Biochar-Based Fertilizer Enhances the Production Capacity and Economic Benefit of Open-Field Eggplant in the Karst Region of Southwest China

Abstract

Biochar as an amendment has been widely applied to enhance crop productivity and improve soil quality. However, the effect of biochar-based fertilizer (BF) on the production capacity and economic benefits of open-field eggplant in the karst region remains unclear. A field experiment was carried out in the karst region of Southwest China from 2020 to 2021 to study the ameliorative roles of different application rates (1875, 2250, 2625, and 3000 kg ha⁻¹) of BF on the fresh yield, quality, fertilizer utilization, and economic benefits of fresh eggplant. The results show that BF increased the yield of fresh eggplant by 3.65–13.76% (2020) and 23.40–49.04% (2021) compared to the traditional fertilization practice (TFP). The application of BF reduced the nitrate content and increased the vitamin C (VC) and soluble sugar content of the fruits, which is beneficial for improving the quality of eggplant fruits. Meanwhile, the application of BF not only increased the nutrient uptake of the eggplant but also significantly improved the fertilizer utilization rates, especially the agronomic efficiency (AE) and recovery efficiency (RE). Moreover, BF could also significantly increase the output value and net income of fresh eggplant, which can help farmers increase their income. In conclusion, a BF application rate of 2544–2625 kg ha⁻¹ could be used to improve the yield, fertilizer efficiency, and economic benefits of open-field eggplant and is recommended for managing agricultural production in the karst region of Southwest China.

1. Introduction

Karst landform is one of the main geomorphic types worldwide. Due to the low rate of soil formation and high leakage, there is no transition layer of weathered parent material between the rock interface and soil, and thus, the ecological environment is extremely fragile [1,2]. China has the widest distribution and largest area of karst landforms, mainly across Guangxi, Guizhou, and Yunnan provinces, accounting for one-third of the total. The exposed rock area in the karst region is large, and the phenomenon of rocky desertification is serious. Simultaneously, the soil in the karst region is easily affected by human activities and the natural environmental processes, which greatly restricts agricultural production herein [3,4].

Biochar is prepared by the pyrolysis of biomass materials under low oxygen or hypoxia conditions [5]. Due to the large specific surface area and rich pore structure, biochar retains soil nutrients and reduces their loss [6]. The surface of biochar has abundant negative charges and a high charge density, resulting in the absorption and attachment of polar or non-polar organic compounds and inorganic ions, such as NH₄⁺- and NO₃⁻- in water, soil, and sediment [7,8]. Therefore, the application of biochar in farmland improves soil quality and carbon fixation, reduces the contents of nitrogen and phosphorus, and improves crop yield [9,10]. Biochar is made by the carbonization of agricultural wastes, and its nutrient content is low; thus, its application alone is insufficient to meet the needs of crop growth [11,12]. However, increasing the crop yield using biochar may require long-term accumulation, and its application for a short period may not increase the yield significantly [13,14]. Therefore, the application of biochar for agricultural production has some limitations.

In recent years, with adjustments to agricultural structure and the development of ecologically circular agriculture in China with the goal to recycle agricultural wastes, the development and application of new high-efficiency fertilizers have been research hotspots in the field of plant nutrition and fertilizer. Biochar-based fertilizers (BF), as a new type of environmental protection fertilizer, have received traction in the fields of agriculture and environmental protection [15,16]. BFs are made by mixing biochar with organic or inorganic fertilizers; these can be used as a slow-release nutrient carrier [17]. BF not only confers the advantage of soil improvement but also continuously supplies nutrients in combination with the fertilizer requirements of crops, thereby reducing nutrient loss and achieving the dual functions of slow release and carbon fixation [18,19]. As a slow-release fertilizer, BF can maintain high nutrient levels in the soil for a long time, which is attributed to the fact that biochar retains nitrogen, phosphorus, potassium, and other nutrients [20,21]. The interaction of biochar with nitrogen, phosphorus, and potassium is through electrostatic interaction, complexation, and mineralization [22]. Biochar can be combined with NH₄⁺-N through electrostatic adsorption, thus reducing the leaching of ammonium ions and improving the utilization efficiency of nitrogen fertilizers [23]. Biochar also complexes with urea to form biochar-based nitrogen fertilizer, and the amino groups on the surface of urea react with the carboxylic anhydride on the biochar for fixation [24]. Additionally, biochar can absorb and fix the ammonia released during the decomposition of urea, which can further prevent the loss of nitrogen fertilizer [25]. When biochar is mixed with phosphorus fertilizer, the former helps capture P-complexing metallic ions, thus reducing the chances of P-fixation [26]. Biochar can also fix K⁺, resulting in the formation of biochar-based potassium fertilizer through π−cation bond action, which helps reduce the rate of loss and improve the utilization rate of potassium fertilizers [27]. Many studies have shown that following BF application in the soil, it oxidizes and decomposes, ultimately leading to nutrient fixation on the biochar, which is gradually released and absorbed by crops [28]. Accumulating evidence shows that BF has good application prospects in improving the physical and chemical properties of soil, reducing chemical fertilizer input, and promoting crop growth [29,30].

