ALBERT

Notes: Leaf 15N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO2 concentration. We tested the hypothesis that elevated CO2 significantly changes foliage δ15N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ15N of foliage of 27 field-grown plant species from six free-air CO2 enrichment (FACE) experiments representing desert, temperate forest, Mediterranean-type, grassland prairie, and agricultural ecosystems. We found that within species, the δ15N of foliage produced under elevated CO2 was significantly lower (P〈0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ15N by life form and growth habit revealed that the CO2 effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO2 concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO2 and leaf δ15N. In a select number of species, we measured bulk soil δ15N at a depth of 10 cm, and found that the observed depletion of foliage δ15N in response to elevated CO2 was unrelated to changes in the soil δ15N. While the data suggest a strong influence of elevated CO2 on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO2 alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ15N responses to elevated CO2 reported here resulted from a general drop in δ15N of the source N, caused by soil-driven processes. There is a stronger possibility, however, that the general depletion of foliage δ15N under high CO2 may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.

Notes: Conclusions Why was Hensen unsuccesful in the quantification of ecological sampling? No aspect of plankton research itself seems to have hindered quantification; both collecting methods and taxonomy were sufficiently advanced. The reason is probably that at the time he began sampling, Hensen had to devise his own statistical methods for expressing the reproducibility and validity of samples. Hensen might have succeeded in this if he had overcome prevalent nineteenth-century attitudes toward randomness. The statistical literature of medicine and physics with which Hensen was probably familiar gave methods for expressing reproducibility and for comparing differences between means of different sets of observations. For example, a student of Poisson writing on medical statistics advocated using Poisson's limit (standard error 2√2) to test the difference between two means56. Other authors suggested that differences between means were most meaningful if very large numbers of observations were used.57 In his laboratory subsampling, Hensen used the propable error as a limit about means. In this and other ways, he seems most indebted to the physicist Ernst Abbe for statistical methods.58 However, all the methodology available to Hensen had been developed for situations in which errors are a property of the measurement or sampling process, and not of the phenomena themselves. The available methods for measuring reproducibility were based on the assumption that differences from the average were small and that they tended to accumulate about the mean in a bell-shaped pattern. Hensen constantly reinvestigated the distribution of plankton numbers about the average using a different method each time. Westergaard points out that medical statisticians did not make such investigations with their biological data.59 To a considerable extent, biological sampling problems forced development of theory because samples afforded the only information on a pattern in water or soil which could not be directly observed. The sampling methods of Laplace and the late nineteenth-century government statisticians contrasted strongly with Hensen's because, either through subjective knowledge of the population sampled or through censuses, they attempted to choose representative or typical samples.60 The high reproducibility and validity of representative sampling is attained by knowing more about a population than a biologist can ordinarily know. The uncertain reproducibility and validity of biological sampling spurred the development of formal sampling theory. A formal sampling theory developed only after change in the general intellectual attitude toward randomness, which was reflected in nineteenth-century statistics.61 The ninteenth-century attitude that randomness is not part of nature changed in the twentieth century to a view of randomness as a property of nature.62 The physicist's incorporation of randomness into physical models in response to this intellectual change late in the nineteenth-century is discussed by Bork.63 In biology, the change was initiated by the attention Darwin focused on morphological variation. The English biometricians — Francis Galton and W. F. R. Weldon, for example — were prominent in developing methods for the analysis of biological variation.64 Most pertinent to the development of sampling theory was Karl Pearson's use of frequency distributions as models of biological variation. In ecology, quantification was brought about by Ronald Fisher more than by anyone else; he incorporated randomness into sampling plans and built upon the methods developed earlier for analysis of individual variation. Fisher's use of random sampling allowed comparison between the sample collections and the collections expected from a model population of known patterning (calculated with a frequency distribution). This is a much more efficient method of determining the validity of a sample than Hensen's comparison of collections with a model uniform collection. Intellectual background and accumulated biological information caused Fisher to find variability where Hensen had seen uniformity. In summary, Victor Hensen became interested in fisheries research because of the economic importance of fishing to Germany. Hensen had considerable understanding of the prerequisites for valid sampling, but the value of his quantitative approach was limited by the general preconceptions shared by most nineteenth-century biologists. Through Hensen's efforts many other biologists were stimulated to undertake quantitative samples, even though the statistical methods for analyzing variation among populations developed only after methods for analyzing variation among individuals had been developed. *** DIRECT SUPPORT *** A8402011 00003

Notes: Abstract We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO2 would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO2 in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO2. The density of protozoa did not respond to soil N but increased marginally (P 〈 0.06) under elevated CO2. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO2. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO2 treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO2, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO2 was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P 〈 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose.

Notes: Abstract Saprophytic fungi have degradative abilities and interspecific interactions which suggest that resource use and yield should increase as species number increases, but previous studies show the opposite. As a test of the possibility that invertebrate activity changes fungal resource use patterns, we grew coprophilous fungi on rabbit feces at the same initial density singly or in mixtures of 2, 4, or 6 species, with or without activity of larvalLycoriella mali (Diptera: Sciaridae). Fungi in mixtures without larvae caused less weight loss in one mixture, and greater weight loss in 2 mixtures than when growing alone; fungi in 4 of 6 mixtures produced fewer spores than when growing alone. Overall, without larvae, weight loss did not increase as number of fungal species increased. Larvae did not change the pattern of weight loss or proportions of spores caused by mixing fungal species. Numbers of larvae surviving to pupate rose as fungal species numbers increased; as a result, weight loss increased with fungal species number in cultures with larvae.