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Applied Soil Ecology 48 (2011) 168–173
Contents lists available at ScienceDirect
Applied Soil Ecology
journal homepage: www.elsevier.com/locate/apsoil
Tillage-induced changes in fungal and bacterial biomass associated with soil
aggregates: A long-term field study in a subtropical rice soil in China
X. Jiang a,b,∗ , Alan L. Wright b , X. Wang a , F. Liang a
a
Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), College of Resources and Environment, Southwest University, 2 Tiansheng Road,
Beibei, Chongqing 400715, China
b
Everglades Research & Education Center, University of Florida, Belle Glade, FL 33430, United States
a r t i c l e
i n f o
Article history:
Received 5 January 2011
Received in revised form 22 March 2011
Accepted 23 March 2011
Keywords:
Soil aggregates
No-tillage
Soil organic carbon
a b s t r a c t
The composition and function of the soil microbial community can be strongly influenced by soil structure
and tillage. Soil total microbial biomass C (MBC), microbial biomass N (MBN), and fungal and bacterial
biomass associated with different soil aggregate-size fractions were measured for a long-term no-till
subtropical rice soil ecosystem to determine how tillage shifts microbial community structure and to
detect the spatial scale at which microorganisms are the most sensitive to disturbance. Surface soil
(0–20 cm) was fractionated into aggregate-sizes (>4.76 mm, 4.76–2.0 mm, 2.0–1.0 mm, 1.0–0.25 mm,
0.25–0.053 mm, <0.053 mm) under three tillage regimes. Soil MBC, MBN, fungal biomass and bacterial
biomass were significantly higher under no tillage (NT) than conventional tillage (CT) and CT flooded
paddy field (FPF) in whole soil (p < 0.05). Microbial biomass C, N and fungal biomass were significantly
higher under NT than CT and FPF for all aggregate sizes. No significant tillage effects on microbial biomass
C:N ratio were observed, but analysis of soil aggregates revealed significant differences due to tillage.
Microbial biomass C:N ratio averaged 8.8, 8.5, and 8.4 for CT, NT, and FPF, respectively, while the ratio
was significantly higher under CT than NT for macroaggregates >1.0 mm, indicating a tillage-induced N
limitation in macroaggregates. The fungal:bacterial biomass ratio ranged from 0.5 to 4.5, and was highest in 4.76–2 mm aggregates for all tillage regimes. Fungal:bacterial ratios were significantly higher for
aggregate sizes >1.0 mm (2.4) than for <1.0 mm aggregates (0.7). Tillage effects on fungal over bacterial
dominance were not significant for whole soil and most aggregate sizes. The NT regime increased microbial biomass, but this increase was proportional for both bacteria and fungi. The soil microbial biomass
and community structure was likely controlled by particle size at the aggregate-scale (0.05–5.0 mm),
while tillage played a role in regulating the microbial community structure.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Agricultural land management is one of the most significant
anthropogenic activities that changes soil characteristics, including
physical, chemical, and biological properties and processes. Conservation tillage practices, such as NT, can increase soil aggregation
and C and N storage, and improve microbial biomass and activity
(Hendrix et al., 1986; Beare et al., 1997; Minoshima et al., 2007;
Zibilske and Bradford, 2007; Wright et al., 2008). Tillage breaks
down aggregates and alters aggregate-size distribution, typically
by decreasing the proportion of macroaggregates in soil (Wright
et al., 2008). However, improved knowledge of how tillage man-
∗ Corresponding author at: Key Laboratory of Eco-environments in Three Gorges
Reservoir Region (Ministry of Education), College of Resources and Environment,
Southwest University, 2 Tiansheng Road, Beibei, Chongqing 400715, China.
Tel.: +86 23 68251025; fax: +86 23 68250444.
E-mail address: [email protected] (X. Jiang).
0929-1393/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsoil.2011.03.009
agement regulates the interaction between soil aggregates and
microbial community structure and function may be helpful to better understand mechanisms for increasing soil C sequestration and
improving fertility in agricultural ecosystems.