Open-field eggplant is an important summer vegetable in the karst region of Guizhou province. However, due to long-term and continuous cropping, insufficient organic fertilizer input, soil carbon/nitrogen (C/N) imbalance, and reduction in soil fertility, the yield, quality, fertilizer utilization, and economic benefits of cultivating eggplant have hardly improved [31,32]. In the present study, distillers grains were used as raw material to prepare biochar through carbonization at a high temperature. Subsequently, the biochar and chemical fertilizers were mixed to a certain proportion to obtain BF. A two-year field experiment was conducted to evaluate the ameliorative roles of BF on the biological and economic benefits of cultivating eggplant in the karst region of Southwest China. In this study, a new technique of fertilizer application using distillers grains was explored, and the findings may provide a reference for the agricultural utility of BF in the karst region of Southwest China.

2. Materials and Methods

2.1. Site Description and Experimental Materials

The field experiment was conducted in Xishan town (27°4′42′′ N, 106°41′56′′ E) in Xifeng County, Guiyang City, Guizhou Province, China in 2020 and 2021. The soil type of the experimental region was as follows: yellow, zonal with a high aluminization intensity, formed under perennial, humid, and bioclimatic conditions in the subtropical zone. Due to intense leaching caused by the perennial humidity, the exchangeable base content was only 20%, and therefore, the base was extremely unsaturated. The nutrient content of the topsoil of the test field was measured; its pH was 5.74, the soil organic matter (SOM) content was 44.58 g kg⁻¹, the total nitrogen (TN) was 0.52 g kg⁻¹, and the available phosphorus (AP) and available potassium (AK) contents were 30.31 mg kg⁻¹ and 107.49 mg kg⁻¹, respectively. The eggplant variety used in the experiment was ‘Ruibao 3’.

The raw materials of the biochar were distillers grains, which comprise biomass waste generated from the production process of distilled spirits (Kweichow Moutai (Group) Circular Economy Industrial Investment and Development Co., Ltd., Zunyi, China). The biochar was prepared by the oxygen-limited cracking method in a biomass carbonization furnace (SSDP-5000-A, Jiangsu Huaian Huadian Environmental Protection Machinery Manufacturing Co., Ltd., Huaian, China). Briefly, we obtained appropriate amounts of distillers grain samples and put them in the equipment, followed by blowing in N₂ for 5–10 min to exhaust the excess air in the furnace. The sample was pyrolyzed at 550 °C for 2 h. After cooling, it was passed through a 100-mesh sieve and placed in the shade until subsequent experiments. The nutrient content of the distillers grain biochar was measured. Its pH was 9.05, the soil organic carbon (SOC) was 265.88 g kg⁻¹, and the total nitrogen (TN), phosphorus (TP), and potassium (TK) contents were 47.51, 11.43, and 25.33 g kg⁻¹, respectively. The structural characteristics of the distillers grain biochar were measured. The specific surface area (SSA) was 2.12 m² g⁻¹, the single point adsorption total pore volume (SPATPV) was 2.95 × 10⁻³ m³ g⁻¹, and the average pore size (APS) was 5.55 nm. According to the local experience of eggplant cultivation, the suitable proportion of N/P₂O₅:/K₂O for growth was 5:3:7. To meet these nutritional requirements, the biochar-based fertilizer was blended with fertilizer and biochar, comprising distillers grains biochar (16%), urea (30%), mono-ammonium phosphate (12%), potassium sulfate (40%), and solid binder (2%). The BF was prepared using a flat grinding extrusion granulator (SKJ-120, Shanghai Jiale Electromechanical Group Co., Ltd., Shanghai, China). Compound fertilizer (N, 15%; P₂O₅, 15%, and K₂O, 15%; Guizhou Xiyang Industrial Co., Ltd., Guiyang, China) and organic fertilizer (comprising mainly cow dung, organic matter ≥ 50%, and N + P₂O₅ + K₂O ≥ 5%; Guizhou Dibao Co., Ltd., Guiyang, China) were also used.

