Nitrogen is a key element controlling the species composition, diversity, dynamics, and functioning of many terrestrial, freshwater, and marine ecosystems. Many of the original plant species living in these ecosystems are adapted to, and function optimally in, soils and solutions with low levels of available nitrogen. The growth and dynamics of herbivore populations, and ultimately those of their predators, also are affected by N. Agriculture, combustion of fossil fuels, and other human activities have altered the global cycle of N substantially, generally increasing both the availability and the mobility of N over large regions of Earth. The mobility of N means that while most deliberate applications of N occur locally, their influence spreads regionally and even globally. Moreover, many of the mobile forms of N themselves have environmental consequences. Although most nitrogen inputs serve human needs such as agricultural production, their environmental conse- quences are serious and long term. Based on our review of available scientific evidence, we are certain that human alterations of the nitrogen cycle have: 1) approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; 2) increased concentrations of the potent greenhouse gas N 2O globally, and increased concentrations of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth; 3) caused losses of soil nutrients, such as calcium and potassium, that are essential for the long-term maintenance of soil fertility; 4) contributed substantially to the acidification of soils, streams, and lakes in several regions; and 5) greatly increased the transfer of nitrogen through rivers to estuaries and coastal oceans. In addition, based on our review of available scientific evidence we are confident that human alterations of the nitrogen cycle have: 6) increased the quantity of organic carbon stored within terrestrial ecosystems; 7) accelerated losses of biological diversity, especially losses of plants adapted to efficient use of nitrogen, and losses of the animals and microorganisms that depend on them; and 8) caused changes in the composition and functioning of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries.

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Human alteration of the global nitrogen cycle: sources and consequences

While ecologists involved in management or policy often are advised to learn to deal with uncertainty, there are a number of components of global environmental change of which we are certain–certain that they are going on, and certain that they are human—caused. Some of these are largely ecological changes, and all have important ecological consequences. Three of the well—documented global changes are: increasing concentrations of carbon dioxide in the atmosphere; alterations in the biogeochemistry of the global nitrogen cycle; and ongoing land use/land cover change. Human activity–now primarily fossil fuel combustion– has increased carbon dioxide concentrations from °280 to 355 mL/L since 1800; the increase is unique, at least in the past 160 000 yr, and several lines of evidence demonstrate unequivocally that it is human—caused. This increase is likely to have climatic consequences–and certainly it has direct effects on biota in all Earth's terrestrial ecosystems. The global nitrogen cycle has been altered by human activity to such an extent that more nitrogen is fixed annually by humanity (primarily for nitrogen fertilizer, also by legume crops and as a by product of fossil fuel combustion) than by all natural pathways combined. This added nitrogen alters the chemistry of the atmosphere and of aquatic ecosystems, contributes to eutrophiction of the biosphere, and has substantial regional effects on biological diversity in the most affected areas. Finally, human land use/land cover change has transformed one—their to one—half of Earth's ice—free surface. This in and of itself probably represents the most important component of global change now and will for some decades to come; it has profound effects on biological diversity on land and on ecosystems downwind and downstream of affected areas. Overall, any clear dichotomy between pristine ecosystems and human—altered areas that may have existed in the past has vanished, and ecological research should account for this reality. These three and other equally certain components of global environmental change are the primary causes of anticipated changes in climate, and of ongoing losses of biological diversity. They are caused in turn by the extraordinary growth in size and resource use of the human population. On a broad scale, there is little uncertainty about any of these components of change or their causes. However, much of the public believes the causes–even the existence–of global change to be uncertain and contentious topics. By speaking out effectively, we can help to shift the focus of public discussion towards what can and should be done about global environmental change.

https://www.jstor.org/stable/pdfplus/1941591.pdf

Beyond Global Warming: Ecology and Global Change

N itrogen emissions to the atmosphere due to human activity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press). The US acid deposition effects

