# Residue Decomposition & Mineralization

Decomposition and mineralization of the fresh organic phosphorus 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:

$$\varepsilon\_{C:N}=\frac{0.58\*rsd\_{ly}}{orgN\_{frsh,ly}+NO3\_{ly}}$$                                                              3:2.2.6

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:

$$\varepsilon\_{C:P}=\frac{0.58\*rsd\_{ly}}{orgP\_{frsh,ly}+P\_{solution,ly}}$$                                                         3:2.2.7

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:

$$\delta\_{ntr,ly}=\beta\_{rsd}*\gamma\_{ntr,ly}*(\gamma\_{tmp,ly}\*\gamma\_{sw,ly})^{1/2}$$                             3:2.2.8

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]$$3:2.2.9

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 P pool is then calculated:

$$P\_{minf,ly}=0.8\*\delta\_{ntr,ly}\*orgP\_{frsh,ly}$$                                     3:2.2.10

where $$P\_{minf,ly}$$ is the phosphorus mineralized from the fresh organic $$P$$ pool (kg P/ha), $$\delta\_{ntr,ly}$$ is the residue decay rate constant, and $$orgP\_{frsh,ly}$$ is the phosphorus in the fresh organic pool in layer $$ly$$ (kg P/ha). Phosphorus mineralized from the fresh organic pool is added to the solution $$P$$ pool in the layer.

    Decomposition from the residue fresh organic P pool is calculated:

$$P\_{dec,ly}=0.2\*\delta\_{ntr,ly}\*orgP\_{frsh,ly}$$                                       3:2.2.11

where $$P\_{dec,ly}$$ is the phosphorus decomposed from the fresh organic $$P$$ pool (kg P/ha), $$\delta\_{ntr,ly}$$ is the residue decay rate constant, and $$orgP\_{frsh,ly}$$ is the phosphorus in the fresh organic pool in layer $$ly$$ (kg P/ha). Phosphorus decomposed from the fresh organic pool is added to the humus organic pool in the layer.

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

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