Co-Application of Bokashi and Biochar Alleviates Water Stress, Improves Soil Fertility and Enhances Wheat Production Under Water-Deficit Conditions

Abstract

Water stress and nutrient stress are major limiting factors affecting crop productivity. Biochar-based organic fertilizers improve soil nutrient availability, water use efficiency (WUE), and crop yields under these adverse conditions. This study investigated the mechanistic effects of biochar–bokashi mixtures under a controlled glasshouse pot experiment on soil fertility, available nutrients, soil moisture, plant water use efficiency (PWUE), and wheat yield parameters under three moisture levels. Four treatments were included, (1) a control, (2) bokashi only, (3) 1% biochar + bokashi, and (4) 2% biochar + bokashi, under 30% (IR₃₀), 50% (IR₅₀), and 60% (IR₆₀) field capacity, totaling twelve treatments in a completely randomized design with three replications. The combined bokashi–biochar application significantly (p < 0.05) improved growth parameters and yields, including plant height, number of fertile tillers (NFT), number of spikes (NS), spike length (SL), 1000-grain weight, biological yield (BY), root biomass, and grain yield (GY), compared to the control and bokashi-only treatments. Bokashi with 1% biochar exhibited superior agronomic performance over the other treatments, including 2% biochar. Biochar addition enhanced soil moisture and PWUE across irrigation levels. Bokashi–biochar treatments under IR₃₀ outperformed the control and bokashi-only treatments under IR₆₀, highlighting biochar’s effectiveness in alleviating water stress and increasing yields. Moreover, co-application significantly increased soil pH while enhancing the organic carbon, total nitrogen, available phosphorous and exchangeable potassium nutrient levels, which positively correlated with yield. Bokashi–biochar mixtures have been proven to be an effective strategy to enhance soil fertility, increase soil moisture to alleviate water stress and support sustainable wheat production under water- and nutrient-limited conditions.

1. Introduction

Wheat (Triticum aestivum L.) is a major cereal crop and ranks first among grain crops worldwide in both area and production [1]. Global wheat productivity is sharply declining due to water shortages [2,3]. Crop growth and productivity in regions with low and irregular rainfall are being affected by abiotic stresses such as drought, raising global concern [4,5]. Due to climate change, drought is expected to increase in future, posing additional challenges to global food security [6]. Improving yield sustainably, especially under water-deficit conditions, is crucial as global population is increasing, intensifying the competition for food demand. However, maintaining continuous productivity for future generations without compromising environmental sustainability is another significant challenge.

Biochar, a carbon-rich product, is widely used as a soil amendment to enhance soil fertility and increase soil carbon sequestration, thereby contributing to food security and climate change mitigation [7,8,9]. The unique properties of biochar, such as its relatively larger surface area and high porosity, enhance nutrient and water retention, making it available to plants for relatively longer periods, thereby alleviating nutrient and water stress in nutrient- and water-deficit areas [8,10]. Biochar application increases water use efficiency (WUE) and plant productivity in water-stressed areas [6,10,11].

To date, numerous studies have demonstrated the net benefits of using biochar, including increased carbon stocks, improved soil physicochemical properties such as pH, organic carbon (OC), cation exchange capacity (CEC), and plant-available water (PAW) [12], enhanced microbial activity [13,14], reduced nutrient leaching [15] and enhanced crop growth [8]. Amending soil with biochar can lead to favorable changes in fine-root proliferation and crop development by decreasing soil compaction, increasing aeration and enhancing soil water retention and nutrient content [16]. The benefits of biochar application in improving the growth and yield of different crops under drought stress conditions have been well documented [16,17,18,19,20]. Studies have shown that biochar alleviates moisture stress and enhances plant growth and yield under water-deficit conditions. Specifically, it improves wheat growth and yield [21]; the plant height and leaf area in okra [18] and maize [22]; growth, yield and quality of tomato [23], winter rapeseed [24], sunflower [20] and chickpea [25]; quinoa growth and seed nutritional value [19]; and the growth and productivity of eggplant [25] under water-deficit conditions. Biochar, when used in combination with other organic fertilizers such as cattle urine, manure, bokashi and compost, has improved soil fertility and increased crop productivity sustainably with minimal environmental impact [26,27], producing positive synergistic effects under water-deficit conditions [28,29].

