Evaluation of Major Soil Nutrients After the Application of Microbial-Inoculated Acidified Biochar Pellets Using a Sigmoid Function

Abstract

This experiment aimed to investigate nutrient dynamics in soil and compare plant growth responses after treatment with acidified biochar pellets inoculated with microorganisms during Kimchi cabbage cultivation, using a sigmoid function model. The treatments included the following: Control–only guano application; ABPM 27 (Pseudomonas fluorescens 22BCO027); and ABPM 86 (Bacillus megaterium 22BCO086). Guano and biochar pellets were applied at 320 kg ha⁻¹, based on the recommended nitrogen application rate for cabbage cultivation. The results showed that the cumulative NO₃-N and P₂O₅ in the ABPM 27 treatment were 27.7% and 12.1% higher, respectively, compared with the control. The maximum cumulative K was not significantly different (p > 0.05) between the treatments. The cumulative NH₄-N and NO₃-N were well fitted (R² > 0.824) to the sigmoid curves, while the cumulative P₂O₅ and K were well described with the linear function (R² > 0.970) regardless of treatment. The highest yield was 77.4 tonnes ha⁻¹ under the ABPM 27 treatment. Therefore, the ABPM 27 treatment is strongly recommended for enhancing cabbage yield in organic farming due to its high capacity for accumulating NO₃-N and P₂O₅.

1. Introduction

Cruciferous vegetables, including nutritious options such as broccoli and cabbage, play a vital role in global food systems and health. In 2020, production was reported as 70.9 million tonnes, and the high nutritional value, fiber content, and potential health benefits of these vegetables make them essential staples in many cuisines around the world [1]. These crops serve as a sustainable source of nutrition, including protein [2]. However, substantial losses of fertilizer components are observed during cultivation: nitrogen (N) by 40–70%, phosphorus (P) by 80–90%, and potassium (K) by 50–70% [3,4]. These losses have contributed to eutrophication due to excessive chemical fertilizer use [5]. Addressing these challenges requires sustainable approaches, such as incorporating organic resources like rice hull biochar as slow-release biofertilizers [6]. Biofertilizers play a crucial role in supporting environment-friendly agroecosystems. Biofertilizers represent an effective alternative to chemical fertilizers, enhancing agricultural productivity in a sustainable manner [7]. Plant growth-promoting rhizobacteria (PGPR), a type of biofertilizer, are particularly promising because they provide benefits to both plants and the environment [8]. PGPR directly promote plant growth through nutrient-absorption mechanisms, including nitrogen fixation and phosphate solubilization [9,10]. The diversity of the soil microbial community is assumed to be altered with biochar pellets inoculated with Pseudomonas fluorescens and Bacillus megaterium. The B. megaterium strain has demonstrated an ability to assimilate nitrate in soil [11]. Through this process, it reduces nitrate to nitrite, thereby enhancing nitrogen availability for plant utilization. Furthermore, P. fluorescens is highly effective in root colonization and plays a vital role in stress responses and nutrient recovery [12]. Additionally, biochar inoculated with B. megaterium has demonstrated an ability to mobilize soil phosphorus (P) by enhancing the diversity of the bacterial community. Beneficial soil bacteria, primarily Bacillus and Pseudomonas, exhibit excellent traits for improving soil quality [13,14]. A crucial component of enhancing soil fertility in the rhizosphere is the ability to supply plants with essential nutrients. Phosphorus and nitrogen deficiencies pose significant constraints in agroecosystems [15], and enhancing the availability of these nutrients can significantly improve soil productivity. Some biochars exhibit extremely alkaline pH levels, which can create unfavorable conditions for soil microbial communities. Therefore, pH adjustment is necessary to support microbial biodiversity and improve the biochar’s capacity to promote the proliferation of beneficial microorganisms. Additionally, acidification of biochar increases the number of positively charged sites, thereby enhancing its ability to adsorb anions [16]. Hence, we propose to load the selected microorganisms onto acidified biochar to enhance the mineralization of key plant nutrients, thereby increasing the yield of napa cabbage to make Kimchi (Brassica rapa subsp. pekinensis) in organic farming practices.

