Como citar este trabajo Torri S, Alvarez R, Lavado R. 2003. Mineralization of Carbon from Sewage sludge in three soils of the Argentine pampas. Commun. Soil Sci. and Plant Anal. (Taylor & Francis, Inc., 325 Chestnut Street, Suite 800, Philadelphia, PA 19106) 34 (13-14): 2035-2043. ISSN (impresa): 0010-3624. ISSN (electronica): 1532-2416.

1. Mineralization of Carbon from Sewage Sludge
in Three Soils of the Argentine Pampas
Silvana Torri,* Roberto Alvarez, and Rau´l Lavado
Universidad de Buenos Aires, Facultad de Agronomı´a,
Buenos Aires, Argentina
ABSTRACT
The mineralization of carbon from sewage sludge was studied in a pot
experiment during an aerobic incubation with three representative soils of
the Pampean Region of Argentina. Surface horizons (0–15 cm) from a
Typic Hapludoll, a Typic Natraquoll and a Typic Argiudoll were used.
Samples were collected a year after sludge addition. Expressed as
percentages, more than 50% of the added sludge carbon mineralized
during the first 60 days. However, 29–45% remained in the soils a year
after application. The best fit to carbon mineralization data was provided
by a first-order exponential plus a constant kinetic model. Percent readily
mineralizable carbon (%CLS) values and first-order rate constants (k) of
the sludge treated soils ranged from 53 to 58 and 0.035 to 0.030 day21
(Typic Hapludoll, Typic Argiudoll) and 71 with a k value of 0.07 (Typic
Natraquoll). Thus, carbon mineralization of sewage sludge in the soils
2035
DOI: 10.1081/CSS-120023235 0010-3624 (Print); 1532-2416 (Online)
Copyright q 2003 by Marcel Dekker, Inc. www.dekker.com
*Correspondence: Silvana Torri, Universidad de Buenos Aires, Facultad de
Agronomı´a, Avda. San Martı´n 4453, Buenos Aires 1417, Argentina; E-mail:
torri@mail.agro.uba.ar.
COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS
Vol. 34, Nos. 13 & 14, pp. 2035–2043, 2003

2. studied was independent of soil texture. The higher pH of the Natraquoll
seemed to have favored an intense microbial activity, and explained the
larger labile carbon pool of sewage sludge in this soil compared to the
other two soils studied.
INTRODUCTION
Land application of sewage sludge offers the possibility of recycling plant
nutrients such as nitrogen, phosphorous and trace elements, some of which are
essential for plant growth.[1– 3]
In addition, organic matter from sludge
generally improves microbial biomass and soil physical properties by
increasing water retention capacity and structural stability.[4,5]
Carbon
mineralization in sewage sludge amended soils depends on the degradation
rate of the carbon compounds present in the sludge, as well as on its nutrient
content.[6]
The mineralization of organic wastes that contain a high percentage
of soluble organic carbon leads to a flush of CO2 production immediately after
their addition to soil.[7]
The strong microbial activity can also promote the
degradation of indigenous soil organic matter, which is known as the priming
effect.[8]
Sewage sludge has been found to increase soil organic matter content
in agricultural and degraded soils.[9,10]
Buenos Aires City and its outskirts are the major source of sludge
production in Argentina. The nearby agricultural region, The Pampas Region, is
located between 328 to 398S and 56 to 678W, with Mollisols developed from
loess-like sediments predominating soils. This zone covers about 20 Mha of
agriculturally useful land, the remaining being either marginally suitable or
unsuitable for cropping, mainly as a result of slight differences in relief. The
lack of public acceptance for cropland application of sewage sludge makes
these uncropped lands suitable for sewage application. However, little
information is available about how this management modifies the organic
amounts of non-cultivated soils.
Carbon mineralization of sewage sludge amended soils may be used to
understand the degree of stabilization of the organic matter provided by the
sludge, in order to estimate the amount of carbon retained in the soil. The
fitting of kinetic equations to mineralization curves makes it possible to
estimate both the potentially mineralizable carbon and its mineralization rate.
Several soil carbon simulation models have been developed to describe soil
carbon cycling processes. Many of these models incorporate soil texture as a
factor that controls soil carbon stabilization.[11]
Clay is assumed to protect
organic matter against decomposition through adsorption or formation of
organic complexes onto clay surfaces[12]
and entrapment of organic particles
Torri, Alvarez, and Lavado2036