Bokashi, a composting method developed in Japan, is a fermented organic amendment created through the anaerobic fermentation of organic waste, such as kitchen waste or agricultural residues, using effective microorganisms. It is a nutrient-rich, odor-free amendment that can be directly applied to the soil [30]. Research indicates that bokashi can improve soil structure, increase available nutrients like N, P and K, and increase microbial activity, leading to better plant growth and increased stress [31,32,33]. Bokashi has a positive influence on the yield and quality of various crops, including arugula (Eruca sativa L.), beet, broccoli and cabbage, as well as soybean, bitter melon, tomato, lettuce, strawberry and radish [32,34,35,36,37,38]. Additionally, bokashi enhances soil nutrients and microbial communities by increasing beneficial microbes and reducing plant pathogens, which improves overall plant growth. Soils amended with bokashi have increased chlorophyll levels and dry biomass in cucumber and kale plants [39].

The individual benefits of biochar and bokashi application are well documented across various crops. However, their combined effects on wheat growth and soil fertility under drought stress remain largely unexplored. To better understand the impact of these amendments on alleviating water stress and promoting wheat growth under various water regimes, a mechanistic study under controlled glasshouse conditions is crucial. This research aims to elucidate how biochar and bokashi amendments influence wheat crops under varying soil moisture regimes and whether these mixtures can alleviate moisture stress while increasing yields. We hypothesize that integrating these amendments will increase soil moisture content, enhance plant water use efficiency (PWUE) and improve soil fertility, thereby enhancing soil nutrient availability, mitigating water stress and enhancing wheat productivity compared to the control and bokashi-only treatments. This research aims to provide a sustainable agricultural practice that could be crucial for regions facing water scarcity.

2. Materials and Methods

2.1. Soil Sampling and Properties

A pot experiment was conducted at Saitama University, Japan, from November 2020 to March 2021 to examine the effects of co-applying biochar and bokashi under different soil moisture levels on winter wheat growth and yield. During the experiment, the average temperature inside the greenhouse was 25.75 °C, with a minimum of 16 °C and a maximum of 35.5 °C, while the average humidity was 76.4%. Before the experiment, soil samples were collected using an auger from a nearby research farm at a 0–25 cm depth. The samples were air-dried, sieved (2.0 mm), thoroughly mixed, and analyzed for texture, pH, organic carbon, total nitrogen (N), available N, total phosphorus (P), and extractable potassium (K) per Japan International Cooperation Agency (JICA) [40]. The soil was classified as well-drained Hafizabad loam with a slightly acidic pH (5.8), containing 6.5 g kg⁻¹ of soil organic carbon, 1.4 g kg⁻¹ of total N, 30.18 mg kg⁻¹ of available N, 22.37 mg kg⁻¹ of available P, and 56.83 mg kg⁻¹ of exchangeable K.

2.2. Production and Characterization of Biochar and Bokashi

The biochar used in this experiment was produced with the pyrolysis of corn stover using a pilot batch-type carbonizer (model ECO500, Meiwa Co. Ltd., Kanazawa, Japan) at 400 °C. The biochar was cooled, ground, sieved (2 mm) and analyzed. Biochar was characterized by pH (9.57), N (11.3 g kg⁻¹), P (0.59 g kg⁻¹) and K (0.97 g kg⁻¹). The bokashi used in this study was sourced from Daiwa Fertilizer Company, Japan, had pH 5.7, and contained 6.0% total nitrogen (N), 4.2% total phosphorus (P) and 2.0% total potassium (K).

2.3. Pot Experiment and Treatments

The experiment was conducted in a glasshouse, where 5 kg of fresh soil was placed into 6 L plastic pots. The pots were arranged in a completely randomized factorial design with three replications per treatment. To prevent shading effects or location biases, the pots were regularly moved to ensure uniform exposure to environmental conditions. In our experiment, we followed the crop fertilization guidelines of the Saitama Prefecture (20 t ha⁻¹ bokashi), applying 44 g of bokashi to each pot, except for the control treatment. Biochar was applied at three rates, 0% (0 g), 1% (50 g), and 2% (100 g), corresponding to 20 and 40 t ha⁻¹, respectively, calculated as a percentage of soil weight using a soil bulk density of 1.51 t m⁻³. Three irrigation regimes (30%, 50% and 60% field capacity) were maintained throughout the 120-day experiment, with irrigation every two days. Manual weeding was performed across all treatments. The wheat cultivar ‘Satonosara’ was sown, and five plants per pot were maintained by thinning on 20 days of sowing. The treatment details are provided in

Table 1. Description of bokashi and biochar treatments under different irrigation regimes.

