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Phosphate solubilization in vitro by isolatedAspergillus niger and Aspergillus carbonarius
Chunqiao Xiao • Yujuan Fang • Ruan Chi
Received: 25 March 2013 / Accepted: 10 September 2013
� Springer Science+Business Media Dordrecht 2013
Abstract Two strains of Aspergillus, A. niger and A. carbonarius, were isolated
from agricultural soil and a lake, respectively, in China. The two isolates could
effectively release soluble phosphate in NBRIP medium containing Ca3(PO4)2 as
the sole phosphorus source. Acidification of the broth seemed to be the major
mechanism for phosphate solubilization by the isolates, and this had a significant
correlation with a drop in pH. High-pressure liquid chromatography analysis indi-
cated the participation of various organic acids, with gluconic acid as the principal
component in the process of acidification in the broth. The isolates displayed trends
of soluble phosphate release that closely matched the substantial increases in glu-
conic acid concentration. Abiotic incubation study using organic or inorganic acid
to solubilize Ca3(PO4)2 indicated that the content of soluble phosphate released was
significantly lower than that of the broth inoculated with the isolates. Higher release
of soluble phosphate and pH reduction have occurred when ammonium rather than
nitrate served as the sole source of nitrogen with Ca3(PO4)2.
Keywords Phosphate solubilization � Aspergillus niger � Aspergillus
carbonarius � Soluble phosphate � Ca3(PO4)2
Introduction
Phosphorus (P) plays a vital role in plant nutrition. P can be tightly bound with soil
cations, particularly calcium, iron, or aluminum, leading to precipitation of P in the
soil. Therefore, despite P being widely and abundantly distributed in the soil in both
C. Xiao � Y. Fang � R. Chi (&)
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of
Technology, Wuhan 430073, People’s Republic of China
e-mail: [email protected]
123
Res Chem Intermed
DOI 10.1007/s11164-013-1395-6
its inorganic and organic forms, it is not readily soluble in the soil and not easily
accessible for plant growth.
Phosphate solubilization by fungi is widespread and, with respect to agriculture,
considerable attention has been paid to this process [1]. The advantage of using
fungi for such a process includes their tolerance to potentially toxic metals, and
better acid and alkali tolerance than bacteria, although fungi might be inferior to
bacteria in their ability to colonize plant roots. Several studies have shown that an
increase in yield or plant growth can be achieved through the inoculation of
phosphate-solubilizing fungi in either pot experiments or under field conditions [2–
4]. Therefore, fungi may have a much better potential to serve as an agent to convert
insoluble phosphates into a soluble form (e.g., HPO42-, H2PO4
-) usable by plants in
the soil.
Fungi are used widely in biotechnology for many processes, including production
of organic acids, antibiotics, enzymes, food products, and alcohol, etc. Organic
acids production by fungi has been reported to be the main mechanism for the
solubilization of inorganic phosphates [5, 6]. These organic acids can either dissolve
inorganic phosphates as a result of anion exchange or can chelate Ca, Fe or Al ions
associated with these inorganic phosphates [7]. However, incongruent observations
have also been reported. Illmer and Schinner [8] reported that, when growing a
phosphate-solubilizing Penicillium sp. in an inorganic phosphates amended
medium, none of the 24 organic acids assayed were produced in significant
amounts. Chen et al. [9] also reported that some bacterial isolates showing
capacities of phosphate solubilization did not produce any kinds of organic acids.
Some reports showed that proton excretion accompanied NH4? uptake was the
possible mechanism causing phosphate solubilization by fungi [10, 11]. However,
there were also some inconsistent reports [12]. Actually, there have been many
different observations about the mechanisms for phosphate solubilization by fungi,
and they should be studied in depth since there is current interest in the use of fungi
capable of solubilizing inorganic phosphates.
