Iris Publishers
Biochar and its Use in Soil: Lessons from Temperate Agriculture
Authored by M Oelbermann
Biochar
Basics
What is biochar?
Biochar is a carbon-rich product
that is obtained when a sustainable source of biomass is heated without oxygen.
Biochar is produced in the same way as charcoal, but its intended use differs.
Biochar is manufactured with a specific set of properties (e.g. adsorption
properties, ion exchange capacity, low bulk density) for its use as soil
amendment and/or adsorbent, whereas charcoal has specific properties (e.g.
generating heat) for its use as a fuel. Humans first used biochar in the form of
charcoal (as a byproduct from cooking) mixed with broken pottery, animal bones
and manure in the Brazilian Amazon. This led to the creation of the Amazonian
Dark Earths or Terra Preta more than 2000 years ago by pre-Columbian cultures
of this region. Whether these soils were created intentionally or if they were
a by-product of human settlements remains unclear. However, Terra Preta soils
are highly fertile and have demonstrated the potential for long-term carbon
sequestration (Figure 1). Based on this premise, researchers have encouraged
the deliberate addition of biochar to tropical soils to enhance their
fertility. From this work, it was found that biochar substantially improved
soil properties and crop productivity, because adding biochar to nutrient-impoverished
tropical soils decreased aluminum toxicity and increased soil pH, resulting in
enhanced microbial activity and nutrient availability. However, temperate soils
have lower iron and aluminum oxide content, a higher pH, high-activity clays
and greater soil organic matter content and will therefore respond differently
to biochar than tropical soils. Amending intensively managed temperate soils
with biochar is a more recent approach to agriculture, with research still in
its infancy. However a soil amendment like biochar that can maintain or improve
soil health, potentially decrease fertilizer input and minimize nutrient
leaching will not only place less pressure on soil resources but also
contribute to environmental sustainability [1].
Biochar
feedstocks
Biochar can be produced from a
variety of feedstocks including residues from the agricultural and forest
industries (straw, rice hulls, wood chips, nut shells, wood pellets, tree bark,
bagasse, manure), organic waste materials and industrial by-products (food
waste, paper sludge, pulp, distillers grains), purpose grown biomass
(miscanthus and switchgrass), or undesirable invasive grasses (phragmites). Due
to biochar’s structure, it is chemically and biologically more stable compared
to the organic matter from which it is made. However, the source of the
feedstock and the conditions during biochar production (pyrolysis) are major
controls on the biochar’s characteristics including its carbon and ash content,
pH, elemental composition and stability (Table 1). Not all biochars are the
same, and the benefits of biochar are not universal. For example, biochar made
from animal manure will have a higher nutrient content and is more likely to
improve soil fertility. Biochar made from wood waste is more suited to carbon
sequestration due to its greater long-term stability. Also, different soil
textures will interact differently when the same type of biochar derived from
the same feedstock is added (Table 2). Adding the same type of biochar to the
same soil type but under different climates will not result in the same
outcome. Recent research shows that adding biochar to coarse and medium
textured soil results in greater fertility improvement than adding biochar to
heavy textured soil [2]. Biochars can also be engineered to have specific
physical and chemical properties by selecting desired feedstock properties and
pyrolysis conditions. For example, biochars can be engineered to function as a
soil amendment or as a sorbent for pollutants, such as heavy metals and
pesticides [1].
Feedstocks
and quality of biochar
Different feedstocks have
different qualities in terms of nutrient composition, and as mentioned above,
if the same feedstock is processed under different pyrolysis processes, the
biochar produced has different characteristics. Even using the same feedstock
and pyrolysis processes can sometimes generate slight differences in biochar
characteristics. This is the case for feedstocks derived from composted food
waste. Therefore, using a high-quality feedstock source is important in order
to avoid any negative effects on soil. Feedstock derived from contaminated
sites or feedstock containing known contaminants which have been converted to
biochar should not be added to agricultural soil. This is because feedstock
containing heavy metals (e.g. lead, zinc, arsenic) and other contaminants will
generate a low-quality biochar containing heavy metals, polycyclic aromatic
hydrocarbons and dioxins, which will negatively affect the soil, plants and
environment. Therefore, it is important to use biochar produced from high
quality feedstock and through a consistent pyrolysis process (Figure 2). The
resultant biochar should also have been characterized (e.g. carbon, nutrient
and heavy metal content). According to the International Biochar Initiative, a
high-quality biochar must contain at least 60% carbon. Inhibition of crop
germination and earthworm avoidance can be used to test biochar safety. In
addition, the use of a suitable biochar as soil amendment should have these two
criteria of carbon (C), oxygen (O) and hydrogen (H) content according to
Schimmelpfennig & Glaser [3]: O/C ratio<0.4 and H/C ratio<0.6.
