1. Introduction
The link between the sharp change in global climate and the gradual increase in greenhouse gases, which is mainly manifested by global warming, is clear at a glance. The Paris Agreement signed by 195 countries in 2016 set the global temperature increase within 2 degrees Celsius as its main target and strives to limit the increase to 1.5 degrees Celsius [
1]. China is a traditional agricultural country, and its farmland soil is an important source of greenhouse gas emissions. Due to the influence of human activities, its annual total greenhouse gas (GHG) emissions have reached 5.1~6.1 Pg CO
2-eq (CO
2 equivalent) (1 Pg = 10
15 g), accounting for 10~12% of the total global greenhouse gas emissions. Therefore, reducing greenhouse gases in farmland soil is of great significance to mitigate global temperature rise [
2].
The organic matter content and basic fertility of most soils in China are relatively low, and straw returning to the field is a common soil fertilization method in the early stage of rural China [
3]. China is a country rich in straw resources [
4] Straw, as an agricultural residue, is rich in nutrients. However, in agricultural systems, the direct application of straw to the soil will increase the content of soil organic carbon, promote microbial decomposition and produce greenhouse gases such as CO
2, CH
4, N
2O, etc. [
5], resulting in the loss of soil nutrients and serious greenhouse effects. To improve the utilization efficiency of straw, crop straw can be converted into biochar. It has been reported that straw biochar can significantly increase the content of soil organic matter and maintain soil quality [
6], and the application of straw-derived biochar was reported to enhance carbon sequestration in paddy soils [
7]. Soil organic carbon mineralization is an important carbon source for CO
2 emissions, and organic carbon fixation is of great significance to the CO
2 sink [
8]. SOC not only provides mineral nutrients for plant growth, but also provides energy for microbial degradation of organic matter. Some studies have shown that biochar can interact with the original soil organic matter after application to the soil, changing the mineralization rate of soil organic matter and thus affecting the release of CO
2 [
9]. Biochar with a developed pore structure and large specific surface area can adsorb and encapsulate soil organic matter to reduce the mineralization of organic matter and carbon emissions [
10], and the adsorption of CO
2 by biochar will also reduce carbon flux with the atmosphere [
11]. This shows that biochar, as a carbon sequestration and emission reduction material, has the potential to be applied to soil over a long period.
Biochar, a carbon-rich solid with a molecular structure that consists of a high number of aromatic rings and porous characteristics, remains carbonized after biomass pyrolysis under anaerobic conditions [
12]. Carbon-rich solids have higher thermal stability and stronger adsorption performance than common organic materials. Biochar is considered a soil remediation material to improve soil structure, maintain soil moisture, sequester carbon, and reduce GHG emissions [
13,
14]. Some studies have shown that the application of biochar to soil improves soil quality, thus increasing crop yield [
15]. The carbon sequestration capacity of biochar prepared from
Miscanthus modified by potassium acetate was 45% higher than that of untreated control [
16]. Compared with the non-application of biochar, the application of biochar resulted in an average increase of 15% and 16% in soil CH
4 and CO
2 emissions, and an average decrease of 38% in N
2O emissions [
17]. The physical and chemical properties of biochar determine the changes in soil properties. Some experimental results show that the yield and characteristics of biochar mainly depend on its pyrolysis temperature and raw materials [
18,
19].
