European Journal of Soil Science, March 2015, 66, 329–338
doi: 10.1111/ejss.12225
Effects of biochar application on greenhouse gas emissions,
carbon sequestration and crop growth in coastal saline soil
X . W . L i n a,b, Z . B . X i e a, J . Y . Z h e n g b, Q . L i u a, Q . C . B e i a & J . G . Z h u a
a
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Road,
Nanjing 210008, China, and b State Key Laboratory of Soil Erosion and Dryland Farming on the Loess, Institute of Soil and Water
Conservation, Chinese Academy of Sciences, 26 Xinong Road, Yangling, 712100, China
Summary
To evaluate the benefits of application of biochar to coastal saline soil for climate change mitigation, the effects
on soil organic carbon (SOC), greenhouse gases (GHGs) and crop yields were investigated. Biochar was applied
at 16 t ha−1 to study its effects on crop growth (Experiment I). The effects of biochar (0, 3.2, 16 and 32 t ha−1 )
and corn stalk (7.8 t ha−1 ) on SOC and GHGs were studied using 13 C stable isotope technology and a static
chamber method, respectively (Experiment II). Biochar increased grain mass per plant of the wheat by 27.7%
and increased SOC without influencing non-biochar SOC. On average, 92.3% of the biochar carbon and 16.8%
of corn-stalk carbon were sequestered into the soil within 1 year. The cumulative emissions of CO2 , CH4 and
N2 O were not affected significantly by biochar but cornstalk application increased N2 O emissions by 17.5%. The
global warming mitigation potential of the biochar treatments (−3.84 to −3.17 t CO2 -eq. ha−1 t−1 C) was greater
than that of the corn stalk treatment (−0.11 t CO2 -eq ha−1 t−1 C). These results suggest that biochar application
improves saline soil productivity and soil carbon sequestration without increasing GHG emissions.
Introduction
Soil carbon (C) sequestration through agricultural landmanagement changes is a cost-effective and environmentally
friendly strategy for mitigating global warming (Lal, 2004). The
incorporation of organic materials into the soil could increase
farmland soil organic C (SOC) (Xie et al., 2007). However, organic
matter amendment could stimulate methane (CH4 ) emissions in
paddy soils and nitrous oxide (N2 O) emissions in some upland
soils (Xie et al., 2010; Cheng et al., 2012). Methane and N2 O have
25 and 298 times greater global warming potential (GWP) than
carbon dioxide (CO2 ) (Forster et al., 2007). Therefore, improving
soil C sequestration and climate change mitigation through residue
incorporation into soils should be limited in farmlands when it
stimulates GHG emissions (Powlson et al., 2011).
Recently, pyrolysis of crop residues in limited oxygen conditions
and storage of the pyrolyzed materials (biochar) in soils has
been suggested as a possible strategy for soil C sequestration
and climate mitigation because of the relative recalcitrance of
biochar to microbial decay as a result of its chemical composition
and strong aromaticity (Woolf et al., 2010; Lehmann et al., 2011).
Some studies have shown that the mean residence time of biochar
Correspondence: Z. B. Xie. E-mail: [email protected]
Received 6 November 2013; revised version accepted 18 November 2014
© 2015 British Society of Soil Science
(hundreds to thousands of years) in soils is greater than that of the
soil organic C (tens to hundreds of years), which mostly comes
from plant residue incorporation into soil (Hamer et al., 2004;
Novak et al., 2010). Therefore, applying biochar to soil will slow
the return of photosynthetically fixed C to the atmosphere (Woolf
et al., 2010; Lehmann et al., 2011). Moreover, biochar is thought to
enhance the climate mitigation potential by offsetting C emissions
through the resulting decrease in agricultural fertilizer demand
and associated energy expenditure, and improve the terrestrial C
sink through the enhancement of crop production with increased
soil fertility (Lehmann et al., 2003; Jones et al., 2011). These
positive effects have promoted application of biochar to soils as a
sustainable management technique for enhancing soil productivity
and long-term C storage (Knoblauch et al., 2011).
Applying biochar to agricultural soils changes soil properties and
microbial activity and populations that could modify the soil C
cycle and the non-CO2 GHG fluxes (Lehmann et al., 2003, 2011).
However, biochar may be produced from diverse sources with
different charring methods and thus its properties and effects on
soil processes can vary greatly (Saarnio et al., 2013). Studies on
the effects of biochar application on soil CO2 emissions and native
SOC are not always consistent, with CO2 emissions and native soil
organic C decreasing, increasing or remaining unchanged when
different types of biochar are applied to different soils (Novak et al.,
2010; Luo et al., 2011; Zimmerman et al., 2011; Troy et al., 2013).
329
330 X. W. Lin et al.
Some studies have shown that biochar application decreases soil
CH4 and N2 O emissions (Yanai et al., 2007; Spokas & Reicosky,
2009; Yu et al., 2013), whereas other studies have shown that
biochar application does not change or even increases soil CH4 and
N2 O emissions (Clough et al., 2010; Bruun et al., 2011; Knoblauch
et al., 2011; Troy et al., 2013). These differences illustrate the
uncertainty regarding the effects of biochar application on the soil C
cycle and GHGs when different biochar materials are incorporated
into different soils. Therefore, the effects of biochar application on
soil C sequestration and GHG emissions should first be investigated
to determine its potential for climate change mitigation with a
full knowledge of its chemical and physical properties before it is
applied to large areas of farmland.
