Geoderma 115 (2003) 129 – 138
www.elsevier.com/locate/geoderma
Microbial biomass and metabolic quotient of soils
under different land use in the Three Gorges
Reservoir area
Tingmei Yan a,*, Linzhang Yang a, C.D. Campbell b
b
a
Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
Soil Quality and Protection, Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
Abstract
Alternative land use systems, and especially agroforestry, are currently being promoted in the Three
Gorges area to enhance soil retention and improve soil quality. The soil microbial biomass (Cmic) is
intimately associated with the build up of organic matter (Corg). Different land uses including mixed
cropping systems were compared with respect to their soil microbial biomass, basal respiration (R), the
Cmic/Corg ratio and metabolic quotient. Microbial biomass C of citrus/wheat soils ranged widely from
54 to 194 Ag g 1 soil and formed 0.48 – 2.11% of the total organic C. For forests, citrus/wheat (wheat
area) and tea bush, microbial biomass C as a percent of soil organic C were roughly four times that for
citrus and citrus/wheat (citrus area). There were no significant differences between tea bush and forest
in microbial biomass C, respiration rate and metabolic quotient. Significant relationships between
microbial biomass C, Cmic/Corg ratio and readily available N were found for all samples except citrus/
wheat (wheat area) and forest soils. Terraced systems and especially terraced agroforestry promoted
the retention of organic matter and this was reflected in the microbial indicators.
D 2003 Elsevier Science B.V. All rights reserved.
Keywords: Agroforestry; Microbial biomass; Respiration; Metabolic quotient; Land use
1. Introduction
The Three Gorges reservoir area is a degraded ecosystem. Though dominated by
mountains, it is densely populated and has been intensively reclaimed and cultivated.
There are more than 80% hillside fields with a slope of greater than 7j (Xu and Liu, 1993),
* Corresponding author. Fax: +86-25-3353608-8011.
E-mail address: Tmyan@issas.ac.cn (T. Yan).
0016-7061/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0016-7061(03)00082-X
130
T. Yan et al. / Geoderma 115 (2003) 129–138
and these steep slopes are the main living and farming space for people. Even land with a
slope of more than 25j is tilled. Survey has shown that the slopes with a gradient of
greater than 25j constituted 17.5% of the farmland or 25% of the dry land (The
Environmental Impact Assessment Department, 1995). Due to long-term human activities
including overuse and inappropriate development, soil erosion has become a serious
environmental issue. Soil erosion concerns more than 90% of the total Three Gorges area,
of which, heavily eroded soils constitute 45.5%, with an erosion coefficient of >4000 t
km 2 year 1 (Shi and Yang, 1989).
Soils in this area usually suffer from nutrient depletion, texture coarsening, decrease
of thickness and soil drought (Zhang, 1996; Yang, 1994; Xu and Liu, 1993; Du et al.,
1994). Fifty percent of the soils have an organic matter content less than 20 g kg 1
(Zhang et al., 1997). For some soils, the rate of erosion exceeds the rate of formation,
materials in those soils may be lost, and the soil profile would become thinner and
thinner. In the reservoir area, most soils are between 30 and 50 cm thick and A and B
horizons and even C horizons are absent (Du et al., 1994). The soil physical structure is
therefore very poor.
Because of the far-reaching and profound impacts exerted by the building of the Three
Gorges Dam Project due for completion in 2010, even more pressure is now being exerted
on this fragile soil resource. To date, there have been no studies on the microbial impact on
nutrient cycling or how different land uses might affect the build up of organic matter and
ameliorate soil erosion.
The important role that soil microorganisms play in the nutrient and energy-flow
relationships of natural as well as man-manipulated environments has given rise to the
need for easily measured biological indicators of ecosystem development and disturbance.
