Nitrogen is allowed to move between the active and stable organic pools in the humus fraction. The amount of nitrogen transferred from one pool to the other is calculated:

$N_{trns,ly}=\beta_{trns}*orgN_{act,ly}*(\frac{1}{fr_{actN}}-1)-orgN_{sta,ly}$ 3:1.2.3

$N_{trns,ly}$ is the amount of nitrogen transferred between the active and stable organic pools (kg N/ha), $\beta_{trns}$ is the rate constant (1×10$^{-5}$), $orgN_{act,ly}$ is the amount of nitrogen in the active organic pool (kg N/ha), $fr_{actN}$ is the fraction of humic nitrogen in the active pool (0.02), and $orgN_{sta,ly}$ is the amount of nitrogen in the stable organic pool (kg N/ha). When $N_{trns,ly}$ is positive, nitrogen is moving from the active organic pool to the stable organic pool. When $N_{trns,ly}$ is negative, nitrogen is moving from the stable organic pool to the active organic pool.

Mineralization from the humus active organic $N$ pool is calculated:

$N_{mina,ly}=\beta_{min}*(\gamma_{tmp,ly}*\gamma_{sw,ly})^{1/2}*orgN_{act,ly}$ 3:1.2.4

where $N_{mina,ly}$ is the nitrogen mineralized from the humus active organic $N$ pool (kg N/ha), $\beta_{min}$ is the rate coefficient for mineralization of the humus active organic nutrients, $\gamma_{tmp,ly}$ is the nutrient cycling temperature factor for layer $ly$, $\gamma_{sw,ly}$ is the nutrient cycling water factor for layer $ly , orgN_{act,ly}$ is the amount of nitrogen in the active organic pool (kg N/ha).

Nitrogen mineralized from the humus active organic pool is added to the nitrate pool in the layer.

Decomposition is the breakdown of fresh organic residue into simpler organic components. Mineralization is the microbial conversion of organic, plant-unavailable nitrogen to inorganic, plant-available nitrogen. Immobilization is the microbial conversion of plant-available inorganic soil nitrogen to plant-unavailable organic nitrogen.

Bacteria decompose organic material to obtain energy for growth processes. Plant residue is broken down into glucose which is then converted to energy:

The energy released by the conversion of glucose to carbon dioxide and water is used for various cell processes, including protein synthesis. Protein synthesis requires nitrogen. If the residue from which the glucose is obtained contains enough nitrogen, the bacteria will use nitrogen from the organic material to meet the demand for protein synthesis. If the nitrogen content of the residue is too low to meet the bacterial demand for nitrogen, the bacteria will use $NH_4^+$ and $NO_3^-$ from the soil solution to meet its needs. If the nitrogen content of the residue exceeds the bacterial demand for nitrogen, the bacterial will release the excess nitrogen into soil solution as $NH_4^+$. A general relationship between C:N ratio and mineralization/immobilization is:

The nitrogen mineralization algorithms in SWAT+ are net mineralization algorithms which incorporate immobilization into the equations. The algorithms were adapted from the PAPRAN mineralization model (Seligman and van Keulen, 1981). Two sources are considered for mineralization: the fresh organic N pool associated with crop residue and microbial biomass and the active organic N pool associated with soil humus. Mineralization and decomposition are allowed to occur only if the temperature of the soil layer is above 0°C.

Mineralization and decomposition are dependent on water availability and temperature. Two factors are used in the mineralization and decomposition equations to account for the impact of temperature and water on these processes.

The nutrient cycling temperature factor is calculated:

$\gamma_{tmp,ly}=0.9*\frac{T_{soil,ly}}{T_{soil,ly}+exp[9.93-0.312*T_{soil,ly}]}+0.1$ 3:1.2.1

where $\gamma_{tmp,ly}$ is the nutrient cycling temperature factor for layer $ly$, and $T_{soil,ly}$ is the temperature of layer $ly$ (°C). The nutrient cycling temperature factor is never allowed to fall below 0.1.

The nutrient cycling water factor is calculated:

$\gamma_{sw,ly}=\frac{SW_{ly}}{FC_{ly}}$ 3:1.2.2

where $\gamma_{sw,ly}$ is the nutrient cycling water factor for layer $ly$, $SW_{ly}$ is the water content of layer $ly$ on a given day (mm H$_2$O), and $FC_{ly}$ is the water content of layer $ly$ at field capacity (mm H$_2$O). The nutrient cycling water factor is never allowed to fall below 0.05.

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.