Journal of Pharmacognosy and Phytochemistry 2020; 9(5): 1556-1562

E-ISSN: 2278-4136
P-ISSN: 2349-8234
www.phytojournal.com Production and characterization of vermicompost
JPP 2020; 9(5): 1556-1562
Received: 20-07-2020 and biochar from rice straw
Accepted: 22-08-2020

Amritha K Amritha K and Jayasree Sankar S

Department of Soil Science &
Agricultural Chemistry, College DOI: https://doi.org/10.22271/phyto.2020.v9.i5v.12557
of Horticulture, Vellanikkara,
Kerala Agricultural University,
Thrissur, Kerala, India Abstract
“Soil Quality” is the capacity of a specific kind of soil to function within natural or managed ecosystem
Jayasree Sankar S boundaries to sustain plant and animal productivity, maintain or enhance water and air quality and support
Department of Soil Science & human health and habitation. One of the major threats for soil quality is the intensive use of agrochemicals
Agricultural Chemistry, College coupled with soil degradation processes. Vermicomposting has emerged as a promising eco-friendly
of Horticulture, Vellanikkara, approach for recovering degraded soils and equally good is the application of biochar, a carbonaceous
Kerala Agricultural University, material produced from pyrolysing biomass for both remediation and for soil carbon storage potentials.
Thrissur, Kerala, India The vermicompost and biochar mentioned in the present study are rice straw based products. The lower
bulk density of straw and its products compared to soil shows its promising role in reducing the soil bulk
density and increasing the porosity besides its capability to hold more water when applied to soil. The
process of vermicomposting helped to increase the nutrients viz., N, P, K, Ca, Mg, S, and silicon and
decrease that of carbon, cellulose and lignin thereby narrowing down the C: N ratio. Conversion of residues
into biochar helped to increase content of most of the nutrients in the final product, while nitrogen, cellulose
and lignin content were found to decrease after pyrolysis. Pyrolysis process imparted more recalcitrant
character by increasing aromatic compounds as evidenced from FT-IR analysis, thus ensuring its suitability
for carbon sequestration.

Keywords: Rice straw, vermicompost, biochar, physico-chemical properties, surface morphology,

