Introduction
About 350 million tons of corn
straw is produced annually by China¡¯s agricultural activity, accounting for
almost 50% of straws¡¯ production. Turning these agricultural waste products
into a valuable resource and source of fertilizer has become a critical focus
of research in the field of biomass resource utilization. Solid-state
fermentation, as green technology, remains one of the best methods for
agricultural waste treatment, and can achieve efficient biotransformation
(Zhang et al. 2016). Tang et al.
(2010) observed that its humification process is similar to that in soil. It
primary begins with mineralization of organic matter which is gradually
transformed into humus (Wang et al. 2015), a final product in
decomposition of organic materials. This process is mainly mediated by the
action of microorganisms.
Corn straw is
composed of lignin, cellulose and hemicellulose. In the solid-state
fermentation, the fungi have a strong ability to decompose and degrade these
compounds due to two mechanisms: (i) production of extracellular enzymes by the
fungi and (ii) and mechanical perforation of the straw by the fungal hyphae
which facilitate the decomposition. Under these conditions, the hydrolysis rate
of organic matter is accelerated, thusly, increasing the decomposition of
refractory organic matter (Johansson et al. 1989; Fu 2012). Therefore,
fungi hold a primordial role in the mineralization and decomposition of corn
straw in solid-state fermentation.
Vijai et al. (2014) reported that Trichoderma
specie is a beneficial fungus, because of their strong ability to break down
cellulose, promotes the fermentation and utilization of organic waste
(Cruz-Quiroz et al. 2017), and degrade complex molecular compounds
(S¨¢nchez–Monedero et
al. 1999). The Trichoderma reesei in particular has been shown to hold
an excellent lignocellulase production capacity (Xin
and Geng 2009; Ortiz et al. 2015) and can
enhance formation of humic acid-like (HAL) content in
a relatively short period (Yang et al.
2019). Moreover, Yang et al. (2019)
demonstrated that T. reesei
during the fermentation process enhanced corn straw humification degree and
fermented corn straw product could be a good source of fertilizer. Latifian et al. (2007) used rice bran as a
substrate and showed that the optimal conditions for cellulase production
occurred at temperatures of 25–30¡ãC and a moisture content of 55–70%. Castro et al. (2010) demonstrated that
cellulose degradation within 72 h, was rich in ¦Â-glucosidase activity, when bagasse was treated with T. harzianum in a solid-state fermentation. Apart from
corn straw, Zeng et al. (2014) showed
that peanut shell powder and corn cob can also be used by T. harzianum under solid-state fermentation. In another
study Zhang et al. (2017) reported that Phanerochaete
chrysosporium is capable of degrading lignin by
excreting extracellular oxidases such as lignin peroxidase, manganese peroxidas, and lactase. Huang et al. (2006) concluded that corn straw treated with P. chrysosporium
decreased significantly the lignin content within 15 days, while Chen et al. (2019) inoculated a spore
solution of P. chrysosporium
to maize straw and canola residue, and observed a lignin degradation rate up to
64.3%. The lignin and cellulose have a profound contribution to the formation
of humic substances (HS). Therefore, dynamic changes
in these compounds will affect HS turn-over rate.
Polyphenol theory
is prevalent in the interpretation of corn straw degradation and humus
formation process. Organic matter is first decomposed into simple organic
compounds under microbial action. Under the action of enzymes, polyphenolsare are oxidized to quinones (Yu et al.
2003) and the latter are condensed with the nitrogen-containing compound to form
HS. On the other, the lignin theory suggests that lignin is the basic
structural unit of humic formation, by first forming humic-acid (HA), then through
further oxidation fuvlic-acid (FA) is formed (Thevenot et al. 2010). Many researchers have
investigated the subject and found that fungi can degrade and transform both
lignin and cellulose, and form quinone compounds (Bahri
et al. 2008). In the early studies, Tuomela et al. (2002), used 14C
labeled lignin to track its decomposition by P. chrysosporium.
They observed that P. chrysosporium was able
to degrade lignin and the 14C was used to synthesize newly formed
HS.
