molecules

Review
A Review on Current Status of Biochar Uses in Agriculture
Tara Allohverdi 1 , Amar Kumar Mohanty 1,2, *, Poritosh Roy 1,2 and Manjusri Misra 1,2, *

1 Bioproducts Discovery and Development Centre, Department of Plant Agriculture, Crop Science Building,
University of Guelph, Guelph, ON N1G 2W1, Canada; tallohve@uoguelph.ca (T.A.);
poritosh@uoguelph.ca (P.R.)
2 School of Engineering, Thornbrough Building, University of Guelph, Guelph, ON N1G 2W1, Canada
* Correspondence: mohanty@uoguelph.ca (A.K.M.); mmisra@uoguelph.ca (M.M.)

Abstract: In a time when climate change increases desertification and drought globally, novel and
effective solutions are required in order to continue food production for the world’s increasing
population. Synthetic fertilizers have been long used to improve the productivity of agricultural soils,
part of which leaches into the environment and emits greenhouse gasses (GHG). Some fundamental
challenges within agricultural practices include the improvement of water retention and microbiota
in soils, as well as boosting the efficiency of fertilizers. Biochar is a nutrient rich material produced
from biomass, gaining attention for soil amendment purposes, improving crop yields as well as for
carbon sequestration. This study summarizes the potential benefits of biochar applications, placing
emphasis on its application in the agricultural sector. It seems biochar used for soil amendment
improves nutrient density of soils, water holding capacity, reduces fertilizer requirements, enhances
soil microbiota, and increases crop yields. Additionally, biochar usage has many environmental



benefits, economic benefits, and a potential role to play in carbon credit systems. Biochar (also known


as biocarbon) may hold the answer to these fundamental requirements.
Citation: Allohverdi, T.;
Mohanty, A.K.; Roy, P.; Misra, M.
Keywords: biochar; soil amendment; agriculture; sustainability
A Review on Current Status of
Biochar Uses in Agriculture.
Molecules 2021, 26, 5584.
https://doi.org/10.3390/
molecules26185584
1. Introduction
Anthropogenic effects of climate change and unsustainable agriculture have caused
Academic Editors: Farid Chemat, drought, fertilizer leaching and lack of food security worldwide [1]. With immediate and
Elena Ibáñez and Sylvain Antoniotti looming future problems, biochar may be the key to developing a sustainable future while
adding valued products to the circular economy model. Researchers desire a potential
Received: 5 August 2021 solution to improve soil quality by applying biochar for soil amendment and improve
Accepted: 9 September 2021 the sustainability of agriculture [2]. Aspects of soil that determine good quality include
Published: 14 September 2021 texture, capacity to retain and sustain microbial activity and ability to retain nutrients and
moisture [3]. Usually, biochar is used without any treatment, but recent research shows that
Publisher’s Note: MDPI stays neutral physically or chemically modified biochar could be applied to improve performance [4].
with regard to jurisdictional claims in
Biochar has been found to have a positive effect on soil health and plant yields, while
published maps and institutional affil-
keeping the health of soil intact [5]. The ability to increase yields without synthetic fertilizers
iations.
or soil additives is a challenge for modern sustainable agricultural methods [6]. In order to
achieve sustainability, retention of water and nutrients in agricultural soils are fundamental
qualities to address [6]. Carbon dioxide (CO2 ) emissions from fossil fuel use is now known
to be the major driving force behind climate change [7]. Capturing this atmospheric carbon
Copyright: © 2021 by the authors. can attenuate the rising greenhouse gas (GHG) emissions. The agricultural sector is one
Licensee MDPI, Basel, Switzerland. area in which huge amounts of biochar can be used to store carbon [7]. The way in which
This article is an open access article
soils can act as a carbon sink depends on a few factors (varying porosity namely) but
distributed under the terms and
ultimately are able to sequester varying degrees of carbon in soils [4]. In general, most
conditions of the Creative Commons
new agricultural land comes from the tropical rainforests that are already under threat
Attribution (CC BY) license (https://
of deforestation [8]. Currently, China and the United States are the forerunners in using
creativecommons.org/licenses/by/
biochar for agricultural purposes [9]. Even though studies exist on biochar amendment
4.0/).

Molecules 2021, 26, 5584. https://doi.org/10.3390/molecules26185584 https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 5584 2 of 17

for purposes of agriculture, they generally focus on single topics such as its effect on
microbiota [10], crop yield [11] or economic assessments [12]. It should be noted that this
study is unique in that multiple aspects are summarized while also elaborating on biochar
decomposition rates and emission rates and how these play into benefits and optimization
of biochar as a soil amendment for agricultural purposes. This study outlines how various
types of biochar have benefits and play a role in agricultural soil amelioration, combating
climate change and future endeavors of biochar use in agriculture (e.g., drought tolerance
and addition to the circular economy model).

2. Historical Use of Biochar for Soil Amendment

The first evidence of biochar use as a soil enhancer was the Terra Preta, also known
as the “Indian black earth”. Terra Preta is a type of soil initially discovered in Western
Amazonia [1]. It can be recognized by its dark color, its high degree of aggregate stability
provided by the presence of higher amounts of carbon and high nutrient content associated
to an increased microbial presence [1].
The most important aspect of the Terra Preta is the fact that biochar was intentionally
introduced to enrich the soil profile by prehistoric indigenous groups [1]. The use of
biochar as a soil amendment has allowed the tropical rainforest to thrive and flourish [13].
The Terra Preta of the Amazon rainforest illustrates how poor soils can be amended to
improve health and biomass fertility [2]. Tropical sandy soils are not naturally fertile,
life is mostly supported by the forest canopy providing organic matter [2]. On the other
hand, it has been found that the Terra Preta possesses higher levels of nutrients (nitrogen,
potassium, calcium and phosphorus) as well as better soil structure and rigidity—a more
secure organization of soil particles [2]. This improved structure is due to the highly stable
organic carbon incorporated in this soil [2]. Lima et al. evaluated the various properties of
the Terra Preta which led to the discovery of flakes from various types of mica in the soil
sub-layers that were once used in pottery [13]. A clear deduction can be made whereby
the clay ovens were indeed used to produce the char and that the clay particles entered in
the soil along with the resultant biochar [13]. It is believed that this practice dated back to
almost 2000 years [3]. The Terra Preta soils contain a vast array of microbial populations [4].
Particularly, there is a significant presence of acido-bacteria species, compared to other soils,
the Terra Preta have 25% increased diversity of bacterial species [4]. This is so important
because a variety of bacteria needs to be preset in soils to provide a nitrogen source to
growing plants [4]. The discovery of the Terra Preta indicates that the original people of
the Amazonia knew the technique of biochar production, or they probably intentionally
introduced this material in the soil to improve fertility [13].

