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Soil Health, Crop Productivity, Microbial Transport,
and Mine Spoil Response to Biochars
J. M. Novak
1
&J. A. Ippolito
2
&R. D. Lentz
2
&K. A. Spokas
3
&C. H. Bolster
4
&K. Sistani
4
&
K. M. Trippe
5
&C. L. Phillips
5
&M. G. Johnson
6
#Springer Science+Business Media New York (outside the USA) 2016
Abstract Biochars vary widely in pH, surface area, nutrient
concentration, porosity, and metal binding capacity due to the
assortment offeedstock materials and thermal conversion con-
ditions under which it is formed. The wide variety of chemical
and physical characteristics have resulted in biochar being
used as an amendment to rebuild soil health, improve crop
yields, increase soil water storage, and restore soils/spoils im-
pacted by mining. Meta-analysis of the biochar literature has
shown mixed results when using biochar as a soil amendment
to improve crop productivity. For example, in one meta-anal-
ysis, biochar increased crop yield by approximately 10 %,
while in another, approximately 50 % of the studies reported
minimal to no crop yield increases. In spite of the mixed crop
yield reports, biochars have properties that can improve soil
health characteristics, by increasing carbon (C) sequestration
and nutrientand water retention. Biochars also have the ability
to bind enteric microbes and enhance metal binding in soils
impacted by mining. In this review, we present examples of
both effective and ineffective uses of biochar to improve soil
health for agricultural functions and reclamation of degraded
mine spoils. Biochars are expensive to manufacture and can-
not be purged from soil after application, so for efficient use,
they should be targeted for specific uses in agricultural and
environmental sectors. Thus, we introduce the designer bio-
char concept as an alternate paradigm stating that biochars
should be designed with properties that are tailored to specific
soil deficiencies or problems. We then demonstrate how care-
ful selection of biochars can increase their effectiveness as a
soil amendment.
Keywords Biochar .Microbiology .Mine-impacted spoils .
Restoration .Soil health
Abbreviations
C Carbon
NNitrogen
SOC Soil organic carbon
USDA-ARS United States Department of
Agriculture-Agricultural Research Service
US EPA United States Environmental Protection
Agency
Introduction
Over the past 10 years, the use of biochar as a soil amendment
has attracted global attention. In this same time span, hundreds
of articles have reported on the potential impact of biochar on
soil properties, greenhouse gas production, and crop yields (ISI
Web of Knowledge at http://apps.webooknowledge.com).
*J. M. Novak
jeff.novak@ars.usda.gov
1
United States Department of Agriculture (USDA)-Agricultural
Research Service (ARS), Coastal Plain Research Center,
Florence, SC 29501, USA
2
USDA-ARS, Northwest Irrigation and Soils Research Laboratory,
Kimberly, ID 83341, USA
3
USDA-ARS, Soil and Water Management Research Unit, St.
Paul, MN 55108, USA
4
USDA-ARS, Food Animal Environmental Systems Research Unit,
Bowling Green, KY 42101, USA
5
USDA-ARS, Forage Seed and Cereal Research Unit,
Corvallis, OR 97331, USA
6
National Health and Environmental Effects Research Laboratory,
Western Ecology Division, United States Environmental Protection
Agency, Corvallis, OR 97333, USA
Bioenerg. Res.
DOI 10.1007/s12155-016-9720-8
Several recent reviews [1–4] have examined the impact of
biochar on soil conditions and crop yield. The consensus of
those reviews is that both crop and soil response to biochar
are variable but can be linked to the biochar attributes and
soil properties. For example, Jeffery et al. [2] conducted a
meta-analysis on 17 biochar studies. They reported crop yield
variability ranged from −28 to +39 %, with an overall average
yield increase of 10 %. A meta-analysis of 114 biochar studies
showed similar results, corroborating that the ability of biochar
to improve crop yield is highly variable [3]. In a recent green-
house study, an 80:20 blend of pine chip (Pinus taeda)and
poultry litter biochar increased above- and below-ground bio-
mass of winter wheat (Triticum aestivum L.) by 81 % compared
to the untreated control in a highly weathered Ultisol [5]. This
result is typical in studies that add biochar to highly weathered
soils where biochar improves soil pH and provides nutrients
[4]. In contrast, variable improvements in crop grain yields over
several growing seasons in Idaho were reported on an Aridisol
(i.e., relatively unweathered soils) as compared to those used by
Sigua et al. [5] suggesting that the specific biochar being eval-
uated was not always effective [6]. In another report, Spokas
et al. [7] reviewed the biochar literature and found that 30 % of
the studies reported no significant differences, and 20 % report-
ed negative yield or growth effects. In summary, the unpredict-
able and variable crop yield response to biochar application
reported in these studies suggest caution toward widespread
adoption of biochar technology for agricultural purposes with-
out a better understanding of the characteristics and specific
biochar properties as well as the potential response by soils
with different inherent properties or management-induced
(e.g., mining) problems.
