Biochar for Maintaining Soil Health

Nguyen Hue

University of Hawaii, Honolulu, Hawaii, USA 96822

(Chapter 2 in Book “Soil Health, 2020” doi: 10.10071978-030-44364-1_2)

(https://www.researchgate.net/publication/341658593_Biochar_for_Maintaining_Soil_Health)

1. Soil Health: Definition and Measureable Indicators

Along with air and water, soil sustains life on earth (Biswas et al. 2019). Soils are

materially (e.g., different minerals, organic fractions) and characteristically (e.g., different

which includes both dynamic and static soil properties) is defined as "the capacity of a

soil to function, within ecosystem and land use boundaries, to sustain productivity,

maintain environmental quality, and promote plant and animal health." Recently, the

definition of soil health has been slightly rephrased as "the continued capacity of the soil

to function as a vital living ecosystem that sustains plants, animals, and humans" as

adapted by the Cornell University Comprehensive Assessment of Soil Health (Cornell-

CASH 2016) and the US Natural Resources Conservation Services (USDA-NRCS

2017). Thus, soil health cannot be assessed by measuring only crop yield, number of

earthworms, soil available nutrient levels, or any single outcome (Dick 2018). It requires

a combination of many indicators. Informative and practical indicators of soil health

nutrient supply and retention for the needs of both plants and microbiota. SOM also

affects many physical properties, such as aggregate stability, water holding capacity and

infiltration, etc. SOM is usually measured as total organic carbon (C), oxidizable C

(Cornell-CASH 2016; Weil et al. 2003; Culman et al. 2012), or CO2 respiration (USDA-

NRCS 2017).

ii. Soil structural stability. This indicator would affect soil erosion, water and air

movement as well as root penetration. Wet-sieving method for aggregate stability, bulk

density measurement, water retention curve, infiltrometry, and penetrometry are

common techniques to assess soil structural stability (Grossman and Reinsch 2002;

Dane and Hopmans 2002).

many other micro-nutrients, can be routinely measured by soil testing laboratories (Hue

et al. 2000).

iv. Microbial population, diversity, and enzyme activity. Metagenomics has been

used to identify microbes that cannot be cultured in the laboratory (Streit and Daniel

2017). Some commercial laboratories have used phospholipid fatty acid analysis to

provide microbial community structural information (Buyer and Sasser 2012). Enzymes,

such as β-glucosidase, N-acetyl- β-D-glucosaminidase, have been used to measure

microbial activity (Deng and Popova 2011; Lammirato et al. 2011).

Since organic C is essential to soil health, and biochar contains large quantity of C,

a fraction of which is labile and reactive (Lorenz and Lal 2018), biochar role in

Harder 2006; Marris 2006). These soils were found to contain burned wood, crop C

residue, and bone from animals (Sombroek et al. 2002). Some archeologists surmised

that these fertile black soils helped sustain a relatively large population of the local

Indians whose land mostly consisted of nutrient-poor Oxisols and Ultisols in the Tropics

(Lehmann and Rondon 2006; Steiner et al. 2007). In fact, the role of biochar (it was

known as charcoal before this millennium) in soil quality/health was acknowledged long

before the 20th century as pointed out by Spokas and Novak (2015).

According to the International Biochar Initiative (IBI), biochar is defined as a solid

material obtained from the thermochemical conversion (i.e., heating or pyrolysis) of

biomass (e.g., wood, crop residue, manure, biosolids, etc.) in an oxygen limited

modern fast reactors (Brown et al. 2015; Oaks 2018; Cox 2019). Simple and inexpensive

charcoal kilns consisted of pits or mounds (Figure 2, A and B). These kilns usually used

wood as feedstock and soil or brick as insulator. The process may take several days or

even weeks, and the finished charcoal is rather low (< 25%) in yield and quality (Oaks

2018).

transfer rates, which can be several hundred degrees (oC) per second (Boateng et al.

different from those produced by slow pyrolysis as discussed in the following sections.