3. Biochar Production Process

There are many ways to produce biochar and the method of production has a huge
impact on the resultant characteristics. Pyrolysis is the thermochemical conversion of
biomass in an oxygen starved/deprived atmosphere. Generally, pyrolysis process produces
bio-oil, syngas and biochar [5]. In dedicated instruments, this process is carried out in
the presence of an inert gas, typically nitrogen [6]. Figure 1 shows a simplified schematic
diagram of a pyrolysis reactor [14]. This type of reactor can be used for either slow or
fast pyrolysis [5]. The schematic portrays the input materials and output materials of the
pyrolysis process.
 Molecules2021,
Molecules 2021,26,
26,5584
x FOR PEER REVIEW 33 of
of1719

Figure1.1.Simple
Figure Simplepyrolysis
pyrolysisreactor
reactorschematic
schematic[14].
[14].[Copyright;
[Copyright;PLOS
PLOSONE,
ONE,2016].
2016].

The
Slow concepts
pyrolysisof slow
producesand fast morepyrolysis
biocharrefer
and to the
less heating
bio-oil andrate of the[15].
syngas process [7]. The
The opposite
heating rate determines if it is to be classified as fast or slow pyrolysis
also tends to be true for the process of fast pyrolysis [15]. There is some evidence to sug- [8]. A few seconds
togest
a few
thatminutes
specificindicate
surfacefast areapyrolysis
and porewhile volumeminutes to hours indicates
are manipulated slowtwo
via these pyrolysis [6].
processes,
During pyrolysis, the natural polymers in biomass (cellulose,
but it seems that other factors (such as maximum temperature and residence time) have ahemicellulose and lignin)
undergo a few transformations [9]. These natural polymers will break apart, cross-link,
greater effect [16]. It’s important to note that biochar characteristics are not always ho-
and then fragment, each at different temperatures [8].
mogenous even if the production method is similar [17]. The factors most affected by tem-
Slow pyrolysis produces more biochar and less bio-oil and syngas [15]. The opposite
perature are that of surface area and pH [18]. pH is affected by temperature since various
also tends to be true for the process of fast pyrolysis [15]. There is some evidence to suggest
functional groups, such as carboxyl, carbonyl and hydroxyl groups, are removed from
that specific surface area and pore volume are manipulated via these two processes, but it
biochar surface at different temperatures. While feedstock most strongly controls organic
seems that other factors (such as maximum temperature and residence time) have a greater
carbon content and mineral aspects [17], these effects seem to be true for biochar made
effect [16]. It’s important to note that biochar characteristics are not always homogenous
from animal waste as well [19].
even if the production method is similar [17]. The factors most affected by temperature are
Low temperature biochar (below 550 °C) has a lower ash content and exhibits less
that of surface area and pH [18]. pH is affected by temperature since various functional
crystalline structure which is more strongly affected by feedstock type [20]. As an exam-
groups, such as carboxyl, carbonyl and hydroxyl groups, are removed from biochar surface
ple, Yoshida et al., 2008, compared biochar yields from two different feedstock sources;
at different temperatures. While feedstock most strongly controls organic carbon content
eucalyptus
and mineral wood
aspectsand[17],banagrass
these effects (Pennisetum
seem to be pirpirem). Under similar
true for biochar made from production condi-
animal waste
tions, mature
as well [19]. banagrass produced higher biochar yields than that of eucalyptus wood
[21].Low
It was postulated that the structure of ◦banagrass is what formed
temperature biochar (below 550 C) has a lower ash content and exhibits less its fixed carbon rate.
The biochar yield can be manipulated through the feedstock selection
crystalline structure which is more strongly affected by feedstock type [20]. As an example, process [21]. Typi-
cally, asettemperature
Yoshida of pyrolysis
al., 2008, compared increases,
biochar yieldsso does
from twothedifferent
available water holding
feedstock sources; capacity
euca-
(WHC) [22]. Additionally, as seen in Figure 2, the biochar yield
lyptus wood and banagrass (Pennisetum pirpirem). Under similar production conditions, is dependent on both feed-
stock type and pyrolysis temperature [15]. The figure indicates
mature banagrass produced higher biochar yields than that of eucalyptus wood [21]. It was how biochar yield is in-
versely related to bio-oil yield at varying temperatures. The general
postulated that the structure of banagrass is what formed its fixed carbon rate. The biochar trend indicates that
with can
yield an increase of pyrolysis
be manipulated through temperature, biochar
the feedstock decreases
selection while[21].
process bio-oil yield increases.
Typically, as tem-
When looking
perature at desirable
of pyrolysis increases, biochar characteristics,
so does the available higher
waterdegree
holding ofcapacity
nutrients(WHC)
is a desired
[22].
outcome [23].asPyrolysis
Additionally, at temperatures
seen in Figure above
2, the biochar 400 is
yield °Cdependent
burns off most of the
on both nitrogen,
feedstock typepo-
tassium, and sulfur molecules and thus pyrolysis temperatures
and pyrolysis temperature [15]. The figure indicates how biochar yield is inversely relatedbelow allow those to exist
toinbio-oil
the resultant
yield atbiochar
varying[23]. temperatures. The general trend indicates that with an increase
of pyrolysis temperature, biochar decreases while bio-oil yield increases. When looking
at desirable biochar characteristics, higher degree of nutrients is a desired outcome [23].
Pyrolysis at temperatures above 400 ◦ C burns off most of the nitrogen, potassium, and
Molecules 2021, 26, 5584 4 of 17

Molecules 2021, 26, x FOR PEER REVIEW 4 of 19

sulfur molecules and thus pyrolysis temperatures below allow those to exist in the resultant
biochar [23].

Effectof
Figure2.2.Effect
Figure ofprocessing
processingtemperature
temperatureononbiochar
biocharand
andbio-oil
bio-oilyield
yield[adapted
[adaptedwith
withpermission
permission
from ref. [15], [Copyright, Elsevier, 2017].
from ref. [15], [Copyright, Elsevier, 2017].

