Characteristics of biochar: biological properties

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Decades of research in Japan and recent stud-
ies in the US have shown that biochar
stimulates the activity of a variety of agricul-
turally important soil microorganisms and
can greatly affect the microbiological proper-
ties of soils (Ogawa et al, 1983; Pietikäinen et
al, 2000).The presence and size distribution
of pores in biochar provides a suitable habitat
for many microorganisms by protecting them
from predation and desiccation and by
providing many of their diverse carbon (C),
energy and mineral nutrient needs (Saito and
Muramoto, 2002;Warnock et al, 2007).With
the interest in using biochar for promoting
soil fertility, many scientific studies are being
conducted to better understand how this
affects the physical and chemical properties of
soils and its suitability as a microbial habitat.
Since soil organisms provide a myriad of
ecosystem services, understanding how
adding biochar to soil may affect soil ecology
is critical for ensuring that soil quality and the
integrity of the soil subsystem are maintained.
Among the ecosystem services that soil
microorganisms provide are decomposing
organic matter; cycling and immobilizing
inorganic nutrients; filtering and bio-remedi-
ating soil contaminants; suppressing and
causing plant disease; producing and releas-
ing greenhouse gases; and improving soil
porosity, aggregation and water infiltration
(Coleman, 1986;Thies and Grossman, 2006;
Paul, 2007). As they interact with plants in
the rhizosphere, bacteria, fungi, protozoa and
nematodes strongly influence the ability of
plants to acquire macro- and micronutrients.
This may occur as a direct result of mutualis-
tic associations between plant roots and
microorganisms, such as with the arbuscular
mycorrhizal (AM) fungi (Glomeromycota;
Robson et al, 1994) or the nitrogen (N2)-
fixing rhizobia bacteria; or through trophic
interactions resulting in nutrient excretion by
secondary feeders, such as protozoa and
nematodes (Brussaard et al, 1990). Clearly,
soil microbial activity strongly affects soil
function and, consequently, crop growth and
yield. The physical and chemical environ-
ment of biochar may alter many of these
biological activities, discussed in detail below.
Introduction
6
Characteristics of Biochar: Biological Properties
Janice E.Thies and Matthias C. Rillig
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The porous structure of biochar (see
Chapter 2 and Figure 6.1), its high internal
surface area and its ability to adsorb soluble
organic matter (see Chapter 18), gases and
inorganic nutrients (see Chapter 5) are likely
to provide a highly suitable habitat for
microbes to colonize, grow and reproduce,
particularly for bacteria, actinomycetes and
arbuscular mycorrhizal fungi (see Figure
6.2). Some members of these groups may
preferentially colonize biochar surfaces
depending upon the physical and chemical
characteristics of different biochars,
discussed below.
The pore space of pyrolysed biomass
increases during charring by several thou-
sand fold and is related to charring
temperature and feedstock materials (see
Chapter 2). Estimates of the resulting surface
area of different biochars range from 10 to
several hundred square metres per gram (m2
g-1) (see Chapter 2), which provides a signifi-
cantly increased surface area for microbial
colonization. Depending upon the size of a
given pore, different microbes will or will not
have access to internal spaces. Several
authors have suggested that the biochar pores
may act as a refuge site or micro-habitat for
colonizing microbes, where they are
protected from being grazed upon by their
The nature and function of soil microbial
communities change in response to many
edaphic, climatic and management factors,
especially additions of organic matter (Thies
and Grossman, 2006). Amending soils with
biochar is no exception. However, the way in
which biochar affects soil biota may be
distinct from other types of added organic
matter because the stability of biochar makes
it unlikely to be a source of either energy or
cell C after any initial bio-oils or condensates
have been decomposed (see Chapter 11).
Instead, biochar changes the physical (see
Chapter 2) and chemical (see Chapters 3 to
5) environment of the soil, which will,in turn,
affect the characteristics and behaviour of the
soil biota.
The effects of biochar on the abundance,
activity and diversity of soil organisms are the
subjects of this chapter.This area of enquiry
has lagged behind other areas of biochar
research. Much of what is known about the
biota in soils containing biochar results from
the pioneering work of the Japanese
researcher M. Ogawa and colleagues and
from research on microbial communities in
the Amazonian dark earths (ADE, also called
‘Terra Preta de Indio’) from Brazil. We
include examples from these works here, with
the aim of forecasting how both the soil flora
and fauna populations may respond to
biochar amendments and to suggest more
fruitful avenues for future research.
86 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
Biochar as a habitat for soil microorganisms
Figure 6.1 The porous struc-
ture of biochar invites
microbial colonization
Source: (left photo) S. Joseph; (right
photo) Yamamoto, with permission
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natural predators (Saito and Muramoto,
2002; Warnock et al, 2007) or where
microbes that are less competitive in the soil
environment can become established
(Ogawa, 1994). The pore size variation
observed across biochar particles from differ-
ent feedstocks and pyrolysis conditions is
such that the micro-flora could, indeed, colo-
nize and be protected from grazing,
especially in the smaller pores (see Table 6.1).
The high porosity of biochar may also
allow it to retain more moisture. Pietikäinen
et al (2000) reported that two biochars, one
prepared from humus and one from wood,
had a higher water-holding capacity (WHC)
(2.9mL g-1 dry matter) than activated carbon
(1.5mL g-1 dry matter) or pumice (1.0mL g-
1 dry matter). An increase in the WHC of
biochar may result in an overall increase in
the WHC of the soils to which it is added (see
Chapter 15). For biochars with a high
mineral ash content, the porosity will
continue to increase as the ash is leached out
over time; thus, the capacity of the biochar to
retain water, provide surfaces for microbes to
colonize, and for various elements and
compounds to become adsorbed will also
likely increase over time. Smaller pores will
attract and retain capillary soil water much
longer than larger pores (larger than 10 µm to
20µm) in both the biochar and the soil.Water
is the universal biological solvent and its pres-
ence in biochar pores increases the
‘habitability’ of biochar substantially.
CHARACTERISTICS OF BIOCHAR:BIOLOGICAL PROPERTIES 87
Table 6.1 Pore diameters in wood and bamboo biochar compared to the ranges in the
diameter of various soil microorganisms
Diameter (µm) Mode
Range
Bamboo biochar – pores10.001–1000 0.1
Wood biochar 10–3000 1495
Bacteria20.3–3 0.5
Fungi22–80 8.0
Protozoa27– 30 20.0
Nematodes23– 30 16.0
Note: 1 See Chapter 2.
