Title: 2.21. Decomposition — ctsm CTSM master documentation

URL Source: https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Decomposition/CLM50_Tech_Note_Decomposition.html

Markdown Content:
Decomposition of fresh litter material into progressively more recalcitrant forms of soil organic matter is represented in CLM is defined as a cascade of \\({k}\_{tras}\\) transformations between \\({m}\_{pool}\\) decomposing coarse woody debris (CWD), litter, and soil organic matter (SOM) pools, each defined at \\({n}\_{lev}\\) vertical levels. CLM allows the user to define, at compile time, between 2 contrasting hypotheses of decomposition as embodied by two separate decomposition submodels: the CLM-CN pool structure used in CLM4.0, or a second pool structure, characterized by slower decomposition rates, based on the fCentury model (Parton et al 1988). In addition, the user can choose, at compile time, whether to allow \\({n}\_{lev}\\) to equal 1, as in CLM4.0, or to equal the number of soil levels used for the soil hydrological and thermal calculations (see Section [2.2.2.1](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Ecosystem/CLM50_Tech_Note_Ecosystem.html#soil-layers) for soil layering).

![Image 1: ../../_images/CLM4_vertsoil_soilstruct_drawing.png](https://escomp.github.io/ctsm-docs/versions/master/html/_images/CLM4_vertsoil_soilstruct_drawing.png)

Figure 2.21.1 Schematic of decomposition model in CLM.[¶](#id1 "Permalink to this image")

Model is structured to allow different representations of the soil C and N decomposition cascade, as well as a vertically-explicit treatment of soil biogeochemistry.

For the single-level model structure, the fundamental equation for carbon balance of the decomposing pools is:

(2.21.1)[¶](#equation-21-1 "Permalink to this equation")\\\[\\frac{\\partial C\_{i} }{\\partial t} =R\_{i} +\\sum \_{j\\ne i}\\left(i-r\_{j} \\right)T\_{ji} k\_{j} C\_{j} -k\_{i} C\_{i}\\\]

where \\({C}\_{i}\\) is the carbon content of pool _i_, \\({R}\_{i}\\) are the carbon inputs from plant tissues directly to pool _i_ (only non-zero for CWD and litter pools), \\({k}\_{i}\\) is the decay constant of pool _i_; \\({T}\_{ji}\\) is the fraction of carbon directed from pool _j_ to pool _i_ with fraction \\({r}\_{j}\\) lost as a respiration flux along the way.

Adding the vertical dimension to the decomposing pools changes the balance equation to the following:

(2.21.2)[¶](#equation-21-2 "Permalink to this equation")\\\[\\begin{split}\\begin{array}{l} {\\frac{\\partial C\_{i} (z)}{\\partial t} =R\_{i} (z)+\\sum \_{i\\ne j}\\left(1-r\_{j} \\right)T\_{ji} k\_{j} (z)C\_{j} (z) -k\_{i} (z)C\_{i} (z)} \\\\ {+\\frac{\\partial }{\\partial z} \\left(D(z)\\frac{\\partial C\_{i} }{\\partial z} \\right)+\\frac{\\partial }{\\partial z} \\left(A(z)C\_{i} \\right)} \\end{array}\\end{split}\\\]

where \\({C}\_{i}\\)(z) is now defined at each model level, and in volumetric (gC m\-3) rather than areal (gC m\-2) units, along with \\({R}\_{i}\\)(z) and \\({k}\_{j}\\)(z). In addition, vertical transport is handled by the last two terms, for diffusive and advective transport. In the base model, advective transport is set to zero, leaving only a diffusive flux with diffusivity _D(z)_ defined for all decomposing carbon and nitrogen pools. Further discussion of the vertical distribution of carbon inputs \\({R}\_{i}\\)(z), vertical turnover times \\({k}\_{j}\\)(z), and vertical transport _D(z)_ is below Discussion of the vertical model and analysis of both decomposition structures is in [Koven et al. (2013)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#kovenetal2013).

![Image 2: ../../_images/soil_C_pools_CN_century.png](https://escomp.github.io/ctsm-docs/versions/master/html/_images/soil_C_pools_CN_century.png)

Figure 2.21.2 Pool structure, transitions, respired fractions (numbers at end of arrows), and turnover times (numbers in boxes) for the 2 alternate soil decomposition models included in CLM.[¶](#id2 "Permalink to this image")