2.2. Experimental Design and Management

The experiment consisted of six treatments, and each treatment was repeated thrice according to a randomized complete block design (RCBD). The treatments included no fertilizer (CK), traditional fertilization practice (TFP, based on the local practices, 3000 kg ha⁻¹ of compound fertilizer, 450.00 kg ha⁻¹ N, 450.00 kg ha⁻¹ P₂O₅, 450.00 kg ha⁻¹ K₂O, and the ratio of the basal to the top dressing was 50:50), BF application at 1875 kg ha⁻¹ (BF1, N 281.25 kg ha⁻¹, P₂O₅ 112.5 kg ha⁻¹, K₂O 375.00 kg ha⁻¹), BF application at 2250 kg ha⁻¹ (BF2, N 337.50 kg ha⁻¹, P₂O₅ 135.00 kg ha⁻¹, K₂O 450.00 kg ha⁻¹), BF application at 2625 kg ha⁻¹ (BF3, N 393.75 kg ha⁻¹, P₂O₅ 157.50 kg ha⁻¹, K₂O 252.00 kg ha⁻¹), and BF application at 3000 kg ha⁻¹ (BF4, N 450.00 kg ha⁻¹, P₂O₅ 180.00 kg ha⁻¹, K₂O 600.00 kg ha⁻¹). According to the local planting traditions, organic fertilizer at 1500 kg ha⁻¹ was used in all treatment groups except for the CK treatment.

Table 1. Fertilizer amounts of different treatments.

Treatments	Basal Dressing Fertilizer (kg ha−1)			Top Dressing Fertilizer (kg ha−1)
	BF	Compound Fertilizer	Organic Fertilizer	Compound Fertilizer
CK	—	—	—	—
TFP	—	1500	1500	1500
BF1	1875	—	1500	—
BF2	2250	—	1500	—
BF3	2625	—	1500	—
BF4	3000	—	1500	—

shows the type and amount of fertilizer used for each treatment.

All the basal dressing fertilizers were spread in the soil simultaneously before planting eggplant seeds and mixed with the topsoil. Eggplant seedlings were transplanted after fertilizer application at a planting density of 18,000 plants ha−1 (with a plant spacing of 70 cm and row spacing of 80 cm). The area of each plot was 56.00 m2 (7.0 m × 8.0 m). After the eggplant seedlings were transplanted, for 30 days, 1500 kg ha−1 of compound fertilizer was applied to the plants in the TFP treatment group. A unified management mode was adopted for all eggplants throughout the growing season to reduce interference due to external factors.

2.3. Sampling and Measurement

2.3.1. Soil Sample Collection and Determination Method

Soil samples between depths of 0 and 20 cm were collected from 10 randomly selected spots on the main experimental area before fertilization. The soil samples were composited and air-dried, ground, and passed through 1 mm and 0.149 mm sieves for determining their physicochemical characteristics. The physical and chemical properties of soil were determined according to the methods described by Bao [33]. The soil pH was measured using a 1:2.5 extraction mixture (soil/water, w/v) with a pH meter (FE20K, Mettler Toledo, Zurich, Switzerland). The soil organic matter (SOM) was determined using the potassium dichromate volumetric–external heating method. The TN was determined using the semi-micro Kjeldahl method. The AP was determined by extraction using hydrochloric acid combined with ammonium fluoride and assessed by molybdenum antimony anti-colorimetry. The soil AK content was determined by extraction using ammonium acetate and assessed using a flame photometer (FP640, Shanghai Aopu Analytical Instrument Co., Ltd., Shanghai, China).

