INTRODUCTION
The process of application of soluble fertilizer along with irrigation
is defined fertigation. Fertigation under drip irrigation is being used
commonly for the application of nitrogenous fertilizers in all fruits
and vegetable crops. Thompson et al. (2003) carried out field experiment
on a sandy loam soil in Southern Arizona with subsurface drip irrigated
broccoli determine the effect of N rate and fertigation frequency on crop
yield, quality and crop N status and estimate N balance. During one of
three season, fertigation frequency affected crop N uptake, but there
was no trend of increasing N uptake with increasing fertigation frequency.
Gardenas et al. (2005) investigated nitrate leaching for various
fertigation scenarios under micro-irrigation. The effect of fertigation
strategy and soil type on nitrate leaching potential for four different
micro-irrigation systems was assessed. It was observed that seasonal leaching
was the highest for coarse-textured soils.
Chopade et al. (1998) results showed drip fertigation with 50%
of the recommended solid fertilizer dose was the best treatment for promoting
growth. Peter et al. (2003) studied the fate of nitrogen applied
to sugarcane by drip irrigation. It was observed that the high soil water
contents maintained with daily application of irrigation water through
the drip system promotes mineralization of soil organic mater and hence
losses of N to the environment. Jiusheng et al. (2003) observed
a uniform distribution of nitrate concentration in the soil in 15 cm around
the point source for a given input concentration. A study of solute transport
in subsurface drip irrigation was carried out by Cote et al. (2003).
The simulation results showed that (1) trickle irrigation can improve
plant water availability in medium and low permeability fine-textured
to account for their soil hydraulic properties, (2) in highly permeable
coarse-texture soils, water and nutrients move quickly downwards from
the emitter, making it difficult to wet the near surface zone if emitters
are buried too deep and (3) changing the fertigation strategy for highly
permeable coarse-texture soils to apply nutrients at the beginning of
an irrigation cycle can maintain larger amounts of nutrient near and above
the emitter , thereby making them less susceptible to leaching losses.
Lafolie et al. (1997) studied modeling of water and nitrogen dynamics
in irrigated salad crops. The model used was one-dimensional and is based
on Richards` equation for describing saturated- unsaturated water flow
in soil. At the soil surface, the model is designed to handle flux-type
or imposed pressure boundary conditions.
Hochmuth (2000) developed guidelines from their study for optimum fertilization.
The simulations showed the effects of different plant water uptake modeling
approaches on water distribution. Simunek et al. (1999) developed
the HYDRUS-2D software package for simulating two-dimensional movement
of water, heat and multiple solute in variably saturated media.
Delgado et al. (2000) developed the NLEAP model and studied the
simulation of nitrate-nitrogen dynamics for cropping systems with different
rooting depths. The model was calibrated and validated the potato, lettuce,
canola, spring wheat and barley conducted on a similar soil depth. The
validated model introduced a new concept of using NLEAP simulations of
best management practices for crops with different rooting depths.
A review study of water and solute dynamics under a drip irrigated crop
has been done by Mmolawa and Or (2000). It was concluded that water and
solute dynamics largely depend on the root distribution and activity of
plants as well as nutrient being introduced in the soil root zone.
A simulation model FUSSIM2 was developed by Heinen (2001) for drip fertigation
to study various fertigation scenarios. Assouline (2002) compared the
effect of three emitter discharges, 0.25, 2.0 and 8 L h-1,
on different aspect of the water regime in daily drip irrigated corn,
relying on field observations and numerical simulations. The study showed
that by changing the fertigation strategy to involve application of non-adsorbed
mobile nutrients at the beginning of an irrigation cycle rather than near
the end of the cycle, larger amounts of nutrients can be maintained near
to and above the drip emitter. Comparison of HYDRUS-2D simulation of drip
irrigation with experimental observations was investigated by Skaggs et
al. (2004). The result support the use of HYDRUS-2D as a tool for
investigating and designing drip irrigation management practices. Many
studies have reported that frequent or continuous fertigation of drip-irrigated
vegetables is an efficient method of fertigation (Tompson et al.,
2003; Ajdary et al., 2007; Halvorson et al., 2002).
The objective of the study was to modeling of the nitrogen distribution
in onion root zone and leaching the N below the root zone from sandy loam
soil for various irrigation and fertigation strategies using a solute
transport model Hydrus-2D. The study involved field experiment and modeling
of nitrogen leaching.