Phosphorus is important for plant metabolic substances such as nucleotides, nucleic acids, and enzymes, as well as for energy transfer. In the soil, phosphorus is a rather immobile element, which bonds with common soil constituents such as calcium (Ca), aluminium (Al), and iron (Fe) [
20]. The utilization of phosphorus is an important mechanism in organic carbon mineralization. The addition of phosphorus will improve soil fertility and enzyme activity, thus promoting carbon mineralization. In highly weathered and humid tropical regions, phosphorus is the most important limiting factor for microbial activity. Some studies have found that phosphorus has a direct impact on microbial activities, and the addition of phosphorus increases the number of microbial communities [
21]. Studies have shown that if the carbon source is effective, the initial rate of microbial respiration will be significantly limited by phosphorus [
22]. Soil enzymes play a key role in the process of soil biochemical reactions, affecting the decomposition and cycling of soil nutrients and soil carbon [
23,
24]. The effect of biochar addition on soil enzyme activity is affected by many factors, including different types of biochar, the interaction between enzymes and biochar, and the soil environment [
24,
25]. A large number of studies that examined the effect of biochar on soil enzyme activity showed that biochar pH, specific surface area, pyrolysis conditions, soil pH, temperature, and microorganisms could significantly change the soil enzyme activity [
26,
27,
28]. There are few reports on the effects of banana straw and cassava straw biochar on soil enzyme activity and C mineralization.
Therefore, the specific purposes of this study are (1) to analyze whether the application of biochar affects soil organic carbon, soil enzyme activity, and soil phosphorus and (2) to explore the mechanism through which biochar produced from banana and cassava straw affects soil carbon mineralization.
2. Materials and Methods
2.1. Soil and Biochar
In April 2019, the serpentine five-point method was used to collect 0~20 cm soil from farmland corn fields in Liuzhou City, Guangxi Zhuang Autonomous Region, China. The organic matter content and basic fertility of the soil are both low. The area belongs to a mid-subtropical monsoon climate with warm and humid conditions throughout the year, an average annual temperature of 17.8 °C, and annual precipitation of 189.9 mm. During soil collection, material on the soil surface was removed first, and then the collected soil was evenly mixed and transported to the laboratory in snakeskin bags for natural air drying. In the process of air drying, plant foliage, root systems, stones, etc., in the soil were removed, and the soil was broken into small clods of approximately 1 cm along the fissures. After the soil samples were air dried and mixed evenly, they were ground through a 60-mesh stainless steel sieve and bagged, and the basic characteristics of the soil were analyzed. As shown in
Table 1, the test soil was acidic.
Using banana straw and cassava straw as raw materials, biochar was prepared by slow pyrolysis in a box-type resistance furnace (model: 4–10 company: Shanghai Dongxing Building Materials Test Equipment Co., Ltd. No.689 Qishen Road, Minhang District, Shanghai) under the condition of 500 °C, 2 h, and oxygen restriction.
Banana straw and cassava straw were obtained from Wulidian Flower and Bird Market in Guilin City, Guangxi. They were naturally dried for two weeks, further dried at 70 °C, and crushed by a crusher through a 60-mesh screen.
2.2. Experimental Design
The experimental design included seven treatments: soil only, soil + 1% banana biochar, soil + 2% banana biochar, soil + 5% banana biochar, soil + 1% cassava biochar, soil + 2% cassava biochar, and soil + 5% cassava biochar, referred to as CK, 1% BSB, 2% BSB, 1% CSB, 2% CSB, and 5% CSB, respectively. All treatments were performed in triplicate.
Test Scheme: A total of 500 g of soil was placed in a 1 L white polyethylene bottle and biochar was added according to the different treatments, i.e., soil only, soil + 1% banana biochar, soil + 2% banana biochar, soil + 5% banana biochar, soil + 1% cassava biochar, soil + 2% cassava biochar, and soil + 5% cassava biochar. After the soil and biochar were mixed evenly, deionized water was added to maintain the field water holding capacity at 40~60%, and water was added by the weighing method. The soil samples were placed in a constant temperature incubator at 25 °C, and the culture period was set at 35 days. Samples were taken and analyzed on the 7th, 14th, 21st, 28th, and 35th days. Another batch of soil samples with the same conditions was set up, with a soil weight of 50 g. A 10 mL beaker filled with a certain concentration of sodium hydroxide solution (0.1 mol·L−1) was placed in a white polyethylene bottle, and carbon dioxide emissions were analyzed on days 1, 3, 5, 7, 10, 15, 20, 25, and 30.
