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Citation: Sun, Y.; Chen, X.; Yang, J.;
Luo, Y.; Yao, R.; Wang, X.; Xie, W.;
Zhang, X. Biochar Effects Coastal
Saline Soil and Improves Crop Yields
in a Maize-Barley Rotation System in
the Tidal Flat Reclamation Zone,
China. Water 2022,14, 3204. https://
doi.org/10.3390/w14203204
Academic Editor: Jian Liu
Received: 17 September 2022
Accepted: 9 October 2022
Published: 12 October 2022
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water
Article
Biochar Effects Coastal Saline Soil and Improves Crop Yields in
a Maize-Barley Rotation System in the Tidal Flat Reclamation
Zone, China
Yunpeng Sun 1, Xiaobing Chen 2,*, Jingsong Yang 1, Yongming Luo 1, Rongjiang Yao 1, Xiangping Wang 1,
Wenping Xie 1and Xin Zhang 1,*
1State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of
Sciences, Nanjing 210008, China
2Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China
*Correspondence: xbchen@yic.ac.cn (X.C.); xzhang@issas.ac.cn (X.Z.)
Abstract:
The summer maize-winter barley (or wheat) rotation system is a conventional farming
method in coastal areas of east China. However, researchers have paid little attention to the increasing
soil degradation after successive crop rotation in coastal saline agriculture. In the current study, a two-
year field experiment was conducted to investigate the changes in soil physio-chemical properties
and crop grain yields under the maize-barley rotation system. Wheat straw derived biochar (BC)
was applied to topsoil (0~20 cm) at four different rates (0, 7.5, 15 and 30 Mg ha
−1
) before summer
maize cultivation, and no biochar was added in the cultivation of the winter barley. Bulk density
(BD), water holding capacity (WHC), water stable aggregate (WSA), soil electrical conductivity (EC),
pH (1:5 water w/v) and soil organic carbon (SOC), at the harvesting time of maize and barley, were
analyzed. The application of biochar increased WHC and macro-aggregate (>2 mm) content after
barley harvest. Soil EC was mainly affected by the rain during maize cultivation and increased only
slightly under BC treatments. However, no difference in EC was found among all treatments after
barley harvest. The application of BC at 30 Mg ha
−1
increased the maize yield by 66% but produced
no difference in the barley yield. We concluded that biochar could be an effective option to mitigate
soil degradation and improve crop productivity in coastal saline agriculture.
Keywords: crop rotation; crop yield; saline soil; soil salinity; biochar
1. Introduction
Land reclamation from the sea provides more lands to satisfy global demands for
resources and it has been practiced since ancient times [
1
]. Nevertheless, soil salinization,
saline intrusion, high sodium adsorption ratio, and a shortage of fresh water are serious
issues restricting the production potential of coastal saline soils [
2
]. Among the stresses on
plant agriculture in coastal reclamation regions, soil salinization is considered the major
limiting factor for crop productivity [
3
]. Salt stress inhibits the growth and development of
plants and even causes damage under plant conditions [
4
]. Many approaches have been
established to maintain and develop sustainable saline agriculture in coastal areas, such as
using saltwater drip irrigation [
5
], brackish ice for salt leaching [
6
], halophytes to absorb
and remove soil salts [
7
], microorganisms for mediating plant salt tolerance [
8
], transgenic
technologies to improve crop yield [9], crop rotation [10].
Crop rotation refers to the continuous cultivation of different crops on the same land,
and it is a simple and effective technique used in organic agriculture. This kind of agronomic
practice is beneficial to crop production, ameliorates the ecosystem of the cropland, and
promotes the combination of soil use and nourishment [
11
]. However, long-term crop
rotation by itself induces soil degradation (e.g., increased soil compaction, reduced water
holding capacity and soil salinization). Due to increased runoff from fertilizers increased
Water 2022,14, 3204. https://doi.org/10.3390/w14203204 https://www.mdpi.com/journal/water
Water 2022,14, 3204 2 of 13
and poor farming methods [
12
–
15
]. Therefore, it is still very important to improve the
soil quality in coastal reclamation regions after consecutive years of cultivation, in order
to develop sustainable saline agriculture. Meanwhile, more attention needs to be paid
to solve the of problems caused by soil degradation in order to ameliorate soil physical
characteristics, enhance soil fertility and mitigate soil salt levels in the plow layer.
Biochar (BC) is an organic carbon-rich porous material produced by biomass py-
rolysis reaction, which is used as a soil modifier to improve and soil productivity and
properties [16].
Biochar can alleviate the effects of salt stress on plants through salt
adsorption [
17
], enhance soil water holding capacity [
18
], and improve soil physical and
hydraulic
properties [19].
Compared with other modifiers, such as gypsum [
20
], humic
acid [
21
], and manure and green waste [
22
], the longevity of biochar in soil provides
obvious advantages for bioremediation [23].
In light of the above mentioned factors, we conducted a two-year field trial with the
addition of biochar to a conventional farming system: a long-term crop rotation process
conducted by native farmers in the coastal tidal flats reclamation region of Jiangsu Province,
China. We hypothesized that biochar application would ameliorate the soil characteristics,
and improve the crop yield and, therefore, promote the development of sustainable saline
agriculture. We aimed to determine the effects of biochar application on soil amelioration
and crop yield in a maize-barley rotation system. As far as we know, the potential of
biochar as a soil modifier to mitigate salt stress and improve soil quality has received little
attention in this field.