MATERIALS AND METHODS
Experimental site: The experiment was conducted in the year 2006
at the Bastam Agricultural Center Farm, Shahrood, Iran which lies the
latitudes of 36 °27`33.29” N and longitudes of 54 °58`31.85”
E. Climate of Shahrood is categorized as semi-arid, subtropical with hot
dry summer and cold winter. The mean annual temperature is 14.4 °C.
July and August are the hottest months with 40 years normal maximum temperature
of 38 °C. January and February are the coldest months with a mean
temperature of -14 ° however, the minimum temperature dips to as low
as -5 °C. Area of each plot was 9 m2. Plant to plant and
row to row spacing were 15 and 30 cm, respectively. The applied fertilizers
were 96 kg ha-1of N, 50 kg ha-1of P and 70 kg ha-1
of K.
In this study to evaluate the physical properties of soil, soil samples
were collected from different layers from surface till the depth of 0.9
m and analyzed to determine physical properties. Values of the physical
properties such as particle size distribution, bulk density, field capacity,
permanent wilting point and hydraulic conductivity are shown in Table
1.
Fertigation schedule: Water requirement of onion crop was estimated
using the pan evaporation data. Irrigation was applied on alternate days
during the crop growing period based on crop water demand. Irrigation
water was applied at the rate of 4 L h-1 through drip emitters
placed on the lateral line. Irrigation was stopped two weeks before harvesting
to allow the crop to mature. Irrigation interval was 48 h. Total amount
of water applied in the entire growing period was 4800 m3 ha-1.
Nitrogen fertilizer was applied on weekly basis at the rate of 96 kg ha-1
through drip fertigation in a split doses in the first twelve weeks
during growing period.
| Table 1: |
Physical properties of soil of the experimental field |
 |
During each fertigation, fertilizer was applied in the beginning of irrigation
for 0.166 h.
Various scenarios considered in the study are given below:
Emitter Discharge rates (L h-1): 2 and 4
Fertigation strategies:
| • |
Alternate day irrigation, weekly fertigation, fertigation
for 10 min after beginning of irrigation |
| • |
Alternate day irrigation, weekly fertigation, fertigation for 10
min before irrigation cutoff |
| • |
Daily irrigation, weekly fertigation, fertigation for 10 min before
irrigation cutoff |
Field observations: Soil samples were collected from different
depths (0-0.15, 0.15-0.30, 0.30-0.45, 0.45-0.60 m) and vertical planes
located at emitter and at 10 and 20 cm away from emitter periodically
(before fertigation, 2, 12, 24, 48, 96 and 168 h after fertigation) using
tube auger from the experimental area to determine spatial and temporal
distribution of water and, available nitrogen in the growing season. These
were analyzed to determine the gravimetric moisture content and, ammonium
and nitrate forms of the available nitrogen. Kjeldahl method (Page et
al., 1982) was used to estimate the ammonium and nitrate forms of
the available nitrogen.
Nitrogen distribution modeling: Solute transport in soil under
surface drip fertigation system is controlled by physical transport. Solute
flow is considered to be influenced mainly by soil properties and drip
emitter discharge rates. In this study, chemical and biological interactions
were not considered. The governing equation for the simulation of the
transport of a single non-reactive ion in homogeneous medium in three
dimensional axi-symmetrical with polar coordinate system, in advection-dispersion
form as given by Bear (1972) and modified by Simunek et al. (1999)
by adding nutrient uptake parameter, is as follows:
where, C [ML-3] is solute concentration in the soil water,
qr and qz [LT-1] are the components of
the volumetric flux density, Drr, Dzz and Drz
[L2T-1] are the components of the dispersion tensor. These components
are given by Bear (1972). First term on the right side is solute flux
due to dispersion, the second term is solute flux due to convection with
flowing water and third term is nutrient uptake by root.
where, |q| [LT-1] is the absolute value of the volumetric
flux density, εL and εT [L] are the longitudinal
and transversal dispersivities. D0 [L2T-1]
is the molecular diffusion coefficient of the solute in free water and
τ is the tortuosity factor. The NU term defines the local passive
nitrate uptake (ML-3T-1) by plant roots, which is
function of space and time and is computed from water uptake value using:
In present study, mineralization gains and denitrification losses sere
neglected.
Selection of model: The modeling of nitrogen distribution from
the onion field under drip fertigation was carried using the computer
simulation model, Hydrus-2D (Simunek et al., 1999). It simulates
three-dimensional axially symmetric water flow; solute transport and root
water and nutrient uptake based on finite-element numerical solutions
of the flow equations. The model can implement a wide range of boundary
conditions, irregular boundaries and soil heterogeneities. The software
package consists of the HYDRUS-2 computer program and the interactive
graphics-based user interface HYDRUS-2D.