Treatments	Irrigation Regime (%)	Bokashi Rate (t ha−1)	Biochar Rate (t ha−1)	Irrigation (% Field Capacity)
Control (C)	IR60	0	0	60
	IR50	0	0	50
	IR30	0	0	30
Bokashi only (B0)	IR60	20	0	60
	IR50	20	0	50
	IR30	20	0	30
Bokashi + 1% biochar (B1)	IR60	20	20	60
	IR50	20	20	50
	IR30	20	20	30
Bokashi + 2% biochar (B2)	IR60	20	40	60
	IR50	20	40	50
	IR30	20	40	30

.

2.4. Data Recording

2.4.1. Plant Growth Parameters

Plant growth and yield parameters such as plant height (HP), number of tillers, fertile tillers (NFT), spikes (NS), spike length (LS), straw length, grains per spike, number of grains, 1000-grain weight (GW), biological yield (BY), and grain yield (GY), were recorded following standard protocols at crop maturity (120 DAS) as explained earlier by Pask et al. [41]. Plant height was measured from the base to the spike tip (excluding awns) and averaged, while total and productive tillers were counted per pot. Spike length and grains per spike were averaged per pot. Grain was harvested and separated from biomass manually. Grains were sun-dried, and the 1000-grain weight was calculated from an electronic balance. Biological yield was measured by sun-drying wheat for a week. Grain yield was determined by air-drying grains to a constant weight, and shoot dry weight was recorded by oven-drying above-ground parts at 80 °C for 48 h. Root traits were assessed by soaking soil clumps, separating roots on a 1 mm mesh sieve, and oven-drying at 70 °C until a constant weight was reached.

Plant water use efficiency (PWUE) was determined using standard calculation as:

$$\mathrm{P}\mathrm{W}\mathrm{U}\mathrm{E} \left(\mathrm{m}\mathrm{g} {\mathrm{m}\mathrm{L}}^{-1}\right)={\displaystyle \frac{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{b}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}}$$

2.4.2. Soil Analysis

After harvest, the soil samples from each pot were air-dried for 72 h. Soil pH and electrical conductivity (EC) were measured in a 1:5 (w/v) water ratio using a pre-calibrated pH/EC meter. Soil moisture (SM) was determined by the Black method, measuring weight differences before and after oven drying at 105 °C until reaching constant weight. Soil organic matter (OM) was determined by Loss on Ignition (LOI), with samples oven-dried at 105 °C, then ignited at 550 °C for 3 h in a muffle furnace. Soil organic carbon was calculated using van Bemmelen’s factor (1.724) based on OM [42]. Total nitrogen content was analyzed using a C–N Corder (Yanaco, MT-500, Yanagimoto Co. Ltd., Kyoto, Japan), while available nitrogen was determined using a QuAAtro39 continuous segmented flow analyzer (BL-Tec, Osaka, Japan; SEAL Analytical, Norderstadt, Germany). Available phosphorus (P) was determined by the Bray 1 method with a spectrophotometer. Extsection ractable and exchangeable potassium concentrations were measured using inductively coupled plasma mass spectroscopy (ICP-MS, Agilent, Santa Clara, CA, USA).

2.5. Statistical Analysis

The data were subjected to a two-way ANOVA using R statistical software (version 3.6.3) to assess the main and interactive effects of treatments and the irrigation regime on plant and soil parameters. The means for the main effect of treatments and irrigation regimes were compared using the least significant difference (LSD) test at the 5% level. Both linear and non-linear regression analyses were performed to investigate the relationships between the selected independent variables and dependent variables to explain the model. A correlation matrix was created combining growth, yield and soil parameters, following the R corr matrix function.

3. Results

3.1. Plant Growth Parameters and Above-Ground Biomass

The effects of bokashi–biochar co-application, with three treatments for various irrigation levels, on plant growth and yield parameters are presented in

Table 2. Effect of biochar application rates on yield parameters of wheat under different irrigation regimes (mean ± SE, n = 3).

