenic N emissions. This study summarizes results of a 25‐yr treatment of an entire central Appalachian hardwood forest watershed via aerial applications of N with a focus on effects of added N on the cover, species richness, and composition of the herbaceous layer. Research was carried out on two watersheds of the Fernow Experimental Forest (FEF), West Virginia. The long‐term reference watershed at FEF (WS4) was used as a reference; WS3 was experimentally treated, receiving three aerial applications of N per year as (NH4)2SO4 totaling 35 kg N ha−1 yr−1, beginning in 1989. Cover of the herbaceous layer (vascular plants ≤1 m in height) was estimated visually in five circular 1‐m2 subplots within each of seven circular 400‐m2 sample plots spanning all aspects and elevations of each watershed. Sampling was carried out in early July of each of the following years: 1991, 1992, 1994, 2003, and 2009—2014, yielding 10 yr of data collected over a 23‐yr period. It was anticipated that the N treatment on WS3 would decrease species richness and alter herb layer composition by enhancing cover of a few nitrophilic species at the expense of numerous N‐efficient species. Following a period of minimal response from 1991 to 1994, cover of the herb layer increased substantially on N‐treated WS3, and remained high thereafter. There was also a coincidental decrease in herb layer diversity during this period, along with a sharp divergence in community composition between WS4 and WS3. Most changes appear to have arisen from unprecedented, N‐mediated increases of Rubus spp., which are normally associated with the high‐light environment of openings, rather than beneath intact forest canopies. These findings support the prediction that N‐mediated changes in the herbaceous layer of impacted forests are driven primarily by increases in nitrophilic species.

• Decades of atmospheric nitrogen (N) deposition in the northeastern USA have enhanced this globally important forest carbon (C) sink by relieving N limitation. While many N fertilization experiments found increased forest C storage, the mechanisms driving this response at the ecosystem scale remain uncertain. • Following the optimal allocation theory, augmented N availability may reduce belowground C investment by trees to roots and soil symbionts. To test this prediction and its implications on soil biogeochemistry, we constructed C and N budgets for a long-term, whole-watershed N fertilization study at the Fernow Experimental Forest, WV, USA. • Nitrogen fertilization increased C storage by shifting C partitioning away from belowground components and towards aboveground woody biomass production. Fertilization also reduced the C cost of N acquisition, allowing for greater C sequestration in vegetation. Despite equal fine litter inputs, the C and N stocks and C : N ratio of the upper mineral soil were greater in the fertilized watershed, likely due to reduced decomposition of plant litter. • By combining aboveground and belowground data at the watershed scale, this study demonstrates how plant C allocation responses to N additions may result in greater C storage in both vegetation and soil.

Nitrogen (N) additions have decreased species richness (S) in hardwood forest herbaceous layers, yet the functional mechanisms for these decreases have not been explicitly evaluated. We tested two hypothesized mechanisms, random species loss (RSL) and non-random species loss (NRSL), in the hardwood forest herbaceous layer of a long-term, plot-scale, fertilization experiment in the central Appalachian Mountains, USA. Using a random thinning algorithm, we simulated changes in species densities under RSL and compared the simulated densities to the observed densities among N-fertilized (+N), N-fertilized and limed (+N+L), and reference (REF) plots in regenerating forest stands. We found a lower S in the +N treatment across all survey years and determined that the reduction in S was a function of NRSL. Furthermore, non-random effects were observed in certain species, as they occurred at densities that were either higher or lower than expected due to RSL. Differential advantages were also observed among species between +N and +N+L treatments, suggesting that species responded to either the fertilization or acidification effects of N, though no consistent pattern emerged. Species nitrophily status was not a useful trait for predicting specific species losses, but was a significant factor when averaged across all treatments and sampling years. Our results provide strong evidence that declines in S in the forest herbaceous layer under N fertilization are due largely to NRSL and not simply a function of species rarity.

The structure and function of terrestrial ecosystems are maintained by processes that vary with temporal and spatial scale. This study examined temporal and spatial patterns of net nitrogen (N) mineralization and nitrification in mineral soil of three watersheds at the Fernow Experimental Forest, WV: 2 untreated watersheds and 1 watershed receiving aerial applications of N over a 25-year period. Soil was sampled to 5 cm from each of seven plots per watershed and placed in two polyethylene bags—one bag brought to the laboratory for extraction/analysis, and the other bag incubated in situ at a 5 cm depth monthly during growing seasons of 1993–1995, 2002, 2005, 2007–2014. Spatial patterns of net N mineralization and nitrification changed in all watersheds, but were especially evident in the treated watershed, with spatial variability changing non-monotonically, increasing then decreasing markedly. These results support a prediction of the N homogeneity hypothesis that increasing N loads will increase spatial homogeneity in N processing. Temporal patterns for net N mineralization and nitrification were similar for all watersheds, with rates increasing about 25–30% from 1993 to 1995, decreasing by more than 50% by 2005, and then increasing significantly to 2014. The best predictor of these synchronous temporal patterns across all watersheds was number of degree days below 19°C, a value similar to published temperature maxima for net rates of N mineralization and nitrification for these soils. The lack of persistent, detectable differences in net nitrification between watersheds is surprising because fertilization has maintained higher stream-water nitrate concentrations than in the reference watersheds. Lack of differences in net nitrification among watersheds suggests that N-enhanced stream-water nitrate following N fertilization may be the result of a reduced biotic demand for nitrate following fertilization with ammonium sulfate.