The soil microbial community is comprised primarily of fungi
and bacteria, which contribute more than 98% to the total soil
microbial biomass (Bardgett and Griffiths, 1997; Gattinger et al.,
2002) and serve unique functions and show variable sensitivity
to environmental change (Hedlund et al., 2004; van der Putten
et al., 2004; Coleman, 2008; Holtkamp et al., 2008). The key differences between bacteria and fungi have been used to identify
fungal over bacterial dominance as an indicator of ecosystem processes (Hendrix et al., 1986; Bardgett and McAlister, 1999; van der
Heijden et al., 2008). This concept has led to the hypothesis that
changes in fungal over bacterial dominance are likely related to
important ecosystem processes such as organic matter decomposition, nutrient cycling, C-sequestration potential, and ecosystem
self-regulation (Hendrix et al., 1986; Bardgett and McAlister, 1999;
Bailey et al., 2002; Six et al., 2006; van der Heijden et al., 2008).
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X. Jiang et al. / Applied Soil Ecology 48 (2011) 168–173
No-tillage management alters soil physical and chemical properties relative to the historic conventional tillage, creating a
significantly altered habitat for bacteria and fungi. Fungal dominance is commonly assumed under NT (Hendrix et al., 1986; Beare
et al., 1997; Frey et al., 2003; Butenschoen et al., 2007), however,
no consistent evidence of tillage effects were found concerning the
fungal over bacterial dominance of microbial communities under
CT compared to NT (Hendrix et al., 1986; Holland and Coleman,
1987; Drijber et al., 2000; Bailey et al., 2002; Helgason et al., 2009).
Helgason et al. (2009) found that both fungal and bacterial biomass
increased under NT, but the abundance of fungi vs. bacteria was
not consistently greater under NT. Similar studies also reported
that NT did lead to an overall increase in microbial biomass compared to CT, but this increase was proportional for both bacteria and
fungi (Pankhurst et al., 2002; Spedding et al., 2004; van Groenigen
et al., 2010). While Bailey et al. (2002) found that fungal dominance
determined via selective inhibition increased under NT practices,
the opposite was true when fungal dominance was determined
using PLFA. Conflicting results were reported for other land uses,
such as pasture and forest (de Vries et al., 2006; Demoling et al.,
2008).
The primary effect of tillage is to physically disturb the soil
structure and specific effects on the soil microbial community will
depend largely on the disturbance that occurs at the spatial scale
to which the microorganisms are most sensitive (Young and Ritz,
2000). Further research is needed, however, to determine how
tillage-induced changes in the soil environment shape microbial
biomass and activity in agroecosystems (Bronick and Lal, 2005).
We hypothesis that soil microbial biomass and community were
controlled by soil structure at the aggregate-scale (0.05–5.0 mm)
since tillage adversely affects microbial dynamics through deterioration of the soil’s structure. The aim of the present study was to
evaluate the long-term tillage effects on soil microbial biomass and
community shifts associated with soil aggregates in a sub-tropical
purple rice soil.
169
of field-moist soil was immersed in water on a set of five nested
sieves (4.76, 2, 1, 0.25, and 0.053 mm) and shaken vertically 3 cm
for 50 times during a 2 min period. The aggregates retained on each
sieve were collected and the silt + clay fraction (<0.053 mm) was
collected after centrifugation.
2.2. Microbial and chemical analysis
Soil microbial biomass (C and N) were determined by the chloroform fumigation extraction method (Anderson and Ingram, 1993;
Vance et al., 1987), and K values of 2.64 and 1.46 were used for
the calculation of MBC and MBN, respectively (Vance et al., 1987;
Brookes et al., 1985).
Fungal ergosterol content was determined according to the procedure of Rossner (1996), who modified the method of Zelles et al.