Treatment	Irrigation Regime (%)	HP (cm)	NFT (Nos.)	NS (Nos.)	LS (cm)	1000 GW (g)	BY (g pot−1)	GY (g pot−1)
Control	IR60	33.8 ± 0.2 f	5 ± 0 e	5 ± 0 f	4 ± 0 f	22.1 ± 0.1 g	29.62 ± 0.1 f	6.8 ± 0 g
	IR50	32.8 ± 0.1 f	5 ± 0 e	5 ± 0 f	3.8 ± 0 fg	21.4 ± 0.1 g	25.7 ± 0.2 fg	5 ± 0 gh
	IR30	24.3 ± 0.1 g	5 ± 0 e	5 ± 0 f	3 ± 0 g	19.4 ± 0.1 g	20.84 ± 0.1 g	3.5 ± 0 h
Bokashi only	IR60	48.2 ± 0.2 d	18 ± 0.6 cd	12 ± 1.5 d	6.1 ± 0.1 e	32.1 ± 0.1 ef	59.14 ± 0.1 cd	17.1 ± 0.1 bc
	IR50	43 ± 0.2 e	15 ± 1 cd	11 ± 0.6 de	5.7 ± 0.4 e	35.3 ± 0.2 e	55.04 ± 0.1 d	14.6 ± 0.1 de
	IR30	37 ± 0.2 f	13 ± 1.2 d	9 ± 0 e	5.2 ± 0 e	31.7 ± 0.1 f	44.23 ± 0.1 e	9.4 ± 0.1 f
Bokashi + 1% biochar	IR60	69.6 ± 4 a	28 ± 1.5 a	23 ± 2 a	12 ± 0.4 a	53.1 ± 3.3 ab	78.72 ± 1.9 a	22.3 ± 0.5 a
	IR50	67.8 ± 2.4 a	19 ± 1.2 bc	20 ± 0.6 b	11.7 ± 0.6 a	54.6 ± 1.1 a	66.93 ± 6.4 bc	18.4 ± 0.9 b
	IR30	54 ± 2.5 c	18 ± 3.6 bc	18 ± 0.6 bc	9.1 ± 0.4 c	43.5 ± 1.3 d	68.59 ± 1 b	15 ± 0.8 de
Bokashi + 2% biochar	IR60	60.4 ± 0.3 b	22.3 ± 1.5 b	18 ± 1.7 bc	10.7 ± 0.2 b	48 ± 1.4 c	70.4 ± 6.1 b	18.6 ± 0.5 b
	IR50	58.7 ± 1.1 bc	19 ± 1.7 bc	18 ± 0.6 bc	10.5 ± 0.3 b	49.9 ± 1.2 bc	64.2 ± 1.4 bc	15.7 ± 1.9 cd
	IR30	54.7 ± 1.2 c	18 ± 1.2 cd	15 ± 1.2 c	7.5 ± 0.3 d	43 ± 0.5 d	59.87 ± 0.7 cd	13.6 ± 0.2 e
Treatment (T)		***	***	***	***	***	***	***
Irrigation regime (IR)		***	***	***	***	***	***	***
T × IR		***	***	**	***	***	*	***
CV		3.33	9.58	7.55	4.17	3.22	4.96	5.05
Root MSE		1.62	1.47	1.00	0.31	1.22	2.66	0.67

Within each column, means followed by different letters are significantly different (* p < 0.05, ** p < 0.01, *** p < 0.001), as determined by two-way ANOVA and Tukey’s test, respectively; HP: height of plant; NFT: number of fertile tillers; NS: number of spikes; LA: length of spike; 1000 SW: thousand-grain weight; BY: biological yield; GY: grain yield; RL: root length CV: coefficient of variation; MSE: mean square error.

. Overall, the B₁ and B₂ treatments significantly improved plant growth and yield parameters such as HP, NFT, NS, LS, 1000 GW, SY, BY and GY under all three irrigation regimes compared to bokashi alone and the control. At IR₆₀, the B₁ treatment resulted in significantly higher HP, NFT, NS, LS, 1000 GW, SY, BY and GY values compared to the B₂ treatment. Moreover, under IR₅₀ and IR₃₀, B₁ resulted in significantly higher HP, LS, 1000 GW and GY compared to B₂. With even further reduced irrigation at 30% (IR₃₀), the bokashi–biochar treatment produced significantly taller plants (HP) and higher NS, LS, 1000 GW, SY, BY and GY values compared to the control and bokashi-only treatments (except GY) under full irrigation at 60% (IR₆₀). Among all the treatments, the B₁ treatment consistently resulted in superior plant growth and yield across all measured parameters (Table 2).