Filamentous fungi, especially genus Aspergillus, are known to secrete certain
organic acids as a part of their metabolic activity, and the solubilization of different
types of inorganic phosphates by them has already been demonstrated [13–15].
Although many fungi can be used to produce organic acids, the genus Aspergillus
remains the main industrial producer [16]. In this study, two species of Aspergillus,
namely A. niger and A. carbonarius, were isolated from agriculture soil and a lake
in China, respectively, and their ability to solubilize Ca3(PO4)2 was studied. The
characteristics and mechanisms for phosphate solubilization by the two isolates
were also investigated.
Materials and methods
Sampling and isolation of probable phosphate-solubilizing fungi
Strain WHAT2 (A. niger) was isolated from soil samples from the rhizosphere of
wheat in a farm located in the suburb of Wuhan city. Strain WHAS3 (A.
C. Xiao et al.
123
carbonarius) originated from water samples from the east lake of Wuhan City.
Isolation and purification were carried out as follows: 10-g or 10-ml samples each
were added to 100-ml sterile saline (0.5 % NaCl), and mixed on the magnetic
blender for 20 min to completely separate microorganisms from the samples. The
serially diluted sample solutions were spread on modified National Botanical
Research Institute’s phosphate growth (NBRIP) agar (pH 7), which contained (per
liter): 5 g Ca3(PO4)2, 10 g glucose, 0.5 g (NH4)2SO4, 0.2 g KCl, 0.5 g
MgCl2�6H2O, 0.25 g MgSO4�7H2O, and 20 g agar [17]. The isolates were incubated
at 30 �C for 3–5 days until the colonies appeared. A single colony with a clear zone
around the fungal colonies from different sites was then picked out for the next
inoculation. Among these fungi, A. niger and A. carbonarius were selected. The
transfer was repeated until the pure culture was obtained. Pure cultures were
maintained on potato dextrose agar slants and kept at 4 �C until required for further
studies. Their identities were confirmed based on the gene sequencing of the internal
transcribed spacer (ITS) regions of the ribosomal DNA (rDNA). The ITS 1, 5.8S
rRNA gene and ITS 2 were amplified by using the polymerase chain reaction
according to White and colleagues. [18]. Amplified products were sequenced and
analyzed using the BLAST searching program at the National Center for
Biotechnology Information website: http://www.ncbi.nlm.nih.gov/BLAST/. Rela-
ted sequences were preliminarily aligned with the default setting of Clustal X (2.0)
[19]. Phylogenetic and molecular evolutionary analyses were conducted using
MEGA v.5 [20].
Broth assays for phosphate solubilization by the isolates
Broth assays were carried out in shake flasks with 50 ml NBRIP medium containing
0.5 g Ca3(PO4)2 as sole P source. The initial pH of the medium was adjusted to 7.
Mycelial discs (10 mm) of each isolate from actively growing colonies after 4 days
on modified NBRIP agar were added as inoculum. Flasks were shaken at 160 rpm at
30 �C for 10 days. Autoclaved, uninoculated medium served as control. Samples
from flasks were taken every other day and the broth was centrifuged at 11,000g for
20 min, and the supernatant was filtered. The filtrate was then assessed for the
soluble phosphate, pH, and organic acids. All experiments were performed in
triplicate.
Abiotic phosphate solubilization by organic and inorganic acids
To study the importance of organic acid in phosphate solubilization, gluconic acid
and acetic acid were added to the above medium containing Ca3(PO4)2 as sole P
source to give a concentration similar to that of the gluconic acid detected in the
broth inoculated with the isolates. To study the importance of acidity in phosphate
solubilization, in another set of P-amended medium, 0.1 M HCl and H3PO4 were
added until the pH was as close as possible to that of the broth inoculated with the
isolates. Each flask was inoculated and incubated at room temperature for 2 h prior
to subsampling. The solution was centrifuged and assessed for the content of soluble
phosphate.