Modeling area: Research field was subdivided into identical volume
elements with a emitter placed at the surface on the plane of symmetry.
Water and nitrogen patterns in the entire field can be described by analyzing
the flow in this single volume element irrigated by single emitter. Because
of the axial symmetry around the vertical axis, the infiltration process
can be viewed as an axi-symmetrical flow with the radius r [L] and the
depth z [L] as key variables. In the present study, radius r was taken
as 30 cm (half of the lateral to lateral spacing) and depth z as 60 cm.
This was done because onion is a shallow rooted crop and nutrient leaching
below 60 cm depth will not be available to the plant. The flux radius
was taken equal to the wetted radius considering emitter in centre. Figure
1 shows the conceptual diagram of simulated area.
 |
| Fig. 1: |
Conceptual diagram of simulated area |
Boundary conditions: In modeling study boundary condition of the modeling
area should be clear. Initial available nitrogen concentration as observed in
various soil layers within the flow domain was given as initial condition for
solute concentration. For all simulations, on the sides of the flow domain,
it was assumed that no flow of water and nitrogen took place and hence no-flux
boundary condition was chosen, which in Hydrus-2D is specified for impermeable
boundaries where, the flux is zero perpendicular to the boundary. In the present
study, water table was situated far below the domain of interest and therefore,
free drainage boundary condition at the base of the soil profile was considered.
Bottom boundary was considered as free drainage boundary.
For simulation of nitrogen distribution in soil layers, Hydrus-2D required
input parameters namely saturated water content (θs), residual water
content (θr), empirical factors (α, n) and saturated hydraulic
conductivity (Ks). Neural Network Prediction option available in Hydrus-2D
was used by assigning the values of clay, silt and sand percentage. Saturated
hydraulic conductivity of sandy loam was obtained from field experiment.
Soils considered for simulation were isotropic. Values of longitudinal
and transverse dispersivity were taken as 0.3 and 0.03 cm, respectively.
This was confirmed through calibration process. Molecular diffusion was
neglected. Values of the hydraulic parameters of the sandy loam soil are
presented in Table 2.
| Table 2: |
Soil hydraulic parameters for sandy loam soil |
 |
RESULTS
Calibration and validation of the model: Hydrus-2D was calibrated
for prediction of nitrogen distribution in soil with the measured nitrogen
in root zone of onion. During calibration runs, simulation period was
kept to 168 h, which included one fertigation (for 0.166 h in the beginning
of irrigation) and two irrigation events (for 0.33 h at the interval of
48 h). Water flux during each irrigation event was equal to 1.3 cm h-1
and duration of irrigation varied (from 0.33 to 2.5 h) to meet crop water
requirement. During fertigation events, duration of nitrogen application
was kept equal to 0.166 h however, concentration of solute flux varied
0.253 to 1.35 mg mL-1 depending on the nitrogen applied at
various crop growth stages. In validation, simulation period was kept
to 36:00 h equal to growing period of onion. Other input parameters were
selected in the same was as in case of calibration. Van Genuchten (1980)
used analytical model without hysteresis for the soil hydraulic properties.
In this study, initial nitrogen concentration in the soil was given as
the total available nitrogen, which was considered as sum of NH4+
and NO3– forms of nitrogen. Though the process
of nitrification is reduced in saturated zone immediately below the emitter
but nitrification occurs in the unsaturated zone around the emitter (Laher
and Avnimelech, 1980). Urea was applied as the source of nitrogen which
is relatively mobile and is not strongly adsorbed by soil colloids. In
soil, urea is hydrolyzed to the ammonium ion and subsequently undergoes
to nitrification. Leaching of nitrogen occurs mostly in the nitrate form,
which is predicted by model. Therefore, in this study, predicted nitrogen
distribution within the root zone and cumulative nitrogen going below
root zone are reported as available nitrogen and amount of nitrogen leached.
Figure 2 and 3 show the simulated
and observed N concentration at various depths at 2, 4, 24, 48, 96 and
168 h after fertigation. The results reveal that simulated and observed
N distributions follow similar trends and N concentration decreases with
increasing depth. These Fig. 2 and 3
also reveal that concentration of N at various points decreases with elapsed
time after fertigation.
|
| Fig. 2: |
Simulated and observed N concentration at the end of
first month after transplanting (a) 2, (b) 4 and (c) 24 h after fertigation |
|
| Fig. 3: |
Simulated and observed N concentration at the end of
first month after transplanting (a) 48 (b ) 96 and (c) 168 h after
fertigation |
| Table 3: |
Comparison of simulated and observed N concentration
at the end of simulation period (Validation of the model) |
 |
For example, simulated and observed N concentrations
below the emitter 4 h after fertigation were 95 and 96 kg ha-1 in the first layer and the same was 94 and 93 kg ha-1 after
24 h. Similar trends were observed in all layers.
