With high rate and expense of N fertilizer presently used in modern agriculture, the nitrate leaching from agricultural soils has long considered as a major environmental problem.
Several investigators have identified the most decisive factors determining the magnitude of leaching losses (Avnimelech and Raveh, 1976; Gustafson, 1983; Bergstrom and Brink, 1986; Bergstrom and Johansson, 1991).
The pollution of groundwater by nitrate is an international problem (Spalding and Exner, 1993; Zhang et al., 1996), where in some countries has worsened in the recent years (Roberts and Marsh, 1987; Betton et al., 1991). One source of nitrate is inorganic nitrogen fertilizers and there is many literature on the link between agriculture and nitrate pollution (Royal Society, 1983; National Research Council, 1993; Criado, 1996).
Soil texture, organic matter content, water flux, fertilizer type, fertilizer application rates and method of fertilizer application can have a major influence on nitrate leaching. The problem encountered when comparing NO3-N leaching from different soils under field conditions (Abdel-Nasser, 2001, 2005; Al-Darby and Abdel-Nasser, 2006). It is known that NO3-N leaching from sandy soil is generally greater than that from clay soils, but it is not generally recognized that macropore flow may be an important reason for this difference (Al-Darby and Abdel-Nasser, 2005; Simmelsgraad, 1998; Hoffmann and Johansson, 1999).
Many regions in the world used the groundwater as the only source of drinking water and agricultural use. Nitrate in drinking water becomes a significant concern only when people drink from a water supply that is highly contaminated with nitrate (such as groundwater). Nitrate poisoning of infants during the first three to four months of life is the major concern, in which nitrate can oxidizes the iron of hemoglobin in blood to form methemoglobin and cause a condition called methemoglobinemia (Shih et al., 1997).
Column and lysimeter studies offer a good way of conducting controlled experiments under laboratory and field conditions (Bergstrom, 1990; Bergstrom and Johansson, 1991). Nitrate leaching from many types of soils or under different N-fertilization can be compared simultaneously in such cases using numerical models.
Therefore, the present study aims to reduce NO3 losses and improves the efficiency of nitrogen fertilizer applied to soil by reuse of date palm by-products after grinding and mixing with sandy loam soil.
MATERIALS AND METHODS
The soil used in the present experiment was collected from surface layer
(0-30 cm) from Kharj Government (Haradh road), Riyadh, Saudi Arabia. The texture
was sandy loam. Some physical and chemical properties performed according to
the methods described in Klute (1986). The results are presented in Table
The PVC transparent columns have an ID of 6.0 cm and a length of 30.0 cm.
The base of column tightly sealed with silicone adhesive. At the base of columns,
a glass tube with 5 mm diameter attached to collect the leachate. The columns
were carefully hand-packed with air-dried soil to the desired bulk density (1.60
Mg m-3) by gentle tapping .The columns filled to a depth of 20.0
Date Palm by-Products Powder
By-products of date palm cultivation were collected from many orchards from
Al-Waseel area, Riyadh, Saudi Arabia and air-dried for one month then grounded
to pass through 0.5 (fine fraction) and 1.0-2.0 mm (coarse fraction) sieve as
a source of organic matter (during the period of May to August, 2008).
fractionation of date palm by-product powder and mean weight diameter
|*Mean weight diameter
The date palm by-products powder were tested at rates namely; zero, 1, 2.5,
5, 7.5 and 10% (w/w). Date palm by-products powder; 0.5 (fine) and 1.0-2.0 mm
(coarse) fractions were mixed with sandy loam soil at desired rates and incubated
for one month before starting the experiments during March 2009. The mixed soil
added to the soil columns as top surface layer of 5 cm. Date palm by-product
powder and the mixed sandy loam soil were analysis for some physical and chemical
properties according to the recommended methods outlined in Klute (1986). Table
2 shows the particle size distribution of the date palm by-products powder.
The results showed that MWD (Mean weight diameter) was 0.26 and 1.21 mm for
fine and coarse fractions, respectively (Van Bavel, 1950).
