Soil & Tillage Research 77 (2004) 137–145
Long-term tillage and crop rotation effects on microbial biomass
and C and N mineralization in a Brazilian Oxisol
Elcio L. Balota a,∗ , Arnaldo Colozzi Filho a , Diva S. Andrade a , Richard P. Dick b
a
Soil Science Department, Instituto Agronômico do Paraná (IAPAR), Caixa Postal 481, 86001-970 Londrina, PR, Brazil
b Crop & Soil Science Department, Oregon State University, Corvallis, OR 97331-7306, USA
Received 12 June 2002; received in revised form 27 November 2003; accepted 9 December 2003
Abstract
Crop rotation and tillage impact microbial C dynamics, which are important for sequestering C to offset global climate
change and to promote sustainable crop production. Little information is available for these processes in tropical/subtropical
agroecosystems, which cover vast areas of terrestrial ecosystems. Consequently, a study of crop rotation in combination with
no tillage (NT) and conventional tillage (CT) systems was conducted on an Oxisol (Typic Haplorthox) in an experiment
established in 1976 at Londrina, Brazil. Soil samples were taken at 0–50, 50–100 and 100–200 mm depths in August 1997
and 1998 and evaluated for microbial biomass carbon (MBC) and mineralizable C and N. There were few differences due to
crop rotation, however there were significant differences due to tillage. No tillage systems increased total C by 45%, microbial
biomass by 83% and MBC:total C ratio by 23% at 0–50 mm depth over CT. C and N mineralization increased 74% with NT
compared to CT systems for the 0–200 mm depth. Under NT, the metabolic quotient (CO2 evolved per unit of MBC) decreased
by 32% averaged across soil depths, which suggests CT produced a microbial pool that was more metabolically active than
under NT systems. These soil microbial properties were shown to be sensitive indicators of long-term tillage management
under tropical conditions.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Basal respiration; Microbial biomass; Carbon and nitrogen mineralization; Tillage systems; Crop rotations
1. Introduction
There is worldwide interest in sequestering C in
soils to offset global climate change. Agroecosystems
represent a large portion of terrestrial ecosystems, are
intensively managed, and provide an opportunity to
stabilize atmospheric CO2 in semi-permanent soil C
pools. Implementing agricultural practices that reduce
soil degradation has the potential to increase agricul∗ Corresponding author. Tel.: +55-43-33762393;
fax: +55-43-33762101.
E-mail address: [email protected] (E.L. Balota).
tural sustainability and soil conservation. A common
conservation practice is no tillage (NT), also called
zero tillage or direct drill. No tillage involves sowing directly through mulch with minimal soil disturbance. Intensive tillage involving plowing and disking
reduces concentration of soil organic matter (SOM).
Consequently, there has been an increase in NT systems in Brazil with >11 Mha planted in 1998/1999
(Derpsch, 2001).
Managing the frequency and type of tillage can
stop soil degradation and improve soil quality (Dick,
1984; Franzluebbers et al., 1999b). Formation and
stabilization of macroaggregates are important for
0167-1987/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.still.2003.12.003
138
E.L. Balota et al. / Soil & Tillage Research 77 (2004) 137–145
protecting and maintaining soil organic matter (Beare
et al., 1994; Gupta and Germida, 1988). Tillage
disrupts soil aggregates exposing organic matter to
microbial degradation. These changes in structure
can affect soil water, temperature, aeration, equilibrium of reactions, and increase soil erosion. Diverse
crop rotations can change soil habitat by affecting
nutrient status, depth of rooting, amount and quality
of residue, aggregation/microbial habitat, and can
stimulate soil microbial diversity and activity.
Soil microorganisms mediate mineralization of
SOM and nutrients. The microbial biomass is a small
but important reservoir of nutrients (C, N, P and S)
and many transformations of these nutrients occur in
the biomass (Dick, 1992). Soil disturbance can cause
significant modifications of soil habitat, which affects the microbial community. This has been shown
in numerous examples where SOM and microbial
biomass decline under agricultural or land disturbance
(Sparling, 1997).
