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ORIGINAL PAPER
Charcoal effects on soil solution chemistry and growth
of Koeleria macrantha in the ponderosa pine/Douglas-
fir ecosystem
Michael J. Gundale &Thomas H. DeLuca
Received: 26 July 2005 /Revised: 10 April 2006 / Accepted: 12 April 2006 / Published online: 3 June 2006
#Springer-Verlag 2006
Abstract We conducted laboratory and greenhouse experi-
ments to determine whether charcoal derived from the
ponderosa pine/Douglas-fir ecosystem may influence soil
solution chemistry and growth of Koeleria macrantha,a
perennial grass that thrives after fire. In our first experi-
ment, we incubated forest soils with a factorial combination
of Douglas-fir wood charcoal generated at 350°C and
extracts of Arctostaphylos uva-ursi with and without the
addition of glycine as a labile N source. These results
showed that charcoal increased N mineralization and
nitrification when glycine was added, but reduced N
mineralization and nitrification without the addition of
glycine. Charcoal significantly reduced the solution con-
centration of soluble phenols from litter extracts, but may
have contributed bioavailable C to the soil that resulted in
N immobilization in the no-glycine trial. In our second
experiment, we grew K. macrantha in soil amended with
charcoal made at 350°C from ponderosa pine and Douglas-
fir bark. Growth of K. macrantha was significantly
diminished by both of these charcoal types relative to the
control. In our third experiment, we grew K. macrantha in
soil amended with six concentrations (0, 0.5, 1, 2, 5, and
10%) of charcoal collected from a wildfire. The data
showed increasing growth of K. macrantha with charcoal
addition, suggesting some fundamental differences between
laboratory-generated charcoal and wildfire-produced char-
coal. Furthermore, they suggest a need for a better
understanding of how temperature and substrate influence
the chemical properties of charcoal.
Keywords Charcoal .Soil solution chemistry .
Douglas-fir and ponderosa pine ecosystems
Introduction
It is well-established that fire alters N cycling in the
ponderosa pine/Douglas-fir (Pinus ponderosa/Psuedotsuga
menziesii) ecosystem (Neary et al. 1999; Hart et al. 2005).
Nitrogen availability has been shown to increase immedi-
ately after fire (Covington and Sackett 1990,1992; DeLuca
and Zouhar 2000) and may remain elevated on the scale of
months to years as a result of enhanced mineralization
(Covington and Sackett 1990,1992; Monleon et al. 1997;
Kaye and Hart 1998; Gundale et al. 2005). Numerous
processes that increase N mineralization after fire have been
identified, including improved substrate quality (White
1991,1994; Fernandez et al. 1997; Pietikainen et al.
2000a), death of roots and soil organisms resulting in a
large labile organic N pool (DeBano et al. 1979; Dunn et al.
1979; Diaz-Ravina et al. 1996; Neary et al. 1999), and a
reduction in C to N ratios due to preferential loss of C
during combustion (Gundale et al. 2005). A potentially
overlooked factor that may also enhance N cycling after fire
is the addition of charcoal to soils.
Several recent studies have shown that charcoal has the
potential to greatly enhance soil fertility. Amazonian forest
soils amended centuries ago with charcoal and manure still
maintain some of the highest biodiversity and productivity
Biol Fertil Soils (2007) 43:303–311
DOI 10.1007/s00374-006-0106-5
M. J. Gundale (*):T. H. DeLuca
Department of Ecosystem and Conservation Sciences
University of Montana,
32 Campus Drive,
Missoula, MT 59812, USA
e-mail: mjgundal@yahoo.com
T. H. DeLuca
e-mail: tom.deluca@cfc.umt.edu
of any soils within the Amazon basin (Glaser et al. 2001,
2002; Mann 2002). In boreal forest soils, charcoal was
shown to enhance N cycling by ameliorating the inhibitory
effects of litter extracts from late-successional species,
which in turn promotes growth of early-successional
species (Zackrisson et al. 1996; Wardle et al. 1998; DeLuca
et al. 2002; Berglund et al. 2004). Recently, DeLuca et al.
(2006) found that the addition of wildfire-formed charcoal
to ponderosa pine forest soils increased nitrification rates.
