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Soil C Saturation: Determining Rates and Limits of Carbon Sequestration

Executive Summary 
Increasing soil C through changes in land use and management is a low cost and environmentally beneficial method of sequestering substantial amounts of atmospheric CO2.



Collaborators: Keith Paustian, Rich Conant, and Eldor Paul  

     
Increasing soil C through changes in land use and management is a low cost and environmentally beneficial method of sequestering substantial amounts of atmospheric CO2. However, it is generally viewed that soils, like other biological sinks (e.g. vegetation stocks), have an inherent upper limit above which no additional C can be stored. The magnitude of this upper or ‘saturation’ limit is crucial to know as it will govern the ultimate significance of the soil sink and the time period over which it can be exploited for CO2 sequestration. However, at present, we have little knowledge of the ‘C carrying capacity’ of soils and moreover we have not quantified how rates of C sequestration differ for soils that are far from, versus close to, some saturation level.  

     
From a synthesis of knowledge on stabilization mechanisms for different soil organic matter pools we have conceptualized a model of SOM dynamics based on four measurable pools (see Figure): 1) a biochemically-protected C pool, 2) a silt- and clay-protected C pool, 3) a microaggregate-protected C pool, and 4) an unprotected C pool (Six et al. 2002). Each pool has its own dynamics and stabilizing mechanisms, which we hypothesize to determine a level at which soil C becomes saturated. However, to develop a quantitative model based on our conceptual model the controls on the dynamics and sizes of our proposed pools need to be better quantified. The formation and degradation processes of microaggregates are less understood than that of macroaggregates and minimal quantitative data is available for these processes. Even though we delineated the four pools as conceptually different pools with different stabilization mechanisms because we believe that they are the main stabilization mechanisms for them, there might be some overlap in stabilization mechanisms between the different pools.  

     
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References:  

     
Six, J., R.T. Conant, E.A. Paul, and K. Paustian. 2002. Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils. Plant and Soil 241:155-176.  

     
For more information, review this document. six_plantandsoil.pdf
  

     

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