Soil characteristics
Some physicochemical characteristics of the soil are given in Table 2. The soil was calcareous in nature with alkaline pH and low content of organic matter. These properties are typical for soils from arid and semi-arid regions of southern Iran (Najafi-Ghiri, 2014). Except for Zn, the content of other micronutrients were in the sufficient range for corn growth. Based on the soil EC value, it was in the category of non-saline soils (Table 2).
Table 2
Some physicochemical characteristics of the soil before cultivation.
Sand (%)
|
58.0
|
Available K (mg kg− 1)
|
251
|
Silt (%)
|
30.0
|
Available P (mg kg− 1)
|
13.0
|
Clay (%)
|
12.0
|
CEC (cmol(+)kg− 1)
|
11.7
|
Soil textural class
|
Sandy loam
|
Fe-DTPA (mg kg− 1)
|
4.64
|
pH(s)
|
7.59
|
Mn-DTPA (mg kg− 1)
|
12.3
|
EC (dS m− 1)
|
2.60
|
Cu-DTPA (mg kg− 1)
|
1.33
|
CCE (%)
|
55.0
|
Zn-DTPA (mg kg− 1)
|
0.64
|
OM (%)
|
0.50
|
Ni-DTPA (mg kg− 1)
|
0.39
|
Notes: EC, electrical conductivity; OM, organic matter; CCE, calcium carbonate equivalent; CEC, cation exchange capacity.
|
Properties of the biochars
The maximum pH and EC values of the biochars were obtained in the SMB treatments, which increased with increasing pyrolysis temperature from 300°C to 500°C (Table 3). This is attributed to the higher content of ash (alkali salts) in the SMB biochars than those for the RHB biochars (Table 3). It is normal for plant-based biochars to contain lower amounts of dissolved solids than animal-based biochars, as confirmed by Boostani et al. (2018a). The concentration of micronutrients (Fe, Mn, Cu and Zn), P and K in the biochars increased with pyrolysis temperature increase, and the highest content was found in the SMB (Table 3). The ash content of biochars is directly related to content of nutrients which are not volatized (Chatterjee et al., 2020)(Table 3). The CEC values of the biochars decreased with increasing pyrolysis temperature, with SMB3 having the highest value (19.70 cmol+ kg− 1) (Table 3). This could be due to the reduction of surface functional groups such as carboxyl and phenol at high pyrolysis temperature, which are the main groups responsible for generating the CEC of biochars (Tomczyk et al., 2020)
With increasing pyrolysis temperature, the C content of the biochars were enhanced while, the H, O and N contents decreased (Table 3). The enhancement in C quantity at higher pyrolysis temperature coincides with the increasing degree of carbonization. While decreasing H and O amount is likely due to dehydration reactions, the decomposition of the oxygenated bonds, and the release of low molecular weight byproducts containing H and O (Zhao et al., 2017). Also, the volatilization of N compounds could be attributed to the decreased N content of the biochars at higher pyrolysis temperatures. The H:C and O:C mole ratios indicate the degree of aromaticity and polarity of the biochars, respectively (Chatterjee et al., 2020). The decreased H:C and O:C mole ratios with increasing the pyrolysis temperature (Table 3) indicates a better degree of carbonization of the biochars (Zhao et al., 2017). The Ni content of the RHB biochars was negligible, but the biochars produced from sheep manure contained a small amount of Ni (Table 3).
Table 3
Some physical and chemical properties of the biochars.
