Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO 2 across four free-air CO 2 enrichment experiments in forest, grassland and desert

The magnitude of changes in carboxylation capacity in dominant plant species under long-term elevated CO 2 exposure (elevated pC a ) directly impacts ecosystem CO 2 assimilation from the atmosphere. We analyzed field CO 2 response curves of 16 C 3 species of different plant growth forms in favorable growth conditions in four free-air CO 2 enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO 2 assimilation ( A ) by +40±5% in elevated pC a (49.5–57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC a in some species. Photosynthesis at a common pC a ( A a ) was significantly reduced in five species growing under elevated pC a , while leaf carboxylation capacity ( V cmax ) was significantly reduced by elevated pC a in seven species (change of −19±3% among these species) across different growth forms and FACE sites. Adjustments in V cmax with elevated pC a were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC a treatment did not affect the mass-based relationships between A or V cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC a on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC a effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC a at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO 2 responses can only be measured experimentally on a small number of species, understanding elevated CO 2 effects on canopy N m and N a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO 2 in different species and plant growth forms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72832/1/j.1365-2486.2004.00867.x.pd

Analysis of gas exchange in seedlings of Acer saccharum : integration of field and laboratory studies

In the field, photosynthesis of Acer saccharum seedlings was rarely light saturated, even though light saturation occurs at about 100 μmol quanta m -2 s -1 photosynthetic photon flux density (PPFD). PPFD during more than 75% of the daylight period was 50 μmol m -2 s -1 or less. At these low PPFD's there is a marked interaction of PPFD with the initial slope (CE) of the CO 2 response. At PPFD-saturation CE was 0.018 μmol m -2 s -1 /(μl/l). The apparent quantum efficiency (incident PPFD) at saturating CO 2 was 0.05–0.08 mol/mol. and PPFD-saturated CO 2 exchange was 6–8 μmol m -2 s -1 . The ratio of internal CO 2 concentration to external ( C i / C a ) was 0.7 to 0.8 except during sunflecks when it decreased to 0.5. The decrease in C i / C a during sunflecks was the result of the slow response of stomates to increased PPFD compared to the response of net photosynthesis. An empirical model, which included the above parameters was used to simulate the measured CO 2 exchange rate for portions of two days. Parameter values for the model were determined in experiments separate from the daily time courses being sumulated. Analysis of the field data, partly through the use of simulations, indicate that the elimination of sunflecks would reduce net carbon gain by 5–10%.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47755/1/442_2004_Article_BF00378907.pd

Environmental geochemistry of radioactive contamination.

This report attempts to describe the geochemical foundations of the behavior of radionuclides in the environment. The information is obtained and applied in three interacting spheres of inquiry and analysis: (1) experimental studies and theoretical calculations, (2) field studies of contaminated and natural analog sites and (3) model predictions of radionuclide behavior in remediation and waste disposal. Analyses of the risks from radioactive contamination require estimation of the rates of release and dispersion of the radionuclides through potential exposure pathways. These processes are controlled by solubility, speciation, sorption, and colloidal transport, which are strong functions of the compositions of the groundwater and geomedia as well as the atomic structure of the radionuclides. The chemistry of the fission products is relatively simple compared to the actinides. Because of their relatively short half-lives, fission products account for a large fraction of the radioactivity in nuclear waste for the first several hundred years but do not represent a long-term hazard in the environment. The chemistry of the longer-lived actinides is complex; however, some trends in their behavior can be described. Actinide elements of a given oxidation state have either similar or systematically varying chemical properties due to similarities in ionic size, coordination number, valence, and electron structure. In dilute aqueous systems at neutral to basic pH, the dominant actinide species are hydroxy- and carbonato-complexes, and the solubility-limiting solid phases are commonly oxides, hydroxides or carbonates. In general, actinide sorption will decrease in the presence of ligands that complex with the radionuclide; sorption of the (IV) species of actinides (Np, Pu, U) is generally greater than of the (V) species. The geochemistry of key radionuclides in three different environments is described in this report. These include: (1) low ionic strength reducing waters from crystalline rocks at nuclear waste research sites in Sweden; (2) oxic water from the J-13 well at Yucca Mountain, Nevada, the site of a proposed repository for high level nuclear waste (HLW) in tuffaceous rocks; and (3) reference brines associated with the Waste Isolation Pilot Plant (WIPP). The transport behaviors of radionuclides associated with the Chernobyl reactor accident and the Oklo Natural Reactor are described. These examples span wide temporal and spatial scales and include the rapid geochemical and physical processes important to nuclear reactor accidents or industrial discharges as well as the slower processes important to the geologic disposal of nuclear waste. Application of geochemical information to remediating or assessing the risk posed by radioactive contamination is the final subject of this report. After radioactive source terms have been removed, large volumes of soil and water with low but potentially hazardous levels of contamination may remain. For poorly-sorbing radionuclides, capture of contaminated water and removal of radionuclides may be possible using permeable reactive barriers and bioremediation. For strongly sorbing radionuclides, contaminant plumes will move very slowly. Through a combination of monitoring, regulations and modeling, it may be possible to have confidence that they will not be a hazard to current or future populations. Abstraction of the hydrogeochemical properties of real systems into simple models is required for probabilistic risk assessment. Simplifications in solubility and sorption models used in performance assessment calculations for the WIPP and the proposed HLW repository at Yucca Mountain are briefly described