Use of sewage sludge to reclaim the organic carbon
content of burnt volcanic soils
Mónica Antiléna, Margarita Briceñob Juan E. Foersterc, Gerardo Galindoc y
Mauricio Escudeyc
a
Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago, Chile.
b
Universidad Arturo Prat, Arturo Prat 2120, Iquique, Chile
c
Universidad de Santiago de Chile, Av. B. O'Higgins 3363, Santiago, Chile
Email:mantilen@uc.cl
ABSTRACT
In this paper, the short term impact of sewage sludge amendment on burnt volcanic
soils was studied, through soil-sewage sludge incubation studies.
In amended soils the original organic carbon content was recovered and the
incubation time effect was more important in the youngest soil (Ralún). The P
chemical distribution showed slight differences with the incubation time,
demonstrating that in volcanic soils there is not redistribution of P from sewage
sludge. The pH and EC increased in amended samples, values decreased with the
incubation time, showing the buffer capacity of volcanic soils. The CEC of soils
increased with the addition of sewage sludge, which can be related to the effect of
pH changes of these variable surface charge soils.
From these studies can be concluded that sewage sludge is a valid option to reclaim
the organic carbon content loss in soils affected by forest fires. However, an
important amount of inorganic P, which is strongly fixed on volcanic soils and it is
not easily available for plants, will be accumulated and could potentially contribute
to soil and water pollution.
Keywords: organic carbon, sewage sludge, Chilean volcanic soils.
INTRODUCTION
Soils derived from volcanic materials are rich in organic matter, with a strong
retention of P, and a variable charge dependent on pH and ionic strenght. Organic
carbon (OC) content is a determinant factor to soils fertility, due to its physicochemical properties and interaction with different components, such as N, P, S, and
available microelements.
From October 2001 to May 2002 more than 4000 forest fires involving native forest
and pine and eucalyptus plantations occurred in southern Chile. It has been reported
that 400ºC may lead to the destruction on the soil organic matter (Mendoza, 1986);
in forest fires temperatures over 700ºC have been reported, resulting in the total
destruction of OC of the soil surface layer.
In developed countries, important amounts of the sewage sludge obtained from
water treatment plants is used as fertilizer, due to its high organic carbon, P, N, K
and micronutrients content. Organic fraction constitutes around of 50% of total
solids in sewage sludge, and in the chemical composition can be found
carbohydrates, fatty acids and proteins (Chang et al, 1981), which are decomposed
and assimilated by soil microorganism.
For the other side, the sewage sludge needs to be disposed, and it should be
considered to ameliorate the OC lost due to forest fires in Chilean soils. The
objective of this paper was to reclaim the original organic carbon content of burnt
volcanic soils, evaluating the short-term impact through soil-sewage sludge
incubation studies.
METHODS
Soil samples from 0-15 cm depth of Collipulli (Ultisol; Fine, mesic, Xeric,
Paleumult), Ralún (Andisol, medial, mesic, Acrudoxic Hapludands) and Diguillín
(Andisol; Medial, mesic, Entic Dystrandept) collected from uncultivated areas were
used. All samples were air-dried and sieved to 2mm.
Municipal sewage sludge was obtained from a water treatment plant from Santiago,
located on the Central region of Chile. The sludge was added to each soil in the
amount needed to recover the OC lost after heated in a furnace at 200ºC and 400°C
(Antilén, 2001). All soils were incubated at room temperature and sampled after two
and four months. Four different treatments for each soil were considered: incubated
control soil (C); soil burnt at 200ºC (S-1) and 400ºC (S-2), soil burnt at 200ºC (S-3)
or 400°C (S-4) with addition of sewage sludge. Samples without time of incubation
(0 months) were also considered.
Soils, sewage sludge, and samples after the incubation time were characterized for
organic carbon content (Walkley-Black method, Sadzawka, 1991), exchangeable
bases, cation exchange capacity (CEC, determined as exchangeable bases plus
extractable acidity at pH 7.2, Sadzawka, 1991), phosphorus chemical fractionation
(Escudey et al., 2004), electric conductivity (EC) and pH (soil water ration 1:2.5).
RESULTS
Characterization
The characterization of samples is presented in Table 1. The sewage sludge has
about three times the OC content of soils. The OC content in Andisols (Ralún and
Diguillin) is higher than in Ultisol (Collipulli). This accumulation is related with
mineralogy dominated by low crystalline compounds.
