Introduction
World population growth and increasing demand for food have led to
excessive use of land for agriculture, a sharp decrease in soil
nutrients, and consequently a diminish in the soil fertility and quality
of agricultural products. The application of chemical and organic
fertilizers is recommended to compensate for soil nutrients. The use of
chemical fertilizers is the fastest and most reliable way to improve
soil fertility and macronutrients, such as nitrogen (N) and phosphorus
(P). However, chemical fertilizers contribute to the non-point source N
and P losses, which increase water and soil pollution, soil
acidification, and accelerate eutrophication. Leaching of nutrients from
the soil to groundwater is caused by several factors, such as
environmental conditions, soil physical and chemical properties,
agricultural activities, and chemical and organic fertilizer application
(Li et al., 2016). The application of organic fertilizers with the
optimum rate of chemical fertilizers is recommended for improving soil
fertility, soil structure, and activity of the organisms. Organic
farming systems that rely on the use of organic matter and the lack of
use of chemical fertilizers are expanding because it is environmentally
friendly and improve the quality of soil and water in comparison with
modern agricultural systems. Various types of organic amendments (sewage
sludge, animal manures, plant residues, biochar, etc.) are used to
improve the physical, chemical, and biological properties of soil.
Sewage sludge is an inexpensive and nutritious organic fertilizer that
is widely used in agricultural land (Opiony et al., 1998; Brady & Weil,
2012). In many countries, the banning of sewage sludge disposal by
burial and the lack of suitable landfill has increased the application
of sewage sludge in agricultural land. One of the advantages of using
sewage sludge in agriculture is the return of essential nutrients and
organic matter to soil (Brockway, 1983; Bramayard, 2002). Poultry
manure, such as sewage sludge, is suggested as a source of nutrients in
some research (Sharplley et al., 2004) and high concentrations of
macronutrients have been reported in these organic manures. But there is
concern about the presence of organic and inorganic pollutants in sewage
sludge and poultry manure. The nutrients and pollutants in the sewage
sludge and poultry manure can be leached when they enter the soil and
cause pollution of groundwater (Li et al., 2016). Agriculture is one of
the primary sources of water pollution and farmers must adopt
sustainable practices to maintain optimum water quality in line with
international standards (Thorburn et al., 2003; Jalali, 2005; Islam et
al., 2011). Water conservation, while maintaining the function of living
systems, is a major challenge for the agriculture industry (Elmi et al.,
2005). Leaching of nutrients is an important environmental concern
because the high concentrations of some ions in drinking water are
harmful to human and aquatic ecosystem health (Jalali & Martipurpour,
2008). Excessive application of organic and inorganic fertilizers in
agricultural lands causes N leaching and other nutrients into surface
and groundwater. Leaching of nitrate (NO3-N) in soils
treated with N-rich fertilizers and organic fertilizers is a serious
concern that has been considered in many investigations (Shephard, 1996;
Corrêa et al., 2006; Samaras et al., 2008; Shepherd and Newell‐Price,
2013; Forge et al., 2016). However, ammonium (NH4-N)
leaching in soils amended with organic and inorganic fertilizers has
received little attention in research. There is also limited knowledge
about the leaching of the main nutrients in sewage sludge and poultry
manure, such as sodium (Na), potassium (K), calcium (Ca), and magnesium
(Mg) (Medalie et al., 1994; McLaren et al., 2003). Although Ca, Mg, and
K nutrients have low environmental and harmful effects on human and
animal health, but their leaching from soil causes the loss of valuable
nutrients as well as the beneficial potential of sewage sludge for
agriculture. Simulation of nutrient leaching using models provides an
opportunity to estimate nutrients leaching and to help to explain the
changes observed during leaching while reducing or eliminating field
measurement costs (Ingraham & William, 2019). The PHREEQC program is a
computer program to simulate chemical reactions and transport processes
in soil and has been used by many researchers (Jalali & Latifi, 2018;
Mazloomi & Jalali, 2017; Embile et al., 2019). A clear understanding of
the potential leaching risk of nutrients from organic amended soils is
needed so that management practices can be modified to protect water
quality. Therefore, this study was carried out to investigate leaching
of NH4-N, NO3-N, Na, K, Ca, and Mg in
soil columns amended with 1%, 3%,and 5% of sewage sludge and poultry
manure. The cations breakthrough curves were also simulated by the
PHREEQC software.