Potential
Uses of biochars
Due to its structure and
composition, there are many promising applications for biochar, such soil
amendment, food or feed additive, composite materials, activated carbon,
electrodes for batteries or electrolysis cells, metallurgical coke substitute,
coal substitute, catalyst for tars, pharmaceutical and cosmetic, and could be
eligible for carbon credits. Nanda et al. [4] have published an extensive
review on the broad applications of biochar.
How are biochars made?
Several reviews have illustrated
the different technologies and processes used experimentally and industrially
to produce biochar [4,5]. These include torrefaction, pyrolysis, gasification,
combustion and hydrothermal carbonization (Table 3). Scale and potential
mobility are important considerations in relation to the feedstock supply,
logistics, seasonality, further refining, products quantities, characteristics
and value, and potential markets.
Torrefaction involves heat
treatment of biomass under atmospheric pressure and within a temperature range
of 200–300 °C, without oxygen or with limited oxygen supplies. It yields char
with less moisture, higher energy density, lower weight, lower O/C and H/C
ratio, increased hydrophobic nature and resistance to biological degradation
with respect to the original feedstock. Typical yields of torrefied biomass
range between 50 and 80%. Gasification is a thermochemical process carried out
at temperatures higher than 750 °C in the presence of a gasifying agent
(typically air, oxygen, or steam) at atmospheric or at elevated pressures.
Under these conditions, biochar yields are not sufficiently significant to
consider gasification an appropriate biochar production process. Similarly,
combustion is not a suitable biochar production process, since, theoretically,
under good combustion conditions, biochar yield should be negligible.
Together with torrefaction,
pyrolysis and hydrothermal carbonization are the major processes used for
biochar production, whose characteristics will greatly depend upon temperature,
heating rate, residence time, feedstock type and physical characteristics, and
reactor configuration. Hydrothermal carbonization is performed with wet
feedstock biomass under water in a sealed confined system and heated at the
temperature range of 175–300 °C up to 16 h under saturated pressure under
subcritical conditions producing tar-free biochar (hydrochar) with large number
of functional groups. Hydrochars contain predominantly aliphatic compounds and
more oxygen functional groups and higher cation exchange capacity than
conventional biochars. On the other hand, they have lower surface area, micro
porosity and carbon stability.
Among the various thermal
technologies, pyrolysis has been the most investigated technique and considered
the best technology to produce biochar. The various modes of pyrolysis include
slow, intermediate, fast, flash, and ultra-pyrolysis, carried out under vacuum,
atmospheric pressure or under pressure. Garcia-Nunez et al. [6] have published
an extensive review on pyrolysis reactor technologies.
Due to the balance between
primary, secondary cracking and recombination reactions, bio-oil yields are
typically maximized (up to 70%) at intermediate temperatures (450-550 °C),
faster heating rates (100-500 °C/s) and short vapor residence times (< 1~2
s), which are characteristic of fast and flash pyrolysis. Under these
conditions, biochar yields are typically of the order of 15~20%. On the other
hand, higher biochar yields (25~40 %) are achieved at moderate temperatures
(300-450 °C), slower heating rates (~ 1 °C/s) and longer vapor residence times
(> 5~10 s), representative of slow or intermediate pyrolysis, when bio-oil
yields vary between 40 and 50% with the balance being gas.