2.3. Soil Chemical Properties and Enzyme Activity Assays and Biochar Characterization by FTIR
Total organic carbon (TOC) in the soil was determined by potassium dichromate oxidation spectrophotometry [
29]. The pH [
30] value was determined by a pH meter at a ratio of soil to water of 1:2.5. Soil was extracted by deionized water, and soluble organic carbon (DOC) [
31] was determined by a TOC analyzer after centrifugation. Available phosphorus was determined by NaHCO
3 extraction-molybdenum antimony colorimetry [
29]. Catalase activity [
32] was determined by potassium permanganate titration. Its calculation formula is as follows:
Urease activity [
32] was determined by indophenol blue colorimetry and was expressed as the amount of urea converted into NH
3-N per gram of soil per hour. Its calculation formula is as follows:
Each treatment was repeated three times, and no substrate (urea) and no soil treatment controls were set up. Soil CO2 emissions were measured by the alkali absorption method. The KBr tableting method was used to characterize the samples by FTIR. FTIR adsorption spectra were recorded from wavelengths 4000 to 400 cm−1, and the resolution was 0.09 cm−1 (Bruker Tensor 27, Ettlingen, Germany). A small amount of biochar samples passed through the 100-mesh sieve were mixed with KBr for FTIR analysis.
2.4. Calculation Method
The formula [
33] for CO
2 emissions is as follows:
where
V0 is the volume of standard hydrochloric acid consumed during blank titration,
V is the volume of standard hydrochloric acid consumed during sample titration,
c is the concentration of standard hydrochloric acid, 0.022 is the molar mass of carbon dioxide (1/2
CO2), M (1/2
CO2) = 0.022 g/mmol, and 22.4/44 is the number of milliliters per gram of CO
2 under standard conditions.
CO2 release rate (mg/kg·d−1) = amount of organic carbon mineralized/; is the culture interval (d).
Cumulative soil CO2 emissions were calculated as the total CO2 emissions from the first day of culture to the day of measurement.
The first-order kinetic equation was applied to fit soil carbon mineralization under different culture conditions:
In the formula, Ct is the cumulative mineralization amount at culture time t (d), C0 is the potential soil carbon mineralization (mg·kg−1); k is the rate constant of soil carbon mineralization, d−1, and t is the culture time, d.
2.5. Statistical Analyses
The average value and standard deviation were calculated using the standard method in Excel 2016. One-way ANOVA was used to study the effects of biochar treatment on soil organic carbon, available phosphorus, and enzyme activities. The Duncan multiple-range test was used for post-test. All statistical tests were carried out using SPSS 24.0, and the mapping was completed using Origin 9.1 mapping software.
3. Results
3.1. FTIR of Biochar
The functional groups of straw biochar were characterized by infrared spectroscopy. As shown in
Figure 1, the two samples had similar functional group structures. Due to the stretching of aliphatic C-H and O-H, their strength decreased, and aromatic C-H formed bands. After pyrolysis, aliphatic O-H peaks disappeared due to dehydration, decomposition, and conversion of functional groups. The peak value of the aromatic C=C skeleton vibration of the CSB sample biochar was more obvious than that of the BSB sample.
3.2. Soil Total Organic C, pH, Available P, Dissolved Organic C
The addition of banana biochar and cassava biochar significantly increased the pH value of the soil (
Table 2), and the pH value increased with the increase in the proportion of biochar. The pH value in response to 1% banana biochar decreased first and then increased with culture time, and the pH value increased by 2.28 units compared with that of CK. The pH value in response to 2% banana biochar increased with culture time and was 4.13 units higher than that of CK. The pH value in response to banana biochar added at 5% increased with culture time and was 5.05 units higher than that of CK. The pH value in response to cassava biochar increased by 1.01 units (1%), by 1.78 units (2%), and by 2.35 units (5%) compared with CK. The pH value of cassava biochar first increased and then decreased. The change in pH in response to banana straw biochar was higher than that in response to cassava straw biochar.