structural chemistry

Introduction
There is a growing interest of late in the agricultural sector to explore the possibility of crop
residue based organic amendment application with thrust on its merits and demerits. Ideally the
crop residue management systems should be chosen in a way with minimal adverse effects on
the environment at the same time optimising crop yields, resembling the site specific nutrient
management.
Rice, the staple food across Asia is the most important human food crop in the world that has
fed more people over a larger time than has any other crop. It also is the most important residue
producing crop in Asia contributing to 84 per cent of total world production. Assuming a harvest
index of 0.5, nearly 200 mt of rice straw is produced in India annually (Benbi and Yadav, 2015)
[2]
.
Crop residues are good sources of nutrients and primary source of organic matter. Rice straw at
harvest contain 0.5-0.8 per cent N, 0.07-0.12 per cent P2O5, 1.16-1.66 per cent K2O, 0.05-0.1
per cent S, and 4-7 per cent silicon. This translates to about 40 per cent of the nitrogen, 30-35
per cent of the phosphorus, 80-85 per cent of the potassium, 40-45 per cent of sulphur and 80
per cent silicon taken up by the plant and which remain in the vegetative parts at maturity
(Dobermann and Fairhurst, 2000)[7]. Rice straw stand out from other straws with its higher
silicon and lower lignin content which designates it as a ligno-cellulosic biomass with 35-40 per
cent cellulose, 25-30 per cent hemicellulose, and 10-15 per cent lignin (Thygesen et al., 2003)
[26]
.
In principle, rice straw can be put into varied uses such as for soil fertility improvement through
carbonisation and composting, in bio energy production, in the making of bio fibre and other
industrially useful products. Soil incorporation of straw is a good proposition for enhancing soil
Corresponding Author: fertility, but the current intensive cropping systems leaves too little time for its proper
Amritha K decomposition and related effects. With hardly few viable options, open field burning is very
Department of Soil Science &
commonly practiced for straw disposal and this has increased dramatically over the last decade
Agricultural Chemistry, College
of Horticulture, Vellanikkara, causing emission and persistence of toxic gases and smog that extends even to adjoining places.
Kerala Agricultural University, In addition, it leads to nutrient losses and killing of beneficial soil flora and fauna.
Thrissur, Kerala, India
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It is in this context that vermicomposting and pyrolysis turn out decomposition with occasional turning at weekly interval
as effective technologies for conversion of rice straw into followed by cowdung slurry application to ensure proper
quality products capable of enhancing soil quality and reducing aeration and moisture content. After three weeks, the
the environmental footprint together with increasing income composting worm Eisenia foetida was introduced into the tank
generation from rice production systems. @2000 Nos. per tank. Turning was done once in five days to
Composting is an excellent waste management strategy, which maintain homogeneity. Care was taken to ensure an optimum
yields biologically stable organic matter. Crop residues contain moisture content of 40 to 50 per cent by sprinkling water. The
the nutrients in their recalcitrant forms which on composting materials gained maturity by 62 days as evidenced by the
get transformed into humified matter through the activity of change in appearance, colour and odour. Sprinkling of water
soil biota. Biochar on the other hand is a novel technology for was stopped at this point to enable the worms to migrate down
agriculture productivity and is a unique weapon to combat and cling to the vermi bed. Composted material was collected
against climate change and global warming via sequestration from the top of the ferrocement tanks without disturbing the
of atmospheric CO2.The most prominent benefit of biochar is vermi bed and kept in shade for two days. The composted rice
its longevity; it can remain in the soil for years. Thus, biochar straw was sieved and stored in the laboratory in plastic
application helps to reduce the repeated addition of soil containers for analysis.
amendments and minimise the possibility of new contaminants The biochar used in the present study was produced from straw
reaching the soil through addition of synthetic soil utilising kilns specially designed and fabricated for the purpose
amendments. Present investigation is an earnest attempt on rice using metallic drums of 87 cm height and 57 cm diameter.
straw management through vermicomposting and pyrolysis Straw was loaded through the inlet at the top and the process of
undertaken in the Department of Soil Science and Agricultural pyrolysis was initiated using a little diesel. With the reduction
Chemistry during 2017-2020. in intensity of smoke produced, closed the inlet to slow down
air entry thus preventing the material getting converted into
Materials and Methods ash. After one hour duration the kiln was allowed to cool and
Straw was collected from the farmers in Thrissur district after the finished product ‘rice straw biochar’ was collected from the
the harvest of rice. Further materials required for the research outlet located towards the bottom side. Pyrolysis temperature
work viz., vermicompost and biochar were produced from the was recorded using an Infrared thermometer and it was found
straw using the methodology furnished underneath. to vary between 350 to 600°C throughout the process. The
Vermicomposting of the rice straw was carried out in product was crushed and passed through 2 mm sieve and
ferrocement tanks of 1m3 diameter and 300 kg capacity. The characterised using standard procedures detailed in Table 1.
bottom portion of the tank up to one foot height was filled with The information on surface composition and topography of the
a layer of coconut husk, positioned with their concave side straw and its products were studied using Scanning Electron
facing upwards. Rice straw and cowdung was mixed in 8:1 Microscope (SEM) that enabled for creating a high resolution
ratio and this mixture was transferred into the ferrocement tank image. The samples were smeared on a small piece of adhesive
to form a layer of 30-45 cm thickness. Cowdung slurry was carbon tape which was fixed on a brass stub. The samples then
sprinkled over this layer. This process was continued till the subjected to gold coating using sputtering unit for 10 seconds
tanks were filled to their fullest capacity maintaining a top layer at 10 mA of current. The gold coated samples were placed in
of cowdung slurry which was then covered using a moistened the chamber of SEM and secondary electron or back scattered
gunny bag. The material was left as such to allow partial electron images are recorded.

