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Adsorption and Desorption of Phosphorus in Biochar-Amended Black Soil as Affected by Freeze-Thaw Cycles in Northeast China

Authors:
  • Kanwon National University

Abstract and Figures

Substantial soil phosphorus (P) losses often occur in the northern temperate regions owing to soil freeze-thaw cycles (FTCs). Presumably, biochar amendment is an efficient method of conserving P and sustaining agricultural production in the black soil region of northeast China. However, how biochar interacts with FTCs to affect soil P adsorption and desorption is unclear. A simulated laboratory FTC experiment was conducted on untreated and biochar-amended soil with varying moisture content to assess their effects on P adsorption and desorption. Soil P adsorption and desorption values were fitted with Langmuir and Freundlich isotherms to determine the interaction of the frequency of FTCs with moisture content and biochar amendment. Higher soil moisture content increased soil P adsorption, whereas biochar amendment mitigated decreased P retention by decreasing soil P adsorption capacity. Biochar amendment significantly increased the desorption ratio (Davg) under all the FTCs. The desorption ratio of soil and biochar-amended soil in saturated moisture content treatment was significantly higher than that of 12 FTCs. The FTCs decreased the P availability of biochar-amended soil by enhancing P desorbability. Our results suggest that biochar amendment in arable black soil should not be conducted during FTCs, particularly during snowmelt.
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sustainability
Article
Adsorption and Desorption of Phosphorus in
Biochar-Amended Black Soil as Affected by
Freeze-Thaw Cycles in Northeast China
Ying Han 1,† , Byoungkoo Choi 2, and Xiangwei Chen 1, *
1School of Forestry, Northeast Forestry University, 26 Hexing Road, Harbin 150040, China;
hanying@nefu.edu.cn
2Department of Forest Environment Protection, Kangwon National University, Chuncheon 24341, Korea;
bkchoi@kangwon.ac.kr
*Correspondence: xwchen1966@nefu.edu.cn; Tel.: +86-451-82191813
These authors contributed equally to this work.
Received: 10 April 2018; Accepted: 14 May 2018; Published: 15 May 2018


Abstract:
Substantial soil phosphorus (P) losses often occur in the northern temperate regions
owing to soil freeze-thaw cycles (FTCs). Presumably, biochar amendment is an efficient method
of conserving P and sustaining agricultural production in the black soil region of northeast China.
However, how biochar interacts with FTCs to affect soil P adsorption and desorption is unclear.
A simulated laboratory FTC experiment was conducted on untreated and biochar-amended soil with
varying moisture content to assess their effects on P adsorption and desorption. Soil P adsorption and
desorption values were fitted with Langmuir and Freundlich isotherms to determine the interaction
of the frequency of FTCs with moisture content and biochar amendment. Higher soil moisture
content increased soil P adsorption, whereas biochar amendment mitigated decreased P retention
by decreasing soil P adsorption capacity. Biochar amendment significantly increased the desorption
ratio (D
avg
) under all the FTCs. The desorption ratio of soil and biochar-amended soil in saturated
moisture content treatment was significantly higher than that of 12 FTCs. The FTCs decreased the P
availability of biochar-amended soil by enhancing P desorbability. Our results suggest that biochar
amendment in arable black soil should not be conducted during FTCs, particularly during snowmelt.
Keywords:
batch equilibrium method; biochar amendment; black soil; freeze-thaw cycle;
phosphorus availability
1. Introduction
Phosphorus (P) is an important nutrient, which determines agricultural productivity, because it
plays key roles in plant metabolism, structure, and energy transformation [
1
,
2
]. Plants can acquire P as
phosphate anions (H
2
PO
4
and HPO
42
) from the soil solution [
3
,
4
]. Previously, the P transformation
rate between soil solution and soil solids was reported to be highly dependent on phosphate adsorption
and desorption characteristics [
5
]. Therefore, P adsorption and desorption restricts the capacity of
supplying soil P, which affects P uptake and utilization by plants [
6
]. A better understanding of
P adsorption and desorption in agricultural systems is critical for improving P sustainability and
increasing crop productivity.