2 See Swift et al (1979).
Figure 6.2 Arbuscular
mycorrhiza fungal hyphae
growing into biochar pores
from a germinating spore
Source: Ogawa (1994)
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In addition to water, a variety of gases,
including carbon dioxide (CO2) and oxygen
(O2), will be dissolved in pore water, occupy
the air-filled pore space or be chemi-sorbed
onto biochar surfaces (Antal and Grønli,
2003); this latter is due to the defect struc-
tures present in the amorphous and
micro-graphene lattices (see Chapter 3).
Depending upon the ratio of air- to water-
filled pore space, the relative concentrations
of the gases, their diffusion rates and the
extent of surface sorption, either aerobic or
anaerobic conditions will predominate in the
biochar pores.Where sufficient O2is avail-
able, aerobic respiration will be the dominant
metabolic pathway for energy generation,
resulting in water (H2O) and CO2as the
primary metabolic end products. As the O2
concentration decreases, facultative aerobes
will begin to use anaerobic respiratory path-
ways as long as suitable terminal electron
acceptors are available.The end products of
anaerobic respiration can be nitric oxide
(NO), nitrous oxide (N2O), nitrogen (N2),
hydrogen sulphide (H2S) and methane
(CH4), among others.Thus, O2diffusion into
biochar pores and the terminal electron
acceptor used during microbial respiration
will, in large part, determine what the remain-
ing pore atmosphere will contain and how
hospitable this environment is likely to be for
its occupants. For further discussion of the
evolution of N2O and CH4from biochar
amended soils, see Chapter 13.
Moisture, temperature and hydrogen ion
concentration (pH) are the environmental
factors that most strongly influence bacterial
abundance, diversity and activity (Wardle,
1998). In a cross-continental study, Fierer
and Jackson (2006) found that the diversity
and richness of soil bacterial communities
differed by ecosystem type, but that these
differences were largely explained by soil pH,
with bacterial diversity highest in neutral soils
and lowest in acidic soils. The activity of
bacterial populations is also strongly influ-
enced by pH. Under both acidic and alkaline
conditions, proteins become denatured and
enzyme activity is inhibited, impairing most
metabolic processes. Biochars vary consider-
ably in their pH, depending upon feedstock
and pyrolysis temperature (see Chapter 5)
and, thus, will also vary in the microbial
communities that develop on and around
them. Under the extremes of pH, fungi will
likely predominate due to their wide range of
pH tolerance; most bacteria prefer circum-
neutral pH. Adding biochar to soil, whether
acid or alkaline, may lead to significant
changes in the soil community composition
by changing the overall ratio of bacteria to
fungi, as well as the predominance of differ-
ent genera within these populations. It may
also significantly alter soil function by affect-
ing enzyme activities and, thus, overall
microbial activity. The influence of biochar
pH on colonizing microbial communities and
their metabolic processes will be an interest-
ing area of future investigation.
Bacteria and fungi rely on their elabo-
rated extracellular enzymes to degrade
substrates in their environment into smaller
molecules that can then be taken up into their
cells and used for various metabolic activities
(Thies and Grossman, 2006; Paul, 2007).
Thus, they become highly ‘invested’ in
remaining in close physical proximity to
where they secrete extracellular enzymes into
their environment. Surfaces become very
important in this regard, whether these are
the surfaces of a soil aggregate, a plant root, a
particle of clay, soil organic matter or biochar.
The activity of extracellular enzymes will
depend upon the molecular location on these
proteins that interacts with the biochar
surface. If the enzyme active site is exposed,
functional and free to interact with its milieu,
then increased activity may occur. However,
if the active site is obscured, reduced activity
may result. It may be that certain classes of
enzymes will be more active and others less
so, based on their molecular composition and
folding characteristics in relation to how (or
whether) they become adsorbed to biochar
88 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
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surfaces.Very little is known currently about
the functionality of microbial extracellular
enzymes interacting with biochar of different
compositions.This is an important area for
future research.
CHARACTERISTICS OF BIOCHAR:BIOLOGICAL PROPERTIES 89
Soil organic C plays a pivotal role in nutrient
cycling and in improving plant-available
water reserves, soil buffering capacity and soil
structure (Horwath, 2007). Researchers used
to regard biochar as a relatively inert
substance that was altered very little by chem-
ical or biochemical processes over time
(Nichols et al, 2000). However, biochar
surface properties do change with time (see
Chapter 10) and it is slowly mineralized over
long periods of time (see Chapter 11). Even
though biochar is not strictly inert, decompo-
sition rates are much slower than for
un-charred organic matter (see Chapter 11).
For example, in studies by Liang et al (2006)
on black C from several Amazonian dark
earths, near-edge X-ray absorption Fine
structure (NEXAFS) spectroscopy was used
to map the spatial distribution of C forms on
black C particle thin sections with a resolution
of up to 50nm. For all the black C particles,
regardless of site age, the C forms in the
centres of the particles were similar; however,
each differed in the amount of surface oxida-
tion according to its age, indicating some
surface degradation, but over very long time
scales.These data support the recalcitrance of
black C and indicate that the stability of these
particles ranges from hundreds to thousands
of years. Hence, the biochar particles them-
selves do not appear to act as significant
substrates for microbial metabolism. Instead,
the residual bio-oils on the particles and the
range of compounds adsorbed to the biochar
surface appear to be the only substrates avail-
able – in the short term – to support microbial
growth and metabolism.
Soil microbial populations can be
affected by both the quality (see Chapters 2
to 5) and quantity of the biochar added to
soil.The qualities of biochar depend largely
upon the feedstock and pyrolysis conditions
(see Chapter 2). Flash carbonizing
(McClellan et al, 2007) and some low-
temperature pyrolysis conditions leave
residual bio-oils and other re-condensed
derivatives on the biochar surfaces (see
Chapter 8; Steiner et al, 2008). Depending
upon the composition of these residual pyrol-
ysis compounds, they may serve as substrates
for microbial growth and metabolism, as
proposed by Ogawa (1994) and Steiner et al
(2008); but they may also be toxic to plants,
as shown by McClellan et al (2007), and
possibly to some microbes.
Populations that establish on the biochar
surface will be those that are able to elaborate
the enzymes necessary to metabolize the
available substrates.The more complex and
unusual a substrate is, the more restricted the
population of organisms will be that can use
it effectively as a source of energy, cell C
and/or nutrients, and the longer it will take to
be completely metabolized. It is likely that
organisms colonizing fresh biochar that has
post-pyrolysis condensates on its surfaces will
differ substantially from those colonizing the
biochar surfaces after these deposits have
been metabolized. While some co-metabo-
lism of the biochar itself has been shown to
be likely over longer periods of time (Hamer
et al, 2004; see Chapter 14), it is the C
substrates and inorganic nutrients that
become adsorbed to the biochar surfaces
after the condensates and/or ash are gone that
will be the dominant ‘foodstuffs’ of later colo-
nizing organisms.The nature of other organic
matter added, soil type and texture, plants
Biochar as a substrate for the soil biota
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cultivated and fire frequency (in forested
systems – e.g. Zackrisson et al, 1996), among
other factors, will also affect the nature of
compounds adsorbed to biochar surfaces and
the organisms that are able to successfully
colonize them. Thus, there will likely be a
succession of organisms colonizing over time
as the characteristics of the surface environ-
ment change.