Soil microorganisms are also agents that promote aggregate stability and good soil
structure. Several studies have shown that the soil microbial biomass changes more
quickly than does soil organic matter as a whole due to changes in soil management. For
example, Powlson et al. (1987) showed that 18 years of straw incorporation increased the
biomass by about 50%, while changes in total organic matter were undetectable. Chander
and Brookes (1991) demonstrated that the ratio of biomass C to soil organic C was a
sensitive indicator of the effects of heavy metals on the microbial biomass using two
different field experiment soils. Powlson et al. (1987) and Wardle (1992) pointed out that
the soil microbial biomass and biomass C/organic C ratio can provide an effective early
warning of the deterioration of soil quality. Killham (1985) and Killham and Firestone
(1984) showed that soil microorganisms divert more energy from growth into maintenance
as stress increases and thus the ratio of respired C to biomass C (the metabolic quotient or
qCO2) can be a much more sensitive indicator of stress. In an analogy to Odum (1971),
who showed that the ratio of basal respiration (R) to basal biomass (B) in an ecosystem
decreases during succession, Insam and Haselwandter (1989) showed that also the soil
qCO2 decreases with succession. The qCO2 has been widely applied in the assessment of
the cultivation regime (Anderson and Domsch, 1990), pollution gradients (Ohtonen,
1994), effect of temperature (Anderson and Domsch, 1986; Anderson and Gray, 1991),
forest ecosystems (Anderson and Domsch, 1993) and acidification (Wolters, 1991).
However, Wardle and Ghani (1995) have questioned the use of qCO2 as a bioindicator,
because it failed to distinguish between effects of disturbance and stress.
T. Yan et al. / Geoderma 115 (2003) 129–138
131
In the present study, different land uses and especially alternative agroforestry systems
in the Three Gorges Reservoir area were compared with respect to their total organic C, N
and nutrient status, soil microbial biomass, basal respiration, the Cmic/Corg ratio and
metabolic quotient ( qCO2) to determine the influence of land use on these relationships.
2. Materials and methods
2.1. Soils
Soil samples for the study were collected from Shui Tianba, Zigui county, in the Three
Gorges reservoir area, which belongs to the humid subtropical monsoon climate zone with
warm winters, early springs, hot and dry summers, and rainy, humid, foggy autumns. The
rainfall in this area is abundant with an annual average of 1100 mm while having an
uneven temporal and spatial distribution. The precipitation from April to October accounts
for over 80% of the annual total rainfall (Chen and Gao, 1988).
The soils were selected to cover the major land uses in this area, i.e., citrus orchard,
citrus plus wheat mixed cropping, wheat, tea bush and forestry. According to US soil
taxonomy, soils for citrus and wheat were Inceptisols and those for tea bush and forestry
were Alfisols (Table 1). At each site, 20 random soil samples were collected to a depth of
20 cm with a 25-mm-diameter stainless steel auger and bulked for each replicate. Four
independent replicates were taken at each site. After sieving (mesh size < 2 mm) and sorting
to remove plant debris and any animals, soil samples were quartered to obtain subsamples
and were then either air dried for chemical analysis or stored at 4 jC before microbiological
Table 1
Soil type, land use, slope, altitude and average basic physico-chemical properties of seven soils under different
crops in different land use systems
Soil no.
Soil type
Land use
Slope
Altitude
(m)
pH
Total C
(g kg 1)
Total N
(g kg 1)
1
2
Purple soil
Purple soil
Terrace
Terrace
280
280
4.98
4.41
10.15
11.40
1.24
1.32
3
Purple soil
Terrace
280
5.47
11.65
1.22
4
Purple soil
Terrace
280
6.14
9.24
1.06
5
6
7
Purple soil
Yellow soil
Yellow soil
citrus—10 years
citrus/wheat
(soybean)—4 years
(citrus area)
citrus/wheat
(soybean)—4 years
(intercrop area)
citrus/wheat
(soybean)—4 years
(wheat area)
wheat (maize) continuous
tea bush—20 years
forestry
(pine/cypress)—20 years
15j
10j
>25j
380
520
500
5.70
4.83
5.24
7.00
8.28
7.15
0.70
0.96
0.52
0.07
0.80
0.05
LSD0.05
Crop descriptions in parenthesis are alternate rotation crops.
132
T. Yan et al. / Geoderma 115 (2003) 129–138
analysis. The soils were incubated for 7 days at 25 jC and their moisture adjusted to 45% of
their water-holding capacity prior to microbial biomass and respiration measurements.