Phosphate solubilization in vitro by isolated A. niger and A. carbonarius
123
Effect of nitrogen sources on phosphate solubilization by the isolates
The effect of nitrogen sources on phosphate solubilization by the isolates was
evaluated by replacement of ammonium sulfate with five nitrogen sources, viz.
ammonium chloride, ammonium nitrate, potassium nitrate, sodium nitrate, and
calcium nitrate, respectively. Culture supernatant was sampled from each broth for
the assessment of the soluble phosphate and pH.
Analytical methods
The content of soluble phosphate was determined by using the vanadium–
ammonium molybdate colorimetric method with a UV–Vis 8500 spectrophotometer
at 490 nm [21]. The pH was recorded with a pH meter equipped with a glass
electrode. Organic acids in the broth were determined by high-pressure liquid
chromatography (HPLC; Agilent 1100) analysis using C18 columns (Thermo
Electron) [22]. Organic acids standards included citric, oxalic, gluconic, formic, a-
ketoglutaric, fumaric, lactic, pyruvic, succinic, tartaric, and ascorbic acids.
Phosphatases activity was determined using the method described by Tabatabai
and Bremmer [23]. Values were given as mean ± standard deviation for triplicate
samples.
Results and discussion
Identification of the isolates
The two isolates were filamentous fungi that produce microscopic spores inside
sacs. They can grow in wide ranges of temperature and pH at 10–45 �C and 2.0–9.5,
and the best temperature and pH are at 28–30 �C and 5.5–6.5, respectively. The
isolates were finally identified as A. niger and A. carbonarious based upon the
results of phenotypic characterization and the phylogenetic tree constructed on the
basis of ITS sequence data (Fig. 1). The sequences were deposited in the GenBank
nucleotide sequence data library under the following accession numbers: JQ929762
(WHAT2) and JQ929763 (WHAS3), respectively.
Phosphate solubilization by the isolates
The two isolates, namely A. niger WHAT2 and A. carbonarius WHAS3, could
effectively solubilize Ca3(PO4)2 in NBRIP medium compared to the abiotic control,
and the content of soluble phosphate released increased significantly during 10 days
of solubilization of Ca3(PO4)2, although there was a slight decrease after 6 days
(Fig. 2). The two isolates varied in their capacities to release soluble phosphate from
Ca3(PO4)2, and the solubilization of Ca3(PO4)2 by strain WHAT2 was better than
that by strain WHAS3 in this study.
Figure 3 shows that the pH in the broth decreased sharply after inoculation
compared to the abiotic control, and remained almost constant after 6 days. A. niger
C. Xiao et al.
123
WHAT2 effected the larger reduction of pH than A. carbonarius WHAS3. The
release of soluble phosphate in the broth was associated with a concomitant
decrease in pH. Simple regression analysis suggest that there is a significant
negative correlation (r = -0.76; P \ 0.01) between the soluble phosphate released
and pH. This observation is consistent with previous reports demonstrating that
phosphate solubilization with decrease in pH [24].
Various organic acids, including gluconic, oxalic, formic, citric, pyruvic,
succinic, and lactic acid, were detected in the broth during solubilization of
Ca3(PO4)2 by HPLC analysis (Table 1). It is presumed that the production of
organic acids plays a vital role in the acidification of the broth. There was a
significant decrease of pH in the broth facilitating the solubilization of Ca3(PO4)2.
gi 161408454 dbj AB369898.1 Aspergillus niger
WHAT2
gi 307088982 gb HM801882.1 Aspergillus sp.