Biochar application to soil can have potential impacts on
water quality and for reclamation of mine spoils. For example,
Novak et al. [8] reported that biochar produced from poultry
litter had a negative impact on shallow ground water quality
by releasing significant concentrations of dissolved phospho-
rus and by increasing movement of fecal bacteria through the
soil [9,10]. On the other hand, biochar produced from pine
chips significantly decreased movement of fecal bacteria
through fine sand [9]. It has also been demonstrated that bio-
chars have an emerging capability for remediating soils im-
pacted by mining [11,12]. However, reclamation of mine
spoils using biochar is a complicated task since the biochar
must be capable of binding heavy metals or reducing toxic
substance concentrations, while also improving soil health
characteristics and thus promoting a more sustainable plant
cover to prevent erosion, leaching, or other unintended, neg-
ative environmental consequence.
The diversity in chemical and physical characteristics of
different biochars can be capitalized on to promote its multi-
functional use in agricultural, industrial, and environmental
sectors [13]. The large variety of potential feedstocks and
biochar production processes must be understood because
they can influence biochar chemical and physical properties
and thus influence selection of an appropriate biochar as a soil
amendment, carbon (C) sequestration agent, or remediation
trigger. Therefore, our objectives are to (1) review the funda-
mentals of biochar manufacture and characterization, (2) dis-
cuss potential impacts of biochar on soil health characteristics,
including crop productivity and soil hydraulics, (3) synthesize
recent findings regarding the impact of biochar on microbial
pathogen movement, and (4) examine the ability of biochar to
reclaim mine-impacted soil which is commonly referred to as
spoil.
Biochar Production and Characterization
Biochar Manufacture
Pyrolysis is the thermal conversion of organic feedstocks for
generation of energy through which the leftover material,
commonly referred to as biochar is created. Pyrolysis of bio-
mass feedstocks occurs at temperatures typically ranging be-
tween 300 and 700 °C under a low oxygen condition [14].
Gasification is another pyrolysis-like technique for making
biochar and subjects feedstocks to gasification reactions at
higher temperatures (>700 °C). Many types of feedstocks
have been used for biochar production including ligno-
cellulosic material such as corn (Zea mays, L), stover, switch-
grass (Panicum virgatum), nut hulls, and crop processing
wastes (e.g., cotton (Gossypium spp.) gin trash, seed screen-
ings, etc.) along with various animal manures (e.g., poultry,
swine, and dairy). Many other municipal and industrial organ-
ic byproducts have also been used for biochar production in-
cluding municipal, wood, and cardboard waste products.
Biochar Characterization
Feedstock diversity and variable pyrolysis conditions result in
biochars with a wide range of chemical and physical charac-
teristics [15]. The carbonization process that converts organic
feedstocks into biochar becomes more intensive as the pyrol-
ysis temperature increases from 300 to 700 °C. In the lower
temperature range (300 to 400 °C), there is some loss of or-
ganic materials due to volatilization, but some ring and car-
boxylic acid compounds are retained. As the pyrolysis tem-
perature increases (400 to 500 °C), most of the volatile mate-
rial is removed as a gas, and the remaining non-volatile solid
material undergoes further structural conversion. Some vola-
tile material can re-condense as tar-like compounds and be-
come associated with the biochar matrix [16]. After the loss of
volatile material, biochars will possess an amorphous core
matrix composed of aromatic and aliphatic compounds that
can have attached carbonyl and hydroxyl functional groups
[17]. Pyrolysis at temperatures from 500 to 700 °C causes the
Bioenerg. Res.
amorphous aromatic sheets to stack up and reform as
turbostratic crystalline-aromatic sheets [18].
Biochar Impact on Soil Health and Crop
Productivity
Soil Health
In this review, we define soil health as the capacity of soil to
function as a living system, to sustain plant and animal pro-
ductivity, maintain or enhance water quantity, and promote
crop productivity. Biochar has the ability to improve soil
health characteristics due to its elemental composition and
ability to improve pH, retain water in its pore space, and bind
nutrients and metals on its functional groups [19]. As men-
tioned previously, biochars produced at lower pyrolysis tem-
peratures (350 °C) will retain some organic carbon structures
that can be decomposed by soil microbes, but as pyrolysis
temperatures increase (>400 °C), the remaining biochar mate-
rial is predominately a C-enriched material that contains or-
ganic structures that resist oxidation and hence can have long
residence times in soil [20]. Amending soils with stable forms
of biochar increases the size of C pools and long-term C se-
questration [21]. Biochars are not totally resistant to decom-
position, as they can be slowly oxidized by biotic [22]and
abiotic mechanisms [23]. In one recent biochar stability study,
Spokas et al. [24] demonstrated that biochars can weather into
pieces through hydration reactions that expand the organic
sheets and eventually exfoliate them as fragments.
Weathering of biochars is an important process because it
leads to formation of carbonyl and carboxylic functional
groups [20] that can consequently increase biochar cation ex-
change capacity and thus improve plant nutrient retention
[15].