|
SMB3
|
SMB5
|
RHB3
|
RHB5
|
pH (1:20)
|
9.96
|
11.0
|
9.0
|
10.3
|
EC (1:20) (dS m− 1)
|
3.94
|
4.28
|
0.84
|
1.17
|
CEC (cmol+ kg− 1)
|
19.70
|
18.94
|
18.94
|
15.33
|
C (%)
|
25.4
|
31.8
|
45.0
|
50.0
|
H (%)
|
1.85
|
0.8
|
2.28
|
1.06
|
N (%)
|
2.10
|
1.57
|
1.30
|
1.10
|
P (%)
|
0.36
|
0.38
|
0.20
|
0.23
|
K (%)
|
2.36
|
2.47
|
0.81
|
1.02
|
Fe (mg kg− 1)
|
1875
|
2019
|
207
|
358
|
Mn (mg kg− 1)
|
236
|
241
|
105
|
139
|
Cu (mg kg− 1)
|
20.1
|
20.8
|
1.5
|
2.8
|
Zn (mg kg− 1)
|
52.1
|
60.7
|
18.2
|
18.5
|
Ni (mg kg− 1)
|
3.0
|
15.4
|
Nd
|
Nd
|
Na (%)
|
0.73
|
0.77
|
0.062
|
0.069
|
Ca (%)
|
5.8
|
7.5
|
0.21
|
0.25
|
Moisture content (%)
|
1.91
|
1.82
|
2.65
|
2.37
|
Ash content (%)
|
53.8
|
60.0
|
34.2
|
44.8
|
H:C mole ratio
|
0.87
|
0.30
|
0.60
|
0.25
|
O + S:C mole ratio
|
0.44
|
0.09
|
0.24
|
0.01
|
C:N ratio
|
12.1
|
20.2
|
34.6
|
45.4
|
Notes: SMB3, sheep manure biochar generated at 300°C; SMB5, sheep manure biochar generated at 500°C; RHB3, rice husk biochar produced at 300°C; RHB5, rice husk biochar produced at 500°C; CEC, cation exchange capacity; EC, electrical conductivity; Nd, non-detectable.
|
Effects of biochars and Si application rates on the soil EC and pH
The main and interaction effects of treatments on the soil EC and pH values were significant (P < 0.05). Addition of all the biochars caused a significant increase in the soil EC values. In particular, the SMB treatment resulted in EC values above 4 dS m− 1 thus resulting in the treated soils to be classified as saline (Qadir and Schubert, 2002). The minimum enhancement was observed in the RHB3 treatment by 21.1% (Table 4). As shown in Table 3, the EC and ash content of the RHB treatments were considerably lower than those of SMB treatments. Previous studies have also found that biochar enhanced soil EC, significantly (Tasneem and Zahir, 2017; Boostani et al., 2020b). Additionally, in all the biochar treatments, application of Si rates from Si0 to Si2, caused a significant increase in the soil EC, with the greatest enhancement was found in the SMB5 + Si2 treatment by 17.8% (Table 4). The increase in soil EC values as a result of Si application rates is attributed to the high solubility of sodium metasilicate. Furthermore, the application of sodium metasilicate to the control soil (CL) without biochar, resulted in a significant pH increase (Table 4). This is attributed to the dissolution of sodium metasilicate in water resulting in the hydrolysis of metasilicate ions to form monosilic acid, along with the formation of sodium hydroxide (Ma et al. 2021) (Eq. 1).
2Na2SiO3 + 6H2O → 2SiOH4 + 4NaOH (Eq. 1)
The interaction effects of treatments showed that in the absence of sodium metasilicate (Si0), all the biochars except RHB3 significantly increased soil pH (Table 4). Whereas, at the Si1 level, the biochars produced at 300°C caused a significant decrease in the soil pH values, and at the Si2 level, the pH values were significantly decreased by application of all the biochars except SMB5 (Table 4). It is likely that in the Si1 and Si2 treatments that the significant addition of salts to the by the biochars (Table 4) resulted in the displacement of exchangeable hydrogen ions present on the surface of the soil particles, resulting in a measured pH decrease. Furthermore, the magnitude of soil pH increase due metasilicate application in the CL treatment (absence of biochar) was much higher than the biochar treatments (Table 4). This could be due to the increase of soil buffering capacity as a result of biochar addition to the soil. Incorporation of biochar with soil increases the soil CEC and, as a result, increases the soil resistance to soil pH changes.
