> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-3-nutrients-pesticides/chapter-3-1-nitrogen/3-1.2-mineralization-and-decomposition-immobilization/3-1.2.2-residue-decomposition-and-mineralization.md).

# 3:1.2.2 Residue Decomposition & Mineralization

Decomposition and mineralization of the fresh organic nitrogen pool is allowed only in the first soil layer. Decomposition and mineralization are controlled by a decay rate constant that is updated daily. The decay rate constant is calculated as a function of the C:N ratio and C:P ratio of the residue, temperature and soil water content.

The C:N ratio of the residue is calculated:

$$\varepsilon *{C:N}=\frac{0.58\*rsd*{ly}}{orgN\_{frsh,ly}+NO3\_{ly}}$$                                                                                                                                        3:1.2.5

where $$\varepsilon\_{C:N}$$ is the C:N ratio of the residue in the soil layer, $$rsd\_{ly}$$ is the residue in layer $$ly$$ (kg/ha), 0.58 is the fraction of residue that is carbon, $$orgN\_{frsh,ly}$$ is the nitrogen in the fresh organic pool in layer $$ly$$ (kg N/ha), and $$NO3\_{ly}$$ is the amount of nitrate in layer $$ly$$ (kg N/ha).

The C:P ratio of the residue is calculated:

$$\varepsilon\_{C:P}=\frac{0.58\*rsd\_{ly}}{orgP\_{frsh,ly}+P\_{solution,ly}}$$                                                                                                                                    3:1.2.6

where $$\varepsilon\_{C:P}$$ is the C:P ratio of the residue in the soil layer, $$rsd\_{ly}$$ is the residue in layer $$ly$$ (kg/ha), 0.58 is the fraction of residue that is carbon, $$orgP\_{frsh,ly}$$ is the phosphorus in the fresh organic pool in layer $$ly$$ (kg P/ha), and $$P\_{solution,ly}$$ is the amount of phosphorus in solution in layer $$ly$$ (kg P/ha).

The decay rate constant defines the fraction of residue that is decomposed. The decay rate constant is calculated:

$$\delta\_{ntr,ly}=\beta\_{rsd}*\gamma\_{ntr,ly}*(\gamma\_{tmp,ly}\*\gamma\_{sw,ly})^{1/2}$$                                                                                                 3:1.2.7

where $$\delta\_{ntr,ly}$$ is the residue decay rate constant, $$\beta\_{rsd}$$ is the rate coefficient for mineralization of the residue fresh organic nutrients, $$\gamma\_{ntr,ly}$$ is the nutrient cycling residue composition factor for layer $$ly$$, $$\gamma\_{tmp,ly}$$ is the nutrient cycling temperature factor for layer $$ly$$, and $$\gamma\_{sw,ly}$$ is the nutrient cycling water factor for layer $$ly$$.

The nutrient cycling residue composition factor is calculated:

$$\gamma\_{ntr,ly}=min\[{exp\[-0.693\*\frac{(\varepsilon\_{C:N}-25)}{25}] ,exp\[-0.693\*\frac{(\varepsilon\_{C:P}-200)}{200}] , 1.0}]$$                                              3:1.2.8

where $$\gamma\_{ntr,ly}$$ is the nutrient cycling residue composition factor for layer $$ly$$, $$\varepsilon\_{C:N}$$ is the C:N ratio on the residue in the soil layer, and $$\varepsilon\_{C:P}$$ is the C:P ratio on the residue in the soil layer.

Mineralization from the residue fresh organic N pool is then calculated:

$$N\_{minf,ly}=0.8\*\delta\_{ntr,ly}\*orgN\_{frsh,ly}$$                                                                                                             3:1.2.9

where $$N\_{minf,ly}$$ is the nitrogen mineralized from the fresh organic N pool (kg N/ha), $$\delta\_{ntr,ly}$$ is the residue decay rate constant, and $$orgN\_{frsh,ly}$$ is the nitrogen in the fresh organic pool in layer $$ly$$ (kg N/ha). Nitrogen mineralized from the fresh organic pool is added to the nitrate pool in the layer.

 Decomposition from the residue fresh organic N pool is calculated:

$$N\_{dec,ly}=0.2\*\delta\_{ntr,ly}\*orgN\_{frsh,ly}$$                                                                                                              3:1.2.10

where $$N\_{dec,ly}$$ is the nitrogen decomposed from the fresh organic N pool (kg N/ha), $$\delta\_{ntr,ly}$$ is the residue decay rate constant, and $$orgN\_{frsh,ly}$$ is the nitrogen in the fresh organic pool in layer $$ly$$ (kg N/ha). Nitrogen decomposed from the fresh organic pool is added to the humus active organic pool in the layer.

 Table 3:1-2: SWAT+ input variables that pertain to mineralization.

| Definition                                                                                   | Source Name | Input Name | Input File |
| -------------------------------------------------------------------------------------------- | ----------- | ---------- | ---------- |
| $$\beta\_{min}$$: Rate coefficient for mineralization of the humus active organic nutrients  | CMN         |            | .bsn       |
| $$\beta\_{rsd}$$: Rate coefficient for mineralization of the residue fresh organic nutrients | RSDCO       |            | .bsn       |
| $$\beta\_{rsd}$$: Rate coefficient for mineralization of the residue fresh organic nutrients | RSDCO\_PL   |            | crop.dat   |