The CENTURY model can simulate labeling by either 14C or 13C. C labeling is specified in the site management. The 14C simulations act as a labeled tracer from atmospheric sources or added organic matter (ASTLBL, omad.100). The c14data file contains a record of atmospheric 14C concentrations which are used by the model to label new plant material, which then flows through the other organic matter pools. Click here to view a sample c14data file.
Simulations using the option for 13C give a constant label to plant material based on the value of DEL13C in the crop parameters and tree parameters. This option will primarily be of use to follow the change in stable isotope signal when there has been a switch from C3 to C4 vegetation or vice-versa. Fractionation of the stable carbon isotopes is included in the model as discussed below.
The 13C/12C ratio in soil organic matter remains close to the ratio in the original vegetation, but fractionation during decomposition of the plant residues and soil organic matter can produce significant changes in the ratio. The magnitude and direction of the change in the ratio may vary with time and the prevailing environmental conditions (Stout and Rafter, 1978; Stout et al., 1981).
13C/12C ratios are expressed relative to a standard as delta 13C values, where
The standard is carbonate from Pee Dee belemnite limestone and units are per mille (). Atmospheric CO2, plant material, and soil organic matter are depleted in 13C relative to the standard and therefore have negative delta 13C values. The more depleted in 13C a material is, the more negative the delta 13C value will be.
Stout et al. (1981) identified four points in the biological carbon cycle where major fractionation of carbon isotopes occurs. The first takes place during photosynthesis with plant tissue being depleted in 13C relative to atmospheric CO2. Of considerable interest is the difference in delta 13C between plants with different photosynthesis pathways (Bender, 1971; Smith and Epstein, 1971). The C3 plants, with the Calvin pathway, have low delta 13C values (-24 to -34), while the C4 plants, with the Hatch and Slack pathway, have high delta 13C values (-6 to -19). This difference in stable carbon isotope signature can be used as a tracer for in situ labelling of soil organic matter when the dominant vegetation type has changed from C3 to C4 species or vice-versa (Cerri et al., 1985; Schwartz et al., 1986; Balesdent et al., 1987; Balesdent et al., 1988; Martin et al., 1990; Balesdent and Balabane, 1992). The CENTURY model has been modified to partition carbon production by plants to the two isotope pools on the basis of a delta 13C value nominated in the crop.100 file for each grassland or crop type.
The second major biological fractionation occurs in the synthesis of the major cell components (Stout et al., 1981). The data of Benner et al. (1987) for a variety of vascular plants showed that cellulose and hemicellulose were typically enriched in 13C by 1 to 2 relative to whole plant material while lignin was depleted by 2 to 6. They observed a greater depletion of 13C in grass lignins than in wood lignins, which they attributed to different amino acid precursors. In the CENTURY model this fractionation in the partitioning of plant material (shoots and roots from crops and grasses, and leaves and fine roots from trees) to the structural and metabolic pools is accounted for as all of the plant lignin is assumed to enter the structural pool. The 13C depletion of lignin relative to the whole plant 13C signature can be altered (DLIGDF, fixed parameters). Because all dead wood and large tree roots enter dead wood pools, which are analogous to the structural pool, there was no need to account for 13C fractionation in wood lignin.
The third major biological fractionation of carbon noted by Stout et al. (1981) is associated with animal consumption of plant material, with animal tissues being depleted in 13C relative to the plant material on which they feed. This is not accounted for this in the model because the important comparison for the CENTURY model is between delta 13C levels in feces and plant material.
The fourth major biological fractionation of carbon takes place during microbial metabolism (Stout et al., 1981). Macko and Estep (1984) examined the isotopic composition of an aerobic, heterotrophic bacteria growing on a variety of amino acid substrates. With most of substrates the bacterial cells were enriched in 13C relative to the amino acid. They suggested that the CO2 respired during the Krebs cycle would be isotopically depleted in 13C. However, in an anaerobic environment methane evolved is very depleted in 13C relative to the organic substrate, but the CO2 evolved is enriched (Games and Hayes, 1976). The net effect on the residual organic matter would depend on the relative size of the fluxes. Environmental effects on fractionation are also reflected in different patterns of stable isotope distribution in soil profiles (Stout and Rafter, 1978). In well-drained mineral soils delta 13C values increase slightly with depth and soil age, which is consistent with respired CO2 being slightly depleted in 13C. In organic soils where decomposition is inhibited the delta 13C values decrease with depth. This could be due to the loss of readily decomposable plant fractions, such as sugars and proteins, with an accumulation of lignin, lipids and waxes in the residual plant material, resulting in depletion of 13C relative to the original plant material (Stout et al., 1981). In other soils, with intermediate levels of drainage and organic matter accumulation, there may be no change in delta 13C values with depth indicating a balance between fractionation due to respiration and accumulation of the depleted plant fractions. All decomposition flows in the CENTURY model are assumed to be the result of microbial activity and have an associated loss of CO2. Fractionation of the carbon isotopes in the loss of CO2 is allowed for (DRESP, fixed parameters). The coefficient for isotope discrimination was calibrated to give a slight increase in the delta 13C value for the total soil organic matter relative to the vegetation.