The corn straw is
our key raw material for simulating humification in which HSs are our final
product. In this study, three types of fungi (P. chrysosporium,
T. harzianum and T. reesei)
were chosen based on their ability to decompose and mineralize corn straw. A
simulation of the short-term humification process was performed in a
solid-state fermentation and we monitored dynamic changes of HS contents in the
residue for 25 days. The aim of this study was; (i) to provide a new reference
to clarify the fungal mechanisms involved in the formation and transformation
of humus, and (ii) determine fungi with the best efficient utilization effect
on corn straw transformation.
Materials and Methods
Site description and corn
straw sampling
The sampling site of corn
straw is presented by a corn cropland located in Jilin Agricultural University,
Nanguan District, Changchun City, Jilin Province,
China (N43¡ã48¡ä43.5¡ä, E125¡ã23¡ä38.50¡ä). The region is characterized by four distinct seasons: a cold wet
spring; short warm summer; sunny, dry and windy fall; and a long cold winter.
The mean annual temperature is 4.8¡ãC, and the mean annual rainfall is 618 mm.
The highest rainfall occurs between July and August. Black soils (Chinese Soil
taxonomy) are the main soil of the region and are classified as Argiudolls according to the USDA Soil Taxonomy (United
States Department of Agriculture 2014).
The corn cultivar
was Zhongjin 368 type (Beijing Golden Grain Seed Co.,
Ltd.), which was planted at the end of April 2017 and harvested in early
October. After harvest, the whole corn straw was naturally air-dried and cut
into 0.5 cm segments. The basic properties of corn straw were: 376.4 g kg-1
organic C, 7.22 g kg-1 total nitrogen, 7.7 g kg-1 total
phosphorus, 4.5 g kg-1 total potassium, and a C/N ratio of 61.8.
Preparation of fungi and
fungal liquid
Three fungal strains used in
this study; (i) T. reesei MCG77,
(ii) P. chrysosporium ATCC 24725 were
purchased from the American Type Culture Collection (ATCC) and (iii) T. harzianum was isolated and purified from fresh soil
collected in Jilin Agricultural University experimental field after one year of
straw returning (T. harzianum provided by
microbial laboratory of Department of Environment and Resource, Jilin
Agricultural University).
Each of the three
strains of fungi was inoculated to a solid bevel tube containing 30 mL of
potato dextrose agar (PDA). The solid bevel tubes were placed in an incubator
at 28¡ãC for 72 h to obtain mature microbial spores (mycelium). Spore solution
preparation process was as follows: 2 mL of sterile distilled water was added
to the inoculated solid bevel tube and vortexed for 2 min. The solution was transferred
to a sterile tube, and the concentration was calculated through a haemocytometer and diluted to a final concentration of 1¡Á107
spores/mL. Finally, the spore suspension was
transferred to liquid medium at a ratio of microbial liquid to liquid medium of
1:10. The culture was incubated on a reciprocating incubator at 100 rpm and
30¡ãC for 6 days. The mycelium-containing broth was used as a backup.
The PDA medium was
prepared from the 200 g peeled potato, which was boiled for 30 min in 1 L of
distilled water, and filtered. Thereafter, 20 g L-1 of glucose and
20 g L-1 of agar were added. Meanwhile, the liquid medium was also
prepared from peeled potato (200 g L-1) boiled for 30 min and
filtered; then 20 g L-1 of glucose was added.
Solid-state fermentation and
collection
The fermentation was conducted
in a BIOTECH-30SS solid fermentation tank (Baoxing
Biological Engineering Equipment Co., Ltd., Shanghai, China). The tank had a
volume of 30 L with a sterilizing function, automatic stirring, controlled humidity
and temperature, and air intake feature. A KQ-C type automatic steam generator
(Shanghai Fengxian Xiexinji
Power Plant) was used to generate steam for sterilization.
Prior to the
fermentation, 1.5 kg of corn straw powder (particle size = 0.5 cm) was
sterilized in the solid fermenter. The sterilization was conditioned at 121¡ãC
for 25 min. After the sterilization, corn straw was thoroughly mixed with each
microbial liquid containing the spore mycelia (0.6 L) and the mineral salt
solution (3.75 L). Each treatment was replicated three times and randomized.
The fermentation was set at 30¡ãC, 60% humidity and 6.0 rpm.