Biochars also contain inorganic ash derived from non-
volatile feedstock constituents [19] from residual bedding ma-
terial mixed with manure feedstocks [25]. As shown in
Tab le 1, biochar produced through gasification of Kentucky
bluegrass (Poa pratensis) seed waste (KBSW) will generally
have a high ash content and alkaline pH value (pH ≈10). High
ash content causes pH values of biochar to be >9 [26,27], thus
making materials such as alkaline KBSW biochar suitable
“designer biochar”for neutralizing acidic soils [28,29].
Furthermore, both biochars presented in Table 1contain C
and nitrogen (N), as well as plant macro- (e.g., Ca, P, and K)
and micronutrients (e.g., Cu and Zn). Of interest to some is the
dissimilarity in nutrient composition between KBSW and
wood biochar, especially with regard to Ca, K, and P concen-
trations (Table 1). Higher N, Ca, K, and P concentrations in
KBSW biochar suggest that it would be a more suitable de-
signer biochar than wood biochar as a soil fertility improve-
ment agent.
The impact of a hardwood biochar on soil fertility character-
istics in a sandy, acidic-Ultisol is shown in Table 2. This hard-
wood biochar caused significant increases in soil pH, organic C,
and plant nutrients such as Ca, K, and Mg concentrations,
supporting the aforementioned changes in soil characteristics
following biochar application. Raising the pH and supplying
Ca, K, and Mg is important because this soil has lost its fertility
due to leaching of base cations [30]. Unfortunately, this
Tabl e 1 Characteristics of two gasified biochars examined in ARS-
Corvallis, Oregon. Biochar from Kentucky bluegrass seed waste
(KBSW) was produced using a small-scale updraft gasification unit at
temperatures of 650 to 750 °C. Biochar from mixed conifer wood was
produced using a small-scale downdraft gasification unit at maximum
char temperatures ranging from 1100 to 1400 °C. The wood biochar
was produced from a mixed conifer logging slash material consisting of
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), Ponderosa pine
(Pinus ponderosa C. Lawson), white fir (Abies concolor (Gord. &
Glend.) Lindl. ex Hildebr.), sugar pine (Pinus lambertiana Douglas),
and incense cedar (Calocedrus decurrens (Torr.) Florin)
Variable Units KBSW biochar Wood biochar
Mean ±SD Mean ±SD
Volatile C % 16.7 0.6 7 0.3
Fixed C % 32.7 1.6 77.5 1
Ash % 50.6 2.1 15.6 0.7
Surface area m
2
g
−1
26.1 1.2 423.3 52.5
pH 10.2 0.1 10.9 0.01
EC mS cm
−1
2.98 0.1 3.56 0.14
CEC cmol kg
−1
43.1 2.1 14.8 4.4
Total C % 35.5 1.5 78 15.9
TSOC g kg
−1
7102 213 3905 117
Total N % 2.2 0.3 0.79 0.4
TSN g kg
−1
410 12 BD
NH
4
-N mg kg
−1
84.2 0.5 5.6 0.2
NO
3
-N mg kg
−1
2.5 0.2 19.4 0.8
Total K mg kg
−1
50,600 2519 34,787 4092
Bray-K mg kg
−1
36,357 1532 21,000 1172
Ca mg kg
−1
13,330 770 43,293 5768
Total P mg kg
−1
15,320 565 5020 849
Bray-P mg kg
−1
935 27 6.3 1
Mg mg kg
−1
6342 97 12787 1477
Smgkg
−1
4650 580 3463 509
Fe mg kg
−1
1125 42 4866 157
Mn mg kg
−1
755 17 2783 366
Na mg kg
−1
411 6 320 34
Zn mg kg
−1
143 2 393 80
Cu mg kg
−1
36 3 42 9
Ni mg kg
−1
BD 25 8
Al mg kg
−1
854 106 1908 411
As mg kg
−1
BD BD
Cd mg kg
−1
BD BD
BD below detection
Bioenerg. Res.
hardwood-based biochar did not produce significant improve-
ments in other important soil plant nutrient (e.g., P and Cu)
concentrations in this highly weathered sandy-textured Ultisol.
Crop Productivity
In anticipation of improving crop productivity, biochars are
commonly applied to soils possessing poor to marginal fertil-
ity characteristics [27,31,32]. The previously cited meta-
analyses have shown divergent results regarding improved
crop productivity after applying biochar to soils [3,7]. Liu
et al. [33] reviewed published data from 59 pot experiments
and 57 field experiments, concluding that crop productivity
was increased by 11 % on average. Uzoma et al. [34]reported
the effects of biochar produced from cow manure on corn
yield, nutrient uptake, and physicochemical properties when
used on a sandy soil. They found that applying 15 or 20 t ha
−1
of biochar significantly increased corn grain yield by 150 or
88 %, respectively, when compare to an untreated control.