(1987). About 2 g of fractionated moist soil was extracted and
saponified with methanol (20 ml), ethanol (5 ml) and KOH (2 g) by
heating at 70 ◦ C for 30 min. After centrifugation, the supernatant
was extracted with hexane (30 ml), separated, and the upper hexane phase evaporated at 40 ◦ C until dryness. The dried ergosterol
was immediately dissolved in 5 ml methanol and quantitatively
transferred to a 5 ml volumetric flask. The ergosterol–methanol
solution was measured by HPLC with a C18 column using a pure
methanol (HPLC grade) mobile phase, a flow rate of 1.5 ml min−1
and UV detector at 282 nm. Ergosterol content was determined
by comparing sample peak areas with external standards. Fungal
biomass C content was calculated from the ergosterol content using
an average conversion factor of 250 (Montgomery et al., 2000). Soil
bacterial biomass was calculated by the difference between total
biomass C and fungal biomass C (Frey et al., 1999; Bittman et al.,
2005).
Data were subjected to ANOVA and mean values separated using
Duncan’s New Multiple Range Test at p < 0.05. Data were normally
distributed with respect to variation. All statistical analyses were
performed by SPSS statistical package.
3. Results
2. Materials and methods
2.1. Tillage regimes and soil sampling
The field experiment has continued since 1990 at Chongqing,
southwest of China, where the soil is a hydargric Anthrosol
(FAO/UNESCO, 1988) with basic properties of pH (H2 O) = 7.1,
organic C = 21.7 g kg−1 , total N = 1.7 g kg−1 , total P = 0.8 g kg−1 , and
total K = 22.7 g kg−1 . The mean annual temperature is 18.2 ◦ C and
precipitation is 1080 mm. The field was planted with rape (Brassica napus L.) in winter and rice (Oryza sativa L.) in summer, with
residues returned to the soil.
Three tillage regimes were imposed: ridge with no-tillage (NT
treatment was imposed on soil that was ridge tilled in 1990, with
the ridges kept intact since then), conventional tillage (CT), and a
rice/fallow rotation for the flooded CT paddy fields (FPF). Briefly,
soils were tilled, irrigated, and puddled twice during the time
between rape harvest and rice transplanting for CT and FPF. After
rice was harvested, soils were tilled again before rape planting for
CT while no tillage or cropping was done for FPF from September to
the next May. For NT, rape and rice were planted on the top of the
ridge and no-till was employed. All plant residues were returned
to the soil. The experiment was laid out in a randomized complete
block design (RCBD) with four replications, and each plot measured
4 m× 5 m.
Five surface soil (0–20 cm) samples were collected along the
plant row from each plot in October, 2009. Fractionation of soil
aggregates was achieved using a wet-sieving procedure (Wright
and Upadhyaya, 1998; Chiu et al., 2006). Approximately 1000 g
3.1. Soil total microbial biomass C and N associated with
aggregates
The MBC and MBN within soil aggregate sizes under different
tillage management are shown in Table 1. In whole soil, MBC was
46% and 80% higher under NT than CT and FPF, respectively. The
MBC in aggregates ranged from 190 to 1025 mg C kg−1 soil, which
was generally highest for macroaggregates >1.0 mm (762 mg C kg−1
soil) and lowest in the three fractions <1 mm (377 mg C kg−1 soil).
This trend occurred for all tillage regimes.
The MBN showed similar trends as MBC in whole soil, and was
50% and 76% higher under NT than CT and FPF, respectively. The
MBN ranged from 36.1 to 135 mg N kg−1 soil, which was highest
in 0.25–0.053 mm size fractions (76, 135 and 82 mg N kg−1 soil for
CT, NT, and FPF, respectively), regardless of tillage regime (Table 1).
No significant difference in MBN was observed among aggregate
sizes >0.25 mm under the same tillage regime. Trends in MBN were
similar to MBC, with higher MBN occurring for NT than CT and FPF.