2. Materials and Methods
2.1. Experimental Site
The field experiment site was located in Huanghai Raw Seed Farm (32
◦
38
0
42.01
00
N,
120
◦
54
0
8.04
00
E), Jiangsu Province, China (Figure 1). The study area was reclaimed in 2005
lying approximately 2 km from the coastline of the China Yellow Sea. The main soil type
was classified as solonchak based on the World Reference Base for Soil Resources (IUSS
Working Group WRB, 2007). The soil texture was mainly silty loam, characterized by low
porosity and infiltration (Table 1). The area was governed by a subtropical oceanic monsoon
climate with large seasonal fluctuations in precipitation. Approximately 70 percent of the
local rainfall occured from June to September every year, and there was a regular annual
variation in the salt content of the soil. The average annual precipitation, evaporation,
temperature, and relative humidity were 1061 mm, 1414 mm, 14.6
◦
C, and 81%, respectively.
Water 2022, 14, x FOR PEER REVIEW 3 of 14
of the local rainfall occured from June to September every year, and there was a regular
annual variation in the salt content of the soil. The average annual precipitation, evapora-
tion, temperature, and relative humidity were 1061 mm, 1414 mm, 14.6 °C, and 81%, re-
spectively.
Figure 1. Location of study area.
Table 1. Basic properties of the original topsoil (0–20 cm) and biochar used for the experiment.
Sample
pHH2O
CEC
BD
SOC
TOC
Total N
Salt
Sand
Clay
Silt
cmol kg−1
g cm−3
g kg−1
g kg−1
g kg−1
g kg−1
Topsoil
9.2
2.4
1.41
2.4
0.3
2.3
191
124
685
Biochar
10.4
21.8
0.65
467
5.9
42.0
ND
ND
ND
Note(s): EC, cation exchange capacity; BD, bulk density; TOC, total organic carbon; ND, not de-
tected.
2.2. Biochar
The biochar used for the field experiment was derived from wheat straw by pyrolysis
at a temperature of 350–550 °C in a vertical kiln made of refractory bricks in Sanli New
Energy Company, Nanyang city, China. With the technology developed by the company,
was expected 35% of the wheat straw dry matter mass to be converted to biochar [24].
Biochar material was ground to pass through a 2 mm sieve and homogenized thoroughly
for the experiment. The basic properties of biochar are presented in Table 1, and more
detailed properties have been reported previously [25,26].
2.3. Experimental Design
A two-year field trial was conducted during the maize-barley rotation. A single factor
randomized complete block design was adopted, involving four treatments with three
replicate plots, for a total of 12 plots. Each treatment plot was 10 × 4 m in the area and
separated by a 1 m wide buffer zone with no crops sowed. The biochar was applied at
rates of 7.5, 15 and 30 Mg ha−1. The control plots were treated with no biochar. Before
summer maize sowing, urea and monoammonium phosphate were applied at 144 kg ha−1
N and 135 kg ha−1 P2O5, respectively. Fertilizers were broadcast and incorporated into the
soil, with no potassium fertilizer application because the soil K content was already suffi-
cient for plant growth [27]. An additional 96 kg ha−1 N of urea was top-dressed at the
Figure 1. Location of study area.
Water 2022,14, 3204 3 of 13
Table 1. Basic properties of the original topsoil (0–20 cm) and biochar used for the experiment.
Sample pHH2O CEC BD SOC TOC Total N Salt Sand Clay Silt
cmol kg−1g cm−3g kg−1g kg−1g kg−1g kg−1
Topsoil 9.2 2.4 1.41 2.4 0.3 2.3 191 124 685
Biochar 10.4 21.8 0.65 467 5.9 42.0 ND ND ND
Note(s): EC, cation exchange capacity; BD, bulk density; TOC, total organic carbon; ND, not detected.
2.2. Biochar
The biochar used for the field experiment was derived from wheat straw by pyrolysis
at a temperature of 350–550
◦
C in a vertical kiln made of refractory bricks in Sanli New
Energy Company, Nanyang city, China. With the technology developed by the company,
was expected 35% of the wheat straw dry matter mass to be converted to biochar [
24
].
Biochar material was ground to pass through a 2 mm sieve and homogenized thoroughly
for the experiment. The basic properties of biochar are presented in Table 1, and more
detailed properties have been reported previously [25,26].
2.3. Experimental Design
A two-year field trial was conducted during the maize-barley rotation. A single factor
randomized complete block design was adopted, involving four treatments with three
replicate plots, for a total of 12 plots. Each treatment plot was 10
×
4 m in the area and
separated by a 1 m wide buffer zone with no crops sowed. The biochar was applied at rates
of 7.5, 15 and 30 Mg ha−1. The control plots were treated with no biochar. Before summer
maize sowing, urea and monoammonium phosphate were applied at 144 kg ha
−1
N and
135 kg ha
−1
P
2
O
5
, respectively. Fertilizers were broadcast and incorporated into the soil,
with no potassium fertilizer application because the soil K content was already sufficient
for plant growth [
27
]. An additional 96 kg ha
−1
N of urea was top-dressed at the jointing
stage. For maize growth, the row spacing and spacing within the rows were 50 cm and
20 cm. The density was approximately six plants per m
−2
. Winter barley was sown by
machine after summer maize harvest. The row spacing was 25 cm and the seeding rate
was 375 kg ha
−1
. Chemical fertilizers containing 168 kg N ha
−1
and 90 kg P ha
−1
were
broadcast and incorporated into the soil. At the jointing-booting stage, 72 kg N ha
−1
urea
was additionally top-dressed as a supplementary fertilizer. During the crop cultivation,
rain was the major water supply, as no irrigation system was used.
