Biochar Applied in Places Where Its Feedstock Was Produced Mitigated More CO₂ Emissions from Acidic Red Soils

Abstract

Agricultural soil is the main source of greenhouse gas emissions, among which carbon dioxide (CO₂) is an important greenhouse gas, impacting the global climate. In China, as a large rice-producing country, carbon sequestration and CO₂ mitigation in paddy soil are crucial for the mitigation of global climate change. While biochar has been widely used in the mitigation of soil greenhouse gas emissions, the application site of biochar, i.e., whether or not it is the same as its feedstocks, may generate different effects on soil CO₂ emissions due to the differences in the element and nutrient concentrations in its feedstocks, especially when applied in fertilized soil. In order to explore the effects of biochar application with different feedstocks on the mitigation of CO₂ emissions from paddy soil, this experiment took paddy soil in a red soil area as the research object, using rice straw and Camellia oleifera fruit shell as raw materials to produce biochar (adding an amount of 40 g kg⁻¹ soil) and urea as an external nitrogen source (adding an amount of 200 mg kg⁻¹ soil). The effects of two different types of biochar derived from feedstocks with different producing origins on the CO₂ emissions from paddy soil were studied via laboratory control incubation studies. The results showed that (1) the effects of rice straw biochar addition on the soil pH, NO₃⁻-N and total available nitrogen (AN) content were significantly higher than those of Camellia oleifera fruit shell biochar (the scale of the increase was higher by 6.40%, 579.7% and 180.1%, respectively). (2) The CO₂ emission rate and cumulative emissions of soil supplemented with rice straw biochar were significantly lower than in that supplemented with Camellia oleifera fruit shell biochar (decreases of 28.0% and 27.5%, respectively). Our findings suggest that the efficiency of emission mitigation of rice straw biochar is better than that of Camellia oleifera fruit shell applied to paddy soil. While future studies considering more types of greenhouse gases will be necessary to expand these findings, this study indicates that biochar prepared from in situ feedstock can be used to reduce greenhouse gas emissions in rice fields, so as to ensure sustainable development and achieve energy conservation and emission reduction goals. This study will benefit future agricultural practices when choosing biochar as a greenhouse gas mitigation strategy in the context of global warming, as well as other global changes following global warming, caused by elevated atmospheric greenhouse gases.

1. Introduction

As one of the most important greenhouse gases, increased atmospheric carbon dioxide (CO₂) is the primary driver of global warming [1]. Human activities, including the combustion of fossil fuels and changes in land utilization, are the primary contributors to the escalating levels of greenhouse gas emissions [2,3]. Soil respiration significantly contributes to CO₂ emissions in the atmosphere, surpassing those from fossil fuel combustion and deforestation combined [4,5]. Based on a report by the World Health Organization in 2020, the concentration of atmospheric CO₂ had increased to 410.5 ppm by the year 2019, while other types of greenhouse gases are also increasing rapidly. Among all sources emitting greenhouse gases, agricultural soil, in particular, is significant [4]. Statistics indicate that 5% to 20% of the annual atmospheric CO₂ originates from agricultural soil [6]. Nevertheless, the persistent elevation of the atmospheric CO₂ levels leads to adverse consequences such as rising sea levels, soil carbon depletion, extreme weather events, and various natural disasters, posing significant threats to both human survival and sustainable development. As the largest rice-producing nation globally, China’s excessive nitrogen application in rice cultivation exacerbates soil acidification and greenhouse gas emissions, potentially intensifying the global climate change trend. Consequently, reducing the soil CO₂ emissions from fertilizer application in rice fields is of the utmost importance in mitigating global warming [7].