On the other hand, some studies have reported no increases
in crop yield following biochar application [35–37]. For ex-
ample, Martinsen et al. [38–2015] reported that biochars cre-
ated from maize (Zea mays, L.) cob or groundnut (Arachis
hypogaea) did not affect maize yields. Similar results were
reported by Jones et al. [39], who used biochar produced from
commercial wood chips as a soil amendment. The mixed per-
formance of biochar as an amendment is related to the wide
diversity of physiochemical characteristics that translates into
variable reactions in soils [8,40].
Variable performance of a hardwood biochar was amply
demonstrated after its application to a Haplocalcid soil in
Idaho [6]. The hardwood biochar was applied to determine if
its addition alone, when mixed with fertilizer and when mixed
with animal manure would improve soil nutrient cycling
resulting in higher corn stover yields. In that study, plots were
treated with a one-time application of animal manure
(42 Mg ha
−1
dry weight, dw), hardwood biochar
(22.4 Mg ha
−1
dw), a blend of the biochar and manure (same
rates), and a control (no amendments). Soil tests were collect-
ed over a 2-year period to determine changes in soil fertility,
total C, total N, and yield of corn silage in Idaho. Lentz and
Ippolito [6] reported the only significant soil fertility increases
due to biochar were in soil Mn and total organic carbon, while
manure-treated soils had significant increases in extractable K,
Mn, Cu, Na, and Zn when compared to the untreated control.
Biochars affect on silage yield was mixed, producing a slight
increase in year one but a 36 % decrease in year two [6].
Further investigations of soil fertility showed that the hard-
wood biochar was unable to adjust the calcareous soil pH to
improve nutrient availability and that it did not significantly
improve available P, N, or any important plant cationic nutri-
ents [6].
The same hardwood-based biochar was applied to a
Paleudalf soil in Kentucky. Over a 3-year (2010, 2011, and
2013) period, corn grain yields were measured (Fig. 1). The
hardwood biochar and the experimental methods were similar
to those used in the Idaho experiment, with the exception that
poultrymanurewasappliedat12to19Mgha
−1
and
224 kg N ha
−1
was applied as liquid urea ammonium nitrate
(UAN). Corn grain yields in 2010 and 2011 were impacted by
drought and unfavorable rainfall distribution (Fig. 1), so no
significant differences were observed among treatments.
However, in 2013, the biochar-treated plots produced signifi-
cantly lower corn grain yield than UAN, poultry litter, and
combinations of biochar with UAN or poultry litter. Because
the addition of the hardwood biochar with chemical fertilizer
and poultry litter did not improve corn grain yield in any ofthe
3 years (Fig. 1), it was concluded that this specific hardwood
biochar did not possess the chemical characteristics capable of
improving soil health characteristics of this Paleudalf and thus
increase corn productivity.
Biochar Influence on Soil Water Hydraulics
Water storage and movement within soil (i.e., hydraulics) is an
important soil health feature that influences water availability
for plants [41], microbial processes [42], and soil nutrient
turnover processes [43]. Long ago, farmers recognized the
importance of improving soil hydraulics with charcoal to ac-
quire higher crop yields. For example, in 1860, Walden [44]
Tabl e 2 Soil fertility characteristics of a Norfolk loamy sand (Ultisol)
after laboratory incubation for 120 days with and without hardwood
biochar (means between columns with a different letters indicate
significant differences at p< 0.05) (Novak, 2015; unpublished data)
Properties Norfolk + 0
biochar
Norfolk + 20 g kg
−1
biochar
pH (H
2
O) 5.6a 6.6b
CEC (cmol kg
−1
) 2.1a 2.3a
Ex. acidity
(cmol kg
−1
)
1.2a 0.9a
Total N (kg ha
−1
) 1473a 1545a
Organic C (kg ha
−1
) 11,500a 65,695b
Macronutrients (kg ha
−1
)
P 70a 57a
K 49a 161b
Ca 288a 440b
Mg 47a 67b
Na 6.7a 7.3a
Micronutrients (kg ha
−1
)
B 0.11a 0.22a
Cu 0.90a 0.90a
Mn 13.4a 14.9a
Zn 4.8a 3.4a
Bioenerg. Res.
reported that the addition of charcoal (i.e., biochar) can im-
prove soil water holding capacities by retaining a “good bal-
ance”of moisture around plant roots. Almost 100 years later,
Tryon [45] was the first to demonstrate that soil texture was a
critical factor controlling the impact of biochar on hydraulic
properties. More recently, considerable attention has been giv-
en to using biochar to modify soil water hydraulics including
water holding capacity and available water content [46–50], as
well as soil hydraulic conductivity [51–54]. Laird et al. [47]
reported that the addition of 1 to 2 % hardwood biochar to a
Midwestern USA Mollisol increased gravity drained water
retention by 15 % relative to the untreated control but did
not affect soil moisture content measured at soil water poten-
tials of 33 or 1500 kPa (field capacity and wilting point, re-
spectively) [41]. Basso et al. [49] reported similar results
(≈23 % increase) in gravity-drained water content in a sandy
Midwest soil relative to an untreated control. Higher water
storage for a sandy soil from the Southeastern USA Coastal
Plain was reported by Novak et al. [48]withanadditional
1.5 cm of water stored per 15 cm of soil after 2 % (w w
−1
)
addition of switchgrass biochar.