Cong et al. 2017). Feedstock for biochar can range from forest products, crop residues,

to animal and municipal wastes. In general, the original biomass structure strongly

influences the final biochar structure. For example, biochar pore structure closely

resembles the cellular structure of its wood-based feedstock (Fuertes et al. 2010). As an

example, Figure 4 shows the scanning electron microscope images of shape and size of

pores from six different wood-based biochars (Berek and Hue 2016). In a review paper,

Chen et al. (2019) mentioned that the volume due to small pores of a rice husk biochar

was 2.1 cm3/g, which was 12.3 times larger than that of a biochar produced from sludge

(biosolids) as measured with the nuclear magnet technology.

biochar increase (Chia et al. 2015). However, when HTT exceeds 700 – 750 oC, some

microporous structures of biochar may break down, reducing its surface area (Huang et

al. 2014). Consequently, the specific surface area of biochar generally ranges from 1.5

to over 500 m2/g (li et al. 2018, Liu et al. 2019), and reaches a maximum then declines

as HTT increases further. At lower HTT and with fast pyrolysis, tars and volatile products

(bio-oils) from the thermal decomposition of biomass may block micro pores and reduce

surface areas. As the temperature increases, the same substances volatile and escape

from the pores, yielding more volume and larger surface areas. Moreover, it should be

noted that having numerous micropores, biochar, especially made at high HTT, could

trap significant amount of water and nutrients, such as nitrate ( Prendergast-Miller et al.

al. (2015), available P ranges from 0.4 to 34% of total P in biochar. In contrast, the

authors also reported that between 55 and 65% of the K, Ca, and Mg available from

biochars can be related to their total concentration. Most of biochar K is water soluble

and readily available, especially when produced from slow pyrolysis (Cantrell et al. 2012;

Berek et al. 2018). Liu et al. (2019) mixed 36 different biochars with water (1:75 mass

carbonate equivalent (CCE). A CCE range of 5.0 – 30.0% is common for many wood-

based biochars (Laird et al. 2010; Berek and Hue 2016). It is worth noting that a biochar

with 8% CCE applied at 2% (w:w) to an acid Ultisol of Hawaii lowered exchangeable

aluminum (Al) from 2 cmolc/kg to virtually zero; thus completely eliminating Al toxicity in

this soil (Berek and Hue 2016; Figure 9).

In fact, Al and to a lesser extent, manganese (Mn) in acid soils can also be

complexed and detoxified by reactive functional groups on the biochar surfaces. Most of

these groups contain oxygen, such as carboxyl, carbonyl, and hydroxyl and closely

related to pyrolysis conditions: their number and density usually decrease as the HTT

increases (Gul et al. 2015; Zhao et al. 2017). In contrast, upon aging and exposed to

exchange, and precipitation) as they do for Al and Mn (Beesley et al 2015; Li et al. 2017).

Figure 9. Exchangeable Al as a function of CaCO3 equivalent of wood-based

biochars applied to an acid Ultisol of Hawaii (adapted from Berek and Hue 2016).

CEC will be markedly increased when biochar is aged and is mixed with soil. Many

studies (Silber et al. 2010; Laird et al. 2010; Mukherjee et al. 2014; Martinsen et al. 2014)

have shown that soil CEC may increase up to 30% on average. However, conflicting

evidence also exists (Mukherjee and Lal 2017). For example, Mukherjee et al. (2014)

reported that after aging for 15 months, biochars made by pyrolysis of wood (oak and

pine) and grass at 250, 400, and 650 oC exhibited 5-fold increases in CEC. When added

measured at pH 6-7 on (a) fresh and “aged” oak and grass biochars produced at 250

and 650 oC, and (b) aged BY soil and BY soil-biochar mixtures. (Adapted from Mukherjee

et al. 2014).

CEC of the amended soil, biochar can effectively retain nutrient cations, such as NH4+,

applied at 0.5 – 2.0% to a Mollisol of Iowa (Laird et al. 2010). Ammonium in wastewater

(concentration range 2-20 mg/L) was removed by 66% by a filter made of peanut hull

Besides being an efficient adsorbent, biochar itself contains nutrients (Table 1).

Depending on feedstock and pyrolysis process, and also on individual nutrient, nutrient

discussed previously, over 50% of total K in biochar is water soluble and readily

bioavailable. Thus, biochar can be a good source of K for crop uptake, especially in

organic farming (Martinsen et al. 2014; Butnan et al. 2015; Berek et al. 2018).

