## 2.21.1. CLM-CN Pool Structure, Rate Constants and Parameters[¶](#clm-cn-pool-structure-rate-constants-and-parameters "Permalink to this headline")
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The CLM-CN structure in CLM45 uses three state variables for fresh litter and four state variables for soil organic matter (SOM). The masses of carbon and nitrogen in the live microbial community are not modeled explicitly, but the activity of these organisms is represented by decomposition fluxes transferring mass between the litter and SOM pools, and heterotrophic respiration losses associated with these transformations. The litter and SOM pools in CLM-CN are arranged as a converging cascade (Figure 15.2), derived directly from the implementation in Biome-BGC v4.1.2 (Thornton et al. 2002; Thornton and Rosenbloom, 2005).

Model parameters are estimated based on a synthesis of microcosm decomposition studies using radio-labeled substrates (Degens and Sparling, 1996; Ladd et al. 1992; Martin et al. 1980; Mary et al. 1993 Saggar et al. 1994; Sørensen, 1981; van Veen et al. 1984). Multiple exponential models are fitted to data from the microcosm studies to estimate exponential decay rates and respiration fractions (Thornton, 1998). The microcosm experiments used for parameterization were all conducted at constant temperature and under moist conditions with relatively high mineral nitrogen concentrations, and so the resulting rate constants are assumed not limited by the availability of water or mineral nitrogen. [Table 2.21.1](#table-decomposition-rate-constants) lists the base decomposition rates for each litter and SOM pool, as well as a base rate for physical fragmentation for the coarse woody debris pool (CWD).

Table 2.21.1 Decomposition rate constants for litter and SOM pools, C:N ratios, and acceleration parameters for the CLM-CN decomposition pool structure.[¶](#id3 "Permalink to this table")     
|  | Biome-BGC
 | CLM-CN

 |  |  |
| --- | --- | --- | --- | --- |
|  | \\({k}\_{disc1}\\)(d\-1)

 | \\({k}\_{disc2}\\) (hr\-1)

 | _C:N ratio_

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

 |
| \\({k}\_{Lit1}\\)

 | 0.7

 | 0.04892

 | 

 | 1

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

 | 0.07

 | 0.00302

 | 

 | 1

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

 | 0.014

 | 0.00059

 | 

 | 1

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

 | 0.07

 | 0.00302

 | 12

 | 1

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

 | 0.014

 | 0.00059

 | 12

 | 1

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

 | 0.0014

 | 0.00006

 | 10

 | 5

 |
| \\({k}\_{SOM4}\\)

 | 0.0001

 | 0.000004

 | 10

 | 70

 |
| \\({k}\_{CWD}\\)

 | 0.001

 | 0.00004

 | 

 | 1

 |

The first column of [Table 2.21.1](#table-decomposition-rate-constants) gives the rates as used for the Biome-BGC model, which uses a discrete-time model with a daily timestep. The second column of [Table 2.21.1](#table-decomposition-rate-constants) shows the rates transformed for a one-hour discrete timestep typical of CLM-CN. The transformation is based on the conversion of the initial discrete-time value (\\({k}\_{disc1}\\) first to a continuous time value (\\({k}\_{cont}\\)), then to the new discrete-time value with a different timestep (\\({k}\_{disc2}\\)), following Olson (1963):

(2.21.3)[¶](#equation-zeqnnum608251 "Permalink to this equation")\\\[k\_{cont} =-\\log \\left(1-k\_{disc1} \\right)\\\]

(2.21.4)[¶](#equation-zeqnnum772630 "Permalink to this equation")\\\[k\_{disc2} =1-\\exp \\left(-k\_{cont} \\frac{\\Delta t\_{2} }{\\Delta t\_{1} } \\right)\\\]

where \\(\\Delta\\)\\({t}\_{1}\\) (s) and \\(\\Delta\\)t2 (s) are the time steps of the initial and new discrete-time models, respectively.

Respiration fractions are parameterized for decomposition fluxes out of each litter and SOM pool. The respiration fraction (_rf_, unitless) is the fraction of the decomposition carbon flux leaving one of the litter or SOM pools that is released as CO2 due to heterotrophic respiration. Respiration fractions and exponential decay rates are estimated simultaneously from the results of microcosm decomposition experiments (Thornton, 1998). The same values are used in CLM-CN and Biome-BGC ([Table 2.21.2](#table-respiration-fractions-for-litter-and-som-pools)).

Table 2.21.2 Respiration fractions for litter and SOM pools[¶](#id4 "Permalink to this table")  
| Pool
 | _rf_

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

 | 0.39

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

 | 0.55

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

 | 0.29

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

 | 0.28

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

 | 0.46

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

 | 0.55

 |
| \\({rf}\_{SOM4}\\)

 | \\({1.0}^{a}\\)

 |

a\\({}^{a}\\) The respiration fraction for pool SOM4 is 1.0 by definition: since there is no pool downstream of SOM4, the entire carbon flux leaving this pool is assumed to be respired as CO2.