Biochar additions to soils have had mixed results with re-
gard to modifying soil hydraulic conductivity (K
sat
)orwater
infiltration rates. Uzoma et al. [52] and Ouyang et al. [53]
reported improvements in K
sat
after biochar additions to a silt
and sandy loam-textured soil, respectively. In contrast, both
Laird et al. [47] and Major et al. [55] reported no significant
change in K
sat
for biochar applied to loam- and clay-textured
soils, respectively. On the other hand, Lim et al. [56]reported
that K
sat
values declined after additions of 1, 2, and 5 % (w
w
−1
) biochar to both a coarse and a fine sand. The decrease
was related to the particle size of the biochar. Repeating the
experiment using a clay loam-textured soil showed that 1 and
2 % biochar additions universally increased K
sat
.Toexplain
these results, Lim et al. [56] modeled the impact of biochar on
K
sat
by incorporating soil pedo-transfer functions with the
biochar-altered soil texture. Using this model, they showed
that soil texture greatly modulates the predicted response on
K
sat
from biochar additions. Similar results were found by
Barnes et al. [54], who showed biochar impacts on K
sat
de-
pending on macro- and meso-porosity of the soil. They also
found that biochar additions decreased K
sat
in sand and organ-
ic soils, but increased K
sat
in a clay-rich soil.
Water infiltration is another important soil health character-
istic because this property regulates water movement into soils
vs. movement across the soil surface. Novak et al. [57]mea-
sured significant improvement in water infiltration rates after
biochar additions to a sandy loam soil. They reported that the
biochar-treated soil had significantly higher infiltration rates
of 0.157 to 0.219 mL min
−1
compared with 0.095 mL min
−1
for an untreated control. For three of the four biochars used in
this study, water infiltration rates declined to values similar to
the control after four water infiltration simulations. This indi-
cates that these biochars have a limited impact on improving
water infiltration and may be a result of the physical clogging
of pores by exfoliated biochar fragments [24].
The matric forces of soil controls the quantity of both total
and plant available water, and although biochar additions to
soils may improve total water retention, plant available water
may be limited [58]. Plant available water is not a binary
process (i.e., on/off) but rather a continuum that changes con-
stantly due to the diverse capillary forces that are a function of
physical pore distribution [59]. Integral water capacity (IWC)
and integral energy (E
i
) are two indices that can be used to
assess this altering function of soil water availability [60,61].
When IWC and integral energy E
i
values were calculated for
biochar-amended soils [62], increases in total soil water hold-
ing capacity comes at the expense of increasing energy
a
ab ab
bbb
a
ab
ab
b
ab
ab
bc
c
a
ab
aa
0
2
4
6
8
10
12
14
Cont Bio CF CF+Bio PL PL+Bio
Corn Grain Yield (Mg ha-1)
Treatment
2010
2011
2013
Fig. 1 Effects of hardwood
biochar (BIO), poultry litter (PL),
chemical fertilizer (CF), and
biochar + poultry litter (CF + Bio)
on corn grain yield (data from
2010, 2011, and 2013 growing
seasons in Bowling Green,
Kentucky; letters after means
indicate significant differences at
aα= 0.5 among treatments in
each year)
Bioenerg. Res.
required to extract the moisture from biochar-amended soil
(Table 3). Data in this table also show that increased IWC is
primarily in the 14 to 33 kPa fraction, which is gravity-drained
water that infiltrates too rapidly to be beneficial for plants. The
salient aspect for designing the appropriate biochar for soil
water holding improvements is to control where these in-
creases in water holding occur. In this manner, one can target
the critical range of plant available soil moisture. As an exam-
ple, sandy soils do benefit from small particle size additions
(<2 mm). Since this size fraction would improve overall plant
available water through the increase in pore tortuosity, it will
also reduce the saturated conductivity and infiltration rates
[56]. For clay-rich soils, the amount of biochar that would
need to be added to improve hydraulic properties is too large
for practical applications [56].
Biochar Impact on Pathogen Transport
and Microbial Properties
Land application of raw animal and human fecal material is a
potential public health risk if humans are exposed to microbial
pathogens contained within these materials. One mode of hu-
man exposure to these pathogens is through consumption of
fecal contaminated groundwater [63,64]. Transport of micro-
bial pathogens into ground water sources may be significantly
reduced by application of biochar [9,10]. Those researchers
evaluated transport of three different E. coli isolates through
laboratory column that were packed with a fine sandy-
textured soil and then amended with poultry litter biochar
produced at two different pyrolysis temperatures (350 or
700 °C). Application of the high-temperature poultry litter
biochar at 2 % (w w
−1
) did not significantly affect transport
behavior of the three E. coli isolates whereas a 10 % biochar
rate reduced column transport for two of the E. coli isolates by
≥99.9 % and reduced transport of the third isolate by 60 %. In
contrast, adding the low-temperature biochar to a soil at either
rate produced about a twofold increase in bacteria transported
for two of the isolates. Transport of the third isolate was un-
affected at the 2 % low-temperature biochar rate but was re-
duced 60 % at the 10 % low-temperature biochar rate. No
correlations between changes in microbial transport and
changes in soil organic matter content, soil solution ionic
strength, pH, and dissolved organic carbon concentration fol-
lowing biochar addition were observed.
