Carbon turnover

Until now we have viewed the TCA cycle as a catabolic process because it oxidizes acetate units to COg and converts the liberated energy to ATP and reduced coenzymes. The TCA cycle is, after all, the end point for breakdown of food materials, at least in terms of carbon turnover. However, as shown in Figure 20.22, four-, five-, and six-carbon species produced in the TCA cycle also fuel avariety of biosynthetic processes. a-Ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate are all precursors of important cellular species. (In order to par-... 

S. W. Leavitt, E. A. Paul, A. Galadima, F. S. Nakayama, S. R. Danzer, H. Johnson, and B. A. Kimball, Carbon isotopes and carbon turnover in cotton and wheat FACE experiments. Plant Soil 787 147 (1996). 

Bird MI, Chivas AR, Head J (1996) A latitudinal gradient in carbon turnover times in forest soils. Nature 381 143-146... 

Most historical studies do not contain the detailed information needed to develop carbon budgets. They are also confounded by erosional losses, changes in the chemical methods to measure SOC, management-induced differences in bulk density, and different methods to calculate turnover kinetics. The consequences of these problems are that it is difficult to compare studies and calculate carbon turnover rates. To overcome these problems simplifying assumptions are often used (Clay et al. 2006 Johnson et al. 2006 Bolinder et al. 2007). Assumptions can reduce the usefulness of the findings. This chapter reviews non-isotopic and 13C isotopic approaches for determining SOC maintenance and implications of simplifying assumptions on SOC turnover calculations. 

Once the carbon inputs are known or estimated, several different methods can be used to determine carbon turnover. SOC turnover can be described using zero and first-order kinetics (Paul and Clark 1989). For zero-order kinetics, the temporal change in the substrate concentration (SSOC/St) is defined by the equation... 

For systems where the mineralization rate is dependent on the substrate concentration, first-order kinetics can be used to describe carbon turnover (Paul and Clark 1989 Six and Jastrow 2002). The first-order rate equation is... 

The Rothamsted Carbon Model (RothC) uses a five pool structure, decomposable plant material (DPM), resistant plant materials (RPM), microbial biomass, humified organic matter, and inert organic matter to assess carbon turnover (Coleman and Jenkinson 1996 Guo et al. 2007). The first four pools decompose by first-order kinetics. The decay rate constants are modified by temperature, soil moisture, and indirectly by clay content. RothC does not include a plant growth sub-module, and therefore NHC inputs must be known, estimated, or calculated by inverse modeling. Skjemstad et al. (2004) tested an approach for populating the different pools based on measured values. 

An alternative approach is to combine models with field measurements to assist in developing carbon budgets (Huggins et al. 1998). Clay et al. (2005) used first-order models to calculate the amount of residue returned to the soil from C3 and C4 plants over an 8-year period. Based on the mineralization rates and when the C3 and C4 residues were returned, the 813C signature of non-harvested biomass was determined. Based on the rates, carbon turnover, the amount of SOC mineralized, and the amount of fresh biomass incorporated into the SOC over an 8-year period were determined. 

Carbon turnover in production fields can be determined, using non-isotopic techniques, by combining historical soil samples, current soil samples, and whole field yield monitor data. Sensitivity analysis of such data shows that the amount of above-ground biomass that could be harvested decreases with root to shoot ratio (Table 8.1). For example, if root biomass is ignored, analysis suggests that only 20-30% of the above-ground biomass can be harvested, whereas if the root to shoot ratio is 1.0, then between 40% and 70% of the residue could be harvested. 

GleixnerG, Bol R, Balesdent J (1999) Molecular insight into soil carbon turnover. Rapid Commun Mass Spectrom 13 1278-1283... 

Skjemstad JO, Spouncer LR, Cowie B, Swift RS (2004) Calibration of the Rathamsted organic carbon turnover model using measurable soil organic carbon pools. Aus J Soil Res 42 79-88 Soon YK, Malhi SS (2005) Soil nitrogen dynamics as affected by landscape position and nitrogen fertilizer. Can J Soil Sci 85 579-587... 

Eklind Y, Sundberg C, Smars S, Steger K, Sundh I, Kirchmann H, Joensson H (2007) Carbon turnover and ammonia emissions during composting of biowaste at different temperatures. J Environ Qual 36 1512-1520... 

The parameters for characterising soil microbial activity used in the reviewed research results are total microbial biomass, diverse enzymatic parameters, carbon turnover parameters andffiycorrhization, ... 

Sinsabaugh. R. L., and S. Findlay. 1995. Microbial production, enzyme activity and carbon turnover in surface sediments of the Hudson River Estuary. Microbial Ecology 30 127-141. 

Veldkamp, E. (1994). Organic-carbon turnover in 3 tropical soils under pasture after deforestation. Soil Sci. Soc. Am. J. 58,175-180. 

Ewing, S. A., Sanderman, I, Baisden, W.T., Wang, Y., and Amundson, R. (2006). Role of large-scale soil structure in organic carbon turnover Evidence from California grassland soils. 

Krull, E. S., Baldock, J. A., and Skjemstad, J. O. (2003). Importance of mechanisms and processes of the stabilisation of soil organic matter for modelling carbon turnover. Fund. Plant Biol. 30(2), 207-222. 

Figure 3.1 Decomposition and carbon turnover in soil A conceptual diagram summarizing the main elements of the initial Rothamsted carbon model (Jen-kinson 1971). To this we have added other small, but potentially functionally important, compartments the volatile organic carbon and the dissolved organic carbon derived during both decomposition of litter and exudation from plants. An inert organic matter pool is added as this appears in later versions of the Rothamsted model.

The bomb pulse method is now considered to be one of the most useful techniques for distinguishing between modern and ancient remains (Geyh 2001 Ubelaker, Buccholz, and Stewart 2006 Wild et al. 1998). Attempts to improve the accuracy of the technique have demonstrated the importance of analyzing a bone fraction that contains a rapid turnover rate (Wild et al. 1998). Analysis of the collagen fraction of recent bone has been shown to be less accurate as a result of its relatively long carbon turnover rate compared with other bone fractions (Ubelaker 2001). Lipids from bone and bone... 

Veldkamp, E. 1994. "Organic carbon turnover in three trap-... 

Figure 4 The global disequilibrium effect. value of CO2 currently fixed into plants (associated with photosynthetic discrimination, is lower than that of older CO2 respired back to the atmospheric CO2 (no fractionation is assumed). This is due to the rapid decrease in atmospheric associated with fossil fuel emissions, on the one hand, and to the slow turnover of carbon in the biosphere, on the other hand. A similar disequilibrium occurs in the ocean where the atmospheric trend influences the values of newly formed Die, while the ocean mean DIG pool lags behind this equilibrium values due to slow mmover rates (not shown). The atmospheric trend shown is based on the best fit line to the data of Francey et al. (1999) the land organic matter trend is obtained by appl3ung global mean = 18%o, and moving it back in time by 27 yr, the first order estimate of global mean soil carbon turnover time. The resulting 0.6%o disequilibrium for the 1990s is within the range of current estimates for both land and ocean.

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