Table 1: Standard procedures employed for characterization

Parameters Methods
Moisture Moisture meter (Model: MB23)
Bulk density Cylinder method (Piper, 1966) [22]
pH Potentiometry
Jackson, 1958[9]
Electrical conductivity Conductometry
Total carbon
CHNS Analyzer (Model : Elementar’s vario EL cube)
Total Nitrogen
Total Phosphorus Colorimetry
Jackson,1958 [9]
Total Potassium Microwave digestion system with HNO3 Flame photometry
Total Calcium (Model: MARSX 250/40) ICP-OES
Total Magnesium (Model: Optima® 8x00 series)
Total Sulphur CHNS Analyzer (Model : Elementar’s vario EL cube)
Silicon Digestion (Ma et al., 2002) [18] and estimation using ICP-OES (Model: Optima® 8x00 series)
Cellulose Sadasivam and Manickam (1996) [23]
Lignin Klason (1923) [12]

Structural chemistry of straw and their products were index was then directed at a certain angle on to the optically
characterized using Fourier Transform Infra-Red spectrometer dense diamond crystal. This reflectance helped to create an
equipped with Attenuated Total Reflectance (FTIR-ATR) evanescent wave that extended beyond the surface of the
containing diamond crystal (Model: Perkin Elmer spectrum crystal on to the sample held in contact with it. The evanescent
100 FT-IR spectrometer with ATR). The methodology wave got alternated in those regions of the IR spectrum where
included transferring samples to the small crystal area located the sample absorbed energy. These alienated beam then
on the ATR top plate, followed by positioning the pressure over returned to the crystal, exist via opposite side of the crystal and
crystal/ sample area and applying force till the pressure gauge got directed to the detector in the IR spectrometer. The detector
registered force sufficient enough to push the sample on the recorded the alienated IR beam as an interferogram signal
diamond surface. An infra-red (IR) beam with a high refractive which could be used to generate an IR spectrum. FT-IR spectra
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were acquired at the middle infra-red region of 4000-400cm-1. reduced the bulk density of biochar (0.64 Mg m-3). The lower
Organic compounds have fundamental vibration bands in the bulk density of straw and products compared to soil indicated
mid infra-red region, because of which this region is widely its promising role in reducing the soil bulk density and
used in IR spectroscopy. increasing the porosity thus its ability to hold more water when
applied to soil.
Results and Discussions
Vermicompost and biochar was produced from rice straw and Electro-chemical properties
it could be seen that the recovery was more (74.38%) when pH is an important electro-chemical property controlling the
straw was converted into compost with the help of residue availability of nutrients. Straw, vermicompost, and biochar
feeding earthworms than as biochar (19.86%) through were alkaline in nature, having a pH value 7.81, 8.71, and 9.24
pyrolysis. Reports say that the biochar yield is highly respectively. The increase in pH after vermicomposting
dependant on the pyrolysis conditions such as temperature, probably resulted from the release of ammonia due to the
heating rate and residence time (Uzun et al., 2006; Tsai et al., proteolytic process. These results are in agreement with the
2007) [29, 28] and is also greatly influenced by physical, chemical findings of Thiyageshwari et al. (2018) [25]. High solubility of
and biological properties of the raw materials used (Lehmann, nutrients in earthworm casts could be another reason for the
2007; Chan and Xu, 2009; Basta et al., 2011; Conz et al., 2017) rise in pH in the present study. The pH of biochar was
[16, 3, 1, 6]
. Elangovan (2014) [8] reported recovery percentage of comparatively higher than the straw as well as vermicompost.
12 to 40, when pyrolysis was done using different biological This might be due to the production of alkali salts during
residues. Phuong et al. (2015) [21] concluded that the decrease pyrolysis process. At a higher temperature, the alkali salts
in biochar yield might be due to the thermal decomposition of begin to separate from the organic matrix thus increasing the
organic material present in the residues. The properties of pH consequently. Highest pH recorded in biochar could be
straw, vermicompost, and biochar are given in Table 2. supported by high calcium and magnesium content as shown in
the results of the present study.
Physical properties Electrical conductivity is a measure of concentration of soluble
Comparatively, higher moisture content was observed in salts. Electrical conductivity of straw, vermicompost, and
vermicompost (23.68%) than straw (7.24%) and biochar biochar was 0.50, 1.15, and 0.86dSm-1 respectively. The higher
(3.62%). Pyrolysis process reduced the moisture content of electrical conductivity might be due to the presence of soluble
final product than the raw materials. However, composting salts. The present study revealed that electrical conductivity