The black soil region in northeast China has been one of the most important crop production
areas in the country owing to the productive physical and chemical capacity of black soil [
7
]. However,
unsustainable land-use practices (e.g., intensive soybean production) have resulted in a substantial
decline in soil fertility during recent decades [
8
]. Phosphorus fertilizers have been intensively applied
to cope with this situation; however, eutrophication [
9
] and low utilization of P fertilizers [
10
] followed.
Sustainability 2018,10, 1574; doi:10.3390/su10051574 www.mdpi.com/journal/sustainability
Sustainability 2018,10, 1574 2 of 10
A relatively understudied method of increasing the efficiency of P fertilization in black soil involves
soil amendment using biochar. Biochar is an important bio-resource that is produced by pyrolysis
under limited air supply [
11
,
12
]. Because of the high physical and chemical capacity of biochar,
it has been used as a potential soil-amending agent for improving soil P availability by reducing soil
P leaching [
13
] and increasing crop productivity [
14
]. Biochar not only alters P availability directly
through its anion exchange capacity or effects on the activity of cations that interact with P [
15
], but also
exerts indirect effects on P retention and release through changes in the soil microbial environment [
16
].
Some investigations involving biochar-amended soil have shown that biochar amendment can reduce
the leaching of PO
4
-P [
13
,
17
,
18
]. Therefore, characterizing P adsorption and desorption in black soil
under biochar amendment is necessary for the development of effective P fertilizer-use strategies.
Phosphorus cycling and leaching in the black soil of northeast China are affected by freeze-thaw
cycles (FTCs) through the associated biochemical and physicochemical processes [
19
]. Previous studies
on biochar amendment have mainly focused on crop productivity during the growing period [20,21].
However, few studies have assessed the effects of biochar amendment on P adsorption and desorption
after FTCs. Soil freezing and thawing is a natural process that occurs regularly in cool temperature
and high-latitude regions [
22
,
23
]. Previous studies suggested that repeated FTCs can increase total
dissolved P and water-extractable P in soil owing to damage caused to plant cells, microbial biomass,
and/or physical disruption of soil aggregates [
22
,
24
27
]. Freeze-thaw cycles can change the extent of P
leaching, and significantly affect the adsorption and desorption characteristics of soil P. Some laboratory
studies have examined the effect of FTCs on soil P adsorption and desorption. Wang et al. [
28
]
observed that FTCs increased P adsorption and decreased P desorption in wetland soils. However,
Qian et al. [29]
and Fan et al. [
30
] showed that decreasing P adsorption would promote the release of
adsorbed P, thereby increasing the risk of soil P losses in the brown and black soils of China.
In the present study, a simulated laboratory freeze-thaw experiment was conducted to assess
the effect of biochar application on P adsorption and desorption in black soil. This study aimed
to (i) identify the effects of FTCs and (ii) examine the interactions among FTC treatments and soil
moisture conditions on P adsorption and desorption after biochar amendment. We expected that our
results would improve our understanding of the effects of biochar on P sustainability in agricultural
production systems. This would be important for the validation of the effect of biochar application
on agriculture.
2. Materials and Methods
2.1. Site Description
The study site is located within Keshan Farm in the black soil region of northeast China
(48
12
0
–48
23
0
N, 125
08
0
–125
37
0
E) [
31
]. The study area was chosen because the soil type and
tillage practices were representative of intensive soybean production in the region. Soil in the study
area was classified as a Mollisol with 22% sand, 33% silt, and 45% clay content. The annual mean
temperature of the study area is 0.9
C, with a lowest monthly mean temperature of
21.4
C in January
and a highest monthly mean temperature of 22.0
C in July. The mean annual precipitation is 501.7 mm,
68.3% of which is concentrated from June to August [
32
]. The soil freezing period typically occurs
from early November to mid-June, and the average annual frost-free period is 115 days. The mean
annual maximum frozen soil depth is 2.5 m [33].
2.2. Experimental Design
2.2.1. Soil Sampling and Analysis
Arable surface soils samples (0–10 cm soil depth) were collected from five randomly assigned
points in the study site. Soil samples were homogenized, air-dried at 25
C, and passed through a 2 mm
sieve. Soil pH was determined using the electrometric method using a suspension in deionized water
Sustainability 2018,10, 1574 3 of 10
(soil: water ratio of 1:2.5, w/v). Soil organic carbon (SOC) was measured using dry combustion using
a total organic carbon analyzer (Elementar, Vario EL cube, Germany). Soil total nitrogen (TN) was
measured using the Kjeldahl distillation method [
34
]. Soil total P (TP) was measured by digestion with a
mixture of acids consisting of H
2
SO
4
and HClO
4
[
35
], followed by the molybdenum blue method [
36
].
