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Seasonal variability of the soil CO2 flux and its isotopic composition in southern Poland

Gorczyca Z., Rozanski K., Kuc T., Michalec B.

Nukleonika

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2003

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Vol. 48, nr 4

187-196

EN

The flux and isotopic composition of soil CO2 has been monitored at three sites located in the southern Poland, during the time period: January 1998 - December 2000. The sites represent typical ecosystems appearing in central Europe: mixed forest, cultivated agricultural field and grassland. To monitor the flux and isotopic composition of soil CO2, the method based on the inverted cup principle was used. The flux of soil CO2 reveals distinct seasonal fluctuations, with maximum values up to ca. 20 mmolźm-2 h-1 during summer and around ten times lower values during winter. Also significant differences among the monitored sites were detected, the CO2 flux being the highest for the mixed forest site and ca. two times lower for the grassland site. The 13C content of the soil CO2 flux is nearly constant throughout the year, with d13C values essentially reflecting the isotopic composition of the soil organic matter and the vegetation type. The 18O content of the soil CO2 flux shows a remarkable seasonality, with distinctly less negative d18O values recorded during summer. This seasonality is highly reduced in the CO2 sampled from different depths of the soil. The radiocarbon content of the CO2 flux turned out to be non-distinguishable, within the uncertainty of the measurements, from current 14C levels in atmospheric carbon dioxide. However, significant reduction of the radiocarbon content was measured in the soil CO2 sampled at 80 cm, when compared to the uppermost layer, reflecting increasing age of soil organic matter with depth.

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Carbon-13 isotope effect for decarboxylation of phenylpropiolic acid (PPA) in concentrated phosphoric acids

Zieliński M., Zielińska A., Papiernik-Zielińska H., Ogrinc N., Kobal I.

Nukleonika

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2000

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Vol. 45, nr 2

121-124

EN

13C isotope effects on the decarboxylation of phenylpropiolic acid (PPA) in 93–97% H3PO4 and in 23% diphosphoric acid in orthophosphoric acid have been investigated from 293 to 353 K. The initial 13C fractionations in all three systems are in agreement with the 13C decarboxylation fractionations expected assuming the loss of the one carbon-carbon bond in the transition state. In 100% H3PO4 and in 23% diphosphoric acid in orthophosphoric acid later fractions of carbon dioxide are depleted in carbon-13 to a greater degree than expected, probably due to isotopic preequilibria between stable and decarboxylating forms of PPA. At 353 K and higher temperature strong deviations of the experimental rate constants and of the experimental 13C fractionations from the values extrapolated from lower temperature are observed. A tentative decarboxylation scheme operating in concentrated phosphoric acid media is proposed.

3

Carbon-13 fractionation in the oxidation of propionate of natural isotopic composition with permanganate and manganate

Zieliński M., Fry A., Zielińska A., Ogrinc N., Papiernik-Zielińska H., Kobal I.

Nukleonika

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2000

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Vol. 45, nr 4

221-224

EN

Carbon-13 fractionation in the oxidation of sodium propionate of natural isotopic composition with permanganate in water solution and with manganate in alkaline solution has been investigated and compared with carbon-14 kinetic isotope effects in the oxidation of (3-14C) – and (2-14C) propionate with permanganate. The experimental carbon-13 isotope ratios, R(13C/12C)cumul., corrected for dilution of R(13C/12C)a by carbon dioxide derived from all external sources and by carbon dioxide derived from (C-1) and from (C-3) of propionate at its complete and partial oxidations, used for calculation of 13C alfa KIE, provided the carbon-13 isotope effects corresponding to initial oxidation of alpha carbon, which correlate well with (2-14C) KIE determined previously, (k12/k13 -1)?100 ? 1/2(k12/k14 -1)?100 ? 1/2(4.6š0.3) at 373 K. The possibility of extension of isotope ratio measurements, using modern double collector mass spectrometers, from geochemical, biomedical, agriculture and environmental research to carbon-13 isotope effects studies in chemical organic reactions is briefly discussed.