Soil Hydraulic Properties
The soil water retention function, θ (h) and the unsaturated hydraulic conductivity function, k (h) of sandy loam soil and soil mixed with date palm by-product powder are determined according to the Mualem-van Genuchten model (Mualem, 1976; Van Genuchten, 1980) using the RETC model (Van Genuchten et al., 1991). Table 3 shows the recorded results.
of soil hydraulic function for sandy loam and soil mixed with date palm
|θr: Residual soil water content, θs:
Saturation soil water content, α and n: Shape parameters, Ks:
Saturated hydraulic conductivity (cm min-1), ι: Pore connectivity
parameter and ρb: Soil bulk density (g cm-3)
picture of the column experiment for nitrate leaching
Water and Nitrate Application
The soil columns saturated by adding water from bottom of each column to
reach saturated conditions, for one day. Then the soil columns left to drain
the excess water for one day to reach a field capacity conditions (this condition
checked by taking a soil samples from a separate columns to check the soil water
content). Nitrate solution (250 NO3 mg L-1) applied for
60 min at steady state rate using multi-syringe pump and then water applied
at the same steady-state rate for 300 min (Fig. 1). The water
or nitrate solution applied at two different constant rates namely; 0.2 and
0.4 cm min-1. The soil columns were monitored for collecting the
leachate at time interval of 15 min for 2 h and then 30 min for the rest time,
4 h (Abdel-Nasser, 2001, 2005).
Water draining through the bottom of the soil columns led to glass collecting
bottles that weighed at different periods to determine the drainage volume.
Sub sample then taken from the accumulated leachate for determination of NO3
concentration. The NO3 flux calculated by multiplying leachate volume
(cm3) by the NO3 concentration (mg L-1). The
NO3 concentration was determined according to the method of Norman
et al. (1985).
At the end of experiment, the soil sectioned at 2.5 cm for 10 cm depth and
at 5.0 cm for the rest of each soil column to determine the concentration of
nitrate by shaking 20 g samples of the soil with 50 mL of 2 M KCl solution for
30 min (MAFF, 1986). The NO3 concentration measured by dual wavelength
method using the scanning spectrophotometer (Norman et al., 1985).
The treatments arranged in three-factor experiment. All the treatments were
present in each of the three fully replicated, randomized blocks of split-split
plot design, with water flux density as the main factor and fraction size and
rates as the splitting factors. Statistical tools used included descriptive
statistics, regression and correlation analysis. The analysis of variance performed
according to SAS (2000). Least significant difference test was also used for
means comparison at 95% confidence level (p = 0.05).
Nitrate Flux at Lower Boundary Condition
The observed NO3 concentrations in the leachate at different
time intervals as affected by water flux density and date palm by-products powder
are shown Table 4 and 5. Both water fluxes
were able to move the NO3 out of the soil columns according to their
concentration (mg L-1) in the leachate of soil amended with
fine fraction of date palm by-products powder at low and high water flux
concentration (mg L-1) in the leachate of soil amended with
coarse fraction of date palm by-products powder at low and high water
concentration at lower boundary increased as water flux increased.
The maximum NO3
flux attained early with highest water application
rate (0.4 cm min-1
) and then delayed as the water flux density decreased.
This result is true for both date palm by-product fractions (fine and coarse fraction),
but the concentration of NO3
in leachate was higher in case of coarse
fraction (1-2 mm size). In addition, the results indicated that increasing the
application rate of date palm by-product powder decreased the NO3
in leachate. The maximum NO3
concentration in the leachate attained
after 75 min in case of low water flux (0.2 cm min-1
) but it attained
after 45 min in the case of high water flux (0.4 cm min-1
Nitrate Distribution Profile
Nitrate distribution profile at different date palm by-product fractions and
water flux shown in Fig. 2a-d. The data
clearly indicate that the NO3 ion was concentrated in the surface
layer of soil columns down to 7.5 cm with low water flux and 10 cm with high
water flux. The NO3 concentration was higher in the top 5 cm of soil
column and then decreased in the following 2.5 cm. Other soil depths are NO3-free
down to the lower end of soil columns. Nitrate concentration in the top soil
layer increased with increasing the application rate of date palm by-product
powder. The soil mixed with fine fraction of date palm by-products powder tends
to contain more NO3 concentration than soil mixed with coarse fraction.