The available P (AP) present in soil was extracted with 0.03 M NH
4
F and 0.025 M HCl [
37
] and
measured using the molybdenum blue method. Soil available nitrogen (AN) was measured using the
alkaline hydrolysis diffusion method. Each analysis was conducted with four replications.
2.2.2. Biochar Production and Amendment
The applied biochar was composed of soybean straw that had been pyrolyzed at 500
C under
anaerobic conditions. The temperature was raised at a rate of approximately 13
C
·
min
1
and
maintained at 500
C for 2 h. The furnace was then turned off, and the sample was allowed to cool
to 25
C. The biochar was ground and passed through a 0.15 mm sieve before application. It was
added uniformly to 400 g soil as a rate of 4% [
38
], and an incubation experiment using 500 cm
3
plastic
incubation containers was conducted for 60 days at 25
C with a moisture content equal to 70% of field
capacity of the soil in the study area. As a control, soil samples without biochar amendment were
incubated under the same conditions.
2.2.3. Freeze and Thaw Experiment
A laboratory-simulated freeze-thaw experiment was conducted after biochar amendment
incubation. Deionized water was added to each assigned soil sample in order to maintain soil
moisture content at 30.5% (MC1), 22.4% (MC2), and 44.8% (MC3), respectively. Thereafter, these soil
samples were pre-incubated at 5
C for 3 days. At the conclusion of pre-incubation, the soil samples
were subjected to freezing at
10
C for 12 h and thawing at 5
C for 12 h. The frequencies of FTCs
were 1 (1 FTC), 3 (3 FTCs), 6 (6 FTCs), and 12 (12 FTCs) as compared with the frequency of the unfrozen
control (0 FTCs), which was kept at 5 C throughout the entire experimental period.
2.2.4. Isothermal Adsorption and Desorption
The P adsorption of each soil sample was examined by placing 1.5 g dried soil in 30 mL of
0.01 mol
·
L
1
KCl (pH = 7) solution that contained 0, 20, 40, 60, 80, 100, 120, 180, and 240 mg
·
L
1
P.
Two drops of chloroform were added to the soil samples to prevent microbial activity. All the samples
were shaken at 25
C for 24 h, centrifuged (5000 r
·
min
1
) for 10 min, and filtered. The P concentration
of the equilibrium solution was then determined by the molybdenum blue method.
Desorption of soil P was measured after the supernatants obtained in the adsorption experiment
were removed, and the residual soil samples were washed twice with 30 mL saturated NaCl to remove
free P. After the samples were centrifuged and filtered, 30 mL of 0.01 mol
·
L
1
KCl (pH = 7) and two
drops of chloroform were mixed with each sample, followed by centrifugation. The supernatants were
examined to determine the desorbed P content. Each analysis was conducted with four replications.
2.3. Data Analysis
2.3.1. Phosphorus Adsorption and Desorption
Adsorption isotherms models, such as the Langmuir and Freundlich isotherms, describe solute
adsorption by solids in aqueous solution at constant temperature and pressure. The P adsorption data
for the soils used in the present study were fitted to the following Langmuir adsorption Equation (1)
and Freundlich adsorption Equation (2).
Ce
Qe
=Ce
Qm
+1
KLQm
(1)
Sustainability 2018,10, 1574 4 of 10
in which C
e
is the equilibrium P concentration in solution (mg
·
L
1
), Q
e
is the mass of P adsorbed per
unit mass of soil (mg
·
kg
1
), K
L
is the Langmuir constant related to bonding energy (L
·
mg
1
), and Q
m
is the sorption maximum (mg
·
kg
1
) calculated by the Langmuir equation. The maximum P buffer
capacity (MBC) of the soil was calculated from the product of Langmuir constants Qmand KL[39].
logQe=logKF+1
nlogCe(2)
in which K
F
is an approximate indicator of adsorption capacity (L
·
mg
1
), and 1/n is a function of the
strength of adsorption in the adsorption process. Freundlich function 1/n is also representative of the
P adsorption intensity. An average value of desorption ratio (D
avg
) was defined as an average ratio of
the desorbed phosphate to the total phosphate adsorbed by the adsorbents.
