The three major forms of nitrogen in mineral soils are organic nitrogen associated with humus, mineral forms of nitrogen held by soil colloids, and mineral forms of nitrogen in solution. Nitrogen may be added to the soil by fertilizer, manure or residue application, fixation by symbiotic or nonsymbiotic bacteria, and rain. Nitrogen is removed from the soil by plant uptake, leaching, volatilization, denitrification and erosion. Figure 3:1-1 shows the major components of the nitrogen cycle.

Nitrogen is considered to be an extremely reactive element. The highly reactive nature of nitrogen results from its ability to exist in a number of valance states. The valence state or oxidation state describes the number of electrons orbiting the nucleus of the nitrogen atom relative to the number present in an electronically neutral atom. The valence state will be positive as the atom looses electrons and will be negative as the atom gains electrons. Examples of nitrogen in different valence states are:

The ability of nitrogen to vary its valence state makes it a highly mobile element. Predicting the movement of nitrogen between the different pools in the soil is critical to the successful management of this element in the environment.

SWAT+ monitors five different pools of nitrogen in the soil (Figure 3:1-2). Two pools are inorganic forms of nitrogen, NH4+ and $NO_3^-$ , while the other three pools are organic forms of nitrogen. Fresh organic N is associated with crop residue and microbial biomass while the active and stable organic N pools are associated with the soil humus. The organic nitrogen associated with humus is partitioned into two pools to account for the variation in availability of humic substances to mineralization.

Figure 3:1-2: SWAT+ soil nitrogen pools and processes that move nitrogen in and out of pools.

Users may define the amount of nitrate and organic nitrogen contained in humic substances for all soil layers at the beginning of the simulation. If the user does not specify initial nitrogen concentrations, SWAT+ will initialize levels of nitrogen in the different pools.

Initial nitrate levels in the soil are varied by depth using the relationship:

$NO3_{conc,z}=7*exp(\frac{-z}{1000})$ 3:1.1.1

where $NO3_{conc,z}$ is the concentration of nitrate in the soil at depth $z$ (mg/kg or ppm), and $z$ is the depth from the soil surface (mm). The nitrate concentration with depth calculated from equation 3:1.1.1 is displayed in Figure 3:1-3. The nitrate concentration for a layer is calculated by solving equation 3:1.1.1 for the horizon’s lower boundary depth.

Organic nitrogen levels are assigned assuming that the C:N ratio for humic materials is 14:1. The concentration of humic organic nitrogen in a soil layer is calculated:

$orgN_{hum,ly}=10^4*(\frac{orgC_{ly}}{14})$ 3:1.1.2

where $orgN_{hum,ly}$ is the concentration of humic organic nitrogen in the layer (mg/kg or ppm), and $orgC_{ly}$ is the amount of organic carbon in the layer (%). The humic organic $N$ is partitioned between the active and stable pools using the following equations:

$orgN_{act,ly}=orgN_{hum,ly}*fr_{actN}$ 3.1.1.3

$orgN_{sta,ly}=orgN_{hum,ly}*(1-fr_{actN})$ 3:1.1.4

where $orgN_{act,ly}$ is the concentration of nitrogen in the active organic pool (mg/kg), $orgN_{hum,ly}$ is the concentration of humic organic nitrogen in the layer (mg/kg), $fr_{actN}$ is the fraction of humic nitrogen in the active pool, and $orgN_{sta,ly}$ is the concentration of nitrogen in the stable organic pool (mg/kg). The fraction of humic nitrogen in the active pool, $fr_{actN}$, is set to 0.02.

Nitrogen in the fresh organic pool is set to zero in all layers except the top 10 mm of soil. In the top 10 mm, the fresh organic nitrogen pool is set to 0.15% of the initial amount of residue on the soil surface.

$orgN_{frsh,surf}=0.0015*rsd_{surf}$ 3:1.1.5

where $orgN_{frsh,surf}$ is the nitrogen in the fresh organic pool in the top 10 mm (kg N/ha), and $rsd_{surf}$ is material in the residue pool for the top 10 mm of soil (kg/ha).

The ammonium pool for soil nitrogen, $NH4_{ly}$, is initialized to 0 ppm.

While SWAT+ allows nutrient levels to be input as concentrations, it performs all calculations on a mass basis. To convert a concentration to a mass, the concentration is multiplied by the bulk density and depth of the layer and divided by 100:

$\frac{conc_N*\rho_b*depth_{ly}}{100}=\frac{kgN}{ha}$ 3:1.1.6

where $conc_N$ is the concentration of nitrogen in a layer (mg/kg or ppm), $\rho_b$ is the bulk density of the layer (Mg/m$^3$), and $depth_{ly}$ is the depth of the layer (mm).

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

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:

3:1.2.3

is the amount of nitrogen transferred between the active and stable organic pools (kg N/ha), is the rate constant (1×10), is the amount of nitrogen in the active organic pool (kg N/ha), is the fraction of humic nitrogen in the active pool (0.02), and is the amount of nitrogen in the stable organic pool (kg N/ha). When is positive, nitrogen is moving from the active organic pool to the stable organic pool. When is negative, nitrogen is moving from the stable organic pool to the active organic pool.