2.3.2. Plant Sample Collection and Determination Method

At maturity, six plants from each experimental plot were sampled before the final harvest, which was used to test the plant’s nutrition and fruit quality. The eggplant plants were divided into two parts, namely the stem-leaf and the fruit, and dried to a constant weight at 60 °C after heating at 105 °C for 30 min. All dried samples were ground and passed through a 0.25 mm sieve and digested in a mixture of concentrated H₂SO₄ and H₂O₂ to determine the concentrations of N, P, and K. The TN concentration was determined by the Kjeldahl nitrogen method; the TP concentration was determined by vanadium molybdenum yellow colorimetry, and the TK concentration using a flame photometer (FP640, Shanghai Aopu Analytical Instrument Co., Ltd., Shanghai, China) [33]. For the determination of nitrate content, 2 g of fresh fruits were taken, to which 10 mL of deionized water was added, and the sample was placed in a boiling water bath for 30 min. The extraction solution (0.1 mL) was taken and mixed with 0.4 mL of 5% salicylic acid-sulfuric acid solution at 25 °C for 20 min. Then, 9.5 mL of 8% NaOH was added to the solution, and the absorbance was measured on a visible spectrophotometer (UV-3600i Plus, Shimadzu, Tokyo, Japan). The VC content was determined by high-performance liquid chromatography (HPLC, LC-2040, Nexera-i, Shimadzu, Japan) after grinding, centrifuging, and filtering with 10 mL 0.2% metaphosphoric acid. For assessing the content of soluble sugar, 0.2 g of fresh leaves were taken, and 10 mL of distilled water was added to the extract in a boiling water bath for 30 min. The extract (0.5 mL) was taken in a test tube, to which 0.5 mL ethyl anthrone and 5 mL sulfuric acid were added. The test tube was placed in a boiling water bath and incubated for 1 min. Subsequently, it was taken out and naturally cooled to room temperature. The absorbance was measured at 630 nm. The content of soluble sugar was determined by anthrone colorimetry. Briefly, 0.6 g of fresh fruit was used for extraction, which was performed twice in 3 mL of 80% ethanol for 60 min. The extracts were then mixed and filtered, and the alcohol was removed by evaporation. The anthrone reagent was added to samples, and after thorough shaking, the absorption of the samples was measured at 625 nm.

2.3.3. Eggplant Yield

When the fresh eggplants were harvested, the yield of each subplot was determined by the actual harvest. According to the growth status of the plants, harvesting was performed multiple times, and the total yield was recorded.

2.4. Calculations and Statistical Analysis

The following parameters and models were calculated according to the method of Zhang et al. [34].

2.4.1. Fertilizer Utilization

$$\mathrm{AE}=\frac{{\mathrm{Y}}_{\mathrm{F}}–{\mathrm{Y}}_{\mathrm{CK}}}{\mathrm{NI}}$$

$$\mathrm{RE}=\frac{{\mathrm{N}}_{\mathrm{F}}–{\mathrm{N}}_{\mathrm{CK}}}{\mathrm{NI}}$$

where AE stands for agronomic efficiency (kg kg⁻¹), RE stands for recovery efficiency (%), Y_F denotes the fresh yield of the fertilization treatment (kg ha⁻¹), Y_CK denotes the fresh yield of the CK treatment (kg ha⁻¹), N_F denotes the nutrient accumulation of the fertilization treatment (kg ha⁻¹), N_CK denotes the nutrient accumulation of the CK treatment (kg ha⁻¹), and NI denotes the nutrient input of fertilization treatment (kg ha⁻¹).

2.4.2. Economic Benefits

$$\mathrm{OV}=\mathrm{Y}\times \mathrm{UP}$$

$$\mathrm{IOV}={\mathrm{OV}}_{\mathrm{F}}-{\mathrm{OV}}_{\mathrm{CK}}$$

$$\mathrm{NET}=\mathrm{OV}-\mathrm{FI}$$

where OV stands for the output value (USD ha⁻¹), Y stands for the fresh yield (kg ha⁻¹), UP stands for the unit price of fresh eggplant (USD kg⁻¹), IOV stands for the increased output value (USD ha⁻¹), OV_F stands for the output value of the fertilization treatment (USD ha⁻¹), OV_CK stands for the output value of the CK treatment (USD ha⁻¹), NET stands for the net income (USD ha⁻¹), and FI stands for the fertilizer input (USD ha⁻¹). In the calculation of economic benefits, the unit price of fresh eggplant was USD 0.4471 kg⁻¹, and the BF, compound fertilizer, and organic fertilizer were 0.4471, 0.2980, and 0.1490 USD kg⁻¹, respectively.

2.4.3. Linear Plus Platform Model

In the experiment, a linear plus platform model was used to fit the response of the yield of fresh eggplant to the BF application rate, so that the best application amount of BF on fresh eggplant could be calculated.

