Nitrogen Fixation

If nitrate levels in the root zone are insufficient to meet the demand of a legume, SWAT+ allows the plant to obtain additional nitrogen through nitrogen fixation. Nitrogen fixation is calculated as a function of soil water, soil nitrate content and growth stage of the plant.

$N_{fix}=N_{demand}*f_{gr}*min(f_{sw},f_{no3},1)$ 5:2.3.9

where $N_{fix}$ is the amount of nitrogen added to the plant biomass by fixation (kg N/ha), $N_{demand}$ is the plant nitrogen demand not met by uptake from the soil (kg N/ha), $f_{gr}$ is the growth stage factor (0.0-1.0), $f_{sw}$ is the soil water factor (0.0-1.0), and $f_{no3}$ is the soil nitrate factor (0.0-1.0). The maximum amount of nitrogen that can be fixed by the plant on a given day is $N_{demand}$.

Growth stage exerts the greatest impact on the ability of the plant to fix nitrogen. The growth stage factor is calculated:

$f_{gr}=0$ when $fr_{PHU} \le 0.15$ 5:2.3.10

$f_{gr}=6.67*fr_{PHU}-1$ when $0.15 <fr_{PHU} \le 0.30$ 5:2.3.11

$f_{gr}=1$ when $0.30 <fr_{PHU} \le 0.55$ 5:2.3.12

$f_{gr}=3.75 -5*fr_{PHU}$ when $0.55 <fr_{PHU} \le 0.75$ 5:2.3.13

$f_{gr}=0$ when $fr_{PHU} > 0.75$ 5:2.3.14

where $f_{gr}$ is the growth stage factor and $fr_{PHU}$ is the fraction of potential heat units accumulated for the plant on a given day in the growing season. The growth stage factor is designed to reflect the buildup and decline of nitrogen fixing bacteria in the plant roots during the growing season.

The soil nitrate factor inhibits nitrogen fixation as the presence of nitrate in the soil goes up. The soil nitrate factor is calculated:

$f_{no3}=1$ when $NO3 \le 100$ 5:2.3.15

$f_{no3}=1.5-0.0005*NO3$ when $100 < NO3 \le 300$ 5:2.3.16

$f_{no3}=0$ when $NO3> 300$ 5:2.3.17

where $f_{no3}$ is the soil nitrate factor and $NO3$ is the nitrate content of the soil profile (kg NO$_3$-N/ha).

The soil water factor inhibits nitrogen fixation as the soil dries out. The soil water factor is calculated:

$f_{SW}=\frac{SW}{.85*FC}$ 5:2.3.18

where $f_{SW}$ is the soil water factor, $SW$ is the amount of water in soil profile (mm H$_2$O), and $FC$ is the water content of soil profile at field capacity (mm H$_2$O).