Materials and Methods
Soil and organic amendments
The sandy loam soil used in this study was collected from 0 - 20 cm
depths of the soil layers in the Azandarian area, Hamadan, western Iran
(Table 1). In this area, various crops and horticultural products,
including grapes, wheat, vegetables, and forage are cultivated. Two
types of organic amendments (sewage sludge and poultry manure) were
widely used to increase soil fertility in Iran. Sewage sludge was
collected from the wastewater treatment plant, Toyserkan, Hamedan, and
poultry manure were purchased from the Netherlands. Farmers in Hamedan
and Iran provide a significant portion of the organic fertilizers from
imported fertilizers. Both organic fertilizers were air-dried, passed
through a 2 mm sieve, and kept to measure their physical and chemical
properties. The total N (Kjeldahl N) was measured by the Hess method
(Hess, 1971). Electrical conductivity (EC) and pH of organic amendments
were measured after extraction with distilled water (1:5 w/v organic
amendments to water extract). Concentrations of NH4-N
and NO3-N in organic amendments were extracted by 1 M
KCl (1:10 w/v organic amendments to water ratio) and determined
colorimetrically (Mulvaney, 1996). These extracted N forms may represent
the available N. Organic amendments samples were digested in nitric acid
(1:20 w/v organic amendments to nitric acid ratio) for extraction of
total basic cations. Total Ca and Mg were determined by the titration
method and total K and Na were measured by flame photometry (Hess,
1971). Characteristics of organic sewage sludge and poultry manure are
shown in Table 2. The exchangeable cations (Ca, Mg, K and Na) in sewage
sludge and poultry manure were extracted using 1 N ammonium acetate
(NH4OAc). The exchangeable cations in the soil before
and after the addition of different rates of sewage sludge and poultry
manure are given in Table 3 (exchangeable Ca, Mg, K and Na were
calculated from the sum of cation exchange capacity (CEC) of soil and
organic amendments, and exchangeable NH4-N was measured
by extraction with 1 M KCl in amended soils). Soil solution properties
(pH, EC, and concentrations of cations) in the control soil and sewage
sludge and poultry manure-amended soils were extracted using distilled
water (1:1 w/v soil to water ratio), as well as the concentration of
available NO3-N by 1 M KCl was extracted (Table 3). The
concentration of NO3-N was measured colorimetrically.
Leaching experiments
The collected soil was air-dried and passed through a 2 mm sieve.
Different rates (1%, 3%, and 5%) of sewage sludge and poultry manure
were mixed homogeneously and manually with soil. Farmers usually add a
lot of organic amendments to increase their crop yield, which is more
than the amount needed for the yield improvement. Therefore, different
rates of sewage sludge and poultry manure were used in this experiment
to compare the leaching of nutrients and the risks of groundwater
pollution with real conditions. The mixtures of soil and organic
amendments were placed into columns consisting of Pyrex tubes, 30 cm in
length and an internal diameter of 4.9 cm. The height and bulk density
of the soil inside the columns were 10 cm and 1.67
g/cm3, respectively. The soils were kept at the end of
the column by a Whatman no. 42 filter paper, which was supported by a
nylon mesh base. To prevent the dispersion of surface soil, a filter
paper was placed on the soil surface of the column. Distilled water was
used for leaching of nutrients from columns. Soil columns were incubated
at room temperature for 20 days and irrigated daily with distilled
water. The leaching experiments were carried out in two replications and
a total of 14 leaching columns were set up. The leachates were collected
daily and stored at 2 °C for measurement of pH, EC,
NH4-N, NO3-N, Na, K, Ca, and Mg
parameters. The pH and EC of leachates were measured using pH and
electrical-conductivity meters, respectively. Concentrations of
NH4-N and NO3-N in leachates were
determined colorimetrically (Mulvaney, 1996). The concentration of Ca
and Mg were determined by the titration method and K and Na were
measured by flame photometry (Hess, 1971).