Saline soils are widespread throughout the world and occupy
about 400 million ha (FAO, 2008). For example, the total area of
saline soil is 7 million ha in Europe (mainly in the Mediterranean
countries) and 195 million ha in Asia, the Pacific and Australia
(FAO, 2008). However, saline soils have smaller SOC concentrations and crop yields than other agricultural soils. For example,
the SOC concentration in surface coastal saline soil is 6 g kg−1 ,
which is only 42% of that of other agricultural soils (Wang et al.,
2004). The poor productivity of saline soil because of its large salt
content and poor structure and porosity limits further C accumulation. Biochar has large potassium and organic C contents and good
porosity. Therefore, applying biochar would hypothetically improve
the productivity of saline soil and increase its soil C sequestration
and global warming mitigation potential (GWMP). However, there
are some uncertainties regarding whether biochar application could
improve crop growth and increase GWMP in saline soils when compared with the traditional incorporation of residue into soils. In this
study, we developed a field microcosm experiment applying biochar
generated from corn stalks and from unmodified corn-stalk residues
to saline soil to determine subsequent soil C sequestration and GHG
emissions and to compare effects on GWMP. This study specifically
aims to (i) investigate the effects of biochar application on crop
(soya bean and wheat) growth in a coastal saline soil, (ii) assess
the effects of biochar and feedstock application on soil C respiration, soil C sequestration and GHG emissions and (iii) evaluate the
GWMP of both materials after biochar or corn stalk application to
a coastal saline soil.
Materials and methods
Study site
The experiment was conducted on coastal saline land, located in the
east of Yancheng City, Jiangsu Province, China (32∘ 59′ 37.04′′ N,
120∘ 47′ 54.14′′ E; 1 m a.s.l.). The local climate is northern
sub-tropical, with an annual mean air temperature of 14.1∘ C,
an annual mean precipitation of 1040 mm and approximately 213
frost-free days. The soil is a loamy sand and is classified as a
Solonchak according to the USDA and FAO soil classification
system (FAO, 1998; Soil Survey Staff, 2006). The average soil
salt concentration is 1.5‰ and the average soil bulk density is
Table 1 Characteristics of the saline soil and the biochar used in this study
Material TOC / g kg−1 TN / g kg−1 TP / g kg−1 TK / g kg−1 C:N pH
Soil
6.3
Biochar 685.0
0.4
13.4
0.58
1.5
19.6
13.4
16.0 9.0
51.1 9.6
Soil and biochar pH was measured in a 1:2.5 (soil:water) and 1:20
(biochar:water) mass ratio, respectively. Soil samples were collected from
the 0–20-cm depth. TOC, TN, TP and TK refer to total organic C, total
nitrogen, total phosphorus and total potassium concentration, respectively.
1.15 g cm−3 at 0–20-cm depth. Other soil chemical properties at
0–20-cm depth are shown in Table 1.
Preparation of biochar
The biochar was produced from corn (maize) stalks. The stalks
were air-dried (10% moisture content), cut into 1-cm pieces,
and pyrolyzed in a carbonization furnace (Model ZBX1, Nanjing
Nianda Furnace Science and Technology Co. Ltd, Nanjing, China).
The furnace was heated to 400∘ C and maintained at this temperature
for 4 hours. During pyrolysis, the furnace was filled with N2 at a rate
of 0.5 l minute−1 to limit the O2 content. The biochar was cooled
overnight after pyrolysis. The biochar was later ground and passed
through a 1-mm sieve. The chemical properties of the biochar are
shown in Table 1.
Microcosm experiments
Two microcosm experiments were conducted from June 2010 to
June 2011; one focused on crop growth (Experiment I) and the other
focused on GHG exchange and SOC dynamics (Experiment II). The
microcosms were made from 5 mm-thick high-density polyvinyl
chloride (PVC) boards with inner dimensions of 23 × 18 × 20 cm3
(length × width × height; without bases). Soil samples from the
0–20-cm depth were separated from plant debris and roots, and
were sieved (< 2 mm) and air-dried. Biochar or corn stalks (cut
into 1 cm-long pieces) were mixed well with 9.0 kg air-dried
soil samples in each microcosm (packing density: 1.15 g cm−3 ).
The microcosms were embedded at 0–20-cm depth in the soil
and approximately 50 cm apart. All microcosms were amended
with N-P-K (15-15-15) compound fertilizer to give 75.0 kg ha−1 N,
32.8 kg ha−1 P and 62.5 kg ha−1 K during crop production at the seed
filling stage of soya bean (on 25th August) and at the growth stage
of wheat (on 30th March). Weeds in the microcosms were removed
by hand during the investigation.
Experiment I. In this experiment, the effect of biochar on crop
yields was determined. We developed six microcosms assigned
to two treatments, 16 t ha−1 biochar + crops (BC16) and no
biochar + crops (Control), each with three replicates and in a
completely randomized design. The experiment followed the local
soya bean–wheat rotation pattern. Soya bean (Glycine max L.,
cultivar Huaidou No. 4) was sown in June 2010 and wheat (Triticum
aestivum L., cultivar Zhengmai No.9023) was sown in November
© 2015 British Society of Soil Science, European Journal of Soil Science, 66, 329–338
Biochar’s effects on GHGs, soil C cycle and crop growth 331
2010 after the harvest of the soya bean. Three soya bean plants in
the soya bean season and six wheat plants in the wheat season were
grown in each microcosm. Soya bean and wheat were harvested in
October 2010 and June 2011, respectively. The biomass and grain
mass per plant were recorded at harvest. Above-ground biomass
was separated into seeds and stalks. Dry mass was determined
after drying at 65∘ C for 72 hours. The yield factors of soya bean
(number of pods and seeds per plant, and hundred-seed mass) and
wheat (number of tillers and seeds per plant, and thousand-grain
mass) were also determined.
