We discuss the physical and enzymatic bases of carbone isotope discrimination during photosynthesis, noting how knowledge of discrimination can be used to provide additional insight into photosynthetic metabolism and the environmental influences on that process

Carbon Isotope Discrimination and Photosynthesis

Preservation of elemental and isotopic source identification of sedimentary organic matter

Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes

Forest soil respiration is the sum of heterotrophic (microbes, soil fauna) and auto- trophic (root) respiration. The contribution of each group needs to be understood to evaluate implications of environmental change on soil carbon cycling and sequestration. Three primary methods have been used to distinguish hetero- versus autotrophic soil respiration including: integration of components contributing to in situ forest soil CO2 efflux (i.e., litter, roots, soil), comparison of soils with and without root exclusion, and application of stable or radioactive isotope methods. Each approach has advantages and disadvantages, but isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots. Pub- lished data from all methods indicate that root/rhizosphere respiration can account for as little as 10 percent to greater than 90 percent of total in situ soil respiration depending on vegetation type and season of the year. Studies which have integrated percent root contribution to total soil respiration throughout an entire year or growing season show mean values of 45.8 and 60.4 percent for forest and nonforest vegetation, respectively. Such average annual values must be extrapolated with caution, however, because the root contribution to total soil respiration is commonly higher during the growing season and lower during the dormant periods of the year. Abbreviations: TScer -t otal soil CO 2 efflux rate; f - fractional root contribution to TS cer; RC - root contribution to TScer

/pdf/separating-root-and-soil-microbial-contributions-to-soil-422u2gsu0s.pdf

Separating root and soil microbial contributions to soil respiration: A review of methods and observations

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/30617/0000257.pdf?sequence=1

Lacustrine organic geochemistry—an overview of indicators of organic matter sources and diagenesis in lake sediments

he efficiency of photosynthesis continues to interest biochemists, biologists, and plant physiologists. Scientists interested in CO2 uptake are concerned about the extent to which the uptake rate is limited by such factors as stomatal diffusion and the chemistry of the CO2 absorption process. The fractionation of carbon isotopes that occurs during photosynthesis is one of the most useful techniques for investigating the efficiency of CO2 uptake. Atmospheric carbon dioxide contains approximately 1.1% of the nonradioactive isotope carbon-13 and 98.9% of carbon-12. During photosynthesis, plants discriminate against C because of small differences in chemical and physical properties imparted by the difference in mass. This discrimination can be used to assign plants to various photosynthetic groups. The isotope fractionation also reflects limitations on photosynthetic efficiency imposed by the various diffusional and chemical components of CO2 uptake. When analyzed in detail, this fractionation provides information .about water use efficiency and indicates that different strategies are needed for improving wateruse efficiency in different kinds of plants. Isotope fractionation in simple physical and chemical processes is well understood and is commonly Current studies include

Carbon Isotopes in PhotosynthesisFractionation techniques may reveal new aspects of carbon dynamics in plants

Biological Magnetic Resonance Biological Applications of Magnetic Resonance R. F. Shulman Biological Magnetic Resonance Lawrence J. Berliner Jacques Reuben

Marion H. O'Leary

Papers

Carbon Isotopes in PhotosynthesisFractionation techniques may reveal new aspects of carbon dynamics in plants

Biological Magnetic Resonance Biological Applications of Magnetic Resonance R. F. Shulman Biological Magnetic Resonance Lawrence J. Berliner Jacques Reuben