Effect of biochar amendment on sorption and leaching of nitrate, ammonium, and phosphate in a sandy soil
When applied to soils, it is unclear whether and how biochar can affect soil nutrients. This has implications both to the availability of nutrients to plants or microbes, as well as to the question of whether biochar soil amendment may enhance or reduce the leaching of nutrients. In this work, a range of laboratory experiments were conducted to determine the effect of biochar amendment on sorption and leaching of nitrate, ammonium, and phosphate in a sandy soil. A total of thirteen biochars were tested in laboratory sorption experiments and most of them showed little/no ability to sorb nitrate or phosphate. However, nine biochars could remove ammonium from aqueous solution. Biochars made from Brazilian pepperwood and peanut hull at 600°C (PH600 and BP600, respectively) were used in a column leaching experiment to assess their ability to hold nutrients in a sandy soil. The BP600 biochar effectively reduced the total amount of nitrate, ammonium, and phosphate in the leachates by 34.0%, 34.7%, and 20.6%, respectively, relative to the soil alone. The PH600 biochar also reduced the leaching of nitrate and ammonium by 34% and 14%, respectively, but caused additional phosphate release from the soil columns. These results indicate that the effect of biochar on the leaching of agricultural nutrients in soils is not uniform and varies by biochar and nutrient type. Therefore, the nutrient sorption characteristics of a biochar should be studied prior to its use in a particular soil amendment project.
Effect of biochar amendment on sorption and leaching of nitrate, ammonium,
and phosphate in a sandy soil
, Bin Gao
, Ming Zhang
, Mandu Inyang
, Andrew R. Zimmerman
Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, United States
Department of Geological Sciences, University of Florida, Gainesville, FL 32611, United States
Effect of biochar on the leaching of nutrients in soils is not uniform.
Sorption of nutrients on biochar varies by biochar and nutrient type.
Nutrient sorption characteristics should be studied prior to biochar application.
Received 9 April 2012
Received in revised form 31 May 2012
Accepted 5 June 2012
Available online 2 July 2012
When applied to soils, it is unclear whether and how biochar can affect soil nutrients. This has implica-
tions both to the availability of nutrients to plants or microbes, as well as to the question of whether
biochar soil amendment may enhance or reduce the leaching of nutrients. In this work, a range of labo-
ratory experiments were conducted to determine the effect of biochar amendment on sorption and leach-
ing of nitrate, ammonium, and phosphate in a sandy soil. A total of thirteen biochars were tested in
laboratory sorption experiments and most of them showed little/no ability to sorb nitrate or phosphate.
However, nine biochars could remove ammonium from aqueous solution. Biochars made from Brazilian
pepperwood and peanut hull at 600 °C (PH600 and BP600, respectively) were used in a column leaching
experiment to assess their ability to hold nutrients in a sandy soil. The BP600 biochar effectively reduced
the total amount of nitrate, ammonium, and phosphate in the leachates by 34.0%, 34.7%, and 20.6%,
respectively, relative to the soil alone. The PH600 biochar also reduced the leaching of nitrate and ammo-
nium by 34% and 14%, respectively, but caused additional phosphate release from the soil columns. These
results indicate that the effect of biochar on the leaching of agricultural nutrients in soils is not uniform
and varies by biochar and nutrient type. Therefore, the nutrient sorption characteristics of a biochar
should be studied prior to its use in a particular soil amendment project.
Ó 2012 Elsevier Ltd. All rights reserved.
Excessive application of fertilizer has caused the release of
nutrient elements, such as nitrogen and phosphorus, from agricul-
tural ﬁelds to aquatic systems. Leaching of nutrients from soils
may deplete soil fertility, accelerate soil acidiﬁcation, increase fer-
tilizer costs for the farmers, reduce crop yields, and most impor-
tantly impose a threat to environmental health (Bhargava and
Sheldarkar, 1993; Ozacar, 2003; Laird et al., 2010). High nutrient
levels in surface and/or groundwater can promote eutrophication,
excessive production of photosynthetic aquatic microorganisms
in freshwater and marine ecosystems (Karaca et al., 2004). It is
therefore very important to develop effective technologies to hold
nutrients in soils.