During the whole culture process, the available phosphorus content in the soil treated with different proportions of the two types of straw biochar increased with the increase in the proportion of biochar (
Figure 2), and the available phosphorus content was the highest on the 35th day. From the first week to the end of the culture period, the available phosphorus content in the 1% BSB, 2% BSB, and 5% BSB treatments increased by 40.75~58.07%, 111.22~146.83%, and 346.30~483.31%, respectively, compared with CK (
p < 0.05). In the third week, the increase in the available phosphorus content in the 1% BSB, 2% BSB, and 5% BSB treatments was 58.07%, 146.83%, and 440.98%, respectively. On the 35th day of culture, the increase in available phosphorus in the soil to which banana straw biochar was added was 52.42% (1%), 120.53% (2%), and 346.30% (5%), which indicated that the promotion effect of banana straw biochar on the available phosphorus content decreased slowly but changed slightly, and the available phosphorus content could be increased to a large extent as well as continuously. Compared with other treatments, the available phosphorus content in the soil treated with 5% CSB fluctuated greatly, indicating that the addition of a high proportion of biochar was beneficial for the accumulation and transport of available phosphorus in the soil. Compared with the control, the available phosphorus content in the soil treated with 1% CSB, 2% CSB, and 5% CSB significantly increased by 94.30%, 180.16%, and 528.13%, respectively. The increase in the soil available phosphorus content ranked as follows: 5% BSB > 2% BSB > 1% BSB, 5% CSB > 2% CSB > 1% CSB. The cassava straw biochar had a greater ability than the banana straw biochar to improve the available phosphorus content.
With respect to total soil organic carbon, the total soil organic carbon content under different biochar ratios increased to different degrees during the culture period, i.e., 5% BSB > 2% BSB > 1% BSB > CK, 5% CSB > 2% CSB > 1% CSB > CK. The total soil organic carbon content of the CSB biochar treatment was higher than that of the BSB treatment. Compared with CK, the total soil organic carbon content of the BSB treatment increased by 32.94~57.99% (1%), 71.42~93.15% (2%), and 170.98~220.09% (5%) (p < 0.05). The increase range of 5% BSB was the largest, which indicated that the effect of increasing total organic carbon was greater with a higher proportion of biochar, and each biochar treatment had a stable promotion effect on the total organic carbon content. The total organic carbon content of soil treated with CSB reached the maximum value in the second week of culture; the values in CK, 1% CSB, 2% CSB, and 5% CSB were 2.31, 4.10, 5.54, and 9.03 g·kg−1, respectively. In the third week of culture, the total organic carbon of soil treated with biochar gradually increased to a stable level with the advancement of time, and the ranges were 61.13~77.55%, 114.79~139.91%, and 209.58~300.16%. The difference between low, medium, and high concentrations was extremely significant. Based on the change trend of the total organic carbon content after biochar addition, the promotion effect of the two types of biochar was cassava straw biochar > banana straw biochar.
As shown in
Figure 2, the soil soluble organic carbon showed an opposite trend in response to banana and cassava straw biochar application. In response to banana straw biochar, the content of soil soluble organic carbon decreased first and then increased with an increase in the application ratio, and the soluble organic carbon content was the lowest when 2% BSB was applied. In response to cassava straw biochar, the soil soluble organic carbon content decreased significantly with an increasing application ratio, and the content was the lowest when 5% CSB was applied. Compared with CK, the soluble organic carbon content of 5% BSB increased 1.69 times in the fifth week of culture, from 34.61 to 58.45 mg·kg
−1, and the soluble organic carbon content of 5% CSB decreased 2.28 times, from 34.61 mg·kg
−1 to 15.16 mg·kg
−1. During the culture period, the DOC content in the soil treated with 5% BSB was significantly higher than that of the control and showed a significant upward trend, with an increase range of 14.93~68.92%. During the culture period, the DOC content in the 1% BSB and 2% BSB treatments began to decline after seven days, reaching a low value on day 14, and then increased to a high value on 28 days and 21 days, respectively. The DOC content of the medium and low concentration treatments was significantly lower than that of the control, and the effect of the medium concentration treatment was the greatest. With respect to cassava straw biochar, the DOC content of the 1% CSB, 2% CSB, and 5% CSB treatments fluctuated greatly. Although the DOC content of the 1% CSB treatment was 4.06% higher than that of the CK treatment on the 35th day, on the whole, compared with that of the control treatment, the DOC content of the 1% CSB, 2% CSB, and 5% CSB treatments decreased by 6.66~29.68%, 7.28~52.55%, and 53.45~83.71%, respectively, i.e., the higher the biochar concentration, the lower the DOC content. The response rate of soil to banana straw and cassava straw biochar was very fast, and the soil DOC content decreased greatly in response to the two types of biochar. The degree of decrease of the soil DOC content was 2% BSB > 1% BSB > 5% BSB, 5% CSB > 2% CSB > 1% CSB. The effect of the two types of biochar on the soluble organic carbon content was cassava straw biochar > banana straw biochar.