The efficiency of biochar in
most applications significantly depends on its carbon and ash content, surface
area, pore size distribution, alkalinity, hydrophobicity, ion-exchange capacity
and elemental composition. These properties are subject to variations depending
on the biochar feedstock, pyrolysis temperature, heating rate, residence time,
potential oxidation medium, preand post-processing treatments. Temperature
plays a prominent role in determining biochar quantity and quality. As the
pyrolysis temperature increases within the broad range between 300 °C and 700
°C, aromatic carbon, ash, surface area, pore volume and pH increase, while
yield, volatile matter, hydrogen and oxygen content, conductivity and cation
exchange capacity decrease. Moisture is also reported to have certain positive
impacts on biochar yield. High moisture containing biomasses are found to improve
the yield of biochar. Studies on woody and agricultural biomass have reported
biochar yields positively correlated with lignin content. Surface area and pore
volume are chief parameters to evaluate the absorption by biochar, particularly
for organic molecules. Although higher processing temperatures increase the
specific surface area, high heating rates favor such increases. Therefore,
biochar derived at higher temperatures are more efficient for adsorption of
organic contaminants due to the higher surface area and pore volume, whereas
biochar generated at lower temperatures is effective for adsorption of
inorganic contaminants due to greater intensity of O-containing functionalities
and higher incidence of cationic complexes in the soil.
Based on these considerations,
the quality of the biochar is strongly influenced by the reactor technology in
which it is produced and on its operating conditions. Although stoves can
produce biochar, its quality is highly questionable as the operating conditions
are not carefully controlled. Biochars produced in batch mode are different
than biochars produced in continuous operations. Mixing is a critically
important parameter to ensure uniform reaction conditions and, consequently,
product quality control. Mixing strongly influences heat transfer and,
therefore, heating rates of the reacting biomass and, as discussed, product
characteristics. Slow pyrolysis is carried out in rotating kilns, intermediate
pyrolysis is auger type or mechanically mixed reactors, whereas fast pyrolysis
is carried out in fluidized, circulating fluidized beds or rotating cone
reactors.
Specific
Uses of Biochar in Agriculture and Horticulture
Soil health is defined as the
capacity of the soil to function as a living system to sustain biological
productivity, maintain environmental quality and promote plant, animal and
human health [7]. The application of biochar to temperate soil has demonstrated
improvement or the potential to improve a variety of soil health indicators
(Figure 3). Biochar can also play an important role in higher-valued
horticultural crops. In the horticultural industry, peat is a favored growing
medium, but due to its long regeneration time, peat is a non-renewable
resource. However, research has shown that 15% (by volume) biochar can be added
to peat and improve crop productivity and soil characteristics [8]. Its labile
carbon content may stimulate microbial activity and interact with roots and
soil bacteria, promoting plant-growth-beneficial microorganisms in peat and in
soil [2,9-11].
The following soil health
characteristics, which can also be linked to the OMAFRA Soil Health Strategy
(http://www.omafra. gov.on.ca/english/landuse/soil-strategy.pdf), have
demonstrated improvement when biochar was added to temperate soil or its use as
a horticultural growing medium [1]:
• increase water holding
capacity and infiltration by adding pore space and supporting aggregation;
• moderation of soil temperature
extremes;
• soil structure through
aggregation (better aggregate stability) and crop root penetration;
• decrease in bulk density;
• increase cation exchange and
nutrient storage;
• moderation of soil acidity;
• changes in microbial community
composition and substrate utilization;
• increase microbial activity
and diversity;
• increase mycorrhizal fungal
colonization;
• increase nitrogen fixation;
• promote beneficial
microorganisms for plant growth;
• reduce greenhouse gas
emissions.
Crop productivity
A recent survey evaluated 1000
different research studies on the effect of biochar on crop yield. Although
biochar showed favorable results on crop productivity, with a 25% increase in
yield in tropical environments this effect was not observed under temperate
conditions. This confirmed that biochar benefits tropical soils but biochar
application to the more fertile temperate soils and its effect on crop yield
remains uncertain [12]. Low yield responses due to biochar addition have been
observed on fertile soils and in some cases low rates of biochar addition did
not result in improved crop productivity. A field study from southern Ontario
(Figure 4) did not find a significant improvement in corn or soybean production
when biochar, derived from pine and spruce wood waste, was added at 3 t/ha and
combined with poultry manure [13]. The same study also found that the ratio of
shoot to root biomass (shoot/root ratio) for corn or soybeans was lower when
soil was amended with biochar. This suggested that crops produced with biochar
experienced lower environmental stress (e.g. water stress) than those produced
without biochar. Most researchers concur that use of a high-quality biochar in
temperate soil, although it may not improve crop yield, will have no negative
effects on crop productivity, but will improve soil health. The application of
biochar in legume-based intercropping system with a cereal crop might be
beneficial for improving cereal growth and reducing N fertilizer supply.
