Effects of Biochar on Soil Organic Carbon Mineralization in Citrus Orchards

Abstract

The primary ecological challenges in citrus orchards include soil acidification, nutrient depletion, and significant carbon dioxide emissions resulting from conventional cultivation practices. To address these challenges, citrus peel residues and cassava stalks underwent pyrolysis at 500 °C to generate biochars. Different proportions of these biochars (1%, 2%, and 4%) were applied under controlled laboratory conditions to assess their impact on the mineralization of soil organic carbon in citrus orchards. The results indicated that both types of biochar effectively regulated the soil pH to approximately 5.5. Significantly, the addition of 4% cassava stalk biochar significantly increased the levels of available phosphorus and potassium. The phosphorus levels rose by 512.55%, and the potassium levels surged by 1434.01%. Additionally, the soil organic carbon increased to 16.7 g/kg. Conversely, the citrus peel biochar decreased the availability of phosphorus but resulted in the highest increase in available potassium, at 1523.75%, and elevated the soil organic carbon content to 13 g/kg. Both types of biochar enhanced the soil organic carbon mineralization rate to varying extents with increasing application ratios, simultaneously boosting the cumulative amount of organic carbon mineralized. Among the treatments, cassava stalk biochar displayed the lowest C₀/SOC ratio, of 0.169, indicating its superior carbon retention capacity. Furthermore, cassava stalk biochar showed inhibitory effects on soil catalase and urease activities within the citrus orchard. Overall, the application of 4% cassava stalk biochar appears to be more beneficial for nutrient regulation and carbon sequestration in citrus orchard soils, while also contributing to the reduction in soil acidification by adjusting pH levels.

1. Introduction

The accelerating rise in global greenhouse gas emissions, particularly carbon dioxide (CO₂), is a primary driver of global climate change. Since the onset of the Industrial Revolution, atmospheric CO₂ concentrations have surged from 278 µmol·mol⁻¹ to 410 µmol·mol⁻¹, with future projections of levels of 537–670 µmol·mol⁻¹ by the end of this century [1]. Soil respiration, driven by the microbial decomposition of organic matter, results in CO₂ emission, a process accelerated by tilling. Converting natural ecosystems to agricultural land reduces carbon storage, as the disturbance of soil and vegetation leads to the release of stored carbon. These processes render agricultural soils significant sources of CO₂ emissions [2,3]. Soil organic carbon (SOC) serves not only as a crucial indicator of soil fertility but also as a key component in the global carbon cycle [4]. The sequestration of SOC is not only crucial for the removal of CO₂ from the atmosphere but also positively impacts soil health and enhances the functioning of ecosystems [5]. Natural SOC influences microbial growth and diversity by affecting the chemical, physical, and biological properties of soil [6]. Understanding SOC mineralization processes is essential for elucidating CO₂ production dynamics and its exchange between soil and the atmosphere [7], which is crucial for managing the global carbon balance.

As of 2021, the area dedicated to citrus orchards in the Guangxi Zhuang Autonomous Region had expanded to 613,000 hectares, making it one of the largest citrus-producing areas in China [8]. However, the extensive cultivation of citrus in the Guangxi region has intensified soil acidification, depleted nutrients, and reduced organic carbon content. Citrus trees flourish in soils with a pH around 5.5, and adequate levels of phosphorus and potassium are crucial for their growth [9]. Agricultural residues in the Southwestern region, including citrus peel and cassava stalks, present significant potential as raw materials. The consumption and processing of citrus produce large volumes of citrus peel residues, which are difficult to utilize and can result in environmental pollution and waste. Cassava is a major industry in Guangxi, responsible for 70% of the nation’s cassava cultivation and processing. However, the improper disposal and burning of cassava stalks after harvest lead to severe environmental pollution.