2.3.2. Statistical Analysis
One-way analysis of variance (ANOVA) with Least Significant Difference (LSD) was used to
assess significant differences in the chemical properties of soil, biochar, and biochar-amended soil,
and significant differences in P adsorption parameters and D
avg
among biochar amendment treatments,
soil moisture content, and FTC treatments. Linear regression analysis was applied to the Langmuir
and Freundlich isotherms of P adsorption on soil and biochar-amended soil with different moisture
contents. A multivariate ANOVA was used to determine the interaction among biochar amendment,
soil moisture content, and FTC treatment on P adsorption and desorption. All statistical analyses were
conducted using SPSS 22.0 (IBM Institute, Armonk, NC, USA) with a significance threshold of p< 0.05.
3. Results and Discussion
3.1. Soil and Biochar Properties
Biochar application improved the general chemical properties of soil. The values of pH, SOC, AP,
and TN in biochar-amended soil were significantly higher than those in the untreated soil (Table 1).
Similarly, Sun and Lu [
38
] reported that the application of biochar to the soil significantly increased
the available N and P content of soils. However, according to Arthur et al. [
21
] and Trazzi et al. [
12
],
the values of pH and SOC increased after biochar amendment. The increases in pH, TP, and TN of soil
after biochar amendment are most likely due to the effects of biochar [
40
,
41
], whereas the increases in
AP and AN were due to the interaction between biochar and soil [15].
Table 1.
Chemical properties of soil, biochar, and biochar-amended soil in the black soil region of
northeast China.
Sample (n= 4) pH
(1:2.5 H2O)
SOC
(g·kg1)
TP
(g·kg1)
AP
(mg·kg1)
TN
(g·kg1)
AN
(mg·kg1)
Soil
(n= 4) 5.77 ±0.09c 51.03 ±1.08c 0.86 ±0.02b 43.05 ±1.29c 3.01 ±0.01c 120.25 ±11.12b
Biochar
(n= 4) 7.82 ±0.07a 169.11 ±3.92a 1.47 ±0.13a 78.96 ±5.95a 6.44 ±0.30a 156.43 ±13.89a
Biochar-amended soil
(n= 4) 6.89 ±0.11b 65.81 ±1.85b 0.95 ±0.04ab 62.79 ±5.23b 4.17 ±0.31b
132.79
±
15.52ab
Note: SOC: soil organic carbon, TP: total P, AP: available P, TN: total N, AN: available N. Different lowercase letters
in the same column indicate significance among soil, biochar, and biochar-amended soil at p< 0.05.
3.2. Phosphorus Adsorption
Adsorption procedures have been suggested for use in predicting the partition of P between
solution and solid phases in the environment [
28
,
42
]. The relationship of P equilibrium concentration
with the amount of adsorbed P was expressed as linear correlations (Figure 1). The P adsorption data
of each sample could be described by the Langmuir (r
2
> 0.65) and Freundlich (r
2
> 0.70) isotherms.
Among the moisture content and biochar amendment, MC1(SB) had the lowest Q
e
(176.73 mg
·
kg
1
),
Sustainability 2018,10, 1574 5 of 10
followed by MC3(SB) (277.99 mg
·
kg
1
), which was comparable to that of MC2(SB) (286.69 mg
·
kg
1
)
at an equilibrium P concentration of 6 mg
·
L
1
. Meanwhile, the Q
e
of MC1(S) (221.78 mg
·
kg
1
)
was lower than that of MC2(S) (355.50 mg
·
kg
1
) and MC3(S) (355.56 mg
·
kg
1
) at equilibrium
concentrations of 13, 8, and 4 mg
·
L
1
, respectively. At >110 mg
·
L
1
equilibrium concentration,
MC1(SB) (654.55 mg
·
kg
1
) had the lowest Q
e
, followed by MC1(S) (738.55 mg
·
kg
1
), which was
comparable to that of MC3(SB) (799.43 mg
·
kg
1
), MC2(SB) (827.12 mg
·
kg
1
), MC2(S) (942.65 mg
·
kg
1
),
and MC3(S) (1009.78 mg
·
kg
1
). Xu et al. [
43
] evaluated the effects of biochar amendment on P sorption
and desorption in three soils with different acidities. The results showed lower P loading on the
biochar-amended black soil, which might be due to differences in biochar production conditions and
feedstock types [44].