3.2. Soil fungal biomass associated with aggregates
Fig. 1 shows soil fungal biomass associated with different sizes
of aggregates under different tillage management. For whole soil,
fungal biomass was 43% and 84% higher under NT than CT and FPF,
respectively. Fungal biomass ranged from 81 to 736 mg C kg−1 soil,
and was significantly higher for macroaggregates >1.0 mm (445,
624 and 424 mg C kg−1 soil for CT, NT, and FPF, respectively) than
the three micro-fractions <1.0 mm (109, 230 and 108 mg C kg−1
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X. Jiang et al. / Applied Soil Ecology 48 (2011) 168–173
Table 1
Total microbial biomass C and N within aggregate under different tillage regimes.
Tillage
Aggregate size (mm)
CT
NT
FPF
CT
NT
FPF
MBC
MBN
Whole soil
>4.76
4.76–2.00
2.00–1.00
1.00–0.25
0.25–0.053
<0.053
553cd
808b
450de
62.9cd
95.0b
53.5cd
680c
743bc
457de
56.5cd
101b
54.7cd
750bc
918ab
695bc
56.6cd
105b
58.4cd
805b
1025a
787b
58.5cd
102b
56.1cd
253e
511d
200ef
45.0de
97.9 b
52.9d
447de
747bc
404de
76.1c
135a
81.9bc
251ef
392de
190f
38.1de
54.4cd
36.1e
Mean values for either MBC or MBN not followed by the same letter are different, p < 5%. CT, conventional tillage; NT, combines ridge with no tillage; FPF, flooded paddy field.
MBC, microbial biomass C; MBN, microbial biomass N.
3.3. Soil bacterial biomass associated with aggregates under
tillage management
Bacterial biomass C ranged from 106 to 464 mg C kg−1 soil, and
exhibited a fluctuating pattern with aggregate size, but was highest
for 1–2 mm and 0.053–0.25 mm aggregates (Fig. 2). For whole soil,
bacterial biomass was 52% and 73% higher under NT than CT and
FPF, respectively. Bacterial biomass exhibited a fluctuating pattern
corresponding to different aggregate sizes. The highest bacterial
biomass was observed in the 2.0–1.0 mm and 0.25–0.053 mm fractions for all tillage regimes. Few tillage effects were observed in
macroaggregates >1.0 mm, with the exception of CT promoting the
highest bacterial biomass in aggregates >4.76 mm. However, tillage
effects were very evident in aggregates <1.0 mm. Under 1 mm, bacterial biomass was significantly highest under NT and lowest under
FPF. For aggregates <1.0 mm, bacterial biomass under NT was 54%
higher than under CT, which was 104% higher than for FPF.
Fungal biomass (mg C kg-1 soil)
900
CT
NT
FPF
800
700
600
500
400
300
200
100
0
whole soil
>4.76
2.0~4.76
1.0~2.0
0.25~1.0 0.053~0.25
<0.053
Soil water-stable-aggregate size (mm)
Fig. 1. Distribution of fungal biomass within soil aggregates under different tillage
management (CT, conventional tillage; NT, combines ridge with no tillage; FPF,
flooded paddy field).
600
Bacterial biomass (mg C kg -1 soil)
soil for CT, NT, and FPF, respectively). The average fungal biomass
for macroaggregates >1 mm (498 mg C kg−1 soil) was 3.3 times
higher than in micro-fractions <1 mm (149 mg C kg−1 soil), regardless of tillage regime. Fungal biomass significantly increased from
>4.76 mm to the 2.0–4.76 mm fractions for all tillage regimes, then
it decreased with decreasing aggregate size until the 0.25–1.0 mm
fraction. Then fungal biomass remained unchanged in fractions
<0.25 mm. For all aggregate size fractions except <0.053 mm, fungal
biomass was higher for NT than other tillage regimes.
CT
NT
FPF
500
400
300
200
100
0
whole soil
>4.76
4.76-2.0
2.0-1.0
1.0-0.25
0.25-0.053 <0.053
Soil water-stable-aggregate size (mm)
Fig. 2. Distribution of bacterial biomass within soil aggregates under different
tillage management (CT, conventional tillage; NT, combines ridge with no tillage;
FPF, flooded paddy field).