According to Liu et al. [14], a maizepeanut intercrop with biochar amendment
allowed a higher nodule weight on peanut roots promoting N fixation, favoring a
higher N transfer from peanut to maize. There is currently no information on
the effects of biochar on pasture production.
Nutrient leaching and crop
nutrient uptake
Biochar has the ability to
retain soil nutrients such as ammonium, nitrate and phosphorus, and this effect
is most readily observed in light-textured (e.g. sandy) soils. This provides an
opportunity for increased nutrient uptake and reduced leaching of nutrients
applied by fertilizers from entering local water sources (e.g. Ontario Great
Lakes Strategy Priority: https://www.ontario. ca/page/ontarios-great-lakes-strategy).
Some research has suggested that the ability of biochar to adsorb nutrients
could result in lower fertilizer requirements; studies are currently underway
to assess this component. A laboratory study found that adding a low quantity
of biochar (1 t/ha) combined with urea ammonium nitrate enhanced corn nitrogen
uptake in a coarse-textured soil (Table 2). This study also found a higher
nitrogen utilization efficiency in fine-textured soil, suggesting that biochar
could serve as a carrier for nitrogen [15]. The size of the biochar particles
also influences nutrient leaching potential. Typically, large (2 to 4 mm) or
medium (1 to 2 mm) biochar particles have a better capacity to adsorb nutrients
and prevent them from leaching, whereas small and very vine (0.05 to 1 mm)
biochar particles are subject to greater movement in soil and can facilitate
the transport of nutrients and other agrochemicals like herbicides [16]. Some
studies showed that the presence of biochar reduced phytotoxicity of herbicide
residues such as atrazine. However, due to the high-binding capacity of some
biochars, it could also render herbicides inactive and make them less effective
in controlling weeds. Biochar with a high specific surface area, high micro
porosity and highly aromatic carbon influences the persistence, release and
bioavailability of herbicides [17]. Further work on this is currently underway.
Greenhouse gases
Currently, only a few field
studies have evaluated temporal changes in greenhouse gas emissions in soil
amended with biochar; and these studies found variable results. For carbon
dioxide (CO2) and nitrous oxide (N2O), studies conducted under field conditions
reported no effect of biochar on greenhouse gas emissions [18]. A 3-year study
from southern Ontario found no difference in CO2 and N2O emissions from soil
amended with biochar (3 t/ha), poultry manure and/or nitrogen fertilizer
compared to soil without biochar [13]. However, greenhouse gas emissions,
particularly N2O, from biochar amended soil are also dependent upon the type of
biochar used, based on feedstock type and pyrolysis processes, its addition
rate, agricultural management practices, and variation in climate depending on
the location of the site. There is also some evidence that the impact of biochar
on N2O emissions can change over time as the biochar ages. Ageing of biochar
can decrease N2O emissions with time due to stabilization of carbon and
nitrogen. However, the use of biochar in mitigating greenhouse gases can also
be integrated with Canada’s Climate Change Mitigation Strategies (https://www.
canada.ca/en/services/environment/weather/climatechange. html).
Carbon sequestration and biochar
stability in soil
Soil carbon sequestration, also
known as carbon farming or regenerative agriculture includes various ways of
land management practices that encourage the long-term storage of carbon.