Currently, there is a strong interest in sequestration
of C in soils as a means to help decrease atmospheric
CO2 and to gain side benefits of improving soil quality
and plant productivity (Burras et al., 2001; Sa et al.,
2001). Microbial biomass measurements can detect
tillage and crop rotation effects on soil earlier than total
organic C or N measurements in soil (Powlson and
Jenkinson, 1981; Carter, 1986; Powlson et al., 1987;
Saffigna et al., 1989; Balota et al., 1998) and therefore
they may be an indicator of potential C sequestration
(Sa et al., 2001). In this context, microbial biomass
can be a valuable tool for understanding changes in
soil properties and in the degree of soil degradation or
soil quality (Smith and Paul, 1990; Doran and Parkin,
1994; Brookes, 1995; Sparling, 1997).
The objective of this study was to evaluate the effect of long-term crop rotations and tillage systems on
microbial biomass C and C and N mineralization in a
tropical soil from Brazil.
2. Materials and methods
A study was established in 1976 at the Experimental Station of Agronomic Institute of Paraná (IAPAR),
district of Londrina, State of Paraná, in the southern
region of Brazil (23◦ 40 S, 50◦ 52 W and 576 m altitude) on an Oxisol (Typic Haplorthox) with 85% clay,
12% silt, 3% sand, pH 4.6 and 12 g organic C kg−1
soil. Long-term means (1976–2002) of temperature
at the site ranged from 23.6 to 25.5 ◦ C in January and
from 16.0 to 24.3 ◦ C in June, while the mean annual
precipitation was 1984 and 2005 mm in 1997 and
1998, respectively (IAPAR—Instituto Agronômico
do Paraná, unpublished results). The design was a
split plot (three replications) with tillage as the main
plot (65 m × 25 m) and crop rotation as the subplot
(8 m × 25 m), which were separated by a 2 m buffer.
Tillage treatments were NT planting into undisturbed
soil by opening a narrow trench and CT with one disc
plowing at 200 mm depth and two light harrowing
for seedbed preparation. Crop rotations were soybean (Glycine max L.)/wheat (Triticum aesativum L.)
(S/W), maize (Zea mays L.)/wheat (M/W), and cotton
(Gossypium hirsutum L. r. latifolium Hutch)/wheat
(C/W). Crop stubbles were retained on the surface
in the NT system or they were tilled conventionally
(plowed to 200 mm depth) following harvest, in both
fall and spring operations each year. Fertilizers were
added according to soil analysis before each cropping
period averaging, 95 kg of N, 55 kg of P and 42 kg
of K per hectare per year for the last 20 years. N
fertilizer was never applied to the soybean crop. Five
sub-samples were taken with a core (55 mm diameter)
randomly within each replicate (0–50, 50–100 and
100–200 mm depths) in August 1997 and in 1998 (at
the end of the winter crop). The fresh soil samples
were sieved through a 4 mm screen with large plant
material removed and stored at 4 ◦ C until analysis. All
determinations were made in triplicate and expressed
on a dry weight basis. Data were averaged across the
two sampling years and were analyzed using the SAS
statistical package (SAS, 1998).