To examine the predictability of the model on seasonal basis, simulation
was carried out to predict the N distribution at the end of growing season
(taking the simulation period of 150 days). The Table 3
reveals that simulated and observed values of N follow similar trend with
not much difference. Simulated and observed N concentrations in the soil
at the time of harvesting varied from 69.5 to 79.5 kg ha-1.
Correlation coefficient between simulated and observed N concentration
varied from 0.992 to 0.997. This also indicates that there is not much
difference between simulated and observed N concentrations. The above
discussion implies that Hydrus-2D can be used to predict the N concentration
in the soil under drip fertigation on seasonal basis also with very good
predictability. After calibration and validation, model was used to predict
the nitrogen distribution and leaching from onion field under drip fertigation
system for various scenarios.
Simulation of nitrogen distribution: N distributions in different
time after fertigation obtained from the simulations were interpreted
to analyze the effect of emitter discharge rates and fertigation strategies
on N distribution. Simulation of N distribution in soil was done in vertical
and radial direction. Selection of values of input parameters and method
of interpretation of the model outputs were done.
Simulation of N distribution in vertical direction: In this simulation,
alternate day irrigation and weekly fertigation schedule was adapted.
Figure 4a shows that initial N concentration decreased
with depth however, after fertigation with pre-decided schedule, N concentration
in the vicinity of emitter, first and second soil layer immediately 4
h after fertigation is increased. The Fig. 4b and c,
show that 4 and 24 h after fertigation N concentration in active root
zone depth (i.e., middle layer) has increased thereby justifying the use
of drip fertigation in maintaining adequate nutrient concentration in
this zone.
|
|
| Fig. 4: |
Simulated nitrogen concentration with 4 L h-1
emitter discharge at the end of second month after transplanting (a)
Initial, (b) 4 , (c) 24 , (d) 48 , (e) 72 and (f) 96 h after fertigation |
It can also be seen that color spectrum of N concentration
in the last layer has not changed much up to 24 h after fertigation. This
indicates that N concentration at the end of second month after transplanting
in the last layers is nearly same to initial even 24 h after fertigation.
This implies that under this scenario, possibility of N leaching is not
much. It may be mention that observed N concentration under this scenario
is at the end of second month was also in the same range. Color spectrum
also revealed that N concentration in the first layer decreased with progress
of time after fertigation (Fig. 4d-f). The data shows
that difference in N concentration is more at the depth of 0-30 cm which
is classified as active root zone. Concentration of N was more at the
vertical plane located at 15 cm from emitter. It may be mentioned that
plant is located at 15 cm from the emitter and this would also mean adequate
N availability near the plant roots.
Figure 5a-c revealed that 5 days after fertigation
amount of N in first and second soil layers slowly is decreased and 7
day after fertigation there was lowest N in active crop root zone. Figure
5c showed that 7 day after fertigation, crop needs nutrient and fertigation
schedule should be repeated.
Figure 5d-f shows that after next fertigation nitrogen
concentration is increasing again in first layer and it is not leaching
to down soil layers.
Simulation of N distribution in radial direction: Figure
6 shows simulation of N distribution in radial direction at the depth
of 15 cm at the end of 2 month after transplanting with emitter discharge
rate of 4 L h-1. This depth was selected because it was assumed
that active root zone of onion crop is this zone. It can be observed that
N concentration at this depth has not changed in radial direction for
a particular soil type. It is observed that N concentration is distributed
uniformly in radial distance at this depth for emitter discharge rates
of 4 L h-1. Figure 6 also revealed that effect
of emitter discharge on N concentration below emitter and at radial direction
is more in case of sandy loam soil. Distribution of N concentration indicated
that effect of emitter discharge was more in the vicinity of emitter in
the upper layer. Effect of discharge on N concentration in the middle
and lower layer was not significant.
|
|
| Fig. 5: |
Simulated nitrogen concentration with 4 L h-1
emitter discharge at the end of second month after transplanting (a)
120, (b) 144, (c) 168 after fertigation, (d) 4, (e) 24 and (f) 48
h after next fertigation |
|
| Fig. 6: |
Simulation of nitrogen distribution in radial direction
at the depth of 15 cm with 4 L h-1 emitter discharge, second
moth after transplanting |
Nitrogen distribution in the soil at the time of harvesting: Color
spectrum revealed that at the harvesting time higher concentration of
nitrogen is distributed in the first and middle layers in case of sandy
loam soil (Fig. 7). This implies that 150 days after
transplanting, nitrogen is distributed mostly in upper soil layer and
less amount of nitrogen is leached below than the active crop root zone.