Experiment II. To determine the effects of biochar and corn stalk
application on the SOC and GHG fluxes, 15 microcosms (without
plants) were assigned to five treatments: control without biochar or
corn stalk (Control); 3.2 t ha−1 biochar (BC3.2); 16 t ha−1 biochar
(BC16); 32 t ha−1 biochar (BC32); and 7.8 t ha−1 corn stalk (CS).
Approximately 3.2 t of biochar was pyrolyzed from 7.8 t corn stalk
(the average annual corn stalk biomass per hectare at the study
site). This experiment was carried out with three replicates in a
completely randomized design.
Measurement of greenhouse gas fluxes
Fluxes of CO2 , CH4 and N2 O in the 15 microcosms (Experiment
II) were measured with opaque static chambers (Conen & Smith,
1998). The static PVC chambers with internal dimensions of
23 × 18 × 50 cm3 (length × width × height) were placed on a water
trough on top of the microcosm during air sampling. A fan (10 cm
in diameter) was installed on the top wall of each chamber to ensure
mixing of gas samples when the chamber was closed. Thermal
insulation materials and aluminum film were added to the exterior
of the PVC cover to reduce the effects of direct radiative heating
during sampling.
Generally, GHG samples were measured every 7–14 days during
spring, summer and autumn and once a month in winter. From 09.00
to 11.00 local time, a 20-ml gas sample was collected with plastic
syringes at 0, 15 and 30 minutes after chamber closure. The samples
were transferred to evacuated 15-ml vials (Nichiden–Rika Glass
Co. Ltd, Kobe, Japan). Carbon dioxide and methane concentrations
were measured with a gas chromatograph (CP 3380, Varian,
Palo Alto, CA, USA) with an electron capture detector. Flux
rates were calculated from the linear increase in CO2 , CH4 and
N2 O concentrations in the chamber headspace over 30 minutes.
Cumulative emissions of GHGs were estimated as the summation
of daily GHG fluxes obtained through linear interpolation between
sampling dates throughout the whole experimental period.
Soil environmental variables and chemical analysis
Soil temperature at 5-cm depth at the experimental site was measured with digital thermometers when gas samples were collected
in the field. The volumetric soil moisture (%) was measured in the
microcosms by time domain reflectometry (TDR, Mpkit-B, Beijing
Channel Scientific Instruments Co. Ltd, Beijing, China).
Soils were sampled (0–20 cm) in microcosms with an auger at
the end of the GHG investigation. Soil samples, with the plant
debris and roots removed by hand, were sieved (< 2 mm) and
air-dried. The SOC content was determined by the Walkley–Black
procedure (Walkley & Black, 1934), which consists of dichromate
and sulphuric acid digestion and ferrous sulphate titration. Soil
pH was determined in water at a soil:water ratio of 1:2.5 using
a combination electrode (Rayment & Lyons, 2011). Soil salinity
was determined by measuring the electrical conductivity of the
extracts (1:5 soil:water mass ratio) with a conductivity meter.
Exchangeable cations (K+ , Na+ , Ca2+ and Mg2+ ) in the soil samples
were determined after extracting 20 g in 100 ml ammonium acetate
(1 m) and then analysing the extracts by inductively coupled plasma
atomic emission spectroscopy (ICP–AES, PerkinElmer Optima
8000, Perkin Elmer, Waltham, USA).
Estimation of the global warming mitigation potential
The saline soil in our study site had a 𝛿 13 C of −22.72‰, because it
had been planted with C3 plants for 9 years (hereafter referred to as
C3 soil). Maize (corn) is a C4 plant with a 𝛿 13 C of −12.07‰; corn
biochar has a 𝛿 13 C of −13.08‰. Therefore, we estimated the SOC
contributed by the C4 materials (corn biochar and corn stalk) to the
soil samples by using the stable isotope technique. Carbonate in soil
samples was removed with 10% HCl, samples were washed with
deionized water and dried overnight (80∘ C) and the 𝛿 13 C values
were determined by isotope ratio mass spectrometry (MAT 251,
Finnigan, Bremen, Germany).
The SOC fractions from the biochar or the corn stalks were
calculated as follows:
( 13
)
𝛿 CA+B − 𝛿 13 CA
F= (
(1)
) ,
𝛿 13 CB − 𝛿 13 CA
where F is the proportion of C from C4 , and 𝛿 13 CA , 𝛿 13 CB and
𝛿 13 CA+B are the 𝛿 13 C values of the corresponding control soil, the
biochar/corn stalk material and the soil samples in the biochar/corn
stalk microcosms, respectively.
The amount of C sequestration derived from C4 sources
(biochar/corn stalk) in soil samples (SB ) was as follows:
SB = SA+B × F,
(2)
where SA+B is the amount of C in the microcosms and F is the
proportion of C from C4 sources.