In recent years, there has been a widespread increase in the utilization of biochar in the mitigation of greenhouse gas emissions, especially in agricultural fertilized soil with intense anthropogenic activity [8,9], which has been widely recognized as a major greenhouse gas emissions source [8,10]. This phenomenon can be attributed to its notable advantages, such as its large surface area, high carbon content, and good porosity [11]. Additionally, it is recognized as a novel measure for the sequestering of carbon and in reducing emissions in agricultural soil. Biochar, a solid substance formed through the high-temperature (250–700 °C) pyrolysis of biomass under anaerobic conditions, is highly aromatic and insoluble [12]. Its diverse sources include tree branches and leaves, fruit shells, crop straws, rice husks, and livestock and poultry manure [13]. The initial comprehension of biochar’s role in carbon sequestration and emissions reduction can be traced back to the black soil in the Amazon Basin. Biochar can effectively be stored in the soil for extended periods, thus influencing the carbon content in the soil and playing a crucial role in mitigating climate change [14]. Studies have indicated that biochar can modify several physicochemical properties of soil, such as water retention, permeability, the adsorption capacity, and the pH value [15,16]. Furthermore, it can impact the activity of soil microorganisms and enzymes, consequently influencing soil CO₂ emissions [9]. Liu et al. [17] discovered that the application of rice straw biochar could effectively decrease soil CO₂ emissions, while Jiang et al. [18] found that the addition of wheat straw biochar significantly promoted soil CO₂ emissions, which increased with the quantity of biochar added. Similarly, Sun et al. [19] observed that the application of wheat straw, willow branches, and coconut shell biochar all resulted in increased soil CO₂ emissions, with the promotion of willow branch biochar showing the most substantial effect. These variations may be attributed to the different structures of biochar derived from varying raw materials, thereby leading to significant differences in their impacts on soil CO₂ emissions. China, a major rice producer with abundant straw resources that are rich in nutrients, faces the challenge of environmental pollution and increased greenhouse gas emissions resulting from burning or incorporating straw directly into the field [20,21]. Considering the potential effects of decomposed plant residue on soil CO₂ emissions and hence greenhouse gas emissions [22], carbonizing rice straw and returning it to the fields not only utilizes local resources to their maximum potential but also offers advantages such as low degradation rates and high stability in paddy soil, leading to a reduction in greenhouse gas emissions in rice fields [23,24].

Additionally, Camellia oleifera (Camellia oleifera Abel), one of the world’s four largest woody oil crops, has a long cultivation history and a wide distribution in China’s subtropical regions [25]. The main by-product of camellia oil production, C. oleifera fruit shell, is often discarded due to its low profitability, leading to adverse environmental effects [26]. However, C. oleifera fruit shells are rich in carbon and nitrogen elements, making them ideal for biochar production [27]. Utilizing C. oleifera fruit shells in the production of biochar not only aids in waste management but also holds the potential to serve as an optimal amendment to reduce soil CO₂ emissions. While numerous studies have been conducted on biochar and greenhouse gas emissions reduction, the mechanisms through which different types of biochar derived from in situ and non-in situ materials impact soil CO₂ emissions in fertilized rice fields remain unclear and have not yet been reported. The feedstock used for biochar production could impact the physicochemical characteristics of the biochar via the different element content, the chemicals produced during the growth period, etc. [10]. When applied in the same ecosystem as its feedstock, there will be more original nutrients taken up by plants and returned back to the soil, further supplying the necessary nutrients. This is different from biochar derived from feedstocks in different ecosystems, thereby differently impacting the soil microbial process and hence the soil CO₂ emissions. To address this gap, this study focused on paddy soil as a test subject and employed rice straw and camellia oil shells as raw materials for biochar production. The study aimed to explore the effects of biochar with different feedstock origins on the CO₂ emissions in fertilized rice fields, thus providing a scientific foundation for both sustainable development and the mitigation of global climate change.

2. Materials and Methods

2.1. Experimental Materials

The tested soil was taken from a typical paddy field in the red soil area, located at the experimental station of Jiangxi Agricultural University (115°55′ E, 28°46′ N). The local climate can be categorized as subtropical humid monsoon, with an average annual temperature of 19.4 °C. The average annual precipitation is about 1665 mm, with uneven distribution throughout the year, and most rain occurs from April to June. The topsoil (0–20 cm) was collected with stones and organic residues, such as roots, removed, and was stored in a cold box for transportation to the laboratory. Subsamples of the fresh soil collected were taken for the determination of ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃⁻-N), available nitrogen (AN, the sum of NH₄⁺-N and NO₃⁻-N), soluble organic carbon (DOC), and soluble organic nitrogen (DON), while the remaining soil samples were air-dried, ground to pass through a 2 mm sieve, and stored in a refrigerator at 4 °C for further use. The soil and biochar pH were determined by the electrode method (soil to water ratio of 1:2.5 and biochar to water ratio of 1:5). Both NO₃^--N and NH₄⁺-N were extracted using a shaking method with a 2 mol L⁻¹ KCl solution and analyzed using an automatic chemical analyzer [10]. The dissolved organic C and DON of the biochar and soil were measured after extraction by deionized water (multi N/C 3100, Jena, Germany) [23]. The total nitrogen (TN) content of the soil and biochar was measured by an automatic intermittent chemical analyzer (Smartchem 200, Rome, Italy) [22]. The basic physicochemical characteristics of the soil were as follows: pH value of 5.85, NH₄⁺-N content of 28.8 mg kg⁻¹, NO₃⁻-N content of 12.4 mg kg⁻¹, DOC content of 24.5 mg kg⁻¹, DON content of 572 mg kg⁻¹, and TN content of 2.35 g kg⁻¹. The biochar used in this study was produced by slow pyrolysis under anaerobic conditions at 450 °C using camellia oil shell and rice straw waste in a muffle furnace (Shanghai Yizhong Electric Furnace Co., Ltd., Shanghai, China). The pyrolysis temperature was programmed to be maintained at 450 °C for 1 h, with a heating rate of approximately 15 °C min⁻¹. The characteristics of the C. oleifera fruit shell biochar were as follows: pH 9.49, NH₄⁺-N 2.15 mg kg⁻¹, NO₃⁻-N 2.52 mg kg⁻¹, DOC 753 mg kg⁻¹, DON 14.5 mg kg⁻¹, and TN 5.13 g kg⁻¹. The characteristics of the rice straw biochar were as follows: pH 10.1, NH₄⁺-N 1.76 mg kg⁻¹, NO₃⁻-N 1.66 mg kg⁻¹, DOC 493 mg kg⁻¹, DON 7.23 mg kg⁻¹, and TN 10.6 g kg⁻¹.