The calculation equation was:

$$\mathrm{Y}=\mathrm{AX}+\mathrm{B} \left(\mathrm{X}\le \mathrm{C}\right); \mathrm{Y}=\mathrm{P} \left(\mathrm{X}>\mathrm{C}\right)$$

where Y stands for the yield of fresh eggplant (kg ha⁻¹), X stands for the BF application rate (kg ha⁻¹), A stands for the slope, B stands for the intercept, C stands for the intersection of the line and the platform, and P stands for the maximum yield (kg ha⁻¹).

2.5. Statistical Analysis

Microsoft Excel 2007 and SPSS 20.0 (SPSS Inc., Chicago, IL, USA) were used for data processing and statistical analysis. The significance of the differences between the soil and the plant indicators was measured by one-way ANOVA. The significance of the differences was tested using Duncan’s new compound extreme difference method, and the significance level was set as α = 0.05. All the figures were constructed with Origin 12.0 (Origin Lab Corporation, Northampton, MA, USA).

3. Results

3.1. Yield of Fresh Eggplants

The TFP and BF treatments both significantly enhanced the yield of fresh eggplants, with an increase of 4905–9081 kg ha⁻¹ in 2020 and 11,036–25,018 kg ha⁻¹ in 2021 compared to the CK treatment group (Figure 1). The yield of fresh eggplant in the BF group increased by 1109–4176 kg ha⁻¹ in 2020 and 6672–13,982 kg ha⁻¹ in 2021 compared to the TFP treatment group, and the increasing rates were 3.65–13.76% in 2020 and 23.40–49.04% in 2021. The fresh yield following the BF3 treatment was the highest in 2020 and 2021, at 34,514 and 42,419 kg ha⁻¹, respectively. The optimal application rate of BF for eggplant in the open field was estimated (Figure 2). The results show that the maximum yield of fresh eggplant was obtained with a BF application of 2625 kg ha⁻¹ in 2020; the rate was 2544 kg ha⁻¹ in 2021.

3.2. Fruit Quality of the Eggplant

Indicators of the fresh fruit quality of eggplant are listed in

Table 2. Effects of BF on quality of fresh eggplant in 2020 and 2021.

Year	Treatments	Nitrate (mg kg−1)	VC (mg kg−1)	Soluble Sugar (mg kg−1)
2020	CK	107.18 ± 4.78 a	99.65 ± 4.44 c	155.24 ± 5.13 b
	TFP	101.13 ± 4.51 a	108.32 ± 4.83 c	161.69 ± 6.43 b
	BF1	90.88 ± 3.61 b	119.45 ± 3.07 b	172.67 ± 4.50 a
	BF2	88.62 ± 3.95 bc	128.16 ± 5.71 ab	180.73 ± 4.08 a
	BF3	85.08 ± 2.33 bc	132.71 ± 5.92 a	181.41 ± 5.14 a
	BF4	83.78 ± 2.17 c	124.31 ± 5.54 ab	176.42 ± 5.70 a
2021	CK	104.97 ± 4.91 a	94.01 ± 3.08 e	153.45 ± 4.99 c
	TFP	104.88 ± 6.16 a	105.44 ± 2.25 d	158.78 ± 8.16 c
	BF1	84.93 ± 3.01 b	121.43 ± 2.09 c	177.16 ± 4.79 b
	BF2	78.40 ± 2.41 bc	138.94 ± 6.60 b	186.19 ± 4.83 ab
	BF3	76.80 ± 3.13 c	148.33 ± 3.98 a	192.12 ± 5.44 a
	BF4	73.23 ± 2.89 c	143.25 ± 5.06 ab	189.82 ± 2.23 a

Note: Different lowercase letters denote significant differences among treatment means at the α = 0.05 level using Duncan’s MRT method. The results are presented as the mean value ± standard error.

. BF application significantly reduced the content of nitrate in fresh eggplant fruits, while the VC and soluble sugar were enhanced. BF reduced the content of nitrate in fresh eggplant fruits by 10.14–17.16% in 2020 and 19.02–30.18% in 2021 compared to the TFP treatment group, and the nitrate content in the BF4 treatment group was the lowest. The contents of VC and soluble sugar in fresh eggplant fruits increased by 10.28–22.52% and 6.79–12.20% in 2020, and by 15.17–40.68% and 11.58–21.00% in 2021, respectively.