Table 4
Effects of biochars and Si application rates on the soil EC (dS m− 1) and pH after corn cultivation.
|
CL
|
SMB3
|
SMB5
|
RHB3
|
RHB5
|
|
|
|
EC
|
|
|
Si0
|
2.55 k
|
4.56 c
|
4.25 d
|
3.07 h
|
3.45 g
|
3.57 C
|
Si1
|
2.68 j
|
4.71 b
|
4.76 b
|
3.15 h
|
3.67 ef
|
3.79 B
|
Si2
|
2.85 i
|
5.07 a
|
5.01 a
|
3.56 fg
|
3.79 e
|
4.05 A
|
Mean
|
2.69 E
|
4.78 A
|
4.67 B
|
3.26 D
|
3.63 C
|
|
|
|
pH
|
|
|
Si0
|
7.69 j
|
7.88 i
|
7.96 gh
|
7.73 j
|
7.84 i
|
7.82 C
|
Si1
|
8.07 de
|
7.99 fg
|
8.13 cd
|
7.90 hi
|
8.04 ef
|
8.02 B
|
Si2
|
8.28 a
|
8.17 bc
|
8.22 ab
|
8.05 ef
|
8.16 bc
|
8.18 A
|
Mean
|
8.01 B
|
8.01 B
|
8.10 A
|
7.89 C
|
8.01 B
|
|
Notes: CL, control; SMB3, sheep manure biochar generated at 300°C; SMB5, sheep manure biochar generated at 500°C; RHB3, rice husk biochar produced at 300°C; RHB5, rice husk biochar produced at 500°C; Si0, without Si application; Si1, addition of 250 mg Si kg− 1 soil; Si2, addition of 500 mg Si kg− 1 soil.
|
Shoot dry matter of Zea mays L. as influenced by biochars and Si application rates
The main effects of biochars and Si application rates on the shoot dry matter of Zea mays L. were not statistically significant, while their interaction effects were significant (P < 0.05). The maximum and minimum amount of shoot dry matter was found in the combined treatments of RHB + Si0 (1.65 g pot− 1) and Cl + Si0 (0.6 g pot− 1), respectively (Fig. 1). Application of all the biochar treatments at the Si0 and Si1 rates caused a significant increase in the shoot dry matter compared to the control (with no biochar application) while at the Si2 application level only SMB5 and RHB3 treatments increased it significantly (Fig. 1). Among the biochars, the RHB3 had the lowest values for pH, EC and Na, and the SMB5 had the maximum content of micro-and macronutrients (Table 3). Boostani et al. (2019b) with application of rice husk and licorice root pulp biochars (2.5%wt.) generated at 350 and 550°C in a Ni-contaminated calcareous soil found that the biochars through increasing micronutrients uptake and decreasing Ni absorption by spinach shoots caused a significant increase in shoot dry matter yield. In another study, addition of pine wood biochar at the rate of 20 Mg ha–1 in a sandy loam soil enhanced corn yields by 14.2% compared to the control (Backer et al., 2016). They mentioned a 67% increase in soil organic matter content in connection with the increase in corn growth.
In the control treatment (with no biochar application), the shoot dry matter was significantly increased by application of Si rates from Si0 to Si2 (16.3%) whereas it was significantly decreased in all the biochar treatments (Fig. 1). This could be due to the considerable increase in soil EC of biochar and Si combined treatments (Table 4). Furthermore, there was a significant and negative correlation between soil pH and shoot dry matter yield (r=-0.41, P < 0.01) as affected by application of treatments. Excessive alkalinity can result in a decrease bioavailability of soil micronutrients and P thus decreasing plant growth. Furthermore, excessive soil Na concentrations result in nutrient antagonisms with other basic cations resulting in plant growth suppression (Qadir and Schubert, 2002). In contrast to this study, Babu and Nagabovanalli (2017) reported that the total biomass of rice plant (grain + husk + straw + root) grown in a Cd-contaminated soil was significantly increased by addition of different levels of Si (0, 250 and 500 kg ha− 1) as Ca-silicates (CaSiO3). They noted that it could be due to soil Cd stabilization caused by Ca-silicate-induced soil pH increase.