Mineralization from the humus active organic pool is calculated:

3:1.2.4

where is the nitrogen mineralized from the humus active organic pool (kg N/ha), is the rate coefficient for mineralization of the humus active organic nutrients, is the nutrient cycling temperature factor for layer , is the nutrient cycling water factor for layer 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.

The majority of plant-essential nutrients are cations which are attracted and sorbed to negatively-charged soil particles. As plants extract these cations from soil solution, the soil particles release bound cations into soil solution to bring the ratio of nutrients in solution and on soil particles back into equilibrium. In effect, the soil buffers the concentration of cations in solution.

In contrast, nitrate is an anion and is not attracted to or sorbed by soil particles. Because retention of nitrate by soils is minimal, nitrate is very susceptible to leaching. The algorithms used by SWAT+ to calculated nitrate leaching simultaneously solve for loss of nitrate in surface runoff and lateral flow also. These algorithms are reviewed in Chapter 4:2.

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

step 1: (Nitrosomonas)

step 2: (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:

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:

$\eta_{tmp,ly}=0.41*\frac{(T_{soil,ly}-5)}{10}$ 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).

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.

Lightning discharge converts atmospheric $N_2$ to nitric acid which can then be transferred to the soil with precipitation. The chemical steps involved are:

step 1: (monoxide)

step 2: (dioxide)

step 3: (nitric acid and monoxide)

More nitrogen will be added to the soil with rainfall in areas with a high amount of lightning activity than in areas with little lightning.

The amount of nitrate added to the soil in rainfall is calculated:

$NO_{3rain}=0.01*R_{NO3}*R_{day}$ 3:1.5.1

where $NO_{3rain}$ is nitrate added by rainfall (kg N/ha), $R_{NO3}$ is the concentration of nitrate in the rain (mg N/L), and $R_{day}$ is the amount of precipitation on a given day (mm H$_2$O). The nitrogen in rainfall is added to the nitrate pool in the top 10 mm of soil.

The amount of ammonia added to the soil in rainfall is calculated:

$NH_{4rain}=0.01*R_{NH4}*R_{day}$ 3:1.5.2

where $NH_{4rain}$ is nitrate added by rainfall (kg N/ha), $R_{NH4}$ is the concentration of ammonia in the rain (mg N/L), and $R_{day}$ is the amount of precipitation on a given day (mm H$_2$O). The nitrogen in rainfall is added to the ammonia pool in the top 10 mm of soil.

Table 3:1-4: SWAT+ input variables that pertain to nitrogen in rainfall.

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.

Dry deposition of nitrate and ammonia is input to the model for each subbasin. Average daily deposition is added to the appropriate surface soil pool.

Table 3:1-5: SWAT+ input variables that pertain to dry deposition.

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:

3:1.2.5

where is the C:N ratio of the residue in the soil layer, is the residue in layer (kg/ha), 0.58 is the fraction of residue that is carbon, is the nitrogen in the fresh organic pool in layer (kg N/ha), and is the amount of nitrate in layer (kg N/ha).

The C:P ratio of the residue is calculated:

3:1.2.6

where is the C:P ratio of the residue in the soil layer, is the residue in layer (kg/ha), 0.58 is the fraction of residue that is carbon, is the phosphorus in the fresh organic pool in layer (kg P/ha), and is the amount of phosphorus in solution in layer (kg P/ha).

The decay rate constant defines the fraction of residue that is decomposed. The decay rate constant is calculated:

3:1.2.7

where is the residue decay rate constant, is the rate coefficient for mineralization of the residue fresh organic nutrients, is the nutrient cycling residue composition factor for layer , is the nutrient cycling temperature factor for layer , and is the nutrient cycling water factor for layer .

The nutrient cycling residue composition factor is calculated:

3:1.2.8

where is the nutrient cycling residue composition factor for layer , is the C:N ratio on the residue in the soil layer, and is the C:P ratio on the residue in the soil layer.

Mineralization from the residue fresh organic N pool is then calculated:

3:1.2.9

where is the nitrogen mineralized from the fresh organic N pool (kg N/ha), is the residue decay rate constant, and is the nitrogen in the fresh organic pool in layer (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:

Table 3:1-2: SWAT+ input variables that pertain to mineralization.

As water evaporates from the soil surface, the water content at the surface drops, creating a gradient in the profile. Water from lower in the profile will move upward in response to the gradient, carrying dissolved nutrients with it. SWAT+ allows nitrate to be transported from the first soil layer defined in the .sol file to the surface top 10 mm of soil with the equation:

3:1.7.1

where is the amount of nitrate moving from the first soil layer to the soil surface zone (kg N/ha), is the nitrate content of the first soil layer (kg N/ha), is the amount of water removed from the first soil layer as a result of evaporation (mm HO), and is the soil water content of the first soil layer (mm HO).