An option to reduce nutrient leaching could be the application
of biochar to soils. Biochar, sometimes called agrichar, is a charcoal
derived from the thermal decomposition of a wide range of carbon-
rich biomass materials, such as grasses, hard and soft woods, and
agricultural and forestry residues. The approach of land application
of biochar in agriculture is receiving increased attention as a way
to create a carbon sink to mitigate global warming, increase soil
water holding capacity, and reduce emissions of NO
well as to control the mobility of a variety of environmental pollu-
tants, such as heavy metals, pesticides and other organic contam-
inants (Lehmann et al., 2006; Verheijen et al., 2009; Inyang et al.,
2010; Van Zwieten et al., 2010). In addition, it is suggested
that application of biochar can increase soil fertility and crop pro-
ductivity by reducing the leaching of nutrients or even supplying
0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
Corresponding author. Tel.: +1 352 392 1864x285.
E-mail address: bg55@uﬂ.edu (B. Gao).
Chemosphere 89 (2012) 1467–1471
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/chemosphere
nutrients to plants (Glaser et al., 2002; Lehmann et al., 2003; Major
et al., 2010).
Only a few studies, however, have investigated the ability of
biochars to retain nutrients, particularly for a range of different
biochars. For example, Lehmann et al. (2003) reported that amend-
ment of biochar produced from secondary forest residuals signiﬁ-
cantly reduced the leaching of fertilizer N and increased plant
growth and nutrition. Ding et al. (2010) showed that bamboo bio-
char sorbed ammonium ions by cation exchange and retarded the
vertical movement of ammonium into deeper soil layers within the
70-d observation time. Laird et al. (2010) reported the addition of
biochar produced from hardwood to typical Midwestern agricul-
tural soil signiﬁcantly reduced total N and P leaching by 11% and
The overarching objective of this work was to determine the ef-
fect of biochar amendment on leaching of nitrate, ammonium, and
phosphate in sandy soils. Biochars were produced from a range of
commonly used feedstock materials. Laboratory batch sorption
experiments were conducted to access the overall aqueous nitrate,
ammonium, and phosphate sorption ability of the biochars. In
addition, laboratory column experiments were used to examine
the leaching dynamics of the three nutrients in a sandy soil
amended with two selected biochars.
2. Materials and methods
Biochar samples were produced from commonly used biomass
feedstock materials: sugarcane bagasse (BG), peanut hull (PH),
Brazilian pepperwood (BP), and bamboo (BB). The raw materials
were oven dried (80 °C) and converted into biochar through slow
pyrolysis using a furnace (Olympic 1823HE) in a N
at temperatures of 300, 450 and 600 °C. The resulting twelve bio-
char samples are henceforth referred to as BG300, BG450, BG600,
PH300, PH450, PH600, BP300, BP450, BP600, BB300, BB450, and
BB600. Another biochar (hydrochar) was produced through the
hydrothermal carbonization of PH submerged in deionized (DI)
water in an autoclave at 300 °C for 5 h and is referred to as HTPH.
All biochar samples were then crushed and sieved yielding a uni-
form 0.5–1 mm size fraction. After rinsing with DI water several
times to remove impurities, such as ash, the biochar samples were
oven dried (80 °C) and sealed in containers for later use. Detailed
information about biochar production procedures were reported
previously (Yao et al., 2011).
Sandy soil was collected from an agricultural ﬁeld at the Univer-
sity of Florida in Gainesville, FL. The soil was sieved through a
1 mm mesh (No. 18) and dried (60 °C) in an oven. Basic properties
of the soil are listed in Table 1.
Nitrate, ammonium, and phosphate solutions were prepared by
dissolving ammonium nitrate (NH
) or potassium phosphate
dibasic anhydrous (K
) in deionized (DI) water. All the chem-
icals used in the study were A.C.S certiﬁed and obtained from Fish-
2.2. Characterization of sorbents
A range of physicochemical properties of the biochar samples
produced were determined. The pH of the biochars was measured
using a biochar to deionized (DI) water mass ratio of 1:20 followed
by shaking and an equilibration time of 5 min before measurement
with a pH meter (Fisher Scientiﬁc Accumet Basic AB15). Elemental
C, N, and H abundances were determined, in duplicate, using a CHN
Elemental Analyzer (Carlo-Erba NA-1500) via high-temperature
catalyzed combustion followed by infrared detection of the result-
gases, respectively. Major inorganic elements
were determined by acid digestion of the samples followed by
inductively-coupled plasma atomic emission spectroscopic (ICP-
AES) analysis. The surface area of the biochars was determined
on Quantachrome Autosorb1 at 77 K using the Brunauer–Em-
mett–Teller (BET) method in the 0.01–0.3 relative pressure range
of the N
2.3. Sorption of nitrate, ammonium, and phosphate
Batch sorption experiments were conducted in 68 mL digestion
vessels (Environmental Express) at room temperature (22 ± 0.5 °C).