Sustainability 2018, 10, x FOR PEER REVIEW 5 of 10
Table 1. Chemical properties of soil, biochar, and biochar-amended soil in the black soil region of
northeast China.
Sample (n = 4)
pH
(1:2.5 H2O)
TP
(g·kg−1)
AP
(mg·kg−1)
TN
(g·kg−1)
AN
(mg·kg−1)
Soil
(n = 4)
5.77 ± 0.09c
0.86 ± 0.02b
43.05 ± 1.29c
3.01 ± 0.01c
120.25 ± 11.12b
Biochar
(n = 4)
7.82 ± 0.07a
1.47 ± 0.13a
78.96 ± 5.95a
6.44 ± 0.30a
156.43 ± 13.89a
Biochar-amended soil
(n = 4)
6.89 ± 0.11b
0.95 ± 0.04ab
62.79 ± 5.23b
4.17 ± 0.31b
132.79 ± 15.52ab
Note: SOC: soil organic carbon, TP: total P, AP: available P, TN: total N, AN: available N. Different
lowercase letters in the same column indicate significance among soil, biochar, and biochar-amended
soil at p < 0.05.
3.2. Phosphorus Adsorption
Adsorption procedures have been suggested for use in predicting the partition of P between
solution and solid phases in the environment [28,42]. The relationship of P equilibrium concentration
with the amount of adsorbed P was expressed as linear correlations (Figure 1). The P adsorption data
of each sample could be described by the Langmuir (r2 > 0.65) and Freundlich (r2 > 0.70) isotherms.
Among the moisture content and biochar amendment, MC1(SB) had the lowest Qe (176.73 mg·kg−1),
followed by MC3(SB) (277.99 mg·kg−1), which was comparable to that of MC2(SB) (286.69 mg·kg−1) at
an equilibrium P concentration of 6 mg·L−1. Meanwhile, the Qe of MC1(S) (221.78 mg·kg−1) was lower
than that of MC2(S) (355.50 mg·kg−1) and MC3(S) (355.56 mg·kg−1) at equilibrium concentrations of 13,
8, and 4 mg·L−1, respectively. At >110 mg·L−1 equilibrium concentration, MC1(SB) (654.55 mg·kg−1)
had the lowest Qe, followed by MC1(S) (738.55 mg·kg−1), which was comparable to that of MC3(SB)
(799.43 mg·kg−1), MC2(SB) (827.12 mg·kg−1), MC2(S) (942.65 mg·kg−1), and MC3(S) (1009.78 mg·kg−1).
Xu et al. [43] evaluated the effects of biochar amendment on P sorption and desorption in three soils
with different acidities. The results showed lower P loading on the biochar-amended black soil, which
might be due to differences in biochar production conditions and feedstock types [44].
Figure 1. Langmuir (a) and Freundlich (b) isotherms of P adsorption on soil and biochar-amended
soils with different soil moisture contents. MC1, MC2, and MC3 represent natural moisture content
(30.5%), half of saturated moisture content (22.4%), and saturated moisture content (44.8%),
respectively. (S) and (SB) represent soil and biochar-amended soil, respectively.
Soil P adsorption parameters with different moisture contents, calculated by Langmuir and
Freundlich isotherms, are shown in Table 2. The response of P adsorption to soil tends to be highly
Figure 1.
Langmuir (
a
) and Freundlich (
b
) isotherms of P adsorption on soil and biochar-amended soils
with different soil moisture contents. MC1, MC2, and MC3 represent natural moisture content (30.5%),
half of saturated moisture content (22.4%), and saturated moisture content (44.8%), respectively. (S) and
(SB) represent soil and biochar-amended soil, respectively.
Soil P adsorption parameters with different moisture contents, calculated by Langmuir and
Freundlich isotherms, are shown in Table 2. The response of P adsorption to soil tends to be highly
dependent on biochar amendment and moisture content. The Q
m
and K
F
of MC2 and MC3 in soils
are significantly higher than natural moisture content (MC1), suggesting that altering soil moisture
content may help to increase soil P adsorption capacity. According to Peltovuori and Soinne [
45
],
the increase in P adsorption in air-dried soil can be attributed to a destruction of organic matter
enveloping the short-range ordered oxide surfaces. In addition, Shukla et al. [
46
] reported that P
adsorption capacity could increase under saturated moisture conditions associated with the change in
soil redox condition. Under saturated moisture conditions, a type of amorphous iron oxide complex
forms [
46
]. As compared with ferric hydroxide, the larger surface areas and increased number of P
adsorption sites of the amorphous iron oxide complex should increase the P adsorption capacity [
47
].