Animal waste is a valuable source of major plant nutrients, offering a sustainable alternative to chemical fertilizers [17]. However, excessive application of manure compost can lead to environmental issues, such as nutrient runoff from croplands. To address this, developing environmentally safe methods to mitigate non-point source pollution is essential. One promising approach is the development of microbial acidified biochar pellets as biofertilizers to enhance nutrient mineralization rates. Several studies have highlighted the synergistic effects of biochar blended with nutrient-rich manure in improving crop yields [18]. Healthy soil preserves microbial biodiversity, which regulates the plant pathogens and plant nutrients for plant growth, promoting positive symbiotic associations with plant roots and ultimately enhancing soil productivity [19]. Biochar is also considered a carrier for plant nutrients and microbial inoculants in soil due to its ability to slowly release nutrients and its massive porous structure for microbial activity [20,21,22]. In normal plant growth, soil nutrient supply should eventually correspond to the sigmoid pattern of nutrient uptake over growing periods. Therefore, the acidified biochar pellets inoculated with microorganisms should facilitate the slow release of nutrients for crop growth, as illustrated in Figure 1.

Biochar application can develop a char sphere that significantly affects soil quality by changing microorganism habitats [23]. Biochar has a well-developed porous structure and high stability, which can substantially change soil porosity and aggregation, subsequently affecting soil microorganisms [24]. Acidified biochar improves the bioavailability of nutrients by altering soil chemical properties [25]. However, highly alkaline biochar may inhibit microbial activity, necessitating pH adjustment for optimal performance [26]. Acidified biochar, when combined with fish powder or other organic materials, has been shown to enhance macronutrient release in soils [27]. The inconsistencies in nutrient release observed with biochar necessitate further exploration of its interactions with microorganisms and environmental factors [28,29]. Moreover, the biochar itself can act as a slow-release nutrient in soil [30], while coexisting ions can enhance P or K release [30,31].

Since major plant nutrients in the soil are easily influenced by various environmental factors, predictive models for cumulative nutrient availability during plant growth are often utilized. Therefore, developing a predictive model for the nutrient capacity of acidified biochar pellets inoculated with microorganisms should account for how the individual fertilizer components influence crop growth. Additionally, parameters in nonlinear models significantly influence fertilizer characteristics. Nonlinear growth models, such as sigmoid curves, have proven effective in predicting cumulative nutrient dynamics in soil and their impact on crop growth [32]. The Gompertz [33] and Logistic [34] models, both sigmoid nonlinear models, are widely used in crop growth analysis and have been applied to crops such as squash (Cucurbita pepo) and fresh pepper (Capsicum annuum) [35]. The parameters in sigmoid models offer effective explanations of the growth dynamics, allowing detailed evaluation of fertilizer characteristics and crop responses. Therefore, sigmoid and linear models could be useful tools for predicting cumulative nutrients in soil treated with microorganisms, such as acidified biochar pellets inoculated with P. fluorescens 22BCO027 and B. megaterium 22BCO086.

This study hypothesizes that acidified biochar pellets inoculated with microorganisms contribute to cumulative nutrient availability and enhance crop growth. The primary aim was to evaluate the cumulative major plant nutrients and growth responses under the application of acidified biochar pellets during cabbage cultivation for Kimchi production.