Co-pyrolysisisisthe
Co-pyrolysis theprocess
processof ofpyrolyzing
pyrolyzingmore morethan
thanone
onetype
typeofoffeedstock
feedstocktotocreate
create
biochar and bio-oil [11]. One of these feedstocks could consist of a polymer
biochar and bio-oil [11]. One of these feedstocks could consist of a polymer such as poly- such as
polyethylene, mostly to improve quality bio-oil since a petroleum-based
ethylene, mostly to improve quality bio-oil since a petroleum-based feedstock adds morefeedstock adds
more characteristics typical of petroleum oil [11].
characteristics typical of petroleum oil [11].
Hydrothermal carbonization (HTC), also referred to as wet-pyrolysis, is a process that
Hydrothermal carbonization (HTC), also referred to as wet-pyrolysis, is a process
creates a sub-category of biochar known as hydrochars, which can also be used for soil
that creates a sub-category of biochar known as hydrochars, which can also be used for
amendment [24]. Additionally, the authors indicate that it is necessary to determine the
soil amendment [24]. Additionally, the authors indicate that it is necessary to determine
influence of hydrochars (and biochar in general) on plant growth since they are often used
the influence of hydrochars (and biochar in general) on plant growth since they are often
for soil amendment purposes to improve soil stability and pollutant removal [24]. The
used for soil amendment purposes to improve soil stability and pollutant removal [24].
ability to predict biochar characteristics is important and it is possible when the process
The ability to predict biochar characteristics is important and it is possible when the pro-
conditions are known [25].
cess conditions are known [25].
Various characteristics of biochar are malleable when production conditions are al-
Various characteristics of biochar are malleable when production conditions are al-
tered [26]. Table 1 represents the properties of different biochar. The slow pyrolysis process
tered [26]. Table 1 represents the properties of different biochar. The slow pyrolysis pro-
produces the highest biochar yield, followed by hydrothermal carbonization and lastly
cess produces the highestthe
co-pyrolysis producing biochar
lowestyield, followed
biochar by hydrothermal
yields [27–30]. carbonization
There seems and
to be no other
lastly co-pyrolysis
discernable trendsproducing
in N, O, H,the
S, lowest
P, K or biochar
pH. It isyields [27–30].
postulated There
that seems to beare
the differences nodue
otherto
discernable trends in N, O, H, S, P, K or pH. It is postulated that the differences
the feedstock type rather than the production conditions. The value of biochar in various are due to
the feedstock depends
applications type rather
on than the production
the biochar conditions. The value of biochar in various
compositions.
applications depends on the biochar compositions.
Table 1. Properties of different biochar.
Table 1. Properties of different biochar.
Feedstock Production Conditions Component, % Reference
Feedstock Production Conditions
C N O Component,
H % S P K Reference
Rice-straw Fast pyrolysis 800 ◦ C 36.2 C N 39.8 O H S P K [27]
CornRice-straw
cob Slow pyrolysis 600 ◦ C 79.1 36.2 4.25 39.8 10.1 [28]
[27]
Corn stover Fast600
Slow pyrolysis pyrolysis
◦C 800 °C
69.8 1.01 0.181 2.461 9.95 [28]
PeanutCorn
hull cob Slow pyrolysis 400 ◦ C 65.5 2.0
79.1 1.16 4.25 0.00162 0.0015 10.0
10.1 [28]
[28]
Corn stover Slow pyrolysis
Slow pyrolysis 300 C◦ 600 °C
59.5 0.137 1.705 7.33 [28]
Corn residue (Stover and cob)
Corn stover HTC 260 ◦ C (30 min) 57.51 ± 1.11 1.62 ± 0.04
69.8 35.12 ± 1.09
1.01 0.23 ± 0.02 0.181 2.461 9.95 [29]
[28]
Rice husk + high density Slow pyrolysis 600 °C
Co-pyrolysis 300 ◦ C 46.802 ± 0.960 0.670 ± 0.003 0.036 ± 0.002 [30]
polyethylene
Peanut hull Slow pyrolysis 400 °C 65.5 2.0 0.00162 0.0015 10.0 [28]
Corn stover Slow pyrolysis 300 °C 59.5 1.16 0.137 1.705 7.33 [28]
Corn residue (Stover and cob) HTC 260 °C (30 min) 57.51 ± 1.11 1.62 ± 0.04 35.12 ± 1.09 0.23 ± 0.02 [29]
Rice husk + high density polyethylene Co-pyrolysis 300 °C 46.802 ± 0.960 0.670 ± 0.003 0.036 ± 0.002 [30]

4. Soil Applications of Biochar

Molecules 2021, 26, 5584 5 of 17

4. Soil Applications of Biochar

Biochar addition can greatly improve soil integrity since soils require a certain degree
of aggregates; solids and organic matter (or humus) to best provide a growing medium
to plants [31]. Furthermore, a variety of particle sizes are required to maintain WHC
along with a certain level of aeration [31]. Biochar can remediate the physical structure of
poor soils. If soil is too compacted, then biochar incorporation may allow better aeration
through its varying degree of porosity [31]. Biochar, when compared to fine, sandy soil
types has a much larger surface area and higher degree of porosity [32]. There are also
benefits to including the compost of biomass in combination with the biochar produced
from that biomass [11]. The addition of compost and biochar proved to be just as favorable
as biochar alone for plant yields due to the more rapidly degrading biomass providing a
steady flow of nutrients for plant uptake until slow release of nutrients from the biochar
began [11]. When soils are amended with biochar, there is a greater amount of oxidation-
reduction reactions that take place within the soil matrix. The persistence of biochar in the
environment and soils overtime is another benefit of biochar and can persist in fields over
several years [23]. As such, biochar may not need to be re-applied yearly, thus proving to
be a cost-efficient alternative.
Overtime soil organic matter content levels diminish. This occurs due to weathering,
farming cultural practices and other anthropogenic activities [24]. The crystalline structure
of biochar makes it particularly stable and withstands in soils [25]. Another interesting
effect is that biochar’s addition to soils causes increased observation of ethylene, a major
plant hormone used to regulate plant growth and ripening [26]. With an increase in
ethylene being produced through biochar amendment, crop yields will increase [26].

4.1. Nutrients and pH

Biochar has the capability to retain and provide bioavailable nutrients for plant uptake.
As an example, the potassium found within biochar is available for plant uptake [23].
Biochar can also have a variety of effects on soil pH, the degree to which is often dependent
on the feedstock and production conditions [33]. There are several species of microbes
(acidobacteria, nematodes and fungi such as mycorrhizae) found within soils [4]. Biochar
has the capacity to help remediate the deficiencies seen in “problem soils”, they possess
qualities such as poor aggregate stability, high salinity, extreme pH levels (too high or too
low) or lacking in nutrients [34]. Problem soils can be defined as soils with poor properties
(biological, physical, or chemical) that hinder plant growth [35]. The long-term health of
soils can be improved with even a single application of biochar [35].
There are a variety of ways biochar can enhance soil health, and with this comes
improved crop productivity [11]. One way this is accomplished is through improved mi-
crobial population diversity [10]. The refuge provided by biochar pores allows populations
to propagate and it also fixes nitrogen for plant uptake [36]. This is particularly important
for crops that are not able to fix their own nitrogen (in the case of non-legumes). It is partic-
ularly interesting to understand that the potassium found in biochar are already in forms
that are available for plant uptake [23]. Biochar also makes nitrogen more available for
plant uptake for crops that are not able to fix their own nitrogen [37]. Borchard et al., 2014
applied biochar at a rate of 15 g/kg of soil and saw an increase in maize yields, an increase
in N, Ca and in maize leaves [38]. In addition, the carbon content in soil increased and
temporarily decreased the soils pH level, although alkaline soils tend to be most favorable
for common cash crops such as maize [38]. Another study by Agegnehu et al., 2016 also
found soil properties to improve with the addition of biochar [11]. When analyzing the soil
physicochemical properties, it was discovered that the treatments including biochar had
increased nitrogen levels when compared to just fertilizer (1.16% vs. 0.15% soil nitrogen).
With a biomass increase of 9–18%, it was found that there were higher levels of nitrogen
within biomass leaves [11]. Using solid forms of carbon also allows soils to improve the
level of nutrients present and how they are retained [39]. For soils that have undergone a
high degree of weathering, nutrient retention is a particularly immense problem and they
Molecules 2021, 26, 5584 6 of 17
Molecules 2021, 26, x FOR PEER REVIEW 6 of 19