A significant interaction effect between treatments and irrigation levels was observed in total above-ground biomass (TB) (Figure 1). Both treatments (B₁ and B₂) showed higher TB compared to bokashi alone and the control. The effectiveness of the treatments varied based on the irrigation regime. Under IR₃₀, bokashi containing 1% biochar significantly outperformed all other treatments, yielding a TB of 46.5 g pot⁻¹ (p < 0.05). However, at IR₅₀, no significant differences in TB were observed among the treatments (p > 0.05). Under IR₆₀, the B₁ treatment continued to exhibit superior performance, yielding a TB of 59.01 ± 1.23 g pot⁻¹, which was significantly higher than the other treatments (p < 0.05).

3.2. Root Length (RL) and Root Biomass (RB)

Significant main and interaction effects (p < 0.05) for both the treatments and irrigation regimes were observed for root length (Figure 2a) and root biomass (Figure 2b). B₀ significantly improved root length compared to the control but was less effective than the B₁ and B₂ treatments. The control treatment exhibited the lowest root length (7.5 ± 0.9 cm at IR₃₀) and biomass (0.9 ± 0 g pot⁻¹ at IR₃₀), whereas the application of bokashi alone significantly improved root growth, reaching 23.8 ± 1.6 cm in root length and a biomass of 3.7 ± 0.1 g pot⁻¹ at IR₅₀. The b₁ treatment further enhanced root development compared to B₂, with the highest RL recorded at IR₆₀ (28.8 ± 1.3 cm) and the maximum RB recorded at IR₆₀ (7.4 ± 0.9 g pot⁻¹). However, there was no significant difference in RL and RB between the B₁ and B₂ treatments.

3.3. Soil Moisture and Plant Water Use Efficiency (PWUE)

The co-application of the bokashi and biochar treatments resulted in significantly higher soil moisture content than the non-biochar treatments (Figure 2c). Across all three irrigation regimes, the soil moisture content was highest in the B₁ and B₂ treatments, with mean values of 23.35% (20 to 27%) and 23.78% (20.5 to 28.0%), respectively. These values were significantly higher than the control (9.65%) and bokashi-only treatments (16.64%).

The bokashi–biochar treatments and varying irrigation levels showed a significant effect on plant water use efficiency (PWUE) (Figure 2d). The B₁ and B₂ treatments increased PWUE by 3.90 and 3.57 times, respectively, compared to the control. Similarly, PWUE increased by 36.86% and 25.08% with the B₁ and B₂ treatments, respectively, compared to the bokashi-only treatment. The highest PWUE value (6.40 mg mL⁻¹) was recorded with B₁ under the IR₃₀ irrigation regime, which was significantly higher than the B₂ treatment. Furthermore, the B₁ treatment had a significantly higher PWUE compared to the B₀ and B₂ treatments.

3.4. Soil Chemical Properties

The co-application of bokashi and biochar significantly influenced soil properties across different irrigation regimes (

Table 3. Effect of biochar application rates on soil parameters of wheat under different irrigation regimes (mean ± SE, n = 3).