2.1. Analytical procedures
Total organic C concentration in soil was measured
by the Walkley–Black potassium dichromate sulfuric acid oxidation procedure (Nelson and Sommers,
1982). Soil microbial biomass C (MBC) was determined by fumigation–incubation (Jenkinson and
Powlson, 1976) on a 20 g (fresh weight) sample using
NaOH to trap CO2 followed by Flow Injection Analysis for electric conductivity. Soil MBC was calculated
using CO2 -C evolved from the fumigated samples
only as suggested by Franzluebbers et al. (1999a), and
E.L. Balota et al. / Soil & Tillage Research 77 (2004) 137–145
a correction factor (kc ) of 0.45 (Jenkinson and Ladd,
1981). Basal respiration was obtained from the measurement of CO2 released from the non-fumigated
control. Metabolic quotient (qCO2 ) was obtained
by dividing the basal respiration by the microbial
biomass C (mg CO2 -C g−1 MBC per day). Mineralization of C was from incubation of 30 g moist soil
in a 350 ml sealed jar in the presence of a NaOH
trap and water in a separate vessel to maintain high
humidity for 3, 6, 10, 17 and 24 days at 30 ◦ C in a
dark chamber. At each sampling period, NaOH was
collected and replaced. The concentration of CO2
released in each period was determined by Flow Injection Analysis. Controls were run on six flasks that
had no soil for the determination of CO2 . Nitrogen
mineralization was from 24-day incubation at 30 ◦ C
of 5 g of moist soil that was extracted with 20 ml of
K2 SO4 solution (0.25 M) by shaking for 50 min at
220 rpm, following by centrifuging and filtering. Concentration of nitrate was determined by the procedure
described by Miyazawa et al. (1985), which consisted
of measuring NO3 − in soil extracts without chemical
reduction by ultraviolet spectrophotometry at 210 nm
(NO3 − + interfering ions) and at a longer wavelengths
(239 nm) where the ultraviolet light absorption due
to NO3 − is negligible and that due to interfering ions
is similar to that measured at 210 nm (with chemical
139
reduction). The difference between absorbance values
obtained at 239 nm (NO3 plus dissolved organics)
and 210 nm (dissolved organics) gave nitrate concentration. The N mineralized was calculated from
the difference between nitrate concentration after and
before the incubation.
3. Results and discussion
No tillage practices increased total C concentrations
over CT by 45, 34 and 14%, in the 0–50, 50–100 and
100–200 mm depths, respectively. There was a trend
for a decrease in total C with soil depth in NT, while
in CT the total C remained almost constant with depth
(Table 1). There was no effect of crop rotation on total
C concentrations in the soil under either tillage system.
3.1. Microbial biomass carbon
Microbial biomass C varied from 163 to 209 ␮g g−1
in the soil under CT and from 204 to 367 ␮g g−1 under NT (Table 1). No tillage resulted in a significant
increase in MBC in all crop rotations (from 11 to
98%) averaged across all depths compared to CT. The
greatest differences between NT and CT occurred in
the surface layer (0–50 mm) where the MBC in NT
Table 1
Total C, microbial biomass C, metabolic quotient (qCO2 ), and MBC:total C ratio (mg g−1 ) in soils under different tillage and crop rotation
systems
Total C (mg g−1 )
MBC (␮g C g−1 )
qCO2 (mg CO2 -C g−1 MBC per day)
MBC:total C (mg g−1 )
CT
NT
CT
NT
CT
NT
CT
NT
15.3 b
14.7 b
13.9 a
20.6 a
22.4 a
20.6 a
177 b
185 b
206 b
347 a
367 a
326 a
5.04 a
3.60 a
3.84 a
2.16 b
2.88 a
2.88 a
11.56
12.58
14.82
16.84
16.38
15.82
Depth: 50–100 mm
S/W
13.4
M/W
15.3
C/W
13.2
17.3
19.0
19.7
194 bA
209 bA
140 bB
280 aB
322 aA
232 aC
3.84
4.80
3.60
3.12
3.12
3.60
14.48
13.66
10.60
16.18
16.95
11.78
Depth: 100–200 mm
S/W
14.4 b
M/W
15.6 b
C/W
13.8 b
16.3 a
17.2 a
16.2 a
192 bA
182 bAB
163 bB
214 aB
272 aA
204 aB
5.04 a
5.52 a
3.60 a
2.88 b
3.12 b
2.64 a
13.33
11.67
11.81
13.13
15.81
12.59
Crop rotationsa
Depth: 0–50 mm
S/W
M/W
C/W
Means within a row followed by a different lower case letter are significantly different at P ≤ 0.05. Means within a column of the same
depth followed by a different upper case letter are significantly different at P ≤ 0.05.
a S: soybean; W: wheat; M: maize; C: cotton. All values represent the mean of data obtained from soil samples collected in 1997 and
1998.