Biochar, characterized by a high specific surface area and remarkable stability, serves as a highly effective soil amendment with exceptional physicochemical properties, capable of enhancing soil fertility and significantly boosting soil carbon sequestration [10,11]. A wealth of studies has demonstrated that the application of biochar to soils can substantially reduce greenhouse gas emissions [12]. Additionally, biochar exhibits a beneficial influence on soil moisture retention as well as the stabilization of soil nitrogen and phosphorus levels [13,14,15]. Laird et al. [16] discovered that biochar can enhance soil quality by increasing the soil’s cation exchange capacity and pH level. Yang et al. [17] compared the effects of straw and biochar amendments on soil phosphorus availability and phosphorus fractions, revealing that, when compared to straw, biochar amendments significantly increased the levels of both available and inorganic phosphorus in the soil. Furthermore, the application of biochar to soil offers essential nutrients and habitats for microorganisms, thereby directly or indirectly influencing the functionality and composition of microbial communities [18]. As a carbon-rich material, biochar can be incorporated into soil to elevate its organic carbon content and boost its carbon storage capacity [19,20]. It is well acknowledged that incorporating biochar into soil exerts both positive and negative priming effects on the soil’s organic carbon content [21]. Regarding positive priming effects, Maestrini et al. [22] observed a 15% increase in soil organic carbon mineralization one year following biochar incorporation. The negative priming effect refers to the suppression of natural soil organic carbon decomposition subsequent to biochar incorporation. For instance, Liu Yuxue et al. [23] reported that the application of 5% biochar to soil led to a reduction in cumulative mineralization of soil organic carbon by 7.95%, to 10.7%. Biochar adsorbs SOC, thereby inhibiting microbial access to organic carbon within its pores, reducing its availability, and subsequently decreasing soil carbon mineralization [24]. The comparative study of the impact of cassava straw biochar and citrus peel biochar on citrus orchard soil is an under-researched area. This study hypothesized that applying citrus peel biochar or cassava stalk biochar to citrus orchard soil would increase SOC content, enhance cumulative carbon mineralization, and accelerate the rate of mineralization. To test this hypothesis, incubation experiments were conducted to evaluate the effects of these biochars on SOC content, enzyme activity, and mineralization in citrus soils.

2. Materials and Methods

2.1. Subject

Soil samples were taken from the 0–30 cm topsoil layer at the core demonstration site for citrus cultivation along the Yulong River, located in Yangshuo County, Guilin City, Guangxi Zhuang Autonomous Region (24°48′17″ N, 110°22′16″ E). The region is characterized by a subtropical monsoon climate, with an average annual temperature of 19.1 °C and an annual rainfall of 1887.6 mm.

During soil sampling, surface weeds and other debris were cleared, and the soil was thoroughly mixed using the quartering method. The mixed soil was subsequently placed in sealed containers for transportation to the laboratory. In the laboratory, plant and animal residues, along with gravel, were meticulously removed from the soil samples. The samples were air-dried and stored in a controlled dry environment for further experimentation. The basic physicochemical characteristics of the soil samples are presented in the

Table 1. Basic physical and chemical properties of tested soil.

pH	Electrical Conductivity (EC) (S·m−1)	Available Phosphorous (AP) (mg·kg−1)	Available Potassium (AK) (mg·kg−1)	Soil Organic Carbon (SOC) (g·kg−1)
4.36	53.8	27.125	38.235	6.78

below.

The citrus peel residue was collected from the same citrus orchard as the soil samples, whereas the cassava straw was sourced from a cassava plantation in Nanning City. Following natural drying, the materials were further dried at 70 °C, and then ground using a grinder and sieved through a 60-mesh screen. Biochar was subsequently produced from these raw materials via slow pyrolysis in a muffle furnace at 500 °C for 2 h under anoxic conditions. The biochar derived from citrus peel residue is termed GBC (citrus peel biochar), while the biochar from cassava straw is termed CBC (cassava straw biochar).

2.2. Experimental Design

The incubation experiment was structured with a total of 21 treatments, encompassing 4 different biochar application rates, each replicated 3 times. Based on the previous work of our research group, the following additions were made to the biochar [25]: Biochar was uniformly mixed into the 0–20 cm soil layer at three concentrations, of 1%, 2%, and 4% (w/w), with detailed data presented in

Table 2. Experimental design.