Charcoal may enhance soil fertility through a variety of
mechanisms. Increased N turnover may occur by charcoal
sorption of high C:N organic molecules from the soil
solution (Zackrisson et al. 1996; Wardle et al. 1998; Glaser
et al. 2002), resulting in reduced microbial N immobiliza-
tion and higher net mineralization and nitrification rates. In
addition, charcoal may remove specific groups of organic
molecules, including polyphenol or monoterpene com-
pounds that are thought to inhibit nitrification (Rice and
Pancholy 1972; Zackrisson et al. 1996; DeLuca et al. 2002;
Berglund et al. 2004). Sorption of organic molecules, along
with the gradual breakdown of charcoal, may initiate
humus formation and, thus, enhance long-term soil fertility
(Glaser et al. 2002). Charcoal may also enhance soil
fertility by creating habitat for microbes within its porous
structure (Pietikainen et al. 2000b).
Despite these potential roles that charcoal may have in
increasing soil fertility, its ecological role in forest ecosys-
tems, such as ponderosa pine/Douglas-fir, has received little
attention. We conducted three separate experiments using
low-temperature charcoal to investigate whether charcoal
influences soil solution chemistry and growth of an early
successional species. In our first experiment, our objective
was to determine whether charcoal had an influence on soil
solution chemistry after addition of the extracts of a late
successional species, Arctostapholos uvi-ursi, via surface
adsorption of phenolic compounds. We hypothesized that
charcoal added to a ponderosa pine forest soil will
effectively sorb the phenol fraction in litter extracts, which
would correspond with enhanced N cycling.
In our second experiment, our objective was to compare
the influence of charcoal made from the bark of two
species, ponderosa pine and Douglas-fir, on growth of
Koeleria macrantha, a perennial grass species that thrives
after fire disturbance in western Montana ponderosa pine/
Douglas-fir forests. Bark charring during low-intensity
wildfire is a potentially significant source of charcoal in
this system. Charred bark may gradually slough from trees
after fire and become incorporated in the soils surrounding
trees. It is recognized that ponderosa pine is a more fire-
adapted species than Douglas-fir; thus, an intriguing
hypothesis is that charred bark of the more fire-adapted
species will have a stronger positive effect on N cycling
processes and plant growth.
In our third experiment, our objective was to determine
whether charcoal generated during a wildfire would have
any effect on K. macrantha growth and to determine
whether this relationship is dependent on soil charcoal
concentration. We hypothesized that wildfire charcoal will
positively influence K. macrantha and that this effect will
increase as a function of soil charcoal concentration.
Collectively, these three experiments address our central
hypothesis that charcoal will alter solution chemistry by
sorbing phenols and enhancing N cycling, which in turn
will improve the growth of early successional species.
Materials and methods
All three experiments utilized field-collected soil, which
was collected from the subsurface horizon (20–30 cm, B
w
Horizon) of a forest soil associated with low elevation
(1,100 m) ponderosa pine/Douglas-fir vegetation in western
Montana, USA. The soil is a sandy-skeletal, mixed, frigid
Typic Dystrustepts. This ecosystem is characterized by low
annual rainfall (<350 mm annually) with approximately
50% falling as snow during the winter months. Soil was
collected during the month of September, returned to the
lab, upon which they were sieved (4 mm) and homoge-
nized. We then added one part sand to three parts field
moist soil (by mass) to decrease fertility and increase
porosity and gas exchange, such that nitrification would not
be limited by low O
2
availability. The sand fraction was
purchased as filter grade silica sand (for pool filters) and
was washed with 1 M HCl, followed by distilled water,
before being homogenized with field collected soil. This
sand-amended soil had a pH of 6.8, electrical conductance
of 91.2 μSm
−1
, and had a textural distribution of 71%
sand, 21% silt, and 8% clay.
All experiments also included the addition of either
laboratory-generated charcoal from Douglas-fir and ponder-
osa pine or charcoal collected in the field after a wildfire.
Laboratory charcoal was generated by burying wood or bark
from these species in silica sand and heating at 350°C for 2 h.
Charcoal was then ground and sieved as specified for each
experiment. Various physical and chemical properties of these
charcoals were measured (Table 1). Charcoal pH was
measured from a 4:1 slurry of deionized water to charcoal.