Simulation of cations leaching by PHREEQC program
For the simulation of leaching of NH4-N, Na, K, Ca, and
Mg in the control and sewage sludge and poultry manure-amended soils,
the PHREEQC program (Parkhurst & Appelo, 2013) was used. The parameters
applied for simulation of cations leaching included the concentrations
of cations in soil solution, the values of CEC, the values of
selectivity coefficients for the exchange of different cations with
exchange sites, soil column characteristics (such as length of soil
columns and leaching time). The selectivity coefficients values (Table
4) applied in the leaching simulation were determined based on the
change of database coefficients and the best fit to the measured data
(Jalali & Latifi, 2018). The cation exchange capacity of the soil and
organic amendments were measured separately and the CEC values used in
the simulation were calculated based on the sum of soil and organic
amendments CEC. The soil solution properties of control and amended
soils were also measured in the laboratory and used for simulation. The
determination coefficient (r2), root-mean-square error
(RMSE), and normalized root-mean-square error (NRMSE) were used to
evaluate the performance of the cation exchange model in PHREEQC program
(Xiong et al., 2006; Mazloomi and Jalali, 2019).
- Results and Discussion
- Electrical conductivity changes in soil leachates
Fig. 1a indicates the EC leachate for different treatments. The results
showed that the application of sewage sludge and poultry manure
increased leachate EC compared to the control soil. Also, increasing the
rates of both amendments increased leachate EC. The leachate EC was very
high on the first day in all soil columns and exponentially decreased on
the second day until a steady trend on the 20th day. Values of leachate
EC in soil columns amended with different rates of poultry manure were
higher than sewage sludge. Poultry manure and sewage sludge contain high
soluble salts and therefore their application in soil increased soil
solutes and thus increased EC. When the solutes of the soil are leached
and the balance between the soil solution and the exchangeable sites is
achieved, the value of leachate EC reaches a constant trend (Civeira &
Lavado, 2008). Many researchers have reported that the use of different
organic amendments increase the concentration of soil solutes and thus
increases the EC in the soil, which is consistent with the results of
the present study (Gallardo-Lara & Nogales, 1987; Selvakumari et al.,
2000; Niklasch & Joergensen, 2001; Atiyeh et al., 2001; Sarwar et al.,
2003; González et al., 2010; Angelova et al., 2013).
pH changes in soil leachates
The results showed that the leachate pH in the control soil fluctuated
in the range of 7.70 to 8.12 and the mean of leachate pH during the
leaching period was 7.93 (Fig. 1b). The application of sewage sludge and
poultry manure decreased leachate pH compared to control soil. Mean
leachate pH in soil columns amended with 1%, 3%, and 5% of sewage
sludge and poultry manure were 7.64, 7.56, 7.47, 7.64, 7.48, and 7.46,
respectively. Adding organic matter to soil buffers soil pH changes.
Soil organic matter has many negative sites that bond with hydrogen ion
in acidic soils and release hydrogen ion in basic soils and this
mechanism neutralizes pH in both soil types. Organic residues usually
contain large amounts of ions and their decomposition initially causes a
temporary increase in soil pH. The soil microorganisms during the
decomposition of plant residues into NH4-N cause a
temporary increase in pH. The conversion of NH4-N to
NO3-N by the nitrification process reduces pH. If the
NO3-N is lost during the leaching process, the pH will
show an even greater reduction. The effects of organic matter on soil pH
depend on the processes of production and loss of N (leaching into the
soil and uptake by the plant), the quantity and quality of soil organic
matter and the initial pH (Pocknee & Sumner, 1997; Paul et al., 2003;
Butterly et al., 2013). Angelova et al. (2013) indicated that organic
acid (amino acids, glycine, cysteine, and humic acid) production during
the process of mineralization of organic matter by heterotrophic
microorganisms and nitrification by autotrophic bacteria decrease soil
pH.