Treatment	Imposer	Apply Proportion (%)
CK	unadded	0
G1%	Citrus peel biochar	1
G2%		2
G4%		4
C1%	Cassava straw biochar	1
C2%		2
C4%		4

G1% (soil + 1% citrus peel biochar). G2% (soil +2% citrus peel biochar). G4% (soil + 4% citrus peel biochar). C1% (soil + 1% cassava straw biochar). C2% (soil + 2% cassava straw biochar). C4% (soil + 4% cassava straw biochar).

.

Soil Incubation Experiment: A total of 500 g of soil was placed into 1 L white polyethylene bottles. Varying proportions of citrus peel biochar and cassava straw biochar were added according to the respective treatment protocols. After thoroughly mixing the soil with the biochar, deionized water was added to maintain a field capacity of approximately 40%. The samples were then incubated at a constant temperature of 25 °C for 35 days. Samples were collected and analyzed on days 1, 3, 5, 7, 14, 21, 28, and 35.

Mineralization Experiment: An additional set of soil samples, each weighing 50 g, was prepared under identical treatment conditions. A 10 mL beaker containing 0.1 mol·L⁻¹ sodium hydroxide solution was placed inside the polyethylene bottles to capture carbon dioxide emissions, which were subsequently analyzed on days 1, 3, 5, 7, 14, 21, 28, and 35.

2.3. Assay Method

Infrared spectral structure in samples was tested by Fourier Transform Infrared Spectroscopy FTIR (Nicolet I S50, Thermo Fisher Scientific, Waltham, MA, USA) in the spectral range of 4000–400 cm⁻¹ using the potassium bromide press method. The soil pH and electrical conductivity (EC) were measured using a pH meter and a conductivity meter, with a soil-to-water ratio of 1:2.5 [26]. Available phosphorus was quantified using the sodium bicarbonate extraction–molybdenum antimony anti-spectrophotometric method [27]. Available potassium was extracted with 1 mol/L ammonium acetate and analyzed by flame photometry (AA-6300, Shimadzu, Kyoto, Japan) [28]. Soil organic carbon (SOC) was quantified through the potassium dichromate oxidation–spectrophotometric method [29]. The soil dissolved organic carbon (DOC) content was measured using a total organic carbon (Multi N/C 3100, Analytik Jena AG, Jena, Germany) analyzer [30]. Soil readily oxidizable organic carbon (ROC) was determined by oxidation with 333 mmol/L potassium permanganate solution [31]. Soil catalase activity was measured by titration with potassium permanganate, and soil urease activity was determined using the indophenol blue colorimetric method [32]. Soil CO₂ emissions were quantified using the alkali absorption method [33].

2.4. Method of Calculation

(1)

Soil organic carbon mineralization (CO₂):

$$\mathrm{SOC} \mathrm{mineralization} \left(\mathrm{m}\mathrm{g}\cdot {\mathrm{k}\mathrm{g}}^{-1}\right)=\left\{\left[\left({\mathrm{V}}_0-\mathrm{V}\right)\times \mathrm{c}\times 0.022\times \left(22.4/44\right)\times 1000\right]\times 2\times 1000\right\}/\mathrm{m}$$

V₀ (mL) represents the volume of standard hydrochloric acid consumed during the blank titration, while V (mL) is the volume consumed during the sample titration. The variable c (mol·L⁻¹) denotes the concentration of the standard hydrochloric acid. The value 0.022 represents the molar mass of carbon dioxide (1/2 CO₂), where M (1/2 CO₂) = 0.022 g/mmol⁻¹. The expression (22.4/44) × 1000 indicates the number of milliliters per gram of carbon dioxide under standard conditions.

(2)

Soil organic carbon mineralization rate (mg·kg⁻¹·d⁻¹) = soc mineralization/$△\mathrm{t}$.