The pH was acidic for all soils, while for sewage sludge the pH was close to
neutrality. The CEC of sewage sludge was almost two times higher than soils, due to
exchangeable bases values, mainly Ca2+. The EC for sewage sludge was ten times
higher than soils, in agreement with the EB values.
Table 1. Characterization of soil and sewage sludge sample
Sample
EB
CEC
OC
(cmol(+)kg-1) (cmol(+)kg-1)
(wt%)
Collipulli
7.80.5
37.74.0
1.80.0
Ralún
3.00.6
44.85.0
6.90.0
Diguillín
10.33.7
54.24.2
5.80.1
Sewage sludge
87.89.8
105.58.2
18.00.5
EC
(mS)
0.10.0
0.20.0
0.20.0
1.90.1
Total P
(mg kg-1)
57615
90136
175889
10200358
pH
4.80.1
4.40.1
5.20.2
6.90.2
The total P in the Ultisol is lower than in Andisols, showing the sequence
Collipulli<Ralún<Diguillín. In the sewage sludge the total P was about two orders
higher than in soils.
Incubation studies
An important loss of easily oxidable OC occurred between 200ºC (S-1) and 400ºC
(S-2) as shown in Table 2. At this temperature range, decarboxylation,
dehydrogenation and dehydroxylation process had been described (Galindo et al.,
1987). In the burnt control soil no changes were observed after the incubation time.
Burnt samples amended with sewage sludge, shown a recuperation of the lost OC
content; the incubation time was important only in Ralún soil at 400ºC where as
much as twice the easily oxidable OC after four month of incubation time was
observed. Thus, from the OC content point of view, sewage sludge can be
considered as an alternative to reclaim the original soil OC content.
Table 2. Easily oxidable OC content in soils and sewage sludge treated samples after 2 and 4
months of incubation
Sample
OC
wt%
0 months
2 months
4 months
Collipulli
C
1.80.0
1.80.0
1.80.0
S-1 (200ºC)
1.60.1
1.50.1
1.60.0
S-2 (400ºC)
0.40.0
0.40.0
0.40.0
S-3 (200ºC)
1.80.1
1.70.2
2.00.1
S-4 (400ºC)
1.80.0
2.30.2
2.20.2
Diguillín
C
5.80.0
5.80.0
5.80.0
S-1 (200ºC)
5.30.5
5.90.0
5.70.2
S-2 (400ºC)
1.70.2
2.40.2
2.10.1
S-3 (200ºC)
5.80.2
6.30.1
6.10.0
S-4 (400ºC)
5.80.0
6.40.2
6.30.1
Ralún
C
6.80.0
6.90.0
6.90.1
S-1 (200ºC)
5.30.1
5.70.1
6.10.0
S-2 (400ºC)
1.00.0
2.30.1
2.00.0
S-3 (200ºC)
6.90.1
6.60.0
7.90.0
S-4 (400ºC)
6.90.0
8.00.3
7.00.3
About 45 to 65% of total P in control soils is present as organic-P, mainly associated
to humic acids (Figure 1). Depending of the soil from 4 to 23% at 200ºC and from
26 to 44% at 400ºC of that organic P was turned into in inorganic P.
The P chemical fractionation of sewage sludge indicated that only 10% of total P
was present as organic P. Around 80% of this organic P is associated to fulvic acids
and 20% to humic acids. Then, the amendment with sludge will increase the
inorganic P accumulation of this type of soils. In agreement with this statement, the
inorganic-P increases in all sewage sludge treated soils. The inorganic-P content in
treated samples is related with the total amount of sewage sludge added to each soil
(Figure 1).
The P forms distribution showed slight differences with the incubation time,
demonstrating that P from sewage sludge is not significantly redistributed to more
available P-forms in volcanic soils after a short period of time.