increased the moisture content. increased on vermicomposting. Decomposition of substrates
Straw is yellow in colour whereas the resultant vermicompost and subsequent release of exchangeable bases would have
was brown in colour which might be due to the presence of increased electrical conductivity of the compost. In a study on
humic substances in vermicompost. With the progress in composting using agricultural by-products, Chandna et al.
composting a series of organic acids are produced that changes (2013) [4] also reported an increase in initial substrate electrical
the colour of the matured compost. Research results conductivity on composting. Loss of biomass through the
conclusively indicates the role of fulvic and humic acids in biotransformation of organic materials and subsequent
colour development. Biochar produced are black in colour due mineralization of nutrient elements could have attributed to the
to the high carbon content. No foul odour was experienced increment in electrical conductivity. The electrical
either from the straw or its products. conductivity of biochar was also higher than the residual
biomass but comparatively lower than vermicompost because
Table 2: Characterization of straw, vermicompost, and biochar of lower nutrient composition in biochar than compost.
Parameters Straw Vermicompost Biochar
1.Physical properties Chemical properties
Moisture (%) 7.24 23.68 3.62 The process of vermicomposting helped to concentrate the
Colour Yellow Brown Black nutrients viz., N, P, K, Ca, Mg, S, and silicon. The increase in
Odour Absence of foul odour nitrogen and decrease in lignin helped to narrow down the C:
Bulk density N ratio from71.20 to 14.83. The process of pyrolysis that
0.80 0.78 0.64
(Mg m-3) yielded biochar also proved to be a nutrient accumulating
2. Electro-chemical properties method. Much alike vermicomposting, the content of lignin
pH 7.81 8.71 9.24 and cellulose got reduced here also. The difference between
EC (dSm-1) 0.50 1.15 0.86 biochar production and vermicomposting in terms of nutrients
3. Chemical properties was the reduction in nitrogen following pyrolysis
Carbon 36.45 18.25 42.17 After vermicomposting, carbon content of straw got reduced
Nitrogen 0.52 1.23 0.44 remarkably due to the combined action of earthworm ingestion
Phosphorus 0.18 0.34 0.22
% and decomposition by microbes. The carbon content in
Potassium 1.29 1.31 1.41
compost is the major source of energy for the microorganisms.
Calcium 543.12 582.45 548.26
Magnesium mg kg-1 249.20 370.17 260.02
The values on carbon content of biochar obtained from the
Sulphur 523.08 540.00 528.98 present study revealed its highly carbonaceous nature. The
Silicon 5.08 13.87 15.38 increased carbon of biochar indicates that pyrolysis
Cellulose 38.10 12.26 2.81 temperature promotes carbonization (Chun et al., 2004) [5].
% This promotion was due to high degree of polymerization
Lignin 12.06 8.84 4.74
C/N ratio 71.20 14.83 95.84 leading to more condensed carbon structure in the biochar
(Lehmann and Joseph, 2009) [17].
The increase in particle size during vermicomposting due to the The increased nitrogen content of the compost is due to the
amalgamation of small particles resulted in reducing the bulk mineralization of proteins present in the substrates to nitrate
density of vermicompost (0.78 Mg m-3). Pyrolysis process also and ammoniacal forms. The nitrogen content in the cowdung
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also contributed to increase in total nitrogen of vermicompost. nutrition. Rice residues contain high quantity of silicon.
It is normally seen that when organic matter reduction is more Vermicomposting and charring enhanced the silicon content.
than the loss of NH3, nitrogen concentration usually increases. Compared to vermicomposting, charring has a great influence
Total nitrogen content was found to decrease with pyrolysis on the release of silicon. Xiao et al. (2014) [30] reported that
process. This might be due to the volatilization loss of nitrogen pyrolysis temperature caused the intense cracking of carbon
during pyrolysis. When plant biomass is subjected to pyrolysis, components, and thus the silicon located in the inside tissue
their nitrogen containing structures, i.e., amino sugars, amino was exposed to cause enhancement of silicon content in the
acids and amines, get transformed into heterocyclic aromatic final product “biochar”.
structures (Koutcheiko et al., 2006) [14]. Lignin and cellulose content decreased after composting and
The phosphorus content of the vermicomposts was higher than charring. The results are in line with the findings of Zhang et
the straw. The phosphorus content in the straw as well as in al. (2015) [31], who reported that thermal degradation of
cowdung might have contributed to increase in phosphorus in cellulose and lignin occurs under high temperature during