After biochar amendment, the variation in Q
m
and K
F
decreased to 817.82–979.21 mg
·
kg
1
and
79.35–135.73 L
·
mg
1
, respectively, and the Q
m
and K
F
for MC2 and MC3 in biochar-amended soils
were lower than those in soils not subjected to biochar amendment treatment. These results indicated
that biochar amendment could reduce soil P adsorption capacity. The decreases in P adsorption with
biochar amendment could be affected by increasing pH [
48
]. Soil surface becomes more negatively
charged, thereby increasing anion repulsion and decreasing P adsorption. Higher pH conditions
depress the formation of HPO42, which is preferentially adsorbed by soil colloids [43].
Sustainability 2018,10, 1574 6 of 10
Table 2.
Parameters of P adsorption on soil and biochar-amended soil with different moisture contents.
Sample
(n= 4)
Moisture
Content
Langmuir Freundlich
Qm(mg·kg1) KL(L·mg1) MBC (L·kg1) R2KF(L·mg1)1/n R2
Soil (n= 4)
MC1 847.74 ±67.53cA 0.032 ±0.006aA 27.23 ±3.32bA
0.75
131.02 ±18.19bA 0.34 ±0.03aB
0.70
MC2
1051.29
±
20.98bA
0.038 ±0.002aA 39.96 ±1.37aA
0.83
208.66 ±11.36aA 0.29 ±0.01bB
0.75
MC3 1152.32 ±65.98aA 0.034 ±0.003aA 39.13 ±2.25abA
0.83
190.95 ±9.53aA 0.33 ±0.02aA
0.82
Biochar-
amended
soil (n= 4)
MC1 817.82 ±19.10bA 0.020 ±0.001aB 16.47 ±0.41bB
0.65
79.35 ±8.59bB 0.41 ±0.03aA
0.74
MC2 979.21 ±30.94aB 0.026 ±0.002bB 25.12 ±1.92aB
0.77
133.31 ±6.98aB 0.35 ±0.01bA
0.80
MC3 943.44 ±63.36aB 0.028 ±0.003bB 26.69 ±1.21aB
0.79
135.73 ±9.01aB 0.35 ±0.02bA
0.78
Note: Q
m
: Langmuir sorption maximum, K
L
: bonding energy constant, MBC: maximum buffer capacity,
K
F
: Freundlich sorption capacity factor, and 1/n: Freundlich function. Different lowercase letters in the same
column indicate significance among different moisture contents at p< 0.05; different uppercase letters in the same
moisture treatment indicate significant differences between soil and biochar-amended soil at p< 0.05.
The effect of moisture content treatments on P adsorption intensity was more obvious in
biochar-amended soil than in the untreated soil. After biochar amendment, the bonding energy
constant (K
L
) of MC2 and MC3 was significantly higher than that of MC1. The value of 1/n in MC2
and MC3 was significantly lower than that in MC1. Our findings suggest that adsorbed P is not easily
desorbed in the moisture content treatment.
Similar to Q
m
and K
F
, the MBC of MC2 and MC3 were higher than that of MC1 in
biochar-amended soil. However, the MBC of biochar-amended soils was significantly lower than that
of untreated soils, revealing that a lower P concentration is required when biochar is added to the soil.
Soil P adsorption parameters can be affected by moisture content and biochar amendment.
However, P adsorption parameters were not significantly different among the five FTCs of the same
soil group (Table 3). A similar study had focused on the effects of freeze-thaw environments on ion
exchange resin stability. Mamo et al. [
49
] showed that FTC had no effects on P adsorption characteristics.
Another study showed that air-drying had a greater effect on soils than freezing had [45].
Table 3.
Changes in P adsorption parameters in response to moisture content (MC), biochar amendment,
and freeze–thaw cycle (FTC) treatment.