Leaching of NH4-N
The breakthrough curves and the cumulative amount of
NH4-N leached from control soil and soils amended with
1%, 3%, and 5% of sewage sludge and poultry manure are shown in Fig.
2a. The results showed that the concentration of NH4-N
in the leachate from the control soil was 0.5 mg/L on the first day and
decreased exponentially over time. Low NH4-N
concentration in the control soil may be due to the low content of
organic matter and low release of NH4-N by
mineralization of organic matter. Also applied NH4-N
fertilizers in light-textured soils are rapidly converted to
NO3-N by nitrification process because these soils have
sufficient ventilation. Therefore, low concentrations of
NH4-N in control soil leachate were expected. Di and
Cameron (2005) indicated that NH4-N leaching rarely
occurs in soil because the concentration of NH4-N in
most surface soils is low and is maintained by exchange sites.
Application of sewage sludge and poultry manure increased
NH4-N leaching compared to control soil. The
concentration of NH4-N on the first day in soils amended
with rates of 1%, 3%, and 5% of sewage sludge and poultry manure were
21.2, 72.0, 207.9, 11.1, 46.8 and 91.2 mg/L, respectively, and decreased
exponentially over time. An exponential decrease in
NH4-N concentration over time may be due to the
reduction in NH4-N concentration in the soil solution
phase, oxidation of NH4-N to NO3-N, and
adsorption of residual NH4-N by soil exchange sites
(Qiang et al., 2004). Soil columns amended with sewage sludge had high
values of NH4-N leaching on day 10 to the end of the
leaching due to lower C/N ratio of sewage sludge, higher total N content
and possibly greater mineralization of sewage sludge compared to poultry
manure. The cumulative amounts of NH4-N leached in soil
columns amended with sewage sludge were significantly higher than
poultry manure (Table 5). The results also showed that in control soil
and soils amended with 1%, 3%, and 5% of sewage sludge and poultry
manure, the cumulative amounts of NH4-N leached was
equal to 14.4%, 87.3%, 131.4%, 133.1%, 33.1%, 55.7% and 67.1% of
soil exchangeable NH4-N, respectively. In soil columns
containing 3% and 5% sewage sludge, the amount of
NH4-N leached from soil was higher than exchangeable
NH4-N, probably due to the decomposition of organic
residues and release of NH4-N by mineralization process.
Zarabi and Jalali (2012) investigated NH4-N leaching in
some native calcareous soils and their results showed that the
cumulative amounts of NH4-N leached were 0.8–7.4 mg/kg.
They also reported that the cumulative amount of NH4-N
leached was about 14.1%–143.8% of the exchangeable
NH4-N in different soils. Many researchers have reported
that NH4-N leaching is high in light-textured soils
treated with organic residues because these soils have low CEC and high
hydraulic conductivity (Deare et al., 1995; Li et al., 1997; Gundersen,
1998; Pathan et al., 2002; Zarabi & Jalali, 2012). Also when the soil
exchange sites are saturated and large amounts of NH4-N
fertilizers enter the soil or in adverse environmental conditions where
nitrification is low in the soil, NH4-N leaching
increases (Wang & Battany, 1995; Wang & Zhang, 2004; Herrmann et al.,
2005; Haynes and Judge, 2008). Oren et al. (2003) reported that high
concentrations of soluble salts and high pH in the soil are the main
factors preventing the oxidation of NH4-N and nitrite
(NO2-N), which increases NH4-N leaching
in the soil. Some studies have reported that NH4-N
leaching from soils treated with sewage sludge and a variety of
livestock manure was negligible (Medalie et al., 1994; Cameron et al.,
1995; Cameron et al., 1996; Carey et al., 1997), which is contrary to
the results of the present study. The cause of the contradiction between
the present study and other researches may be due to differences in soil
texture, type of organic residues, amount of NH4-N in
the organic residues, degree of decomposition of organic residues, and
duration of leaching. There were positive correlations (Table 6) between
cumulative NH4-N leaching losses and exchangeable
NH4-N, Ca, and Mg, while positive correlations were
found with solution NH4-N, NO3-N, and Mg
in the control and amended soil.