2. Materials and Methods

2.1. Preparations of the Acidified Biochar Pellets Inoculated with a Microorganism (ABPM)

The biochar, guano, and citric acid used in this study are registered as certified organic farming materials in Korea. Rice hull biochar was produced using the top-to-bottom pyrolysis method [36], while guano was obtained from NOUSBO Co. (Suwon, Kyoung Gi Do, Republic of Korea). To acidify rice hull biochar, it was treated with 0.3 M citric acid in a 1:1.7 ratio (biochar/solution), followed by drying at room temperature. The biochar’s pH was adjusted to 2.5, after which the biochar was mixed with guano in a 6:4 ratio. The microorganisms selected for this study were isolated from soil that is typically used for planting Kimchi cabbage. These antagonistic microorganisms were identified through a bioassay test conducted in a pot experiment. The strains were identified using 16S rRNA gene sequencing and compared against the NCBI GenBank database through next-generation sequencing (NGS). The identified strains, Pseudomonas fluorescens 22BCO027 and Bacillus megaterium 22BCO086, were confirmed as antagonistic microorganisms (Figure 2).

P. fluorescens naturally suppresses some soilborne pathogens, while B. megaterium, a rod-shaped Gram-positive bacterium, produces exoenzymes. Both B. megaterium and P. fluorescens are crucial for disease resistance and nutrient mineralization [37]. P. fluorescens 22BCO027 and B. megaterium 22BCO086 were cultured using tryptic soy agar (TSA) and laked brucella blood agar (LBA), respectively, under dark conditions. Each cultured medium was continuously mixed and sprayed onto the acidified rice hull biochar and guano mixture in a 3:1 ratio (mixture/medium) using a blender. These mixtures were processed into pellets using a machine (7.5 KW, 10HP, KemKang Engineering Pellet Mill Co., DaeGu, Republic of Korea) to produce ABPM 27 (acidified biochar pellet inoculated with P. fluorescens 22BCO027) and ABPM 86 (acidified biochar pellet inoculated with B. megaterium 22BCO086). The physicochemical properties of ABPM 27 and ABPM 86 are shown in

Table 1. Physicochemical properties of ABPM 27 and ABPM 86.

Biochar Pellets *	pH (1:20)	EC (dS m−1)	N	C	P2O5	K
			g kg−1		mg kg−1
Guano	7.45	6.00	135.2	163.2	33.2	0.90
Rice hull biochar	10.68	0.88	2.2	562.7	0.1	0.02
ABPM 27	7.54	6.00	43.5	192.6	0.6	0.20
ABPM 86	7.34	5.93	43.6	192.6	0.6	1.10

* ABPM 27 and 86: acidified biochar pellets (rice hull biochar/guano in a 6:4 ratio) inoculated with microorganisms P. fluorescens 22BCO027 and B. megaterium 22BCO086, respectively.

. For ABPM 27 and ABPM 86, the pH values were 7.54 and 7.34, respectively, and the nitrogen content was approximately 4.4%. Notably, the potassium concentration in ABPM 86 was 5.8 times higher than in ABPM 27.

2.2. Cabbage Cultivation

Young cabbage plants (30 days old) were transplanted into an experimental upland field at the National Institute of Eco-friendly Environmental Microbiology, Gyeonggi-do, Republic of Korea. The cabbage variety used, “Hyang-Gem Yellow”, is widely preferred for Kimchi production. Planting was spaced at intervals of 30 cm × 60 cm. The experimental design employed a randomized complete block with three replications. The area of each plot was 12 m² (6 m × 2 m). Treatments included the following: (1) Control (guano application only); (2) ABPM 27 (acidified biochar pellets inoculated with P. fluorescens 22BCO027); and (3) ABPM 86 (acidified biochar pellets inoculated with B. megaterium 22BCO086). Guano and ABPM were applied at N 320 kg ha⁻¹, in accordance with nitrogen recommendations for cabbage cultivation [38]. Before transplanting, guano and ABPM were uniformly distributed and thoroughly mixed with the surface soil using a rotary machine. Additionally, these were incorporated as a basal application across all treatments.

Irrigation was provided periodically via a drip system with nozzles connected to a pump throughout the cultivation period. In the absence of rainfall, the experimental field received irrigation on a weekly basis (Figure 3). Cabbage growth metrics, including bulb height, bulb width, and yield, were measured.

Table 2. Physicochemical properties of the soil used.