The global warming potential (GWP) of non-CO2 greenhouse
gases in the different treatments was calculated by using factors
of 25 for CH4 and 298 for N2 O (298) (Forster et al., 2007). The
global warming mitigation potential (GWMP) in the biochar/corn
stalk treatments was calculated as follows:
(
)
GWP- SOCBeq -SOCCeq
GWMP =
,
(3)
OC
where GWP is the GWP of non-CO2 greenhouse gases in the
biochar/corn stalk treatments, SOCBeq is the SOC (CO2 equivalents)
© 2015 British Society of Soil Science, European Journal of Soil Science, 66, 329–338
332 X. W. Lin et al.
in the biochar/corn stalk treatments, SOCCeq is the SOC (CO2
equivalents: CO2 –eq.) in the control, and OC is the organic C input
in the biochar/corn stalk treatments.
Table 2 Above-ground biomass and yield compositions of wheat and soya
bean
Crop
Biomass and yield compositions Treatment Mean
P value
Soya
Biomass per plant / g
0.089
Data analysis
General linear model–repeated measures define factors (SPSS 13.0,
SPSS Inc., Chicago, IL, USA) were used to assess the significance of the effects of treatment, sampling day and their interactions on GHG emissions, wherein sampling day was treated
as a within-subject variable and treatment was treated as a
between-subject variable. For each measurement of GHG fluxes
and the cumulative GHG emission, the significant difference among
treatments was assessed using one-way anova and least significance difference (LSD). Pearson correlation coefficients between
GHG fluxes and soil temperature and soil moisture were calculated. Differences in biomass and grain mass per plant between
the biochar treatments and the control were compared with a
t-test. All significant differences noted in the text are at the
0.05 level.
Grain mass per plant / g
Number of pods per plant
Number of seeds per plant
Hundred seed mass / g
Wheat Biomass per plant / g
Grain mass per plant / g
Number of tillers per plant
Number of seeds per plant
Thousand grain mass / g
Results
Crop biomass and grain mass
Biochar application (16 t ha−1 biochar) increased the above-ground
biomass and grain mass per plant of soya bean by 11% (P = 0.089)
and 24% (P = 0.057), respectively, when compared with the control
(no biochar + crops) (Table 2). Biochar application increased the
wheat above-ground biomass by 32% (P = 0.043) and its grain mass
per plant by 28% (P = 0.040) when compared with the control
treatment. The increase in grain mass per plant induced by biochar
application resulted mainly from increased pod and grain numbers
per plant for soya bean and thousand-grain mass for wheat.
Soil organic carbon
Biochar application at 16 and 32 t ha−1 significantly increased the
SOC concentration (Table 3). Biochar application was linearly
correlated with the SOC (r = 0.981, P < 0.001), increasing the SOC
concentration by 31–298% compared with the control (P = 0.060,
P < 0.001 and P < 0.001 for BC3.2, BC16 and BC32, respectively).
Incorporating a large amount of corn stalk (7.8 t ha−1 ) into the soil
did not increase the SOC significantly after 1 year. Biochar and corn
stalk application increased the 𝛿 13 C value of the SOC significantly.
The biochar contributed 18.5% of the SOC in the BC3.2 treatment,
53.9% of the SOC in the BC16 treatment and 72.3% of the SOC
in the BC32 treatment, whereas the corn stalks contributed 7.3% of
the SOC in the CS treatment. Most of the biochar C (82.3–99.4% in
the biochar treatments) was sequestered in the saline soil, but only
16.8% of the corn stalk C was sequestered as SOC. Furthermore,
1 year of biochar and corn stalk application did not significantly
change the C3 -SOC concentrations.
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
15.7 ± 0.4
14.1 ± 0.6
6.4 ± 0.4
5.2 ± 0.2
24.5 ± 2.5
17.5 ± 1.8
33.7 ± 1.7
27.0 ± 1.5
19.0 ± 0.1
19.1 ± 0.1
6.3 ± 0.5
4.7 ± 0.2
2.9 ± 0.2
2.2 ± 0.1
2.5 ± 0.3
2.1 ± 0.1
58.5 ± 4.7
49.2 ± 2.2
49.0 ± 1.3
45.5 ± 0.3
0.057
0.085
0.042
0.888
0.043
0.040
0.277
0.151
0.061
BC16, 16 t ha−1 biochar application; Control, no biochar application; P
value, the significant levels of the t-test.
Carbon dioxide
The soil CO2 emissions varied significantly with the season, with
the mean daily CO2 emission rate varying from 52 mg m−2 hour−1
at the end of January to 781 mg m−2 hour−1 in the middle of August
(Figure 1). Soil CO2 emission rates were affected significantly
by the interaction between the treatment and the sampling day
(Table 4). For example, the BC3.2, BC32 and CS treatments
increased the soil CO2 emissions on two to eight occasions during
the whole investigation period. However, the BC3.2, BC16 and
BC32 treatments decreased the soil CO2 emissions significantly
on one or two occasions. The cumulative CO2 emissions were
greatest in the CS treatment during soya bean (36% more than the
control) and wheat (14.1% more than the control) production and
CO2 emissions in CS exceeded that of the control by 6.7 t CO2 ha−1
over two production cycles (Figure 2; Table 5). Biochar application
had no significant impact on the cumulative CO2 emissions during
soya bean growth, wheat growth and the whole investigation period.