Before sowing the seeds, biochar and base fertilizers were sprinkled onto the soil
surface and immediately mixed into the tilling layer (0–20 cm). Maize (Suyu 21) was seeded
on 12 June 2015, and harvested on 28 September 2015. Barley (Supi 4) was also seeded on
30 October 2015, with the continued application of fertilizer but no further addition of
biochar, and it was harvested on 22 April 2016. The crop growth management was consis-
tent across the plots.
2.4. Soil Sampling and Analysis
To measure soil salinity, a composite sample at a depth of 0–20 cm was obtained from
five subsamples collected using a soil auger from each treatment plot on June 10 (seeding
stage), July 2 (seedling stage), July 28 (elongation stage), and September 30 (mature stage) in
2015, and April 25 (harvest stage) in 2016. Each date represented, respectively, the seedling
stage, jointing stage, filling stage, and ripening stage of the maize, and the ripening stage
of the barley. Samples were air-dried and ground to pass through a 2 mm sieve, and a
portion was then ground to pass through a 0.15 mm sieve for soil fertility measurement.
At the ripening stage of the maize and barley, undisturbed soil cores (d= 5 cm, l= 5 cm;
v= 100 cm3)
were taken from topsoil to measure bulk density (BD) and soil water holding
capacity (WHC). For water stable aggregate content measurement, soils (0–20 cm) were
sampled on 25 April 2016.
Water 2022,14, 3204 4 of 13
Soil electrical conductivity (EC) and acidity (pH) were determined by a HANNA
EC215 conductivity meter and a HANNA pH 211 microprocessor pH meter using 1:5 (w/v)
soil: water suspensions. The electrical conductivity from 1:5 samples was converted to total
soil salt content using the equation proposed by [28]:
TS =2.47 EC1:5 +0.26 (1)
where, EC
1:5
is the electrical conductivity of the soil water extract (dS m
−1
) and TS is the
total salt content (g kg−1).
The soil organic carbon (SOC) was obtained by the Walkley-Black dichromate oxida-
tion method [
29
]. The sizes of water stable aggregates included four fractions: (1) large
macro-aggregates (>2 mm); (2) small macro-aggregates (0.25–2 mm); (3) micro-aggregates
(0.25–0.053 mm) and (4) silt + clay (<0.053 mm).
2.5. Statistical Analysis
Statistical analysis was performed using SPSS, version 19 (IBM SPSS Statistics,
New York, NY, USA, 2010). All analytical data are presented as a mean plus/minus
one standard deviation. Any significant differences among treatments were determined by
one-way analysis of variance (ANOVA). The post-hoc test was carried out using Dunnett’s
method at the level of significance of p< 0.05.
3. Results
3.1. Soil Physical Properties
3.1.1. Soil Bulk Density
There was no significant reduction in soil bulk density (BD) under biochar application
treatment compared to the control after the maize harvest in 2015 (Figure 2). Meanwhile,
BD increased by 3.3% and 3.7% under the 7.5 and 15 Mg ha
−1
biochar treatments compared
to the control. However, the 15 and 30 Mg ha
−1
biochar treatments resulted in significant
decreases in BD (5.9% and 6.2%, respectively) compared to the untreated plots after the
barley harvest in 2016. Furthermore, there were no difference for the 7.5 Mg ha
−1
biochar
treatment across the maize—barley rotation system, but a small reduction (2.4%) compared
to CK was found in 2016.
Water 2022, 14, x FOR PEER REVIEW 5 of 14
compared to the control. However, the 15 and 30 Mg ha−1 biochar treatments resulted in
significant decreases in BD (5.9% and 6.2%, respectively) compared to the untreated plots
after the barley harvest in 2016. Furthermore, there were no difference for the 7.5 Mg ha−1
biochar treatment across the maize—barley rotation system, but a small reduction ( 2.4%)
compared to CK was found in 2016.
Figure 2. Bulk density (BD) as affected by biochar (BC) treatment. Error bars respresent one standard
deviation from the mean. Different letters indicate significant difference (p < 0.05) between control
and treatments in different years.
3.1.2. Soil Water Holding Capacity
Increases were found in WC and FC in 30 Mg ha−1 BC treated plot compared to the
control plot, after maize harvest (Table 2). Compared to CK, the 30 Mg ha−1 BC treatment
increased the WC from 29.1% to 31.0% and the FC from 32.4% to 37.5% in the maize-
planting experiment in 2015. In the subsequent barley cultivation period, the WC was in-
creased compared to CK from 21.1% to 24.7% and FC from 23.7% to 26.7% in the 30 Mg
ha−1 BC treatment plot. Moreover, no differences were observed in the WC, SWC, and FC
under the 7.5 and 15 Mg ha−1 treatments across the maize–barley rotation compared to
CK.