Bio-oils, ash, pyroligneous acids (PAs)
(Steiner et al, 2008) and volatile matter (VM)
(McClellen et al, 2007), among others, are
terms given by various researchers to the
variety of residues remaining on biochar
surfaces immediately following pyrolysis.
Surface-adhering pyrolysis condensates can
include water-soluble compounds such as
acids, alcohols, aldehydes, ketones and sugars
that are easily metabolized by soil microbes.
However, depending upon feedstock and
pyrolysis conditions, they may also contain
compounds such as polycyclic aromatic
hydrocarbons, cresols, xylenols, formalde-
hyde, acrolein and other toxic carbonyl
compounds that can have bactericidal or
fungicidal activity (Painter, 2001). Ogawa
(1994) and Zackrisson et al (1996) have
shown that these substances can, and do,
serve as C and energy sources for selected
microbes. The turnover time of these
substrates is likely to be on the order of one to
two seasons and, thus, will not determine
community composition for any length of
time.
Smith et al (1992) suggested that vari-
ability in the adsorption dynamics of nutrient
and C-containing substrates by biochar (see
also Chapter 5) might alter the competitive
interactions between microbes and change
their overall community structure and
dynamics. Pietikäinen et al (2000) explored
the ability of biochar made from Empetrum
nigrum, biochar made from humus, activated
carbon and pumice to adsorb dissolved
organic carbon (DOC) and to support
microbial populations.These four materials
were added to mesocosms in the laboratory.
Non-heated humus was placed on top of
each absorbent and the mesocosms were
watered with leaf litter extract. The most
DOC was removed by the activated carbon,
whereas the least was removed by the
pumice, with the two biochars intermediate
between these two treatments. All of the
adsorbents were colonized by microbes after
one month; but the respiratory activity was
highest in the two biochar-amended treat-
ments. Phospholipid fatty acid (PLFA)
profiles and substrate utilization patterns (i.e.
Biolog Ecoplate®, Hayward, CA) demon-
strated that different communities developed
on the different adsorbents. Principal compo-
nent analysis of the PLFA profiles showed
that the communities in the two biochars
were most similar to each other and that both
harboured communities divergent from those
on pumice and activated carbon.
Communities colonizing pumice and acti-
vated carbon also diverged substantially from
each other (Pietikäinen et al, 2000).Thus, the
type and availability of substrates associated
with the different adsorbents led to coloniza-
tion by different microbial communities.
Differences in these surface communities
may,in turn, result in changes in the availabil-
ity of nutrients to plants and nutrient cycling,
in general, in the soils to which these adsor-
bents are added. More work is needed to
better understand which organisms colonize
biochar in its initial phases, how these
communities change over time in different
soils under varying management, and how
such changes affect the agronomic outcomes
in biochar-amended soils.
90 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
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The methodological issues that may arise
when analysing biological communities in
biochar amended soils are many and varied.
Most of these issues will be associated with
the capacity of biochar to adsorb a wide range
of organic and inorganic molecules. Most of
the assays typically used to detect the abun-
dance of soil biota in general (i.e. microbial
biomass) and their activities (e.g. adenosine
tri phosphate (ATP) as a measure of soil
energy charge and CO2evolution as a meas-
ure of soil respiratory activity) can be
confounded by the strong sorption of the
molecules being extracted or evolved and
measured as surrogates for the specific
processes involved.The sorption capacity of
biochar is therefore likely to introduce signifi-
cant biases into most methods used to assess
the abundance, activity and diversity of the
soil biota, including extraction of DNA from
soil and follow-on molecular analyses. Since
biochar can adsorb many inorganic nutrient
elements (e.g. NH4+, HPO4and H2PO4),
DOC, as well as chemi-sorb CO2and O2, our
ability to fully extract these compounds (or
measure gases released) is likely to be
compromised; hence, we are likely to underes-
timate the values derived from most assays
conducted on biochar-amended soils. Such
sorption can affect an assay as straightforward
as measuring inorganic N contents (typically
a KCl or K2SO4extraction) to more complex
assays, such as using cell C contents liberated
by fumigation to estimate microbial biomass
and measuring CO2captured in the head-
space of contained samples to calculate soil
respiratory activity.We are studying the sorp-
tion capacity of a mineral soil from Auburn,
New York, to which corn biochar was added at
12t ha-1.The ability to extract DOC added to
the soil was significantly reduced in the pres-
ence of biochar, indicating that the capacity of
the amended soil to adsorb DOC was very
high (see Figure 6.3).
These preliminary results illustrate that
our estimates of microbial biomass derived
from soil DOC extracts may be seriously
underestimated as the proportion (and likely
type) of biochar added to soil increases. Use
of internal standards, such as spiking with
specific marker molecules and assessing their
recovery, will be necessary to improve esti-
mates of microbial parameters that are based
on extractions.
CHARACTERISTICS OF BIOCHAR:BIOLOGICAL PROPERTIES 91
Methodological issues
Figure 6.3 Time course of
dissolved organic carbon
(DOC) adsorption in slurr ies
of soil with 30t biochar ha-1
added compared to
un-amended soil
Note: 10g soil shaken with 40mL of
0.05M K2SO4containing 100mg DOC
L-1 for either 5, 10, 15, 20, 30, 60, 90,
120, 150, 240 or 360 minutes; DOC
measured by oxidation and infrared gas
analysis (N = 3).
Source: Jin et al (2008)
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In practical terms, this means that values
derived from most microbial assays will be
underestimated when these measured vari-
ables are derived from either soil extracts or
headspace gas measurements.Thus, caution
in interpreting data derived from these types
of assays is clearly warranted.