Electrical conductance (EC) was measured from charcoal
paste (2:1 distilled water and charcoal). Cation exchange
capacity (CEC) was estimated on charcoal samples via NHþ
4
replacement where 1 g of charcoal was rinsed twice with
25 ml of 1 M ammonium acetate (pH 7) to saturate exchange
sites. Excess saturating solution was removed from charcoal
samples with three consecutive washes with 25 ml of 95%
ethyl alcohol. Sorbed NHþ
4was then extracted with 25 ml of
2 M KCl and analyzed on a segmented flow analyzer (Auto
304 Biol Fertil Soils (2007) 43:303–311
Analyzer III, Bran Luebbe, Chicago, IL) using the Berthelot
reaction (Willis et al. 1993). Charcoal density was measured
by measuring dry mass of intact charcoal pieces and
measuring volume displacement in deionized water. Total
C was measured via dry combustion on a Fissions Elemental
Analyzer (Milano, Italy). Soluble and total phenols were
measured by extracting 1 g of ground charcoal in 25 ml of
deionized water and 50% methanol, respectively, and were
analyzed using the Prussian Blue Method (Stern et al. 1996).
Extractable NHþ
4and NO
3(Mulvaney 1996)wereextracted
by shaking 1 g of charcoal for 30 min in 25 ml of 2 M KCl,
and then filtering through Whatman #2 filters. The extracts
were analyzed for NHþ
4
–N using the Berthelot reaction
(Willis et al. 1993)andNO
3
–N by the cadmium reduction
method (Willis and Gentry 1987) on a segmented flow
analyzer (Auto Analyzer III). Soluble PO3
4was extracted by
placing1gofcharcoalin25mlof0.01MCaCl
2
for 30 min.
Extracts were filtered through Whatman #42 filter paper and
then analyzed on a segmented flow analyzer using the
molybdate method as described by Kuo (1996).
Experiment 1: charcoal sorption potential
We conducted a laboratory incubation study using the soil
described above, where Douglas-fir charcoal and extract of
Arctostaphylos uva-ursi were added in a factorial combi-
nation yielding four treatments (Charcoal/Extract, Charcoal/
No extract, No Charcoal/Extract, and No Charcoal/No
extract). Each treatment was replicated five times and
consisted of 300 g of soil and placed into mason jars. The
treatments receiving charcoal addition received a 2%
charcoal amendment (20 g/kg). Charcoal was generated in
a muffle furnace by submerging Douglas-fir wood in sand
and heating it at 350°C for 2 h. Charcoal was ground and
sieved through a 4.75-mm sieve. A. uva-ursi extract was
made by extracting 100 g of A. uva-ursi leaves in 1 l of
deionized water for 24 h and filtering this extract through
Whatman #42 filters. The total phenol concentration of this
extract was 267.5 mg/l. Extract treatments received 25 ml
of this extract. No-extract treatments received an equivalent
volume of deionized water. Soils were homogenized
following this addition. This addition brought the soil in
each mason jar to a water content of approximately 60%
WHC. Mason jars were incubated in the dark for 14 days
after which a portion of the soil was extracted and analyzed.
This entire experiment was repeated exactly as described
above but with glycine added to all mason jars as a source of
highly labile organic N to stimulate a more marked N
response. Glycine, a simple amino acid that is readily
mineralized to NHþ
4, was added to each mason jar at a rate
of 75 mg/jar (250 mg/kg of soil). These two experiments will
hereafter be referred to as the glycine and no-glycine trials.
Experiment 2: effects of bark charcoal on plant growth
This greenhouse experiment consisted of three treatments
(Douglas-fir charcoal, ponderosa pine charcoal, and a
control) using the sand-amended soil described above to
evaluate the influence of charcoal source on K. macrantha.
Each treatment consisted of 20 replicate pots where each
pot received 1.5 kg of soil, and charcoal treatments received
a 2% (by mass) charcoal amendment. One percent of this
charcoal was homogenized into the soil, while the other 1%
was evenly distributed on the soil surface. We made
charcoal from Douglas-fir and ponderosa pine in the
laboratory by burying bark of each species in silica sand
and heating to 350°C in a muffle furnace for 2 h. Charcoal
was ground and sieved (<1 mm) using a Wiley mill.