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Nitrogen Saturation in Temperate Forest Ecosystems

http://people.forestry.oregonstate.edu/richard-waring/sites/people.forestry.oregonstate.edu.richard-waring/files/publications/Landsberg%20%26%20Waring%201997%20For%20Ecol%20Mgmt.pdf

A generalised model of forest productivity using simplified concepts of radiation-use efficiency, carbon balance and partitioning

http://andrewsforest.oregonstate.edu/pubs/pdf/pub1374.pdf

Stabilization and destabilization of soil organic matter: mechanisms and controls

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Retention or Loss of N in IFS Sites and Evaluation of Relative Importance of Processes

The objectives of this study were to investigate the impacts of in· ternal nitrification on soil and soil solution acidity and on the rate of nutrient export through NO, -mediated leaching. This was achieved by comparing soil chemical properties and soil solution composition within a naturally N·rich red alder (Alnus rubra Bong.) ecosystem to those of an adjacent Douglas·fir IPseudotsuga menziesii (Mirbel) Francol forest where soil N levels were significantly lower and no measurable HNO, production could be observed. In the red alder sys· tern, where> 100 kg ha-' yr-' of N were added through symbiotic N, fixation, the net annual NO, - leaching past the 40·cm soil depth amounted to 3460 mol charges ha-', and NO, -concentrations in the solutions collected below 40 cm periodically exceeded drinking water standards of 10 mg L -'. The H' and NO, - release was most pro· nounced in the forest floor and top 10 em of the soil under alder oc­cupancy and caused significant acidification of percolating solutions. Less than 1010 of the total H' input from internal (nitrification) and external (atmospheric) sources leached below the 40-cm depth, which was indicative for the strong buffering capacity of this particular soil. The cation displacement reactions involved in this pH buffering caused a 15% decline in base saturation and a significant acidification of the upper part of the soil profile. The presence of large amounts of mobile NO, -in solution triggered accelerated cation leaching, causing a selective redistribution of primarily exchangeable Ca" from the A to the B horizon. These field studies lead us to conclude that the rate and the selectivity of NO, -mediated leaching in a red alder system could significantly lower the exchangeable cation pool in the rooting zone or

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The Impact of Nitrification on Soil Acidification and Cation Leaching in a Red Alder Ecosystem

Anion mobility in soils: Relevance to nutrient transport from forest ecosystems

Effects of Acid Rain Forest Nutrient Status

Precipitation, throughfall, forest floor, and soil leachate samples were monitored continuously in 1981 and 1982 in a N-poor Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forest and a red alder (Alnus rubra Bong.) forest growing adjacently on a glacial soil in western Washington. The purpose of the study was to quantify the relative importance of atmospheric vs. natural sources of H⁺ input to forest soil acidification, and to determine the role of N transformation processes in the overall H⁺ balance of soils with different N status. Rainwater samples had an avg pH of 4.7 and annual H⁺ deposition via precipitation averaged 320 mol H⁺ ha⁻¹ yr⁻¹. This was modest compared to internal H⁺ production associated with HCO⁻₃ and NO⁻₃ formation. In the soil under alder cover, which was naturally enriched in N through symbiotic N₂-fixation, nitrification released up to 4500 mol H⁺ ha⁻¹ annually to the solution percolating through the upper part of the soil profile. In the N-poor soil no nitrification could be observed and N transformation processes had a minor influence on the soil H⁺ balance. The main internal acidification source in this case was H₂CO₃ dissociation releasing 420 mol H⁺ ha⁻¹ yr⁻¹. In both instances, soil solutions appeared well buffered against these external and internal acidification sources, and few H⁺ leached below the 40-cm soil depth.

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Dale W. Cole

Papers

Retention or Loss of N in IFS Sites and Evaluation of Relative Importance of Processes

The Impact of Nitrification on Soil Acidification and Cation Leaching in a Red Alder Ecosystem

Anion mobility in soils: Relevance to nutrient transport from forest ecosystems

Effects of Acid Rain Forest Nutrient Status

Acidification Sources in Red Alder and Douglas Fir Soils-Importance of Nitrification