To examine the leaching potential, N concentration in the last layer can
be used as an indicator. Maximum amount of N concentration in this layer
for sandy loam soil was 78 kg ha-1.
| Table 4: |
Percentage of N leached below than root zone with different
fertigation strategy |
 |
This implies that more
permeable soils are prone to leaching compared to the less permeable soils.
However, it may be mentioned that N concentration in active root zone
is adequate.
Nitrogen leaching below the root zone depth: To find out the leaching
potential of sandy loam soil in the end of crop growing period under various
emitter discharge rates, amount of N going below 60 cm depth were determined
and are shown in Table 4. Amount of N going below the
root zone depth was obtained from the cumulative drainage boundary flux
component available in post processing files of Hydrus-2D. Nitrogen leaching
percentage was taken as ratio of cumulative N going below 60 cm depth
and applied N.
|
| Fig. 7: |
Simulated nitrogen concentration at the time of harvesting
with emitter discharge of (a) 2 L h-1 and (b) 4 L h-1) |
The table revealed that in all cases percentage of N leached
below the root zone depth was much less. N leaching increased with increase
in discharge rate under all fertigation strategies for sandy loam soil.
For the same soil with 2 L h-1 emitter discharge, the N leaching
was highest in case of alternate day irrigation-weekly fertigation (strategy
B ) given 10 min before irrigation cut off. However, at 4 L h-1,
N leaching was highest in case of alternate day irrigation-weekly fertigation
(strategy A ) given 10 min after beginning of the irrigation. This implies
that in case of permeable soils like sandy loam, fertigation strategies
play role in N leaching. Therefore, while designing drip fertigation system
in this soil, above discussion should be considered.
DISCUSSION
Results of this study showed that emitter discharge played a significant
role in influencing N concentration in middle and lower layers for sandy
loam soil. However, in case of 2 and 4 L h-1 emitter discharge,
N concentration in the active root zone (i.e., middle layer) is comparatively
more. Though, N concentrations for 2 and 4 L h-1 in the middle
layer (i.e., in active root zone) are adequate, leaching percentages of
N below 60 cm will be more for the discharge of 4 L h-1. This
may be due to the fact higher emitter discharge may have pushed the N
below the 60 cm.
Table 4 revealed that N leaching increased with increase
in discharge rate under all fertigation strategies for sandy loam soil.
For the same soil with 2 L h-1 emitter discharge, the N leaching
was highest in case of alternate day irrigation-weekly fertigation (strategy
B ) given 10 min before irrigation cut off. However, at 4 L h-1,
N leaching was highest in case of alternate day irrigation-weekly fertigation
(strategy A ) given 10 min after beginning of the irrigation. This implies
that in case of permeable soils like sandy loam, fertigation strategies
play role in N leaching. Therefore, while designing drip fertigation system
in this soil, above discussion should be considered.
CONCLUSION
Results presented in this study showed that if the drip system designs
properly, it will distribute uniformly nutrient in radial and vertical
direction of soil surface. Calibration and validation results show that
Hydrus-2D can be used for simulation of water and nitrogen distribution
and nutrient leaching in soil. Results revealed that in the one week fertigation
schedule adequate amount of nitrogen was available in onion root zone.
Results also revealed in sandy loam soil fertigation strategy is effecting
the nitrogen leaching. N leaching increased with increase in emitter discharge
rate under all fertigation strategies. With 2 L h-1 emitter
discharge, the N leaching was highest in case of alternate day irrigation-weekly
fertigation (strategy B ) given 10 min before irrigation cut off. However,
at 4 L h-1, N leaching was highest in case of alternate day
irrigation-weekly fertigation (strategy A ) given 10 min after beginning
of the irrigation. This implies that in case of permeable soils like sandy
loam, fertigation strategies play role in N leaching. Therefore, while
designing drip fertigation system in this soil, above discussion should
be considered.
ACKNOWLEDGMENTS
I acknowledge Dr. D.K. Singh, A.K. Singh and Manoj Khanna Water Technology
Center , Indian Agricultural Research Institute, New Delhi-India. I am
thankful Mr. Hassan Goley Irrigation Engineer, Shahrood University of
Technology , Shahrood. Iran, for his helps in study work.
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