Δt is the culture interval (d).

(3)

Cumulative mineralization of soil organic carbon.

Accumulated mineralization of organic carbon $=\sum _1^n$ soc mineralization.

(4)

The first-order kinetic equation was used to study the kinetic data of organic carbon cumulative mineralization.

$${\mathrm{C}}_{\min }={\mathrm{C}}_0\left(1-{\mathrm{e}}^{-\mathrm{k}\mathrm{t}}\right)$$

where C_min represents the cumulative mineralization of organic carbon at time t (days); C₀ is the potential mineralizable organic carbon in the soil, expressed in mg·kg⁻¹; k is the soil carbon mineralization rate constant, in d⁻¹; and t is the incubation time in days (d).

2.5. Statistical Analysis

The data were statistically analyzed using Excel 2020 and SPSS 25.0. One-way analysis of variance (ANOVA) was performed to compare the differences between treatments. The least significant difference (LSD) method was employed for significance testing, with a significance level set at p < 0.05. Graphs were generated using Origin 2021.

3. Results

3.1. GBC and CBC Characterization Analysis

As shown in Figure 1, both GBC and CBC exhibit absorption peaks around 3400 cm⁻¹, corresponding to -OH stretching vibrations, and approximately 1600 cm⁻¹, indicative of C=O bond stretching [34,35]. The peak observed near 1340 cm⁻¹ mainly reflects the stretching vibrations of aromatic hydrocarbons, suggesting that both biochars possess a low degree of aromatization and heterocyclization [36].

3.2. Effects of Adding Different Proportions of GBC and CBC on Soil Properties

As depicted in Figure 2a,b, the application of two amendments led to a marked increase in citrus orchard soil pH during the initial 7 days of the incubation period, subsequently exhibiting fluctuations within the typical range. By the conclusion of the incubation period, the soil pH in the biochar-amended samples was observed to be higher than that of the control (CK). The elevation in soil pH demonstrated a positive correlation with the application rates of GBC and CBC, following the sequences G4% > G2% > G1% and C4% > C2% > C1%. Specifically, the incorporation of 4% GBC and 4% CBC elevated the soil pH to 5.82 and 5.50, respectively, corresponding to increments of 18.78% and 12.24% relative to the CK.

As demonstrated in Figure 2c,d, the available phosphorus content in the soil initially increased in the GBC treatment group during the incubation period, but a decline was observed after 21 days. The variations among the different proportions were minimal, and by the conclusion of the incubation period, the available phosphorus content in all the GBC treatments was lower than that in the control group. Conversely, in the CBC treatments, the available phosphorus content in the soil exhibited a significant increase with higher CBC application rates, following the sequence C4% > C2% > C1%, corresponding to increments of 512.55%, 177.42%, and 21.51%, respectively, relative to the CK. The overall trend of the two biochar types exhibited a similarity, with peak values recorded on the third and 28th days.

As illustrated in Figure 2e,f, the available potassium content in the soil exhibited a significant increase with the application rates of both GBC and CBC, eventually stabilizing after seven days of incubation. Relative to the control group, the available potassium content in the GBC treatment groups exhibited increases of 394.02%, 865.99%, and 1523.75%, respectively. In the CBC treatment groups, the available potassium content increased by 405.18%, 942.25%, and 1434.01%, respectively.

3.3. Effects of Adding Different Proportions of GBC and CBC on Soil Organic Carbon Mineralization

As depicted in Figure 3a,b, the application of GBC and CBC induced an increase in organic carbon mineralization within the citrus orchard soil. Throughout the incubation period, all the treatment groups demonstrated a gradual decrease over time, ultimately reaching stabilization. By the conclusion of the incubation period, the organic carbon mineralization rate in the treated soil samples surpassed that of the untreated control group, although no significant differences were detected among the various treatments. The G2% and G4% treatments exhibited nearly identical effects, each reflecting a 22% increase relative to CK, whereas the C4% treatment led to a 14.7% increase compared to CK.