Increasing carbon in soil can be achieved when the rate of addition of organic
matter is greater than its rate of decomposition in soil. Biochar has a
molecular structure that makes it more resistant to microbial decomposition,
compared to non-pyrolyzed organic matter (e.g. crop residues, manure and cover
crops), allowing it to persist in soil for 1000 to 10,0000 years. Biochar
contributes to carbon stabilization by promoting aggregate stability and
through its association with organo-mineral complexes. Some researchers have
speculated that the presence of aged biochar reduces decomposition rates of
crop-derived residue carbon in soil and increases crop residue stabilization in
soil aggregates [19]. In southern Ontario, a 3-year study did not find an
increase in soil organic carbon when biochar was added to soil [13]. This is
because measurable changes in soil organic carbon are difficult to detect over
the short-term (<10 years), and variation among years have been observed
since the soil is not in equilibrium. However, the field study from southern
Ontario found a greater concentration of extractable carbon in soil amended
with biochar, indicating that a readily available carbon source for the
microbial community was present. This suggests that a greater accumulation of
decomposing organic matter was likely derived from the biochar and/or its
interaction with soil organic carbon and crop roots.
The stability of biochar applied
to soil depends on the biochars properties and soil mineralogical composition.
Evaluating the impact of biochar on soil carbon sequestration and stabilization
is difficult since most of these changes are not readily observed over the
short-term. Simulation models, if calibrated to the study site’s environmental
conditions (e.g. climate and soil characteristics) and land management
practices can provide insight into the potential long-term effect of biochar on
soil carbon. For example, Dil & Oelbermann [20] simulated the effect of
biochar addition on soil organic carbon stocks over 150 years based on soil
collected in southern Ontario from Elora (medium texture), Delhi (coarse
texture) and Vineland (fine texture). Using the Century soil organic matter
model, they found that a once application of maple-oakbirch derived biochar at
2 t/ha, compared to other management practices including maize-soybean
rotation, continuous maize, addition of manure, and no till, led to a greater
increase and longterm stabilization of soil organic carbon at the Elora and
Delhi sites. They also found that the quantity of carbon stabilized was
influenced by the soil texture, and soil texture also influenced whether carbon
was stabilized in active, slow or passive carbon fractions.
Other uses of biochar in
agriculture
Recently, the use of biochar as
a livestock feed supplement has also received some interest from the
agricultural community. The benefits of biochar as a feed supplement may
influence animal health for gastrointestinal decontamination and nutrition.
Some studies have shown an improvement in production and livestock health.
There has also been some research into using biochar to help reduce ruminant
methane production. In southern Ontario, University of Guelph’s Dr. A.
Carpenter (Ridgetown campus) is investigating various aspects of biochar as a
feed supplement in dairy cows.
Some Precautions
Biochar storage and handling
Biochar can form explosive
mixtures with air in confined spaces. There is also a danger of spontaneous
heating and ignition when biochar is tightly packed. Fresh biochar quickly
adsorbs oxygen and moisture, which can lead to high temperatures and ignition.
Volatile compounds in biochar can also present a fire hazard, which is more of
a concern for low quality biochar. When biochar is mixed with other amendments
such as manure or composts, its potential to become flammable is greatly
reduced.
Incorporation of biochar with
soil
Depending on the feedstock and
the process used to generate biochar, the resultant product can take on many
forms ranging from coarse to powdered material. Researchers suggest that it is
important to retain a coarser structure of the biochar as this maintains its
ability to hold water, nutrients and encourages microbial activity. If biochar
is incorporated into the soil as a powder (less than 50 microns in size), then
its capacity to hold water and nutrients are lost. However, biochars can also
be generated for a specific purpose e.g. enhanced nutrient adsorption.
Uniform topsoil mixing can be
achieved by mechanically applying biochar using a spreader and then mixing it
into the soil by ploughing or disking. When mixing biochar with other
amendments, the same approach of uniform topsoil mixing can be used. In certain
management systems e.g. orchards or vineyards, the biochar is mixed with the
other amendment(s) and top dressed between rows of trees or vines.
Incorporation of biochar with liquid manures can be done when the biochar is
applied to the soil surface in a uniform layer or incorporated into the soil.