experience low Cation Exchange Capacity (CEC) due to the reduced mineral component
component
found in that found in that
soil [39]. It soil
also[39]. It alsoto
is related is improved
related to improved
WHC butWHC the verybutsurface
the verybonding
surface
bonding that occurs with improved CEC adds to the
that occurs with improved CEC adds to the nutrient retention [39]. nutrient retention [39].
Another
Another property
property of of biochar
biochar is is its
its ability
ability toto directly
directly provide
provide nutrients
nutrients forfor plant
plant up-
up-
take
take [2].
[2]. For
For example,
example, thethe potassium
potassiumthat thatisispresent
presentininbiochar
biocharfrom fromitsitsoriginal
original feedstock
feedstock is
is generally found in forms available for plant uptake [23]. As can be
generally found in forms available for plant uptake [23]. As can be seen in Figure 3, whether seen in Figure 3,
whether pH increases
pH increases or decreases
or decreases afteramendment
after biochar biochar amendment
depends depends on the characteris-
on the characteristics of the
tics of the biochar [33]. Generally, biochars formed from agricultural
biochar [33]. Generally, biochars formed from agricultural residues tend to be more residues tend to be
alkaline
more
and therefore help increase the pH of soils [23]. These types of biochar have higherhave
alkaline and therefore help increase the pH of soils [23]. These types of biochar ash
higher
contentash content
which whichmore
provides provides
basicmore
saltsbasic
to skewsaltsmore
to skew more[23].
alkaline alkaline [23]. In contrast,
In contrast, biochars
biochars created
created from fromresidues,
animal animal residues,
such as suchchickenas chicken
litter orlitter or bovine
bovine manure, manure, are signif-
are significantly
icantly moredue
more acidic acidic duefunctional
to the to the functional
groups groups they provide
they provide to biochar to biochar
[34]. [34].

Figure 3. A variety of effects caused by biochar surface chemistry [33]. [Copyright; MDPI, 2020].
Figure 3. A variety of effects caused by biochar surface chemistry [33]. [Copyright; MDPI, 2020].
Extremely saline soils also pose a risk to crop production [40]. A field experiment
Extremely
spanning acrosssaline
2 yearssoils also pose
discovered that a arisk to crop production
combination of wheat straw [40]. biochar
A field andexperiment
poultry
spanning
manure was across 2 years
able discovered
to decrease the that
extremea combination
salinity ofofcentral
wheat straw
Chinese biochar
soils and poultry
in order to
manure was able to decrease the extreme salinity of central Chinese
improve the growth of maize [40]. What was most interesting here was the fact that leaf soils in order to im-
prove the growth
bioactivity of maize
increased, and the[40]. Whatleaf
maize wassapmost
hadinteresting here was the fact
increased concentrations that leaf
of nitrogen,
bioactivity increased, and the maize leaf sap had increased
phosphorus and potassium. The increased nitrogen coming from the chicken manure was concentrations of nitrogen,
phosphorus
taken up more andeasily
potassium.
by theThe increased
biochar nitrogen
[40]. Soil salinitycoming from the
is reduced viachicken manure
the presence of was
Na+
taken up more
ions, biochar easily
binds to by
thethe
Nabiochar
+ ions and [40]. Soilthem
stops salinityfromis reduced
being taken via up
theby presence
the plant of [41].
Na+
ions, biochar
On acidic binds
soils, corntostover
the Naand+ ions and stops them
switchgrass biochar fromwas being
foundtaken up by the
to increase plant
soil pH [41].
and
On
otheracidic soils, corn
properties over stover
a periodand of switchgrass
165 days, which biochar was foundshort
is a relatively to increase
amountsoil pH [42].
of time and
other properties
The hydrogen over
and a period atoms
aluminum of 165 days,
free inwhich is a relatively
soils cause acidity andshort amount
limit crop of time [42].
growth [41].
The hydrogen and aluminum atoms free in soils cause acidity
They bind to essential plant nutrients and halt them from uptake [43]. Soils that have and limit crop growth [41].
They
pH lessbind to essential
than plant nutrients
5.0 are strongly and halt
acidic, which them
are the from uptakewhere
conditions [43]. Soils that have
the most damagepH
less than
occurs 5.0 The
[44]. are strongly
quality and acidic,
yield which
of manyare the
cropsconditions
are severely where the most
hindered damage
at this pointoccurs
[44].
[44]. The
Evenquality and that
legumes, yieldfix
of their
manyown crops are severely
nitrogen benefithindered at this point
nutritionally from the[44].addition
of biochar. The ratethat
Even legumes, of nitrogen
fix theirfixation
own nitrogenof commonbenefit beans increasedfrom
nutritionally afterthe
addition
addition with
of
biochar [45].
biochar. The rateAt aof
biochar
nitrogenaddition
fixation ofof g kg−1 , the
90common beansnitrogen fixation
increased afterincreased
addition fromwith 50%
bio-
to 72%
char [45].
[45]. AtEven without
a biochar the use
addition ofof90traditional
g kg−1, thechemical
nitrogenfertilizers, biochar can
fixation increased from aid50%
in the
to
growth
72% [45].ofEven
cropswithout
[46]. In general,
the use of nitrogen retention
traditional chemicalincreases with the
fertilizers, addition
biochar can ofaidbiochar.
in the
Biocharof
growth addition in aInpot
crops [46]. experiment
general, nitrogen on retention
rice allowed the crop
increases with tothe
take up more
addition ofnitrogen
biochar.
fertilizeraddition
Biochar [47]. Thisinresulted in 23–27% increase
a pot experiment of nitrogen
on rice allowed the uptake
crop toand takefurthermore caused
up more nitrogen
an 8–10%[47].
fertilizer increase in rice yield.
This resulted Interesting
in 23–27% increaseto note that without
of nitrogen uptake theand
application
furthermore of nitrogen
caused
fertilizer, there was no positive yield effect on the rice [47].
Molecules 2021, 26, 5584 7 of 17