3.3. Soil Enzyme Activity
As shown in
Figure 3, the soil catalase activity was the highest in response to the 5% straw biochar treatments, followed by the 2% treatments, the 1% treatments, and the CK treatment. There were significant differences among the different BSB treatments in different periods. The catalase activity of the 5% BSB biochar treatment was the highest in the third week of culture, 40.17 mL·g
−1, and the catalase activity of the 2% BSB biochar treatment was the highest in the fifth week of culture, 43.73 mL·g
−1. CK, 1% CSB, 2% CSB, and 5% CSB showed significant differences at different stages. The catalase activity in the 5% CSB and 2% CSB treatments reached the highest level in the third week of culture, 33.30 mL·g
−1 and 27.67 mL·g
−1, respectively. Throughout the whole culture period, the trend of the catalase activity in different treatments was similar.
As shown in
Figure 3, the soil urease activity in the different straw biochar treatments was obviously different. The urease activity ranked as 2% BSB > 1% BSB > CK > 5% BSB with respect to the banana straw biochar, compared with 5% CSB > 2% CSB ≈ 1% CSB ≈ CK with respect to the cassava straw biochar treatment. There was a significant difference in the urease content among the different proportions of banana straw biochar in the first week of culture. In the third week of culture, there was no significant difference in the urease content between the 5% BSB treatment and the CK treatment. Urease activity in the 2% BSB and 1% BSB treatments was significantly higher than that in the CK treatment, i.e., by 134.99% and 108.47%, respectively. In the fourth week of culture, there was no significant difference in the soil urease content between the 1% BSB and 2% BSB treatments. The urease activity in the 2% BSB and 1% BSB treatments was significantly higher than that in CK, i.e., by 109.29% and 118.60%, respectively. There was no significant difference between the CK, 1% CSB, and 2% CSB treatments during the whole culture period. Except for the third week of culture, there was no significant difference between the CK, 1% CSB, and 2% CSB treatments. The urease activity of the 5% CSB treatment increased by 184.33% compared with CK.
3.4. Soil Carbon Mineralization
During the 35-day culture period, the change trend of cumulative soil carbon mineralization in the two different biochar treatments was roughly similar (
Figure 4) and gradually increased over time. The cumulative soil carbon mineralization in the 1% CSB treatment increased slowly after 0–10 days of culture and tended to be stable in the later period. The cumulative soil carbon mineralization in the 2% CSB treatment increased slowly in the first 15 days and tended to be stable in the later period. At the end of the culture period, the cumulative carbon mineralization in the BSB treatment was 11.41~52.57 µg·g
−1 and that in the CSB treatment was 11.41~35.03 µg·g
−1. During the whole culture process, the cumulative carbon mineralization in soil treated with different proportions of biochar was significantly higher than that of CK, and after the fifth week of culture, the cumulative carbon dioxide emissions from soil treated with BSB were in the order of 2% BSB > 1% BSB > 5% BSB. The cumulative carbon dioxide emissions from soil treated with CSB were in the order of 5% CSB > 2% CSB > 1% CSB. Compared with CK, the cumulative mineralization of soil carbon in the 1% BSB, 2% BSB, and 5% BSB treatments increased by 72.63%, 78.40%, and 71.66%, respectively. The cumulative mineralization of soil carbon in the 1% CSB, 2% CSB, and 5% CSB treatments increased by 19.09%, 54.51%, and 71.19%, respectively.