On organic N mineralization, biochar can have positive, neutral, or negative effects

(Prommer et al. 2014; Maestrini et al. 2014). For example, an increase of 7% in N

mineralization was obtained when 5% of a wheat straw biochar (slow pyrolysis at 525

oC) was mixed with a sandy loam soil (Spodosol) , while a 43% reduction was resulted

from the application of the same feedstock but fast pyrolyzed biochar after 65 days of

incubation (Bruun et al. 2012). Similarly, the direct contribution of N from biochar has a

mixed result, particularly in terms of plant responses (Gul and Whalen 2016; Hood-

Nowotny et al. 2018). Since the C/N ratio in many biochars is much greater than the 25-

30 range, which deems optimal for N mineralization, N deficiency in crops due to N

immobilization in biochar amended soils may occur, at least in the short term (Deenik et

causes and expected effects of biochar on soil biological properties.

Figure 11. Schematic diagram showing the effect of biochar application on soil

microorganisms and microbial responses (adapted from Palansooriya et al. 2019).

3.3.1. Biochar as a potential habitat and growth promoter for soil biota.

The porous structure of biochar, its large internal surface area, and its high capacity

to retain water provide favorable habitats for soil biota (Quilliam et al. 2013; Jaafar et al.

2015). Bacteria (size 0.3 - 3 mm) and hyphae (< 16µm) of different fungi can colonize

mites and nematodes (Ezawa et al. 2002; Jaafar et al. 2015; Ogawa and Okimori 2010).

SEM images from Palansooriya et al. (2019) clearly show fungal hyphae grown on the

surface of a peanut shell biochar (Figure 12).

Figure 12. SEM images of fungal hyphae (pointed by white arrow) grown on peanut

shell biochar (pyrolyzed at 500 oC) (adapted from Palansooriya et al. 2019).

also help some microbes that are less competitive in the “hostile” environment of the

unamended soils become established (Ogawa and Okimori 2010; Wong et al. 2017).

Maestrini et al. (2014) used 13C-labelled ryegrass biochar to estimate microbial uses of

biochar C in a forest soil. The authors found that 4.3% of biochar 13C was mineralized

after a 5-month incubation, of which 0.45% was in microbial biomass. The

(SOM), causing a priming effect, which can speed up or slow down the mineralization of

native SOM (Whitman et al. 2015).

the bacteria to fungi ratio, microbial communities (Chen et al. 2013; Wong et al 2019).

Microbial feeders and their predators may also change as a result (Thies et al. 2015).

3.3.2. Biochar effects on soil enzyme activities and microbial community structures.

There are strong interactions between biochar and extracellular enzymes (Bhaduri

et al. 2016). These enzymes are needed to degrade substrates, particularly C- and/or N-

containing materials (e.g., cellulose, proteins) for their food. Biochar will affect the activity

of these enzymes in various ways, depending on the relative location (folding

conformation) of the active sites of the enzyme and the reactive functional groups on

biochar surfaces, pH, and concentration of ionic species in the surrounding environment

2014; Bhaduri et al. 2016; Gasco et al. 2016).

Using 13C-labelled phospholipid fatty acid (PLFA) analysis to study microbial

community and their main C food source in a very acidic soil (pH 3.7) amended with

groups (Gram positive, Gram negative bacteria, actinobacteria, and fungi) were more

the C from the 350 oC biochar, but not the 700 oC biochar as substrate. Similar findings

were reported by Gomez et al. (2014) in a study using 4 soils from the Midwest, USA,

bacteria, relative to Gram+ and fungi. Also, biochar can serve as a C substrate for

microbial activity.

library analysis, denaturing gradient gel electrophoresis (DGGE) and quantitative real-

time PCR assay (qPCR) as reported by Chen et al. (2013). The authors found that gene

copy numbers of bacterial 16S rRNA was increased by 28% and 64% and that of fungal

18S rRNA decreased by 35% and 46% under biochar applications of 20 and 40 ton/ha,

respectively, over the control in a rice paddy of China.

holding capacity, root penetration, and reduced erosion. Second, given commonly high

pH value, abundant reactive functional groups, relatively high CEC, ash content, and

desirable characteristics, application of biochar to soils, especially highly weathered,

diversity and activity. On the other hand, since climate and time also affect biochar

properties and performance via oxidation by water and oxygen, long-term field

time); also feedstock (e.g., wood, crop residues, animal manure) should be clearly

identified and publicized (Spokas and Novak 2015).

processes or changing policy that would monetize the value of carbon sequestration in

soils where biochar has demonstratively served as an effective amendment for

maintaining and enhancing soil quality.

This work was partially supported by a grant (No. SW16-021) from the USDA-

Western Region Sustainable Agricutlure Research and Education (W-SARE) given to the

author from 2016 to 2019.

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