This can, however, cause blockages of flow in the liquid manure applicator if
biochar particles are large or a high quantity of biochar is used. Therefore,
testing of viscosity and flow of the biochar-liquid manure mixture is
recommended prior to its application at the field scale. Deep banding of
biochar allows for the placement of the biochar directly into the rhizosphere
of the crop and reduces risk of biochar loss via erosion. Top dressing biochar
is a desirable approach in no-till systems. However, with top dressing there is
a risk of water and wind erosion, especially if biochar particles have a fine
texture. This can be avoided if biochar is mixed with moist manure.
Application rates and
combination with other amendments
The optimal rate of biochar
application to agricultural soil has not yet been established. Average
application rates ranged from as low as 3 t/ha to 50 t/ha. Optimal rates will
depend on biochar type, soil characteristics and management objectives. To
date, the majority of studies that evaluated the impact of biochar on temperate
soil have added large quantities of biochar (10 t/ha), but this may also not be
a cost-effective approach to produce field crops. However, the horticultural
industry that produces high-value vegetable crops, the use of biochar as a
growing medium may be a more cost-effective approach compared to currently used
growing media [8].
For field crops, researchers are
in agreement that adding only biochar to temperate agricultural soil will
likely result in a minimal, if any, improvement especially in fine-textured
(clay) soil. Instead, biochar is more effective when mixed with other soil
amendments such as solid or liquid manure, compost and/or fertilizers [21].
This will improve the efficiency of the biochar and the other amendment(s) than
when applied alone. Adding biochar also reduces the odor of some manures.
Biochar can be applied as a single application which can provide benefits for
several years due to its recalcitrance. Since biochar ages in soil and its
interaction with soil changes with time, it is not necessary to apply biochar
at each crop seeding. However, field data is currently not available if it is
more effective to apply a large dose of biochar at once, or if yearly
applications at lower rates is more desirable.
Next Steps: How to Integrate
Biochar on Your Farm
Which biochar to use?
The performance and stability of
biochar in soil are highly dependent on soil types, plant species, and climate.
Growers interested in using biochar on their property should apply it to a
small area of their farm and then monitor results in subsequent years [22].
Several key points should be kept in mind when considering the use of biochar
as a soil amendment:
• soil texture and fertility;
• nutritional requirements of
the plant;
• local climate;
• feedstock quality and
pyrolysis process;
• biochar particle size;
• which other amendments will
the biochar be mixed with mineral fertilizer, manure, or other types of organic
amendments such as compost;
• what is the objective of using
biochar: nutrient retention, improved soil health, carbon sequestration and/or
greenhouse gas reduction, etc. For example, biochar used for carbon
sequestration should be derived from wood feedstock (high-carbon source)
produced at high pyrolysis temperatures for long-term stability. Using biochar
with the intended use of increasing soil health/ fertility should be derived
from a feedstock with a greater nutrient content such as manure. It is recommended
that available nutrients, rather than total nutrients, in the biochar should be
measured [23].
Where to obtain biochar?
There is an increasing interest
in integrating biochar and biochar-products in agriculture. However, logistical
difficulties in obtaining biochar and the current high costs of biochar, since
it is still mostly an experimental product [1], have led to some smallscale
on-site pyrolysis projects. Although do-it-yourself biochar production may be
more cost effective, there are some potential problems with this approach:
• lack of controlled pyrolysis
conditions that result in biochar with variable characteristics and therefore
an inconsistent product;
• inability to capture toxic
and/or greenhouse gases produced during pyrolysis;
• contamination of feedstock
with heavy metals;
• lack of biochar
characterization.
Although biochar obtained from
external sources is more expensive, there are several advantages when using
biochar derived from an established and reputable producer:
• availability of biochar from
different and high-quality feedstocks;
• consistent and high-quality
biochar;
• characterization of biochars
attributes;
• capture of gases produced
during pyrolysis.
As the biochar market continues
to advance, including the development of cost-effective mobile units that
reduce transportation costs, applications at the farm-scale will become more
economical in time.
To read more about this article: https://irispublishers.com/wjass/fulltext/biochar-and-its-use-in-soil-lessons-from-temperate-agriculture.ID.000610.php
Indexing List of Iris Publishers: https://medium.com/@irispublishers/what-is-the-indexing-list-of-iris-publishers-4ace353e4eee
Comments
Post a Comment