Methane and nitrous oxide
The coastal saline soil was a weak source of CH4 in all treatments, with the daily mean CH4 flux varying from −35 to
350 μg m−2 hour−1 (Figure 3). The biochar and corn stalk applications did not influence the CH4 fluxes significantly during most of
the sampling days. Corn stalk application increased the cumulative
CH4 emissions by 30.0% more than the control during the whole
investigation period, but the increase was not significant (Figure 4).
© 2015 British Society of Soil Science, European Journal of Soil Science, 66, 329–338
Biochar’s effects on GHGs, soil C cycle and crop growth 333
Table 3 Total soil organic carbon (SOC), the amount of biochar C and C3 SOC in the biochar or corn stalk treatments and summary of the analyses of variance
Treatment
BC3.2
BC16
BC32
CS
Control
anova
Degrees of freedom
Sum of squares
Mean squares
F value
P value
Total SOC / g kg−1
𝛿 13 C value / ‰
C4 in SOC / %
C3 in SOC / g kg−1
C4 sequestration rate / %
4.5 ± 0.1
8.9 ± 0.3
13.8 ± 0.7
3.8 ± 0.3
3.5 ± 0.1
−20.93 ± 0.23
−17.52 ± 0.08
−15.75 ± 0.20
−21.94 ± 0.15
−22.72 ± 0.21
18.5 ± 2.5
53.9 ± 0.8
72.3 ± 2.2
7.3 ± 1.4
–
3.7 ± 0.2
4.1 ± 0.2
3.8 ± 0.1
3.5 ± 0.3
3.5 ± 0.1
82.3 ± 9.2
95.2 ± 2.0
99.4 ± 7.7
16.8 ± 2.1
–
4
237.53
59.38
157.85
< 0.001
4
107.92
26.98
214.25
< 0.001
3
8253.99
2751.33
274.90
< 0.001
4
0.81
0.20
1.20
0.369
3
13329.12
4443.04
39.27
< 0.001
BC3.2, BC16, BC32, CS and Control refer to 3.2, 16, 32 t ha−1 biochar application, 7.8 t ha−1 corn stalk application and the control, respectively.
Figure 1 Effects of biochar and corn stalk application on soil CO2 fluxes.
BC3.2, 3.2 t ha−1 biochar; BC16, 16 t ha−1 biochar; BC32, 32 t ha−1 biochar;
CS, 7.8 t ha−1 corn stalk; Control, control treatment (microcosms without
crop). Each bar represents LSD (least significant difference) at 5%.
Figure 2 Effects of biochar and corn stalk application on the cumulative
soil CO2 emissions. Vertical bars are standard errors.
Table 5 Summary of the analyses of variance of the cumulative soil CO2 ,
CH4 and N2 O emissions from one-way anova
Table 4 Summary of the analysis of variance of CO2 , CH4 and N2 O fluxes
from repeated-measure anova
Model
Degrees of Sum of
freedom
squares
CO2 Treatment
4
Date
34
Date × treatment 136
4
CH4 Treatment
Date
34
Date × treatment 136
4
N2 O Treatment
Date
34
Date × treatment 136
Mean
square
F
value
P
value
293848.6 73462.2 6.04
0.010
16690288.8 490890.8 66.46 < 0.001
2301414.8 16922.2 2.29 < 0.001
74711.0 18677.8 0.61
0.667
4676952.7 137557.4 9.59
0.000
1393137.8 10243.7 0.71
0.988
14658.2
3664.5 3.86
0.038
691989.7 20352.6 14.81 < 0.001
230956.0
1698.2 1.24
0.065
The cumulative CH4 emissions did not differ significantly between
the biochar treatments and the control during soya bean growth,
wheat growth and across the two growth seasons.
Large temporal variations in soil N2 O emissions were observed in
all treatments and the mean daily N2 O emission rates ranged from
15 to 200 μg m−2 hour−1 (Figure 5). Two N2 O emission pulses were
Period
CO2
In soya bean season
In wheat season
Across two seasons
CH4 In soya bean season
In wheat season
Across two seasons
N2 O In soya bean season
In wheat season
Across two seasons
Degrees of Sum of Mean F
P
freedom
squares square value value
4
4
4
4
4
4
4
4
4
75.63
5.95
109.63
3.71
29.05
27.72
1.33
0.94
3.78
18.91
1.49
27.41
0.93
7.26
6.93
0.33
0.23
0.95
6.37
5.35
7.55
0.30
1.37
0.59
2.14
1.97
4.45
0.008
0.014
0.005
0.869
0.312
0.675
0.150
0.176
0.025
observed for all treatments after fertilizer application. The effects
of biochar and corn stalk applications on N2 O emissions varied
with the sampling day. For example, CS significantly increased the
N2 O emissions more than the control or the biochar treatments on
five occasions. The BC16 and BC32 treatments had significantly
larger N2 O emissions than the Control or the BC3.2 treatments
on one occasion, whereas the BC16 and BC32 treatments had
significantly smaller N2 O emissions than the Control or the BC3.2
treatments on one to two occasions. Biochar application did not
© 2015 British Society of Soil Science, European Journal of Soil Science, 66, 329–338
334 X. W. Lin et al.
Figure 6 Effects of biochar and corn stalk application on the cumulative
soil N2 O emissions. Vertical bars are standard errors.