Soil Type	pH (1:10)	EC (mS m−1)	O.M (mg kg−1)	NH4-N	NO3-N	P2O5	K
				mg kg−1
Silty loam	6.7	0.03	14.0	14.3	45.9	116.1	12.2

shows the physicochemical properties of the soil used in the experiment.

2.3. Analysis of Physicochemical Properties

Surface soil samples were collected weekly throughout the cultivation period. Dried soil, ABPM, and guano samples were crushed and then sieved through a 2 mm mesh. The pH and electrical conductivity (EC) of rice hull biochar were simultaneously measured in a 1:20 suspension (sample/deionized water) using a pH meter (Thermo Scientific, Waltham, MA, USA, Orion 4-star). Total carbon and nitrogen contents were measured using a CHINS element analyzer (Vario Macro Cube, Langenselbold, Germany). Organic matter content was calculated by multiplying the carbon content by a conversion factor of 1.724. Dried soil samples were extracted with 2 M KCl solution, and the extractants were then analyzed for NH₄-N and NO₃-N concentrations using a Bran-Luebbe Auto Analyzer (Seal Analytical Ltd., Mequon, WI, USA). The extractants using the Mehlich III method [39] were analyzed for PO₄ and K concentrations using UV spectrophotometer kits (C-Mac, Daejeon, Republic of Korea). P₂O₅ concentrations were calculated by multiplying PO₄ values and the conversion factor, 2.29.

2.4. Statistical Analysis

Statistical analyses were performed using one-way analysis of variance (ANOVA) and Duncan’s multiple range test, which are more sensitive than more conservative methods (e.g., Tukey’s HSD or Bonferroni) for detecting subtle differences between multiple treatments. SAS version 9.4 (SAS Institute, Cary, NC, USA) was used for the analyses.

To evaluate nutrient accumulation patterns, three candidate models—rectangular hyperbola (Michaelis–Menten), 4-parameter logistic, and Gompertz—were fitted to the cumulative NH₄⁺-N and NO₃⁻-N data as a function of days after transplanting using non-linear least squares in R. Model performance was assessed based on the coefficient of determination (R²), root mean square error (RMSE), and Akaike information criterion (AIC). These metrics were used to compare the cumulative NH₄⁺-N and NO₃⁻-N data. Among the tested models, the three-parameter sigmoid curve for NO₃⁻-N data provided the best overall fit, achieving the highest R² (0.964) and the lowest RMSE (48.56) and AIC values (95.74). Therefore, sigmoid curves with three parameters were applied to fit the cumulative NH₄⁺-N and NO₃⁻-N data in soils treated with ABPM 27 and ABPM 86 using the following equation:

$$Y={\displaystyle \frac{a}{1+exp(-(\frac{t-t0}{b}\left)\right)}}$$

(1)

where Y(t) is the cumulative nutrient amount (mg) at time t; a is the asymptotic maximum (the plateau value as t → ∞); b is a scale parameter controlling the steepness of the curve; and t₀ is the time at which Y reaches 50% of the maximum value (a/2).

Linear equations were used to evaluate cumulative P₂O₅ and K:

Y = ax + b

(2)

where Y (mg) is the cumulative nutrients, x is the days after transplant, a is the regression coefficient, and b is the intercept with the F-axis.

Standard deviations were calculated to compare cumulative nutrients across treatments. Comparisons of the treatments were analyzed using ANOVA and a subsequent Duncan’s multiple range test, based on the collected data.

3. Results and Discussions

3.1. Cumulative Plant Nutrients in the Soil

Ammonium nitrogen (NH₄-N) is mineralized from organic resources under oxidation conditions and serves as an essential macronutrient for plant growth. The changes in NH₄-N concentrations (a) and cumulative NH₄-N (b) in plots treated with ABPM 27 and ABPM 86 during cabbage cultivation for Kimchi production are shown in Figure 4. In the control, NH₄-N concentration dropped sharply from 85.2 mg kg⁻¹ to 10.2 mg kg⁻¹ within four days post-transplanting. For all treatments, NH₄-N concentrations gradually decreased after day 15 until harvest. However, cumulative NH₄-N increased rapidly until day 15 and continued to rise more gradually throughout the growing period.