Table 2. Parameters of soil water content (WC), saturated water capacity (SWC) and field capacity
(FC) measured in October 2015 and March 2016 by ring–cutting method after maize and barley har-
vest.
Maize
Barley
Biochar Rate
WC
SWC
FC
WC
SWC
FC
Mg ha−1
g 100 g−1
g 100 g−1
g 100 g−1
g 100 g−1
g 100 g−1
g 100 g−1
0
29.1±1.3 b
33.7±1.8 a
32.4±1.3 b
21.1±0.7 b
25.2±0.4 a
23.7±0.6 b
7.5
29.6±1.4 ab
35.5±1.1 a
33.7±0.5 b
22.4±2.4 ab
26.9±3.5 a
25.1±3.2 ab
15
29.3±1.1 ab
37.0±3.8 a
34.6±2.6 ab
23.2±0.5 ab
26.7±0.2 a
25.1±0.5 ab
30
31.0±1.3 a
37.5±5.9 a
36.4±2.1 a
24.7±0.7 a
29.1±1.5 a
26.7±0.2 a
Note(s): Different letters in the same column indicate significant differences (p < 0.05) between the
treatments.
Figure 2.
Bulk density (BD) as affected by biochar (BC) treatment. Error bars respresent one standard
deviation from the mean. Different letters indicate significant difference (p< 0.05) between control
and treatments in different years.
Water 2022,14, 3204 5 of 13
3.1.2. Soil Water Holding Capacity
Increases were found in WC and FC in 30 Mg ha
−1
BC treated plot compared to the
control plot, after maize harvest (Table 2). Compared to CK, the 30 Mg ha
−1
BC treatment
increased the WC from 29.1% to 31.0% and the FC from 32.4% to 37.5% in the maize-planting
experiment in 2015. In the subsequent barley cultivation period, the WC was increased
compared to CK from 21.1% to 24.7% and FC from 23.7% to 26.7% in the
30 Mg ha−1
BC
treatment plot. Moreover, no differences were observed in the WC, SWC, and FC under the
7.5 and 15 Mg ha−1treatments across the maize-barley rotation compared to CK.
Table 2.
Parameters of soil water content (WC), saturated water capacity (SWC) and field capacity (FC)
measured in October 2015 and March 2016 by ring–cutting method after maize and barley harvest.
Maize Barley
Biochar
Rate WC SWC FC WC SWC FC
Mg ha−1g 100 g−1g 100 g−1g 100 g−1g 100 g−1g 100 g−1g 100 g−1
0 29.1 ±1.3 b 33.7 ±1.8 a 32.4 ±1.3 b 21.1 ±0.7 b 25.2 ±0.4 a 23.7 ±0.6 b
7.5
29.6
±
1.4 ab
35.5 ±1.1 a 33.7 ±0.5 b
22.4
±
2.4 ab
26.9 ±3.5 a
25.1
±
3.2 ab
15
29.3
±
1.1 ab
37.0 ±3.8 a
34.6
±
2.6 ab 23.2
±
0.5 ab
26.7 ±0.2 a
25.1
±
0.5 ab
30 31.0 ±1.3 a 37.5 ±5.9 a 36.4 ±2.1 a 24.7 ±0.7 a 29.1 ±1.5 a 26.7 ±0.2 a
Note(s): Different letters in the same column indicate significant differences (p< 0.05) between the treatments.
3.2. Water Stable Aggregate Content
Figure 3shows the effects of the BC treatments on the distribution of the different
aggregate size fraction in the topsoil (0–20 cm) after the winter barley harvest in 2016. The
silt + clay fraction dominated the size distribution (65%), followed by micro-aggregates
(29%), then small macro-aggregates (3%). The large macro-aggregates were the least
represented fraction (2.8%). Remarkably, significant increases were observed in large
macro-aggregates (>2 mm) (103% and 104%, respectively) for the 15 and 30 Mg ha
−1
biochar treated–plots compared to the control. Biochar addition had no significant effect
on silt + clay, micro-aggregates, and small micro-aggregates proportion. The 7.5 and
15 Mg ha
−1
biochar treatments were themselves not significantly different. There were
decreases in Micro-aggregates by 9%, 7% and 17% under the 7.5, 15 and 30 Mg ha
−1
biochar
treatments compared to CK, respectively.
Water 2022, 14, x FOR PEER REVIEW 6 of 14
3.2. Water Stable Aggregate Content
Figure 3 shows the effects of the BC treatments on the distribution of the different
aggregate size fraction in the topsoil (0–20 cm) after the winter barley harvest in 2016. The
silt + clay fraction dominated the size distribution (65%), followed by micro-aggregates
(29%), then small macro-aggregates (3%). The large macro-aggregates were the least rep-
resented fraction (2.8%). Remarkably, significant increases were observed in large macro-
aggregates (>2 mm) (103% and 104%, respectively) for the 15 and 30 Mg ha−1 biochar
treated–plots compared to the control. Biochar addition had no significant effect on silt +
clay, micro-aggregates, and small micro-aggregates proportion. The 7.5 and 15 Mg ha−1
biochar treatments were themselves not significantly different. There were decreases in
Micro-aggregates by 9%, 7% and 17% under the 7.5, 15 and 30 Mg ha−1 biochar treatments
compared to CK, respectively.