Leaching of NO3-N
The breakthrough curve and the cumulative amount of
NO3-N leached are shown in Fig. 2b. The concentration of
NO3-N in the control soil in the first leachate was 14
mg/L and decreased to less than 10 mg/L over time. The rapid movement of
NO3-N ions in the soil is due to the repulsive force
between the NO3-N anion and the negative charge
particles in the soil. Zarabi and Jalali (2012) investigated
NO3-N leaching in some native calcareous soils with
different textures. Their results showed that the NO3-N
concentration was high in the first leachate and in the subsequent
leachates the NO3-N concentration was significantly
reduced, which is in agreement with the results of the present study.
Razzaque and Hanafi (2004) reported that NO3-N
concentration in the first leachate of peat soil was 13 mg/L and
decreased abruptly. Application of sewage sludge and poultry manure
amendments increased NO3-N leaching. Concentrations of
NO3-N in the first leachate in columns amended with 1%,
3%, and 5% of sewage sludge and poultry manure were 32.5, 65.7, 125.1,
45.3, 150.2 and 347.6 mg/L, respectively. Nitrate concentration in
poultry manure was higher than sewage sludge, so NO3-N
leaching was higher in the presence of poultry manure as expected.
Nitrate concentration in all soil columns reached equilibrium after
about 10 days. High levels of NO3-N in leachate even
after 20 days indicate the possibility of groundwater contamination with
high rates of sewage sludge and poultry manure. The cumulative amount of
NO3-N leached in control soil and soil columns amended
with 1%, 3% and 5% of sewage sludge and poultry manure were 9.3,
25.9, 39.4, 62.5, 27.8, 75.4 and 173.9 mg/kg, respectively and
significantly higher amounts of NO3-N was leached from
soil amended with 5% poultry manure (Table 5). The results also showed
that in the control soil and soils amended with 1%, 3% and 5 % of
sewage sludge and poultry manure, the amounts of NO3-N
leached was equal to 94.0%, 123.3%, 121.2%, 123.4%, 104.6%, 128.9%
and 150.2% of available NO3-N, respectively. Nitrate in
the soil is readily transported by diffusion and mass flow process, so a
large proportion of N fertilizers are exposed to leaching. Groundwater
contamination by NO3-N has been reported by many
researchers as a result of the overuse of organic fertilizers )Tong et
al., 1997; González et al., 2009; Masaka et al., 2013; Aronsson et al.,
2014; Parvage et al., 2015). González et al. (2009) reported a high
concentration of NO3-N in leachate in a study on soil
treated with poultry manure. In this study, NO3-N
leaching increased significantly with increasing the application rates
of sewage sludge and poultry manure. Some researchers have reported that
there is a positive relationship between the applied rate of fertilizers
and the leaching of NO3-N (Gondek & Konpee, 2009;
Shepherd & Newell‐Price, 2013). One of the reasons for high
NO3-N leaching in the present study is due to the sandy
texture of the soil, which caused a large amount of
NO3-N in sewage sludge and poultry manure to be leached
from the soil columns. Shepherd and Newell‐Price (2013) indicated that
NO3-N leaching in soil was affected by soil texture and
was higher in sandy soils. In sandy soils, N mineralization and
NO3-N leaching are high because high temperatures and
sandy soil texture increase the mineralization of organic N. The effect
of soil texture on N mineralization is due to the amount of clay
(Breland & Hansen, 1996). In soils with high clay content, a physical
shield prevents the mineralization of organic matter (Stenger et al.,
1995). Sadej (2009) indicated that the amount and dominant form of N
leaching in soils amended with different types of sewage sludge and
sewage sludge compost depends on the application rate, application
method, and type of sewage sludge.