The supplementation of biochar alone alters the bioavailability of various nutrient

uptake [48]. One study was able to find that soil surface available phosphorous increased
by 45% when possessing a viable amount of phosphorous [48]. Available nutrients are
necessary to improve the yield of crops and thus increase food production [49]. This
phenomenon is not limited to Zea mays, but increased levels of nutrients through biochar
amendment leading to higher plant heights, higher wet biomass weight and higher dry
biomass weight [49]. There are many cases in which the addition of biochar can reduce the
motility of various fertilizer compounds [50]. Often this is in reference to less fertilization
requirements, since less compounds of fertilizer are moving through and out of soils.
For example, for the growth of sugar beet and potatoes, the optimal level of nitrogen
fertilization was quite high at 300 kg ha−1 [51]. Referring to Table 1, the percent weight of
nitrogen found in corncob biochar pyrolyzed slowly at 600 ◦ C, was 4.25%. This would mean
7058.82 kg ha−1 of this corncob biochar could replace the traditional nitrogen fertilizer. Of
course, this is a hefty amount of biochar but at the very least it means food producers could
reduce the application rate of traditional fertilizers and replace a portion of it with biochar.
It is more persistent in soils and will aid in reducing fertilizer motility as well. Compared to
high temperature pyrolysis, low temperature pyrolyzed biochar tends to degrade at slower
rates. This would mean an initial increase in biochar addition will last for multiple growing
seasons instead of having to re-apply traditional fertilizers every growing period [15].
Due to the CEC of many biochars, surface functional groups, such as hydroxyl and
carboxyl groups, can bind to nitrogen and phosphorous in many forms found in fertiliz-
ers [20]. For example, it was found that biochar formed from pine-chips at 300 ◦ C created
char with lower surface area (16.5 m2 g−1 ) along with lower cation exchange capacity
(39.5 cmol kg−1 or centimoles per kilogram). This can account for the higher pH (8.2) since
there were fewer functional groups on biochar surface to add to the acidity of the soil
through amendment [20].
Greater nutrient retention by biochar is what increases the nutrient content of biochar
amended soils [52]. It seems that biochar is effective at reducing the leaching of nitrogen,
in the form of NH3 . Wheat straw biochar applied at rates of 0.5% to 1% were enough to
reduce N leaching in various forms. The following forms of nitrogen were assessed as a
percentage, NH4 + by 11.6–24%, -N 13.2–29.7% and NO3 − —N 14.6–26% [53]. This effect can
be explained by the fact that biochar ends up covalently bonding to ammonia and helps
retain it in soils [54]. Another study by Yao et al., 2012 discovered that peanut hull and
pepperwood biochar pyrolyzed at 600 ◦ C were able to sorb a variety of nutrients and reduce
leaching in a column experiment [55]. Results showed that the amount of nitrate leaching
reduced by 34%, ammonium by 34.7% and phosphate by 20.6% [55]. A study carried out
on calcium rich soils known to have issue with nutrient retention indicated that bagasse
biochar is effective at reducing nitrate leaching [56]. Pyrolyzed at 400–800 ◦ C, this material
was analyzed for its degree of CEC (measured at pH 7). In general, biochar application
reduced the level of fertilizer needs [57]. With the addition of corn cob derived biochar
and from fig tree derived biochar, the growth of maize was improved [12]. Produced via
slow pyrolysis at 600 ◦ C, these two types of biochar were lower in pH and their surface
due to increased surface functionality. Through this greater surface functionality, there
is a higher degree of free carboxylic carbon that will keep cations on biochar surface [12].
Using biochar can be an easy way to improve the organic matter content of soils [58]. When
organic matter is not added, the microbiome will change to microbes that are predominantly
anaerobic. This is a problem because this causes ammonium nitrogen to accumulate in the
growing medium. This ammonium nitrogen when exposed to air quickly turns into a more
mobile NO3 -N. Overall, without the addition of biochar this can cause excessive leaching
of various substances used as fertilizer [58].

4.2. Water Holding Capacity

One of the most important ways biochar is able to abate the effects of climate change
induced drought is through its ability to retain water [59]. An effective example of this is use
Molecules 2021, 26, 5584 8 of 17

of biochar in sub-Saharan Africa to combat dry soils [60]. In this region, soils are generally
poor because of higher concentrations of sand from granite-rock parent material [60]. Not
only are these soils naturally dry, but also acidic in nature (often with a pH below 4) and
produce low plant yields. The soils in this area are also stressed due to repetitive drought.
The combination of these two effects produces massive food shortages to the area [60].
It is known that the WHC of biochar is most strongly influenced by the level of biochar
porosity [11]. This porosity is made up of macro and micro pores [11]. This porosity is
vital to increase soil water capacity, up to 14.6% increase of water content was found when
biochar was used in combination with fertilizer [11]. Water is the primary vessel with
which plants take up nutrients, additionally with lower WHC and improving soil health
further increases other positive effects [39]. When there is a significant loss of water in soils,
the plants residing will experience salt stress [61].
With the addition of biochar there is on average an increase of about 18% WHC [62].
It is known that biochar improves water-holding capacity through surface area and poros-
ity [62]. Even across soil types, WHC improves with biochar amendment [63]. With just 9%
addition of biochar (yellow pine wood pyrolyzed at 400 ◦ C) there was a 100% increase of
WHC [62]. Meaning that there was a doubling of WHC [62]. Another study discovered
that the water holding capacity was increased by 30% when sunflower husk biochar was
applied at 9.52% weight [64]. This is another example of how ~10% dry weight biochar
application is generally optimal to remediate or improve soils [64].

4.3. Microbiome
Many factors of biochar affect microbial populations such as feedstock, pyrolysis
conditions, particle size and soil properties [23]. There is evidence to suggest that biochar
enrichment can help mycorrhizal and rhizobial populations at the root level [34]. With these
microbial populations present, a variety of reactions take place within the soil matrix [20].
These occur at the interface between root hairs and microbes in soil. Through sorption,
various organic compounds that are bonded to biochar structures can be used by the
plant [23]. Addition of organic matter generally helps microbial species [65]. When
assessing the health and abundance of mycorrhizal communities, biochar amendment
has been shown to benefit them in the following ways; they provide a refuge through
porosity, for species, they detoxify the soils of heavy metals and can alter the soil’s physic-
chemical properties [66]. With the addition of biochar pyrolyzed at 350 ◦ C, there were more
bacteria (both gram negative and gram positive) in soils compared to the soil amended
with biochars produced at lower or higher temperatures [36]. Additionally, greater aeration
and more soil pores create the soil-water interface in which these microorganisms live [67].
Microbial populations can also aid in degradation of fertilizers thus, reduce issues of
nutrient leaching [66]. Their presence in soils is vital for the health of food crops [65].
In general, the prognosis is good when it comes to improving the diversity and count
of bacterial genes in biochar amended soils [68]. Chen et al., 2013 discovered that field of
rice, biochar applied at 20,000 kg/ha and 40,000 kg/ha altered the microbiome populations.
This shift included the favor of bacterial populations instead of fungal populations [68].
Another source indicated that there was no increase in microbial diversity but rather
increased microbial biomass with the addition of biochar from various biomass sources
in a meta-analysis by Li et al., 2020 [10]. It is thought that the reason for this is because
bacterial groups in soils are more readily affected and sensitive to biochar, while fungi may
not be [10].