Soil Ni availability and shoot Ni concentration as affected by biochars and Si application rates
The main effects of treatments and their interactions on the soil Ni-DTPA and shoot Ni-concentration of corn were significant (P < 0.05). Generally, with increasing Si application rates from Si0 to Si2, the soil Ni-DTPA was significantly decreased (14.8%), and as a result, the concentration of Ni in the aerial parts of corn also decreased significantly (32.0%) (Table 5). In addition, application of all the biochar treatments caused a significant decrease in the content of soil Ni-DTPA and corn Ni-shoots (Table 5). The lowest concentrations of soil Ni-DTPA and corn Ni-shoots were associated with the combined treatment of SMB5 + Si2, with their concentrations decreasing by 32.4% and 57.2% compared to the control treatment (with no biochar and Si application), respectively. The SMB5 treatment had the highest pH and ash content among the biochars (Table 3). The Soil Ni-DTPA (r=-0.56, P < 0.01) and corn Ni-shoots (r=-0.60, P < 0.01) had a negative and significant correlation with soil pH. This confirms that one of the main reasons for decreasing the shoot Ni content and soil Ni-DTPA is rising soil pH as influenced by Si and biochar application. Furthermore, in the conditions of this experiment, the formation of insoluble Ni-silicates could decrease soil Ni-DTPA concentration (Sparks et al., 2023). In agreement with our results, Boostani et al. (2020a) found that the content of soil Ni mobility factor (5–15%) and shoot Ni concentration of spinach (54–77%) were significantly decreased by addition of plant residue biochars (licorice root pulp and rice husk biochar each applied at 2.5%) in a Ni-contaminated calcareous soil. They concluded that the increased soil pH as affected by biochars via transforming soil Ni chemical fractions from bioavailable forms (soluble and exchangeable) to fractions with higher stability (residual) caused a significant decrease in soil Ni bioavailability. Additionally, it was hypothesized that the high content of nano-Si in the rice husk biochar helped to immobilize the Ni.
Table 5
Soil Ni availability (Ni-DTPA) (mg Ni kg− 1 soil) and shoot Ni concentration (mg Ni kg− 1 DM) as affected by biochars and Si application rates.
|
CL
|
SMB3
|
SMB5
|
RHB3
|
RHB5
|
|
|
|
Ni-DTPA
|
|
Mean
|
Si0
|
25.6 a
|
24.2 b
|
22.0 d
|
22.3 cd
|
20.4 f
|
22.9 A
|
Si1
|
22.7 c
|
21.2 e
|
17.7 i
|
19.2 g
|
18.6 g-i
|
19.9 B
|
Si2
|
22.3 cd
|
20.6 ef
|
17.3 i
|
18.7 gh
|
18.4 h
|
19.5 B
|
Mean
|
23.5 A
|
22.0 B
|
19.0 D
|
20.1 C
|
19.2 D
|
|
|
|
Shoot Ni concentration
|
|
|
Si0
|
10.4 a
|
7.35 bc
|
9.85 a
|
7.55 bc
|
7.65 b
|
8.56 A
|
Si1
|
7.65 b
|
6.90 bc
|
6.60 cd
|
7.05 bc
|
7.35 bc
|
7.11 B
|
Si2
|
7.20 bc
|
5.05 ef
|
4.45 f
|
5.80 de
|
6.60 cd
|
5.82 C
|
Mean
|
8.41 A
|
6.43 C
|
6.96 BC
|
6.80 BC
|
7.20 B
|
|
Notes: CL, control; SMB3, sheep manure biochar generated at 300°C; SMB5, sheep manure biochar generated at 500°C; RHB3, rice husk biochar produced at 300°C; RHB5, rice husk biochar produced at 500°C; Si0, without Si application; Si1, addition of 250 mg Si kg− 1 soil; Si2, addition of 500 mg Si kg− 1 soil.