There were positive correlations (Table 6) between cumulative
NO3-N leaching losses and exchangeable Na and EC, while
positive correlations were found with solution Na, K, and Ca in the
control and amended soil.
Leaching of Na and K
The breakthrough curves of Na and the cumulative amounts of Na leached
are shown in Fig. 3a. The results showed that in control and amended
soils maximum concentration of Na was observed in the first leachate.
The high concentrations of Na in the leachate of poultry manure-amended
soils are due to the higher Na content in poultry manure compared to
sewage sludge (Table 2). The concentration of Na in the leachate in all
soil columns decreased exponentially until day 6-14 and after this time
remained constant. The results showed that significantly higher amounts
of Na were leached from soil amended with 3% and 5% poultry manure
(Table 5). Cumulative amounts of Na leached in control soil and soil
columns containing 1%, 3% and 5% sewage sludge and poultry manure
were 2.6, 9.5, 23.3, 38.9, 41.7, 125.8 and 201.1 mg/kg (Table 5),
corresponding to 1.9%, 6.9%, 14.5%, 24.2%, 25.9%, 60.8% and
79.5%, respectively. There were positive correlations (Table 6) between
cumulative Na leaching losses and exchangeable Na, while positive
correlations were found with pH, EC, and solution Na, K, and Ca in the
non-amended and amended soil.
The breakthrough curves of K and the cumulative amounts of K leached are
shown in Fig. 3b. The results showed that a significant amount of K was
leached from amended soils. Over time, K concentration in control and
amended soils decreased exponentially and K concentration in the
leachates in poultry manure amended-soils was higher than that of sewage
sludge. The results showed that significantly higher amounts of K were
leached from soil amended with 3% and 5% poultry manure (Table 5). The
cumulative amounts of K leached in control soil and soils containing
1%, 3% and 5% of sewage sludge and poultry manure were 57.6, 68.9,
129.4, 168.5, 211.9, 746.5, 1123.4 mg/kg (Table 5), corresponding to
8.1%, 9.7%, 18.2%, 23.7%, 29.8%, 100.5% and 158.6%, of
exchangeable K, respectively. The poultry manure used in the present
study was enriched by K fertilizer, so in the soils amended with 3% and
5% poultry manure the amount of leached exchangeable K was higher than
the soil exchangeable K. Potassium leaching in the soil depends on the
concentration of K in the soil solution, the type of soil clay minerals,
the dry and wet conditions of soil, and the type of cations (especially
Ca, Mg, and Na) present in the solution (Evangelou & Lumbanraja, 2002).
The addition of various organic residues such as poultry manure and
sewage sludge increases the concentration of K in the solution phase and
thus increases its leaching (Adegbidi et al., 2003; González et al.,
2009). Adding these residues also increases the concentration of Ca, Mg,
and Na in the soil and thus increases the release of K from the soil
exchange sites. Jones et al. (2012) reported that the application of
poultry manure increased the concentration of Na and K in soil solution,
and K had a higher concentration than Na in leachate, which is
consistent with the results of the present study. Zhang et al. (2004)
investigated the effect of animal manure on EC and solute concentration
in soil leachates. Their results showed that the application of high
levels of animal manure increased the concentration of nutrients such as
Na and K in the leachate, and the highest concentration of Na and K were
observed in the first leachate. The addition of sewage sludge to soil
also increases the leaching of Na and K into the soil because sewage
sludge is the source of these cations (Medali et al., 1994; Mclaren et
al., 2003; Ahlberg et al., 2006).