Organic horizons (O
i
,O
e
, and O
a
) were added to the surface
of each pot to add an additional and substantial mineraliz-
able pool of plant essential nutrients, as well as to provide a
source of bioavailable organic C that may influence soil
nutrient transformations. This organic material was ran-
domly collected (as described in Gundale et al. 2005) from
a ponderosa pine/Douglas-fir forest that had not been
exposed to fire for approximately 80 years and originated
from numerous species, including understory and overstory
species, but appeared to be primarily composed of
undecomposed ponderosa pine and Douglas-fir litter. The
organic material was homogenized and 100 g was added to
the surface of each pot. A mixed bed ionic resin capsule
(Unibest, Bozeman, MT) was placed in the center of each
pot to sorb nutrients throughout the duration of the
experiment.
K. macrantha was grown in these pots between October
2004 and March 2005 under ambient light conditions. An
average greenhouse temperature of 21°C was maintained. K.
macrantha seeds (Western Native Seeds, Coaldale, CO)
were germinated in a separate soil medium, and a single
seedling was transplanted into each pot. Pots were watered
3 days a week throughout the duration of the experiment. At
Table 1 Chemical properties of four charcoal types (df Douglas-fir,
pp ponderosa pine, wildfire wildfire collected) used in the laboratory
and greenhouse experiments
df Wood df Bark pp Bark Wildfire
pH 4.15 4.18 4.81 5.74
EC (μSg
−1
) 20.5 24.2 111.6 141.5
CEC (cmol
c
kg
−1
) 20.66 19.42 34.48 29.35
Density (g cm
−3
) 0.30 0.08 0.21 0.29
Total C (%) 72.9 72.4 71.5 82.3
Soluble phenols (μgg
−1
) 34.9 30.7 43.4 48.2
Total phenols (μgg
−1
) 441.7 148.1 810.4 393.8
PO3
4(μgg
−1
)0.94 0.84 2.46 0.95
NHþ
4(μgg
−1
)7.12 9.0 5.6 38.3
NO
3(μgg
−1
)0.12 0.3 0.0 4.4
Biol Fertil Soils (2007) 43:303–311 305
the end of the experiment, resin capsules were recovered,
and soil was rinsed from roots. Plants were oven-dried at 65°C,
and above- and belowground masses were measured.
Experiment 3: effect of wildfire charcoal on plant growth
Charcoal collected from a wildfire site was added to the soil
described above at a rate of 0, 0.5, 1, 2, 5, and 10%, and
placed in greenhouse pots seeded with K. macrantha to
determine whether an increase in soil charcoal content has
any influence on the growth of K. macrantha.Each
treatment (n=10) was established by adding 1.0 kg of
charcoal-amended soil per pot. The charcoal used in this
experiment differed from both previous experiments because
it was collected after a wildfire rather than generated in the
laboratory. Large particles (>5-cm diameter) of charcoal
were collected in the spring of 2004 from the Black
Mountain Fire (August 2003), Missoula, MT, (DeLuca et
al. 2006). It was impossible to decipher the species origin of
this charcoal, but it was likely primarily Douglas-fir and
ponderosa pine wood and bark char. The charcoal particles
were crushed, using a mallet, producing fragments ranging
from a diameter of 2 cm to microscopic. No attempt was
made to discriminate against any size class in an attempt to
simulate the range of charcoal particle sizes likely incorpo-
rated into the soil under natural conditions. Organic horizon
materials (50 g) were collected from a forest stand not
exposed to fire for over 80 years and added to the surface of
each pot as described earlier. All other experimental
conditions were run identically to experiment 2.
Laboratory analyses
At the end of experiment 1, 30 g of soil were extracted with
2 M KCl and analyzed for NHþ
4and NO
3as described
above. Amino N was measured on these same extracts
using the ninhydrin method (Moore 1968). Soluble phenols
were extracted by shaking 30 g of soil for 1 h with 50 ml of
deionized water followed by filtration. Sorbed phenols were
extracted by shaking 30 g of soil with 50% methanol for
24 h followed by filtration. Phenols in these extracts were
measured using the Prussian blue method (Stern et al.
1996). Respiration was measured at the end of the
incubation by incubating 50 g dry weight equivalent soil
in a sealed container with 20 ml 1 M NaOH traps for 3 days
(Zibilske 1994).
Mixed bed ionic resin capsules (Unibest) were used in
experiments 2 and 3 to determine solution NHþ
4;NO
3, and
PO
4throughout the duration of the experiments. Capsules
were placed in the center of each pot, directly beneath each
plant, and were removed and extracted in 10 ml of 2-M KCl
three consecutive times. We analyzed NHþ
4;NO
3,and
PO3
4from these extracts as described previously.