As illustrated in Figure 3c,d, compared to the CK group, the application of GBC and CBC significantly enhanced the cumulative organic carbon mineralization within the soil. This study revealed that during the initial 7 days of incubation, the cumulative organic carbon mineralization in the treated groups was nearly indistinguishable from that of the untreated control group. However, after the 7-day mark, the cumulative organic carbon mineralization in the biochar-amended soils began to rise, with no significant differences observed among the varying application rates. For the GBC treatments, the effects followed the sequence of G4% > G1% > G2%, corresponding to increases of 20.34%, 17.12%, and 14.20%, respectively, relative to the control group. For the CBC treatments, the C1% and C4% applications demonstrated similar effects, each reflecting a 13.2% increase relative to the CK, whereas the C2% treatment yielded a 9.5% increase.

As indicated in the

Table 3. Soil carbon mineralization kinetic parameters.

Treatment	Fitting Parameters
	C0/mg·kg−1	k/d−1	R2	C0/SOC
CK	2190.6 ± 263.76	0.056 ± 0.013	0.958	0.302
G1%	2730.7 ± 317.37	0.048 ± 0.010	0.974	0.252
G2%	2966.7 ± 405.28	0.038 ± 0.008	0.981	0.242
G4%	2971.8 ± 332.12	0.043 ± 0.008	0.982	0.229
C1%	2557.4 ± 247.61	0.053 ± 0.010	0.977	0.189
C2%	2667.6 ± 347.35	0.044 ± 0.010	0.974	0.175
C4%	2816.9 ± 362.01	0.043 ± 0.009	0.977	0.169

, the model’s fitting coefficient was R² > 0.958, indicating a satisfactory fit. Following the application of GBC, the soil mineralization rate constant (k) ranged from 0.038 to 0.048 d⁻¹, with each value being lower than that of the control (CK). The potential mineralizable carbon (C₀) in the soil surpassed that of the control, following the sequence of G4% > G2% > G1%. Following the application of CBC, the soil mineralization rate constant (k) ranged from 0.043 to 0.053 d⁻¹, with each value being lower than that of the control (CK). The potential mineralizable carbon (C₀) in the soil exceeded that of the control, following the sequence of C4% > C2% > C1%. By the conclusion of the incubation period, the C₀/SOC ratio in the CK soil was 0.302, whereas the soils treated with biochar exhibited lower C₀/SOC values compared to the control. The C₀/SOC values in the GBC-treated soils followed the sequence of G1% > G2% > G4%, reflecting reductions of 16.63%, 19.88%, and 24.33% relative to the CK. The C₀/SOC values in the CBC-treated soils adhered to the sequence of C1% > C2% > C4%, exhibiting reductions of 37.46%, 41.94%, and 44.02% relative to the CK.

3.4. Effects of Adding Different Proportions of GBC and CBC on Different Carbon Fractions in Soil

As depicted in Figure 4a,b, the soil organic carbon (SOC) content increased with higher application rates of GBC and CBC, exhibiting the trend of 4% > 2% > 1% > CK. By the conclusion of the incubation period, the SOC in the G4% treatment had risen by 179.46% compared to the control, whereas the C4% treatment demonstrated an even greater increase of 229.97%.

Figure 4c,d illustrates that the soil dissolved organic carbon (DOC) content progressively increased over time in soils treated with both types of agricultural waste. However, at specific time points, the DOC content in the treatment groups was lower than that of the control group. Nevertheless, by the conclusion of the incubation period, the DOC content in all the treatment groups was significantly higher than that in the control, adhering to the sequence of G4% > G1% > G2% > CK and C1% > C2% > C4% > CK.