3.3. Nutrients Accumulation

The application of BF affected the nutrient accumulation in eggplant, and the N, P, and K contents in the eggplant plants are shown in Figure 3. The application of fertilizer improved the accumulation of N, P, and K by 62.93–157.44%, 54.07–92.68%, and 110.04–225.41%, respectively, in 2020 and 58.05–269.51%, 60.07–172.91%, and 118.05–339.71% in 2021 compared to the CK treatment group. Furthermore, the N, P, and K accumulation following BF application increased by 19.77–58.01%, 0.70–25.05%, and 16.11–54.93% in 2020, and 43.94–133.78%, 18.42–70.50%, and 35.54–101.65% in 2021, respectively, compared to the TFP treatment group. The N, P, and K accumulation rates were the highest in the BF3 treatment group, at 222.30, 69.08, and 379.20 kg ha⁻¹, in 2020, and 248.35, 84.74, and 445.21 kg ha⁻¹, in 2021, respectively.

3.4. Fertilizer Utilization

Table 3. Effects of BF on agronomic efficiency and recovery efficiency in 2020 and 2021.

Year	Treatments	AE (kg kg−1)			RE (%)
		AEN	AEP	AEK	REN	REP	REK
2020	CK	—	—	—	—	—	—
	TFP	10.90 ± 3.37 b	10.90 ± 3.37 b	10.90 ± 3.37 b	12.08 ± 1.03 d	4.31 ± 0.44 c	28.49 ± 1.68 d
	BF1	21.38 ± 4.62 a	53.46 ± 11.56 a	16.04 ± 3.47 ab	29.21 ± 0.15 c	17.58 ± 0.53 b	44.71 ± 0.16 b
	BF2	19.95 ± 4.63 a	49.86 ± 11.58 a	14.96 ± 3.47 ab	31.78 ± 1.80 b	19.22 ± 1.87 ab	49.09 ± 2.09 a
	BF3	23.06 ± 0.18 a	57.66 ± 5.44 a	17.30 ± 1.63 a	34.53 ± 0.89 a	21.09 ± 0.81 a	50.03 ± 1.35 a
	BF4	17.26 ± 2.21 ab	43.16 ± 5.52 a	12.95 ± 1.66 ab	28.59 ± 1.03 c	17.60 ± 0.93 b	37.75 ± 1.27 c
2021	CK	—	—	—	—	—	—
	TFP	24.53 ± 2.19 d	24.53 ± 2.19 d	24.53 ± 2.19 d	8.67 ± 1.10 d	4.15 ± 0.53 e	26.56 ± 2.12 e
	BF1	62.96 ± 3.78 a	157.41 ± 9.46 a	47.22 ± 2.84 a	30.47 ± 1.53 c	24.72 ± 1.68 c	52.80 ± 2.01 c
	BF2	57.66 ± 1.63 b	144.15 ± 4.07 b	43.25 ± 1.22 b	40.20 ± 2.05 b	27.52 ± 1.68 b	57.04 ± 2.30 b
	BF3	63.54 ± 2.42 a	158.85 ± 6.05 a	47.65 ± 1.82 a	46.00 ± 1.22 a	34.09 ± 0.95 a	65.52 ± 1.73 a
	BF4	45.74 ± 1.70 c	114.34 ± 4.25 c	34.30 ± 1.27 c	32.75 ± 0.79 c	20.50 ± 0.49 d	47.27 ± 0.91 d

Note: Different lowercase letters denote significant differences among treatment means at the α = 0.05 level using Duncan’s MRT method. The results are presented as the mean value ± standard error.

shows the AE and RE of different fertilization treatment groups. Compared to the TFP treatment group, the AE_N, AE_P, and AE_K following BF treatment increased by 58.35–111.56%, 295.96–428.99%, and 18.81–58.72% in 2020, and 86.47–159.03%, 366.12–547.57%, and 39.83–94.25% in 2021, respectively. The AE_N, AE_P, and AE_K in the BF3 treatment group were the highest, at 23.06, 57.66, and 17.30 kg kg⁻¹ in 2020, and 63.54, 158.85, and 47.65 kg kg⁻¹ in 2021, respectively. Similarly, the RE_N, RE_P, and RE_K following BF treatment increased by 136.67–185.84%, 307.89–389.33%, and 32.50–75.61% in 2020, and 251.44–430.57%, 393.98–721.45%, and 77.97–146.69% in 2021, respectively, compared to the TFP treatment group. The RE_N, RE_P, and RE_K in the BF3 treatment group were also the highest, at 34.53%, 21.09%, and 50.03% in 2020, and 46.00%, 34.09%, and 65.52% in 2021, respectively.