2.2. Experimental Design

The experiment was conducted with two levels of nitrogen application: N1, which involved the application of nitrogen at a rate of 200 mg kg⁻¹ soil [24,28], and N0, which did not involve any nitrogen application. Additionally, two levels of biochar were included: rice straw biochar (RB) and C. oleifera fruit shell biochar (FB), applied at a rate of 40 g kg⁻¹ soil [23]. The experimental design comprised four treatments and four replications, namely N0RB (no nitrogen + rice straw biochar), N0FB (no nitrogen + camellia oil shell biochar), N1RB (nitrogen + rice straw biochar), and N1FB (nitrogen + C. oleifera fruit shell biochar).

The air-dried soil was carefully transferred into 150 mL conical flasks, allotting 25 g of soil per flask. Deionized water was added accordingly to attain soil moisture content equivalent to 120% of the water holding capacity (WHC). The conical flasks were sealed with cling film and equipped with small openings to sustain aerobic conditions. Following a 2-day pre-incubation period, the flasks were positioned in a dark environment, maintaining a constant temperature of 25 °C for 45 days. In addition, four sets of samples were prepared under identical conditions to assess the soil pH, NO₃-N, and NH₄⁺-N levels on days 3, 10, 14, and 45. The daily measurement of the total mass of the conical flasks was conducted to ensure consistent soil moisture content throughout the entire study period [24,28].

2.3. Determination Methods

Before sampling, the conical flask was internally ventilated and sealed with a rubber stopper. Fresh air, 40 mL in volume, was introduced into the flask by using a plastic syringe, and thorough mixing ensued. Subsequently, an equivalent volume of gas was collected from the conical flask, which was then placed back in the incubator for a 2 h cultivation period prior to the second sampling [22,24]. The concentration of CO₂ in the gas samples was analyzed using a gas chromatograph (Agilent 7890B, CA, USA).

The formula for the calculation of the soil CO₂ emission rate is as follows:

$$F=P\times V\times {\displaystyle \frac{\mathsf{\Delta }c}{\mathsf{\Delta }t}}\times {\displaystyle \frac{1}{RT}}\times M\times {\displaystyle \frac{1}{m}}$$

(1)

where F represents the soil CO₂ emission rate (mg kg⁻¹ h⁻¹); P and V correspond to standard atmospheric pressure (Pa) and the empty volume of the conical flask (cm³); Δc/Δt denotes the change in the CO₂ concentration per unit time; R represents the universal gas constant; T signifies the absolute air temperature (K); M stands for the molecular mass of soil CO₂ (g mol⁻¹); m denotes the dry weight of the soil sample (g).

The formula for the calculation of the cumulative soil CO₂ emissions is as follows [27]:

$$M=\sum {\displaystyle \frac{(F_i+F_{i+1})}{2}}\times \left(t_{i+1}-t_i\right)\times 24$$

(2)

In this equation, M represents the cumulative soil CO₂ emissions (mg kg⁻¹); F represents the CO₂ emission rate (mg kg⁻¹ h⁻¹); i represents the i-th gas collection; t_i+1-t_i represents the number of days between two consecutive gas collections.