Statistical analyses
Data in experiment 1 meeting assumptions of normality and
homoscedasticity were analyzed using two-factor analysis
of variance (ANOVA), where extract and charcoal were
entered as fixed factors under the general linear model.
Variables not meeting these assumptions were analyzed
using a Kruskal–Wallis test (K–W test). This analysis tests
for differences among treatments but does not evaluate the
significance of individual factors or interactions between
factors.
Data in experiments 2 and 3 were analyzed using one-
factor ANOVA followed by the Student–Newman–Keuls
post hoc procedure. Different letters are used to display post
hoc differences. Data not meeting assumption of normality
and homoscedasticity were compared using K–W tests,
which were not followed by post hoc procedures. All
analyses were conducted using SPSS 12.0 software.
Results and discussion
Experiment 1: low temperature charcoal sorption potential
Both charcoal and litter extract significantly influenced
numerous soil chemical variables (Fig. 1). In both glycine
and no-glycine trials, litter extract negatively influenced
extractable NO
3concentrations. The negative influence of
A. uva-ursi on extractable NO
3reported here is consistent
with our previous studies in ponderosa pine forest soils
(DeLuca et al. 2006) and with studies that showed that litter
from late-successional boreal species, such as the ericaceous
shrub Empetrum hermaphroditum, diminishes net nitrifica-
tion (DeLuca et al. 2002; Berglund et al. 2004). Charcoal
had an unexpected negative effect on NO
3in the no-glycine
trial. In contrast, the addition of charcoal increased NO
3
concentrations in the glycine trial. These results may be a
function of the charcoal we used in this study, which was
generated at a low temperature (350°C). Charcoal contains a
significant concentration of bioavailable C, specifically
soluble phenols (Table 1) that may have caused net NO
3
immobilization (Schimel et al. 1996) in the no-glycine trial
where low NHþ
4concentrations existed (Rice and Tiedje
1989). The NO
3immobilization effect did not occur in the
glycine trial because NHþ
4limitations were drastically
reduced with glycine addition. In addition, higher rates of
nitrification in the glycine trial likely occurred because this
process was not limited by a lack of substrate availability
(glycine additions resulted in high NHþ
4concentrations).
The higher rate of nitrification associated with charcoal
in the glycine trial is consistent with the finding reported by
DeLuca et al. (2006), which suggests that charcoal may
sorb compounds from litter extract and the soil solution that
306 Biol Fertil Soils (2007) 43:303–311
are inhibitory to nitrifying bacteria, or sorb carbon-rich
molecules that would otherwise stimulate microbial immo-
bilization of N.
A. uva-ursi extract had a strong positive effect on NHþ
4
in both no-glycine and glycine trials because it likely
contained some NHþ
4and substrates that are rapidly
mineralized to NHþ
4. Charcoal had a strong negative effect
on NHþ
4in both no-glycine and glycine trials. The
mechanisms for this pattern may differ between the two
trials. In the no-glycine trial, the most likely explanation for
reduced NHþ
4is that immobilization occurred as a function
of N limitations in these soils. In the glycine trial, higher
rates of nitrification associated with charcoal likely con-
tributed to lower NHþ
4concentrations.
