Nitrification & Ammonia Volatilization
Last updated
Last updated
Nitrification is the two-step bacterial oxidation of to .
step 1: (Nitrosomonas)
step 2: (Nitrobacter)
Ammonia volatilization is the gaseous loss of that occurs when ammonium, , is surface applied to a calcareous soil or when urea, (, is surface applied to any soil.
surface applied to a calcareous soil:
step 1:
step 2:
Urea surface applied to any soil:
step 1:
step 2:
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:
if 3:1.3.1
where is the nitrification/volatilization temperature factor, and is the temperature of layer (°C).
The nitrification soil water factor is calculated:
if 3:1.3.2
if 3:1.3.3
where is the nitrification soil water factor, is the soil water content of layer on a given day (mm HO), is the amount of water held in the soil layer at wilting point water content (mm HO), and is the amount of water held in the soil layer at field capacity water content (mm HO).
The volatilization depth factor is calculated:
3:1.3.4
where is the volatilization depth factor, and 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:
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:
3:1.3.6
and the volatilization regulator is calculated:
3:1.3.7
where is the nitrification regulator, is the volatilization regulator, is the nitrification/volatilization temperature factor, is the nitrification soil water factor, and 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):
3:1.3.8
where is the amount of ammonium converted via nitrification and volatilization in layer (kg N/ha), is the amount of ammonium in layer (kg N/ha), is the nitrification regulator, and is the volatilization regulator.
To partition between nitrification and volatilization, the expression by which is multiplied in equation 3:1.3.8, is solved using each regulator individually to obtain a fraction of ammonium removed by each process:
3:1.3.9
3:1.3.10
where is the estimated fraction of nitrogen lost by nitrification, is the estimated fraction of nitrogen lost by volatilization, is the nitrification regulator, and is the volatilization regulator.
The amount of nitrogen removed from the ammonium pool by nitrification is then calculated:
3:1.3.11
and the amount of nitrogen removed from the ammonium pool by volatilization is:
3:1.3.12
where is the amount of nitrogen converted from to in layer (kg N/ha), is the amount of nitrogen converted from to in layer (kg N/ha), is the estimated fraction of nitrogen lost by nitrification, is the estimated fraction of nitrogen lost by volatilization, and is the amount of ammonium converted via nitrification and volatilization in layer (kg N/ha).