Figure 3. Changes in water stable aggregate percentages of soil as affected by biochar (BC) treat-
ment. Error bars represent one standard deviation from the mean. Different letters indicate signifi-
cant difference (p < 0.05) between control and treatments in different years.
3.3. Soil Salinity
Although no significant difference was observed during the crop cultivation, a higher
EC value was found for the BC treatments compared to the untreated plot (Table 3). Mean-
while, similar trends of EC and pH changes were found at the depth of 0~20 cm from the
beginning to the end of the maize–barley rotation period under all treatments. The EC and
pH were also inversely related. Moreover, the EC of all treatment plots \ was significantly
reduced (by 43%, 42%, 37% and 25%) at a depth of 0~20 cm on 25 April 2016 compared to
June 2015 under the 0, 7.5, 15, and 30 Mg ha−1 biochar treatments, respectively. Reductions
in the EC at the moderate depth of 20~40 cm by 25%, 36%, 23%, and 35% were seen after
barley harvest compared to the maize ripping period, under CK, 7.5, 15 and 30 Mg ha−1
BC treated plots. The effects of biochar addition on salt-affected soil amelioration were
better in deep soil than in topsoil. However, the decrease in pH at 0~20 cm was higher
than the decrease in pH in depth at 20~40 cm. The largest reduction in pH was seen un-
der the 15 Mg ha−1 BC treatment (from 9.70 to 9.12 in the surface soil layer (the top 20 cm),
and from 9.20 to 9.16 in the soil layer of 20~40 cm).
Figure 3.
Changes in water stable aggregate percentages of soil as affected by biochar (BC) treatment.
Error bars represent one standard deviation from the mean. Different letters indicate significant
difference (p< 0.05) between control and treatments in different years.
Water 2022,14, 3204 6 of 13
3.3. Soil Salinity
Although no significant difference was observed during the crop cultivation, a higher
EC value was found for the BC treatments compared to the untreated plot (Table 3).
Meanwhile, similar trends of EC and pH changes were found at the depth of 0~20 cm
from the beginning to the end of the maize-barley rotation period under all treatments.
The EC and pH were also inversely related. Moreover, the EC of all treatment plots
\
was
significantly reduced (by 43%, 42%, 37% and 25%) at a depth of 0~20 cm on
25 April 2016
compared to June 2015 under the 0, 7.5, 15, and
30 Mg ha−1
biochar treatments, respectively.
Reductions in the EC at the moderate depth of 20~40 cm by 25%, 36%, 23%, and 35%
were seen after barley harvest compared to the maize ripping period, under CK, 7.5, 15
and
30 Mg ha−1
BC treated plots. The effects of biochar addition on salt-affected soil
amelioration were better in deep soil than in topsoil. However, the decrease in pH at
0~20 cm
was higher than the decrease in pH in depth at 20~40 cm. The largest reduction in
pH was seen under the 15 Mg ha
−1
BC treatment (from 9.70 to 9.12 in the surface soil layer
(the top 20 cm), and from 9.20 to 9.16 in the soil layer of 20~40 cm).
3.4. SOC and Crop Yield
Biochar addition improved soil organic carbon (Figure 4). With an initial average value
of 2.32 g kg
−1
, SOC was greatly increased by 5%, 13% and 25% in 2015 and 8%, 14% and 34%
in 2016 compared to the control, in 7.5, 15 and 30 Mg ha
−1
BC treated plots, respectively.
The higher the biochar addition, the greater the improvement in SOC. In addition, there
were increases in SOC by 7%, 11%, 9%, and 15% under 0, 7.5, 15 and
30 Mg ha−1
biochar
treatments, respectively, in 2016 compared to 2015.
Water 2022, 14, x FOR PEER REVIEW 8 of 14
3.4. SOC and Crop Yield
Biochar addition improved soil organic carbon (Figure 4). With an initial average
value of 2.32 g kg−1, SOC was greatly increased by 5%, 13% and 25% in 2015 and 8%, 14%
and 34% in 2016 compared to the control, in 7.5, 15 and 30 Mg ha−1 BC treated plots, re-
spectively. The higher the biochar addition, the greater the improvement in SOC. In addi-
tion, there were increases in SOC by 7%, 11%, 9%, and 15% under 0, 7.5, 15 and 30 Mg ha−1
biochar treatments, respectively, in 2016 compared to 2015.
Figure 4. Soil organic carbon (SOC) as affected by biochar (BC) treatment. Values are means ± SD (n
=3). Bars with different letters in each year are significantly different according to LSD at p < 0.05.
The maize grain yield varied from 2.79 Mg ha−1 in the control to 4.62 Mg ha−1 in 30
Mg ha−1 BC plot, and the barley yield varied from 2.65 Mg ha−1 in CK to 3.13 Mg ha−1 in 7.5
Mg ha−1 BC treated plot (Figure 5). There were increases in the maize yield by 1.54 per
hectare and 1.84 per hectare under 15 and 30 Mg ha−1 biochar treatments compared to the
control. The highest maize yield was found under the highest biochar-dose.
Figure 4.
Soil organic carbon (SOC) as affected by biochar (BC) treatment. Values are means
±
SD
(n= 3).
