## 2.25.3. CH4 Production[¶](#ch4-production "Permalink to this headline")
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Because CLM does not currently specifically represent wetland plant functional types or soil biogeochemical processes, we used gridcell-averaged decomposition rates as proxies. Thus, the upland (default) heterotrophic respiration is used to estimate the wetland decomposition rate after first dividing off the O2 limitation. The O2 consumption associated with anaerobic decomposition is then set to the unlimited version so that it will be reduced appropriately during O2 competition. CH4 production at each soil level in the anaerobic portion (i.e., below the water table) of the column is related to the gridcell estimate of heterotrophic respiration from soil and litter (RH; mol C m\-2 s\-1) corrected for its soil temperature (\\({T}\_{s}\\)) dependence, soil temperature through a \\({A}\_{10}\\) factor (\\(f\_{T}\\)), pH (\\(f\_{pH}\\)), redox potential (\\(f\_{pE}\\)), and a factor accounting for the seasonal inundation fraction (_S_, described below):

(2.25.2)[¶](#equation-24-2 "Permalink to this equation")\\\[P=R\_{H} f\_{CH\_{4} } f\_{T} f\_{pH} f\_{pE} S.\\\]

Here, \\(f\_{CH\_{4} }\\) is the baseline ratio between CO2 and CH4 production (all parameters values are given in [Table 2.25.1](#table-methane-parameter-descriptions)). Currently, \\(f\_{CH\_{4} }\\) is modified to account for our assumptions that methanogens may have a higher Q\\({}\_{10}\\) than aerobic decomposers; are not N limited; and do not have a low-moisture limitation.

When the single BGC soil level is used in CLM (Chapter [2.21](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Decomposition/CLM50_Tech_Note_Decomposition.html#rst-decomposition)), the temperature factor, \\(f\_{T}\\), is set to 0 for temperatures equal to or below freezing, even though CLM allows heterotrophic respiration below freezing. However, if the vertically resolved BGC soil column is used, CH4 production continues below freezing because liquid water stress limits decomposition. The base temperature for the \\({Q}\_{10}\\) factor, \\({T}\_{B}\\), is 22°C and effectively modified the base \\(f\_{CH\_{4}}\\) value.

For the single-layer BGC version, \\({R}\_{H}\\) is distributed among soil levels by assuming that 50% is associated with the roots (using the CLM PFT-specific rooting distribution) and the rest is evenly divided among the top 0.28 m of soil (to be consistent with CLM’s soil decomposition algorithm). For the vertically resolved BGC version, the prognosed distribution of \\({R}\_{H}\\) is used to estimate CH4 production.

The factor \\(f\_{pH}\\) is nominally set to 1, although a static spatial map of _pH_ can be used to determine this factor ([Dunfield et al. 1993](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#dunfieldetal1993)) by applying:

(2.25.3)[¶](#equation-24-3 "Permalink to this equation")\\\[f\_{pH} =10^{-0.2235pH^{2} +2.7727pH-8.6} .\\\]

The \\(f\_{pE}\\) factor assumes that alternative electron acceptors are reduced with an e-folding time of 30 days after inundation. The default version of the model applies this factor to horizontal changes in inundated area but not to vertical changes in the water table depth in the upland fraction of the gridcell. We consider both \\(f\_{pH}\\) and \\(f\_{pE}\\) to be poorly constrained in the model and identify these controllers as important areas for model improvement.

As a non-default option to account for CH4 production in anoxic microsites above the water table, we apply the Arah and Stephen (1998) estimate of anaerobic fraction:

(2.25.4)[¶](#equation-24-4 "Permalink to this equation")\\\[\\varphi =\\frac{1}{1+\\eta C\_{O\_{2} } } .\\\]

Here, \\(\\varphi\\) is the factor by which production is inhibited above the water table (compared to production as calculated in equation [(2.25.2)](#equation-24-2), \\(C\_{O\_{2}}\\) (mol m\-3) is the bulk soil oxygen concentration, and \\(\\eta\\) = 400 mol m\-3.

The O2 required to facilitate the vertically resolved heterotrophic decomposition and root respiration is estimated assuming 1 mol O2 is required per mol CO2 produced. The model also calculates the O2 required during nitrification, and the total O2 demand is used in the O2 mass balance solution.

Table 2.25.1 Parameter descriptions and sensitivity analysis ranges applied in the methane model[¶](#id14 "Permalink to this table")      
| Mechanism
 | Parameter

 | Baseline Value

 | Range for Sensitivity Analysis

 | Units

 | Description

 |
| --- | --- | --- | --- | --- | --- |
| Production

 | \\({Q}\_{10}\\)

 | 2

 | 1.5 – 4

 | 

 | CH4 production \\({Q}\_{10}\\)

 |
|  | \\(f\_{pH}\\)

 | 1

 | On, off

 | 

 | Impact of pH on CH4 production

 |
|  | \\(f\_{pE}\\)

 | 1

 | On, off

 | 

 | Impact of redox potential on CH4 production

 |
|  | _S_

 | Varies

 | NA

 | 

 | Seasonal inundation factor

 |
|  | \\(\\beta\\)

 | 0.2

 | NA

 | 

 | Effect of anoxia on decomposition rate (used to calculate _S_ only)

 |
|  | \\(f\_{CH\_{4} }\\)

 | 0.2

 | NA

 | 

 | Ratio between CH4 and CO2 production below the water table

 |
| Ebullition

 | \\({C}\_{e,max}\\)

 | 0.15

 | NA

 | mol m\-3

 | CH4 concentration to start ebullition

 |
|  | \\({C}\_{e,min}\\)

 | 0.15

 | NA

 | 

 | CH4 concentration to end ebullition

 |
| Diffusion

 | \\(f\_{D\_{0} }\\)

 | 1

 | 1, 10

 | m2 s\-1

 | Diffusion coefficient multiplier (Table 24.2)

 |
| Aerenchyma

 | _p_

 | 0.3

 | NA

 | 

 | Grass aerenchyma porosity

 |
|  | _R_

 | 2.9\\(\\times\\)10\-3 m

 | NA

 | m

 | Aerenchyma radius

 |
|  | \\({r}\_{L}\\)

 | 3

 | NA

 | 

 | Root length to depth ratio

 |
|  | \\({F}\_{a}\\)

 | 1

 | 0.5 – 1.5

 | 

 | Aerenchyma conductance multiplier

 |
| Oxidation

 | \\(K\_{CH\_{4} }\\)

 | 5 x 10\-3

 | 5\\(\\times\\)10\\({}^{-4}\\)\\({}\_{ }\\)\- 5\\(\\times\\)10\-2

 | mol m\-3

 | CH4 half-saturation oxidation coefficient (wetlands)

 |
|  | \\(K\_{O\_{2} }\\)

 | 2 x 10\-2

 | 2\\(\\times\\)10\-3 - 2\\(\\times\\)10\-1

 | mol m\-3

 | O2 half-saturation oxidation coefficient

 |
|  | \\(R\_{o,\\max }\\)

 | 1.25 x 10\\({}^{-5}\\)

 | 1.25\\(\\times\\)10\\({}^{-6}\\) - 1.25\\(\\times\\)10\\({}^{-4}\\)

 | mol m\-3 s\-1

 | Maximum oxidation rate (wetlands)

 |