WHAS3
gi 349844858 gb JF Aspergillus carbonarius
gi 158144421 gb EF661429 Aspergillus ochraceopetaliformis
gi 158144422 gb EF661430 Aspergillus insulicola
gi 158138942 gb EU02161
838359.1
.1
.1
6.1 Aspergillus flocculosus41
100
99
99
88
Fig. 1 ITS rDNA-based phylogenetic relationship between the isolates and representatives of otherrelated taxa (GenBank accession numbers in parentheses). The numbers at the nodes indicate the levels ofbootstrap support based on data for 1,000 replicates; values inferred greater than 50 % are only presented.Scale bar 0.01 substitutions per nucleotide position
0 2 4 6 8 100
50
100
150
200
250
300
Con
tent
of s
olub
le p
hosp
hate
(m
g l-1
)
Time (d)
Abiotic control WHAT2 WHAS3
Fig. 2 Content of soluble phosphate in the broth inoculated with the isolates during 10 days ofsolubilization of Ca3(PO4)2. Bars standard errors
Phosphate solubilization in vitro by isolated A. niger and A. carbonarius
123
Among the organic acids detected, gluconic acid was predominantly produced by
the two isolates in this study. The results in Fig. 4 show that the concentration of
gluconic acid in the broth inoculated with the isolates increased significantly during
10 days of solubilization of Ca3(PO4)2, although there was a slight reduction after
6 days. A. niger WHAT2 produced the highest concentration of gluconic acid
compared with A. carbonarius WHAS3 during solubilization of Ca3(PO4)2.
The two isolates displayed trends of release of soluble phosphate that closely
matched the substantial increases in gluconic acid concentration. Simple regression
analysis shows that there is a strong positive correlation between the content of
soluble phosphate released and the concentration of gluconic acid (r = 0.86;
P \ 0.01).
Abiotic phosphate solubilization by organic and inorganic acids
The production of organic acids, especially gluconic acid, played a vital role in the
solubilization of Ca3(PO4)2 in the present study. This result prompted us to
investigate the effect of organic and inorganic acids on the solubilization of
Ca3(PO4)2 under sterile conditions. In the present study, the importance of organic
acid in the solubilization of Ca3(PO4)2 was assessed by adding gluconic acid and
acetic acid to the medium containing Ca3(PO4)2, at a concentration similar to the
level of gluconic acid detected in the medium inoculated with the two isolates.
However, this strategy failed to affect the release of soluble phosphate in the broth
to the comparable levels in the presence of the isolates. As shown in Table 2, it was
obvious that the organic acid solubilized less Ca3(PO4)2 than by A. niger WHAT2.
Similar results were observed in the presence of A. carbonarius WHAS3 (data not
0 2 4 6 8 103.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
pH
Time (d)
Abiotic control WHAT2 WHAS3
Fig. 3 pH in the broth inoculated with the isolates during 10 days of solubilization of Ca3(PO4)2. Barsstandard errors
C. Xiao et al.
123
Ta
ble
1O
rgan
icac
ids
det
ecte
d,
and
thei
rco
nce
ntr
atio
n(m
gl-
1)
inth
eb
roth
ino
cula
ted
wit
hth
eis
ola
tes
afte
r6
day
so
fin
ocu
lati
on
Cit
ric
acid
Ox
alic
acid
Glu
con
ic
acid
Fo
rmic
acid
a-K
etoglu
tari
c
acid
Fu
mar
ic
acid
Lac
tic
acid
Py
ruv
ic
acid
Su
ccin
ic
acid
Tar
tari
c
acid
Asc
orb
ic
acid
WH
AT
21
1.3
±0
.99
0.1
±6
.43
97
.1±
20
.73
0.2
±1
.5–
–1
9.4
±1
.71
3.0
±1
.51
5.2
±0
.9–
9.5
±0
.7
WH
AS
3T
race
70
.2±
5.2
32
5.7
±2
1.4
24
.6±
1.9
––
16
.1±
1.7
12
.5±
1.1
11
.7±
1.1
–T
race
Res
ult
sre
pre
sen
tth
em
ean
of
thre
ere
pli
cate
s±
stan
dar
dd
evia
tio
n
–N
ot
det
ecte
d
Phosphate solubilization in vitro by isolated A. niger and A. carbonarius
123
shown). The results confirmed some reports demonstrating that the mere presence of
organic acids does not account for all the soluble phosphate that is solubilized by
microorganisms [25].