As seen in
Table 3, the first-order kinetic equation accurately simulated the mineralization dynamics of soil organic carbon during the 30-day culture period. In general, the mineralization potential (Cp) of soil treated with different proportions of biochar was obviously different. The range of soil treated with BSB biochar was 12.617~67.918 µg·g
−1 and that of soil treated with CSB biochar was 12.617~42.318 µg·g
−1. It can be seen that the mineralization potential of soil increased with the increase in the proportion of biochar. However, the rate constant (k) of soil carbon mineralization showed an opposite trend. The k value of soil treated with BSB biochar varied from 0.128 to 0.036 d
−1, while that of soil treated with CSB biochar varied from 0.128 to 0.132 d
−1, which is consistent with the trend of the soil carbon mineralization rate in
Figure 4. Compared with CK, the mineralizable potential of the 1% BSB, 2% BSB, 5% BSB, 1% CSB, 2% CSB, and 5% CSB treatments increased by 72.758%, 79.383%, 81.423%, 6.686%, 52.835%, and 70.185%, respectively.
The dynamic trend of the carbon dioxide mineralization rate in soil treated with the two types of straw biochar was basically the same. Overall, the CO2 emission rate of each treatment during the whole culture period can be roughly divided into three stages: the soil CO2 emission rate decreased rapidly until day 5, decreased slowly from 5–10 days, and gradually reached a stable state from 10–30 days, and then the CO2 emission rate of each treatment approached zero.
The carbon sequestration capacity of biochar can be compared by analyzing the ratio of soil CO
2 emissions to total soil organic carbon in a certain period of time [
34]. The higher the ratio, the weaker the carbon sequestration capacity, and vice versa. By comparing the ratio of CO
2 emissions and soil TOC content in the middle stage of culture with different biochar and concentration treatments, the carbon sequestration capacity of each treatment during the whole culture period can be roughly analyzed. As shown in
Table 4, the ratio of the treatments was ranked as follows: 1% BSB > CK > 2% BSB > 2% CSB > 1% CSB > 5% CSB > 5% BSB. The carbon fixation capacity of banana straw biochar was ranked as 5% BSB > 2% BSB > CK > 1% BSB, and the carbon fixation capacity of cassava straw was ranked as 5% CSB > 1% CSB > 2% CSB > CK.
3.5. Correlation Analysis between Soil Organic Carbon, Soil Enzymes, and Soil Physical and Chemical Properties
As shown in
Table 5, in the soil treated with banana straw biochar, available phosphorus was significantly positively correlated with pH, organic carbon, and soluble organic carbon and was significantly negatively correlated with catalase and urease activities. pH was significantly positively correlated with total organic carbon and soluble organic carbon and was significantly negatively correlated with catalase. Catalase had a highly significant negative correlation with organic carbon and a significant negative correlation with soluble organic carbon. Urease was negatively correlated with organic carbon and soluble organic carbon. Organic carbon and soluble organic carbon were significantly positively correlated.
As seen from
Table 6 in the soil with cassava straw biochar, available phosphorus had a very significant positive correlation with pH, urease, and organic carbon and a very significant negative correlation with catalase and soluble organic carbon. pH was positively correlated with urease and organic carbon and negatively correlated with catalase and soluble organic carbon. Catalase had a very significant positive correlation with soluble organic carbon and a very significant negative correlation with urease and organic carbon. Urease had a very significant positive correlation with organic carbon and a very significant negative correlation with soluble organic carbon. Organic carbon and soluble organic carbon were significantly negatively correlated with each other.
As seen from
Table 6, soil catalase and CO
2 emissions were significantly positively correlated in the banana straw biochar treatment, and soil CO
2 emissions under the cassava straw biochar treatment were extremely significantly positively correlated with urease and catalase.