2.2. Soil properties analyses
Soil pH was measured using a glass electrode with a soil/water ratio of 1:2.5. Particle
sizes were determined by a pipette method and soil ash after loss on ignition. Soil total C,
total N, reactive C and readily available N, P and K were measured by K2Cr2O7 – H2SO4
oxidation, Kjeldahl digestion, H2SO4 dilute heat, NaCl –Zn, FeSO4 distillation, NaHCO3
extraction (colorimetric detection) and NH4OAc extraction (flame photometry detection)
methods, respectively (Agro-chemistry Speciality Committee in Soil Science Society of
China, 1983; Institute of Soil Science, CAS, 1978).
2.3. Microbial biomass and basal respiration
Soil microbial biomass C (Cmic) was analysed by the fumigation– extraction method
(Vance et al., 1987). C contents, extracted with K2SO4 from the CHCl3-treated and untreated
soils, were determined by an automated TOC Analyser (Shimazu, TOC-500) and a Kec value
of 2.22 was used to convert the measured flush of C to biomass C (He et al., 1997). Basal
respiration (CO2 evolution) was measured in duplicate on 20-g samples of soil in 100 cm3
soil jars after 7 days by using gas chromatography to measure the headspace CO2 that
accumulated over 24 h at 25 jC. Soil microbial respiration (Rmic) and soil microbial biomass
(Cmic) ratio was used to calculate the metabolic quotient ( qCO2), which is the amount of
CO2 – C produced per unit of microbial biomass carbon (Anderson and Domsch, 1986).
2.4. Statistical analyses
A one-way analysis of variance was used to compute means and least significant
differences (LSD) with different land use as factors.
3. Results and discussion
Soil pH varied from 4.83 under the forest to 6.14 under wheat in the citrus/wheat
terraced fields. The soil under citrus trees was more acid than under wheat and was
intermediate in the intercropped areas in the citrus/wheat agroforestry, such that there was
a pH gradient of increasing pH going from the citrus trees to the wheat rows (Table 1).
Total C was generally higher in the terraced soils under citrus trees but was significantly
lower under the wheat, tea bush and forestry on unterraced slopes. A similar trend was
observed for total N. Similarly, reactive C and readily available nutrients were higher in
the terraced soils under citrus trees, agroforestry and wheat (Table 2). The clay content was
significantly higher under the terraced wheat crops than in the other soils.
The microbial biomass C content of intercropped citrus/wheat soils (soils 2, 3, 4)
ranged from 54 to 194 Ag g 1 soil and increased significantly from the citrus area to the
wheat area (Table 3). The highest Cmic was found under the wheat crop rows in the
T. Yan et al. / Geoderma 115 (2003) 129–138
133
Table 2
Average basic physico-chemical properties of seven soils under different crops in different land use systems
(continued)
Soil no.
Reactive C
(g kg 1)
Readily available N
(mg kg 1)
Readily available P
(mg kg 1)
Readily available K
(mg kg 1)
Ash
(%)
Clay
(%)
1
2
3
4
5
6
7
LSD0.05
7.54
8.50
8.63
7.25
4.10
6.38
4.99
0.47
127.0
122.8
26.6
10.8
9.8
8.9
5.7
7.4
83.18
54.79
30.77
18.86
15.56
3.15
0.98
1.50
252.2
248.7
122.8
112.8
86.9
71.9
93.1
3.3
95.1
95.4
95.4
95.6
95.6
94.8
95.2
0.01
30.25
24.20
21.35
21.30
25.70
26.55
23.05
3.20
agroforestry system. The tea bush and forest soils also had higher Cmic than the citrus or
cropped soils irrespective of whether they were grown in terraces or on slopes. In the
intercropped citrus/wheat system, the Cmic formed 0.48– 2.11% of the total organic C.
Soils from citrus and citrus/wheat (citrus area) (soils 1, 2) had lowest Cmic among all soils.
Microbial biomass C as a percent of soil organic C was 2.31%, 2.11%, 1.78% for forestry
(soil 7), citrus/wheat (wheat area) (soil 4) and tea bush (soil 6), respectively, which were
roughly four times those for citrus, citrus/wheat (citrus area). There were no significant
differences between tea bush and forest soils for any of the measured biological parameters
except the Cmic/Corg ratio.