Figure 3 Effects of biochar and corn stalk application on soil CH4 fluxes.
Each bar represents LSD (least significant difference) at 5%.
Table 6 Pearson correlation (r) coefficients between greenhouse gas
(GHG) fluxes and soil temperature and moisture
Treatment
CO2
CH4
Figure 4 Effects of biochar and corn stalk application on the cumulative
soil CH4 emissions. Vertical bars are standard errors.
N2 O
BC3.2
BC16
BC32
CS
Control
BC3.2
BC16
BC32
CS
Control
BC3.2
BC16
BC32
CS
Control
Soil moisture
Soil temperature
r
r
−0.260
−0.175
−0.095
−0.097
−0.031
−0.306
−0.280
−0.253
−0.273
−0.312
0.067
0.174
0.120
0.177
0.033
P value
0.009
0.083
0.349
0.422
0.760
0.002
< 0.001
0.020
0.006
0.002
0.508
0.085
0.235
0.080
0.748
0.664
0.659
0.688
0.658
0.673
0.128
0.102
0.112
−0.012
0.130
0.220
0.244
0.192
0.147
0.253
P value
<
<
<
<
<
0.001
0.001
0.001
0.001
0.001
0.194
0.300
0.083
0.900
0.186
0.024
0.012
0.050
0.134
0.009
P value, the significant levels of Pearson correlation coefficients.
Figure 5 Effects of biochar and corn stalk application on soil N2 O fluxes.
Each bar represents LSD (least significant difference) at 5%.
significantly affect the cumulative N2 O emissions (Figure 6). The
cumulative N2 O emissions were greatest under the CS treatment,
increasing the N2 O emissions by 20.9% more than the control
during soya bean growth and by 13.6% during wheat growth, but
the increases were not significant. The corn stalk treatment emitted
significantly more N2 O than the control (17.5%) throughout the
entire sampling period.
Relationships between GHG fluxes and soil temperature
and moisture
Soil CO2 fluxes were positively correlated with soil temperature
(r = 0.658–0.688, P < 0.05) for all treatments, while only the soil
CO2 fluxes in the BC3.2 treatment were weakly negatively correlated (r = −0.260, P < 0.05) with soil moisture (Table 6). A weak
negative correlation between CH4 fluxes and soil moisture was
observed for all treatments (r = −0.253 to −0.316, P < 0.05). There
was a weak correlation between N2 O fluxes and soil temperature
for most of the treatments (r = 0.220–0.253, P < 0.05), except for
the CS treatment (r = 0.147, P > 0.05).
Global warming mitigation potential of the treatments
No significant difference was observed for CO2 equivalents (GWP)
of non-CO2 greenhouse gases between the biochar treatments
and the control in each season and across both growth seasons
(Table 7). Corn stalk application significantly increased GWP by
19.2% more than the control. All biochar treatments had negative
global warming mitigation potentials (GWMPs) (−3.84 to −3.17 t
CO2 -eq. ha−1 t−1 C), with no significant differences between the
different biochar treatments. However, the GWMP of the corn stalk
treatment (−0.11 CO2 -eq ha−1 t−1 C) was significantly less than that
of the biochar treatments.
© 2015 British Society of Soil Science, European Journal of Soil Science, 66, 329–338
Biochar’s effects on GHGs, soil C cycle and crop growth 335
Table 7 Global warming potential (GWP) (t CO2 -eq ha−1 ) of non-CO2
greenhouse gases (GHGs) and per area annual global warming mitigation
potential (GWMP)a (t CO2 -eq. ha−1 t−1 C) and summary of the analyses of
variance
Treatment
BC3.2
BC16
BC32
CS
Control
anova
Degrees of freedom
Sum of squares
Mean squares
F value
P value
a The
GWP in the GWP in
soya bean the wheat
season
season
Total
GWP
GWMPa
0.83 ± 0.12
0.97 ± 0.02
0.82 ± 0.01
1.05 ± 0.08
0.89 ± 0.03
0.74 ± 0.03
0.72 ± 0.10
0.75 ± 0.05
1.00 ± 0.12
0.83 ± 0.08
1.57 ± 0.09
1.69 ± 0.11
1.58 ± 0.05
2.05 ± 0.12
1.72 ± 0.03
−3.17 ± 0.67
−3.84 ± 0.28
−3.74 ± 0.26
−0.11 ± 0.89
–
4
0.11
0.03
2.03
0.165
4
0.16
0.04
2.12
0.153
4
0.46
0.11
4.89
0.019
3
27.99
9.33
8.92
0.006
Table 8 Soil pH, soil salt concentration (‰) and soil exchangeable cations
(K+ , Na+ , Ca2+ and Mg2+ ) (mg kg−1 soil) in the biochar-treated (16 t ha−1 )
and control microcosms with crops (Experiment I) in June 2011
Properties
Treatment
Mean
P value
pH
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
Control
BC16
9.2 ± 0.09
9.2 ± 0.11
1.3 ± 0.04
1.3 ± 0.05
87 ± 5
121 ± 11
48 ± 5
49 ± 2
1113 ± 89
1137 ± 88
154 ± 10
169 ± 17
0.920
Salt concentration
K+
Na+
Ca2+
Mg2+
0.920
0.043
0.792
0.861
0.487
P value is the significant levels of the t-test.
calculation of GWMP is shown in the Materials and methods.