Bars with different letters in each year are significantly different according to LSD at p< 0.05.
The maize grain yield varied from 2.79 Mg ha
−1
in the control to 4.62 Mg ha
−1
in
30 Mg ha−1
BC plot, and the barley yield varied from 2.65 Mg ha
−1
in CK to
3.13 Mg ha−1
in 7.5 Mg ha
−1
BC treated plot (Figure 5). There were increases in the maize yield by
1.54 per hectare and 1.84 per hectare under 15 and 30 Mg ha
−1
biochar treatments compared
to the control. The highest maize yield was found under the highest biochar-dose.
Water 2022,14, 3204 7 of 13
Table 3.
Values of electrical conductivity (EC) and pH values of soils sampled at different points during crop cultivation. EC and pH were measured in 1: 5 w/vratio
soil suspensions.
Property Biochar 10 June 2015
Seeding Stage
2 July 2015
Seedling Stage
28 July 2015
Elongation Stage
30 September 2015
Mature Stage
25 April 2016
Harvest Stage
Rate Mg ha−10~20 cm 20~40 cm 0~20 cm 20~40 cm 0~20 cm 20~40 cm 0~20 cm 20~40 cm 0~20 cm 20~40 cm
EC
dS m−1
0 0.43 ±0.18 a 0.64 ±0.27 a 0.30 ±0.11 a 0.55 ±0.28 a 0.49 ±0.19 a 0.56 ±0.25 a 0.68 ±0.27 a 0.52 ±0.32 c 0.25 ±0.06 a 0.48 ±0.26 a
7.5 0.51 ±0.34 a 0.79 ±0.29 a 0.36 ±0.12 a 0.48 ±0.17 a 0.47 ±0.21 a 0.69 ±0.33 a 0.79 ±0.30 a 0.66 ±0.34 a 0.30 ±0.14 a 0.51 ±0.22 a
15 0.50 ±0.54 a 0.78 ±0.20 a 0.34 ±0.18 a 0.49 ±0.15 a 0.51 ±0.34 a 0.37 ±0.13 a 0.73 ±0.30 a 0.75 ±0.20 a 0.32 ±0.07 a 0.60 ±0.11 a
30 0.47 ±0.14 a 0.83 ±0.19 a 0.39 ±0.25 a 0.53 ±0.23 a 0.58 ±0.25 a 0.50 ±0.08 a 0.82 ±0.24 a 0.62 ±0.20 a 0.33 ±0.03 a 0.54 ±0.11 a
pH
0 9.8 ±0.3 a 9.3 ±0.1 a 9.3 ±0.1 a 9.2 ±0.2 a 9.1 ±0.3 a 9.0 ±0.1 b 8.9 ±0.1 a 9.1 ±0.1 a 9.2 ±0.1 a 9.3 ±0.1 a
7.5 9.5 ±0.7 a 9.5 ±0.5 a 9.1 ±0.3 a 9.4 ±0.4 a 9.2 ±0.2 a 9.3 ±0.4 ab 9.1 ±0.6 a 9.4 ±0.5 a 9.2 ±0.3 a 9.4 ±0.3 a
15 9.7 ±0.4 a 9.2 ±0.3 a 9.2 ±0.2 a 9.2 ±0.1 a 9.1 ±0.1 a 9.0 ±0.0 ab 8.9 ±0.1 a 9.2 ±0.1 a 9.0 ±0.1 a 9.2 ±0.1 a
30 9.7 ±0.5 a 9.4 ±0.6 a 9.5 ±0.4 a 9.4 ±0.2 a 9.3 ±0.3 a 9.4 ±0.4 a 9.4 ±0.4 a 9.4 ±0.4 a 9.1 ±0.3 a 9.4 ±0.3 a
Note(s): Data are means ±standard deviation. Lowercase letters indicate significant differences at p< 0.05 in EC or pH for different biochar application rates.
Water 2022,14, 3204 8 of 13
Water 2022, 14, x FOR PEER REVIEW 9 of 14
Figure 5. Crop yields of maize and barley as affected by biochar (BC) treatment. Values are means
± SD (n =3). Bars with different letters in each year are significantly different according to LSD at p
< 0.05.
4. Discussion
4.1. Effects of Biochar on Improvement of Soil Physical Properties
Soil physical properties are important indicators of soil quality, which can be defined
as the ability of soil to support crop growth without causing soil degradation [30]. Soil
degradation in coastal areas induced by crop rotation was investigated in the study, and
biochar was considered a suitable material for mitigating or improving soil limiting fac-
tors [31,32]. Physical changes in soil bulk density, water holding capacity, and water stable
aggregates were investigated in this field experiment.