Both charcoal and extract significantly influenced con-
centrations of amino N that represent a highly labile
Phenols (
µ
µ
g g-1)
0
5
10
15
20
Soluble Phenols
Sorbed Phenols
Soluble Phenols
KW test **
Sorbed Phenols
Charcoal ***
Extract ***
Charcoal x Extract NS
0
5
10
15
20
Soluble Phenols
KW test *
Sorbed Phenols
Charcoal ***
Extract **
Charcoal x Extract NS
0
50
100
150
200 Amino N
Charcoal ***
Extract *
Charcoal X Extract NS
Ammonium
Charcoal ***
Extract **
Charcoal x Extract NS
Nitrate
KW test ***
Extractable N (
µ
µ
g g-1)
0
5
10
15
20
Amino N
NH4
+
NO3
-
Amino N
Charcoal *
Extract ***
Charcoal x Extract **
Ammonium
KW test ***
Nitrate
Charcoal ***
Extract ***
Charcoal x Extract *
S S + E S + C S + C + E
Respiration (
µ
µ
g CO2 day-1)
0.0
0.1
0.2
0.3
0.4
0.5
Respiration
Charcoal NS
Extract *
Charcoal x Extract NS
S S + E S + C S + C + E
0.0
0.1
0.2
0.3
0.4
0.5
Respiration
Charcoal NS
Extract NS
Charcoal x Extract **
ab
cd
ef
No-Glycine Trial Glycine Trial
Fig. 1 Extractable amino N, NH þ
4, and NO
3[mean (SE)] without
(a) and with (b) glycine addition; soluble (water extracted) and sorbed
phenols (methanol extracted) [mean (SE)] without (c) and with (d) glycine
addition; and basal soil respiration [mean (SE)] without (e) and with
(f) glycine addition, from a 14-d soil incubation experiment where soils
were amended with a factorial combination of charcoal and extracts from
Arctostaphylos uva-ursi leaves (Ssoil only, S+E soil plus extract, S+C soil
plus charcoal, S+C+E soil plus charcoal plus extract). Data were analyzed
with a two-factor ANOVA where significance was tested for Charcoal,
Extract, and Charcoal × Extract interaction. Data that did not meet
parametric assumptions of normality or homoscedasticity were analyzed
using a Kruskal–Wallis (KW) test. Asterisks represent statistical signifi-
cance (pvalue, ns >0.1, *<0.05, **<0.01, ***<0.001)
Biol Fertil Soils (2007) 43:303–311 307
fraction of organic N that can be rapidly mineralized.
Glycine, which is a simple amino N molecule, stimulated
rapid rates of N mineralization and resulted in increased
amino N concentrations, which suggests that the added
glycine was not completely utilized and that substrate
limitations were eliminated during this trial. In glycine and
no-glycine trials, the litter extract resulted in higher
concentrations of amino N to soils. The effect of charcoal
on amino N, however, differed in glycine and no-glycine
trials. In the no-glycine trial, charcoal significantly in-
creased amino N concentrations. This response may have
occurred because charcoal sorbed phenolic molecules that
otherwise would form insoluble complexes with amino N
groups. In contrast, charcoal had a negative effect on amino
N in the glycine trial, which is likely the result of charcoal
enhancing microbial utilization of glycine.
As expected, A. uva-ursi extract significantly increased
phenols (soluble and sorbed) in both trials. The addition of
charcoal to soil significantly diminished the soluble phenol
concentration while increasing the pool of sorbed phenol.
This result is consistent with several studies in the boreal
forest that have demonstrated a high capacity of charcoal to
adsorb phenolic compounds (Zackrisson et al. 1996; Wardle
et al. 1998; DeLuca et al. 2002; Berglund et al. 2004).
Solubility of these fractions likely influences the degree to
which they are bioavailable and, therefore, their ability to
interfere with N transformations (Harborne 1997). It is
interesting to note that total phenols (sorbed and soluble)
was higher in the charcoal-only treatment of both trials than
the control, demonstrating that charcoal itself adds a
substantial amount of total phenol to the soil (Table 1).
These phenols are likely derived from the components of
wood, such as lignin that are degraded during charcoal
formation. It is unclear what effect these phenols have on
soil processes, but it is likely that they could be utilized as a
food source by microbes, stimulating N immobilization.
Soil respiration showed little response to charcoal in
glycine or no-glycine trials. In the no-glycine trial, the extract
significantly increased soil respiration. Extract and charcoal
had no individual effect on soil respiration in the glycine trial;
however, the interaction between charcoal and extract showed
a significant effect. We speculate that this response may
reflect that amines and degradable carbon substrates were
better utilized by microbes when phenolic molecules in the
same extract were sorbed by charcoal.
These data demonstrate that low-temperature charcoal
effectively sorbs soluble phenols from A. uva-ursi extracts,
which in turn stimulates nitrification, provided nitrification is
not substrate-limited. Our results are consistent with
Berglund et al. 2004 and DeLuca et al. (2002), who showed
that the effect of charcoal on nitrification only occurred when
a labile N source was also present. These studies are also
consistent with the Terra Preta phenomenon reported in the
Amazonian basin where charcoal and manure (high labile N
concentration) were historically incorporated into the soil
(Glaser et al. 2001,2002). Today, these soils maintain the
highest fertility in the region, which may in part be a
function of the interactive effect of charcoal and manure.