Increased availability of monomeric aluminum (Al³⁺) in forest soils is an important adverse effect of acidic deposition that reduces root growth and inhibits nutrient uptake. There is evidence that Al³⁺ exposure interferes with NO₃⁻uptake. If true for overstory trees, the reduction in stand demand for NO₃⁻ could increase NO₃⁻ discharge in stream water. These effects may also differ between species that tolerate different levels of soil acidity. To examine these ideas, we measured changes in relative uptake of NO₃⁻ and NH₄⁺ by six tree species in situ under increased soil Al³⁺ using a ¹⁵N-labeling technique, and measured soluble soil Al levels in a separate whole-watershed acidification experiment in the Fernow Experimental Forest (WV). When exposed to added Al³⁺, the proportion of inorganic N acquired as NO₃⁻ dropped 14% across species, but we did not detect a reduction in overall N uptake, nor did tree species differ in this response. In the long-term acidification experiment, we found that soluble soil Al was mostly in the free Al³⁺ form, and the concentration of Al³⁺ was ~65 µM higher (~250%) in the mineral soil of the acidified watershed vs. an untreated watershed. Thus, increased levels of soil Al³⁺ under acidic deposition cause a reduction in uptake of NO₃⁻ by mature trees. When our ¹⁵N uptake results were applied to the watershed acidification experiment, they suggest that increased Al³⁺ exposure could reduce tree uptake of NO₃⁻ by 7.73 kg N ha⁻¹ year⁻¹, and thus increase watershed NO₃⁻ discharge.

Nitrogen deposition in the northeastern US changed N availability in the latter part of the twentieth century, with potential legacy effects. However, long-term N cycle measurements are scarce. N isotopes in tree rings have been used as an indicator of N availability through time, but there is little verification of whether species differ in the strength of this signal. Using long-term records at the Fernow Experimental Forest in West Virginia, we examined the relationship between soil conditions, including net nitrification rates, and wood [delta].sup.15N in 2014, and tested the strength of correlation between tree ring [delta].sup.15N of four species and stream water NO.sub.3.sup.- loss from 1971 to 2000. Higher soil NO.sub.3.sup.- was weakly associated with higher wood [delta].sup.15N across species, and higher soil net nitrification rates were associated with higher [delta].sup.15N for Quercus rubra only. The [delta].sup.15N of Liriodendron tulipifera and Q. rubra, but neither Fagus grandifolia nor Prunus serotina, was correlated with stream water NO.sub.3.sup.-. L. tulipifera tree ring [delta].sup.15N had a stronger association with stream water NO.sub.3.sup.- than Q. rubra. Overall, we found only limited evidence of a relationship between soil N cycling and tree ring [delta].sup.15N, with a strong correlation between the wood [delta].sup.15N and NO.sub.3.sup.- leaching loss through time for one of four species. Tree species differ in their ability to preserve legacies of N cycling in tree ring [delta].sup.15N, and given the weak relationships between contemporary wood [delta].sup.15N and soil N cycle measurements, caution is warranted when using wood [delta].sup.15N to infer changes in the N cycle.

Nitrogen deposition in the northeastern US changed N availability in the latter part of the twentieth century, with potential legacy effects. However, long-term N cycle measurements are scarce. N isotopes in tree rings have been used as an indicator of N availability through time, but there is little verification of whether species differ in the strength of this signal. Using long-term records at the Fernow Experimental Forest in West Virginia, we examined the relationship between soil conditions, including net nitrification rates, and wood δ¹⁵N in 2014, and tested the strength of correlation between tree ring δ¹⁵N of four species and stream water NO₃⁻ loss from 1971 to 2000. Higher soil NO₃⁻ was weakly associated with higher wood δ¹⁵N across species, and higher soil net nitrification rates were associated with higher δ¹⁵N for Quercus rubra only. The δ¹⁵N of Liriodendron tulipifera and Q. rubra, but neither Fagus grandifolia nor Prunus serotina, was correlated with stream water NO₃⁻. L. tulipifera tree ring δ¹⁵N had a stronger association with stream water NO₃⁻ than Q. rubra. Overall, we found only limited evidence of a relationship between soil N cycling and tree ring δ¹⁵N, with a strong correlation between the wood δ¹⁵N and NO₃⁻ leaching loss through time for one of four species. Tree species differ in their ability to preserve legacies of N cycling in tree ring δ¹⁵N, and given the weak relationships between contemporary wood δ¹⁵N and soil N cycle measurements, caution is warranted when using wood δ¹⁵N to infer changes in the N cycle.

