## 2.21.2. Century-based Pool Structure, Rate Constants and Parameters[¶](#century-based-pool-structure-rate-constants-and-parameters "Permalink to this headline")
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The Century-based decomposition cascade is, like CLM-CN, a first-order decay model; the two structures differ in the number of pools, the connections between those pools, the turnover times of the pools, and the respired fraction during each transition (Figure 15.2). The turnover times are different for the Century-based pool structure, following those described in Parton et al. (1988) ([Table 2.21.3](#table-turnover-times)).

Table 2.21.3 Turnover times, C:N ratios, and acceleration parameters for the Century-based decomposition cascade.[¶](#id5 "Permalink to this table")    
|  | Turnover time (year)
 | C:N ratio

 | Acceleration term (\\({a}\_{i}\\))

 |
| --- | --- | --- | --- |
| CWD

 | 4.1

 | 

 | 1

 |
| Litter 1

 | 0.066

 | 

 | 1

 |
| Litter 2

 | 0.25

 | 

 | 1

 |
| Litter 3

 | 0.25

 | 

 | 1

 |
| SOM 1

 | 0.17

 | 8

 | 1

 |
| SOM 2

 | 6.1

 | 11

 | 15

 |
| SOM 3

 | 270

 | 11

 | 675

 |

Likewise, values for the respiration fraction of Century-based structure are in [Table 2.21.4](#table-respiration-fractions-for-century-based-structure).

Table 2.21.4 Respiration fractions for litter and SOM pools for Century-based structure[¶](#id6 "Permalink to this table")  
| Pool
 | _rf_

 |
| --- | --- |
| \\({rf}\_{Lit1}\\)

 | 0.55

 |
| \\({rf}\_{Lit2}\\)

 | 0.5

 |
| \\({rf}\_{Lit3}\\)

 | 0.5

 |
| \\({rf}\_{SOM1}\\)

 | f(txt)

 |
| \\({rf}\_{SOM2}\\)

 | 0.55

 |
| \\({rf}\_{SOM3}\\)

 | 0.55

 |