As depicted in Figure 4e,f, the soil readily oxidizable carbon (ROC) content increased following the initial application of GBC and CBC. The impact of varying the GBC application rates on the soil ROC content was particularly pronounced. As the incubation period progressed, the ROC content exhibited a downward trend. By the conclusion of the incubation period, the ROC content in the soils treated with GBC and CBC was significantly higher than that in the control. The GBC treatments adhered to the sequence of G1% > G2% > G4%, whereas the CBC treatments followed the sequence of C2% > C1% > C4%. Notably, the ROC content increased by 115.69% and 127.45% in the C1% and C2% treatments, respectively, relative to the control.

3.5. Effects of Adding Different Proportions of GBC and CBC on Soil Enzyme Activities

As illustrated in Figure 5a,b, the addition of GBC had differential effects on the urease activity by the conclusion of the incubation period. The G4% treatment significantly enhanced the urease activity, by 108.7%, whereas the G1% treatment resulted in a 21.74% reduction in urease activity. Following the CBC application, the urease activity in all the treatment groups was lower than that of the control by the conclusion of the incubation period.

Figure 5c,d demonstrates that the GBC application led to an increase in catalase activity, particularly in the G4% treatment, for which the activity rose by 30%. In contrast, the effects of the CBC application diverged, generally resulting in lower catalase activity relative to the control throughout the incubation period. Notably, in the C4% treatment group, the catalase activity declined by 17.54% by the conclusion of the incubation. Overall, the catalase activity adhered to the sequence of C1% > C2% > CK > C4%.

3.6. Analysis of Relationship

As illustrated in Figure 6a, after the application of the citrus peel biochar to the citrus orchard soil, the available potassium demonstrated a highly significant positive correlation with pH (p ≤ 0.01). Th SOC exhibited a highly significant positive correlation with pH, available potassium, urease, and catalase (p ≤ 0.01), and a significant positive correlation with available phosphorus (p ≤ 0.05). The urease exhibited a highly significant positive correlation with both pH and available potassium (p ≤ 0.01). The catalase demonstrated a highly significant positive correlation with pH (p ≤ 0.01) and a significant positive correlation with available potassium and urease (p ≤ 0.05).

As illustrated in Figure 6b, with the application of the cassava straw biochar to the citrus orchard soil, the pH exhibited a highly significant positive correlation with both available phosphorus and available potassium (p ≤ 0.01). The SOC was significantly positively correlated with available potassium (p ≤ 0.05), positively correlated with pH and available phosphorus, and negatively correlated with ROC and catalase. The DOC exhibited a negative correlation with pH, available phosphorus, and available potassium, but a positive correlation with SOC. The urease demonstrated a significant positive correlation with available potassium (p ≤ 0.05), a positive correlation with pH, available phosphorus, and SOC, and a negative correlation with catalase. The catalase exhibited a significant negative correlation with available potassium (p ≤ 0.05) and a negative correlation with pH and available phosphorus. The cumulative organic carbon emissions were highly significantly positively correlated with DOC (p ≤ 0.01).

4. Discussion

4.1. Effects of GBC and CBC Application on Soil Physical and Chemical Properties

The application of citrus peel biochar and cassava straw biochar to citrus orchard soil demonstrated a proportional increase in soil pH relative to the biochar dosage, indicating a robust positive correlation. This finding aligns with prior research [37], which has established that biochar, owing to its inherent alkalinity, can neutralize acidic components in soil, thus raising the soil pH to approximately 5.5, the optimal range for citrus growth. The incorporation of both types of biochar resulted in an elevated concentration of base cations in the soil. These cations underwent hydrolysis, leading to the generation of hydroxide ions (OH⁻), which played a crucial role in the elevation of soil pH [38]. Furthermore, the acidic functional groups within biochar can interact with carbonates and organic anions, thereby amplifying its alkaline properties [39,40]. However, the organic matter in biochar also catalyzes the decomposition of soil organic matter, leading to the formation of organic acids, which in turn diminishes the efficiency of biochar in elevating soil pH.