3.5. Economic Benefits

The application of BF affected the economic benefits of fresh eggplant (

Table 4. Effects of BF on economic benefits of eggplant in 2020 and 2021.

Year	Treatments	Output Value (USD ha−1)	Increased Output Value with Fertilizer (USD ha−1)	Fertilizer Inputs (USD ha−1)	Net Incomes (USD ha−1)
2020	CK	11,372 ± 457 d	—	—	11,372 ± 457 d
	TFP	13,565 ± 221 c	2193 ± 677 b	1118	12,447 ± 221 c
	BF1	14,060 ± 398 bc	2688 ± 582 b	1062	12,998 ± 398 bc
	BF2	14,381 ± 485 b	3009 ± 699 b	1230	13,151 ± 485 b
	BF3	15,432 ± 211 a	4060 ± 383 a	1397	14,035 ± 211 a
	BF4	14,547 ± 428 b	3175 ± 240 ab	1565	12,982 ± 428 b
2021	CK	7812 ± 314 e	—	—	7812 ± 314 e
	TFP	12,747 ± 252 d	4935 ± 440 d	1118	11,629 ± 252 d
	BF1	15,730 ± 201 c	7918 ± 476 c	1062	14,668 ± 201 c
	BF2	16,513 ± 318 b	8701 ± 245 b	1230	15,283 ± 318 b
	BF3	18,998 ± 296 a	11,186 ± 426 a	1397	17,601 ± 296 a
	BF4	17,014 ± 400 b	9202 ± 342 b	1565	15,449 ± 400 b

Note: Different lowercase letters denote significant differences among treatment means at the α = 0.05 level using Duncan’s MRT method. The results are presented as the mean value ± standard error.

). The output of fresh eggplant in the BF treatment group increased by 495–1867 USD ha⁻¹ in 2020 and 2983–6251 USD ha⁻¹ in 2021 compared to the TFP treatment group. The corresponding rates of increase were 3.65–13.76% in 2020 and 23.40–49.04% in 2021. After the cost of the fertilizers was deducted, the net income from fresh eggplants treated with BF increased by 535–1588 USD ha⁻¹ in 2020 and 3039–5972 USD ha⁻¹ in 2021, and the increase rates were 4.30–12.76% in 2020 and 26.13–51.35% in 2021. The net income in the BF3 treatment group was the highest in both years.

4. Discussion

4.1. Developmental Potential of Biochar-Based Fertilizer

Biochar has obvious advantages for carbon sequestration and emission reduction, water and fertilizer preservation, and soil improvement, thus addressing the difficulties in sustainable agricultural development, environmental protection, and governance [35,36]. However, biochar application has some limitations for agricultural production at present due to the following fundamental reasons: (i) the application amount of biochar and input costs are high, (ii) the application method of biochar is controversial, and (iii) the economic benefit and output following biochar application is unclear [37,38,39,40]. Biochar-based fertilizer is produced through the secondary processing of biochar and other mineral fertilizers. Thus, the granulation effect and the quantity ratio effects are not only exerted but also temporally and spatially consistent, which is beneficial for reducing agricultural production costs and improving the commercial utility of biochar [38]. Therefore, biochar-based fertilizer may be a new developmental direction in agriculture.

4.2. Biochar-Based Fertilizer for Improving Yield and Quality of Crops

The advantages of BF in improving crop production and soil environmental quality have been confirmed [41,42]. The present study showed that the yield and quality of fresh eggplant following BF treatment improved significantly (Figure 1, Table 2). Due to its loose and porous characteristics, biochar can improve the physical properties and soil porosity of clayey yellow soil in the karst region [31]. Moreover, the characteristics of biochar with a large SSA and high adsorption capacity make it possible to absorb fertilizer nutrients through pore closure and surface adsorption following granulation with chemical fertilizers, thus delaying the fertilizer efficiency of BF [43,44]. This view was confirmed in the study of Chew et al. [45] in another experiment. They pointed out that the nutrient release rate of chemical fertilizer was much higher than that of BF, and most of the released nutrients were leached and lost in the early stage of crop growth. However, the nutrient release rate of BF was slow, which can provide the nutrients needed by plants in the later stage of growth, promote the accumulation of fruit nutrients, and improve the quality of crops [45]. Melo et al. [46] observed that BF was able to further contribute to an increase in productivity beyond that of conventional fertilizers, especially when involving N fertilizers. This is mainly attributed to the following: (i) BF increases the photosynthetic rate of crop leaves, which is caused by BF reducing the limitation of photosynthesis by non-stomatal factors, thus increasing the accumulation of carbohydrates, and (ii) increases in the specific root surface area, root branching, and fine roots under field conditions in soils result in higher crop productivity [47]. Biochar not only contains several elements such as N, P, and K but also contains abundant mineral nutrients, including Ca, Mg, and Zn, contributing to a balanced nutrient supply [48]. Meanwhile, the mineral elements in biochar could promote the synthesis of related enzymes in plants, thus promoting an increase in the VC and soluble sugar contents in fruits and reducing the content of acids in fruits [49]. Notably, biochar could affect the processes in the N cycle in the agricultural ecosystem, which may reduce the excessive nitrate uptake by plants [50,51].