Increasing water flux tends to decrease the NO3 concentration in
soil columns. The high water flux was able to move more NO3 out of
Nitrate concentration in solution of soil amended with date palm by-products
at two water flux rate
Mass balance recovery (mg) of nitrate in soil columns amended
with fine fraction of date palm by-products powder
Mass Balance Recovery
Table 6 and 7 show the mass balance
recovery calculations of NO3 in soil columns for different treatments.
The results presented the NO3 balance sheet (inflow and outflow calculations).
The results indicated that mixing the date palm by-products powder with soil
decreased the amount of NO3 moved out the soil column and increased
the amount of NO3 retained in soil. The NO3 mass recovery
(%) was ranged between 95.91 and 100.71%. This means that the difference between
added nitrate and leached NO3 was less than 5%. This error accepted
in case of laboratory experiments.
Statistical analysis of data presented in Table 8 and 9 indicated that all experimental treatments have highly significant effects on reducing NO3 leaching out the soil columns and increased the soil retention of NO3. The differences between treatments also are highly significant (Table 10). Table 11 shows the quadratic polynomial equations for the effect of date palm residues powder on nitrate content in leachate and soil solution. The results indicated that fine fraction of date palm by-products was more efficient by 1.5 times than coarse fraction in reducing nitrate losses by leaching.
balance recovery (mg) of nitrate in soil columns amended with coarse fraction
of date palm by-products powder at low water flux (0.2 cm min-1)
of variance for nitrate content in leachate
of variance for nitrate content in soil solution
effect and significance of the experimental treatments
The results, indicated that the percent of NO3 leaching ranged from
96.71 to 82.36% with low water flux (0.2 cm min-1) and from 98.50
to 92.71% with high water flux (0.4 cm min-1), in case of soil mixed
with fine fraction of date palm by-products powder as application rate increased
from 0 to 10% (w/w).
polynomial equations for the effect of date palm residues on nitrate in
leachate and soil solution
is the rate of date palm residues powder (%, w/w). Y is the NO3
content in leachate or soil solution (mg)
Volume of leachate and mean moisture content of soil amended
with fine and coarse fractions of date palm residue at two water flux
It resulted in decreasing the NO3 leaching by about 14.35 and 5.79%
for low and high water flux, respectively. The corresponding values for soil
mixed with coarse fraction were from 96.71 to 87.29% and from 99.81 to 94.34%
for low and high water flux, respectively. The decreasing in NO3
leaching was 9.42 and 4.34%, respectively.
Soil Moisture Distribution
The results of the soil moisture distribution in soil columns as affected
by water flux density and date palm by-products rate shown Table
12. The data clearly indicate that the mean soil moisture content increased
as rate of application increased. The values were ranged from 0.360 to 0.443
cm3 cm-3 with low water flux density and from 0.373 to
0.456 cm3 cm-3 with high water flux density as the rate
of fine fraction increased from 0 to 10%. The soil moisture contents were higher
in case of fine fraction than coarse fraction. The values for coarse fraction
were ranged from 0.355 to 0.392 cm3 cm-3 with low water
flux density and ranged from 0.372 to 0.407 cm3 cm-3 with
high water flux density as the rate of coarse fraction increased from 0 to 10%.
This means that increasing rate of date palm by-products increased the mean
soil moisture content. In addition, the soil mixed with fine fraction has more
water content than soil mixed with coarse fraction. The soil moisture content
reflected on the volume of leachate moved out the soil columns. The results
indicated that volume of leachate decreased as increasing of date palm by-products
rate and increased with coarse fraction than fine fraction.
Nitrate leaching strongly affected by the particle size distribution, the soil porosity and the occurrence of preferential flow paths (Cameria et al., 2003). Soils have varied retentive properties depending on their texture and organic matter content (Gaines and Gaines, 1994). Due to the higher proportion of large pores, coarse soils are usually more vulnerable to leaching than clayey soils (Wu et al., 1997). Fine soil does not specifically retain nitrate, but water does not pass easily through fine soil. Large surface areas of the individual fine particles and the large number of very small pore spaces can hold a large amount of water. Water filled pores of fine soils lack oxygen. Lacking oxygen, a group of soil bacteria, called facultative anaerobes, substitute nitrate for oxygen for respiration. When bacteria use nitrate as substitute foe oxygen, they convert nitrates to nitrogen gas through a process called denitrification. Nitrate loss through denitrification in fine soils reduces the amount of nitrates that can potentially leach to groundwater (Bhumbla, 2006).