The available phosphorus content in the soil exhibited a positive proportional relationship with the amount of biochar applied [41,42]. This phenomenon occurs because biochar contains phosphorus components and raises soil pH, thereby reducing the adsorption of phosphorus by iron and aluminum oxides in soil. Additionally, the application of biochar enhances the microbial environment, thereby stimulating phosphatase–potassium-solubilizing bacteria and phytase activities, which subsequently increases phosphorus and potassium availability [43,44]. Concerning the available potassium content, both types of biochar provided a significant amount of base cations and minerals, which displaced the potassium in the soil, thus increasing the level of available potassium [45]. The application of 4% cassava straw biochar exerted the most pronounced effect on increasing the available phosphorus and potassium levels in the citrus orchard soil. The majority of the water-soluble phosphorus and potassium in biochar is released into the soil within the first week [46]. The elevated potassium content in the soil increases the concentration of calcium ions, which facilitates the formation of calcium phosphate. As the incubation time progresses, the pH surrounding the biochar rises, leading to the adsorption of calcium ions from the soil by the biochar [47]. Consequently, phosphorus release is enhanced after 28 days.

4.2. Effects of GBC and CBC on Soil Organic Carbon Mineralization

The enhancement of soil organic carbon (SOC) mineralization by biochar is governed by multiple factors, particularly the physicochemical properties and inherent stability of the biochar [48]. In this study, the incorporation of both citrus peel biochar and cassava straw biochar facilitated the mineralization of SOC. During the initial stages of its application, increasing the amount of biochar resulted in an elevated respiration rate of soil microbes and a corresponding increase in cumulative mineralization [49]. Across various treatments, the SOC mineralization rate initially decreased and subsequently stabilized over time, aligning with the findings of Ameloot et al. [50]. The soil carbon mineralization rate constant (k) is widely employed as an indicator of SOC turnover. In this experiment, the application of citrus peel biochar and cassava straw biochar introduced stable carbon sources into the soil, influencing their availability to microbes and, consequently, reducing the organic carbon turnover rate.

The application of both biochar types elevated the dissolved organic carbon content in the soil, thereby promoting microbial activity and leading to the release of additional carbon dioxide [51]. Biochar can protect water-soluble organic matter from mineralization. However, higher biochar content results in more DOC being released into the soil, leading to the 2% biochar treatment showing lower DOC release compared to the other two higher proportions [52,53]. The C₀/SOC ratio serves as a metric for soil carbon sequestration capacity; a lower ratio indicates a stronger sequestration potential. The application of both citrus peel biochar and cassava straw biochar enhanced the carbon sequestration capacity of citrus orchard soil, with cassava straw biochar exerting a more pronounced effect. Cassava straw is rich in lignin and cellulose [54], whereas citrus peel is abundant in fruit acids and volatile compounds [55]. Lignin remains relatively stable in biochar, and during the carbonization process, cassava straw forms more stable structures due to its higher carbonization degree, thereby enhancing the soil’s carbon sequestration capacity [56].

4.3. Effects of GBC and CBC on Soil Carbon Fractions

This study demonstrated that the application of agricultural waste to citrus orchard soil effectively enhanced the soil organic carbon (SOC) content. The enhancement of SOC provided by citrus peel biochar (GBC) and cassava straw biochar (CBC) was proportional to the application rate, in line with the findings of Novak et al. [57]. As a carbon-rich material, biochar introduces exogenous organic carbon into soil [58]. Biochar possesses a highly stable chemical structure that resists microbial decomposition, enabling it to persist in soil over extended periods and mitigate organic carbon loss [59]. Furthermore, biochar’s porous structure and distinctive aromatic composition facilitate the adsorption of SOC and the formation of stable complexes, thereby enhancing SOC sequestration [60]. In the later stages of biochar treatment, the biochar induced a negative effect on SOC, inhibiting the mineralization of natural SOC and consequently enhancing its sequestration capacity [61]. Different types of biochar exhibit varying capacities in modifying soil properties, with straw-derived biochar demonstrating the highest efficiency in enhancing SOC [62,63].