4. Discussion
4.1. Interaction between microbial biomass, community structure
and soil aggregate
Soil total microbial biomass, fungal biomass, and bacterial
biomass were heterogeneously distributed among soil aggregates.
Furthermore, the distribution patterns of soil microbial biomass,
including biomass fungal and bacterial biomass, in aggregatesize fractions were similar regardless of tillage regime. However,
soil total biomass, fungal biomass, and bacterial biomass exhibited different distribution patterns within aggregates. Soil total
microbial biomass was lowest for 1.0–0.25 mm aggregates and the
<0.053 mm (silt + clay) fraction, while the peak concentrations were
expressed in the 2.0–1.0 mm fraction. However, bacterial biomass
was mainly concentrated within microaggregates, while fungal
biomass was concentrated in macroaggregates.
Singh and Singh (1995) reported that microbial biomass C was
greater in the macroaggregate than the microaggregate sizes for
forest, savanna and crop soils. They hypothesized that different size
classes may support two different types of microorganisms (fungidominated and bacteria-dominated) of the food web in soil. Our
results clearly demonstrated that fungi dominate in macroaggregates while bacteria dominate in microaggregates.
Microbial biomass C:N ratio is relatively consistent, varying
between 2 and 30, but ratios typically range between 8 and 12
(Paul and Clark, 1996; Wright and Coleman, 2000; Cleveland and
Liptzin, 2007). Soil aggregate-size fractions in this study had microbial biomass C:N ratios ranging from 3.9 to 13.8, with an average
of 8.3 (Fig. 3). Macroaggregate size fractions (>1.0 mm) had significantly higher microbial biomass C:N ratio for all tillage regimes
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X. Jiang et al. / Applied Soil Ecology 48 (2011) 168–173
18
Ratio of microbial biomass C to N
CT
16
NT
PFP
14
12
10
8
6
4
2
0
whole soil
>4.76
4.76-2.0
2.0-1.0
1.0-0.25 0.25-0.053
<0.053
Soil water-stable-aggregate size (mm)
Fig. 3. Ratio of microbial biomass C to N within soil aggregates under different
tillage management (CT, conventional tillage; NT, combines ridge with no tillage;
FPF, flooded paddy field).
(11.1) than microaggregates (5.6). Lower biomass C:N ratio can also
be attributed to N limitation (Devi and Yadava, 2006). However, this
was not likely the case in our study, since C:N ratios were lowest for
the 0.25–1.0 mm fraction under all tillage regimes, but total N concentrations in this fraction were the highest of all aggregate sizes
(Jiang et al., 2011). In a recent review, Cleveland and Liptzin (2007)
suggested that, as a broadly defined group, the soil microbial community is homeostatic and variations in soil element ratios do not
significantly affect soil microbial biomass element ratios. Generally, C:N ratios of fungal biomass range from 10 to 15 (Jenkinson
and Ladd, 1981; McGill et al., 1981) and that of bacteria between 3
and 6 (Jenkinson and Ladd, 1981; McGill et al., 1981), meaning that
fungi generally have a greater C:N ratio than bacteria. For this reason, our results may provide more evidence to confirm that fungi
dominate in macroaggregates and bacterial dominate in microaggregates, irrespective of tillage regime.
The fungal:bacterial biomass ratio ranged from 0.5 to 4.5 (Fig. 4).
The ratio of fungal to bacterial biomass was significantly higher
for 4.76–2.0 mm aggregates, and was greater than 1 for aggregate
sizes >1.0 mm (2.4), which was 3.5 times higher than for <1.0 mm
Ratio of fungal biomass to bacterial biomass
6
CT
5
NT
FPF
4
3
2
1
0
whole soil
>4.76
4.76-2.0
2.0-1.0
1.0-0.25 0.25-0.053
<0.053
Soil water-stable-aggregate size (mm)
Fig. 4. Ratio of fungal biomass to bacterial biomass within soil aggregates under
different tillage management (CT, conventional tillage; NT, combines ridge with no
tillage; FPF, flooded paddy field).