About 0.1 g of each biochar sample was added into the vessels and
mixed with 50 mL 34.4 mg L
nitrate and 10.0 mg L
solution or 30.8 mg L
phosphate solution. Vessels without either
biochar or nutrient elements were included as experimental con-
trols. The mixtures were shaken at 55 rpm in a mechanical shaker
for 24 h, and then ﬁltered through 0.22
m nylon membrane ﬁlters
(GE cellulose nylon membrane).
In addition to pH, concentrations of nitrate in the supernatants
were determined using an ion chromatograph (Dionex Inc. ICS90).
Concentrations of ammonium and phosphate in the supernatants
were measured using the phenate method (APHA et al., 1992)
and the ascorbic acid method (ESS Method 310.1; (USEPA,
1992)), respectively, using a dual beam UV/VIS spectrophotometer
(Thermo Scientiﬁc, EVO 60). Nutrient elements concentrations on
the solid phase were calculated based on the initial and ﬁnal aque-
ous concentrations. All the experimental treatments were carried
out in duplicate and the average values are reported. The variance
between any duplicate measurements in this study was smaller
2.4. Leaching of nutrients from soil columns
Two biochar samples, PH600 and BP600, were selected to study
their effect on nutrients retention and transport in a sandy soil. Soil
columns were made of acrylic cylinders measuring 16.5 cm in
height and 4.0 cm in diameter, and the bottom of the columns
were covered with a stainless steel mesh with 60
m pore size to
prevent soil loss. The sandy soil with (2% by weight) or without
biochars was wet-packed into the column (200 g total) following
procedures reported previously (Tian et al., 2010). These columns
were ﬂushed with 10 pore-volumes of DI water before use to pre-
condition the column. A nutrient solution containing 34.4 mg L
nitrate, 10.0 mg L
ammonium and 30.8 mg L
then applied to these laboratory soil columns to study biochar ef-
fect on nutrients retention and transport. About one pore-volume
of DI water was poured into the soil columns on the ﬁrst day. On
days 2 and 3, same amount of nutrient solution was applied to
the soil columns. After that, the columns were ﬂushed with one
pore-volume DI water each day for another 4 d. All the leachate
samples were collected from the outlet at the bottom of the col-
umns and immediately ﬁltered through 0.22
m ﬁlters for further
analyses. The nitrate, ammonium and phosphate concentrations in
leachate samples were measured using the same method described
Basic properties of the sandy soil used in this study.
Sandy 94.0 3.0 3.0 2.4 1.0
1468 Y. Yao et al. / Chemosphere 89 (2012) 1467–1471
Properties and elemental composition of biochars used in this study.