P
e
r
c
e
n
t
(
%
)
P
e
r
c
e
n
t
(
%
)
1
0
0
(
a
)
(
b
)
8
0
6
0
4
0
2
0
0
1
0
0
(
c
)
(
d
)
8
0
6
0
4
0
2
0
T
r
e
a
t
m
e
n
t
s
S-2( C
0
S-2(m)
2m)
S-2(
4
S-4( m)
2
S-4(m)
4m)
S-1(0 C
S-1(2m)
m
S-1(4)
S-3( m)
2
S-3(m)
4m)
0
I
n
o
r
g
a
n
i
c
P
F
u
l
v
i
c
P
H
u
m
i
c
P
T
r
e
a
t
m
e
n
t
s
Figure 1. P forms distribution in Collipulli (a), (b) and Ralún (c) and (d) soils with
different treatments
The pH increases in all burnt samples (Table 2), and in sewage sludge treated
samples. In general, a decreases with incubation time is observed, showing the
buffering capacity of soils.
Table 2. Evolution of pH and electrical conductivity in
samples with two (2m) and four months (4m) of incubation.
Sample
pH
EC (mS)
2m
4m
2m
4m
Collipulli
C
S-1
S-2
S-3
S-4
5.460.02
6.140.02
6.010.03
6.040.02
6.610.02
5.360.01
5.890.01
5.790.01
5.910.02
6.200.01
0.080.01
0.110.00
0.170.01
0.300.01
1.080.01
0.080.00
0.120.01
0.290.00
0.260.02
1.480.01
Ralún
C
S-1
S-2
S-3
S-4
4.960.01
5.680.01
6.710.02
5.780.01
6.600.01
5.100.01
5.840.02
5.960.01
5.500.03
6.810.02
0.220.01
0.200.00
0.350.01
0.710.02
2.250.01
0.360.01
0.250.01
0.460.01
0.800.00
1.940.01
Diguillín
C
S-1
S-2
S-3
S-4
6.020.01
6.590.02
6.470.02
6.360.03
6.940.03
5.840.02
5.970.03
6.580.02
5.770.01
6.810.01
0.240.02
0.200.01
0.430.01
0.490.01
1.940.01
0.250.01
0.530.02
0.730.01
0.880.01
2.010.02
The EC of burnt soils increases with heating temperature and incubation time, due to
cations and anions coming from organic carbon combustion (mainly exchangeable
bases and carbonate). In all sewage sludge treated soils the EC increases about two
times, because of the ionic release of sewage sludge; also incubation time increases
the EC as result of a continuous slow delivery of ionic species from sewage sludge,
controlled by their solubility constants.
The CEC decreases when soil heating temperature increases, the OC destruction not
only results in a lost of active sites, but also results in the exposition of inorganic
active surface sites as Al-OH and Fe-OH with pKa higher than carboxylic or
phenolic groups, and consequently with positive surface charge at the equilibrium
pH observed (Galindo et al., 1987).
The CEC of soils increases with the addition of sewage sludge, which can be related
to the exchangeable bases and the organic matter content increment because of
amendment.
7
0
(+) kg -1 )
6
0
C
o
llip
u
lli2
m
C
o
llip
u
lli4
m
R
a
lú
n
2
m
R
a
lú
n
4
m
5
0
CEC(cmol
4
0
3
0
2
0
1
0
C
S
1
S
2
C
S
1
S
2
7
0
(+) kg -1 )
6
0
5
0
CEC(cmol
4
0
3
0
2
0
1
0
T
r
e
a
tm
e
n
t
Figure 2. Changes in CEC in burnt and amended Collipulli and Ralún soils in 2 and
4 months.
CONCLUSIONS
From these experimental data can be concluded that the sewage sludge is a valid
option to reclaim the organic carbon lost in soils affected by forest fires.
Additionally the sewage sludge produced changes in pH, electric conductivity, CEC
and P forms.
The pH changes observed with the temperature and with the sewage sludge could be
considered appropriate to soils derived from volcanic materials, due to their acidic
characteristics which may result in aluminum toxicity problems. The incubation
time decreased the pH to initial values of all sewage sludge treated samples,
showing the buffer capacity of volcanic soils.
The organic-P is turned into inorganic-P with temperature. The inorganic-P
significantly increased in amended soils, which is strongly fixed on volcanic soils
and it is not easily available for plants. It will be accumulated and could potentially
contribute to soil and water pollution. The increment of EC of treated soils will not
result in a salinity problem due to the pluviometry in the region where soils are
located.
The use of sewage sludge increased the total amount of Mg, K, and Zn in soils.
ACKNOWLEDGEMENTS
This study was supported by DICYT-USACH, FONDECYT (grant 1030778) and
DIPUC Nº2003 16/E2.
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