92 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
In the Amazonian dark earths, which are
rich in biochar, microbial community activ-
ity, biomass and composition are
significantly different from those in adjacent
un-amended soils (Thies and Suzuki,
2003). In studies with ADE, different
researchers have shown that these soils have
a higher microbial biomass and abundance
of culturable bacteria and fungi, but signifi-
cantly lower respiratory activity and, thus, a
higher metabolic efficiency (O’Neill, 2007;
Liang, 2008). For example, Liang (2008)
measured CO2evolved over a 532-day
period from four ADE of varying ages and
their adjacent background soils that were
low in biochar. Regardless of site age, the
microbial activity of the four ADE was simi-
lar and 61 to 80 per cent (p<0.05) lower
than any of the adjacent soils on a per unit C
basis. However, microbial biomass was 43 to
125 per cent higher (p<0.05) overall in the
ADE than the adjacent soils. Thus, the
metabolic quotient (the ratio of C evolved as
CO2to microbial biomass C) was signifi-
cantly lower in the ADE, indicating a higher
metabolic efficiency of the microbial
community. It is this reduction in CO2
evolved by a larger microbial biomass that is
proposed to lead to the increased retention
and stabilization of organic matter in the
ADE over time, relative to the typically
impoverished state of the highly weathered
soils of the Amazon region.
In field studies in Aurora, New York,
mineral soil was amended with varying rates
(0, 1t ha-1, 3t ha-1, 12t ha-1 and 30t ha-1) of
corn-derived biochar. Soils were sampled at
the end of the first cropping year and soil
respiration was measured over an eight-week
period (Jin et al, 2008).Total respiration and
the respiratory rate decreased with increasing
biochar added (see Figure 6.4) – just as
observed by Liang (2008) with the ADE
soils.
The decreased respiratory activity we
observed in response to adding biochar could
Effects of biochar on the activity of the soil biota
Figure 6.4 Soil respiration
rate decreases as the rate of
biochar applied increases;
incubations of 20g of soil at
50 per cent water holding
capacity, CO2captured in
0.5M NaOH and
quantification of electrical
conductivity as a measure of
trapped CO2(N = 3)
Source: Jin et al (2008)
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indicate that the biochar is inhibiting the
activity of biochar-colonizing microorgan-
isms, changing bacterial to fungal ratios (or
population structure), increasing C use effi-
ciency, and decreasing population abundance
or some combination of these responses.
Changes may also result from chemi-sorption
of respired CO2to the biochar surface. If
sorbed, CO2would not be recovered in the
assay and, thus, artificially reduce the estimate
of respiratory activity.Which of these scenar-
ios is the primary driving mechanism for
reduced CO2release from biochar amended
soils is yet to be resolved. Evidence from the
ADE soils (O’Neill, 2007; Liang, 2008) and
our measurements of microbial biomass in the
Aurora, New York, experiment suggest that
microbial abundance increases in soils rich in
biochar; thus, decreased abundance is not
among the driving mechanisms. This is
substantiated by the results of Zackrisson et al
(1996), who investigated the effects of
biochar on soil microbial properties at six
sites.They found that microbial biomass was
consistently enhanced in humus when it was
placed adjacent to biochar particles.
Steiner et al (2008) studied the effect of
adding different combinations of biochar,
kaolin and PA on substrate-induced respira-
tion (SIR) of the microbial community in a
highly weathered Amazonian upland soil. In
three separate mesocosm experiments, basal
respiration was measured for 11 to 18 hours
before adding glucose and measuring SIR.
Basal respiration did not differ between treat-
ments composed of:
• varying rates of wood biochar added;
• varying combinations of kaolin and wood
biochar added’ or
• varying combinations of biochar, water
and PA (a potential microbial substrate)
added to soil mesocosms.
When glucose was added, however, the
substrate-induced respiratory activity of the
soil biota as measured by the total CO2
evolved over the following 34 hours increased
with increasing amounts of biochar (0, 50g
kg-1, 100g kg-1 and 150g kg-1) added to soil,
with and without kaolin substitution. Adding
only water to biochar did not increase micro-
bial respiratory activity. However, when easily
metabolizable organic matter (glucose) was
added to the soil amended with biochar +
water, soil microbial activity increased expo-
nentially over the following 15-hour period.
Amending soil with biochar, water and PA
together increased microbial respiratory
activity for a short period (10 hours) before it
dropped back to the basal rate.When glucose
was added to this treatment, an exponential
increase in activity that was sustained over 15
hours was observed; but the respiratory rate
was significantly higher than that of the
biochar + water + glucose treatment.Thus,
the PA added appeared to be a metabolizable
substrate for the microbial community.
Steiner et al (2008) used these SIR data to
calculate microbial biomass and concluded
that PA stimulated microbial growth above
that of adding biochar alone; thus, PA must
contain easily degradable substrates able to
support microbial colonization, in general.
SIR has been used as a means to calculate soil
microbial biomass in many agricultural soils
(Anderson and Domsch, 1978). However,
the possibility that biochar may chemi-sorb
CO2directly or that CO2may be fixed by
chemolithotrophs associated with biochar
particles has not been adequately explored;
thus, the use of the exponential respiratory
rate to calculate microbial biomass in experi-
ments where biochar is added to soil must be
done with caution.
Reduced respiratory activity in biochar-
amended temperate soils (see Figure 6.4) and
in the ADE soils of the tropics (Liang, 2008)
contrasted with the respiratory response to
added glucose observed by Steiner et al
(2008) suggests that reduced respiratory
activity in biochar-amended soils may, in
part, be due to changes in substrate quality
and/or availability.
CHARACTERISTICS OF BIOCHAR:BIOLOGICAL PROPERTIES 93
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Effects of chemi-sorption on
soil biotic activity
The presence of biochar in soil enhances the
adsorption of DOC (see Figure 6.3), inor-
ganic nutrients, various gases, as well as
potentially toxic compounds, such as pesti-
cides, heavy metals and toxic secondary
metabolites (see Chapters 14 and 16), all of
which can influence the abundance, diversity
and activity of soil organisms. For example,
Wardle et al (1998) investigated the short-
term effects on plant growth and microbial
biomass of adding biochar to boreal forest
soils in glasshouse studies.They found that
adding biochar to humus collected from
three forested systems differing in under-
storey vegetation increased soil microbial
biomass and plant growth in the test system.
They suggested that the biochar acted to
adsorb secondary metabolites and phenolics
that were produced by the decomposing
ericaceous vegetation, with the net result of
increasing soil nutrient availability. This is
discussed in greater detail in Chapter 14.
Whether an adsorbed chemical is bio-
available or not, and, hence, whether its
adsorption increases or decreases microbial
activity, will depend upon the molecular
structure of the chemical, the binding sites on
the molecule and biochar surface, the type of
biochar and the characteristics of the
microorganisms in question (see Figure 6.5).
The strength of binding will also vary in rela-
tion to the type of molecular surface
interaction dominating (i.e. hydrophobic
interactions, covalent bonding, van der Waals
forces, cation or anion exchange, or ion
substitution) (see Chapter 16).