Atmospheric deposition occurs when airborne chemical compounds settle onto the land or water surface. Some of the most important chemical pollutants are those containing nitrogen or phosphorus. Nitrogen compounds can be deposited onto water and land surfaces through both wet and dry deposition mechanisms. Wet deposition occurs through the absorption of compounds by precipitation as it falls carrying mainly nitrate () and ammonium (). Dry deposition is the direct adsorption of compounds to water or land surfaces and involves complex interactions between airborne nitrogen compounds and plant, water, soil, rock, or building surfaces.

The relative contribution of atmospheric deposition to total nutrient loading is difficult to measure or indirectly assess and many deposition mechanisms are not fully understood. Most studies and relatively extended data sets are available on wet deposition of nitrogen, while dry deposition rates are not well defined. Phosphorus loadings due to atmospheric deposition have not been extensively studied and nation-wide extended data set were unavailable at the time of data preparation for the CEAP project. While research continues in these areas, data records generated by modeling approaches appear to be still under scrutiny.

A number of regional and local monitoring networks are operating in the U.S. mainly to address information regarding regional environmental issues. For example, the Integrated Atmospheric Deposition Network (IADN) (Galarneau et al., 2006) that estimates deposition of toxic organic substances to the Great Lakes. Over the CONUS (conterminous United States), the National Atmospheric Deposition Program (NADP) National Trends Network (NTN) (NADP/NTN, 1995; NADP/NTN, 2000; Lamb and Van Bowersox, 2000) measures and ammonium in one-week rain and snow samples at nearly 240 regionally representative sites in the CONUS and is considered the nation’s primary source for wet deposition data.

The U.S. EPA Clean Air Status and Trends Network (CASTNET), developed form the National Dry Deposition Network (NDDN), operates a total of 86 operational sites (as of December 2007) located in or near rural areas and sensitive ecosystems collecting data on ambient levels of pollutants where urban influences are minimal (CASTNET, 2007). As part of an interagency agreement, the National Park Service (NPS) sponsors 27 sites which are located in national parks and other Class-I areas designated as deserving special protection from air pollution.

Groundwater flow entering the main channel from the shallow aquifer can contain nitrate. With SWAT+ the nitrate pool in the shallow aquifer is modeled, allowing for fluctuations in nitrate loadings in the groundwater over time.

Nitrate enters the shallow aquifer in recharge from the soil profile. Water that moves past the lowest depth of the soil profile by percolation or bypass flow enters and flows through the vadose zone before becoming shallow and/or deep aquifer recharge. SWAT+ assumes there is no change in nitrate concentration of the recharge as it moves through the vadose zone.

An exponential decay weighting function proposed by Venetis (1969) and used by Sangrey et al. (1984) in a precipitation/groundwater response model is utilized in SWAT+ to account for the time delay in aquifer recharge once the water exits the soil profile. The delay function accommodates situations where the recharge from the soil zone to the aquifer is not instantaneous, i.e. 1 day or less. This same relationship is used to account for the delay in nitrate movement from the soil profile to the aquifers.

The nitrate in recharge to both aquifers on a given day is calculated:

3:1.9.1

where is the amount of nitrate in recharge entering the aquifers on day (kg N/ha), is the delay time or drainage time of the overlying geologic formations (days), is the total amount of nitrate exiting the bottom of the soil profile on day (kg N/ha), and is the amount of nitrate in recharge entering the aquifers on day (mm HO). The total amount of nitrate exiting the bottom of the soil profile on day is calculated using the percolation equation given in Chapter 4:2.

Nitrate in the shallow aquifer may be remain in the aquifer, move with recharge to the deep aquifer, move with groundwater flow into the main channel, or be transported out of the shallow aquifer with water moving into the soil zone in response to water deficiencies. The amount of nitrate in the shallow aquifer after all these processes are taken into account is:

3:1.9.2

while the amount of nitrate lost in groundwater flow is

3:1.9.3

the amount of nitrate lost in revap to the soil profile is

3:1.9.4

and the amount of nitrate transported to the deep aquifer is

3:1.9.5

Because nitrogen is a very reactive element, nitrate in the shallow aquifer may be lost due to uptake by bacteria present in the aquifer, chemical transformations driven by a change in redox potential of the aquifer, and other processes. To account for losses of nitrate due to biological and chemical processes, a half-life for nitrate in the aquifer may be defined that specifies the number of days required for a given nitrate concentration to be reduced by one-half. The half-life entered for nitrate in the shallow aquifer is a lumped parameter that includes the net effect of all reactions occurring in the aquifer.

Nitrate removal in the shallow aquifer is governed by first-order kinetics:

Table 3:1-5: SWAT+ input variables that pertain to nitrogen in the shallow aquifer.

Legumes are able to obtain a portion of their nitrogen demand through fixation of atmospheric performed by rhizobia living in association with the plant. In exchange for nitrogen, the plant supplies the bacteria with carbohydrates.

The complexity of the nitrogen cycle and nitrogen’s importance in plant growth have made this element the subject of much research. The nitrogen cycle is a dynamic system that includes the water, atmosphere and soil. Plants require nitrogen more than any other essential element, excluding carbon, oxygen and hydrogen. Nitrogen is modeled by SWAT+ in the soil profile and in the shallow aquifer.