Source df
Langmuir Freundlich
Qm(mg·kg1) KL(L·mg1) MBC (L·kg1) KF(L·mg1)1/n
F Sig. F Sig. F Sig. F Sig. F Sig.
MC 2 42.62 0 33.09 0 167.12 0 130.91 0
33.75
0
TrT 1 99.97 0 84.65 0 448.50 0 285.28 0
42.93
0
FTCs 4 1.43 0.23 1.38 0.25 0.90 0.47 1.01 0.41 2.17 0.08
MC ×TrT 2 4.99 0.01 0.30 0.74 12.23 0 9.49 0 0.03 0.97
MC ×FTCs 8 5.50 0 3.14 0.004 3.31 0.002 3.16 0.003 3.61
0.001
TrT ×FTCs 4 0.57 0.69 0.85 0.50 1.10 0.36 1.78 0.14 1.65 0.17
MC ×TrT ×FTCs 8 4 0 2.03 0.05 2.25 0.03 1.26 0.28 1.15 0.34
Note: df: degrees of freedom, Q
m
: Langmuir sorption maximum, K
L
: bonding energy constant, MBC: maximum
buffer capacity, K
F
: Freundlich sorption capacity factor, 1/n: Freundlich function, MC represents natural
moisture content, a half of saturated moisture content, and saturated moisture content; TrT represents soil and
biochar-amended soil; FTCs include 1, 3, 6, and 12 FTCs and an unfrozen control.
3.3. Phosphorus Desorption
Desorption of P in soil is a reversible process, which is directly related to adsorbed P re-use and
P bioavailability in soil [
50
]. The desorbability of P in soil can be used to indicate the degree of P
desorption from the adsorptive materials [
51
]. Phosphorus desorbability is mainly affected by soil
moisture content, biochar amendment, and the frequency of FTCs. However, FTCs interacted with
moisture treatment and biochar amendment treatments (Figure 2). The D
avg
values of biochar-amended
soil were always higher than those of untreated soil, which indicated that biochar amendment could
enhance soil P desorption ability. The D
avg
values of MC2 and MC3 were relatively lower than that
of MC1 in all the FTC treatments. The D
avg
values of untreated and biochar-amended soil increased
Sustainability 2018,10, 1574 7 of 10
owing to the increasing frequency of FTCs under each moisture content treatment. After 12 FTCs,
the D
avg
values of saturated moisture content in untreated and biochar-amended soil had significantly
increased from 9.63% to 10.75% and 15.93% to 17.44%, respectively. Our findings suggest that both
untreated soil and biochar-amended soil with higher moisture content are more sensitive to FTCs.
Sustainability 2018, 10, x FOR PEER REVIEW 7 of 10
Table 3. Changes in P adsorption parameters in response to moisture content (MC), biochar
amendment, and freezethaw cycle (FTC) treatment.
Source
df
Langmuir
Freundlich
Qm
(mg·kg−1)
KL (L·mg−1)
MBC (L·kg−1)
KF (L·mg−1)
1/n
F
Sig.
F
Sig.
F
Sig.
F
Sig.
F
Sig.
MC
2
42.62
0
33.09
0
167.12
0
130.91
0
33.75
0
TrT
1
99.97
0
84.65
0
448.50
0
285.28
0
42.93
0
FTCs
4
1.43
0.23
1.38
0.25
0.90
0.47
1.01
0.41
2.17
0.08
MC × TrT
2
4.99
0.01
0.30
0.74
12.23
0
9.49
0
0.03
0.97
MC × FTCs
8
5.50
0
3.14
0.004
3.31
0.002
3.16
0.003
3.61
0.001
TrT × FTCs
4
0.57
0.69
0.85
0.50
1.10
0.36
1.78
0.14
1.65
0.17
MC × TrT × FTCs
8
4
0
2.03
0.05
2.25
0.03
1.26
0.28
1.15
0.34
Note: df: degrees of freedom, Qm: Langmuir sorption maximum, KL: bonding energy constant, MBC:
maximum buffer capacity, KF: Freundlich sorption capacity factor, 1/n: Freundlich function, MC
represents natural moisture content, a half of saturated moisture content, and saturated moisture
content; TrT represents soil and biochar-amended soil; FTCs include 1, 3, 6, and 12 FTCs and an
unfrozen control.