Adsorption of both substrate and
microorganisms to biochar surfaces may
result in a higher concentration of substrate
near the attached bacterial cells and, there-
fore, may increase substrate use
(Ortega-Calvo and Saiz-Jimenez, 1998).
Purines, amino acids, and peptides that enter
the interlayer region of expanding clays, such
as montmorillonite, may not affect microbial
metabolism because the cells cannot access
the substrate.This may also be the case for
these compounds in the porous structure of
biochar.
It is still not clear if the adsorption of
compounds to biochar inhibits microbes,
increases nutrient immobilization, or simply
provides microbes a protected site with
adequate resources away from predation
(Pietikäinen et al, 2000;Warnock et al, 2007).
Considering the complexity of interactions
among biochar, inorganic nutrients, minerals
and microorganisms in soils, many questions
still remain to be answered regarding the
94 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
Figure 6.5 Potential
simultaneous adsorption of
microbes,soil organic matter,
extracellular enzymes and
inorganic nutrients to biochar
surfaces:Availability of C, energy
and nutrients for colonizing
microorganisms will depend upon
the nature and strength of these
interactions and, in the case of
enzymes,if adsorption affects
access to the enzyme active site(s)
Source: chapter authors
ES_BEM_13-1 13/1/09 15:42 Page 94

mechanisms governing the direct effects of
biochar on soil organisms (e.g. surface inter-
actions with microbial cell walls or capsular
materials) and the indirect effects that may
result from changes in adsorption of organic
matter, nutrients and clays and other miner-
als (see Chapters 3, 10 and 11). Research on
these topics will be critical for increasing our
understanding of the potential benefits of
biochar as a soil ameliorant.
CHARACTERISTICS OF BIOCHAR:BIOLOGICAL PROPERTIES 95
Soil biological communities are complex
assemblages of bacteria, archaea, fungi, algae,
protozoa, nematodes, arthropods and a diver-
sity of invertebrates. Interactions among the
members of these populations and soil chem-
ical and physical properties will determine
overall ecosystem function and productivity.
The chemical and physical characteristics of
different biochars will add another layer of
complexity to soil food web interactions by
altering the availability of soluble and partic-
ulate organic matter (substrates), mineral
nutrients, pH, soil aggregation and the activ-
ity of extracellular enzymes (see Figure 6.5),
and, thus, will affect diversity, abundance and
distribution of associated microbial commu-
nities.
Bacteria and Archaea
Work on characterizing Bacteria and Archaea
populations associated with biochar is in its
infancy. Pietikäinen et al (2000) showed that
biochar-associated communities differed
from those associated with pumice or acti-
vated carbon (see above) in terms of their
phospholipid fatty acid (PLFA) and C
substrate utilization profiles, but did not iden-
tify specific populations involved beyond the
large groupings based on PLFAs predomi-
nating on the various substrate surfaces.
Steiner et al (2008) examined respiratory
activity and projected biomass estimates, but
did not identify key groups colonizing
biochar- or PA-amended Brazilian soils.
Much of what we know about how the pres-
ence of biochar changes in bacterial and
archaeal communities comes from work with
the ADE soils in Brazil (Thies and Suzuki,
2003; Kim et al, 2007; Grossman et al,
submitted). Grossman et al compared micro-
bial community compositions between four
anthrosols and adjacent background soils
with the same mineralogy under four differ-
ent land uses, using microbial community
DNA fingerprinting followed by cloning and
sequencing. Microbial communities from
ADE were similar to each other regardless of
site, and these communities were distinct
from those in the adjacent soils. Archaeal
communities in the adjacent soils diverged by
over 90 per cent from those characterized
from the ADE. Clearly, factors common to
the ADE are stronger drivers of microbial
community composition than factors associ-
ated with soil type, sampling depth or land
use – factors that normally strongly influence
microbial community composition in soils
without biochar. Indeed, bacterial communi-
ties in the adjacent background soils
separated primarily by soil type and/or land
use. Sequencing of taxa unique to particular
samples showed that both anthrosols and
adjacent soils contained organisms that are
taxonomically distinct from those found in
sequence databases (i.e. GenBank). Most
sequences obtained were novel and matched
those in databases at less than 97 per cent
similarity. Sequences obtained only from the
ADE grouped at 93 per cent similarity with
the Verrucomicrobia, a genus commonly
found in rice paddies in the tropics and
increasingly being shown to be present in
agricultural soils. Proteobacteria and
Cyanobacteria spp were found only in adja-
Diversity of organisms interacting with biochar
ES_BEM_13-1 13/1/09 15:42 Page 95

cent background soils, and Pseudomonas,
Acidobacteria, and Flexibacter spp were
common to both soil types.The predominant
difference between the ADE and adjacent
background soils was the presence of biochar
in the ADE.The high similarity in bacterial
and archaeal community composition in the
ADE suggests that biochar-amended soils
will also select for distinct microbial commu-
nities. Much work is needed to identify
population differences arising from biochar
amendments and what soil processes may be
affected by changes in microbial community
composition and dynamics.
Kim et al (2007) examined the gross
diversity of bacterial populations extant in an
ADE as compared to an undisturbed forest
site in the western Amazon. They used
oligonucleotide fingerprinting of 16S rRNA
gene sequences amplified from soil DNA
extracts, which indicated that,while there was
considerable overlap in the broad groups of
bacteria identified, the ADE soil bacterial
population was 25 per cent more diverse than
that in the undisturbed forest soil (see Figure
6.6).
In studies on the ADE, we used the
BacLight™ fluorescent staining assay to visu-
alize live and dead microbial cells on the
surface of biochar particles picked out of the
ADE. In Figure 6.7, live (green fluorescence)
and dead (red fluorescence) bacteria, fungi
and fine roots can be clearly seen.This illus-
trates the capacity of biochar to support
active microbial populations and retain dead
organisms briefly on the biochar surfaces.
Nitrogen (N2) fixation by diazotrophs
The use of biochar as a soil ameliorant can
potentially have many different effects on N2-
fixing bacteria (diazotrophs), root nodulation
and N2fixation. Diazotrophs are a special-
ized group of bacteria with a diverse
phylogeny, but the common functional
capacity of sequentially reducing atmos-
pheric N2to ammonia (NH3), which is often
used immediately to produce amino acids.
Diazotrophs fix N2either as free-living soil
bacteria (e.g. Azotobacter sp or Azospirillum
sp) or as mutualists in association various
plants, such as the rhizobia that form N2-
fixing nodules on legume roots and the
actinorrhizal association of Frankia sp with
the roots of various tree species. Only organ-
isms in the domains Bacteria and Archaea
have the genetic capacity to produce the
enzyme nitrogenase, which is required to fix
atmospheric N2. Nitrogenase is, however,
deactivated in the presence of O2and
requires Fe and Mo to produce.