Changes in the nitrogen (N) status of forest ecosystems can directly and indirectly influence their carbon (C) sequestration potential by altering soil organic matter (SOM) decomposition, soil enzyme activity, and plant–soil interactions. However, model representations of linked C–N cycles and SOM decay are not well validated against experimental data. Here, we use extensive data from the Fernow Experimental Forest long-term whole-watershed N fertilization study to compare the response to N perturbations of two soil models that represent decomposition dynamics differently (first-order decay versus microbially explicit reverse Michaelis–Menten kinetics). These two soil models were coupled to a common vegetation model which provided identical input data. Key responses to N additions measured at the study site included a shift in plant allocation to favor woody biomass over belowground carbon inputs, reductions in soil respiration, accumulation of particulate organic matter (POM), and an increase in soil C:N ratios. The vegetation model did not capture the often-observed shift in plant C allocation with N additions, which resulted in poor predictions of the soil responses. We modified the parameterization of the plant C allocation scheme to favor wood production over fine-root production with N additions, which significantly improved the vegetation and soil respiration responses. Additionally, to elicit an increase in the soil C stocks and C:N ratios with N additions, as observed, we modified the decay rates of the POM in the soil models. With these modifications, both models captured negative soil respiration and positive soil C stock responses in line with observations, but only the microbially explicit model captured an increase in soil C:N. Our results highlight the need for further model development to accurately represent plant–soil interactions, such as rhizosphere priming, and their responses to environmental change.

Background and objectives: aggregation and structure play key roles in the water-holding capacity and stability of soils and are important for the physical protection and storage of soil carbon (C). Forest soils are an important sink of ecosystem C, though the capacity to store C may be disrupted by the elevated atmospheric deposition of nitrogen (N) and sulfur (S) compounds by dispersion of soil aggregates via acidification or altered microbial activity. Furthermore, dominant tree species and the lability of litter they produce can influence aggregation processes. Materials and methods: we measured water-stable aggregate size distribution and aggregate-associated organic matter (OM) content in soils from two watersheds and beneath four hardwood species at the USDA Forest Service Fernow Experimental Forest in West Virginia, USA, where one watershed has received (NH4)2SO4 fertilizer since 1989 and one is a reference/control of similar stand age. Bulk soil OM, pH, and permanganate oxidizable carbon (POXC) were also measured. Research highlights: fertilized soil exhibited decreased macro-aggregate formation and a greater proportion of smaller micro-aggregates or unassociated clay minerals, particularly in the B-horizon. This shift in aggregation to soil more dominated by the smallest (<53 µm) fraction is associated with both acidification (soil pH) and increased microbially processed C (POXC) in fertilized soil. Intra-aggregate OM was also depleted in the fertilized soil (52% less OM in the 53–2000 µm fractions), most strongly in subsurface B-horizon soil. We also document that tree species can influence soil aggregation, as soil beneath species with more labile litter contained more OM in the micro-aggregate size class (<250 µm), especially in the fertilized watershed, while species with more recalcitrant litter promoted more OM in the macro-aggregate size classes (500–2000 µm) in the reference watershed. Conclusions: long-term fertilization, and likely historic atmospheric deposition, of forest soils has weakened macro-aggregation formation, with implications for soil stability, hydrology, and storage of belowground C.

Nitrogen additions have caused species composition changes in many ecosystems by facilitating the growth of nitrophilic species. After 24 years of nitrogen fertilization in a 40 year-old stand at the Fernow Experimental Forest (FEF) in Central Appalachia, USA, the cover of Rubus spp. has increased from 1 to 19 % of total herbaceous-layer cover. While Rubus spp. are generally associated with high-light conditions that are created after a disturbance event, some species are also known to be nitrophilic. We investigated whether the increase in cover in Rubus spp. was due to either nitrogen, light, or an interaction between these two factors. To test for the effect of nitrogen and light on Rubus spp. cover, we compared the relative cover of Rubus spp. among fertilized and unfertilized watersheds and among fertilized and unfertilized experimental plots, using estimates of canopy openness as a covariate. Rubus spp. plants were also grown ex situ in a field experiment using a 2-way factorial design, measuring leaf area, and using two levels of nitrogen and three levels of light. The effect of nitrogen fertilization on relative Rubus spp. cover depended on canopy openness in the watersheds (F = 17.57, p = 0.0002) and experimental plots (F = 25.04, p = 0.0047). A similar effect for leaf area was also observed among plants grown in the field experiment (F = 4.12, p = 0.0247). Our results confirm that, although Rubus spp. at FEF are nitrophilic, they require sufficient light to increase their cover. Furthermore, the dominance of Rubus spp. in the herbaceous layer likely contributes to the observed decline in species diversity.