In a follow-up study, Abit et al. [65] investigated the role of
biochar feedstock (poultry litter and pine chips), pyrolysis
temperature (350 and 700 °C), application rate (1 and 2 %),
and soil moisture content (50 and 100 %) on the transport of
two E. coli isolates through a fine sand soil. The authors re-
ported that both high-temperature biochars reduced E. coli
transport at the 2 % application rate, with substantially greater
reductions observed with the pine chip biochar. Application of
the low-temperature poultry litter biochar either had no signif-
icant effect or increased transport of both E. coli isolates—
results consistent with those discussed above [10]. Changes in
transport behavior following biochar addition were quantita-
tively similar for both saturated and unsaturated soils, but for
all treatments, the effect was more pronounced in partially
saturated columns. A strong inverse correlation between soil
organic carbon (SOC) and transport of isolates under partially
and fully saturated conditions was observed (r
2
values ranging
Tabl e 3 Calculation of integral water capacity (IWC) and integral energy (E
i
) for a sequence of 5 and 10 % (w w
−1
)pinechip(Pinus taeda) biochar
additions to a sandy-textured, highly weathered, Norfolk loamy sand (Ultisol)
Integral water capacity (IWC, cm
3
/cm
3
)
5 % biochar addition (size fraction, mm) 10 % biochar addition (w w
−1
)
kPa Control <0.25 0.25–0.50 0.5–1.0 1.0–2.0 <0.25 0.25–0.50 0.50–1.0 1.0–2.0
0–14 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.02 0.01
14–33 0.01 0.05 0.05 0.03 0.01 0.05 0.04 0.04 0.08
33–250 0.03 0.05 0.07 0.04 0.04 0.01 0.02 0.04 0.05
250–1200 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.01 0.01
1200–1500 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Total 0.05 0.11 0.16 0.09 0.07 0.07 0.08 0.10 0.15
Integral energy: (E
i
,Jkg
−1
)
0–14 5.83 1.73 1.41 2.45 4.67 0.45 1.52 1.89 0.58
14–33 5.20 9.00 6.77 8.30 4.97 11.13 9.58 7.71 9.02
33–250 35.28 32.0 42.62 32.70 41.3 8.99 23.42 35.7 25.7
250–1200 14.47 19.4 52.74 19.78 26.4 1.22 12.11 31.62 20.2
1200–1500 0.26 0.49 2.36 0.50 0.70 0.01 0.27 1.1 0.64
Total 61.0 62.6 105.9 63.7 78.0 21.8 46.9 78.0 56.2
Bioenerg. Res.
from −0.85 (p= 0.07) to −0.95 (p= 0.015)) suggesting that
decreases in E. coli transport following biochar additions to
soil were due, in part, to increased sorption of bacteria to the
added biochar. A statistically significant (p<0.001) correla-
tion between bacterial sorption and bacterial transport through
the columns was also observed, further supporting the hypoth-
esis that sorption-related mechanisms significantly contribut-
ed to observed changes in bacterial transport through biochar-
amended soils. These results suggest that application of bio-
char to agricultural fields may affect retention and transport
behavior of enteric bacteria through soils, in addition to
changing other soil properties previously mentioned.
Mine-Impacted Spoils and Biochar
There are approximately 500,000 abandoned mines across the
USA [66]. Accompanying these abandoned mines are waste
piles (tailings) and mine spoils. Mine spoils commonly con-
tain residual metals (e.g., Cd, Cr, Cu, and Zn) or sulfide bear-
ing minerals (e.g., FeS and CuS) that typically undergo hydra-
tion, oxidation, and acidification reactions. This causes the
spoils to acidify and, in the presence of water, generates acidic
mine drainage that can release heavy metals to the environ-
ment. The low spoil pH can reduce or eliminate vegetation
cover, further enhancing sediment transport and thus off-site
heavy metal movement via wind or, water erosion, and
leaching. The large number of abandoned mine sites, as well
as the extent of mine-impacted landscapes, create a challenge
to develop management strategies that promote re-
establishment of a vegetative cover and remediation of
mine-impacted spoil. Biochar may play an important role in
mine land remediation based on its ability to increase pH, soil
water retention, nutrient availability, and binding of heavy
metals and thus support plant establishment and growth.
Several studies have reviewed the potential ability of bio-
char to minimize off-site movement of heavy metals and to
function as a neutralizing agent for acidified mine spoils
[67–69]. These studies reported that biochar can bind heavy
metals, ameliorate acidic soils, and improve soil health char-
acteristics. Gwenzi et al. [70] showed that biochar, alone or in
combination with compost, was effective in absorbing heavy
metals from mine tailings. Additionally, Jain et al. [71]report-
ed that alkaline biochars incorporated into sulfur containing
mine waste were effective at neutralizing acidic compounds.