For free-living diazotrophs, the fine pores
of biochar create a habitat where reduced O2
tensions are likely. If Fe and Mo are available
in sufficient supply, the fixation of atmos-
pheric N2will increase an organism’s
competitiveness in the biochar environment
and, thus, their proportional representation
within the biochar and soil community.
For mutualists, such as rhizobia, avail-
ability of N will strongly influence nodulation
96 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
Figure 6.6 Taxonomic cluster
analysis of 16S rRNA gene
sequences from Amazonian dark
earths (ADE) and adjacent pris-
tine forest soil based on
oligonucleotide fingerprinting of
16S rRNA gene sequences
Source: Kim et al (2007), with permission
from the publisher
ES_BEM_13-1 13/1/09 15:42 Page 96

and subsequent N2fixation because legumes
will preferentially take up inorganic N from
the soil solution.The low N content of most
biochars and the exchange of NH4+between
the biochar surface and soil solution are likely
to modify N availability to plant roots and
may stimulate nodulation and N2fixation in
legumes and actinorrhizal plants.
Matsuo Ogawa has worked with N2-
fixing bacteria and mycorrhizal fungi for over
30 years. In 1994, he summarized his work
on the response of these organisms to adding
biochar to soil. He reported that sprinkling
biochar over field soils increased his ability to
culture and isolate N2-fixing bacteria from
soil samples on to an N-free medium. He
reported that Azotobacter sp was identified
from among these isolates. He has tested the
use of biochar as a carrier for N2-fixing
rhizobia inoculant and as a carrier material
for mycorrhizal fungi. Inoculation of soybean
with biochar-based rhizobia preparations
increased nodulation and N2fixation. Adding
biochar to soil also appeared to stimulate the
N2-fixing activity of free-living diazotrophs.
Ogawa (1994) proposed that these bacteria
might be poorer competitors whose survival
in soil may be enhanced by their ability to
colonize the biochar pores. Most biochars are
very low in inorganic N content, giving
diazotrophs a competitive advantage for
surface colonization. Increased N2fixation by
rhizobia in association with plants may result,
in part, from the transient adsorption of
NH4+on biochar surfaces that could lower
the inorganic N concentration in the soil
solution and improve root nodulation and
subsequent nodule activity. Biochar may also
sorb important signalling molecules, such as
nod factors, increasing their longevity in soil
and the likelihood that they will interact with
compatible rhizobia bacteria and improve
nodulation.
Rondon et al (2007) examined the effect
of biochar additions on N2fixation by rhizo-
bia-nodulating Phaseolus vulgaris in
Colombia. Increasing rates of biochar (0,
30kg ha-1, 60kg ha-1 and 90kg ha-1) increased
the proportion of nitrogen derived from fixa-
tion (percentage NdF) from 50 per cent in
the control to 72 per cent in the 60kg ha-1
treatment and increased bean yields by 46
per cent. They attributed these findings to
increased availability of Mo and B (for nitro-
genase function), increased soil pH and
increased N immobilization. Rhizobia tend to
prefer circum-neutral pH; thus, increasing
pH in an otherwise strongly acidic soil may
be a major factor in improving nodulation
and N2fixation in these trials.These authors
also examined whether arbuscular mycor-
rhizal fungi colonization was increased by
adding biochar, but did not observe any
significant effects.
CHARACTERISTICS OF BIOCHAR:BIOLOGICAL PROPERTIES 97
Figure 6.7 Bacteria, fungi and
fine roots readily colonize biochar
surfaces
Source: Tsai et al (2008)
ES_BEM_13-1 13/1/09 15:42 Page 97

Use of biochar as an inoculant
carrier
Many microorganisms have been used to
increase crop production through batch
culturing, adding the inoculum to an appro-
priate carrier and either placing the inoculum
in the planting furrow or adhering it to seeds
immediately prior to planting. Both mutualis-
tic and free-living N2-fixing bacteria, other
plant growth promoting rhizobacteria
(PGPR), such as Paenibacillus,Bacillus and
Pseudomonas, and saprophytic (e.g.
Trichoderma harzianum) and mycorrhizal
fungi have been used as inoculants applied to
field soils. Ogawa (1994) has used biochar as
a carrier substrate for both rhizobia and for
arbuscular mycorrhizal (AM) over the past
20+ years with excellent success. Additional
studies conducted in Japan (Takagi, 1990)
and in Syria (Beck, 1991) have shown that
biochar is a suitable carrier for the N2-fixing
root nodule bacteria Rhizobium,
Mesorhizobium and Bradyrhizobium. It is not
difficult to speculate on the variety of appli-
cations biochar inoculants may have in
agriculture and environmental remediation.
Biochar may prove a most efficient inoculant
delivery system and may also improve
outcomes of bioremediation efforts by
increased sorption of organic pollutants onto
biochar impregnated with bacteria selected
for their capacity to degrade the target pollu-
tants.
Fungi
Soil fungi are a heterogeneous group both
functionally and phylogenetically (Thorn and
Lynch, 2007), encompassing members of the
phyla of the Eumycota, as well as non-
Eumycotan phyla (notably the Oomycota).
In terms of function, the fungi can be
coarsely divided into saprophytes, plant
pathogens and mycorrhizal fungi. Each one
of these groups could exhibit very different
responses to biochar application that need to
be understood.While briefly addressing each
of these groups, the main discussion is
focused on mycorrhizal fungi, whose interac-
tion with biochar has been studied the most.
Saprophytic fungi
Saprophytic fungi, as decomposers, are
particularly important as they may influence
the persistence and modification of biochar
materials in soil. In contrast to bacteria, fungi
have a hyphal, invasive growth habit (aptly
likened to tunnelling machines; Wessels,
1999), which gives them access to the interior
of solid materials. This means that sapro-
phytic fungi could be effective colonizers of
the interior of biochar particles. Fungi also
have exceptional enzymatic capabilities, and
this further highlights the need to study fungi
as decomposers of biochar. For example,
Laborda et al (1999) showed that fungi
(Trichoderma and Penicillium spp) could
contribute to depolymerization of coal (hard
coal, sub-bituminous coal and lignite) via
production of enzymes such as Mn-peroxi-
dase and phenoloxidase. Hockaday (2006)
reported degradation of biochar by fungal
laccase.