Treatment	Irrigation Regime (%)	SOC (g kg−1)	pH	EC (dS m−1)	Total N (g kg−1)	Available N (mg kg−1)	Available P (mg kg−1)	Exchangable K (mg kg−1)
Control (C)	IR60	6.1 h	5.82 d	0.9 i	1.30 i	13.8 ± 0.3 ab	18.1 ± 0.3 e	40.6 ± 0.4 e
	IR50	6 h	5.82 d	0.9 j	1.33 h	13.9 ± 0.6 ab	19.6 ± 1 e	40.6 ± 0.3 e
	IR30	5.7 i	5.78 d	0.8 k	1.35 h	15 ± 1.5 a	19.8 ± 0.8 e	43.1 ± 0.1 de
Bokashi only (B0)	IR60	8.3 f	6.02 abc	1.3 g	2.53 ef	11.8 ± 0.5 b	19.8 ± 2.1 e	108.2 ± 0 bcde
	IR50	8.5 e	6.01 bc	1.4 f	2.55 e	11.8 ± 0.7 b	22 ± 2 de	89.7 ± 0.2 cde
	IR30	7.4 g	5.98 c	1.2 h	2.48 g	13.7 ± 1 ab	23.9 ± 0.8 d	117.2 ± 1.1 bcd
Bokashi + 1% biochar (B1)	IR60	9.7 bc	6.11 ab	1.9 c	2.58 cd	8.6 ± 0.9 cd	29.3 ± 0.1 c	135.4 ± 35.8 bc
	IR50	10.8 a	6.09 abc	1.8 d	2.60 bc	8.6 ± 0.2 cd	32.1 ± 1.5 bc	138.2 ± 28.9 bc
	IR30	8.1 f	6.04 abc	1.7 e	2.52 f	9 ± 0.9 c	25.3 ± 2.7 d	170.4 ± 45 ab
Bokashi + 2% biochar (B2)	IR60	9.8 b	6.13 a	2.2 a	2.62 b	5.9 ± 0.4 e	35.2 ± 0.4 ab	154 ± 6.1 abc
	IR50	9.6 cd	6.11 ab	1.9 c	2.68 a	6 ± 0.7 e	38 ± 0.2 a	160.4 ± 18.6 abc
	IR30	9.4 d	6.05 abc	2 b	2.56 de	6.5 ± 0.2 de	30.6 ± 1 c	216.1 ± 56 a
Treatment (T)		***	***	***	***	***	***	***
Irrigation regime (IR)		***	**	***	***	**	***	*
T × IR		***	ns	***	***	ns	***	ns
CV		0.77	0.48	0.19	0.43	7.20	5.14	21.44
Root MSE		0.06	0.04	0.00	0.01	0.75	1.34	25.26

Within each column, means followed by different letters are significantly different at * p < 0.05, ** p < 0.01, *** p < 0.001, as determined by a two-way ANOVA and Tukey’s test, respectively; ns: not significant. SOC: soil organic carbon; EC: electrical conductivity; N: soil nitrogen; P: soil phosphorous; K: soil potassium; CV: coefficient of variation; MSE: mean square error.

). The B₁ treatment (9.53 g kg⁻¹) and B₂ treatment (9.57 g kg⁻¹) significantly increased soil organic carbon (SOC) compared to the control (5.90 g kg⁻¹) and bokashi-only treatment (8.01 g kg⁻¹). Moreover, SOC varied significantly across three irrigation regimes for each treatment (p < 0.001), with the highest levels of SOC under IR₅₀ (10.2 g kg⁻¹) and IR₆₀ (9.7 g kg⁻¹), compared to IR₃₀ (8.1 g kg⁻¹), demonstrating irrigation’s influence on SOC retention.

Soil pH increased slightly from 5.8 to 6.1 with the co-application of bokashi and biochar at 1% and 2% for all irrigation regimes. The co-application of bokashi and biochar significantly influenced the soil electrical conductivity (EC) under various irrigation levels. The highest soil EC was observed in the B₂ treatment (2.0 dS m⁻¹), followed by B₁ (1.8 dS m⁻¹), and the lowest was observed in the control (0.9 dS m⁻¹). Compared to the bokashi-only treatment (1.3 dS m⁻¹), the B₂ and B₁ treatments increased soil EC by 56.41% and 38.46%, respectively (Table 3).

The co-application of bokashi and biochar significantly increased total soil N content in all treatments (p < 0.001). The B₂ (2.68 g kg⁻¹) and B₁ (2.60 g kg⁻¹) treatments exhibited the highest total N, showing a 98–106% increase compared to the control (1.30–1.35 g kg⁻¹), while B₀ treatments showed intermediate levels (2.48–2.55 g kg⁻¹). Among the irrigation regimes, IR₅₀ promoted the highest total N retention (2.68 g kg⁻¹ in B₂), followed by IR₆₀ (2.62 g kg⁻¹) and IR₃₀ (2.56 g kg⁻¹). Contrary to the total N trends, available N decreased significantly in the amended soils (p < 0.001). The control maintained the highest available N (13.8–15.0 mg kg⁻¹), while the B₁ (8.6–9.0 mg kg⁻¹) and B₂ treatments (5.9–6.5 mg kg⁻¹ and 6.0–9.0 mg kg⁻¹) showed 41–57% reductions, whereas the B₀ treatments retained intermediate levels (11.8–13.7 mg kg⁻¹). IR₃₀ preserved 10–15% more available N than IR₆₀ across all treatments (Table 3). A significant interaction effect of the bokashi–biochar treatments and irrigation levels was observed for available P. The average available P increased by 50.78% and 80.52% in the B₁ (28.8 mg kg⁻¹) and B₂ (34.6 mg kg⁻¹) treatments, respectively, compared to the control (19.1 mg kg⁻¹) and by 31.96% and 57.99%, respectively, compared to the B₀ treatment (21.9 mg kg⁻¹) across the three irrigation regimes. Significant main effects for the bokashi–biochar treatments and irrigation levels were observed, but not for their interaction with extractable potassium (K). Extractable K was 3.6 and 4.3 times higher in the B₁ (148.1 mg kg⁻¹) and B₂ (176 mg kg⁻¹) treatments, respectively, compared to the control (41.43 mg kg⁻¹). Moreover, extractable K increased by 40.91% and 68.36%, respectively, in these treatments compared to the B₀ treatment (105.3 mg kg⁻¹).