171
aggregates (0.7). A fungal to bacterial ratio of 1 indicates that the
fungi and bacteria are contributing equally to the microbiological
activity of the soil (Bailey et al., 2002). Here, it was clearly demonstrated that fungi contributed more to the microbial biomass in
macroaggregates regardless of tillage regime.
According to the hierarchical model proposed by Tisdall and
Oades (1982), the formation of micro- and macro-aggregates are
interrelated processes. Qualitative differences in microbial communities between microaggregates and macroaggregates are likely
related to substrate availability and pore-size distribution. The
major determinants of C and nutrient turnover in soils are thought
to be physical protection of substrates and the spatial distribution
of microorganisms (primary decomposers) and microfaunal predators (secondary decomposers) among pore sizes (Ladd et al., 1993).
The distribution of pore sizes among aggregates and the associated microbial community provides a theoretical basis (Hattori,
1988) for the change in the distribution of microbial biomass among
aggregate sizes shown in our study. Other evidence from a previous study may substantiate this speculation, where similar patterns
of soil pore-size distribution within different sizes of aggregates
occurred under NT and CT (Shi et al., 2008).
Data reported here demonstrate that the distribution patterns
of soil microbial biomass, including biomass fungal and bacterial
biomass, in aggregate size fractions were similar under all tillage
regimes. Therefore, we draw the conclusion that soil microbial
biomass and community structure associated with aggregates was
most probably determined by the size of aggregates at this scale
(mm).
4.2. Tillage effect on the interaction between soil microbial
biomass and aggregates
It has been widely reported that microbial biomass can be
altered through tillage practices and is higher in soils under NT
than CT (Mullen et al., 1998; Kandeler et al., 1999; Feng et al., 2003;
Spedding et al., 2004; Nyamadzawo et al., 2009; González-Chávez
et al., 2010). Our study confirms this finding. Soil total microbial
biomass C, N, fungal biomass, and bacterial biomass were significantly higher under NT than CT and FPF for whole soil. Higher
biomass under NT could be attributed to higher soil organic C
under NT (Jiang et al., 2011) and the deposition and accumulation of residues in surface soil. However, soil microbial biomass
change associated with aggregates was different than the response
for whole soil. Soil total microbial biomass C, N and fungal biomass
were significantly higher under NT than CT and FPF in all aggregate
sizes, whereas bacterial biomass did not show significant differences in macroaggregates (>1.0 mm) for all tillage regimes. Soil
total microbial biomass was mainly concentrated within macroaggregates, and tillage initiated a shift from soil macroaggregates
to microaggregates and individual particles, leading to a decrease
of 67% in macroaggregates (Jiang et al., 2011). Therefore, it was
likely that tillage decreased soil microbial biomass mainly by
decreasing the proportion of soil macroaggregates in the whole
soil.
Tillage did not influence the soil microbial biomass C:N ratio of
whole soil, with average ratios of 8.8, 8.5 and 8.4 for CT, NT, and FPF,
respectively. These values were similar to the 60:7 ratio estimated
by Cleveland and Liptzin (2007). However, microbial biomass C:N
ratio was significant higher under CT than NT for macroaggregates >1.0 mm. According to Cleveland and Liptzin (2007), the C:N
ratio of soil microbial biomass may be even more constrained than
that observed for soils and plant tissues. Therefore, they suggested
the proportion of C to N in the microbial biomass may represent another useful tool for assessing nutrient limitation in soils
(Cleveland and Liptzin, 2007). As macroaggregates were dominated
by fungi for all tillage regimes, a higher microbial biomass C:N ratio
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X. Jiang et al. / Applied Soil Ecology 48 (2011) 168–173
under CT may be a reflection of N limitation in macroaggregates relative to NT. Fungal biomass was significantly lower under CT than
NT, which could be evidence of nutrient limitation. HernándezHernández and Lopez-Hernández (2002) also reported that MBN
significantly decreased under CT while increasing under NT, especially in macroaggregates >0.25 mm during the rainy season. Their
microbial biomass C:N ratio was higher under CT (6.0) than NT (4.6)
in macroaggregates.
