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:

3:1.2.6

where is the C:P ratio of the residue in the soil layer, is the residue in layer (kg/ha), 0.58 is the fraction of residue that is carbon, is the phosphorus in the fresh organic pool in layer (kg P/ha), and is the amount of phosphorus in solution in layer (kg P/ha).

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

3:1.2.7

where is the residue decay rate constant, is the rate coefficient for mineralization of the residue fresh organic nutrients, is the nutrient cycling residue composition factor for layer , is the nutrient cycling temperature factor for layer , and is the nutrient cycling water factor for layer .

The nutrient cycling residue composition factor is calculated:

3:1.2.8

where is the nutrient cycling residue composition factor for layer , is the C:N ratio on the residue in the soil layer, and is the C:P ratio on the residue in the soil layer.

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

3:1.2.9

where is the nitrogen mineralized from the fresh organic N pool (kg N/ha), is the residue decay rate constant, and is the nitrogen in the fresh organic pool in layer (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:

3:1.2.10

where is the nitrogen decomposed from the fresh organic N pool (kg N/ha), is the residue decay rate constant, and is the nitrogen in the fresh organic pool in layer (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.