Likewise, the effect of inorganic acid (HCl and H3PO4) on the solubilization of
Ca3(PO4)2 under sterile conditions was also studied. When the pH of the medium
was finally adjusted to different pH values similar to the level detected in the
medium inoculated with A. niger WHAT2, only a small amount of soluble
phosphate was released, which was significantly smaller than the content detected in
the medium incubated with the test fungus (Table 2). Similar results were observed
in the presence of A. carbonarius WHAS3 (data not shown). Our results further
demonstrated multiple actions in addition to acid production of phosphate-
solubilizing microorganisms in the phosphate solubilization.
Effect of nitrogen sources on phosphate solubilization by the isolates
Better phosphate solubilization and pH reduction were observed when NH4? rather
than NO3- was used as the nitrogen source (Table 3). The results further confirmed
that H? extrusion by NH4? assimilation may also be involved in the solubilization
of inorganic phosphates. However, the hypothesis that phosphate solubilization is
linked to acidification caused by NH4? assimilation does not hold true for all
microorganisms [12, 26].
Phosphate-solubilizing microorganisms are known to produce phosphatases,
which are hydrolytic enzymes, responsible for the breakdown of insoluble
phosphates. Achal et al. [27] reported that phosphate solubilization by Aspergillus
tubingensis is due to the lowering of the pH of the broth and the activity of the acid
phosphatase and phytase. Rinu and Pandey [28] have also shown that phosphatases
0 2 4 6 8 100
100
200
300
400
500
Con
cent
ratio
n of
glu
coni
c ac
id (
mg
l-1)
Time (d)
WHAT2 WHAS3
Fig. 4 Concentration of gluconic acid in the broth inoculated with the isolates during 10 days ofsolubilization of Ca3(PO4)2. Bars standard errors
C. Xiao et al.
123
Ta
ble
2C
om
par
iso
no
fso
lub
iliz
atio
no
fC
a 3(P
O4) 2
by
A.
nig
erW
HA
T2
and
abio
tic
solu
bil
izat
ion
foll
ow
ing
the
add
itio
no
fo
rgan
ico
rin
org
anic
acid
Day
A.
nig
erW
HA
T2
Glu
con
icac
idA
ceti
cac
idH
Cl
H3P
O4
Con
cen
trat
ion
of
glu
con
icac
id
det
ecte
d(m
gl-
1)
pH
So
luble
ph
osp
hat
e
rele
ased
(mg
l-1)
Co
nce
ntr
atio
n
(mg
l-1)
So
luble
ph
osp
hat
e
rele
ased
(mg
l-1)
Con
cen
trat
ion
(mg
l-1)
So
luble
ph
osp
hat
e
rele
ased
(mg
l-1)
pH
So
luble
ph
osp
hat
e
rele
ased
(mg
l-1)
pH
So
lub
le
ph
osp
hat
e
rele
ased
(mg
l-1)
23
80
.7±
18
.74
.35
±0
.17
12
2.7
±5
.53
80
.78
8.3
±3
.23
80
.76
.5±
0.2
4.3
51
2.8
±0
.64
.35
5.4
±0
.1
44
49
.1±
25
.34
.09
±0
.12
23
5.6
±1
1.2
44
9.1
97
.4±
4.1
44
9.1
7.7
±0
.44
.09
30
.4±
0.8
4.0
96
.1±
0.4
64
68
.5±
24
.53
.91
±0
.10
27
2.8
±1
5.2
46
8.5
10
1.2
±5
.84
68
.57
.9±
0.7
3.9
13
1.2
±1
.13
.91
7.7
±0
.5
84
41
.8±
20
.64
.07
±0
.14
26
3.7
±1
3.4
