Dr. Frank S Gilliam

Nitrogen (N) deposition can alter N availability, soil pH, and microbial activity and composition in soils, which directly and indirectly affects soil N transformations and influences forest ecosystem processes and functions. Understanding the effects of N deposition on soil N transformations in forest ecosystems has important implications for the ecological and environmental impacts of N deposition. In this chapter, the effects of N deposition on net and gross soil N transformations are synthesized based on the published literature. Results show that N deposition increases both net and gross soil N mineralization and nitrification rates, but do not affect gross NH4+ immobilization rates and gross NO3− immobilization rates. Responses of soil N transformations to N deposition varied depending on climate (such as mean annual precipitation, mean annual temperature, climate region), environmental factors (especially ambient atmospheric N deposition rate), soil properties (soil C:N, soil pH, soil horizon), and experimental N addition (e.g., chemical form of N, N addition duration). The duration of N addition plays an important role affecting the responses of soil N transformations to such additions. Thus, the temporal changes in the effects of N deposition are specifically reviewed based on several case studies. In future research, the observations under long-term time scales are needed to clarify the effects of N deposition on soil N transformations in forest ecosystems. In addition, the role of microbes in the responses of soil N transformations to N deposition also should be investigated.

Historical increases in emissions and deposition of oxidized and reduced nitrogen (N) provided the impetus for global-scale research on the effects of excess N in terrestrial and aquatic ecosystems. Much of the eastern U.S. has been susceptible to negative effects of excess N. The Clean Air Act and associated rules have led to decreases in emissions and deposition of oxidized N, especially in the eastern U.S., representing a research challenge and opportunity for ecosystem ecologists. This chapter predicts changes in the structure and function of North American forest ecosystems in response to decreased N deposition. Hysteresis is a property of a system wherein output is not a strict function of corresponding input, incorporating a time lag, particularly when responses to decreasing input vary from responses to increasing input. A conceptual hysteretic model predicts varying lag times in recovery of soil acidification and nutrient leaching, surface water nitrogen concentrations and export, plant diversity, soil microbial communities, and forest carbon and N cycling toward pre-N-impact conditions. These processes are expected to respond notably to reductions in N deposition, most showing a degree of hysteresis, with the greatest delays in the response occurring in those tightly linked to “slow pools” of N in wood and soil organic matter. Some responses, especially nitrate concentrations in stream flow, have already become apparent in regions of northeastern U.S. Because experimental studies of declines in N loads in forests of North America are lacking and due to the expected hysteresis, it is difficult to generalize from experimental results to patterns expected from declining N deposition. Responses to declining N will be long-term and difficult to distinguish from concurrent environmental changes affecting the N cycle, e.g., elevated atmospheric CO2, climate change, reductions in acidity, invasive species, and vegetation responses to disturbance.

Human activity in the last century has increased nitrogen (N) deposition to a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. We synthesized current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and estimated associated empirical critical loads of N for several receptors: freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Biogeochemical responses included increased N mineralization and nitrification, increased gaseous N losses, and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root-shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species. The range of critical loads of nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1–39 kg N ha−1 yr−1, spanning the range of N deposition observed over most of the country. The empirical critical loads of N tend to increase in the following sequence: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.

The herbaceous layer is simultaneously exposed to and responds to many forest disturbances, ranging from microscale disturbances such as frost heaving and trampling by large vertebrates to more extensive and intensive disturbances resulting from herbivory, tree mortality, inundation, periodic fire, catastrophic wind damage, and forest management practices, including timber harvesting and use of herbicides. This chapter provides a conceptual overview of many of these disturbances.