It is normally assumed that more energy is diverted from growth and production to
maintenance under stress conditions. Soil pH of citrus and citrus/wheat (citrus area) soils
was lower than that of the other soils and also had a lower Cmic. Citrus/wheat intercropped
system may have made the soil environment more favorable for microbial growth
compared to the single wheat system as the combined cropping reduces soil erosion
and maintains higher moisture contents and nutrient levels. Higher respiration rates and
Table 3
Microbial biomass C, basal respiration, metabolic quotient ( qCO2) and Cmic/Corg of seven different soils
Soil no.
Land use
1
2
citrus
citrus/wheat –
citrus area
citrus/wheat –
intercrop area
citrus/wheat –
wheat area
wheat
tea bush
forestry
3
4
5
6
7
LSD0.05
Microbial
biomass C
(Ag g 1 soil)
Basal Respiration
(CO2 – C 10 2
Ag g 1 h 1)
Metabolic
quotient
(10 4 h 1)
Cmic/
Corg (%)
56.6
53.9
0.64
0.96
1.1
1.8
0.56
0.48
114.9
1.05
0.91
1.00
194.2
1.18
0.61
2.11
97.9
147.9
150.6
25.5
0.69
1.38
1.18
0.26
0.70
0.95
0.72
0.26
1.38
1.78
2.31
0.36
134
T. Yan et al. / Geoderma 115 (2003) 129–138
microbial biomass C were found in the agroforestry system especially in the interrows of
the trees and wheat and under the wheat rows themselves (Table 3).
The low values of CO2 –C recorded for these soils overall probably reflect their low
total C content due to the frequent soil erosion events and poor structure (Table 3). The
lowest respiration was recorded in the citrus orchard and wheat-only systems, which were
unterraced. The soils from the agroforestry system had intermediate respiratory activity,
which was, however, significantly higher than that of either the citrus tree or wheat
monocultures alone. This suggests greater benefit in terms of soil organic matter inputs
and microbial activity were obtained in the terraced intercropped system. Similarly, more
CO2 – C was respired from the tea bush (soil 6) and the forest (soil 7) soil. Tea bush soils
are known to be inhibitory to soil microorganisms and have been shown to have high
qCO2 (Yao et al., 2000). We also found that the tea bush soil had a higher qCO2 than the
forest soil with comparable Corg but this was not significantly different. This might be
explained by the fact that the tea bush plantation in our study was much younger than that
studied by Yao et al. (2000). The highest qCO2 was found under the citrus trees of the
agroforestry system (Table 3).
The more extensive rooting systems and protective canopies of the tea bush, forest and
citrus trees will protect the soil from erosion and maintain more favorable conditions for
plant and microbial growth. Sparling (1981) considered basal respiration to be representative of the active part of Cmic. However, Anderson and Domsch (1985) stated that both
the dormant and active microbial communities contribute to the basal respiration. A
significant negative relationship was found between the qCO2 and soil pH with the best fit
being a polynomial function (Fig. 1). Such a negative relationship has been found in
studies of other ecosystems, hence, differences in pH probably explain some of the
variation in qCO2. Conversely, a positive correlation was found between the Cmic/Corg
ratio and soil pH for all soils (Fig. 2). An even stronger correlation between Cmic/Corg ratio
and soil pH could be obtained if the forest and tea bush soils growing on Yellow soils were
omitted from the analysis suggesting an interaction with the land use and/or soil type (Fig.
2) and this was a stronger relationship than found for the qCO2 and pH.
For most of the soils studied, no significant relationships of microbial biomass and
Cmic/Corg ratios with total organic C and total N were found (Table 4). However, there
were significant positive relationships between microbial biomass C, Cmic/Corg ratios and
readily available N for all samples except those from citrus/wheat (wheat area, soil 4) and
also the forest soil (soil 7) (Table 4). Soil C is usually the limiting factor for microorganisms in agricultural soils and the effect of fertilization on Cmic is indirect, acting via
an altered C input from the crops. Thus, an enhancement of N supply may first increase
plant growth as well as roots and residues, and then microbial biomass C and its proportion
in soil organic C can be augmented.