Like cumulative Na leached, there were positive correlations (Table 6)
between cumulative K leaching losses and exchangeable Na, while positive
correlations were found with pH, EC, and solution Na, K, and Ca in the
non-amended and amended soil, indicating the similarities between these
ions.
Leaching of Ca and Mg
The breakthrough curves and the cumulative amounts of Ca and Mg in
control and amended soils are shown in Figs. 4. Application of
amendments increased the concentration of Ca and Mg in the leachates and
the leaching of both cations in all soil columns decreased exponentially
over time. Increasing rates of amendments also increased the
concentrations of Ca and Mg in the soil leachates. The results showed
that the cumulative amounts of Ca and Mg leached in the columns treated
with poultry manure were higher than the sewage sludge. The results
showed that significantly higher amounts of Ca were leached from soil
amended with 5% poultry manure (Table 5). The amounts of exchangeable
Ca leached in the control soil and the soils amended with 1%, 3%, and
5% of sewage sludge and poultry manure were 16.2%, 18.3%, 21.5%,
23.1%, 16.4%, 23.5% and 25.7%, respectively, and the amounts of
exchangeable Mg leached in these treatments were 33.5%, 37.8%, 32.9%,
48.4%, 38.3%, 59.9% and 63.5%, respectively. The trend of Mg
concentration in leachate was similar to that of Ca, and the same
leaching trend is due to the similar behavior of these two cations in
soil (Mclaren et al., 2003; González et al., 2009). The results showed
that significantly higher amounts of Mg were leached from soil amended
with 3% and 5% poultry manure (Table 5). In some studies, the effects
of organic residues on the leaching of Ca and Mg have been reported.
Zhang et al. (2004) reported that animal manure application had a
significant effect on the leaching of nutrients such as Ca and Mg, and
cumulative amounts of cations leached increased with the increasing rate
of manure. Haynes and Judge (2008) investigated the effect of surface
application of poultry manure (0-5 cm depth) on nutrient and salinity
leaching in soil columns, and measured nutrient concentrations in 4
leachate volumes on days 9, 37, 58 and 86 after the leaching. Their
results showed that the addition of poultry manure increased the
leaching of different nutrients such as Ca and Mg compared to control
soil. Ahlberget al. (2006) investigated the leaching of various cations
from surface soil treated with sewage sludge. Their results showed that
soil treated with sewage sludge increased the leaching of nutrients such
as Ca and Mg from the soil and the amount of Ca and Mg leached was
equal.
There were positive correlations (Table 6) between cumulative Ca
leaching losses and exchangeable NH4-N, K, Ca, and Mg,
while positive correlations were found with EC and solution
NO3-N, Ca, and Mg in the non-amended and amended soil.
There were positive correlations (Table 6) between cumulative Mg
leaching losses and exchangeable Na, while positive correlations were
found with EC and solution Na, K, Ca, and Mg in the non-amended and
amended soil.
In general, based on the proportion of cumulative nutrients leached
relative to the exchangeable ones, the order of nutrients leached in
sewage sludge-amended soils was NH4-N >
NO3-N > Mg > Na >
K > Ca, whereas the order for poultry-amended soils was K
> NO3 > Na >
NH4-N > Mg > Ca. Thus, more
attention should be paid to the potential risk of soil nutrient leaching
especially NH4-N, and K, NO3-N posed
after application of sewage sludge and poultry manure, respectively.