44
1.8
95
.3±
5.5
44
1.8
7.0
±0
.34
.07
30
.1±
0.7
4.0
76
.4±
0.3
10
42
8.7
±2
1.8
4.1
4±
0.2
02
55
.9±
12
.14
28
.79
0.9
±3
.64
28
.76
.8±
0.3
4.1
42
9.7
±1
.04
.14
5.6
±0
.4
Res
ult
sre
pre
sen
tth
em
ean
of
thre
ere
pli
cate
s±
stan
dar
dd
evia
tio
n
Phosphate solubilization in vitro by isolated A. niger and A. carbonarius
123
Ta
ble
3E
ffec
to
fam
mo
niu
man
dn
itra
teo
nth
ere
leas
eo
fso
lub
lep
ho
sph
ate
fro
mC
a 3(P
O4) 2
by
the
isola
tes
afte
r6
day
sin
ocu
lati
on
(NH
4) 2
SO
4N
H4C
lN
H4N
O3
Ca(
NO
3) 2
KN
O3
NaN
O3
WH
AT
22
83
.7±
11
.9
(4.1
9±
0.0
7)
27
1.6
±1
0.0
(4.2
4±
0.1
0)
24
3.4
±1
1.2
(4.3
7±
0.2
5)
20
1.3
±1
2.4
(4.4
8±
0.1
0)
20
1.3
±1
0.5
(4.4
7±
0.0
8)
20
2.8
±1
2.4
(4.4
5±
0.1
3)
WH
AS
32
47
.2±
10
.8
(4.5
0±
0.0
9)
24
1.5
±9
.3
(4.6
2±
0.1
3)
22
1.5
±1
1.7
(4.7
0±
0.1
7)
15
8.7
±9
.7
(4.7
7±
0.0
9)
17
8.5
±1
0.7
(4.7
8±
0.0
8)
16
2.3
±8
.3
(4.7
7±
0.1
1)
Res
ult
sre
pre
sen
tth
em
ean
of
thre
ere
pli
cate
s±
stan
dar
dd
evia
tio
n.
Fig
ure
sin
par
enth
eses
indic
ates
pH
of
the
bro
thaf
ter
6d
ays
ino
cula
tio
n
C. Xiao et al.
123
excreted by Paecilomyces hepiali were involved in the solubilization of insoluble
phosphates. However, we failed to detect any phosphatases in the broth in the
present study.
Generally, phosphate solubilization by microorganisms is not a simple phenom-
enon and may be determined by many factors, such as nutritional, physiological,
and the growth conditions of the microorganisms. Therefore, it could be
accomplished by a range of mechanisms and need further study.
Conclusion
Two isolated strains, namely A. niger WHAT2 and A. carbonarius WHAS3, can
effectively solubilize Ca3(PO4)2 and release soluble phosphate in NBRIP medium.
The main mechanism of phosphate solubilization is the production of organic acids
(principally gluconic acid), which played a vital role in the acidification of the broth,
followed by the decrease in pH, and thus further facilitated the solubilization of
Ca3(PO4)2. An abiotic study using organic or inorganic acid to solubilize Ca3(PO4)2
indicated that the content of soluble phosphate released was significantly lower than
was detected in the broth inoculated with the isolates. The enhancement of the
release of soluble phosphate and pH reduction was observed when ammonium
rather than nitrate acts as the sole source of nitrogen with Ca3(PO4)2 in NBRIP
medium.
Acknowledgments This research work was kindly supported by National Natural Science Foundation
of China (51004078), Program for New Century Excellent Talents in University (NCET-11-0965),
National Natural Science Foundation of Hubei province (2012FFA101), Program for Changjiang
Scholars and Innovative Research Team in University (No. IRT0974) and National Basic Research
Program of China (No. 2011CB411901).