Biochars with greater ash contents, such as those produced at
high temperatures [72], in the presence of oxygen [73], or
from feedstocks with high conversion efficiency, such as from
manure, will tend to have higher pH values [25]. This can
have important consequences for acidic mine land reclama-
tion, since alkaline biochars may be an effective substitute for
lime. As an example, Ippolito (2015; unpublished data)
showed that adding increasing amounts of biochar produced
from beetle-killed lodge pole pine (Pinus contorta Dougl. Var.
latifolia Engelm.) to four different acidic mine land soils (from
Creede and Leadville, Colorado, and Northern Idaho) signif-
icantly increased soil pH (Fig. 2). Concomitantly, significant
reductionsin bioaccessible heavy metals (i.e., Cd, Cu, Mn, Pb,
and Zn) were observed (Table 4). In this same experiment,
they observed similar results using biochar produced from
switchgrass or tamarisk (Tamarix spp)feedstock.
Biochars can also have metal sorption properties when
used as a soil amendment [11,73,74]. Their ability to bind
metals arises from their porosity, surface area, and surface
functional groups [75,76]. Metals will electrostatically bind
to the surface functional groups and can be physically held in
biochar pore spaces [11]. Additionally, alkaline biochars pro-
mote metal precipitation as carbonate, oxide, or hydroxide
phases at pH values greater than 7 [75]. Ippolito et al. [75]
used extended X-ray absorption fine structure analysis to
show that at a solution pH of 6, pecan shell biochar seques-
tered Cu similar to being bound to organic surface functional
groups. As the system pH increased to 9, Cu carbonate and
oxide species dominated. Biochars made from manure can
have substantial soluble P [27] that can promote the formation
of insoluble metal-phosphate mineral phases such as pyromor-
phite (i.e., Pb
5
(PO
4
)
3
Cl) and reduce Pb bioavailability [77]. In
this way, the mobility of Pb with percolating water is reduced,
thus lowering the potential of water quality degradation.
Another advantage of using biochar as a tool in mine land
remediation is that it is a more stable form of C as compared to
other remediation products. Commonly used soil amendments
for remediating contaminated mine tailings include biosolids,
manures, composts, digestates (i.e., the remains of anaerobic
digesters), papermill sludges, yard, and wood wastes [12,69].
Although these amendments are effective in the short term,
each eventually decomposes and thereby reduces their long-
term remediation efficacy. On the other hand, black carbon (a
biochar-like material formed via wildfires) can have a soil
residence time of hundreds to thousands of years [78]and,
thus as compared with other amendments, may be a more
remediation amendment for mine land reclamation.
Designing Biochar with Specific Characteristics
In this review, we have drawn attention that biochars are not
always an effective amendment at improving soil health char-
acteristics and that crop yield improvements are inconsistent.
Nonetheless, biochar is still globally heralded as an effective
soil amendment in spite of contradictory results. To make
biochar amendments more consistently beneficial, Novak
et al. [27,30] theorized that biochars could be engineered
through single or multi-feedstock selection, blending feed-
stocks, choosing appropriate physical states (e.g., pellets and
dust), and modifying pyrolysis temperatures, to produce
Bioenerg. Res.
biochar materials that target specific soilhealth characteristics.
The theory was termed “designer biochar”and the concept
was vetted through several journal publications [1,79–82].
Moreover, Sohi et al. [83] expanded the “designer biochar
concept”by offering the “systems fit”paradigm for biochar
development and utilization. Sohi et al. [83] explained that the
Lodgepole Pine Biochar Application Rate (% by Wt.)
0 5 10 15
Soil pH
4
5
6
7
8
Creede, CO
Leadville CO A
Leadville CO B
Northern ID
Lodgepole Pine Biochar pH = 7.8
c
a
bb
c
b
aa
c
b
aa
c
b
aa
Fig. 2 Effects of biochar
produced from lodge pole pine on
soil pH values in several mine-
impacted soils (significant
differences at α=0.05between
biochar rates for individual soils
are denoted by different letters
above each bar)
Tabl e 4 Effect of increasing lodgepole pine (Pinus contorta)biochar
application rate on 0.01 M CaCl
2
-extractable (i.e., bioaccessible) metals
in the Creede, Colorado soil, the Leadville, Colorado A and B soils, and a
soil from Northern Idaho (for comparison within a column and for a
specific soil, different letters indicate significant differences at α<0.05
(Ippolito, 2015; unpublished data))
Lodge pole pine biochar application rate
(% by wt.)