|
Uptake of macro- and micronutrients by corn shoots in a Ni-polluted calcareous soil as affected by biochars and Si application rates
The main effects of Si application rates showed that with increasing soil Si levels from Si0 to Si2, the uptake of K, Ca and P by corn shoots were significantly decreased by 34.1%, 23.1% and 18.5%, respectively (Table 6). The decrease in the absorption of Ca and K nutrients as influenced by the increase in the level of Na-silicate application, is likely due to the increase in Na in the soil solution, and the competition of Na with Ca and K for absorption by the corn roots (Qadir and Schubert, 2002). Furthermore, there was a significant and negative correlation between soil pH and P uptake by corn shoots (r=-0.36, P < 0.05). Application of Na-silicate levels caused a significant increase in soil pH values (Table 4). This can enhance the soil P fixation and thus, decreasing its bioavailability for the plant (Qadir and Schubert, 2002). The main effects of biochars indicated that all the biochars caused a significant increase in the uptake of Ca, K and P by corn shoots, so that the highest increase was associated with the RHB3 treatment (Table 6). The RHB3 contained the least ash and soluble salts among the biochars (Table 3), thus it resulted in the smallest increases in soil EC and pH (Table 4). This explains why RHB3 resulted in the greatest increase in shoot dry matter (Fig. 1) and ultimately better absorption of nutrients such as P, K and Ca. The interaction effects of treatments showed that the combined treatment of RHB3 + Si0 had the highest uptake of K (5.19 mg pot− 1), Ca (1.46 mg pot− 1) and P (0.40 mg pot− 1) by the plant shoots (Table 6).
Table 6
Uptake of K, Ca and P (mg pot− 1) by corn shoots in a Ni-polluted soil as affected by biochars and Si application rates.
|
CL
|
SMB3
|
SMB5
|
RHB3
|
RHB5
|
|
|
|
K
|
|
|
Si0
|
2.10 f
|
2.76 e
|
4.39 b
|
5.19 a
|
4.19 b
|
3.72 A
|
Si1
|
2.05 f
|
2.67 e
|
3.16 d
|
3.85 c
|
2.67 e
|
2.88 B
|
Si2
|
1.58 g
|
2.02 f
|
2.80 e
|
3.90 c
|
1.98 f
|
2.45 C
|
Mean
|
1.91 E
|
2.49 D
|
3.45 B
|
4.31 A
|
2.95 C
|
|
|
|
Ca
|
|
|
Si0
|
0.45 g
|
0.68 de
|
1.03 b
|
1.46 a
|
1.10 b
|
0.95 A
|
Si1
|
0.47 g
|
0.85 c
|
0.75 c-e
|
1.45 a
|
1.08 b
|
0.92 A
|
Si2
|
0.49 g
|
0.67 ef
|
0.55 fg
|
1.15 b
|
0.81 cd
|
0.73 B
|
Mean
|
0.47 D
|
0.74 C
|
0.78 C
|
1.36 A
|
1.00 B
|
|
|
|
P
|
|
|
Si0
|
0.15 h
|
0.18 gh
|
0.29 c
|
0.40 a
|
0.32 bc
|
0.27 A
|
Si1
|
0.17 gh
|
0.22 ef
|
0.24 de
|
0.31 bc
|
0.20 fg
|
0.23 B
|
Si2
|
0.17 gh
|
0.17 gh
|
0.26 d
|
0.33 b
|
0.18 gh
|
0.22 B
|
Mean
|
0.17 E
|
0.19 D
|
0.26 B
|
0.35 A
|
0.24 C
|
|
Notes: CL, control; SMB3, sheep manure biochar generated at 300°C; SMB5, sheep manure biochar generated at 500°C; RHB3, rice husk biochar produced at 300°C; RHB5, rice husk biochar produced at 500°C; Si0, without Si application; Si1, addition of 250 mg Si kg− 1 soil; Si2, addition of 500 mg Si kg− 1 soil.
|
The main effects of treatments and their interactions were significant (P < 0.05) on the uptake of Fe, Mn, Cu and Zn by corn shoots. The uptake of Fe (42.8%), Mn (32.6%), Cu (39.8%) and Zn (41.7%) were significantly decreased by application of Si rates from Si0 to Si2 (Table 7). This is attributed to the following two reasons: (1) the enhancement of soil pH as affected by Si application levels subsequently decreasing soil micronutrient bioavailability (Table 4). The uptake of Fe (-0.57**), Mn (-0.58**), Cu (-0.60**) and Zn (-0.49**) uptake by corn shoots were negatively correlated with soil pH. (2) The increase of soil soluble Na concentration resulting in micronutrient antagonisms for plant uptake (Qadir and Schubert, 2002).