The results clearly indicate that mixing sandy loam soil with date palm by-products powder, fine (less than 0.5 mm size) or coarse (1.0-2.0 mm size) increased the retained water in soil column then reduced the water moved out the soil columns ,therefore reduced the nitrate leaching because of more retention of nitrate in soil. The increased retention of nitrate attributed to the ability of soil mixed with date palm by-product powder to retain more water and consequently more nitrate against leaching.
The large amount of irrigation water used in agriculture makes the risk of leaching nitrates and other chemicals potentially greater in areas that irrigated. The increase in soil moisture that results from irrigation dissolves excess nitrate present in the soil profile and makes it more susceptible to leaching (Casey et al., 2002). Higher moisture content will also raise microbial activity including mineralization (Skopp et al., 1990). The increase in mineralization rates directly affects nutrient leaching (Doran, 1980).
Mixing the sandy loam soil (which had a coarser texture and lowest organic matter content) with organic matter (date palm by-product) would alter the soil moisture retention characterization (the mixed soil had finer texture and more organic matter content). The mixed soil more retained of nitrate; therefore, the leached nitrate was less than the sandy loam soil. This result confirms the fact that excessive rates of NO3-N fertilizer avoided for sandy soils than for finer soils due to the low NO3-N retention (Gaines and Gaines, 1994). The present result is logical because the NO3-N is non-reactive solute (McMahon and Thomas, 1974). Macro-pore flow may be an important reason for this difference (Hoffman and Johansson, 1999). These differences attributed to the differences in pore size distribution. At pore scale, the variation of water and solute flow may be due to the different velocities of water and solute because of pore groups in soil. Mixing soil with organic matter resulted in decreasing the pore size distribution therefore restricted the water and solute flux in soil. The retention of NO3-N to soil particles depends on soil type (high content of silt or clay), soil organic matter content and cation exchange capacity.
Nitrates that lost through leaching to groundwater can contribute to the groundwater nitrate pollution. The current public health standards for safe water require that Maximum Contaminant Level (MCL) should not exceed nitrate concentrations of 10 mg L-1 as NO3-N or 45 mg L-1 NO3 (USEPA, 1991, 1996; WHO, 2008).
Nitrate leaching from fertilizer depends upon the fertilizer types (ammonium, nitrate or organic), method of application and climate condition. Nitrate leaching may be greater when fertilizer contains the nitrate compound to the situations where ammonium nitrogen is the major component of a fertilizer (Bhumbla, 2006). Nitrate losses are likely to be higher when all nitrogen applied in one application compared to split application.
To avoid the groundwater pollution, frequent application of light rates of N-fertilizer performed to minimize the losses of NO3 through soil profile (Petrovic, 1989). Thus, careful matching of nitrogen fertilizer application to crop needs can reduce nitrate leaching. In addition, application of organic matter to soil helps to reduce nitrate leaching. The more efficient technology to reduce the NO3 leaching may be using the nitrification inhibitors, which when bed with fertilizers, slow the conversion of ammonium into leachable nitrate (Abdel-Nasser and El-Shazly, 1994; El-Shazly and Abdel-Nasser, 2000; Al-Darby and Abdel-Nasser, 2006).
The results clearly indicate that application of organic matter (date palm by-products) to soil increased the retained water in soil column then reduced the water moved out the soil columns, therefore reduced the nitrate leaching because of more retention of nitrate in soil. The increased retention of nitrate attributed to the ability of soil mixed with organic matter to retain more water and consequently more nitrate against leaching.
The present column experiment is useful for assessing relative behavior of NO3 in soil, at different agricultural practices. Nevertheless, may not be suitable for describing chemical transport in the field scale, since it does not account for many chemical processes; normally occur under natural field conditions, include immobilization, mineralization, nitrification and plant uptake, which result in different amount of nitrogen available for leaching.
The authors would like to express great thanks to the Agricultural Research Center and the Deanship of Scientific Research, King Saud University for funding this research through the project supported by Saudi Basic Industries Corporation (SABIC).