Experiment 2: effects of bark charcoal on plant growth
In this experiment, we unexpectedly found that charcoal
from both species diminished growth of K. macrantha
relative to the control with reduced mass in both above-
ground and belowground growth (Table 2). K. macrantha
growing in pots with Douglas-fir charcoal had a signifi-
cantly higher root to shoot ratio than the other treatments
that appeared to be primarily driven by low aboveground
biomass. This data suggests that there is likely no difference
in the effect of ponderosa pine and Douglas-fir charcoal on
plant species in this ecosystem.
We found that resin-sorbed NHþ
4and NO
3were
significantly higher in the Douglas-fir charcoal treatment
relative to the ponderosa pine charcoal treatment and the
control. Resin-sorbed PO3
4was significantly higher in both
Douglas-fir and ponderosa pine charcoal treatments than
the control. These results may be interpreted in several
ways. First, they may indicate higher mineralization and
nitrification rates in the presence of charcoal as suggested
by experiment 1. If higher mineralization occurred in the
presence of charcoal, it is unclear why a corresponding
increase in plant growth did not occur. It is possible that
some toxic substance was generated during charcoal
formation that inhibited root growth of K. macrantha,
despite a positive effect on nutrient availability (Fritze et al.
Table 2 Plant mass and resin sorbed nutrients (mean±SE, n=20) from
a greenhouse experiment where soil was amended with 2% charcoal
made from Douglas-fir (df) and ponderosa pine (pp) bark at 350°C
df
Charcoal
pp
Charcoal
no
Charcoal
p
value
Total mass (g) a1.6 (0.2) a1.9 (0.1) b2.5 (1.0) <0.001
Root mass (g) a0.8 (0.1) a0.9 (0.1) b1.2 (0.1) <0.05
Aboveground
mass (g)
a0.7 (0.1) b1.0 (0.1) c1.3 (0.1) <0.001
Root to shoot
ratio
a1.1 (0.1) b0.9 (0.1) b0.9 (0.1) <0.01
NHþ
4(μg resin
capsule
−1
)
a4.6 (0.9) b1.4 (0.6) b2.2 (0.8) <0.05
NO
3(μg resin
capsule
−1
)
a1,770.8
(286.2)
b935.8
(241.9)
b581.5
(211.8)
<0.01
PO3
4(μg resin
capsule
−1
)
a5.1 (1.4) a5.6 (0.9) b0.5 (1.2) <0.05
Letters in bold indicate differences using the Student–Newman–Keuls
post hoc procedure
308 Biol Fertil Soils (2007) 43:303–311
1998; Villar et al. 1998). These toxic substances are likely
to be more abundant in low temperature charcoals, such as
used in this experiment, and may be prone to volatilization
at higher temperatures. An additional explanation is that
charcoal may have enhanced soil macroporosity, allowing
more soil solution to pass through capsules, resulting in
misleading resin-sorbed nutrient concentrations.
Experiment 3: effect of wildfire charcoal on plant growth
In support of our hypothesis, natural charcoal collected from a
wildfire showed a positive effect on growth of K. macrantha
(Table 3). Both total and aboveground masses were signifi-
cantly higher in pots amended with 5 and 10% charcoal
addition than the control. Pots with lower charcoal content
(0.5–2%) showed an intermediate growth response. No
significant shift in allocation to above- or belowground
structures was detected across the charcoal gradient. As in
experiment 2, resin-sorbed NO
3and PO3
4decreased as plant
growth increased. These results suggest that these measure-
ments do not reflect any direct effect charcoal may have on
nutrient cycling, but are rather indicative of the solution nutrient
concentration as influenced by plant uptake. No difference in
resin sorbed NHþ
4occurred across the charcoal gradient.