Despite different land uses, soils with low pH released more CO2 – C per unit microbial
biomass both for Purple soils (citrus, citrus/wheat, wheat) and Yellow soils (tea bush,
forestry) (Fig. 1), and also the ratio of microbial C to total soil C increased with increasing
soil pH (Fig. 2). These results were in accordance with the work done by Wolters (1991),
who found an increase in the metabolic quotient of CO2 – C after acid-rain treatment on
beech forest soils. Anderson and Domsch (1993) also found that microbial communities
released more CO2 – C per unit microbial biomass under acidic soil condition than under
T. Yan et al. / Geoderma 115 (2003) 129–138
135
Fig. 1. Relationship between metabolic quotient ( qCO2) and soil pH for seven soils under citrus (o), citrus/
wheat – citrus area (.), citrus/wheat – intercrop area (5), citrus/wheat – wheat area (n), wheat (D), tea bush (E)
and forestry (w). Fitted polynomial function for all soil (curve).
Fig. 2. Relationship between Cmic/Corg ratio and soil pH for soils under citrus (o), citrus/wheat – citrus area (.),
citrus/wheat – intercrop area (5), citrus/wheat – wheat area (n) wheat (D), tea bush (E) and forestry ( w). Fitted
power function for all soils (upper curve) and for all Yellow soils, i.e., excluding tea bush and forestry soils
(bottom curve).
136
T. Yan et al. / Geoderma 115 (2003) 129–138
Table 4
Correlation coefficients of soil microbial biomass and Cmic/Corg ratio with Total organic C, Total N and readily
available N for seven soils under different land use
Soil no.
1
2
3
4
5
6
7
Total organic C
(g kg 1)
biomass C (Ag
Cmic/Corg (%)
biomass C (Ag
Cmic/Corg (%)
biomass C (Ag
Cmic/Corg (%)
biomass C (Ag
Cmic/Corg (%)
biomass C (Ag
Cmic/Corg (%)
biomass C (Ag
Cmic/Corg (%)
biomass C (Ag
Cmic/Corg (%)
g
1
)
g
1
)
g
1
)
g
1
)
g
1
)
g
1
)
g
1
)
0.54
0.77*
0.42
0.52
0.16
0.07
0.12
0.77*
0.70*
0.28
0.26
0.41
0.51
0.03
Total N
(g kg 1)
0.41
0.12
0.82*
0.87*
0.44
0.84*
0.47
0.82*
0.40
0.34
0.49
0.54
0.33
0.36
Readily available N
(mg kg 1)
0.82*
0.87*
0.91*
0.92*
0.76*
0.77*
0.05
0.32
0.81*
0.62*
0.67*
0.71*
0.52
0.46
(1) Citrus orchard—10 years, (2) citrus/wheat (citrus area)—4 years, (3) citrus/wheat (intercrop area)—4 years,
(4) citrus/wheat (wheat area)—4 years, (5) wheat, (6) tea bush—20 years, (7) forestry—20 years.
* Statistically significant at the 95% level.
more neutral pH conditions had a positive relationship with soil pH. Interestingly, the
relationship between pH and the Cmic/Corg ratio appeared to be affected by soil type/land
use and this would complicate its interpretation as an indicator for all soils.
In conclusion, the agroforestry systems on terraced soils appeared to protect and
enhance the build up of organic matter and this was reflected in the response of the
microbial indicators. The relationship between the qCO2 of the microbial biomass and pH
means that careful management of soil pH is also required to ensure a healthy microbial
biomass and continued soil fertility.
Acknowledgements
This work was sponsored by grants from the UK Royal Society and Chinese Academy of
Sciences, the Magnitude Program of Chinese Academy of Sciences (KZ951-A1-202) and
the China National Key Research Program (G1999011802). M.S. Davidson, C.M. Cameron,
A. Norrie and R. MacDougall are gratefully appreciated for their technical assistance. CDC
is funded by the Scottish Executive Environment and Rural Affairs Department.
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