The cumulative NH₄-N in the ABPM 27 and ABPM 86 treatments were reduced by 8.9% and 13.8%, respectively, compared with the control on day 15 after transplanting. These releasing patterns might be attributable to high nitrogen mineralization in the control. Sigmoid curves modeling the cumulative NH₄-N for ABPM 27 and ABPM 86 treatments exhibited strong fits (R² > 0.862). However, a hyperbolic model has previously been used for evaluating nutrient models involving blended biochar pellets under similar conditions [40].

The equations for cumulative NH₄-N using Equation (1) in the plots incorporated with the ABPMs throughout the cultivation period are presented in

Table 3. The sigmoid equations fitting three parameters for cumulative NH₄-N during cultivation.

Treatments	Sigmoidal Equations	p-Values	R2
Control	Y = 322.1/(1 + exp(−(t + 0.73)/3.6))	<0.0001	0.825
ABPM 27	Y = 236.8/(1 + exp(−(t + 9.0)/18.7))	<0.0001	0.924
ABPM 86	Y = 277.6/(1 + exp(−(t − 1.7)/1.8))	<0.0001	0.862

Note: Y represents the cumulative NH₄-N. ABPM 27 and ABPM 86 denote biochar pellets inoculated with microorganisms P. fluorescens 22BCO027 and B. megaterium 22BCO086, respectively (ANOVA).

. The highest cumulative NH₄-N was 322.1 mg in the control treatment. In addition, the cumulative NH₄-N in the ABPM 27 and ABPM 86 treatments was low due to biochar’s absorption capacities and microbial immobilization. The sigmoid curves fitted well with the measured NH₄-N in the ABPM 27 and ABPM 86 treatments (R² > 0.862). The high R² values indicate that the sigmoid model provided an excellent fit with the observed cumulative NH₄-N data. The modified hyperbola equation was used for evaluating the nutrient accumulation with a blended biochar pellet [41]. Additionally, the hyperbola model fitted well for accumulated NH₄-N in the batch column experiments under reduction conditions with precipitation periods.

The changes in NO₃-N concentrations (a) and cumulative NO₃-N (b) for plots treated with ABPM 27 and ABPM 86 are illustrated in Figure 5. In the ABPM 27 treatment, NO₃-N concentrations peaked at day 15 after transplanting, declined sharply until day 36, and then remained stable throughout the harvest period. Additionally, the cumulative NO₃-N in the control increased sharply until 15 days after transplanting and then remained steady. However, in the ABPM 27 treatment, it continued to rise until day 36 post-transplant before stabilizing. This suggests that the cumulative NO₃-N in the ABPM 27 treatment was prolonged by 16 days due to biochar’s sorption capacity. Also, the ABPM 27 treatment exhibited a 27.7% higher release than in the control, indicating enhanced microbial activity of P. fluorescens under the application of acidified biochar pellets throughout the growing season.

Nitrate nitrogen (NO₃-N) is crucial for plant growth but poses environmental risks, including water contamination [42]. Controlling NO₃-N concentrations is critical for mitigating these risks, while supporting crop productivity. The observed increase in cumulative NO₃-N under the ABPM 27 treatment highlights the potential of P. fluorescens to enhance soil nitrogen dynamics. Although acidified biochar has been shown to reduce ammonia losses by up to 60% [43], its effects on extractable NH₄-N and NO₃-N concentrations vary depending on soil conditions.

Sigmoid equations for the cumulative NO₃-N, as determined using Equation (1) in the plots incorporated with the ABPM treatments, are presented in

Table 4. The sigmoidal equations fitting three parameters for cumulative NO₃-N during cultivation.