5. Biochar Decomposition
Even though biochar in soil can serve as a carbon sink, biochar is not a permanent
fixture because it degrades overtime [48]. For soil amendment application, biochar de-
composition rates depend on the state of biochar, properties of soil and the climate [15].
When amended with biochar, soils were able to improve available phosphorous content
minimally, although a very important finding, this only lasted for less than 6 months, and
 Even though biochar in soil can serve as a carbon sink, biochar is not a permanent
fixture because it degrades overtime [48]. For soil amendment application, biochar de-
Molecules 2021, 26, 5584 composition rates depend on the state of biochar, properties of soil and the climate 9 of[15].
17

When amended with biochar, soils were able to improve available phosphorous content
minimally, although a very important finding, this only lasted for less than 6 months, and
thusisisaashort-term
thus short-termnegative
negativeeffect
effect[48].
[48].Simulated
Simulatedaging
agingofofa afield
fieldindicated
indicatedthatthatbiochar
biochar
produced from rice husk, modified with sulfur, was able to continually
produced from rice husk, modified with sulfur, was able to continually benefit soil health benefit soil health
over 50 years of time. This was produced via slow pyrolysis
over 50 years of time. This was produced via slow pyrolysis at 550–600 C [69]. at 550–600 ◦ °C [69].
Pyrolysis
Pyrolysis time
time influences
influences the the resultant
resultant decomposition
decomposition rate, asrate, as illustrated
illustrated in Tablein Table 2
2 [70,71],
[70,71],
even when even when temperature
temperature is kept the is same.
kept the same. created
Biochar Biocharvia created via fast pyrolysis
fast pyrolysis has a
has a slower
slower rate of decomposition in soil than biochar formed through
rate of decomposition in soil than biochar formed through slow pyrolysis [70,71]. It is slow pyrolysis [70,71].
It is important
important to then to suggest
then suggest fast pyrolyzed
fast pyrolyzed biochar
biochar for purposes
for purposes of remediating
of remediating prob-
problem
lem soils in order to have the beneficial effects of amendment last
soils in order to have the beneficial effects of amendment last longer. In this way, therelonger. In this way, there
is not only an environmental benefit but also an economic
is not only an environmental benefit but also an economic one. An example of biocharone. An example of biochar
persistencein
persistence inthe
the environment
environment is is the
theaftermath
aftermathofofforest fires
forest burning
fires burning woody
woody biomass
biomass that
incorporates
that incorporates intointo
thethe
soilsoil
toptop
layers
layers[72]. Deadwood
[72]. Deadwood biomass
biomass from
fromblack
blackspruce
spruce trees
trees ig-
nited through
ignited throughforestforest fires
fires create
create a more stable stable form
form ofof solid
solidcarbon
carbonas asopposed
opposedtotoraw raw
biomass.Knowing
biomass. Knowingthat that this
this raw carbonaceous matter matter is
is less
lessstable
stablethan
thanbiochar
biocharignited
ignitedin
the absence of oxygen indicates
in the absence of oxygen indicates that one that one is able to harness the power of creating
harness the power of creating stable stable
formsofofsolid
forms solidcarbon
carbontotoremain
remainininsoilssoils[72].
[72].Since
Sincebiomass
biomassisisrenewable,
renewable,capturing
capturingititinina a
solidform
solid formallows
allowssoilssoilstotoact
actasasa acarbon
carbonsink,
sink,when
whengenerally
generallyagricultural
agriculturallands
landsare
arenot not
usedininthis
used thisway
way(Figure
(Figure4)4)[73].
[73].

Table2.2.Decomposition
Table Decompositionrates
ratesofofbiochar
biocharfrom
fromeucalyptus
eucalyptusand
andoak
oakfeedstock
feedstockproduced
produced under
under varying
varying conditions.
conditions.

Feedstock
Feedstock Production
Production Decomposition
Decomposition RateRate
Eucalyptus
Eucalyptus Pyrolyzed 450
Pyrolyzed °C,
450 0.7
◦ C, 0.7hh 0.0039
0.0039 [70][70]
Eucalyptus
Eucalyptus Pyrolyzed 450
Pyrolyzed 450 ◦ C,3 3hh
°C, 0.0081
0.0081 [70][70]
◦
OakOak Pyrolyzed
Pyrolyzed 450
450 °C,C,3 3hh 0.003 [71][71]
0.003
Pyrolyzed 450 ◦ C, 0.7 h
OakOak Pyrolyzed 450 °C, 0.7 h 0.0047 [71]
0.0047 [71]
Eucalyptus Pyrolyzed 450 ◦ C, 0.7 h 0.0049 [70]
Eucalyptus
Eucalyptus Pyrolyzed
Pyrolyzed 450 C,0.7
450 °C,
◦ 0.7hh 0.0039 [70][70]
0.0049
Eucalyptus Pyrolyzed 450 °C, 0.7 h 0.0039 [70]

Figure4.4. The cyclical

Figure cyclical and
andrenewable
renewablenature
natureofof
biochar
biocharas as
a soil amendment
a soil [73].[73].
amendment [Copyright;
[Copyright;
ItechOpen,2020]
ItechOpen, 2020]Cycle
Cycleofofbiochar
biochardecomposition
decompositionandandformation
formationwhen
whenamended
amendedinto
intosoils.
soils.

6. Environmental Benefit
Since 1750, there has been an increase of 31% atmospheric CO2 [74]. Biochar has the
ability to sequester this CO2 in the form of solid carbon in soils [75]. This allows it to use
soils as a receptacle to sequester carbon. Carbon dioxide is not the only GHG that can
be sequestered through biochar, nitrous oxide is also able to be sequestered [34]. This is
Molecules 2021, 26, 5584 10 of 17

because feedstock types that are higher in nitrogen content (including chicken litter, animal
manure and municipal sewage sludge) pass that nitrogen content on to the plants [34].
It is important to understand the entire lifecycle of biochar to prove the sustainability of
using this material. The carbon footprint of biochar must include production, persistence
in soil, rate of degradation in soil and degree of soil fertility. An example of this was a study
carried out in mainland China in which four paddy rice fields and 3 maize fields were
amended with biochar [76]. The addition of biochar reduced the amount of released carbon
by 18,479.35–37,457.66 kg of carbon dioxide [76], i.e., a reduction of 47% and 57% for both
rice and maize, respectively [75]. Spokas and Reicosky, 2009 looked at 16 different types of
biochar and their effect on GHG emissions in soils [77]. Feedstock, pyrolysis conditions
and surface area all did not influence the amount of methane released from the growing
medium [77]. In this study, after the addition of fertilizer, there would be a net increase
of GHG emissions. Other studies prove that even when accounting for the GHG release
during production, there remains a net decrease in GHG release when biochar is added to
agricultural soils [77]. Over a 100-year period of this practice, the removal of the emissions
from fallen branches reduced from 340 to 70 kg CO2 eq. MWh−1 . This is a significant
decrease in emissions from a very indirect and passive form of biomass breakdown [78].
Previously mentioned applications of biochar include filtration and truly this is a way
in which to benefit the environment as well, to remove pollutants from the environment
through porosity and CEC of the material [79]. Toxin removal is accomplished through
waste-water treatment, reduced GHG emissions, controlled degradation of land (through
adding stability and aggregates to soils), reduced nutrient leaching, release controlled
fertilizer and heavy metal and pollutant removal [79]. Previously mentioned Ebeheakey
et al., 2018 noted a great reduction in lead acetate, ferric chloride, saponins, flavonoids
and triterpernoids after biochar amendment [80]. At 12 weeks post amendment, the
soils did not contain any of these substances [80]. Additionally, it is possible that altered
biochar can more effectively sorb pollutants from waterways. The high porosity of slow
pyrolysis biochar formed from cotton and sewage sludge had maximum sorption ability of
1.761 mg g−1 and 2.586 mg g−1 [81]. Another source indicated that bamboo biochar was
able to reduce nitrogen species from water [82]. The maximum sorption ability in this case
was 10.35 mg g−1 in the unaltered biochar but greater for modified biochar [82].
Global warming potential (GWP) indicates how much energy the emissions of one
ton of carbon dioxide absorbs [24]. As can be seen in Table 3, the miscanthus feedstock
produced the lowest GWP while aiding in using soils as a carbon sink [15]. Meanwhile,
peat moss feedstock has the most GWP. Observing the intermediate GWP value for the
co-pyrolyzed biochar (formed from both miscanthus and peat moss feedstock) is proof of
the predictability of biochar qualities based on feedstock, production type and conditions.
The amount of stored carbon increased while decomposition rate decreases. Obstinately, a
lower amount of carbon can be stored with a greater decomposition rate [15]. The effects
of biochar on crop yields is also a fundamental benefit that overall aids in protecting the
environmental system [42].