Bulk density not only affects the availability of soil nutrients and moisture but also
indirectly reflects productivity [33]. As a porous abundant material, biochar performs well
in improving soil physical properties [34] and can alleviate the negative physical effects
induced by soil–compaction [35]. However, in the maize–barley rotation system, the bulk
density only decreased in the 30 Mg ha−1 biochar treated plots, when compared to the
control, after summer maize cultivation in 2015. At the same time, lower biochar rates of
7.5 and 15 Mg ha−1 increased the BD by 0.04 and 0.05 g cm−3, respectively. However, after
the winter barley harvest the following year, significant decreases were observed under
all biochar treatments compared to CK (Figure 2). The bulk density of soil depends greatly
on the soil particle size, mineralogy, organic matter content, and management. During the
ripping period of the maize, salt accumulated in the topsoil (0–20 cm), and a higher salt
content existed in the biochar–treated plots than in the control plots (Table 3). Addition-
ally, the rain was thought to be a key factor, as the rainy season fell during the summer
maize cultivation. As the climate conditions and planting management were consistent,
we hold the opinion that rainfall leaching caused the soil-compaction and thus made the
soil heavier in 2015. During the subsequent barley cultivation, very little rain fell, and the
soil salt decreased under the biochar treatments compared to CK. Thus, a higher reduction
in BD after the barley harvest was observed with the highest biochar addition rate.
The results showed that the addition of biochar increased the soil water content
(WC), saturated water capacity (SWC), and field capacity (FC), relative to the no-biochar
controls (Table 2). Biochar addition improved soil water–holding capacity due to its
highly porous structure, in another experiment [36]. However, significant differences
were only observed in plots treated with 30 Mg ha−1 biochar treated plots compared to CK
Figure 5.
Crop yields of maize and barley as affected by biochar (BC) treatment. Values are means
±
SD (n= 3). Bars with different letters in each year are significantly different according to LSD
at p< 0.05.
4. Discussion
4.1. Effects of Biochar on Improvement of Soil Physical Properties
Soil physical properties are important indicators of soil quality, which can be de-
fined as the ability of soil to support crop growth without causing soil degradation [
30
].
Soil degradation in coastal areas induced by crop rotation was investigated in the study,
and biochar was considered a suitable material for mitigating or improving soil limiting
factors [31,32].
Physical changes in soil bulk density, water holding capacity, and water
stable aggregates were investigated in this field experiment.
Bulk density not only affects the availability of soil nutrients and moisture but also
indirectly reflects productivity [
33
]. As a porous abundant material, biochar performs well
in improving soil physical properties [
34
] and can alleviate the negative physical effects
induced by soil–compaction [
35
]. However, in the maize-barley rotation system, the bulk
density only decreased in the 30 Mg ha
−1
biochar treated plots, when compared to the
control, after summer maize cultivation in 2015. At the same time, lower biochar rates of
7.5 and 15 Mg ha
−1
increased the BD by 0.04 and 0.05 g cm
−3
, respectively. However, after
the winter barley harvest the following year, significant decreases were observed under all
biochar treatments compared to CK (Figure 2). The bulk density of soil depends greatly
on the soil particle size, mineralogy, organic matter content, and management. During the
ripping period of the maize, salt accumulated in the topsoil (0–20 cm), and a higher salt
content existed in the biochar–treated plots than in the control plots (Table 3). Additionally,
the rain was thought to be a key factor, as the rainy season fell during the summer maize
cultivation. As the climate conditions and planting management were consistent, we hold
the opinion that rainfall leaching caused the soil-compaction and thus made the soil heavier
in 2015. During the subsequent barley cultivation, very little rain fell, and the soil salt
decreased under the biochar treatments compared to CK. Thus, a higher reduction in BD
after the barley harvest was observed with the highest biochar addition rate.
The results showed that the addition of biochar increased the soil water content (WC),
saturated water capacity (SWC), and field capacity (FC), relative to the no-biochar controls
(Table 2). Biochar addition improved soil water–holding capacity due to its highly porous
structure, in another experiment [
36
]. However, significant differences were only observed
in plots treated with 30 Mg ha
−1
biochar treated plots compared to CK in the maize-barley
Water 2022,14, 3204 9 of 13
rotation system. The soil water holding capacity in 2016 was lower than that in 2015,
indicating soil degradation. Biochar application mediated the soil degradation. The higher
the biochar rate, the higher the water holding capacity of the soil, and the total porosity
of the soil increased with the decrease in bulk density [
37
] over the experimental period.
During barley cultivation, the topsoil was disturbed by plowing and sowing. However,
there was a low level of rainfall from October to April in this study area, which limited the
formation of soil capillaries. Therefore, the lower soil water content after the barley harvest
may have been caused by a reduction in the capillary porosity.
After one maize—barley rotation, the 15 and 30 Mg ha
−1
biochar treatments sub-
stantially increased the soil large macro-aggregates (>2 mm) content compared to the
7.5 Mg ha−1
and no-biochar treatments (Figure 3). No significant differences were found
in the small macro-aggregates (0.25–2 mm), micro-aggregates (0.25–0.053 mm) and silt
+ clay (<0.053 mm) contents, across all treatments compared with CK. Our results were
in line with those of other studies [
38
,
39
], which reported no effect of biochar on micro-
aggregation improvement. However, some studies have reported different effects of biochar
on water stable aggregates. For example, (Dong et al.) [
40
] reported increased aggregation
under the selected biochar supplementation rates, and the soil organic carbon extracted
from the biochar was considered to be an important factor to improve the stability of
aggregates. A previous study [
41
] indicated that SOC initially induces the formation
of micro-aggregates (<250
µ
m) by connecting polyvalent cations and clay particles, and
the subsequent formation of macro-aggregates (>250
µ
m) comes as a result of the com-
bination of micro-aggregates. Liu et al. [
42
] observed incremental aggregation under
40 Mg ha−1
BC treatment, but no effect under 20 Mg ha
−1
treatment, which is similar to
our results. We postulated that the differences in the effect of biochar addition might have
been due to the application rate [
42
], application time [
43
,
44
], incubation method [
45
],
climate [
46
], soil properties [
47
,
48
] and the texture of the biochar used [
49
]. Additionally,
appropriate biochar application rates are a feasible method for accelerating water stable
aggregate formation [50].