pH Elemental composition (%, mass based)
NP K Ca MgZn CuFe Al
BG300 33.4 5.2 7.2 69.5 4.2 24.5 0.9 0.05 0.27 0.46 0.14 0.01 0.00 0.02 0.10
BG450 28.0 15.3 7.9 78.6 3.5 15.5 0.9 0.07 0.25 0.83 0.18 0.01 0.00 0.06 0.11
BG600 26.5 4.2 7.9 76.5 2.9 18.3 0.8 0.08 0.15 0.91 0.21 0.01 0.00 0.05 0.11
PH300 38.4 0.8 7.8 73.9 3.9 19.1 1.6 0.09 0.86 0.32 0.13 0.00 0.00 0.00 0.06
PH450 21.7 21.8 8.2 81.5 2.9 13.0 1.0 0.09 0.94 0.33 0.13 0.00 0.00 0.00 0.06
PH600 30.8 27.1 8.0 86.4 1.4 10.0 0.9 0.10 0.71 0.34 0.12 0.00 0.00 0.00 0.06
HTPH300 44.9 5.6 6.8 56.4 5.6 36.7 0.9 0.08 0.00 0.20 0.02 0.00 0.00 0.07 0.07
BP300 51.5 81.1 6.6 59.3 5.2 34.1 0.3 0.03 0.10 0.73 0.12 0.01 0.00 0.04 0.03
BP450 32.0 0.7 7.3 75.6 3.6 17.2 0.3 0.07 0.25 1.32 0.23 0.00 0.00 0.05 0.03
BP600 28.9 234.7 9.1 77.0 2.2 17.7 0.1 0.09 0.12 1.81 0.29 0.00 0.00 0.08 0.03
BB300 73.2 1.3 6.7 66.2 4.7 27.7 0.4 0.24 0.30 0.22 0.14 0.01 0.00 0.00 0.08
BB450 26.3 18.2 5.2 76.9 3.6 18.1 0.2 0.36 0.35 0.29 0.19 0.01 0.00 0.00 0.04
BB600 24.0 470.4 7.9 80.9 2.4 14.9 0.2 0.50 0.52 0.34 0.23 0.01 0.00 0.00 0.04
Determined by weight difference assuming that the total weight of the samples was made up of the tested elements only.
Fig. 1. Removal of nitrate (a), ammonium (b), and phosphate from aqueous solution by different types of biochars (see text for abbreviations).
Y. Yao et al. / Chemosphere 89 (2012) 1467–1471
3. Results and discussion
3.1. Biochar properties
The biochar production rate ranged 21.7–51.5% on a mass basis
(Table 2). In general, more biochar was yielded at the lower pyro-
lysis temperatures due to lower losses of volatile components
(Antal and Gronli, 2003; Novak et al., 2009). The pH of the biochars
ranged from 5.2 to 9.1 (Table 2). Most of the biochars were alkaline,
which is common for thermally produced biochars (Lehmann and
Joseph, 2009). While two biochars had considerable N
(BP600 and BB600, 234.7 and 470.4 m
, respectively), the sur-
face areas of most biochars were relatively very small ranging from
0.70 to 81.1 m
(Table 2). Positive correlation between N
sured surface area and pyrolytic temperature was found for all
tested biochars, which is consistent with the results of several pre-
vious biochar studies (Brown et al., 2006; Li et al., 2011; Mukherjee
et al., 2011).
Elemental composition analysis indicated all the biochar sam-
ples to be carbon-rich with carbon compositions ranging 56.4–
86.4% (Table 2), which is typical of pyrolyzed biomass (Inyang
et al., 2011; Zimmerman et al., 2011). The oxygen and hydrogen
contents of all the samples ranged 10.0–36.7% and 1.4–5.6%,
respectively. As reported in the literature, some of these oxygen
and hydrogen contents are likely in organic functional groups on
biochar surface (Inyang et al., 2011; Uchimiya et al., 2011). The
biochar samples contained relatively small amount of nitrogen
(0.1–1.6%) and relatively low levels of phosphorous (0.03–0.5%)
and metal elements (Table 2).
3.2. Adsorption of nitrate, ammonium and phosphate by biochars
The four biochars made at a higher temperature (600 °C),
BG600, BB600 PH600, and BP600 could remove nitrate from aque-
ous solution with removal rates of 3.7%, 2.5%, 0.2%, and 0.12%,
respectively (Fig. 1a). The rest of the biochars (nine) showed no ni-
trate removal ability, and even released nitrate into the solution.
Thus, increase in pyrolysis temperature may improve the sorption
ability of biochars to aqueous nitrate. Mizuta et al. (2004) reported
that bamboo biochar made at 900 °C had relatively higher nitrate
adsorption capacity even compared to a commercial activated car-
bon, which is consistent with the ﬁndings of this study.
Nine of the thirteen tested biochars showed some ammonium
sorption ability, with removal rate ranged 1.8–15.7% (Fig. 1b).
The BP biochars had the best overall ammonium sorption perfor-
mance with removal rates of 3.8%, 15.7% and 11.9% for BP300,
BP450 and BP600, respectively. There was no apparent pyrolysis
temperature trend in the ammonium sorption data.