140
E.L. Balota et al. / Soil & Tillage Research 77 (2004) 137–145
was on average 83% higher than CT plots. Soil samples from NT plots also were 55 and 28% greater in
MBC than CT at 50–100 and at 100–200 mm depth,
respectively. The M/W crop rotation increased MBC
over other crop rotations at 50–100 mm depth under
CT and 100–200 mm depth under NT. The values of
MBC were similar to those observed under a range
of ecosystems from temperate and tropical regions
(Prasad et al., 1994; Alvarez et al., 1995; Miller and
Dick, 1995; Franzluebbers et al., 1995; Balota et al.,
1998; Mendes et al., 1999). Alvarez et al. (1995),
studying soils of the Argentinean pampas cultivated
under different tillage regimes for 12 years, observed
that NT resulted in 50% greater organic C and 181%
greater MBC than CT.
The increase of MBC under NT over CT could be
attributed to several factors. An important factor for
tropical soils may be the effect of the surface litter
in an NT system lowering soil temperature, and increasing water content, soil aggregation and C content
over CT systems. The microbial biomass in the surface
soil under NT represents either a sink or source for
plant-available nutrients, depending on cropping and
climatic changes in the soil environment. The accumulation of crop residues at the surface provides substrates for soil microorganisms, which accounts for the
higher MBC at the surface under NT. Increased MBC
under NT is consistent with the results of Ferreira et al.
(2000), who observed greater bradyrhizobia diversity
at the same experimental site.
3.2. Metabolic quotient
The values of metabolic quotient, soil respiration
per unit of microbial C (qCO2 ) varied from 3.6 to 5.5
under CT and from 2.16 to 3.60 mg CO2 -C g−1 MBC
per day under NT (Table 1). Averaged across crop
rotations and depths, qCO2 in soil under NT was
32% lower than under CT. At 0–50 mm depth qCO2
values under NT were from 20 to 57% lower than under CT with all crop rotations. At 50–100 mm depth,
qCO2 for NT was 19–35% lower than CT, while at
100–200 mm values under NT were from 27 to 43%
lower. Crop rotation effects on qCO2 were not found
at any soil depth. The inverse relationship between
MBC and qCO2 was similar to the results of Prasad et
al. (1994), who observed an increase in MBC of 106%
and decrease of 39% in the qCO2 in different ecosys-
tems of India. On the other hand, Alvarez et al. (1995)
observed that under NT there was an increase of 60%
in qCO2 in the first 50 mm depth while at 50–150 mm
depth, it was reversed with NT having qCO2 value
21% less than CT. Generally, it is viewed that
qCO2 decreases in more stable systems (Insam and
Domsch, 1988), while the incorporation of residues
to soil increases qCO2 (Ocio and Brookes, 1990).
According to Saffigna et al. (1989), soils under CT
provide less organic matter and MBC, but a larger
metabolic quotient than NT. These differences may
be due to differences of accessibility to C substrates
by microorganisms, changes in the metabolic rates
and changes in microbial community composition
(Alvarez et al., 1995). No tillage also may have a
greater amount of larger aggregates, which protects
microorganisms from adverse conditions. This has
important implications for agriculture, because if less
C is evolved from the soil, more C can be stored in
soil organic matter. An increase of MBC combined
with a decrease of qCO2 suggests that the soil ecosystem under NT was becoming more stable (Sparling,
1997).