Following the application of GBC and CBC, the soil dissolved organic carbon (DOC) content exhibited a general upward trend over time, indicating that these biochars contributed to the increase in soil DOC. This effect is primarily attributed to the unstable organic carbon generated during biochar production from agricultural waste, which becomes adsorbed onto the biochar surface and serves as a source of soil DOC [64]. Specifically, the application of 4% citrus peel biochar significantly elevated DOC levels in the later stages, whereas the effects of the three cassava straw biochar ratios were relatively consistent. The two materials did not exhibit a clear pattern, suggesting that biochar, unlike raw biomass materials, displays significant variability.

Readily oxidizable carbon (ROC) is a component intimately linked to bioavailability and is highly responsive to soil changes [65,66]. The results indicated that the addition of both types of biochar increased the soil ROC content, implying that ameliorating the acidic pH conditions of the soil can enhance ROC content, suppress soil enzyme activity, and decelerate ROC decomposition.

4.4. Effects of GBC and CBC on Soil Enzyme Activities

Soil enzyme activity serves as a crucial biological indicator of soil health, with its fluctuations typically influenced by the soil’s physicochemical properties. In studies examining the effects of biochar on soil enzyme activity, Yao et al. [67] reported that biochar application can effectively enhance the activity of soil catalase and urease. However, in this study, compared to the control group, the urease activity decreased following the application of both biochar types. This reduction in urease activity could be attributed to the elevated nitrogen content in the soil resulting from biochar addition, which may inhibit urease catalytic activity [68]. Additionally, this decline might stem from the distinct impacts of the two biochar types on soil microbial communities.

The citrus peel biochar, by increasing the exogenous carbon sources, elevated the DOC content, thereby accelerating microbial metabolic rates and positively influencing catalase activity. In contrast, cassava straw biochar, owing to its higher stability, provided less DOC and ROC, thereby reducing microbial availability. As the application rate of the cassava straw biochar increased, its positive impact on the catalase activity gradually diminished, and at higher application rates, its effect fell below that of the control. This indicates an inhibitory effect on specific microbial activities, resulting in reduced catalase production [69].

5. Conclusions

The application of different proportions of citrus peel biochar and cassava straw biochar effectively adjusted the pH of citrus orchard soil, bringing it into an optimal range for citrus growth. All the application rates for citrus peel biochar were observed to decrease the available phosphorus content in the soil, while a 4% application of cassava straw biochar had a significant impact on enhancing the available phosphorus and potassium levels in the soil. Despite the increased soil mineralization rate and cumulative mineralization resulting from the introduction of exogenous carbon sources, both biochar types significantly boosted the soil organic carbon (SOC) content. Furthermore, both biochars enhanced the carbon sequestration potential of the citrus orchard soil, with the cassava straw biochar exhibiting a more pronounced effect than the citrus peel biochar. The most substantial increase in SOC was achieved with a 4% application of cassava straw biochar. The citrus peel biochar primarily enhanced the content of active organic carbons, such as dissolved organic carbon (DOC) and readily oxidizable carbon (ROC), while the cassava straw biochar was more effective at increasing the total organic carbon content in the citrus orchard soil. Specifically, a 4% application of cassava straw biochar increased the soil organic carbon by 229.7% compared to the control (CK), and the biochar derived from the citrus peels increased the organic carbon content by 179%. Regarding the soil enzyme activity, the cassava straw biochar exhibited an inhibitory effect on both the urease and the catalase activity, whereas the citrus peel biochar had little impact on the urease activity but significantly enhanced the catalase activity. In conclusion, a 4% application of cassava straw biochar demonstrated the best overall performance in increasing soil organic carbon content, enhancing soil carbon sequestration potential, improving nutrient levels, and regulating soil pH in citrus orchards.

The experimental conditions applied in this study may differ from those in real-world agricultural settings, necessitating further field trials to validate the applicability of these findings. Additionally, using citrus peels and cassava straw from a single location may have imposed regional constraints on the results. Future studies could incorporate materials from multiple locations and diverse sources to enhance the generalizability and robustness of the findings.