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Empirical critical loads for N deposition effects and maps showing areas projected to be in exceedance of the critical load (CL) are given for seven major vegetation types in California. Thirty-five percent of the land area for these vegetation types (99,639 km(2)) is estimated to be in excess of the N CL. Low CL values (3-8 kg N ha(-1) yr(-1)) were determined for mixed conifer forests, chaparral and oak woodlands due to highly N-sensitive biota (lichens) and N-poor or low biomass vegetation in the case of coastal sage scrub (CSS), annual grassland, and desert scrub vegetation. At these N deposition critical loads the latter three ecosystem types are at risk of major vegetation type change because N enrichment favors invasion by exotic annual grasses. Fifty-four and forty-four percent of the area for CSS and grasslands are in exceedance of the CL for invasive grasses, while 53 and 41% of the chaparral and oak woodland areas are in exceedance of the CL for impacts on epiphytic lichen communities. Approximately 30% of the desert (based on invasive grasses and increased fire risk) and mixed conifer forest (based on lichen community changes) areas are in exceedance of the CL. These ecosystems are generally located further from emissions sources than many grasslands or CSS areas. By comparison, only 3-15% of the forested and chaparral land areas are estimated to be in exceedance of the NO(3)(-) leaching CL. The CL for incipient N saturation in mixed conifer forest catchments was 17 kg N ha(-1) yr(-1). In 10% of the CL exceedance areas for all seven vegetation types combined, the CL is exceeded by at least 10 kg N ha(-1) yr(-1), and in 27% of the exceedance areas the CL is exceeded by at least 5 kg N ha(-1) yr(-1). Management strategies for mitigating the effects of excess N are based on reducing N emissions and reducing site N capital through approaches such as biomass removal and prescribed fire or control of invasive grasses by mowing, selective herbicides, weeding or domestic animal grazing. Ultimately, decreases in N deposition are needed for long-term ecosystem protection and sustainability, and this is the only strategy that will protect epiphytic lichen communities.

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Archived soil samples (1937-1999) and historic air quality data from the Los Angeles Basin were used for reconstructing the record of change between atmospheric NO(x) loads, soil delta(15)N values and the diversity of arbuscular mycorrhizae (AM), which are ubiquitous plant-fungus mutualists that control plant community productivity. A tripling of atmospheric NO(x) loads between 1937 and the 1970s was paralleled by soil nitrogen enrichment (delta(15)N = 3.18). From 1975 onwards, atmospheric NO(x) declined, but soils became nitrogen saturated (delta(15) N = -4 and NO(3)-nitrogen = 171mgkg(-1)). The shifts in the AM community followed 28 years of atmospheric nitrogen enrichment and coincided with the onset of soil nitrogen saturation. Such changes were manifest in the loss of AM productivity, species richness (one species per year), three genera (Acaulospora, Scutellospora and Gigaspora) in the spore community and Gigaspora within the roots. Nitrogen enrichment also enhanced the proliferation of potentially less mutualistic species of Glomus. Autoregressive models implied that such patterns will persist and be driven by soil nitrogen cycling patterns. Chronic nitrogen enrichment from air pollution thus alters the diversity and mutualistic functioning of AM communities, which, in turn, may influence the plant community.

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The goal of this study was to determine the interaction of mycorrhizae and two N sources, ammonium (NH(4)(+)) and nitrate (NO(3)(-)), on the growth of a coastal sage scrub (CSS) species, Artemisia californica, and an exotic annual grass, Bromus madritensis ssp. rubens. Anthropogenic nitrogen deposition may be influencing the decline of CSS and replacement by exotic grasses, but the extent to which mycorrhizae are involved in shrubland decline is unknown. NO(3)(-) is the dominant form of deposition in southern California, although the native, uneutrophied soils have a greater concentration of NH(4)(+). Seeds of each species were germinated in pots of sterile soil, inoculated with native soil containing mycorrhizal spores and infective root fragments, and fertilized with 50 µg/g of either NO(3)(-) or NH(4)(+). NH(4)(+) enhanced the growth of both mycorrhizal species, while NO(3)(-) did not. Control plants of B. madritensis under low N had a significant response to mycorrhizae, but A. californica did not. Nitrate increased the growth of nonmycorrhizal A. californica as much as the mycorrhizal NH(4)(+)-treated plants. There is no evidence in this study to suggest that the decline of A. californica or increase in B. madritensis is due to a mycorrhizal response to NO(3)(-). Other life history traits of the two species must be used to explain the invasive behavior of the annual grass. Mycorrhizae may be more important in controlling plant growth in native uneutrophied soils dominated by NH(4)(+) rather than NO(3)(-).

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Society relies on telecommunications to such an extent that telecommunications software must have high reliability. Enhanced measurement for early risk assessment of latent defects (EMERALD) is a joint project of Nortel and Bell Canada for improving the reliability of telecommunications software products. This paper reports a case study of neural-network modeling techniques developed for the EMERALD system. The resulting neural network is currently in the prototype testing phase at Nortel. Neural-network models can be used to identify fault-prone modules for extra attention early in development, and thus reduce the risk of operational problems with those modules. We modeled a subset of modules representing over seven million lines of code from a very large telecommunications software system. The set consisted of those modules reused with changes from the previous release. The dependent variable was membership in the class of fault-prone modules. The independent variables were principal components of nine measures of software design attributes. We compared the neural-network model with a nonparametric discriminant model and found the neural-network model had better predictive accuracy.