3.5. Relationship Between Yield Parameters and Soil Properties

A correlation analysis was conducted to examine relationships among multiple variables, including HP, GY, SY, OC, pH, EC, TN, AN, AP, EK, SM, and PWUE. The correlation matrix, based on Pearson’s correlation coefficient, revealed strong positive correlations between grain yield (GY) and HP (r = 0.93), SM (r = 0.94) and OC (r = 0.94), indicating their critical roles in enhancing wheat productivity. Soil pH (r = 0.80), EC (r = 0.83) and TN (r = 0.86) also showed strong to moderate positive correlations with GY, while AP (r = 0.63), EK (r = 0.63) and PWUE (r = 0.60) exhibited weaker but significant positive correlations, highlighting their contributions to yield.

Notably, available nitrogen (AN) displayed a strong negative correlation with most parameters, ranging from −0.74 to −0.84, including GY (r = −0.74), HP (r = −0.84), OC (r = −0.85) and EC (r = −0.95), suggesting that while total nitrogen (TN) was positively correlated with yield, AN alone was not a direct determinant of crop performance. Soil organic carbon and soil moisture were strongly positively correlated with pH, EC and TN (r ranging from 0.81 to 0.93), emphasizing their interconnected roles in soil fertility (Figure 3).

4. Discussion

4.1. Plant Growth Parameters

Our findings demonstrate the significant positive effect of bokashi and biochar mixtures on wheat yield parameters (Table 2, Figure 1). The sole application of bokashi showed higher yields compared to the control, showing that bokashi compost is an efficient organic fertilizer influencing yields, which is in line with previous studies [34,38,43,44]. However, with B₁ and B₂, the yield parameters were almost two-fold higher compared to the B₀ treatment, highlighting the potential benefits of incorporating biochar into bokashi. For most of the yield parameters, B₁ showed a superior agronomic effect over B₂ (Table 2).

The combination of B₁ increased plant height by 110.56% and 49.3% compared to the control and B₀, respectively (Table 2). These findings are consistent with previous studies that reported similar increases in plant height following the application of biochar–compost or bokashi–biochar combinations. For instance, Pandit et al. [45], in a pot experiment under greenhouse conditions, observed a 20–40% increase in plant height for maize treated with a biochar–compost mixture. Similarly, Agegnehu et al. [46], in a field experiment, reported a comparable increase in plant height for barley treated with a biochar–compost mixture. Andayani et al. [47], in another pot experiment, also found a 20–40% increase in plant height for soybean treated with bokashi–biochar combinations. Additionally, Pagliaccia et al. [44], in a glasshouse setting, observed a 30–40% increase in plant height in Carrizo citrange plants treated with bokashi–biochar combinations. Similarly, the bokashi–biochar mixtures in our study showed a significant positive effect (p < 0.05) over B₀ and the control on other crop growth parameters such as NFT, NS, LS, 100 GW, SY, BY and GY under three different irrigation regimes. The co-application of biochar with bokashi considerably enhanced wheat yield. Compared to B₀, the B₁ and B₂ treatments increased yield by 35.55% and 16.57%, respectively. When compared to the control, the yield improvements were even greater-264.05% for B₁ and 231.07% for B₂. Similar improvements have been reported in other crops under different experimental conditions, with greenhouse studies showing a 40–50% increase in maize yield [45] and a 305% biomass increase in quinoa with biochar–compost application [48]. Likewise, field experiments demonstrated a 30% increase in soybean yield with bokashi–rice husk application [43], while a glasshouse study on Carrizo citrange reported a 5–50% biomass improvement [49] The yield increase observed in this study likely resulted from improved nutrient retention, enhanced water-holding capacity, and stimulated microbial activity effects attributed to the co-application of biochar and bokashi compost which aligns with previous findings on biochar–organic amendment interactions. These results highlight the potential of biochar–bokashi amendments to enhance crop resilience under water-limited conditions. The positive agronomic effect of the bokashi–biochar amendment is possibly due to improved soil fertility, enhanced microbial activities and nutrient recycling, increased nutrient bioavailability and higher water retention capacity [14,45,49]. When biochar is mixed with organic fertilizers, organic coatings are formed on the biochar surface, and this increases the nutrient retention capacity of the amendments. Nutrients retained in the pores and micropores of biochar are gradually released in response to the dynamic balance between nutrient availability and plant demand [26,50,51].