The mineral salt
solution at a final pH of 5 was prepared as follows: urea 4.2 g L-1,
ammonium sulfate 19.6 g L-1, calcium chloride 0.028 g L-1,
potassium dihydrogen phosphate 28 g L-1, magnesium sulfate 4.2 g L-1,
ferrous sulfate 0.07 g L-1, manganese sulfate 0.021 g L-1,
zinc sulfate 0.019 g L-1, cobalt chloride 4.2 g L-1 and
yeast extract 7 g L-1.
Sampling and analysis
Small portions of straw
samples were collected from each bevel tubes on the first day and every 5 days.
Before each sampling, corn straw and the liquid were uniformly mixed. The
monitoring lasted for 25 days and the samples were analyzed separately for each
day of collection. Collected corn straw samples were oven-dried at 55¡ãC for 6
h, then fine grounded to pass through a 0.25 mm sieve.
Analytical methods
Extraction and determination
protocol of humus in corn straw was based on the modified method of soil humus
composition (Kumada et al. 1967). Water-soluble substances (WSS) were extracted from
corn straw with distilled water under permanent shaking for 1 h. The Humic-like substances (HLS) were extracted with a mixture
of 0.1 M sodium hydroxide and 0.1 M sodium pyrophosphate. The humic acid-like (HAL) and fulvic acid-like (FAL) were
separated with 0.5 M sulfuric acid. The residues left were considered as humin-like (HML).
The carbon content
of the total organic carbon (TOC), WSS, humus-like extracted (HLE), HAL and HML
components of corn straw was determined by potassium dichromate oxidation
method (Lu 2000), and the carbon content of FAL was determined by difference in
HLE and HAL. The PQ (%) value was computed as:
and relative content of WSS was calculated as:
Data processing and
statistical analysis
Microsoft Office Excel 2017
was used for data processing and statistical analysis was performed by SPSS
Statistics 22.0. Significance differences among treatment means were evaluated
using the least significant difference test with DUCAN adjustment at P < 0.05.
Results
Change in TOC in corn straw
during fermentation
The TOC content decreased with
fermentation time across all treatments (Fig. 1). Corn straw treated with fungi
showed an exponential decrease in TOC content, while non-treated corn showed
gradual decrease. After 5 days, T. reesei showed a significant decrease in TOC content
compared with other treatments. Between the 5th, 10th,
and 15th days, no significant difference was observed in TOC content
between P. chrysosporium and T. harzianum. At
the end of the fermentation period, the TOC of T. reesei,
P. chrysosporium and T. harzianum,
respectively decreased by 15.92, 9.31 and 7.71% compared with that of CK.
Effects of fungi on humic-like substances in corn straw composition
The carbon content at the
first day after inoculating corn with P. chrysosporium,
T. harzianum and T. reesei
increased in the following order: HML>WSS>FAL>HAL (Table 1). The C
content in FAL of the three fungal inoculations were
higher than that of CK throughout the fermentation time, except that of T. harzianum and T. reesei which was lower than CK in the 20th
and 25th days. The C content of HAL showed an inverse trend, that
is, HAL was markedly higher in corn straw treated with fungi than CK during the
fermentation period but not always (Table 1). The C content of WSS did
not exhibit distinct distribution pattern, some treatments increased WSS to a
particular day and started to decrease again. However, the C content in HML
appeared to decrease throughout fermentation period in all treatments. The C
content in HAL of T. harzianum and T. reesei treatments exceeded that of FAL on the 15th
and 20th days. Meanwhile, CK and P. chrysosporium showed greater FAL carbon contents
than HAL C content throughout the fermentation period (Table 1)
During the
fermentation, the relative contents of WSS under the three fungi inoculations
showed different degrees of increasing trend meanwhile that of CK decreased
slowly (Fig. 2a). The organic C in corn straw inoculated with fungi was
transformed into water-soluble organic C at varying degrees and was stabilized
after 20 days. At the end of fermentation, the relative content of WSS in T.
reesei, T. harzianum
and P. chrysosporium increased by 6.07, 3.84
and 2.38%, respectively, compared with CK.