To what extent do biochar particles serve
as a habitat for soil fungi? Ogawa and Yamabe
(1986) suggested that biochar may be an
unsuitable habitat for saprophytic fungi, but
not for mycorrhizal fungi. However, this will
greatly depend upon the nature of the
biochar, as well as upon the amount of labile
organic molecules that sorb to biochar in soil,
which may thus serve as a source of C and
energy for soil microbes. For example,
Pietikäinen et al (2000) found that naturally
produced biochar from forest wildfires
hosted microbial communities, but no
specific emphasis was placed on colonizing
fungi. More recently, in approximately 100-
year old biochar, Hockaday et al (2007)
visualized filamentous growth of unidentified
microbes inside of aged biochar particles by
scanning electron microscopy. Given the
scale in the figure, these filaments are about
98 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
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4Ìm in diameter, and therefore likely to be
fungi, rather than actinomycetes. Given that
fungi can inhabit both the exterior and the
interior of biochar particles, questions
regarding the community composition and
identity of these (presumably) saprophytic
fungi arise. It is not clear whether they can
modify the biochar material chemically
through the secretion of extracellular
enzymes, or whether the composition of
fungal consortia and a suite of extracellular
enzymes are important in this process.
Fungi, due to their mycelial nature, can
also help to stabilize biochar in the soil matrix
and within soil aggregates. Fungal filaments
and metabolic products serve as binding
agents at the level of meso- and macro-aggre-
gates (Tisdall and Oades, 1982; Rillig and
Mummey, 2006).
Pathogenic fungi
Fungal pathogens are widely recognized for
their role in agro-ecosystems (Agrios, 1997)
and increasingly also in natural ecosystems.
Disease magnitude is a function of the inter-
play of host susceptibility,pathogen virulence
and environmental conditions (the disease
triangle), and some or all of these factors
could be affected by biochar additions.
Nevertheless, there are very few studies that
have examined biochar effects on fungal root
pathogens.
Matsubara et al (2002) provided the
most detailed account of the interaction of
biochar materials and pathogenic fungi using
the Fusarium oxysporum-asparagus pathosys-
tem. The authors also included inoculation
with arbuscular mycorrhizal (AM) fungi as a
treatment.They found that in AM pre-inocu-
lated plants, disease indices were strongly
reduced, and even further reduced when
biochar was added. The results thus
suggested that biochar may enhance the abil-
ity of AM fungi to help plants resist fungal
pathogen infection. Steiner et al (2008)
examined the role of smoke condensates
from biochar production (PA) on soil
microbes because previous reports cited in
this chapter had indicated a ‘soil sterilizing’
effect. Stimulated microbial populations and
activity in response to PA addition were
found (see discussion above); but the study
did not include effects on potential
pathogens.This may be a promising research
avenue.
Given the scarcity of data, there are clear
needs for research on the effects of biochar
on pathogenenic fungi: several selected
pathosystems, especially those involving soil-
borne phytopathogens, should be examined
for how the interaction of pathogen and host
could be affected, and what the specific
mechanisms are. Additionally, it would be
highly desirable to include monitoring of root
lesions or other plant disease symptoms in
fields where biochar is applied.
Mechanisms could, to some degree, be
similar to the ones described for other
biotrophic fungi-forming mycorrhizae (see
the following sub-section). Biochar-mediated
increases in water-holding capacity could
also favour certain pathogens with zoospores,
such as the Pythium or Phytophthora in the
Oomycota. If biochar alters root architecture
(e.g. through nutrient effects at the individual
plant level, or increased abundance of fine-
rooted plant species at the plant community
level), this could also have consequences for
host susceptibility since finer root systems
may offer a greater surface area for attack by
soil-borne pathogens (Newsham et al, 1995).
Mycorrhizal fungi
Mycorrhizae are common root-fungal mutu-
alisms with key roles in terrestrial ecosystems
(Rillig, 2004). There are several types of
mycorrhizas, the most common of which are
arbuscular mycorrhizae (AM) and ectomyc-
orrhizae (EM) (Smith and Read, 1997).
These two groups are distinct morphologi-
cally, physiologically and ecologically with
respect to the plant hosts, and also in regard
to phylogeny of the fungal partner.Thus, it is
highly likely that they also respond differently
CHARACTERISTICS OF BIOCHAR:BIOLOGICAL PROPERTIES 99
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to biochar additions.
There has been keen interest in the
effects of biochar on mycorrhizae, with
pioneering work coming primarily from
Japanese researchers.The interest in mycor-
rhizae and biochar is probably due to three
reasons. First, mycorrhizal fungi are ubiqui-
tous key components in virtually all biomes
(Treseder and Cross, 2006).Therefore, it is
important to understand how any soil addi-
tive, including biochar, may affect their
performance. AM fungi colonize most of the
important crop species (corn, rice, wheat,
etc.) so that they are also of interest from a
perspective of agro-ecosystem productivity
and sustainability. Second, mycorrhizae are
sensitive to management interventions
(Schwartz et al, 2006), such as adding
biochar, and it is tempting to speculate on the
possible synergistic effects of mycorrhizal
inoculation and biochar application in
enhancing soil quality and plant growth.
Applying biochar to soil stimulated the colo-
nization of crops by AM fungi. Nishio and
Okano (1991) reported that root infection by
AM fungi significantly increased alfalfa yield
by 40 to 80 per cent when 1kg m-2 of biochar
was added to an alfalfa field in a volcanic ash
soil. Third, the majority of the studies
reported in the literature show a strongly
positive effect of biochar on mycorrhiza
abundance (Warnock et al, 2007), which is
intriguing from a mechanistic perspective.
Warnock et al (2007) summarized the
literature on responses of mycorrhizae to
biochar additions and provided several mech-
anisms for biochar effects on mycorrhizae.
Some of these had been proposed previously,
but very few have been thoroughly tested.
Here, we regroup mechanisms of interaction
into physical, chemical and biological; these
are, of course, strongly interrelated and
would be acting concurrently.
Physical effects
Saito (1989) reported that hyphae and spores
of AM fungi were visible on extracted
biochar particles following a field application
of biochar in a C-rich soil, and that it
appeared that AM fungi had colonized these
particles. It was suggested that the porous
nature of the biochar particles or the reduced
competition from saprophytes (for which this
habitat was presumed to be less suitable)
could have contributed to this (Saito, 1989;
Saito and Marumoto, 2002). Saito and
Marumoto (2002) suggested that biochar
particles act as a microhabitat for AM fungi
and enable them to survive, and may also
provide protection from predator grazing.