4.3. The Functions of BF in Improving Fertilizer Utilization Rate and Economic Benefits

Based on the results of this study, the AE and RE of nutrients following BF treatment increased significantly, indicating that BF could significantly improve the utilization rates of fertilizers in the karst region, especially N and K fertilizers (Table 3). Studies have shown that biochar in BF can adsorb nitrogen (NH₄⁺ or NO₃⁻) in soil and fertilizer on its surface with a relatively large number of exchange ions and active carboxyl, hydroxyl, and other functional groups, thus preventing nitrogen from leaching downward into the subsoil, reducing fixed and gaseous losses of nitrogen, and improving the efficiency of nitrogen utilization [52,53]. Moreover, biochar can stimulate the activity of bacteria related to nitrogen, and owing to its porosity and large surface area, it can provide a habitat for microorganisms, which is conducive to adsorbing microorganisms, ultimately affecting the processes of the nitrogen cycle in the soil system [54]. Biochar application can provide a carbon source for soil nitrogen-fixing microorganisms, which can also promote their growth [55]. Some studies have shown a significant positive correlation between the abundance of soil nitrogen-fixing bacteria and the activity of nitrogen-fixing enzymes and the soil carbon content. An increase in the SOC content is crucial for biochar to promote biological nitrogen fixation and improve nitrogen utilization efficiency [55]. Some studies have shown that K on the surface of biochar can be quickly released for absorption and utilization by plants after application into the soil, which is related to the high availability of K in biochar [56,57]. The carboxyl functional groups in biochar can improve the adsorption capacity of soil for cations, thus increasing the probability of K⁺ entering the soil lattice, which is conducive to improving the utilization rate of potassium [58,59]. However, the increase in AE and RE may be the result of the inherent nutrient composition of the biochar. Therefore, during the preparation of biochar-based fertilizers, nitrogen and potassium can replace chemical fertilizers in the future.

The economic benefit is an important index to measure the increase in farmers’ output and income. The results show that the net income following BF treatments increased by 4.30–12.76% in 2020 and 26.13–51.35% in 2021 (Table 4), similar to the results of previous studies [60,61]. At present, most research on biochar focuses on soil and environmental effects but studies on economic benefits are relatively scarce. This is mainly because the production and transportation costs of biochar are relatively high; its large-scale application will lead to higher production costs in the early stage, and the economic benefits cannot be increased rapidly [62,63]. However, the application of BF not only ensures a balanced supply of various nutrients but also improves the crop yield and quality following a reduction in the amount of fertilizer, thus achieving a comprehensive utilization of fertilizer and agricultural wastes; this has obvious ecological and economic benefits [64,65]. The optimal application amount of BF was estimated using a linear model, and the results show that the application of 2544–2625 kg ha⁻¹ was optimal (Figure 2), which can be popularized in agricultural production in the karst region. Furthermore, future research needs to focus on the exploration of new types of BF to improve its nutrient-controlled release performance to the maximum. In addition, functional biochar-based fertilizers should be prepared according to the soil status and production demands of different regions.

5. Conclusions

In conclusion, biochar-based fertilizer significantly improved the yield, quality, fertilizer utilization, and economic benefits of eggplant cultivation in open fields in the karst region of Southwest China. The application of biochar-based fertilizer is a nutrient-efficient management strategy for open-field eggplant cultivation in Southwest China, which increases the production capacity and economic benefits while reducing nutrient leakage. Although there have been many reports on the impact of biochar-based fertilizer on crop growth, studies on the underlying mechanism of action are lacking. Therefore, more fieldwork should be conducted to provide experimental support for the application of biochar-based fertilizer.