The relative contents of HAL
under the four treatments increased with time, and the magnitude of change
followed the order of: T. reesei>T. harzianum>P. chrysosporium>
CK (Fig. 2b). At day 25, the HAL of T. reesei,
Table 1: Effect of different fungi on humic-like
substance in corn straw
Treatments |
Time |
WSS |
HAL |
FAL |
HML |
(d) |
(g kg-1) |
(g kg-1) |
(g kg-1) |
(g kg-1) |
|
CK |
0 |
68.28 ¡À 1.28a |
23.86 ¡À 0.98c |
41.44 ¡À 0.37d |
224.44 ¡À 1.79a |
5 |
67.73 ¡À 1.20a |
25.89 ¡À 0.67b |
44.13 ¡À 0.66d |
221.25 ¡À 1.52ab |
|
10 |
67.11 ¡À 0.92a |
26.98 ¡À 1.43b |
45.83 ¡À 0.82c |
219.92 ¡À 0.99b |
|
15 |
66.42 ¡À 0.94ab |
29.17 ¡À 0.16ab |
46.64 ¡À 0.99c |
218.87 ¡À 0.95c |
|
20 |
65.98 ¡À 1.33b |
30.26 ¡À 0.94a |
47.91 ¡À 0.26b |
216.87 ¡À 1.22d |
|
25 |
65.01 ¡À 0.69b |
31.49 ¡À 0.67a |
48.81 ¡À 0.26a |
215.37 ¡À 1.35e |
|
P. chrysosporium |
0 |
68.28 ¡À 1.76a |
22.84 ¡À 1.40c |
46.33 ¡À 0.77c |
222.60 ¡À 0.35e |
5 |
69.40 ¡À 1.10a |
23.45 ¡À 1.01c |
47.09 ¡À 0.72c |
217.70 ¡À 1.84e |
|
10 |
69.57 ¡À 0.97a |
25.40 ¡À 1.22c |
50.33 ¡À 0.74b |
212.08 ¡À 1.51d |
|
15 |
69.19 ¡À 1.02a |
29.13 ¡À 1.83b |
50.71 ¡À 0.59b |
197.99 ¡À 4.33c |
|
20 |
69.67 ¡À 1.46a |
36.22 ¡À 1.74a |
53.89 ¡À 1.33a |
185.97 ¡À 0.44ab |
|
25 |
70.05 ¡À 2.27a |
37.71 ¡À 1.85a |
54.73 ¡À 0.99a |
176.24 ¡À 1.33a |
|
T. harzianum |
0 |
68.28 ¡À 0.91c |
23.00 ¡À 2.88f |
46.08 ¡À 0.26b |
224.57 ¡À 0.67a |
5 |
69.50 ¡À 2.80c |
25.19 ¡À 2.59e |
47.11 ¡À 0.59a |
217.81 ¡À 0.71b |
|
10 |
70.83 ¡À 0.99bc |
31.69 ¡À 1.46d |
48.08 ¡À 0.97a |
207.63 ¡À 2.43c |
|
15 |
72.62 ¡À 2.79abc |
38.85 ¡À 0.88c |
45.50 ¡À 1.01bc |
191.10 ¡À 4.00d |
|
20 |
75.91 ¡À 3.15ab |
51.06 ¡À 2.39b |
37.89 ¡À 0.91d |
183.81 ¡À 5.78d |
|
25 |
76.42 ¡À 3.97a |
59.06 ¡À 2.90a |
31.99 ¡À 0.66e |
178.08 ¡À 6.05d |
|
T. reesei |
0 |
68.28 ¡À 1.26c |
23.54 ¡À 0.79f |
46.25 ¡À 0.11b |
223.61 ¡À 0.28a |
5 |
69.90 ¡À 2.29c |
34.04 ¡À 1.51e |
47.66 ¡À 1.49a |
196.29 ¡À 3.71b |
|
10 |
71.89 ¡À 2.54b |
44.33 ¡À 1.31d |
45.94 ¡À 1.13c |
170.27 ¡À 3.11c |
|
15 |
74.68 ¡À 3.35b |
52.55 ¡À 1.60c |
41.29 ¡À 0.88d |
158.48 ¡À 3.34d |
|
20 |
76.16 ¡À 3.02ab |
59.20 ¡À 1.58b |
38.75 ¡À 0.54e |
141.98 ¡À 0.72e |
|
25 |
76.62 ¡À 2.81a |
68.03 ¡À 2.54a |
33.27 ¡À 1.98f |
133.66 ¡À 3.62f |
Mean¡Àstandard
deviation. Values sharing
same letter(s) within each treatment are not significantly different at P>0.05.