Cd Cu Mn Pb Zn
mg kg
−1
Creede, CO soil
0 6.38a 0.12 32.4a 8.47a 541a
5 2.22b ND 1.01b 2.27b 178b
10 1.52b ND ND 0.52c 110b
15 2.02b ND 3.60b 0.36c 149b
Leadville, CO soil A
0 27.4a 3.21a 485a ND 2410a
5 7.50b 0.25b 162b ND 784b
10 4.19b 0.01b 86.8b ND 432b
15 4.94b 0.01b 108b ND 510b
Leadville, CO soil B
0 9.72a 2.73a 163a 0.09 887a
5 2.89b 0.34b 48.0b ND 295b
10 2.45b 0.02c 41.5b ND 233b
15 2.87b 0.04c 51.9b ND 298b
Northern ID soil
0 2.29a 0.78a 100a 32.0a 146a
5 0.73b 0.06b 14.8c 4.60b 32.1b
10 0.46c 0.01b 6.58d ND 15.1c
15 0.87b 0.05b 24.8b 0.43c 37.7b
ND non-detectable
Bioenerg. Res.
“system fit paradigm”causes biochars to be developed
through a combination of biochar and non-biochar ingredi-
ents. The goal is to fit the biochar into a particular soil-crop
system by considering the interaction of relevant waste
streams (different feedstock selections), production technolo-
gy (pyrolysis vs. gasification), and considering specific soil
constraints (pH, CEC, etc.). Others have adopted the designer
biochar concept and the commercial production of custom-
blended biochars [84] has been initiated.
Designer biochars are useful as a soil amendment when they
possess physicochemical properties that can target a specific
soil improvement [80,83]. For instance, biochars made from
switchgrass using a range of pyrolysis temperatures and mate-
rial sizes showed the most significant improvement in soil
moisture storage in an Ultisol and two Aridisols [49] compared
to other biochars. If soil C sequestration and low soil pH are the
target variables for improvement, then the appropriate “design-
er biochar”could be made through high-temperature pyrolysis
(700 °C) of pecan (Carya illinoinensis) or peanut (Arachis
hypogaea)shells[30]. In areas with high animal manure pro-
duction and soils containing excessive plant available P con-
centrations, the manure could be blended and pelletized with
lingo-cellulosic feedstocks (i.e., hardwoods, pine shavings,
etc.) prior to pyrolysis [80]. Blending the manure and creating
pellets is a biochar production strategy to reduce extractable P
concentrations, thus causing a rebalancing of soil P contents
[80]. On the other hand, for infertile soils that rapidly need
SOC, pH, and plant available P improvements, then the un-
blended animal manure could be pyrolyzed using a mid tem-
perature (≈500 °C) setting and the biochar applied as a dust-size
material (0.25 mm). These examples show the utility of design-
ing biochars based on feedstock type, blends, and material size
to improve a targeted soil limitation. By employing the design-
er biochar concept, it is our hope that more consistent results
with biochars ability to improve soils, remediate mine spoils,
and increase crop yields will be obtained.
Conclusions
Biochars have the capacity to be a useful soil amendment for
both agricultural and environmental purposes. While there is
not a “one-size-fits-all biochar,”they have the ability to im-
prove soil health characteristics such as raising SOC contents,
adjusting soil pH values, and increasing soil nutrient and water
retention. Improvement in these salient soil health character-
istics in some soils will result in crop yield increases, which is
particularly important as soils continue to degrade through
anthropogenic activity and by climate change. Biochars also
have been used to reduce the movement of microorganisms
through soil thereby decreasing the potential for human health
risks. Additionally, biochar can be used as a remediation agent
for mine spoils by sequestering metals, raising soil pH, and
improving nutrient content on mine-impacted soils.
Several meta-analysis investigations using results from the
recent biochar literature has revealed a wide range of soil and
crop yield responses—some negative and some positive.
Overall, these results indicate that biochar performance is de-
termined by the initial soil fertility level, soil texture, and degree
of soil weathering. Negative soil health and crop yield re-
sponses are undesirable considering the need to improve crop
yields to sustain a growing human population. We suggest that
biochars can be made more effective as an amendment if they
are designed to have specific chemical and physical properties.
In conclusion, this review shows that biochars can be used
in several ways to address problems with soil health, low crop
productivity, C sequestration, contaminant movement, and en-
vironmental impacts of mine spoils. It remains to be seen what
other crop or soil roles will be developed for biochars, but
their use in the horticultural, industrial, health, and environ-
mental sector continues to grow [13].
Acknowledgments Sincere gratitude is expressed to the support staff at
involved ARS and United States Environmental Protection Agency (US
EPA) locations whose hard work made this research article possible. Parts
of the information in this article have been funded through an Interagency
Agreement between the United States Department of Agriculture-
Agricultural Research Service (60-6657-1-024) and the US EPA (DE-
12-92342301-1). It has been subject to review by scientists of the
United States Department of Agriculture-Agricultural Research Service
(USDA-ARS) at multiple locations and by the National Health and
Environment Effects Research Laboratory’s Western Ecology Division
and approved for journal submission. Approval does not signify that the
contents reflect the views of the USDA-ARS or the US EPA, nor does
mention of trade names or commercial products constitute endorsement
or recommendation for their use.
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