This article describes ways in which excess nitrogen from fossil fuel combustion may stress the biosphere. Nitrogen emissions can have a direct effect on air quality through both the oxidizing potential of nitrogen oxides and the role these compounds play in the formation of ozone. The complexity of these effects on water quality and on forest nutrition is discussed.

https://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/publications/pdfs/Aber_BioScience_1989.pdf

Nitrogen saturation in northern forest ecosystems

Until recently, the common view of the terrestrial nitrogen cycle had been driven by two core assumptions—plants use only inorganic N and they compete poorly against soil microbes for N. Thus, plants were thought to use N that microbes “left over,” allowing the N cycle to be divided cleanly into two pieces—the microbial decomposition side and the plant uptake and use side. These were linked by the process of net mineralization. Over the last decade, research has changed these views. N cycling is now seen as being driven by the depolymerization of N-containing polymers by microbial (including mycorrhizal) extracellular enzymes. This releases organic N-containing monomers that may be used by either plants or microbes. However, a complete new conceptual model of the soil N cycle needs to incorporate recent research on plant–microbe competition and microsite processes to explain the dynamics of N across the wide range of N availability found in terrestrial ecosystems. We discuss the evolution of thinking abou...

Nitrogen mineralization: challenges of a changing paradigm

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

/pdf/nitrogen-saturation-in-temperate-forest-ecosystems-29ixssxci4.pdf

Nitrogen Saturation in Temperate Forest Ecosystems

▪ Abstract The use of stable isotope techniques in plant ecological research has grown steadily during the past two decades. This trend will continue as investigators realize that stable isotopes can serve as valuable nonradioactive tracers and nondestructive integrators of how plants today and in the past have interacted with and responded to their abiotic and biotic environments. At the center of nearly all plant ecological research which has made use of stable isotope methods are the notions of interactions and the resources that mediate or influence them. Our review, therefore, highlights recent advances in plant ecology that have embraced these notions, particularly at different spatial and temporal scales. Specifically, we review how isotope measurements associated with the critical plant resources carbon, water, and nitrogen have helped deepen our understanding of plant-resource acquisition, plant interactions with other organisms, and the role of plants in ecosystem studies. Where possible we also...

Stable Isotopes in Plant Ecology

Preface.- Contributors.- List of Abbreviations.- Section 1: Basic Principles: Introduction.-Sources of Heavy Metals and Metalloids in Soils.- Chemistry of Heavy Metals and Metalloids in Soils.- Methods for the Determination of Heavy Metals and Metalloids in Soils.- Effects of Heavy Metals and Metalloids on Soil Organisms.- Soil-Plant Relationships of Heavy Metals and Metalloids.- Heavy Metals and Metalloids as Micronutrients for Plants and Animals.-Critical Loads of Heavy Metals for Soils.- Section 2: Key Heavy Metals And Metalloids: Arsenic.- Cadmium.- Chromium and Nickel.- Cobalt and Manganese.- Copper.-Lead.- Mercury.- Selenium.- Zinc.- Section 3: Other Heavy Metals And Metalloids Of Potential Environmental Significance: Antimony.- Barium.- Gold.- Molybdenum.- Silver.- Thallium.- Tin.- Tungsten.- Uranium.- Vanadium.- Glossary of Specialized Terms.- Index.

Heavy metals in soils : trace metals and metalloids in soils and their bioavailability

To assess the response of forests to a changing chemical environment, a means is needed for separating the total cation export from the watershed into a component derived from mineral weathering reactions and a component due to the removal of exchangeable (plant-available) cations in the soil1–3. We show that this separation may be possible by using 87Sr/86Sr ratios as a tracer of cation sources in stream water. Our measurements from a high-elevation forest ecosystem in the Adirondack mountains, New York, indicate that mineral weathering reactions contribute about 70% and soil cation-exchange reactions about 30% of annual strontium exports. Based on these results and the ratios of major cations to strontium in the local glacial till, we estimate the release of Ca2+, Mg2+, K+ and Na+ owing to weathering. The present weathering rate seems adequate to replace annual losses of cations from the total soil exchangeable pool, suggesting that the watershed is not in immediate danger of acidification from atmospheric deposition. But as our strontium isotope data indicate that 50–60% of the strontium in the organic-soil-horizon exchangeable and vegetation cation pools has an atmospheric origin, reduction of atmospheric cation inputs4 coupled with continued strong-acid anion inputs5 may result in significant depletion of this cation reservoir.

Determination of soil exchangeable-cation loss and weathering rates using Sr isotopes

Observations made over the past 5 years suggest that the most recent needles of red spruce (Picearubens Sarg.) in the Green Mountains of Vermont, U.S.A., are prone to browning and loss owing to cold stress and (or) winter desiccation. Particularly severe events occurred in the winters of 1981 and 1984 and the latter event was observed in detail. On many trees in all size classes, a large proportion of the 1983 needles turned reddish-brown between late February and early April, 1984. This pattern of discoloration and subsequent defoliation, which appears to be repeated periodically, can account for the visual symptoms of declining red spruce in the montane forests of the northeastern United States. A predisposition to winter damage appears to be a component of the decline and could result from natural or possibly anthropogenic factors.

Winter damage to foliage as a factor in red spruce decline

Decreased use of leaded gasoline has caused a rapid reduction in atmospheric Pb deposition to terrestrial and aquatic ecosystems of the northeastern United States. In forest soils, the response to decreased Pb deposition has been more rapid than was anticipated based on previous studies of Pb cycling in forested ecosystems. To better understand the observed ecosystem response, we estimated both short-(25 y) and long-term (190 y) time series of the regional average atmospheric Pb concentrations in precipitation, cloudwater, and air. We combined these time series with modeled wet and dry deposition velocities to calculate the time-dependent Pb flux at different elevations in montane forests of the northeastern United States

Lead migration in forest soils: response to changing atmospheric inputs.

This review focuses on the long-range atmospheric transport of metals to organic-rich surface soils (mostly 50–90% organic matter) in the temperate, coniferous, and boreal zones of North America an...

Metal contamination of natural surface soils from long-range atmospheric transport: Existing and missing knowledge

Atmospheric deposition of lead (Pb) throughout the 1900s resulted in elevated amounts of this toxic metal even in remote forest soils of the northeastern United States. Soils can act as a net sink for metals and thus minimize groundwater and surface water contamination. Recent studies utilizing forest floor temporal data and models of total Pb in precipitation, surface soils, and streams have estimated the time scale of Pb release from soils. However, due to the limited availability and spatial variability of forest floor survey data, other methods for quantifying anthropogenic Pb movement are needed. This study uses the isotopic composition (206Pb/207Pb) of soil Pb and measurements of 210Pb and 226Ra to directly trace the transit of atmospherically deposited Pb in the soil profile. We also report on the recovery of an enriched 207Pb dose applied in 1984 to the surface of a soil plot in the coniferous forest at Camels Hump in Vermont. The isotopic composition of soil Pb in low elevation deciduous forests ...

Andrew J. Friedland

Papers

Determination of soil exchangeable-cation loss and weathering rates using Sr isotopes

Winter damage to foliage as a factor in red spruce decline

Lead migration in forest soils: response to changing atmospheric inputs.

Metal contamination of natural surface soils from long-range atmospheric transport: Existing and missing knowledge

Using stable and radioactive isotopes to trace atmospherically deposited Pb in montane forest soils.