References
1. F.P. Coutinhoa, W.P. Felix, A.M. Yano-Melob, Ecol. Eng. 42, 85 (2012)
2. G.O. Mendes, C.S. Dias, I.R. Silva, J.I.R. Junior, O.L. Pereira, M.D. Costa, World J. Microbiol.
Biotechnol. 29, 43 (2013)
3. H.J. Zhu, L.F. Sun, Y.F. Zhang, X.L. Zhang, J.J. Qiao, Bioresour. Technol. 111, 410 (2012)
4. H. Singh, M.S. Reddy, Eur. J. Soil Biol. 47, 30 (2011)
5. H.S. Hamdy, Ann. Microbiol. 63, 267 (2013)
6. J.M. Scervino, M.P. Mesa, I.D. Monica, M. Recchi, N.S. Moreno, A. Godeas, Biol. Fertil. Soils 46,
755 (2010)
7. K. Kpomblekou, M.A. Tabatabai, Soil Sci. 158, 442 (1994)
8. P. Illmer, F. Schinner, Soil Biol. Biochem. 24, 389 (1992)
9. Y.P. Chen, P.D. Rekha, A.B. Arun, F.T. Shen, W.A. Lai, C.C. Young, Appl. Soil Ecol. 34, 33 (2006)
10. J.E. Cunningham, C. Kuiack, Appl. Environ. Microbiol. 58, 1451 (1992)
11. A. Pandey, N. Das, B. Kumar, K. Rinu, P. Trivedi, World J. Microbiol. Biotechnol. 24, 97 (2008)
12. I. Reyes, L. Bernier, R.R. Simard, H. Antoun, FEMS Microbiol. Ecol. 28, 281 (1999)
13. K. Rinu, A. Pandey, Mycoscience 51, 263 (2010)
14. K. Rinu, M.K. Malviya, P. Sati, S.C. Tiwari, A. Pandey, ISRN Soil Sci. (2013). doi:10.1155/2013/
598541
15. F.C. Ogbo, Bioresour. Technol. 101, 4120 (2010)
16. G.S. Dhillon, S.K. Brar, M. Verma, R.D. Tyagi, Biochem. Eng. J. 54, 83 (2011)
Phosphate solubilization in vitro by isolated A. niger and A. carbonarius
123
17. C.S. Nautiyal, FEMS Microbiol. Lett. 170, 265 (1999)
18. M.A. Innis, D.H. Gelfand, J.J. Sninsky, T.J. White, PCR Protocols: a Guide to Methods and
Applications (Academic Press, San Diego, 1990), pp. 315–322
19. J.D. Thompson, T.J. Gibson, F. Plewniak, F. Jeanmougin, D.G. Higgins, Nucleic Acids Res. 24, 4876
(1997)
20. K. Tamura, D. Peterson, N. Peterson, G. Stecher, M. Nei, S. Kumar, Mol. Biol. Evol. 10, 2731 (2011)
21. L.H. Jiang, S.L. Zhao, J.H. Zhu, Chem. Eng. 85, 50 (2001). (in Chinese)
22. C.Q. Xiao, R.A. Chi, H. He, G.Z. Qiu, D.Z. Wang, X.W. Zhang, Appl. Biochem. Biotechnol. 159,
330 (2009)
23. M.A. Tabatabai, J.M. Bremner, Soil Biol. Biochem. 1, 301 (1969)
24. E. Perez, M. Sulbaran, M.M. Ball, L.A. Yarzabal, Soil Biol. Biochem. 39, 2905 (2007)
25. P. Illmer, F. Schinner, Soil Biol. Biochem. 27, 257 (1995)
26. C.C. Chuang, Y. Kuo, C.C. Chao, W.L. Chao, Biol. Fertil. Soils 43, 575 (2007)
27. V. Achal, V.V. Savant, M.S. Reddy, Soil Biol. Biochem. 39, 695 (2007)
28. K. Rinu, A. Pandey, World J. Microbiol. Biotechnol. 27, 1055 (2011)
C. Xiao et al.
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