CK represent a corn straw not inoculated with fungi; The T. reesei, T. harzianum,
and P. chrysosporium
denotes corn straw treated with these fungi and incubated for 25 days. WSS is
water soluble substance, HAL denotes humic acid like,
FAL represent fluvic acid like, and HML is humin like
Fig. 1: The TOC content of corn straw over the course of the
fermentation
The mean total
organic carbon (TOC) content of corn straw over the course of the fermentation
plus standard deviation. Bar means
followed by different capital letter-cases are significantly different during
the fermentation period for the same treatment at P < 0.05. Means followed by different small letters are
significantly different among each day of fermentation per treatment
T.harzianum and P. chrysosporium respectively increased by 15.24, 10.89
and 4.98% compared with CK treatment.
The relative
contents of FAL under CK and P. chrysosporium
increased with time (Fig. 2c), while that of T. harzianum
and T. reesei treatments decreased after day
10. At the end of fermentation period, T. reesei
and T. harzian treatments decreased FAL by
1.78 and 2.70%, respectively. While P. chrysosporium
increased FAL by 3.72% compared with CK.
The carbon content
of HML decreased along with the fermentation process (Fig. 2d). The magnitude
change followed the order of: T. reesei>T.harzianum>P. chrysosporium>
CK. The three fungi treatments respectively, decreased HML by 17.18, 8.41 and
7.50% compared with CK treatment in day 0.
Effects of different fungi
inoculant on the PQ of humic-like substances
The PQ value represents the
percentage of HAL in HLE which can reflect the degree of humification of straw
residues (Fig. 3). The PQ values
of corn straw inoculated with fungi showed different degrees of increasing
trend, implying that the formation rate of HAL is eventually higher than FAL.
The humification degree was highest in T. reesei treatment followed by T. harzianum, P. chrysosporium
and CK. At the end of fermentation, the PQ values of T. reesei,
T. harzianum and P. chrysosporium
respectively increased from 33.73, 33.27 and 33.01 in 0 day to 67.16, 64.86 and
40.78% in the 25th dat. The CK treatment showed a slight change in
PQ (%), increasing from 36.52 to 39.21%.
Fig. 2: Changes of relative content of each HLS component in
different treatments: WSS (a); HAL (b); FAL (c); HML (d)
Fig. 3: Changes in PQ value under different treatments
The mean PQ value
of corn straw over the course of the fermentation plus standard deviation. Bar means followed by different capital letter-cases
are significantly different during the fermentation period for the same
treatment at P < 0.05. Means
followed by different small letters are significantly different among each day
of fermentation per treatment
Discussion
The TOC content of corn straw
decreased during the 25 days of fermentation, which could be related to dynamic
changes in C content of humus components. Hu et al. (2017) suggested that the C content of HAL in organic matter
play an important role in SOC storage. Similar to our study Li et al.
(2015), found that microbial inoculant significantly reduced TOC and HML but
increased the content of HA in fermented organic materials. In the present
study, the highest C content of HAL in corn straw across all treatments was
measured in the 25th day and T.
reesei showed greatest HAL than other treatments.
Yang et al. (2019) also observed
greater HAL content in straw inoculated with T. reesei. This could be ascribed to
greater polymerization of FAL into a more complex and relatively stable HAL
fraction and is consistent with the Polyphenolic theory of humus formation. It
could mean the T. reesei
and T. harzianum in our study preferentially
utilized HAL as a source of C and energy, thusly enabling synthesis of HAL
fractions. These suggestions are supported by a rapid decline in FAL content in
corn straw treated with T. reesei and T. harzianum.