Tillage effects on fungal over bacterial dominance were not significant for whole soil and most aggregate sizes, except for the
4.76–2.0 mm fraction for NT. The results indicated that NT increased
microbial biomass, but this increase was proportional for both bacteria and fungi. Similar results were reported in other studies using
PLFA based on determination of fungal over bacterial dominance.
They found that changes to NT or reduced tillage led to an overall
increase in microbial biomass, but this increase was proportional
for bacteria and fungi (Pankhurst et al., 2002; Spedding et al., 2004;
Helgason et al., 2009). van Groenigen et al. (2010) assessed fungal and bacterial dominance using several different techniques and
found minor differences between conventional and reduced tillage.
They found that reduced tillage was associated with an increase
of both bacterial and fungal biomass in surface soils. The functional dominance of fungi, determined via growth-based measures,
did not increase under reduced tillage. It was then concluded that
reduced tillage could promote soil C storage due to a proportional
increase in both bacterial and fungal biomass (van Groenigen et al.,
2010).
Tillage effects on microbial biomass C:N ratio and fungal over
bacterial dominance were not significant for whole soil, but analysis of soil aggregates revealed significant differences in microbial
community structure as a function of aggregate size. The greater
sensitivity of soil aggregates to tillage effects is not surprising since
analysis of components usually yields more information than analysis of the whole. Lupwayi et al. (2001) also reported no significant
tillage effects on bacterial diversity in whole soils but significance
in aggregates. However, tillage effects on soil microorganisms are
complicated. The primary effect of tillage is to physically disturb
the soil structure, so tillage disrupts soil aggregates and lowers
concentrations of soil organic C and nutrients that contribute to
aggregation (Filho et al., 2002; Jiang and Xie, 2009; Six et al., 2006;
Jiang et al., 2011). The specific effect will depend largely on the
disturbance that occurs at the spatial scale to which the microbes
are sensitive (Young and Ritz, 2000). The present results suggest
that soil microbial biomass and fungal over bacterial dominance
associated with aggregates are mainly determined by the size of
aggregate fractions, whereas tillage plays a role in altering the distribution patterns of aggregates.
5. Conclusions
Soil microbial biomass, fungal biomass and bacterial biomass
were heterogeneously distributed among soil aggregates while
their distribution patterns associated with aggregates were similar.
Soil microbial biomass was mainly associated with macroaggregates. Furthermore, fungi dominated in macroaggregates and
bacteria dominated in microaggregates. Soil microbial biomass and
community structure associated with aggregates were most probably determined by aggregate size at this scale (0.05–5.0 mm) and
was mainly attributed to pore-size distribution.
The NT increased microbial biomass, and this increase was
proportional for both bacteria and fungi. Conventional tillage significantly decreased fungal biomass in macroaggregates, and higher
microbial biomass C:N ratios were observed in macroaggregates
under CT than NT, which indicated a potential N limitation in
macroaggregates that was induced by tillage.
Tillage effects on microbial biomass C:N ratio and fungal over
bacterial dominance were not significant for whole soil, but analysis of soil aggregates revealed significant differences due to tillage.
This indicates that characterization of soil aggregates according to
size fraction is a useful tool for process-oriented research into soil
biological dynamics.
Acknowledgements
We thank Prof. David C. Coleman, University of Georgia, for
reviewing this manuscript. This research was financially supported by the Natural Science Foundation of Chongqing, China
(CSTC-2008BA1024), Program for New Century Excellent Talents
in University (NCET-08-0817), and Natural Science Foundation of
China (no. 40871112). We wish to acknowledge useful suggestions
by the reviewers and the editor.
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