Nitrogen (N) saturation arises when atmospheric inputs of N exceed biological N demand, resulting in loss of NO3- in streams, accompanied by the loss of nutrients (Ca and Mg) that are essential to forest health. Previous studies have shown that some watersheds the Fernow Experimental Forest (FEF), West Virginia, USA, are among the more N-saturated sites in North America. Research from the Gilliam laboratory at Marshall University (West Virginia, USA) began focusing specifically on N biogeochemistry in 1993 with establishment of plots at FEF to carry out long-term in situ (“buried bag”) incubations in three watersheds: two control (WS4, WS7) and one treatment (WS3). This was done in conjunction with the Fernow Watershed Acidification Study, established by the USDA Forest Service in 1989 to treat an entire watershed (WS3) with aerial applications of 35 kg N ha –1 year  –1. The initial period (1993–1995) exhibited increases in rates for all watersheds, but especially in treated WS3. This period has been followed by declines in net nitrification, which is consistent with current declines in stream NO3– and has been especially pronounced in WS3 since 1998. Also during this time, sampling of the herbaceous layer (vascular plants ≤ 1 m in height) has revealed pronounced changes in response to N treatments on WS3, especially in the increase of the shade-intolerant Rubus spp. Future work will investigate the effects of freezing on soil N dynamics. Preliminary results indicate that freezing exacerbates the symptoms of N saturation already seen in soils at FEF, further increasing already high rates of net nitrification.

This chapter reports the findings of a Working Group on how atmospheric nitrogen (N) deposition affects both terrestrial and freshwater biodiversity. Regional and global scale impacts on biodiversity are addressed, together with potential indicators. Key conclusions are that: the rates of loss in biodiversity are greatest at the lowest and initial stages of N deposition increase; changes in species compositions are related to the relative amounts of N, carbon (C) and phosphorus (P) in the plant soil system; enhanced N inputs have implications for C cycling; N deposition is known to be having adverse effects on European and North American vegetation composition; very little is known about tropical ecosystem responses, while tropical ecosystems are major biodiversity hotspots and are increasingly recipients of very high N deposition rates; N deposition alters forest fungi and mycorrhyzal relations with plants; the rapid response of forest fungi and arthropods makes them good indicators of change; predictive tools (models) that address ecosystem scale processes are necessary to address complex drivers and responses, including the integration of N deposition, climate change and land use effects; criteria can be identified for projecting sensitivity of terrestrial and aquatic ecosystems to N deposition. Future research and policy-relevant recommendations are identified.

This chapter explores variations in and dynamics of herb layer assemblages across the North Carolina Piedmont landscape. It focuses on three questions: (1) What are the key environmental factors influencing herb distributions at various stages in the succession following land abandonment? (2) What do these patterns tell us with respect to the mechanisms that underlie the dynamics of herb populations? (3) Are Piedmont forest herbaceous communities changing in ways not related to old-field plant succession? To address these questions, the chapter draws on several strands of research carried out over the last 80 years to provide a more comprehensive picture of the successional patterns and mechanisms of change in herb-layer communities of the forested landscapes of the Piedmont region of North Carolina.

Nitrogen deposition, along with habitat loss and climate change, constitute a major threat to Earth’s biodiversity. Fossil fuel combustion and modern agriculture add more nitrogen to terrestrial and aquatic ecosystems than all natural processes combined. Because nitrogen often limits productivity, this enrichment it likely to have major ecological impacts. In terrestrial ecosystems, nitrogen deposition can lead to increased growth of often weedy species, cation depletion in the soil, nutrient imbalances in plant tissue, and soil acidification among other effects. These processes often reduce plant biodiversity and homogenize communities, which can propagate through food webs and impact entire ecosystems.

In this chapter, the impact of watershed acidification treatments on WS3 at the Fernow Experimental Forest (FEF) and at WS9 on vegetation is presented and summarized in a comprehensive way for the first time. WS7 is used as a vegetative reference basin for WS3, while untreated plots within WS9 are used as a vegetative reference for WS9. Bioindicators of acidification impacts that will be considered include several measures of tree and stand growth rates, foliar chemistry, bolewood chemistry, and herbaceous species composition and diversity. These studies enhance our understanding of the inter-relationships of changes in soil conditions caused by the acidification treatment and the condition of forest vegetation.