3.3. Ratio microbial biomass C:total C
The values of microbial biomass C:total C varied
from 11.6 to 14.8 mg g−1 under CT and from 11.8 to
16.8 mg g−1 under NT (Table 1). No significant change
in ratio of MBC to total C was observed due to tillage.
Crop rotation did not affect proportion of MBC as total
C under either CT or NT at any soil depth. The trend
of increasing percentage of total C as MBC due to NT
followed the results of Carter (1986) where increasing
tillage intensity decreased both MBC and percentage
of total C as MBC. Variations in percentage of total
C as MBC have been related to monoculturing and
crop rotation. The percentages of total C as MBC were
lower than other reports with values of 1.8 and 2.3%
(Insam et al., 1989) and of 2.3 and 2.9% (Anderson and
Domsch, 1989) for monoculturing and crop rotations,
respectively. However, other studies on different soils
and management systems showed a wide range for the
percentage of total C as MBC (e.g. 0.6–3.7% by Insam,
1990; 0.3–5.4% by Anderson and Domsch, 1989). The
wide ranges for this relationship suggest it is sensitive
to soil management, time of sampling, and analytical
methods.
E.L. Balota et al. / Soil & Tillage Research 77 (2004) 137–145
141
Table 2
Basal respiration, mineralization of C and N during 24 days and C:N mineralization ratio in soils under different tillage and crop rotation
systemsa
Crop rotationsc
Depth: 0–50 mm
S/W
M/W
C/W
BR (␮g CO2 -C g−1
per day)
C mineralization
(␮g g−1 per day)
N mineralizationb
(␮g g−1 per day)
C:N mineralization
ratio (g g−1 )
CT
NT
CT
NT
CT
NT
CT
NT
7.6
10.9
9.3
3.30 bA
3.00 aAB
2.00 aB
6.50 aA
2.90 aB
2.00 aC
0.089 bA
0.082 aA
0.052 aB
0.171 aA
0.079 aB
0.081 aB
37 a
37 a
38 a
38 a
37 a
25 b
9.1 a
10.2 a
8.7 a
1.50 bB
3.30 aA
3.20 aA
4.00 aA
4.10 aA
3.60 aB
0.058 b
0.065 a
0.051 a
0.087 a
0.083 a
0.070 a
26 bB
51 aAB
64 aA
46 aB
49 aAB
51 aA
2.30 bB
3.40 aA
3.30 aA
7.10 aA
4.30 aB
3.70 aC
0.052 bA
0.054 bA
0.061 bA
0.216 aA
0.110 aB
0.113 aB
44 a
63 a
54 a
33 a
39 a
33 b
9.0
6.9
7.8
Depth: 50–100 mm
S/W
7.3 aB
M/W
10.3 aA
C/W
5.1 bB
Depth: 100–200 mm
S/W
9.5 aA
M/W
10.2 aA
C/W
5.8 aB
6.4 bB
8.6 aA
5.5 aB
Means within a row followed by a different lower case letter are significantly different at P ≤ 0.05. Means within a column of the same
depth followed by a different upper case letter are significantly different at P ≤ 0.05.
a Values are means of three replications.
b Only NO . All values represent the mean of data obtained from soil samples collected in 1997 and 1998.
3
c S: soybean; W: wheat; M: maize; C: cotton.
Changes in the percentage of total C as MBC may
be related to organic matter formation and efficiency of
conversion of recalcitrant C pools into MBC (Sparling,
1992). The percentage of total C as MBC has been suggested as an indicator of whether soil organic matter is
decreasing, increasing or in a steady state (Anderson
and Domsch, 1989; Insam, 1990). Jenkinson and Ladd
(1981) suggested 2.2% as a threshold value for when
soil is in equilibrium. All of our results were ≤1.7%,
which was lower than this threshold. Nonetheless,
there was 45% greater organic C at 0–50 mm under
NT than CT. Generally, if a soil is intensively disturbed, MBC will decline faster than the organic matter, and the percentage of total C as MBC will decrease
(Powlson and Jenkinson, 1981; Sparling, 1992). Our
results suggest that ratio of MBC:total C under tropical/subtropical conditions may have a different threshold as an indicator of C accumulation than in temperate regions.