Table 3. Global warming potential (GWP) of different processed biomass applied for soil amendment [15].

Feedstock Production Method Application GWP/t Feedstock (Kg CO2 eq)

Peat moss and Miscanthus Hydrothermal carbonization Soil amendment 79.51
Miscanthus Hydrothermal carbonization Soil amendment −320.86
Peat moss Hydrothermal carbonization Soil amendment 714.64

As seen in Figure 4, biochar can not only reduce CO2 emissions but also reduce nitrous
oxide [33,72]. If food producers can increase crop yields within current crop land area and
without expanding agricultural lands, this helps protect current forested land [42]. Carbon
content in soils depleting is already a concern in agroforest ecosystems [83]. For example,
in India, rain fed crops in fact rely upon soil organic matter in the form of carbon to thrive.
Molecules 2021, 26, 5584 11 of 17

For every addition of 1 Mg ha−1 of soil organic carbon (SOC) there are grain crop increases.
1000 kg ha−1 of SOC increases the yield of groundnut by 13 kg ha−1 , finger millet by
101 kg ha−1 , sorghum by 90 kg ha−1 , pearl millet by 145 kg ha−1 , soybean by 18 kg ha−1 ,
and rice by 160 kg ha−1 [83]. This relatively small addition of organic matter in the form of
carbon allows for not only crop yield increase but also reduction of GHG emissions. Even
in instances where biochar addition does not decrease GHG emissions, there still exists
an increase in crop yields without any increase in those same emissions [84]. When straw
derived biochar was amended into sandy-loam soils (low in organic matter) a five-year
wheat and maize crop rotation resulted in a decrease of N2 O emissions, crop yields did
increase but overall GWP did not decrease [84]. This is due to biochar production emissions
essentially. Despite this, the evidence still remains that the outcome of amendment is either
neutral or beneficial in terms of crop yield, GWP, emissions rates and SOC rates. Other
aspects of conservation farming exist, these include no tillage, crop rotation, permaculture,
etc. [85]. The issue is that none of these strategies alone have been able to offset emission
rates and SOC degradation rates in any meaningful capacity. Therefore, it should be
highlighted that biochar has yet to be used in a widespread way in order to accomplish
the same goals [85]. Furthermore, when discussing aspects of traditional agriculture, there
exists many constraints when utilizing typical chemical fertilizers. Yes, the addition of
biochar in soils reduces the immediate GWP as compared to compost, but when looking
at even broader analysis, overall GWP is also reduced [86]. Therefore, it can be said that
optimization of fertilizer application is important to control GHG emissions [87].

7. Economic Benefit
Even though biochar seems costly, its agricultural applications provides long-term
economic benefits. An economic assessment carried out by Keske et al., 2019 was able to
determine that biochar application for agricultural purposes, had a 99% probability of be-
coming profitable [12]. When using biochar created from forest biomass waste (in this case
black spruce) to grow beets, crop yields increased [12]. Biochar was applied at 10,000 kg/ha,
and the resultant beet yields increased from 2900 kg/ha to 11,004 kg ha−1 [12]. The net
return for this process was up to $4953 ha−1 . Conversely, the same study indicated that the
costs of application were covered for beet production but not potato production [12]. In
general, when being produced from waste biomass, biochar production is economically
beneficial [22]. It is also important to note that using soils as a carbon sink creates a more
economically sustainable method to manage waste [88]. Poland is an example of a nation
that would benefit greatly from establishing biochar production within a circular economy
model [89]. Additionally, some work has been carried out to determine that biochar use
in soils can act as carbon sequestration in the Polish climate and soil type which is very
promising [89].
As mentioned in Table 3, the use of different production conditions can help create a
type of biochar that is more persistent in soils and thusly will not require yearly application
allowing for more fiscal saving. However, the current market price of biochar may restrict
its use for either amending soil or for energy generation and this requires exploration.

8. Discussion
Despite all the benefits biochar can provide, there is controversy surrounding biochar
amendment in agricultural soils. To keep up with growing food demands, it is necessary to
have a focus on improving the yield of staple crops. Additionally, Chan et al., 2007 carried
out a pot trial to study green waste biochar effects on radish yields (Raphanus sativus) [88].
The biochar was applied at 10, 50 and 100 t/ha. The soils the study was carried out on had
a history of regular cropping and were of the soil type alfisol. In this case the yield of radish
was not improved, even at the highest application rate of 100 t/ha. The most interesting
interaction involved nitrogen. When biochar was applied together with nitrogen fertilizer,
the yields improved [5]. Time of residence for biochar can also vary. As an example,
some sources indicate that wood biochar can have a longevity of anywhere from 100 to
Molecules 2021, 26, 5584 12 of 17