3.3. Phosphorus Desorption
Desorption of P in soil is a reversible process, which is directly related to adsorbed P re-use and
P bioavailability in soil [50]. The desorbability of P in soil can be used to indicate the degree of P
desorption from the adsorptive materials [51]. Phosphorus desorbability is mainly affected by soil
moisture content, biochar amendment, and the frequency of FTCs. However, FTCs interacted with
moisture treatment and biochar amendment treatments (Figure 2). The Davg values of biochar-
amended soil were always higher than those of untreated soil, which indicated that biochar
amendment could enhance soil P desorption ability. The Davg values of MC2 and MC3 were relatively
lower than that of MC1 in all the FTC treatments. The Davg values of untreated and biochar-amended
soil increased owing to the increasing frequency of FTCs under each moisture content treatment.
After 12 FTCs, the Davg values of saturated moisture content in untreated and biochar-amended soil
had significantly increased from 9.63% to 10.75% and 15.93% to 17.44%, respectively. Our findings
suggest that both untreated soil and biochar-amended soil with higher moisture content are more
sensitive to FTCs.
Figure 2. Average desorption ratio among moisture content, biochar amendment, and freezethaw
cycles (FTCs). S and SB represent soil and biochar-amended soil, respectively. Different lowercase
letters in the same FTCs indicate significant differences among different moisture contents within the
same sample at p < 0.05. Different uppercase letters in the same moisture content indicate significant
differences among different FTCs at p < 0.05.
Figure 2.
Average desorption ratio among moisture content, biochar amendment, and freeze–thaw
cycles (FTCs). S and SB represent soil and biochar-amended soil, respectively. Different lowercase
letters in the same FTCs indicate significant differences among different moisture contents within the
same sample at p< 0.05. Different uppercase letters in the same moisture content indicate significant
differences among different FTCs at p< 0.05.
In the present study, the desorbability of P increased after biochar amendment. This result may
be due to the significantly lower bonding energy of adsorption (Table 2). Moreover, FTC treatment
enhanced P desorbability, because the stability of soil aggregates decreased with repeated FTCs,
and such a disturbance could increase the release of soil inorganic and organic substrates [
52
,
53
].
It has been suggested that FTCs damage and lyse microbial cells, leading to the release of potential
dissolved nutrients [
54
]. The destruction of soil aggregates and decomposition of organic matter leads
to increased competition between phosphates and fulvic acids for adsorption sites, thereby increasing
P desorbability [55].
4. Conclusions
The biochar amendment decreased the P adsorption capacity and intensity of black soil.
Freeze-thaw cycles aggravated the release of P in biochar-amended soil by increasing P desorbability.
However, the effects of FTCs on P release are more pronounced at saturated moisture levels in
black soil. Therefore, biochar amendment in arable black soil should not be conducted during FTCs,
particularly during snowmelt. The P fraction of biochar-amended black soil under FTCs should be
further investigated to verify the underlying mechanisms.
Author Contributions:
Y.H. and X.C. conceived and designed the experiments; Y.H. wrote the manuscript;
and B.C. offered many suggestions and revised the manuscript. All authors have read and approved the
final manuscript.
Acknowledgments:
This work was supported by National Natural Science Foundation of China (Nos. 41271293
and 41302222).
Conflicts of Interest: The authors declare no conflict of interest.
Sustainability 2018,10, 1574 8 of 10
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article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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... On the other hand, the mean soil surface temperature significantly affected the migration behaviors of different P forms in mollisol as indicated by the difference in the ratio of the leaching loss of each P form relative to TP leaching loss in the third and fourth periods during which the rainfall was comparable but where the mean soil surface temperature was higher in the fourth period ( Figure 4). The importance of soil surface temperature in regulating soil extracellular enzyme activities, as well as the adsorption and desorption equilibrium on P availability has been reported [51,52]. A model using changes in soil surface temperature within the Everglades also predicts that the mineralization of organic P forms strengthens with temperature [53]. ...
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... Soil phosphorus is one of the important limiting factors of karst plant growth. The temporal and spatial distributions of phosphorus content can not only reflect the soil structure and availability but also affect the growth of wetland vegetation, the formation of the environment, and the process of vegetation succession [11,[17][18][19][20]. Therefore, the above problems can be addressed by studying the change characteristics of soil phosphorus and their inherent correlation with soil nutrients in the Caohai Nature Reserve before and after vegetation restoration. ...
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