3:1.3 Nitrification & Ammonia Volatilization

Nitrification is the two-step bacterial oxidation of $NH_4^+$ to $NO_3^-$.

step 1: $2NH^+_4+3O_2\xrightarrow{-12e^-}2NO^-_2+2H_2O+4H^+$ (Nitrosomonas)

step 2: $2NO^-_2+O_2\xrightarrow{-4e^-}2NO^-_3$ (Nitrobacter)

Ammonia volatilization is the gaseous loss of $NH_3$ that occurs when ammonium, $NH_4^+$, is surface applied to a calcareous soil or when urea, ($NH_2)_2CO_2$, is surface applied to any soil.

$NH_4^+$ surface applied to a calcareous soil:

step 1: $CaCO_3+2NH^+_4X\longleftrightarrow(NH_4)_2CO_3+CaX_2$

step 2: $(NH_4)CO_3\longleftrightarrow2NH_3+CO_2+H_2O$

Urea surface applied to any soil:

step 1: $(NH_2)_2CO+2H_2O\xleftrightarrow{urease\,enzyme}(NH_4)_2CO_3$

step 2: $(NH_4)_2CO_3\longleftrightarrow2NH_3+CO_2+H_2O$

SWAT+ simulates nitrification and ammonia volatilization using a combination of the methods developed by Reddy et al. (1979) and Godwin et al. (1984). The total amount of nitrification and ammonia volatilization is calculated, and then partitioned between the two processes. Nitrification is a function of soil temperature and soil water content while ammonia volatilization is a function of soil temperature, depth and cation exchange capacity. Four coefficients are used in the nitrification/volatilization algorithms to account for the impact of these parameters. Nitrification/volatilization occurs only when the temperature of the soil layer exceeds 5°C.

The nitrification/volatilization temperature factor is calculated:

$\eta_{tmp,ly}=0.41*\frac{(T_{soil,ly}-5)}{10}$ if $T_{soil,ly}>5$ 3:1.3.1

where $\eta_{tmp,ly}$ is the nitrification/volatilization temperature factor, and $T_{soil,ly}$ is the temperature of layer $ly$ (°C).

The nitrification soil water factor is calculated:

$\eta_{sw,ly}=\frac{SW_{ly}-WP_{ly}}{0.25*(FC_{ly}-WP_{ly})}$ if $SW_{ly}<0.25*FC_{ly}-0.75*WP_{ly}$ 3:1.3.2

$\eta_{sw,ly}=1.0$ if $SW_{ly} \ge 0.25*FC_{ly}-0.75*WP_{ly}$ 3:1.3.3

where $\eta_{sw,ly}$ is the nitrification soil water factor, $SW_{ly}$ is the soil water content of layer $ly$ on a given day (mm H$_2$O), $WP_{ly}$ is the amount of water held in the soil layer at wilting point water content (mm H$_2$O), and $FC_{ly}$ is the amount of water held in the soil layer at field capacity water content (mm H$_2$O).

The volatilization depth factor is calculated:

$\eta_{midz,ly}=1-\frac{z_{mid,ly}}{z_{mid,ly}+exp[4.706-0.0305*z_{mid,ly}]}$ 3:1.3.4

where $\eta_{midz,ly}$ is the volatilization depth factor, and $z_{mid,ly}$ is the depth from the soil surface to the middle of the layer (mm).

SWAT+ does not require the user to provide information about soil cation exchange capacity. The volatilization cation exchange factor is set to a constant value:

$\eta_{cec,ly}=0.15$ 3:1.3.5

The impact of environmental factors on nitrification and ammonia volatilization in a given layer is defined by the nitrification regulator and volatilization regulator. The nitrification regulator is calculated:

$\eta_{nit,ly}=\eta_{tmp,ly}*\eta_{sw,ly}$ 3:1.3.6

and the volatilization regulator is calculated:

$\eta_{vol,ly}=\eta_{tmp,ly}*\eta_{midz,ly}*\eta_{cec,ly}$ 3:1.3.7

where $\eta_{nit,ly}$ is the nitrification regulator, $\eta_{vol,ly}$ is the volatilization regulator, $\eta_{tmp,ly}$ is the nitrification/volatilization temperature factor, $\eta_{sw,ly}$ is the nitrification soil water factor, and $\eta_{midz,ly}$ is the volatilization depth factor.

The total amount of ammonium lost to nitrification and volatilization is calculated using a first-order kinetic rate equation (Reddy et al., 1979):

$N_{nit|vol,ly}=NH4_{ly}*(1-exp\lfloor-\eta_{nit,ly}-\eta_{vol,ly}\rfloor)$ 3:1.3.8

where $^{N_{nit|vol,ly}}$ is the amount of ammonium converted via nitrification and volatilization in layer $ly$ (kg N/ha), $NH4_{ly}$ is the amount of ammonium in layer $ly$ (kg N/ha), $\eta_{nit,ly}$ is the nitrification regulator, and $\eta_{vol,ly}$ is the volatilization regulator.

To partition $^{N_{nit|vol,ly}}$ between nitrification and volatilization, the expression by which $NH4_{ly}$ is multiplied in equation 3:1.3.8, is solved using each regulator individually to obtain a fraction of ammonium removed by each process:

$fr_{nit,ly}=1-exp\lfloor-\eta_{nit,ly}\rfloor$ 3:1.3.9

$fr_{vol,ly}=1-exp\lfloor-\eta_{vol,ly}\rfloor$ 3:1.3.10

where $fr_{nit,ly}$ is the estimated fraction of nitrogen lost by nitrification, $fr_{vol,ly}$ is the estimated fraction of nitrogen lost by volatilization, $\eta_{nit,ly}$ is the nitrification regulator, and $\eta_{vol,ly}$ is the volatilization regulator.

The amount of nitrogen removed from the ammonium pool by nitrification is then calculated:

$N_{nit,ly}=\frac{fr_{nit,ly}}{(fr_{nit,ly}+fr_{vol,ly})}*N_{nit|vol,ly}$ 3:1.3.11

and the amount of nitrogen removed from the ammonium pool by volatilization is:

$N_{vol,ly}=\frac{fr_{vol,ly}}{(fr_{nit,ly}+fr_{vol,ly})}*N_{nit|vol,ly}$ 3:1.3.12

where $N_{nit,ly}$ is the amount of nitrogen converted from $NH_4^+$ to $NO_3^-$ in layer $ly$ (kg N/ha), $N_{vol,ly}$ is the amount of nitrogen converted from $NH_4^+$ to $NH_3$ in layer $ly$ (kg N/ha), $fr_{nit,ly}$ is the estimated fraction of nitrogen lost by nitrification, $fr_{vol,ly}$ is the estimated fraction of nitrogen lost by volatilization, and $^{N_{nit|vol,ly}}$ is the amount of ammonium converted via nitrification and volatilization in layer $ly$ (kg N/ha).