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:

$N_{dec,ly}=0.2*\delta_{ntr,ly}*orgN_{frsh,ly}$ 3:1.2.9

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.

Variable Name	Definition	File Name
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