3. in aggregates.[13]
The objective of the present research was to study carbon
mineralization of added sewage sludge in three representative soils of the
Pampean Region in order to assess the amount of carbon retained.
MATERIALS AND METHODS
Three representative soils of the Pampean Region were used. The soils
were Mollisols (U.S. Soil Taxonomy): Typic Hapludoll, Typic Natraquoll and
Typic Argiudoll belonging to a non cultivated area and had the same origin
and mineralogical composition.[14,15]
Composite soil samples (10 subsamples,
0–15 cm depth) were air dried and passed through a 2 mm sieve. Selected soil
characteristics are shown in Table 1.
SewagesludgefromtheoutskirtsoftheurbanareaofBuenosAiresCitywas
providedbyAguasArgentinasS.A.Thesludgewasdriedat608Cbeforegrinding
and sieving (,2 mm), and was homogeneously mixed with 100 g of each soil at
proportions equivalent to the following field application rates: control (no
sludge) and sewage sludge (150 t DM ha21
). Applied sludge presented the
following analytical data (dry basis): total C: 251 mg g21
; total N: 19.3 mg g21
;
total P: 0.052 mg g21
; Ca: 22.5 mg g21
; Mg: 5.6 mg g21
; K: 10.7 mg g21
; CEC:
11.95 cmol(c) kg21
; pH: 5.8; Cd: 10.01 mg kg21
; Cu: 490.57 mg kg21
; Cr:
229 mg kg21
; Ni: 157 mg kg21
Pb: 407.64 mg kg21
and Zn: 2500 mg kg21
.
Table 1. Selected properties of the A horizon (0–15 cm).
Typic
Hapludoll
Typic
Natraquoll
Typic
Argiudoll
Clay (%) 19.2 27.6 32.7
Silt (%) 23.2 43.0 57.5
pH 5.12 6.21 5.44
Organic carbon (mg g21
) 28.6 35.31 23.9
Electrical conductivity
(dS m21
)
0.61 1.18 0.90
Cation exchange
capacity (cmol(c) kg21
)
22.3 22.3 15.3
Exchangeable cations
Ca2þ
(cmol(c) kg21
) 5.2 9.1 11.0
Mg2þ
(cmol(c) kg21
) 2.0 5.4 1.8
Naþ
(cmol(c) kg21
) 0.3 3.1 0.1
Kþ
(cmol(c) kg21
) 2.8 1.6 2.2
Mineralization of Carbon from Sewage Sludge 2037

4. The pot incubation was performed in a greenhouse at air temperature and
was arranged in completely randomized blocks with three replications. Mean
annual minimum and maximum temperatures were 11 and 248C respectively.
Soil moisture was maintained at 80% of field capacity through daily irrigation
with distilled water. Samples were obtained at day 1, 30, 60, 150, 270, and 360
after sludge application. At each sampling period, three entire pots of each
treatment were removed from the system. Carbon in soil samples was
determined by wet digestion.[16]
As the three soils presented different initial carbon content, Eq. 1 shown
below was used to estimate residual sewage sludge carbon in soil at each
sampling date.
%CRSSðtÞ ¼
CSðtÞ 2 CCðtÞ
CSSðt ¼ 0Þ
£ 100 ð1Þ
Where: %CRSS: percent residual carbon from sewage sludge in soil; CS:
carbon content in soil from sewage sludge treatment (mg C g21
soil); CC:
carbon content from control soil (mg C g21
soil); CSS: carbon initially added
as sewage sludge (mg C g21
soil); t: time after sludge application (days).
The results were analyzed by ANOVA and when significant differences
were detected the means were compared by Tukey test. Statistical significance
was set at p , 0:05: In addition, the percentage of residual sewage sludge
carbon was analyzed by fitting the experimental values to several kinetics
models commonly used to test decomposition data.[17– 19]
The kinetic model
according to Eq. 2 provided the best fit to carbon mineralization data for the
three soils. This model was originally proposed by Jones[17]
for N
mineralization, and was found to be suitable in describing the decomposition
process of many types of organic materials.[20,21]
A non-linear regression
analysis was used (Statgraphic 6.0, 1992), and the coefficients were tested by
the F value.
%CRSSðtÞ ¼ %CLS e2kt
þ %CRS ð2Þ
Where: %CRSS: percent residual carbon from sewage sludge in soil; %CLS:
initial percent of sewage sludge carbon in the labile pool; %CRS: percent of
sewage sludge carbon in the resistant pool; k ¼ mineralization
constant, fraction mineralized per time unit (day21
); t: time after sludge
application (days).
Torri, Alvarez, and Lavado2038

