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A multi-layer box model of carbon dynamics in soil

Kuc T.

Nukleonika

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2005

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Vol. 50, nr 2

49-55

EN

Abstract A multi-layer box model (MLB) for quantification of carbon fluxes between soil and atmosphere has been developed. In the model, soil carbon reservoir is represented by two boxes: fast decomposition box (FDB) and slow decomposition box (SDB), characterised by substantially different turnover time (TT) of carbon compounds. Each box has an internal structure (sub-compartments) accounting for carbon deposited in consecutive time intervals. The rate of decomposition of carbon compounds in each sub-compartment is proportional to the carbon content. With the aid of the MLB model and the 14C signature of carbon dioxide, the fluxes entering and leaving the boxes, turnover time of carbon in each box, and the ratio of mass of carbon in the slow and fast box (Ms/Mf) were calculated. The MBL model yields the turnover time of carbon in the FDB (TTf) ca. 14 for typical investigated soils of temperate climate ecosystems. The calculated contribution of the CO2 flux originating from the slow box (Fs) to the total CO2 flux into the atmosphere ranges from 12% to 22%. These values are in agreement with experimental observations at different locations. Assuming that the input flux of carbon (Fin) to the soil system is doubled within the period of 100 years, the soil buffering capacity for excess carbon predicted by the MLB model for typical soil parameters may vary in the range between 26% and 52%. The highest values are obtained for soils characterised by long TTf, and well developed old carbon pool.

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Diurnal variations and vertical distribution of δ13C, and concentration of atmospheric and soil CO2 in a meadow site, SE Poland

Szaran J., Dudziak A., Trembaczowski A., Niezgoda H., Hałas S.

Geological Quarterly

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2005

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Vol. 49, No. 2

135--144

EN

We provide the results of 24-hour observations made in a meadow site located in a small river valley in Central Europe. Samples of atmospheric air were taken from three horizons: near the soil (0.05 m), in the grass (0.5 m) and above the meadow (2 m) at two-hour intervals. At the same time, samples of soil air were collected from two horizons: -0.1 and -0.5 m. We have found a variation of δ¹³C above the ground from -6‰ during the day to -20‰ late at night accompanied by variations in CO₂ concentration from 270 ppm during the day, to various levels late at night at different heights above the ground. The maximum concentration was 1430 ppm at the ground level. The correlation coefficient between δ¹³C and reciprocal of concentration was the highest (R2 = 0.984) for the samples collected 2 m above the ground, the regression line clearly indicating CO₂ mixing from the two sources: atmospheric and biogenic reservoirs. The intercept of the mixing line yields δC = -23.0‰ for the biogenic CO₂. In contrast, the diurnal variations in the soil were relatively small, δ¹³C varied from -21.6 to -23.4‰, while CO₂ concentration from 4300 to 8200 and from 24 700 to 34 500 ppm at depths of 0.1 m and 0.5 m respectively, which is less than 2-fold. Small diurnal variations are characteristic of dry soils, where δ^13C is weakly correlated with CO₂ concentration (in our case R2 was 0.30 and 0.54, respectively).

3

Carbon dynamics in soil recorded by 14C: model calculations

Kuc T., Gorczyca Z., Kapusta M.

Geochronometria

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2004

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Vol. 23

45-50

EN

Time records of 14CO2 from soil respiration collected in the years 1998 to 2003 on sites representing different ecosystems (mixed wood, agricultural field and grassland, southern Poland), were analysed by use of MLB model built for this purpose. The modelled turnover time (TT) is maximum 22 yr for the agriculturally cultivated soil and only 14 yr for soil under grassland and mixed forest. About 22% of an old component of 1500 yr was admixed to the agriculturally cultivated soil while it was only 12% under mixed forest. Estimated ratio of carbon content in slow and in fast decomposition boxes varies from 22 to 40 in southern Poland pointing to slow decomposing organic compounds as the major pool of carbon in soil. The lowest ratio was observed for soil of low proportion of organics content in mixed forest, and the highest for grassland of well developed soil profile. D14C time records for not cultivated soils (under grassland and mixed forest) showed higher values than local atmospheric CO2, moreover 14CO2 from the mixed forest soil respiration remarkable exceeded "clean air" reference level for Central Europe.

4

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.