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Oxalic acid is produced by some species of plants and mycorrhizal fungi and it may solubilize unavailable soil phosphorus (P) bound by cations (Ca++, Al++, Fe+++). Field and greenhouse experiments were conducted to show whether oxalate produced by the annual Salsola tragus or added oxalic acid would solubilize P from the inorganic-bound soil P pool, making it available for uptake by Stipa pulchra. Oxalate could be leached in the laboratory from the senescent canopy of Salsola, and leaching by rainfall was hypothesized to be a source of potential increased soil P under the Salsola canopy. Both oxalate leached from the canopy of Salsola and added oxalic acid increased the availability of soil P in greenhouse experiments. The source of the increase in available soil P in the greenhouse experiment was shown to be the inorganic-bound P pool, as the total P concentration of the soil decreased with increasing oxalate. There were significant increases in Stipa shoot P in response to Salsola leachates and in response to added oxalate in the greenhouse studies. These results suggest an important role for oxalate in P cycling. On disturbed sites where Salsola invades it may act to facilitate the establishment of later seral species like Stipa by creating a nutrient island of available P.

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Self-consistent (Hartree-Fock) calculations of the process of decomposition of protonated and lithiated syn-N-nitrosourea show that the presence of cations perturbs the electron distribution significantly. The decomposition of nitrosourea is facilitated when a proton or lithium ion is positioned at the oxygen of the nitroso group. These results may suggest clinical experimentation with nitrosoureas used in conjunction with lithium salts.

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E- and Z-2-haloethyldiazotates--electrophilic species hitherto suggested as intermediates in the reactions of 2-haloethylnitrosoureas (HENUs) under physiological conditions--were synthesized and characterized by 1H-, 15N- and 13C-NMR (nuclear magnetic resonance). They were stabilized and solubilized in organic solvents as their 18-crown-6 ether complexes. Characterization of the Z-2-fluoroethyldiazotate by 19F- and 13C-NMR, and comparison with the Z-2-chloroethyl compound, confirmed facile cyclization to the 1,2,3-oxadiazoline and subsequent decomposition to nitrogen and ethylene oxide. The E-2-haloethyldiazotates form DNA interstrand cross-links at a rate, and to an extent, and with a DNA base dependence, which parallels the behaviour of the parent HENUs, while the Z isomers alkylate DNA but show minimal cross-linking. Both E-and Z-(2'-chloroethyl)thioethyldiazotates, neither of which can undergo cyclization, cross-link DNA efficiently. Self-consistent-field (SCF) ab initio calculations provided optimized geometries, atomic charges and LUMO (Lowest Unoccupied Molecular Orbital) atom contributions for the E- and Z-2-haloethyldiazohydroxides. The HSAB (Hard and Soft Acids and Bases) theory, in conjunction with HOMO (Highest Occupied Molecular Orbital) values on key DNA base sites, accounted for the observed site-selectivity in the formation of identified cross-links produced by 1,3-bis-(2-chloroethyl)-1-nitrosourea. Independent chemical studies on cytosine derivatives corroborated the predicted site selectivity of attack by electrophiles and the formation of ethanocytidine cyclic adducts.

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E and Z-2-haloethyldiazotates, which have been postulated as the ultimate electrophiles responsible for the biological activity of 2-haloethylnitrosoureas (HENUs), have been synthesized and characterized by 15N-nmr. Their stability and solubility in organic solvents are increased by forming crown ether complexes. While E and Z forms are configurationally stable in solution the Z form cyclizes at greater than or equal to -20 degrees C to a 1,2,3-oxadiazoline. The E isomers cross-link DNA, in contrast to the Z isomers. However, both E and Z (2'chloroethyl)thioethyldiazotates (neither of which may cyclize) cross-link DNA extremely efficiently. The cross-linking by these agents is a two-step process and increases with the (G + C) content of the DNA. E-2-chloroethyldiazotate exhibits activity against P388 leukaemia in vivo, lending credence to the suggestion that it is the ultimate electrophile from HENUs. Ab initio calculations predicted the optimized geometry, LUMO energies and atom contributions and the net atomic charges for the diazohydroxides and the HOMO energies and atom contributions for the alternative DNA base sites. An analysis based on Frontier Orbital methods invoking the Hard and Soft Acids and Bases theory permitted an interpretation of the formation of a cross-link site and several modified bases isolated from the reaction of HENUs with DNA.

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The effects of vesicular-arbuscular mycorrhizal inoculum and competition from non-mycorrhizal annual plants were tested on Agropyron smithii Rydb. and Agropyron dasystachyum (Hook.) Scribn. growing in an inoculum-poor soil. Mycorrhizal A. smithii decreased stomatal resistance (rs ) and increased leaf water potential compared to the non-inoculated plants, but only during the driest portion of the growing season. The presence of annuals caused increased rs of A. smithii, but only of non-inoculated plants. Mycorrhizal plants had the same rs with or without annuals, indicating that mycorrhizas may alleviate the detrimental effects of competition on rs of A. smithii. By contrast, neither inoculation nor annuals significantly affected water relations of A. dasystachyum. Mycorrhizas also did not increase N and P concentrations or percentage cover of either grass, but both had delayed phenology when mycorrhizal. This research was done during a wet year when the effects of mycorrhizas may be subtle. Long-term observations are necessary to determine the importance of mycorrhizas under field conditions.

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