final vermicompost. Mineralization and mobilization of pyrolysis. The extent of reduction in cellulose was 67.80 per
phosphorus by bacterial and phosphatase activity of cent in vermicompost. Compared to the vermicomposts, the per
earthworms could be the main reason of phosphorus increase cent reduction of cellulose was high after charring (92.62 %).
in vermicompost (Tripathi and Bhardwaj, 2004) [27]. When The extent of reduction in lignin was lower compared to
organic matter passes through the gut of earthworm, some cellulose in both vermicompost (26.69 %) and biochar (60.69
phosphorus is converted into more available form. The release %). Lignin is a relative complex compound having a cross-
of phosphorus in the available form is performed partly by linked phenolic-type structure which does not easily
earthworm gut phosphatase and further release of phosphorus breakdown. Due to its aromatic structure, it is more chemically
can be ascribed to the phosphorus solubilizing microorganisms stable and heat resistant than cellulose.
present in the worm casts (Suthar, 2008) [24]. Charring enhances Carbon to nitrogen ratio serves as a reliable parameter for the
P availability from residues. This is because with combustion, maturity of compost. A remarkable change in the C: N ratio
there is disproportionate volatilization of carbon which leads to was noticed after composting. The C: N ratio of vermicompost
cleavage of organic phosphorus bonds thus yielding biochar was 14.83. The improvement in nitrogen and lowering of
rich in soluble salts of phosphorous (Knoepp et al., 2005) [13]. carbon resulted in the lowering of the C: N ratio, which is an
The increase in potassium content in the vermicomposts important criterion for a compost to be fully mature. The results
suggests that earthworms has symbiotic gut microflora with are in conformity with the findings of Thiyageshwari et al.
secreted mucus and water to increase the degradation of (2018) [25]. The C: N ratio of biochar was higher than the
ingested substrates and release of metabolites (Khwairakpam vermicompost and straw. The increase in carbon content and
and Bhargava, 2009) [11]. Chandna et al. (2013) [4] also opined decrease in nitrogen by pyrolysis process might be the reason
that during the composting of agricultural substrates, organic for high C: N ratio.
carbon decreased, whereas total N, P and K increased with
time. The nutrient content in biochar was comparatively lower Surface morphology
than the vermicomposts. Whereas, the potassium content was The SEM micrograph of straw exhibited a complex
found to be more in biochar. This might be due to the ash morphology with cell wall composition (Figure 1).
content in the biochar.
Upon vermicomposting, calcium content was found to be
increased. Calcium enrichment occurs when the substrates pass
through the digestive tract of earthworms. Earthworms were
reported to captivate calcium in excess from their food and
transfer it to calciferous glands, which contain carbonic
anhydrase enzyme which catalyse the fixation of CO2 as
CaCO3 concretions before being excreted through the digestive
tracts (Padmavathiamma et al., 2008) [20]. The bicarbonates
produced in excess of earthworm metabolic requirement were
excreted as cast material, thus increasing the calcium content
in the final vermicompost. The indistinguishable significance
of composting and vermicomposting in enriching the compost
with calcium content was earlier reported by Mayadevi (2016)
[19]
. Pyrolysis process increases the calcium content in the final
product. The increase in calcium content in the biochar might
be due to the release of calcium during pyrolysis.
Magnesium content was highest in vermicompost than biochar
and straw. Only slight variation in sulphur content was
Fig 1: SEM micrograph of rice straw
observed among the straw and their products such as
vermicompost and biochar. Both vermicomposting and
SEM image of vermicompost (Figure 2) showed a highly
pyrolysis go in favour of increasing nutrient content in the final
fragmented, porous and disaggregated structure contrary to the
product, though its often comparatively higher in
rice straw. This might be due to the activity of earthworms
vermicompost. This might be due to the biological as well as
during vermicomposting.
thermal decomposition of straw during vermicomposting and
Biochar (Figure 3) exhibited a highly disordered and complex
charring respectively.
morphology with longitudinal channels and pores under 50µm
Silicon is considered as a beneficial element for crop growth,
resolution. The particles gave a broken or distorted appearance
especially for crops under Poaceae family. Rice is a typical
thus resembling the plant structure with remains of vessels, the
silicon accumulating plant and it benefits from silicon
larger diameter tubes used for the transport of fluids and
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nutrients. The results are in conformity with the findings of