Simulation of cations leaching by PHREEQC program
The measured and simulated breakthrough curves of NH4-N,
Na, K, Ca, and Mg is shown in Figs. 5-9. The values of
r2, RMSE, and NRMSE obtained from the cations leaching
simulation are shown in Tables 7–9. The main mechanism to simulate the
leaching of these cations is the cation exchange that controls the
concentrations of the cations in leachates. The addition of organic
amendments to soil increased CEC of soil columns, which was defined for
different soil columns in the PHREEQC program. Some researchers also
reported that the use of organic residues increased soil CEC (Ouédraogo
et al., 2001; Walker & Bernal, 2005; Clark et al., 2007). The results
showed that NH4-N leaching was well predicted (Fig. 5)
in soils treated with sewage sludge and poultry manure
(r2=0.84-0.97, RMSE= 1.23-24.11). But the simulation
of NH4-N leaching in the control soil was not very accurate
(r2=0.61, RMSE=0.14). The NH4-N curve tailing did not
match well with the measured data in soil columns treated with 3% and
5% of sewage sludge, which may be due to mineralization and
ammonification of sewage sludge over time. The breakthrough curves of Na
were less accurate (Fig. 6) than the other cations (r2= 0.51- 0.67, RMSE=1.7-93.38), and the Na leaching was overestimated.
The breakthrough curves of K (Fig. 7) and Ca (Fig. 8) indicated that K
and Ca leaching in control soil and soils amended with sewage sludge and
poultry manure were well predicted (High r2 and low
RMSE). Simulation of Mg (Fig. 9) leaching showed that there was no good
agreement between measured and simulated data in some treatments and the
simulation had low accuracy. For a better comparison of cation
simulation results, the NRMSE was calculated (Table 9). The NRMSE is
used to compare models with different scales or with different
observation values. NRMSE values, similar to RMSE, indicate that
simulation of leaching of NH4-N, K, and Ca (mean NRMSE
< 1) by cation exchange model was better than Mg and Na (mean
NRMSE > 1). Many studies have used PHREEQC program to
simulate the adsorption, transport, and leaching of different elements
(Ferrell et al., 2002; Komnitsas et al., 2006; Congzhi et al., 2006;
Martens et al., 2011; Mazloomi & Jalali, 2017; Jalali & Latifi, 2018).
Ferrel et al. (2002) used the PHREEQC program to predict lead (Pb)
transport in the clay layer of a landfill and simulated the effect of
different parameters (CEC, selectivity coefficients, diffusion
coefficient, and different amounts of Pb) on Pb transport. Congzhi
(2006) investigated the transport of K, Na, Ca, and Mg through soil
column experiments and simulated the measured data using PHREEQC and
CXTFIT programs. He reported that the simulation results were in good
agreement with the measured data and PHREEQC simulated the measured data
better than CXTFIT. Martens et al. (2011) measured leaching of K, Na,
Ca, and Mg cations into the clay column over 2600 days (clay column was
leached by different concentrations of sodium nitrate) and then
simulated the measured results by PHREEQC program. They indicated that
cation exchange is the predominant mechanism for control of cations
leaching into the clay column.
Conclusions
Sewage sludge and poultry manure are widely used to provide nutrients
for crops and improve soil quality. However, an excess application may
increase the risk of nutrient loss and groundwater pollution. Since our
research was performed in the laboratory, which does not exactly
replicate conditions in the field, we were able to determine the
relative risk of leaching among the tested organic amendments. The
content of nutrients and the application rate of sewage sludge and
poultry manure were influential factors on the values of leaching of
nutrients. The maximum value of NH4-N leaching was
observed in the soil columns amended with 5% sewage sludge and the
maximum values of NO3-N, Na, K, Ca and Mg leaching were
observed in the columns amended with 5% poultry manure. In general, the
order of nutrients leached in sewage sludge-amended soils was
NH4-N > NO3-N
> Mg > Na > K > Ca,
whereas the order for poultry-amended soils was K >
NO3-N > Na >
NH4-N > Mg > Ca, indicating
the potential risk of soil nutrient leaching, especially
NH4-N, K and NO3-N posed after
application of sewage sludge and poultry manure, respectively, should be
given greater attention. The output of the PHREEQC program showed that
the simulated breakthrough curves, particularly for
NH4-N, K, and Ca were well matched to the measured data.
The main mechanism to simulate the leaching of these cations was the
cation exchange that controls the concentration of the cations in
leachates.