Discussion
Above-ground biomass
Biochar application affects crop growth depending on the type
of biochar, the crop and the type of soil (Van Zwieten et al.,
2010). In this study, applying biochar to the coastal saline soil
significantly increased wheat yield and increased the soya bean
yield (although not significantly, P = 0.057) during the first year.
Some studies have shown that biochar application improves the pH
of acidic soil and the exchangeable K+ , Ca2+ and Mg2+ (mostly
coming from biochar), which stimulates crop growth (Major et al.,
2010; Van Zwieten et al., 2010). Applying biochar (16 t ha−1 ) to
the saline soil did not affect the soil pH, the exchangeable Na+ ,
Ca2+ and Mg2+ concentrations or the salt concentration, but it
significantly increased the exchangeable K+ concentration by 44%
by the end of the study (Table 8). Increasing the soil exchangeable
K+ increases the K+ :Na+ ratio in plants, which improves their
salt tolerance and subsequently increases plant growth (Bohra &
Doerffling, 1993). Therefore, the larger wheat grain mass per plant
could result from more soil-available K+ introduced to soil by
biochar with large amounts of available K+ and enhancement of
crop K+ uptake (Lehmann et al., 2003). Biochar also improves
soil fertility by increasing cation exchange capacity, water-holding
capacity, nutrient retention through cation adsorption, fertilizer-use
efficiency and symbiotic microorganisms and earthworm habitats,
which would improve plant growth (Van Zwieten et al., 2010;
Knoblauch et al., 2011; Lehmann et al., 2011). Therefore, the other
mechanisms behind the increase in crop growth induced by biochar
application to the saline soil should be studied.
Soil organic C and respiration
Some studies have shown that only 2–21% of plant residue C enters
the SOC pool annually through humification (Lal, 2004; Xie et al.,
2010). In our study, approximately 17% of the corn stalk C was
captured by soil when the corn stalk was directly returned to the
saline soil in 1 year. However, when plant residues were transformed into biochar (41% C recovery rate) and applied to soils,
approximately 63% of the plant C was captured. Therefore, converting plant biomass into biochar could increase the C sequestration in
saline soil by about four-fold, as indicated by the investigation in
the first year. Several studies have suggested that biochar stimulates
the loss of the native SOC (Hamer et al., 2004; Luo et al., 2011).
In this study, biochar application did not significantly affect the
non-biochar SOC.
As found by Zimmerman et al. (2011), biochar application facilitated soil respiration during the first several months in our study.
The initial increase in CO2 emission caused by biochar application may be caused by both mineralization of labile-C in biochar
and stimulation of microbial activity and thereby enhanced mineralization of the soil organic C (Jones et al., 2011; Zimmerman et al.,
2011; Case et al., 2012; Troy et al., 2013). However, biochar application did not affect the non-biochar SOC in this study. Thus, we
suggest that most of the extra CO2 from biochar application during the soya bean season probably came from the rapid use of a
small labile component of biochar (Zimmerman et al., 2011). However, the proportion of labile-C in biochar is small (0.1–6%) and
always decomposed quickly (Jones et al., 2011; Knoblauch et al.,
2011; Troy et al., 2013). In our study, this increase in soil respiration only occurred during the first few months and ceased during
the later part of the investigation, while biochar application did not
increase the cumulative CO2 emissions across both seasons.
Non-CO2 greenhouse gases
Some studies have shown that biochar application decreases
methane emissions in paddy and tropical soils (Spokas et al.,
2009), whereas others have reported increased methane emissions
(Knoblauch et al., 2011; Yu et al., 2013). Such conflicting results
© 2015 British Society of Soil Science, European Journal of Soil Science, 66, 329–338
336 X. W. Lin et al.
may be explained by the variable responses of methanogenic activity and methanotrophic activity to biochar application (Spokas
et al., 2010). There are also changes in microbial and faunal populations (Lehmann et al., 2011), which are probably driven by different
changes in soil physicochemical characteristics and the microbiological circumstances when different physicochemical biochars are
applied to different soils (Van Zwieten et al., 2009; Spokas et al.,
2010). For example, the increase in CH4 emissions induced by
biochar application could be explained by the increased activities
of methanogens because biochar increased soil pH and by emission of the CH4 from biochar pore spaces formed during production
(Spokas & Reicosky, 2009; Yu et al., 2013). However, the improvement of soil aeration and porosity caused by biochar amendment
may also increase methanotrophic activity and thereby decrease
CH4 emission (Troy et al., 2013). Our results showed that the CH4
emissions were weakly and negatively correlated with soil moisture, similar to results reported by Cheng et al. (2010). However,
biochar application did not significantly affect soil moisture (data
not shown), which is in agreement with the field study of Hardie
et al. (2014). Therefore, results from our experiment do not provide evidence that soil moisture has a role in biochar’s impact on
soil CH4 emissions. In this study, biochar addition could increase
anaerobic conditions without having an impact on soil pH (Table 8),
which would favour CH4 oxidation. However, biochar application
to the saline soil did not affect CH4 emissions, which is similar to
the results from other agricultural and forest nursery soils (Spokas
& Reicosky, 2009; Troy et al., 2013).
