Effects of nitrogen on temporal and spatial patterns of nitrate in streams and soil solution of a central hardwood forest

This study examined changes in stream and soil water NO₃⁻ and their relationship to temporal and spatial patterns of NO₃⁻ in soil solution of watersheds at the Fernow Experimental Forest, West Virginia. Following tenfold increases in stream NO₃⁻ concentrations over a 13-year period (1969–1981) on untreated WS4, concentrations have declined through 2006. Following fourfold increases in stream NO₃⁻ on treatment WS3 from pretreatment levels to a 1998 maximum, concentrations have declined through 2006, despite additions of N. Concentrations of soil water NO₃⁻ were consistently lower for WS4 compared to WS3. Data for soil water NO₃⁻ on WS3 versus WS4 followed patterns of net mineralization and nitrification for these watersheds. Nitrogen additions to WS3 decreased spatial heterogeneity of N processing, which was largest in the pretreatment year and decreased significantly to a minimum by 2000-2001. Concurrently, soil water NO₃⁻ increased on WS3 from 1.3mg NO₃⁻-N L⁻¹ in pretreatment 1989 to a maximum of 6.4mg NO₃⁻-N L⁻¹ in 2001. Spatial heterogeneity in soil water NO₃⁻ on WS4 remained high during this period. Data suggest that temporal patterns of stream NO₃⁻ may be influenced by spatial heterogeneity of watershed processes which vary over time in response to N availability.Journal ArticleFinal article publishe

Response of soil fertility to 25 years of experimental acidification in a temperate hardwood forest

The effects of enhanced acid deposition from the atmosphere, and associated elevated inputs of N, are widely evident, especially for forests where excess N has led to a variety of deleterious effects. These include declines in biodiversity, a response that will likely require considerable time for recovery. The purpose of this study was to determine responses of plant nutrient availability in surface mineral soil to 25 yr of experimental acidification and N addition in a central Appalachian hardwood forest ecosystem. We hypothesized that chronic additions of (NH₄)₂SO₄ will increase mineral N, decrease soil pH, P, and base cations, increase micronutrients (Mn²⁺ and Fe²⁺), and increase levels of A1³⁺. Results supported these predictions, although Mn²⁺ did not vary significantly. Earlier work on these plots found no response of any of the extractable nutrients to 3 yr of treatment, yet after 25 yr, our results suggest that impacts are apparent in the top 5 cm of the A horizon. We surmise that impacts in these soils may have lagged behind the onset of acidification treatments or that several years of treatment were required to overcome preexisting differences in soil ions. Generally, current findings confirm that (NH₄)₂SO₄ treatments have lowered the pH, enhanced levels of exchangeable A1³⁺, and increased stream-water exports of NO₃⁻ and base cations—a process that further acidifies soil. The combination of these changes in surface soils, with their high proportion of fine roots, may contribute to the reduced growth and competitiveness of some hardwood species at the acidified site.Technical ReportFinal article publishe

Nitrogen (N) dynamics in the mineral soil of a Central Appalachian Hardwood Forest during a quarter century of whole-watershed N additions

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.Journal ArticleFinal article publishe

Twenty-five-year response of the herbaceous layer of a temperate hardwood forest to elevated nitrogen deposition

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.Journal ArticleFinal article publishe