Treatments	Sigmoidal Equations	p-Values	R2
Control	Y = 612.3/(1 + exp(−(t − 5.2)/3.5))	<0.0002	0.927
ABPM 27	Y = 863.6/(1 + exp((t − 7.8)/4.2))	<0.0001	0.982
ABPM 86	Y = 663.1/(1 + exp(−(t − 4.1)/2.1))	<0.0001	0.972

Note: Y is the cumulative NO₃-N from sigmoidal equations. Means values indicate significant differences (p < 0.001) between treatments (ANOVA).

. The maximum cumulative NO₃-N in the control was 612.3 mg, and this value was the lowest among the treatments during the cultivation periods. The cumulative NO₃-N from all the treatments fitted the sigmoid model closely (R² > 0.927). The high R² values indicate that the sigmoid model provided an excellent fit with the observed cumulative NO₃-N data. This suggests that the results may have coincided with plant growth curves during cultivation periods. Moreover, the ABPM 27 treatment resulted in a prolonged and elevated supply of NO₃⁻-N relative to the control. The hyperbola model was also fitted to cumulative NO₃-N in batch column experiments with blended biochar pellets, as described by [41].

Figure 6 shows the changes in P₂O₅ concentrations (a) and cumulative P₂O₅ (b) using linear equations in the plots incorporated with the ABPM 27 and ABPM 86 treatments. In the control and ABPM 27 treatments, P₂O₅ concentrations decreased until day 8 after transplanting, peaked on day 15, and then gradually declined. The highest P₂O₅ concentration of 185.2 mg kg⁻¹ was recorded in the ABPM 27 treatment on day 15.

Phosphorus (P) is one of three macronutrients essential for plant growth. It plays critical roles in plant cultivation, affecting soil fertility and crop productivity. The P solubility of biochar is very low due to binding either with lime or with clay particles [44], in which Ca-phosphates are the most common forms of P. However, runoff from excessive application of P fertilizer in the cropland causes eutrophication in rivers. Biochar incorporation is an eco-friendly option for overcoming this problem in a sustainable agricultural system [45]. However, acidified biochar can enhance phosphorus availability, making it a sustainable alternative to chemical P fertilizers [46]. This improvement may result from its ability to lower pH and introduce organic functional groups (e.g., carboxyl groups from citric acid), which facilitate the dissociation of calcium phosphate compounds. By promoting the separate solubilization of calcium and phosphate, acidified biochar increases the pool of plant-available phosphorus, especially in calcareous or alkaline soils where P is often immobilized.

Acidified biochar with sulfuric acid increases phosphorus (P) solubility and enhances P recovery by 2 to 35 times [47]. Phosphate-solubilizing bacteria increase the abundance of genes associated with inorganic P solubilization and organic P mineralization. The potential of inoculating biochar with phosphate-solubilizing bacteria to improve plant phosphorus uptake has been discussed [48,49]. Moreover, organic material could enhance the soil’s bacterial biodiversity [50,51].

Linear equations to describe the cumulative P₂O₅ in soil incorporated with the ABPMs using Equation (2) were described in Figure 5b. The cumulative P₂O₅ in the ABPM 27 treatment was 1481.68 mg, which was 12.1% higher than the control. Linear regression models for all treatments showed excellent fits (R² > 0.970).

Potassium (K) is essential for vegetable productivity, and biochar has been shown to enhance its availability by reducing leaching losses [52,53]. Changes in K concentrations (a) and fitting with the linear model on cumulative K (b) are shown in Figure 7. K concentrations fluctuated inconsistently during cultivation, ranging from 13.2 mg kg⁻¹ to 9.7 mg kg⁻¹ across treatments. There were no significant differences between treatments (p > 0.001). The cumulative K was well fitted to linear models (R² > 0.999). However, ABPM treatments had limited impact on cumulative K, indicating a minimal influence on soil potassium dynamics during cabbage cultivation. Despite the high potassium content in ABPM treatments (Table 1), their limited effect on cumulative soil K may be attributed to the relatively low microbial involvement in K cycling and the strong adsorption of K⁺ onto soil colloids, which reduces leaching and immobilization compared with N and P [54,55].