3.4. Basal respiration and carbon mineralization
The values of basal respiration (BR) varied from 5
to 10 ␮g CO2 -C g−1 per day under CT and from 5 to
10 ␮g CO2 -C g−1 per day under NT (Table 2). There
was significantly greater BR in the C/W rotation under
NT compared with CT at 50–100 mm depth, but the
opposite occurred in the S/W rotation at 100–200 mm
depth. Crop rotation had a significant effect on BR
at 50–200 mm layer. Under CT, the M/W rotation
at 50–100 mm depth and S/W and M/W rotation at
100–200 mm had greater BR than other crop rotations,
while under NT, BR was 40% greater in the M/W rotation at 100–200 mm depth than other rotations. On
average, there was no difference in BR between NT
and CT. Franzluebbers and Arshad (1996) observed a
tillage effect that varied from 4 to 28 ␮g CO2 g−1 under CT and from 4 to 31 ␮g CO2 g−1 under NT. It is
suggested that BR be regarded as an index of potential
CO2 loss or a microbial index, but should not be regarded as an indicator of C accumulation or loss from
soils under field conditions. Basal respiration can indicate soil quality since soil quality is preserved with
high BR and reduced with low BR.
The values of C mineralization released as CO2 during 24-day incubation varied from 1.5 to 3.4 ␮g C g−1
per day under CT and from 2.0 to 7.1 ␮g C g−1 per
day under NT (Table 2). No tillage resulted in a significant increase in C mineralization under the S/W
rotation at all depths compared with CT. On average
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E.L. Balota et al. / Soil & Tillage Research 77 (2004) 137–145
the C mineralization under NT was 50% greater than
CT. The C mineralized was affected by both tillage
and crop rotation which is consistent with research
of Franzluebbers et al. (1995) who obtained values
from 8 to 11 ␮g C g−1 per day under CT and from
8 to 10 ␮g C g−1 per day under NT in 24-day incubations. In general, C mineralization decreases with
depth, mainly due to the total C be lower in deeper
layer, however we observed a slight increase of C mineralization at deeper depth even though total C was
somewhat lower in this layer under NT.
Basal respiration and C mineralization values vary
widely in the literature, depending on a variety of
factors including soils, climates, and experimental conditions. For example, BR varied from 8 to
84 ␮g CO2 g−1 per day (Gupta and Germida, 1988;
Insam et al., 1991; Prasad et al., 1994), while C mineralization ranged from 21 ␮g C g−1 per day under
cultivated soils to 34 ␮g C g−1 per day under native
soils during 14-day incubations (Gupta and Germida,
1988).
Although NT had greater C mineralization than did
CT, the experimental manipulations would not have
reflected field conditions, particularly for NT, which
would have minimal disturbance under field conditions. The disturbance of sampling and processing
would likely have exposed protected soil organic matter in NT soils; artificially stimulating C mineralization. Incorporated crop residues decompose 1.5 times
faster with less immobilization and greater mineralization of nutrients than do surface placed residues
(Holland and Colleman, 1987; Kushwaha et al., 2000).
Reicosky (1997) observed that under field conditions,
tillage released 13.8 times greater release of CO2 from
soil than did NT for the first 19 days after moldboard
incorporation of residues. Bayer et al. (2000) adjusted
a first-order exponential model to predict total organic
C based on C losses and C inputs under subtropical
field conditions of Brazil. They observed that the coefficient for C loss under NT was one-half as large as
the coefficient under CT, providing support for higher
concentrations of total C under NT than CT.
C mineralization rates were highest when soybean
was in the rotation despite having significantly less
biomass input to soil than with maize. Alvarez et al.
