Denitrification

Denitrification is the bacterial reduction of nitrate, $NO_3^-$, to $N_2$ or $N_2O$ gases under anaerobic (reduced) conditions. Denitrification is a function of water content, temperature, presence of a carbon source and nitrate.

In general, when the water-filled porosity is greater than 60% denitrification will be observed in a soil. As soil water content increases, anaerobic conditions develop due to the fact that oxygen diffuses through water 10,000 times slower than through air. Because the rate of oxygen diffusion through water slows as the water temperature increases, temperature will also influence denitrification.

Cropping systems where water is ponded, such as rice, can lose a large fraction of fertilizer by denitrification. For a regular cropping system, an estimated 10-20% of nitrogen fertilizer may be lost to denitrification. Under a rice cropping system, 50% of nitrogen fertilizer may be lost to denitrification. In a flooded cropping system, the depth of water plays an important role because it controls the amount of water oxygen has to diffuse through to reach the soil.

SWAT+ determines the amount of nitrate lost to denitrification with the equation:

$N_{denit,ly}=NO3_{ly}*(1-exp\lfloor-\beta_{denit}*\gamma_{tmp,ly}*orgC_{ly}\rfloor)$

if $\gamma_{sw,ly}\ge \gamma_{sw,thr}$ 3:1.4.1

$N_{denit,ly}=0.0$ if $\gamma_{sw,ly}<\gamma_{sw,thr}$ 3:1.4.2

where $N_{denit,ly}$ is the amount of nitrogen lost to denitrification (kg N/ha), $NO3_{ly}$ is the amount of nitrate in layer $ly$ (kg N/ha), $\beta_{denit}$ is the rate coefficient for denitrification, $\gamma_{tmp,ly}$ is the nutrient cycling temperature factor for layer $ly$ calculated with equation 3:1.2.1, $\gamma_{sw,ly}$ is the nutrient cycling water factor for layer $ly$ calculated with equation 3:1.2.2, $orgC_{ly}$ is the amount of organic carbon in the layer (%), and $\gamma_{sw,thr}$ is the threshold value of nutrient cycling water factor for denitrification to occur.

Table 3:1-3: SWAT+ input variables that pertain to denitrification.

Variable Name	Definition	Input File
SOL_CBN	$orgC_{ly}$: Amount of organic carbon in the layer (%)	.sol
CDN	$\beta_{denit}$: Rate coefficient for denitrification	.bsn
SDNCO	$\gamma_{sw,thr}$: Threshold value of nutrient cycling water factor for denitrification to occur	.bsn