Application of all the biochar treatments caused a significant increase in the uptake Fe, Mn, Cu and Zn by corn shoots compared to the control, with RHB3 treatment having the greatest uptake (Table 7). The biochars used in this research contained significant amounts of the micronutrients, Fe, Mn, Cu, and Zn (Table 3), which could increase their absorption by plant roots. Although RHB3 contained the least micronutrients (Table 3), it enhanced micronutrient uptake to the greatest extent likely due to the resultant lower soil pH and EC values (Table 4) as previously discussed. The interaction effects of treatments showed that the combined treatment of RHB3 + Si0 had the highest uptake of Fe (78.1 µg pot− 1), Mn (145 µg pot− 1), Cu (7.83 µg pot− 1) and Zn (30.2 µg pot− 1) by the plant shoots (Table 7). Boostani et al. (2019b) also showed that the application (2.5 wt %) of licorice root pulp and rice husk biochars prepared at 350°C and 550°C, resulted in a significant increase in the uptake of Zn, Cu, Fe and Mn by spinach shoots in a Ni-polluted calcareous soil. The increase of the nutrients absorption by the plant as a result of adding biochars to the soil may be due to the enhancement of plant growth, as well as, enhanced soil microbial activity, or the release of nutrients from the biochars during the growing season (Adejumo et al., 2016).
Table 7
Uptake of Fe, Mn, Cu and Zn (µg pot− 1) by corn shoots in a Ni-polluted soil as affected by biochars and Si application rates.
|
CL
|
SMB3
|
SMB5
|
RHB3
|
RHB5
|
|
|
|
Fe
|
|
|
Si0
|
29.5 f-h
|
36.2 d-f
|
56.5 b
|
78.1 a
|
57.8 b
|
51.6 A
|
Si1
|
34.2 ef
|
34.9 ef
|
40.9 de
|
50.1 bc
|
30.3 f-h
|
38.1 B
|
Si2
|
31.6 fg
|
25.0 gh
|
22.4 h
|
43.7 cd
|
25.1 gh
|
29.5 C
|
|
31.8 C
|
32.1 C
|
39.9 B
|
57.3 A
|
37.7 B
|
|
|
|
Mn
|
|
|
Si0
|
48.6 f
|
65.4 cd
|
72.7 c
|
145 a
|
117 b
|
89.6 A
|
Si1
|
54.9 d-f
|
70.9 c
|
55.1 d-f
|
107 b
|
64.2 c-e
|
70.4 B
|
Si2
|
43.7 f
|
52.7 ef
|
43.6 f
|
108 b
|
53.8 d-f
|
60.4 C
|
|
49.1 E
|
63.0 C
|
57.1 D
|
120 A
|
78.2 B
|
|
|
|
Cu
|
|
|
Si0
|
3.05 e-g
|
4.55 cd
|
6.35 b
|
7.83 a
|
6.47 b
|
5.65 A
|
Si1
|
3.27 e-g
|
3.85 d-f
|
3.43 e-g
|
4.97 c
|
3.99 c-e
|
3.90 B
|
Si2
|
2.91 fg
|
2.77 g
|
3.23 e-g
|
4.81 cd
|
3.25 e-g
|
3.40 C
|
|
3.07 D
|
3.72 C
|
4.33 B
|
5.87 A
|
4.57 B
|
|
|
|
Zn
|
|
|
Si0
|
16.5 b-d
|
17.7 bc
|
30.2 a
|
30.2 a
|
33.7 a
|
25.6 A
|
Si1
|
18.1 bc
|
13.2 c-e
|
21.5 b
|
21.4 b
|
13.5 c-e
|
17.5 B
|
Si2
|
16.3 b-d
|
9.70 e
|
16.2 b-d
|
22.4 b
|
10.1 de
|
14.9 C
|
|
17.0 B
|
13.5 C
|
22.6 A
|
24.6 A
|
19.1 B
|
|
Notes: CL, control; SMB3, sheep manure biochar generated at 300°C; SMB5, sheep manure biochar generated at 500°C; RHB3, rice husk biochar produced at 300°C; RHB5, rice husk biochar produced at 500°C; Si0, without Si application; Si1, addition of 250 mg Si kg− 1 soil; Si2, addition of 500 mg Si kg− 1 soil.
|