(1995) found that maize produced 9.7 Mg ha−1 per
year and soybean produced 5.7 Mg ha−1 per year of
dry residue. At our site soybean had 42% less crop
residue (straw and root) than maize (data not shown).
Higher C mineralization when soybean was in the rotation may have been due to its lower C:N ratio and
ready decomposability. Furthermore, decomposition
rates are regulated by other residue properties such as
lignin, polyphenol, and silica content, and available
evidence suggests that soybean has suitable chemical characteristics for rapid C mineralization (Aulakh
et al., 1991; Tian et al., 1992).
The values of C mineralized to total C during 24
days varied from 2.7 to 5.8 mg g−1 under CT and from
2.3 to 10.4 mg g−1 under NT (Table 3). On average
under CT, the ratio of total C mineralized to MBC
was 4.7 g g−1 while under NT it was 5.6 g g−1 MBC
during 24-day incubations. The percentage of MBC
mineralized during 24 days varied from 18 to 55%
under CT and from 15 to 80% under NT. On average,
across crop rotations and depths, the percentage of
MBC mineralized in 24 days, was approximately the
same (38%) under both tillage systems.
3.5. Nitrogen mineralization
The values of N mineralization during 24 days of
incubation varied from 0.05 to 0.09 ␮g N g−1 per day
under CT and from 0.07 to 0.22 ␮g N g−1 per day under NT (Table 2). The S/W rotation under NT had
significantly greater (54–364%) N mineralization at
all depths than CT. On average, NT had 78% greater
N mineralization than CT, while S/W had 35 and
63% greater N mineralization than M/W and C/W rotations, respectively. Greater N mineralization under
NT has been observed previously, with values ranging from 8 to 13 ␮g N g−1 per day under CT and
from 10 to 22 ␮g N g−1 per day under NT in different
aggregate-size fractions during a 20-day incubation
(Beare et al., 1994) and from 0.2 to 1.0 ␮g N g−1 per
day under CT and from 0.4 to 1.2 ␮g N g−1 per day
under NT in 24-day incubation (Franzluebbers et al.,
1995). Gupta and Germida (1988) observed an average value of 3.1 ␮g N g−1 per day under native soils
and 1.6 ␮g N g−1 per day under cultivated soils during 14 days of incubation. Mendes et al. (1999) found
values from 0.3 to 1.9 ␮g N g−1 per day in 16-day
incubations among different aggregate-size fractions
in soil collected in different seasons. The majority of
these studies analyzed the amount of mineralizable N
as NH4 -N plus NO3 -N, whereas Gupta and Germida
E.L. Balota et al. / Soil & Tillage Research 77 (2004) 137–145
143
Table 3
Ratio C mineralization (MinC) during 24 days to total C or microbial biomass C in soils under different tillage and crop rotation systems
Crop rotationsa
MinC:total C (mg g−1 per 24 days)
MinC:MBC (g g−1 MBC per 24 days)
CT
CT
NT
NT
Depth: 0–50 mm
S/W
M/W
C/W
5.18 bA
4.90 aA
3.45 aA
7.57 aA
3.10 bB
2.33 aC
0.447 aA
0.389 aA
0.233 aA
0.450 aA
0.190 bB
0.147 bB
Depth 50–100 mm
S/W
M/W
C/W
2.69 bA
5.17 aA
5.82 aA
5.55 aA
5.18 aA
4.39 aB
0.185 aB
0.379 aAB
0.549 aA
0.343 aA
0.305 aA
0.372 bA
Depth 100–200 mm
S/W
M/W
C/W
3.83 bB
5.23 bA
5.74 aA
10.45 aA
6.00 aB
5.48 aB
0.288 bB
0.448 aB
0.486 aA
0.796 aA
0.379 aB
0.435 aB
Means within a column of the same depth followed by a different upper case letter are significantly different at P ≤ 0.05. Means within
a row followed by a different lower case letter are significantly different at P ≤ 0.05.
a S: soybean; W: wheat; M: maize; C: cotton. All values are mean of data obtained from soil samples collected in 1997 and 1998.