4.2. Effects of Biochar on Soil Salinity
Our results indicated that the addition of biochar to soils had a poor effect on mitigat-
ing the soil salt content during maize cultivation. The more biochar was added, the higher
the EC in soil. However, the EC measured after the barley harvest showed no differences
between all biochar treatments (Table 3). At the same time, the pH decreased in all biochar
treated plots compared to CK. A large proportion of rainfall occurred during maize culti-
vation, and the soil salt content changed significantly over time as shown in Table 3. The
soil salt content was higher at a depth of 20~40 cm than at 0~20 cm on 30 September 2015.
Nevertheless, an inverse relationship between the EC values of the two soil depths was
shown in 2016. The soil pH in saline soil is closely related to carbonate and bicarbonate [
51
],
so the soil pH decrease in 2016 may have been caused by the corresponding salt transfer
from topsoil to the deeper soil. Biochar showed a delayed effect on soil salinity elimination
in our study, demonstrating no benefit in the short–time experiments. Unlike the controlled
incubation conditions in laboratory studies [
31
,
52
,
53
], some natural field factors such as
rain and groundwater changed continually during the crop cultivation period in our study
area. The soil salinity was mainly determined by the meteorological conditions during
the maize-barley rotation system. Therefore, long-term experiments should be carried
out to study the continuous effects of biochar on soil salinity mediation. Additionally,
due to its stability, large specific surface area, and porosity [
54
], the biochar added to the
soil is primarily used for nutrient storage [
55
], microbial activity improvement [
56
], root
growth [
57
] and soil physical culture modification [
58
], thus accelerating nitrogen uptake
by crops [59,60] and blocking salt upward movement through capillary water.
Water 2022,14, 3204 10 of 13
4.3. Effects of Biochar on Carbon Sequestration and Yield
In good agreement with [
54
,
61
], which showed that biochar added to soil significantly
increased the SOC, the SOC in our study area increased continuously during the crop
rotation cultivation period (Figure 3). Applying biochar to agricultural soils could retard
the soil C cycle and the non-CO
2
greenhouse gas (GHG) emissions [
62
]. Some reports
have shown that biochar application decreased soil GHGs, including CH
4
, N
2
O, and NO
fluxes [
60
,
63
]. Global warming caused by GHGs could lead to sea level rises, and seawater
intrusion would bring more serious problems in coastal saline soil agriculture. Furthermore,
recent estimates suggested that by 2100, the global average sea level may have risen by
more than 2 m [
64
], which will result in serious harm to coastal ecology, agriculture, fishing,
and human life. Therefore, we suggest that biochar could play a role in slowing down
climate change by ensuring the sustainable development of coastal agriculture.
It is well documented that biochar modifies soil to improve crop productivity [
65
–
67
].
Biochar supplementation at rates of 7.5, 15, and 30 Mg ha
−1
exerted a significant positive
effect on improving maize grain yield by 1.70%, 55.14%, and 65.59%, respectively, but
only small increases in barley yield were achieved in the subsequent cultivation period
following no further biochar amendment (Figure 4). Humified materials (humic–acid–like
material and fulvic–acid–like materials) in biochar could improve crop growth [
68
], and
Wang, et al. [
69
] also reported that biochar amendment promoted humic acid synthesis.
However, low temperatures could limit biochar performance, as has been reported by
Nelissen et al. [70].
5. Conclusions
Soil degradation was observed in the maize-barley rotation system in our study area,
and this phenomenon was especially obvious in terms of soil water holding capacity
changes. We showed that biochar is a potentially suitable soil amendment for coastal saline
agriculture, because it improved the physical properties of the soil in this study. Moreover,
biochar is an effective treatment for coastal saline soil agriculture because it increased the
large macro-aggregate content and SOC in the soil, which resulted in an improvement in
the water stable aggregates. Furthermore, the grain yield of the maize and barley cultivated
in biochar–treated soil was improved.
Author Contributions:
Conceptualization, Y.S. and X.C.; methodology, Y.S. and J.Y.; software, Y.S.;
validation, X.Z., Y.L., Y.S., W.X., X.W. and R.Y.; formal analysis, Y.S.; investigation, Y.S.; resources,
X.Z.; data curation, X.C.; writing—original draft preparation, Y.S.; writing—review and editing, Y.S.,
X.C. and Y.L.; visualization, X.C.; supervision, X.Z.; project administration, J.Y. and X.Z.; funding
acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the National Key Research & Development Program of China,
grant number 2019YFD1002702; the Natural Science Foundation of China, grant number U1806215,
and the National Natural Science Foundation of China (General Program), grant number 41977015.
Acknowledgments:
The authors would like to acknowledge Dongtai Coastal Saline Soil Institute for
working place and equipment for this study.
Conflicts of Interest: The authors declare no conflict of interest.
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