1000 years [45]. This broad range means that determining re-application rates for croppers
can be difficult. There is very little data concerning the time of residence for other types
of biochar as well [45]. Jeffery et al., 2015 included a study in which the application of
biochar was not able to improve hydrological qualities of a sandy soil [90]. Critique of this
study may include the fact that the study site in question was in the Netherlands where
soil quality is often thought to be of the highest caliber. These kinds of confounding results
are a major constraint when it comes to biochar soil amendment. El-Nagger, 2019 analyzed
multiple sources in order to create a review of the outcome of biochar amendment on soils
with low fertility [91]. The benefits of biochar amendment depend on the product used for
the specific situation. Particular situations require specialized biochar products, whether it
is activated or not [21].
Much of the soils found on earth are highly acidic and would benefit from the addition
of biochar [92]. This is as a result of mainly anthropogenic activities [93]. Biochar tends
to be more alkaline and helps increase the pH in order to reduce acidity [94]. When fava
beans and turnip rape were grown with the addition of pine tree biochar, there was an
increase of yield as a result of increased water holding capacity [93]. When categorizing
soil types, 38% of tropical regions were able to benefit from biochar amendment while
only 10% of temperate soils saw the same fertility benefits. However, there is very little
work carried out to fully understand why these differences occur. N2 O emission could be
triggered due to increased nitrogen fixation by microbes, whose populations are stimulated
by biochar addition [92]. A lot of the benefits of biochar addition into soils are illustrated
in Figure 5. For example, using soils as a carbon sink to reduce GHG emissions, reduced
fertilizer leaching into waters and soils, or improved nutrient availability for crops, just to
point out a few [92]. Future endeavors of biochar use in agriculture include climate change
mitigation, drought tolerance and addition to the circular economy. As discussed earlier,
the method of capturing carbon in a solid form helps reduce, or at the very least, delay
GHG emissions. If nations that participate in carbon tax programs (such as Canada and
Zambia) make biochar readily available to food producers, this system could continue to
thrive [95]. While carbon taxes or carbon credits are seemingly focused on manufacturing
processes, agriculture remains as one of the most emission-heavy industries [96]. This
is an unexplored option to reduce emissions without penalizing other emission-heavy
industries.
Molecules 2021, 26, x FOR PEER REVIEW While certain businesses may leave nations in which penalization
13 of 19 may occur,
food production will remain sturdier in their locations due to the inherent need for food.
No matter the economic situation, food production is vital, and agriculture remains.

Figure 5. TheFigure
multitude
5. Theof benefitsofbiochar
multitude benefitssoil amendment
biochar provides.
soil amendment [adapted
provides. with permission
[adapted from
with permission ref.ref.
from [92], [Copyright,
[92], Copyright,Elsevier, 2011].
Elsevier, 2011].

Furthermore, biochar very effectively fits into the circular economy model. The bio-
char production process can play a role in multiple industries, but biochar production
most strongly has a role in agriculture. The growth of food will produce agricultural res-
idue, which when valorized/polymerized forms biochar as one of the co-products. After
soil amendment, there is an improvement of soil properties, nutrient retention, microbial
Molecules 2021, 26, 5584 13 of 17

Furthermore, biochar very effectively fits into the circular economy model. The biochar
production process can play a role in multiple industries, but biochar production most
strongly has a role in agriculture. The growth of food will produce agricultural residue,
which when valorized/polymerized forms biochar as one of the co-products. After soil
amendment, there is an improvement of soil properties, nutrient retention, microbial
activity and crop yield as demonstrated in Figure 6. Overall, this benefits the farming
economy. In addition, reduction of waste from the agricultural sector will occur. In fact, a
case study surrounding a small-scale olive farm enacted what was essentially a circular
economy model regarding olive farming residues and bioenergy production [97]. This is
Molecules 2021, 26, x FOR PEER REVIEW 14 of 19
an example of biochar providing value-added products into the agricultural economy to
stimulate the circular economy model.

Figure6.6.The
Figure Therole
rolebiochar
biocharplays
playsininthe
thecircular
circulareconomy
economymodel.
model.

9.9.Conclusions
Conclusions
Biochar
Biocharcan canimprove
improveagricultural
agriculturalsoils
soilsinina avariety
varietyofofways.
ways.These
Thesemethods
methodsinclude,
include,
but
butare
arenotnotlimited
limitedto,to,improving
improvingwaterwaterholding
holdingcapacity,
capacity,improving
improvingsoil soilstability
stabilitythrough
through
addition
additionofof aggregates
aggregates andand
solids, increasing
solids, microbiome
increasing microbiomepopulations and controlling
populations fungi
and controlling
populations, reducing the need for fertilizer and reduce fertilizer leaching.
fungi populations, reducing the need for fertilizer and reduce fertilizer leaching. One One is also ableis
toalso
acknowledge the immense
able to acknowledge thebenefits
immense tobenefits
crop yields,
to croptheyields,
reduced theGHG
reducedemissions and the
GHG emissions
role
andplayed
the rolein the circular
played economy
in the circularmodel.
economy Aside from Aside
model. agriculture,
from biochar can be
agriculture, used for
biochar can
water filtration
be used purposes,
for water filtrationremoving
purposes, heavy metalsheavy
removing from metals
the environment, and removing
from the environment, and
pharmaceuticals from the environment.
removing pharmaceuticals It is essentialItfor
from the environment. is the futurefor
essential of the
biochar application
future of biochar
toapplication
learn how to manipulate biochar properties so as to tailor the amendment
to learn how to manipulate biochar properties so as to tailor the amendment to each region,
climate, crop type and soil. Overall, the literature indicates that biochar
to each region, climate, crop type and soil. Overall, the literature indicates that biochar has beneficial
effects on soil quality
has beneficial effectsand cropquality
on soil yields,and
but crop
possible constraints
yields, needconstraints
but possible to be explored.
need The
to be
explored. The variability of biochar properties should be viewed as its best asset. Biochar
seems to be a potential material that can be tailor-made to solve unique agricultural chal-
lenges.

Author Contributions: Investigation, data analysis, writing—original draft preparation, T.A.; writ-
ing—review and editing, T.A., A.K.M., P.R. and M.M.; project conceptualization, administration,
Molecules 2021, 26, 5584 14 of 17

variability of biochar properties should be viewed as its best asset. Biochar seems to be a
potential material that can be tailor-made to solve unique agricultural challenges.

Author Contributions: Investigation, data analysis, writing—original draft preparation, T.A.;

writing—review and editing, T.A., A.K.M., P.R. and M.M.; project conceptualization, administration,
funding acquisition and supervision, A.K.M. and M.M. All authors contributed to the discussion,
reviews and All authors have read and agreed to the published version of the manuscript.
Funding: (i) The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)-Canada/University
of Guelph-Bioeconomy for Industrial Uses Research Program Theme (Project Nos. 030331, 030332);
(ii) the Ontario Research Fund, Research Excellence Program; Round-7 (ORF-RE07) from the Ontario
Ministry of Research Innovation and Science (MRIS), Canada (Project Nos. 052644 and 052665); and
(iii) the Natural Sciences and Engineering Research Council (NSERC), Canada Discovery Grants
(Project Nos. 401111); and (iv) the NSERC, Canada Research Chair (CRC) program Project No. 460788.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this article is available on request from the
corresponding author.
Acknowledgments: This study has also benefited from the facility funding to the Bioproducts
Discovery and Development Centre, University of Guelph supported by FedDev Ontario, Canada;
OMAFRA Canada; the Canada Foundation for Innovation (CFI); Federal Post-Secondary Institutions
Strategic Investment Fund (SIF), Canada; and Bank of Montreal (BMO).
Conflicts of Interest: The authors declare no conflict of interest.

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