Phuong et al. (2015) [21]. Structural chemistry
The structure of straw, vermicompost, and biochar were
analysed using FT-IR. Each peak in FT-IR is assigned with
corresponding functional groups. The functional group of
cellulose, hemicellulose and lignin could be seen in the
spectrum. In FT-IR spectrum presence of silicon is illustrated
by Si-O-Si and Si-H bond. In the present study such bonds were
identifiable in both the straw and its products (Figure 4-6).
Silicon is a major component in chemical structure of rice, and
is typical of its recalcitrant property (Jindo et al., 2014) [10].
In vermicompost, the O-H stretching of hydroxyl groups from
alcohols and carboxylic acid, and N-H stretching vibrations
from amides and amines are indicated by a band from 3300-
3500 cm-1. Vermicompost had significant level of nitrogen rich
compounds and low level of aliphatic or aromatic compounds
compared to straw, which was confirmed by the FT-IR
analysis. The peaks at 2943.75 cm-1 and 2896.31 cm-1 are
assigned to aliphatic methylene groups, found to be decreased
Fig 2: SEM micrograph of vermicomposted straw in vermicompost compared to the straw.
The reduction in methylene peaks might be due to the decrease
in CH2 and CH3 groups, which suggested the decomposition of
aliphatic compounds after composting. The easily
biodegradable compounds are decreased after
vermicomposting. The presence or absence of spectral peaks
for functional groups indicated the stabilization or degradation
of residue during bioconversion process (Mayadevi, 2016) [19].
The complete disappearance of O-H stretching (>3000 cm-1)
and almost disappearance of aliphatic C-H stretching (3000-
2500 cm-1) was noted in the FT-IR spectrum of biochar. While,
peaks arising from the aromatic stretching became more
apparent. This implies that greater dehydration and increased
aromatization occurred during pyrolysis process. Lee et al.
(2010) [15] reported that charring temperature modifies the
functional groups, and thus aliphatic carbon groups decreases
but aromatic carbon increases. Pyrolysis process created more
recalcitrant character by increasing aromatic compounds, and
is thus suitable for carbon sequestration.
Fig 3: SEM micrograph of rice straw biochar

Fig 4: Fourier-transform infrared (FT-IR) spectrum of rice straw

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Fig 5: FT-IR spectrum of vermicomposted rice straw

Fig 6: Fourier-transform infrared (FT-IR) spectrum of rice straw biochar

Acknowledgements 5. Chun Y, Sheng GY, Chiou CT, Xing BS. Compositions

The authors are grateful to Kerala Agricultural University and and sorptive properties of crop residue-derived chars.
Kerala State Council for Science, Technology, and Environ. Sci. Technol. 2004; 38:4649-4655.
Environment for providing technical and financial assistance 6. Conz RF, Abbruzzini TF, deAndrade CA, Milori DM,
during the course of investigation. Cerri CE. Effect of pyrolysis temperature and feedstock
type on agricultural properties and stability of biochars.
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