At the early
stages of fermentation (0–10 days), the C content in FAL across all treatments
was nearly 2 times higher than that in HAL, and humification degree (PQ) was
less than 50%. Tuomela et al. (2002) showed that the FA content in immature compost is
high than HA content and Hu et al. (2019),
found that corn straw application resulted to effective accumulation of FA in
the soil. However, as straw decomposition progressed, a gradual decrease in FAL
content in corn straw inoculated with T. reesei
and T. harzianum and increase in HAL and PQ
was observed, which coincides with previous studies
(Yang et al. 2019). Zhang and Dou
(2005) investigated the fermentation of corn straw for 15 days and also found
that, in the initial stage of straw decomposition HA was produced at a lower
rate than FA. However, over time FA was slowly transformed into HA, and Wei et al. (2018) showed that formed HA
could be in a loosely bound from or stable from. In the present study the
results suggest that T. reesei and T. harzianum were
more effective in the transformation of FA into stable HA fraction.
While, P. chrysosporium favoured accumulation of both HAL and FAL
contents with the duration of fermentation. Huang (2006) discussed that
the degradation of lignin by P.chrysosporium
occurs in the secondary metabolic stage, and the degradation is dominated by
oxidation. Therefore, it could mean corn straw was firstly decomposed into FA,
or directly decomposed into CO2, and then FA was converted to HA
fraction.
The utilization of
HML C is the leading cause to the decrease in TOC content. Before the 10th
day, the three treatments except T. reesei has
used less than 3% of HML as source of C and energy, which confirms that HML is
a stable fraction. Another alternative would suggest that microorganisms
proliferate at an extremely rapid rate in the early stages of fermentation and
a part of microbial biomass carbon might have replenished C losses in HML which
has been utilized by the microorganisms. Thusly, changes in HML were slightly
insignificant in the first 10 days of fermentation. According to Paul (2002)
part of dead microorganisms are not recycled through the microbial pool and
hold the possibility of entering humus C. Our results further showed that in
the later stages of straw fermentation, the relative content of HML decreased
rapidly, because the readily available sources of C were nearly depleted.
Therefore, microorganisms could have started to utilize the HML as a source of
C and energy. Among the three fungal strains, the T. reesei demonstrated the strongest
ability to utilize and transform HML, while P. chrysosporium and T. harzianum
showed seldom difference.
In the present
study, we found that the composition of corn straw HLS in the solid
fermentation process possessed high C content in WSS. The T. reesei, T. harzianum
and P. chrysosporium inoculants can
effectively degrade the C found primarily in HLS, which leads to the conversion
of the small molecular substrate detected in a ¡°free state¡± in WSS. Because T.
reesei has the strongest ability to reduce HLS
contents, its corresponding WSS increment was also the largest compared with
treatments. It is well established that WSS is an important C substrate for
microbes (Dou et al. 2007). Gregorich et al. (2003) pointed out that WSS is
easily utilized by microbes shortly after inoculation. However, in this study,
the increase in WSS content after fungi inoculation could be attributed to the
degradation preference by fungi. The fungi inoculants might have preferred to
degrade complex molecules, resulting in the continuous accumulation of new WSS
during corn straw decomposition. The increase in WSS in fungi treatments tended
to be steady on the days 20 and 25, which confirmed the idea of easy
consumption of WSS proposed by Gregorich et al. (2003).
Conclusion
In the 25 day solid-state
fermentation, the three fungi inoculants significantly increased the relative
content of WSS, FAL in corn straw and reduce the relative content of HML. The T.
reesei have the most significant effect on humus
formation. During decomposition and transformation of corn straw into humus,
FAL is formed first, then later transformed into HAL. P.chrysosporium preferentially promote
formation of FAL, while the two species of Trichoderma preferentially
promote formation of HAL. In the last day of fermentation, T. reesei improved corn straw
humification, HAL, and WSS carbon contents by 40.62, 53.71 and 15.15%,
respectively, compared with CK. Fungi can effectively promote the
transformation of corn straw into humus, and their transformation dynamics is
consistent with polyphenol theory. The T. reesei
has the best ability to synthesize HAL and WSS fractions in HLS.
Acknowledgement
The Project is supported by
National Key Research and Development Program of China (2016YFD0200304) and
National Natural Science Foundation of China (41571231;
31670527).
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