(1988), Mendes et al. (1999) and our study measured
only NO3 -N.
The C:N mineralization ratio showed few effects
due to tillage or crop rotations. Nonetheless, there was
a wider C:N mineralization ratio under C/W and M/W
rotations, which may have been due to microbial populations using organic matter that was less labile with
wide C:N ratios. A lower soil mineralizable C:N ratio can suggest that the flow of C and N through the
mineralizable fraction has become less stable, which
may decrease conservation of C and N in the long
term (Franzluebbers and Arshad, 1996). All the C:N
mineralization ratios were greater than 25, which according to Franzluebbers and Arshad (1996) indicate
that the crop residues and resulting soil organic matter
fractions were low in concentrations of N.
There was also an interactive effect of tillage, crop
rotation, and soil depth on the C:N ratio mineralized
(Table 2). Crop rotation using C/W under CT resulted
in a higher C:N mineralization ratio than under NT
at 0–50 and 100–200 mm depth, while a greater ratio
was found under NT with the S/W rotation in the
50–100 mm depth (Table 2). There was a crop rotation
effect only at 50–100 mm depth where C/W rotation
under CT and M/W rotation under NT showed a higher
ratio than other crop rotation.
Our results of greater N mineralization under NT
are in agreement with previous research (Beare et al.,
1994; Franzluebbers et al., 1995; Franzluebbers and
Arshad, 1996). However, nitrification can be inhibited
in NT soils under field conditions, because of accumulation of organic matter and nutrients, such as N, at or
near the soil surface that may restrict N-mineralization
(Kushwaha et al., 2000).
Although the samples were collected at wheat harvest time (winter crop), nearly 3 months after tillage,
there was a crop rotation effect on N mineralization.
Again, as with C mineralization, the high N mineralization rates with soybean may have been due to its
narrow C:N ratio and other favorable chemical characteristics for rapid release of N.
4. Perspectives
One explanation for the effect of NT on microbial
biomass and nutrient mineralization is that NT systems provide a more favorable habitat for microorganisms. Previous studies at the same experimental site
showed that NT systems compared with CT increased
size of macroaggregates (Castro Filho et al., 2002).
Macroaggregates probably provided an improved microhabitat for microorganisms, since macroaggregates
have been reported with greater enzyme activities, respiration and MBC than in microaggregates (Gupta
and Germida, 1988; Dick, 1992; Miller and Dick,
144
E.L. Balota et al. / Soil & Tillage Research 77 (2004) 137–145
1995; Franzluebbers and Arshad, 1997; Mendes et al.,
1999).
Crop rotation within a tillage regime had small effects on total C. However, inclusion of maize increased
MBC (50–100 mm depth) and increased C mineralization for both the 50–100 and 100–200 mm depths
over other rotations. It was surprising that inclusion
of maize in the crop rotation had little effect on MBC
in the surface layer since it had substantially higher
biomass inputs. It is likely that in a tropical setting
with year-round warm soils there is rapid decomposition and high losses regardless of crop inputs. However, the significant effect of maize in the lower depths
suggests there is either leaching or root-derived input of organic compounds to support high microbial
biomass. Conversely, inclusion of soybean stimulated
C and N mineralization in the surface depth, which
likely reflects its higher quality and narrow C:N ratio
to increase microbial activity.
The results indicate that for this subtropical site in
southern Brazil, tillage is the critical factor in sequestering C and microbial activities. Disturbance had a
greater impact than the type of C inputs from crop
rotations for maintaining or improving soil microbial
biomass and activity.
Acknowledgements
To CNPq—Conselho Nacional de Desenvolvimento
Cientı́fico e Tecnológico (Brazilian National Research
Council), of E.L. Balota for Post-Doctorate fellowship
to United States, process 200440/00-4.
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