3.2. Plant Growth Responses

The growth responses of cabbage under the treatments are shown in

Table 5. Growth responses of Kimchi cabbage under the different treatments.

Treatments	Bulb Height (cm)	Bulb Width (cm)	Yield (tonnes ha−1)
Control	35.8 a	19.8 a	71.3 b
ABPM 27	36.1 a	19.0 ab	77.4 a
ABPM 86	34.9 a	15.8 c	70.2 bc
F	4.9	10.4	12.5
p-values	<0.004	<0.00002	<0.001

Mean values followed by different letters indicate significant differences (p < 0.001) between treatments in bulb height, bulb width, and yield (ANOVA and subsequent Duncan’s multiple range test). Bulb height and width (n = 15), and yield estimated as bulb weight (n = 9) × 1450.

. Plant growth parameters, including bulb height, bulb width, and yield were measured to evaluate the effects of ABPM treatments. The lowest values for bulb height (34.9 cm plant⁻¹), bulb width (15.8 cm plant⁻¹), and yield (70.2 tonnes ha⁻¹) were observed in the ABPM 86 treatment. However, no significant differences in bulb height or bulb width were noted between the control and ABPM 27 treatments (p > 0.05). The highest yield (77.4 tonnes ha⁻¹) was observed in the ABPM 27 treatment, likely due to increased NO₃-N and P₂O₅ concentrations in the soil.

Application of ABPM 27 enhanced cabbage yield by 8.6% compared with the control in this study, which is lower than the increase reported in other studies. This might be attributed to the cultivation methods and crop types. The application of biochar has been shown to enhance crop production by approximately 10% in agroecological systems [56]. However, excessive application (>30 tonnes ha⁻¹) can reduce nitrogen use efficiency and crop yield [57]. Acidified biochar, when used appropriately, has been reported to increase shoot biomass by up to 3.5 times compared with controls [47]. Additionally, the use efficiency of N, P, and K depends on the amount of plant nutrients supplied by the biochar-based microbial fertilizer [58].

In this study, the yield of cabbage increased by 8.6% under ABPM 27 treatment compared with the control. While environmental factors and nutrient-microorganism interactions can influence plant growth, these variables were controlled to minimize their impact on growth responses. Further research should include investigations into its impact on soil microbial community structure and function beyond the initial inoculation, as well as evaluations of its economic viability for farmers.

4. Conclusions

The application of ABPM (acidified biochar pellets inoculated with microorganisms) was evaluated for major nutrient dynamics in soil using sigmoid and linear equations and plant growth responses during cabbage cultivation for Kimchi production. Experimental results showed that the cumulative NO₃-N in the ABPM 27 treatment was 27.7% higher than in the control, while the cumulative P₂O₅ increased by 12.1% compared with the control. Additionally, the cumulative NO₃-N in the ABPM 27 treatment was delayed by 16 days due to biochar’s sorption characteristic. Such an effect is beneficial, as it indicates prolonged nitrogen availability in the root zone, potentially enhancing crop uptake efficiency and minimizing nitrate leaching losses. Cumulative NH₄-N and NO₃-N were well fitted to sigmoid models (R² > 0.825), whereas cumulative P₂O₅ and K aligned closely with linear models (R² > 0.970). The maximum cumulative K showed no significant differences (p > 0.05) between treatments. Additionally, the ABPM 27 treatment enhanced cabbage yield by 8.6% compared with the control. The ABPM 27 treatment demonstrated potential benefits as an organic fertilizer, enhancing NO₃-N and P₂O₅ concentrations in the soil while improving cabbage yield. These findings support the use of ABPM 27 in organic farming systems, along with nutrient management, as the NO₃-N component was supplied more steadily and in greater amounts than in the control. Further research is recommended to optimize application rates and assess the long-term effects of this approach on soil health and crop productivity.