Only ﬁve biochars had ability to remove phosphate from aque-
ous solution, with the rest of the biochars releasing phosphate into
the solution (Fig. 1c). The BG450 biochar had the highest removal
rate of 3.1%. The HTPH, BG300, PH600, and the three bamboo
biochars released relatively large amount of phosphate into the
solution (>2%). The hydrothermally produced biochar, HTPH,
showed no nutrient sorption ability and released the greatest
amount of nitrate and phosphate.
It is well-accepted that biochar can be used as a soil amend-
ment to improve soil fertility and crop productivity. Some previous
studies attributed this function to the ability of biochar to retain
nutrients in soils (Steiner et al., 2008, 2009; Beesley et al., 2011;
Lehmann et al., 2011). The sorption experimental results in this
work, however, showed that the ability of biochar to adsorb nutri-
ent elements is not universal, but depends on both the nutrient and
the biochar type. In fact, most of the biochars tested in this work
showed little/no sorption ability to phosphate or nitrate, but per-
formed slightly better in removing ammonium from aqueous
solutions. Perhaps it not surprising that biochars are more effective
at removing cationic species from solution given that most bioch-
ars have been reported to have a net negative surface charge (Bees-
ley et al., 2011; Lehmann et al., 2011).
3.3. Transport in soil columns
Two biochars (PH600 and BP600) with relatively good sorption
ability for nutrients were selected for the soil column leaching
study. When applied to the sandy soil, the two biochars reduced
the leaching of both nitrate and ammonium ions from the column
(Fig. 2a and b). Compared to the columns without biochar, after
6 d, the PH600 and BP600 amended soil columns released about
34.3% and 34.0% less of total nitrate and 14.4% and 34.7% less
ammonium, respectively. These results are in line with ﬁndings
of the batch sorption experiment that both biochars could remove
nitrate and ammonium from aqueous solutions (Fig. 1).
Fig. 2. Cumulative amounts of nitrate (a), ammonium (b), and phosphate (c) in the
leachates from biochar-amended and unamended soil columns (see text for
1470 Y. Yao et al. / Chemosphere 89 (2012) 1467–1471
The two biochar’s effect on the leaching of phosphate from the
soils columns was different (Fig. 2c). BP600 reduced the total
amount of phosphate in the leachates by about 20.6%, whereas
PH600 increased the amount of phosphate leached from the soil
columns by about 39.1%. These results are also consistent with
the results of the batch sorption experiment (Fig. 1). Although mul-
tiple mechanisms could be responsible to the enhanced or reduced
retention of nutrients in the biochar amended soil (Sposito, 1989),
several recent studies have suggested that, when applied to soils,
biochar may not only affect soil ion exchange capacity but also pro-
vide refugia for soil microbes to inﬂuence the binding of nutritive
cations and anions (Liang et al., 2006; Atkinson et al., 2010). Fur-
ther investigations are still needed to unveil the governing mech-
anisms of nutrient retention and leaching in biochar amended
Biochar land application is commonly assumed to be an effec-
tive way to sequester carbon and improve soil fertility by reducing
nutrient leaching. The ﬁnding from this work, however, suggests
that the effect of biochar on the retention and release of nutrient
ions (i.e., nitrate, ammonium, and phosphate) varies with nutrient
and biochar type. Of the thirteen biochars tested in this study, most
of them showed little or no nitrate or phosphate sorption ability.
However, nine biochars removed aqueous ammonium. When two
selected biochars with relatively good sorption ability were used
in soil columns, they could effectively reduce the leaching of ni-
trate and ammonium. Only one biochar, however, could reduce
the leaching of phosphate from the soil columns. The results ob-
tained from the leaching column study were consistent with ﬁnd-
ing from the sorption experiments, suggesting the effect of biochar
on nutrients in soils could be determined through laboratory batch
sorption studies. It is also recommended that sorption ability of
biochars to nutrients should be determined before their applica-
tions to soils as amendment.
This research was partially supported by the USDA through
Grant 58-3148-1-179 and T-STAR-2009-34135-20192 and the
NSF through Grant CBET-1054405. The authors also thank the
anonymous reviewers for their invaluable insight and helpful
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