2.4. Data Analysis

Two-way analysis of variance (ANOVA) with nitrogen and biochar addition as treatments was performed to examine the impacts of nitrogen addition, biochar with different feedstocks, and their interactions on the soil pH, nitrogen content, NO₃⁻-N, NH₄⁺-N, available nitrogen, and CO₂ emission rates or cumulative CO₂ emissions. Tukey’s honestly significant differences test was used to compare the means between the biochar treatments with different feedstocks. Statistical analyses were performed using JMP 9.0.

3. Results

3.1. Effects of Different Treatments on Soil Physicochemical Properties

Nitrogen addition significantly impacted the soil NO₃⁻-N, NH₄⁺-N, and AN (p < 0.05). Similarly, biochar addition exhibited a significant effect on the soil pH, NO₃⁻-N, and AN (p < 0.05). However, the interaction between nitrogen addition and biochar did not influence changes in the soil pH, NO₃⁻-N, NH₄⁺-N, and AN (p > 0.05). The influence of RB on the soil pH, soil NO₃⁻-N, and AN content was noticeably higher than that of FB. Without nitrogen application, the soil pH in the N0RB treatment was 0.46 higher than that in the N0FB treatment, and the soil NO₃⁻-N and AN in the N0RB treatment were 579.7% and 180.1% higher than those in the N0FB treatment, respectively. Under nitrogen application, the increases were 0.49%, 34.0%, and 13.7%, respectively (Figure 1).

3.2. Effects of Different Treatments on Soil Cumulative CO₂ Emissions and CO₂ Emission Rate

Nitrogen addition and biochar addition showed a significant impact on the soil cumulative CO₂ emissions (p < 0.05) and emission rate (p < 0.05). However, the interaction between nitrogen addition and biochar did not affect the soil cumulative CO₂ emissions and emission rate (p > 0.05). The influence of N0RB on the soil cumulative CO₂ emissions and emission rate was notably lower than that of N0FB, resulting in reductions of 27.5% and 28.0%, respectively. For N1RB and N1FB, these reductions were 17.6% and 32.3%, respectively (Figure 2). Notably, the cumulative CO₂ emissions from fertilized soil with the N1FB treatment amounted to 627.58 mg kg⁻¹, while that with the N1RB treatment was 517.20 mg kg⁻¹ (Figure 2). Comparatively, the cumulative CO₂ emissions from unfertilized soil with the N0FB treatment amounted to 514.17 mg kg⁻¹, while that with the N0RB treatment was 373.23 mg kg⁻¹ (Figure 2). The initial cultivation period exhibited the highest CO₂ emission rate, gradually decreasing and stabilizing thereafter (Figure 3).

4. Discussion

4.1. Effects of Different Biochar Additions on Soil CO₂ Emissions and Soil Physicochemical Properties in Paddy Field

Different types of biochar have different structural properties, which, therefore, have different impacts on the physicochemical properties of soil and consequently have different effects on soil CO₂ emissions [29,30]. In this experiment, the soil CO₂ emissions were significantly higher when applying FB than RB. This may be due to the higher DOC content in FB, which provides a more abundant carbon source for soil microbes, resulting in more CO₂ emissions. Additionally, soil CO₂ emissions are not only influenced by DOC, but also related to the soil pH [31]. Research has shown that biochar contains abundant alkaline groups and metal cations (such as K⁺, Ca²⁺, and Mg²⁺), which can neutralize acidic groups in the soil, improve the soil base saturation, and consequently increase the soil pH. According to the results of this experiment, the soil pH was significantly higher when adding RB compared to adding FB. This may be because RB contains more alkaline groups. However, despite the increase in the soil pH, the CO₂ emissions were higher when adding FB compared to adding RB. This may be because the increase in the soil pH decreases the microbial activity and respiration, leading to a decrease in soil CO₂ emissions.

Biochar possesses the ability to alter the soil aeration conditions and contains a large amount of organic carbon, leading to improved soil porosity upon application [30]. This facilitates increased oxygen availability for nitrobacteria, serving as a valuable carbon source. Consequently, nitrification is significantly enhanced, resulting in the elevated production of NO₃⁻-N. Moreover, biochar can adsorb NO₃⁻ through both chemical and physical processes, mitigating the loss of NO₃- in the soil and augmenting the levels of soil NO₃⁻-N and AN. Within the scope of this experiment, significant disparities were observed in the levels of soil NO₃⁻-N and AN between the RB and FB treatments. This discrepancy may be attributed to the higher presence of ketone and furanone substances on the surface of RB or its improved porosity, thereby endowing it with a superior adsorption capacity for NO₃⁻. Furthermore, the heightened soil pH resulting from the RB treatment potentially hindered the enzymatic activity involved in the conversion of NO₃⁻ and NO₂⁻ to N₂O, ultimately leading to increased levels of NO₃⁻-N in the RB-amended soil. Nevertheless, it is important to consider that the preparation of biochar may result in minimal CO₂ emissions, necessitating further investigation in future studies [8].