The different responses of K. macrantha to charcoal in
experiments 2 and 3 suggest that charcoal produced in a
laboratory may be greatly different from charcoal generated
during wildfire. Differences in charring conditions may
influence the chemical and structural nature of charcoal and
may therefore change its influence on soil solution
chemistry. One potentially important difference between
laboratory- and wildfire-collected charcoal was the ratio of
soluble phenols to NHþ
4concentration extracted from the
charcoals (Table 1). While all charcoal had relatively
similar soluble phenol contents, which may stimulate
microbial N immobilization, high NHþ
4concentrations
may have offset this immobilization effect when wildfire
charcoal was used. Another potentially important difference
is the different pH of laboratory charcoal and wildfire
charcoal (Table 1). The low pH associated with the lab
charcoals may have indirectly diminished P availability
in these treatments. Another difference between the
charcoal used in experiments 2 and 3 was the range
of charcoal particle size used. Experiment 3 incorporated
charcoal ranging from large (1–2cm)tomicroscopic
fractions. We noted substantial root penetration into large
charcoal particles at the end of this greenhouse experiment,
which suggests that some resource, such as water, is more
available inside large charcoal particles. It is also possible
that grinding charcoal to a smaller size class, in some way,
eliminates its beneficial effects on soil fertility. For
instance, grinding may enhance the availability of organic
carbon because it is very immobile, whereas N ions are
significantly more mobile; thus, nutrient immobilization
may be more substantial when charcoal is ground.
Conclusion
It is clear that charcoal has the potential to significantly alter
soil solution chemistry and growth of K. macrantha. Charcoal
did not appear to stimulate N cycling in a low-nutrient
setting, but when glycine was added to soil, charcoal greatly
enhanced N mineralization and nitrification. This result may
indicate that low temperature charcoal contributes bioavail-
able carbon that causes N immobilization under low nutrient
conditions. As hypothesized, charcoal effectively sorbed
soluble phenols from litter extracts. This sorption may
effectively reduce the inhibitory effect of litter extracts on
soil microorganisms, plants, and biogeochemical processes.
Low-temperature, laboratory-generated charcoal had a nega-
tive effect on growth of K. macrantha, possibly as a result of
Table 3 Plant mass and resin sorbed nutrients (mean±SE, n=10) from a greenhouse experiment where soil was amended with 0, 0.5, 1, 2, 5, and
10% charcoal collected from a wildfire
Percent charcoal
0% 0.5% 1% 2% 5% 10% p
Total mass (g) a0.5 (0.2) ab1.0 (0.3) ab1.1 (0.2) ab1.1 (0.2) b1.3 (0.1) b1.4 (0.1) <0.05
Root mass (g) 0.3 (0.2) 0.6 (0.2) 0.7 (0.1) 0.7 (0.1) 0.8 (0.1) 0.8 (0.1) >0.05
Aboveground mass (g) a0.2 (0.1) ab0.4 (0.1) ab0.4 (0.1) ab0.4 (0.1) b0.5 (0.1) b0.6 (0.1) <0.01
Root to shoot ratio 1.5 (0.2) 1.5 (0.3) 1.8 (0.2) 1.7 (0.3) 1.6 (0.2) 1.3 (0.1) >0.05
NHþ
4(μg resin capsule
−1
)55.6 (4.0) 49.8 (4.6) 36.9 (6.0) 42.7 (2.2) 43.0 (1.3) 44.4 (2.9) <0.05
a
NO
3(μg resin capsule
−1
)a1,539.8
(463.4)
b947.9
(128.4)
bc552.3
(116.3)
bc556.1
(93.4)
bc561.8
(278.7)
c248.6
(29.8)
<0.001
PO3
4(μg resin capsule
−1
)a10.1 (1.4) a8.8 (2.1) ab5.8 (1.2) ab6.5 (1.7) bc1.7 (1.0) c0.0 (1.2) <0.001
Letters in bold indicate differences using the Student–Newman–Keuls post hoc procedure
All pvalues are for one-way ANOVA, unless otherwise noted
a
Kruskal–Wallis test pvalue
Biol Fertil Soils (2007) 43:303–311 309
a toxicity effect caused by some compound formed during
low temperature charring or by N immobilization, as
suggested by the no-glycine soil incubation. In contrast,
charcoal created during a wildfire had a positive effect on the
growth of K. macrantha, suggesting low-temperature, labo-
ratory charcoal may not adequately represent field-collected
charcoal. Field-collected charcoal may have been generated
in a higher oxygen, higher temperature environment and may
have been exposed to leaching by rainwater and occlusion by
soil organic compounds before collection. Further investiga-
tion is required to evaluate how charcoal formation conditions
alter its effect on soil processes and plant growth and how
these processes manifest themselves in natural ecosystems.
Acknowledgements We thank V. Kurth, D. Mackenzie, and T.
Burgoyne for their assistance in the laboratory and greenhouse. We
also acknowledge funding from the NSF (NSF-DEB-03171108) and
the USDA Joint Fire Sciences Program (FFS #107) for this research.
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