Ezawa et al (2002) reported that AM fungal
root colonization was increased in the pres-
ence of ground biochar as opposed to
non-ground material, and also attributed this
effect mainly to the porous nature of biochar;
however, high application rates of 30 per cent
volume per volume (v/v) were used. Apart
from these observations, it seems that a quan-
titative or functional assessment of hyphal
colonization of these particles remains lack-
ing. Porous particles, such as expanded clay,
are used often in AM inoculum production
(as a carrier material) because of the docu-
mented association of AM fungi with these
particles (e.g. Baltruschat, 1987).Therefore,
it seems likely that surface phenomena and
micro-pore habitats could play an important
role in improving mycorrhizal interactions
with plant roots; but the mechanisms are not
yet understood.
Chemical effects
There is considerable evidence for the impor-
tance of chemical changes in the effects of
biochar on mycorrhiza abundance (e.g.
nutrient availability and pH changes;
reviewed in Warnock et al, 2007). Biochar has
frequently been documented to alter, and
often increase, the availability of N and P in
the rooting zone. Depending upon the gener-
ation temperature and feedstock properties,
biochar itself may add nutrients (see Chapter
5). Mycorrhizal fungi and C supply from
their hosts can react sensitively to these vari-
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ables. A different chemical effect pertains to
signalling in the rhizosphere, the soil infor-
mation ‘superhighway’ (Bais et al, 2004).
Adsorption of inhibitory compounds or their
sequestration and the slow release of positive
signalling molecules are examples of how
biochar could interfere with root-fungus or
other signal exchanges. AM fungi can
respond to a number of chemical signals
(such as flavonoids, sesquiterpenes and
strigolactones), which alter growth or
branching (e.g. Akiyama et al, 2005).While a
likely mechanism, no direct evidence has
been provided for biochar signal interference
in the soil.
Biological effects
From the perspective of organism interac-
tions, effects at the same, lower or higher
trophic levels could influence organism abun-
dance and functioning. Competition or
facilitative interactions could be altered in the
presence of biochar or as a consequence of
effects on physico-chemical soil properties.
Mycorrhizal fungi could, for example,
compete with saprophytes (Gadgil and
Gadgil, 1971), and this interaction could be
altered by biochar.Alternatively, mycorrhiza-
tion helper bacteria (Garbaye, 1994) could
aid mycorrhizal fungi in colonizing roots, and
this group of facilitative organisms could
become stimulated.The trophic level below
mycorrhizal fungi consists of roots, and fungi
in the soil are hypothesized to be controlled
mostly from the bottom up (Wardle, 2002).
Carbon allocation to obligate biotrophic AM
fungi and EM fungi (which do include
species with saprophytic abilities) would be
pivotal for mycorrhizal fungal abundance,
and this allocation is regulated in numerous
ways (Koide and Schreiner, 1992), including
through delivery of fungal services, such as
nutrient acquisition (Javot et al, 2007).Fungi,
including mycorrhizal fungi, are subject to
grazing by soil fauna. Altered grazing interac-
tions – for example, through toxic effects on
grazers or provision of refugia (enemy-free
space) in biochar particles – could lead to
changes in abundance.
There are pressing needs for research on
the effects of biochar on mycorrhizal
symbioses. It is important to explore the full
parameter space (e.g. biochar feedstock,
production temperature, application rate, soil
nutrient status and ecosystem type) in terms
of effects of biochar on this symbiosis.This
also includes the reporting of negative or
neutral effects; perhaps there has been biased
reporting towards positive effects. In order to
understand what effects biochar has on
mycorrhiza, mycorrhizal response variables
need to be examined in a more differentiated
way.This entails measuring different phases
of the fungus (e.g. for AM fungi the extra-
radical and the intra-radical phases), the
fungal community composition and the func-
tioning of the fungal interaction with the host
plant. Studies need to address specific mech-
anistic hypotheses, thus moving beyond mere
phenomenological assessment of effects.
Only in this way will causes of biochar effects
be understood and, thus, can clear manage-
ment recommendations be made.
Finally, the above discussion and that in
Warnock et al (2007) has mostly centred on
the mycorrhizae at the individual host plant
level; but mycorrhizal effects in ecosystems
manifest themselves in a hierarchical fashion
(O’Neill et al, 1991). Thus, AM fungi are
known to be important in mediating interac-
tions among co-occurring plants, including
weeds (e.g. Marler et al, 1999). It is therefore
worth considering how biochar could – via
affecting mycorrhizae – also alter the
competitive balance among plant community
members, such as weeds and crops. At the
ecosystem level, effects on individual host
plants are important, as well as plant commu-
nity changes; but mycorrhizal fungi can also
influence a variety of ecosystem-level
processes in multiple ways (Rillig, 2004).
One that may be of particular interest in the
context of biochar and soil C storage is soil
aggregation (Rillig and Mummey, 2006).
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Mycorrhizal fungi, through various mecha-
nisms, including physical, biological and
biochemical pathways, can influence soil
aggregation and, thus, the storage of C within
these aggregates (Rillig and Mummey,
2006). One biochemical mechanism includes
the production of the protein glomalin;
concentrations of glomalin-related soil
protein are often highly correlated with soil
aggregate water stability (Wright and
Upadhyaya, 1998). If biochar enhances the
functionality of mycorrhizal fungi in this
regard (e.g. by enhancing glomalin produc-
tion), more C other than that contained in the
biochar itself could be stored.This is a topic
worthy of pursuit in future studies.
Soil fauna
Biochar could affect soil fauna directly or
indirectly, but relatively little direct data are
available yet. Indirectly, soil fauna could be
affected by altered biotic resources. Energy
and matter flow through soil food webs is, at
a coarse level, organized into energy chan-
nels: the fungal-based and bacteria-based
energy channels. Thus, if shifts between
fungi and bacteria in response to biochar
occur, these will likely ripple on to changes
at the higher trophic levels within each
energy channel. Directly, soil fauna could be
influenced by ingesting biochar particles.
This is the case for geophagous fauna, such
as earthworms (Topoliantz and Ponge,
2003, 2005). Here, it may be an interesting
question to examine how biochar may inter-
fere with the intricate associations of
earthworm gut microbes. Further direct
effects include all documented physico-
chemical changes caused by biochar
additions.
102 BIOCHAR FOR ENVIRONMENTAL MANAGEMENT
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Soil management strategies, such as amend-
ing soil with biochar, should aim to support
and enable the soil biota to carry out the key
ecosystem functions that they moderate in
order to ensure long-term soil fertility and
sustained crop production. Amending soil
with biochar needs to involve a careful selec-
tion of the feedstock and pyrolysis conditions
to find an optimal match of biochar type to
the intended ecosystem goal(s). Among the
many other aims of adding biochar to soil, it
is important to ensure sustained functioning
of the soil biota so that critical ecosystem
functions are maintained. In future work, the
effects of biochar on various soil biota
groups, their diversity and functioning need
to be carefully considered.
Conclusions
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