In this study, corn stalk application increased soil N2 O emissions,
which may be associated with enhanced denitrification rates (Cheng
et al., 2012). Some studies show that the application of biochar
derived from high-N manure or biochar/high-N animal excreta combinations increases soil N2 O emissions (Spokas & Reicosky, 2009;
Clough et al., 2010; Bruun et al., 2011). The stimulation mechanism
of biochar could be attributed to disturbance of nitrification, increasing N availability, improvement in the aeration of soil and stimulation of N2 O-producing activity of microbes or nitrifiers (Clough
et al., 2010; Case et al., 2012; Saarnio et al., 2013). Other studies
show that biochar application reduces soil N2 O emissions (Clough
et al., 2010; Case et al., 2012). The mechanisms of N2 O reduction
in biochar-amended soils could be attributed to reduced N availability due to biochar’s adsorption of substrates such as ammonium
and nitrate (Bruun et al., 2011; Case et al., 2012), changes in microbial community structure (Bruun et al., 2011), a decrease in soil
redox potential (Case et al., 2012) or microbial inhibition by volatile
organic compounds contained in biochar (Spokas et al., 2010). At
present, the mechanisms of biochar’s effect on N2 O emissions are
not yet fully understood. The different physicochemical characteristics of biochar, soil types, biochar application rates and agricultural management measures could explain the different responses of
N2 O emissions observed in these studies. Although biochar application changed some soil physicochemical properties in this study, its
application did not affect the N2 O emission, which is in agreement
with other field studies (Saarnio et al., 2013). The largest biochar
application rate was approximately 1.5% by mass of soil in this
study, whereas other studies have shown that N2 O emissions are
restricted when biochar is applied at larger rates (> 4%) (Yanai
et al., 2007; Spokas et al., 2009). Therefore, whether or not biochar
application at greater rates could depress N2 O production in saline
soil and enhance the GWMP of biochar needs further study.
Global warming mitigation potential
Applying corn stalks to saline soil may increase the SOC with
increasing soil N2 O emissions. Therefore, the GWMP of corn stalk
application to saline soil is nearly zero because of the amount
of C sequestration that is offset by its contribution to the GWP
of non-CO2 GHG emissions. The GWMP of biochar application
to saline soil is greater than corn stalk application because of
the greater C sequestration, without increasing N2 O emissions.
The GWMP of biochar application to saline soil in this study
was approximately 28–34 times greater than the value of corn
stalk application. Charring plant residues and applying biochar
to the soil instead of incorporating untreated harvest residues
in saline soil may be a powerful strategy for mitigating global
warming. However, GHG emissions during biochar production
should be considered when calculating the GWMP of biochar
application in soils (Woolf et al., 2010). Biochar production would
lead indirectly to CO2 -equivalent emissions because of power
requirements (electricity from fossil fuel), although there would be
some offset of equivalent CO2 emissions by recovering pyrolytic
gases (H2 , CO, CH4 , C2 H6 and C2 H4 ) (Woolf et al., 2010). From
some previous studies, the net CO2 -equivalent emissions during
biochar production were estimated as 0.44 t CO2 - to 4.20 t CO2 -eq.
t−1 biochar (unpublished data) but depended on pyrolysis methods.
When net CO2 -equivalent emissions during biochar production
are minimized (slow pyrolysis with less energy input), the annual
GWMP including the biochar production and application to saline
soil was estimated as −2.90 t CO2 -eq ha−1 t−1 C, which was still
greater (26 times) than that of the corn stalk treatment. In a system
that uses electric heating with greater energy input, the annual
GWMP including the biochar production and application to saline
soil was estimated as 2.50 t CO2 -eq. ha−1 t−1 C, which would be
the net CO2 -equivalent emissions. Therefore, biochar application
would have the greater potential for mitigating global warming in
saline soils when using the efficient pyrolysis techniques (such as
continuous slow pyrolysis) with synthetic gas recovery and minimal
energy input during its production.
Conclusions
Agricultural saline topsoils inherently have small SOC contents
because of decades of little incorporation of biomass residue into
the soil. However, biochar application quickly increases the SOC
of saline soil, depending on the biochar application rate. Biochar
application potentially increases soil C respiration loss during the
early stage of treatment, but no significant effect was observed
thereafter. Overall, the early CO2 loss only represents a small
fraction of the biochar C, indicating that most of the biochar carbon
© 2015 British Society of Soil Science, European Journal of Soil Science, 66, 329–338
Biochar’s effects on GHGs, soil C cycle and crop growth 337
could be sequestered in saline soil. Biochar application to saline soil
had positive impacts on crop growth with increasing above-ground
C sequestration, which would lead to more C sequestration in
soil when the increased biomass is converted to biochar. Biochar
amendment in saline soil did not affect the CH4 and N2 O cumulative
emission significantly. The GWMP of the biochar treatments was
greater than that of the corn stalk treatment. Thus, amending soils
with biochar pyrolyzed from plant residues could be used as a
means to manage C sequestration and mitigate global warming
without increasing N2 O production in saline soils. However, the
long-term effects of biochar application in the saline soil should be
investigated.
Acknowledgements
This research was supported by the ‘Strategic Priority Research
Program-Climate Change: Carbon Budget and Relevant Issues’ of
the Chinese Academy of Sciences (XDA05050509), the National
Natural Science Foundation of China (41105100, 41171191), the
open foundation of the State Key Laboratory of Soil Erosion and
Dryland Farming on the Loess Plateau (10501-286) and the Blue
Moon Fund, USA.
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