5. RESULTS AND DISCUSSION
A rapid decomposition of the added carbon was observed during the first
days after application (Fig. 1), indicating a high proportion of easily
degradable organic components in the sludge. No significant differences in
residual carbon from sewage sludge were observed between day 30 and 360
(Natraquoll), day 60 and 360 (Hapludoll) and day 60–150 and 360
(Argiudoll). However, 29–45% of the added carbon remained in the soils one
year after application (Fig. 1). Similar results were observed by other authors
in sewage sludge amended soils.[2,22]
The added carbon consisted of two fractions of different degrees of
biodegradability (Eq. 2, Table 2): a labile fraction (53–71%) that mineralized
quickly and followed a first order kinetic process and a resistant fraction (29–
45%), apparently not available to microorganisms.
Carbon mineralization from added substrates has been shown to be more
rapid in soils with low compared with high clay content.[23]
Franzluebbers[24]
reported similar results for the mineralization of soil organic matter. Residual
substrate and decomposition products may become stabilized by sorption onto
mineral particles and by incorporation into soil aggregates, being physically
Figure 1. Percent residual carbon from sewage sludge (%CRSS) in a Typic
Hapludoll, a Typic Natraquoll and a Typic Argiudoll as a function of time of
incubation. Same letters for each date of measurement show no significant differences
between treatments.
Mineralization of Carbon from Sewage Sludge 2039

6. inaccessible to microbial turnover.[25]
However, sewage sludge carbon
mineralization in the three studied soils did not depend on soil texture (Fig. 1,
Table 2). The Typic Hapludoll, which had a lower clay and silt content than
the other two soils, showed no significant differences in percent residual
sludge carbon content compared to the Typic Argiudoll between days 150 and
360. These soils, in turn, retained a significantly higher added carbon content
than the Natraquoll in the same period of time. We suggest that the recently
introduced organic carbon was located in larger pores and less entangled in
aggregates than native soil organic matter. Thomsen et al.[26]
studied the
decomposition of partially stabilized 14
C-labeled ryegrass residues at four
different soil water matric potentials using twelve differently textured soils of
similar mineralogical composition. Differences in the turnover of 14
C-ryegrass
residues were better explained by soil moisture parameters than by soil
texture. They found that the decomposition of 14
C-labeled ryegrass residues
and rye grass derived microbial metabolites only involved water in pores
.0.2 mm. As the water content of the three soils studied was periodically
adjusted according to water holding capacity, water availability was high and
did not limit microbial activity. Thus, no relationship between soil texture and
sewage sludge mineralization was observed during the first year of
application.
The Typic Natraquoll in this research presented a mineralization constant
value, which doubled that in the other two soils (Table 2). Although the pH of
the three soils increased by the addition of sludge, the Natraquoll exhibited a
higher pH value than the other two soils over the sampling period. At day 360,
the pH values of the sludge amended soils were pH ¼ 5:0 (Hapludoll), pH ¼
5:5 (Argiudoll) and pH ¼ 6:0 (Natraquoll). Liming has been shown to
increase the solubility and leaching of dissolved organic carbon in several
experiments with forest soils[27,28]
and to reduce total organic carbon.[29]
Motavalli et al.[30]
found evidence that increased acidity of soils ðpH , 6:5Þ
linearly reduced decomposition rates of freshly-added organic materials. It
appeared that the higher pH of the Natraquoll stimulated microbial activity,
Table 2. Estimated parameters according to Eq. [2] for sewage sludge carbon
mineralization.
Soil %CLS
k
(day21
) %CRS R2
p
Typic Hapludoll 58.4 0.035 41.5 0.99 ,0.001
Typic Natraquoll 71.5 0.071 28.5 0.99 ,0.001
Typic Argiudoll 53.0 0.030 45.4 0.92 ,0.010
Torri, Alvarez, and Lavado2040

7. increasing carbon mineralization of incorporated sewage sludge. It was
concluded that, in these soils, pH was the main factor regulating the
decomposition of the organic matter added as sewage sludge. Slightly acid
soils retained more sewage sludge carbon than soils with a higher pH.
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