4.2. Effects of Interaction between Biochar and Nitrogen Fertilizer on Soil CO₂ Emissions in Paddy Field

Soil respiration primarily includes the chemical oxidation of carbon-containing substances and the respiratory activity of soil animals, plant roots, and soil microorganisms. Soil respiration is closely related to the emission of CO₂ from the soil. Nitrogen addition in this experiment significantly promoted the emission of soil CO₂. Since this experiment was conducted in an indoor static cultivation environment, visible plant roots and soil animals were eliminated before the experiment, so the main source of soil CO₂ was the respiratory activity of soil microorganisms. The growth and reproduction of microorganisms in the soil are limited by carbon and nitrogen sources [32]. Nitrogen addition alleviated the nitrogen limitation of microorganisms to some extent and enhanced the microbial activity. At the same time, nitrogen input increased the content and altered the ratio of organic and inorganic nitrogen in the soil. Studies have shown that the addition of inorganic nitrogen reduces heterotrophic respiration, but, as the proportion of organic nitrogen increases, it promotes microbial respiration [33]. On the other hand, the transfer of carbon pool components in the soil largely determines the emission rate of soil CO₂.

The addition of organic nitrogen can potentially facilitate the transfer from stable carbon pools to active carbon pools, while also increasing the content of organic nitrogen in the soil, which may, to some extent, alleviate the microbial limitation on carbon degradation [34] and enhance the transformation of carbon elements in the soil into gaseous carbon. Therefore, nitrogen addition in this experiment may have increased the proportion of organic nitrogen in the soil, alleviating the nitrogen limitation of microorganisms, increasing the microbial activity, promoting soil microbial respiration, and consequently increasing the total soil respiration, leading to an increase in CO₂ emissions [12]. At the global scale, however, the addition of nitrogen not only impacts soil CO₂ emissions via changes in the transfer from stable carbon pools to active carbon pools, but also regulates the temperature sensitivity of soil CO₂ emissions [7]. This was not included in this study but needs to be considered to obtain the overall effects of nitrogen addition on soil CO₂ emissions and hence to evaluate the mitigation capacity of agricultural soil CO₂ emissions.

It should be noted that the transformation of urea and the nitrogen addition rate in the soil potentially impact soil CO₂ emissions [7,24], which may generate different results if other types of nitrogen sources are used in future studies. In addition, in most agricultural practices, organic fertilizer is widely used, which will introduce nitrogen into soil ecosystems with various types. Consequently, most of the nitrogen transformations will occur in the same place simultaneously, causing complicated effects on soil respiration via which CO₂ will be emitted. Thereby, to draw a full picture of the interactive effects of nitrogen and biochar on soil CO₂ emissions and hence the soil carbon sink capacity, more studies considering other types of nitrogen sources with different rates [7] and prolonged experimental periods will be necessary, so as to expand this study’s findings to other types of ecosystems. Moreover, this study considered only one type of greenhouse gas, and other types, including nitric oxide (NO), nitrous oxide (N₂O), and methane (CH₄), still need more work to evaluate the response; based on this, the emission budgets and mitigation capacities of greenhouse gases in paddy soil ecosystems can be obtained.

5. Conclusions

Various types of biochar exert different impacts on the enhancement of paddy field soil. The incorporation of rice straw biochar leads to a significant elevation in the soil pH from 5.85 to 7.70, as well as notable increases in soil NO₃⁻-N from 12.4 mg kg⁻¹ to 16.9 mg kg⁻¹ when compared to unfertilized soil. Additionally, the cumulative CO₂ emissions from soil with the N1FB treatment amounted to 627.58 mg kg⁻¹, while that with the N1RB treatment was 517.20 mg kg⁻¹. The cumulative CO₂ emissions from soil with the N0FB treatment amounted to 514.17 mg kg⁻¹, while that with the N0RB treatment was 373.23 mg kg⁻¹, both showing substantial mitigation effects due to the RB addition treatments. Conversely, the application of nitrogen fertilizer contributes to an increase in CO₂ emissions from the soil; however, more studies are needed considering other types of nitrogen with longer study periods. To summarize, the utilization of biochar derived from locally sourced materials proves effective in mitigating CO₂ emissions in paddy field soil, although more work will be necessary to expand these findings to different types of agricultural ecosystems.