The increased plant height and increases in other yield parameters with bokashi–biochar amendments under the three different irrigation regimes illustrate the potential of biochar to increase water holding capacity and plant-available water (PAW), thereby increasing yields. Most importantly, at reduced irrigation of 30%, the B₁ treatment produced significantly taller plants (HP) and higher NS, LS, 1000 GW, SY, BY and GY values compared to the control and B₀ treatments with full irrigation at 60% (Table 2). These findings suggest that moderate irrigation combined with biochar amendments can optimize wheat growth and yield by alleviating water stress and increasing PAW during critical crop growth periods. Several previous studies have reported the potential of biochar in increasing water retention capacity and PAW in coarse-textured weathered soils, thereby increasing crop yields [45,52,53,54].

The correlation analysis illustrated the significant positive relationship between grain yield and various yield parameters, underscoring the importance of these growth parameters in determining wheat yield (Figure 3). Moreover, soil pH, OC, N, P and K showed a positive correlation with grain yield upon biochar addition, aligning with previous studies [26,55,56]. This finding is further supported by Pandit et al. [57], who reported that biochar application improved soil nutrient availability, particularly nitrogen (N), phosphorus (P) and potassium (K), in a pot experiment. The enhanced nutrient retention and reduced nutrient leaching alleviated nutrient stress, leading to increased crop yields in low-productivity soils. These results align with the present study, where the co-application of biochar and bokashi likely contributed to improved soil fertility, supporting higher wheat yields.

The co-application of bokashi and biochar showed enhanced root length and root biomass compared to both the control and B₀ treatments (Figure 2a,b). This enhanced root development can be attributed to improved soil aeration, moisture retention and enhanced microbial activities (mycorrhizal fungi) upon biochar addition, which create a more favorable environment for root growth [49]. Biochar enhances root proliferation by improving soil physical properties and providing a habitat for beneficial microorganisms [58]. Our research shows that the co-application of bokashi and biochar increased RL and RB by 31.1% and 83.9%, respectively, compared to bokashi alone. This aligns with previous studies reporting a 40–50% increase in root biomass in soybeans with bokashi and biochar [47], emphasizing the synergistic effect of bokashi and biochar’s role in root growth and nutrient uptake. Similar trends have been observed in previous studies, where soybean root biomass increased by 40–50% with bokashi and biochar application under greenhouse conditions [47], while bokashi alone improved tomato root length by 19.48–24.37% [59]. Field studies further support these findings, showing that the addition of 1% biochar increased root length in cotton by 10% compared to manure alone, whereas a higher application rate of 3% was less effective [60]. Furthermore, EM bokashi increased peanut root dry matter by 46.67% over chemical fertilizers in field conditions [61], reinforcing the benefits of organic amendments in promoting root development. While this study demonstrates the positive effects of bokashi and biochar on plant growth and yield-attributing parameters under controlled greenhouse conditions, future multi-season field trials across diverse agro-ecological regions are necessary to evaluate the scalability of these benefits.

4.2. Soil Moisture and Plant Water Use Efficiency (PWUE)

The co-application of biochar and bokashi significantly increased soil water content (SWC) and plant water use efficiency (PWUE) over non-biochar treatments (Figure 2c,d). Biochar’s high porosity and surface area contribute to its ability to retain water, which is critical under varying irrigation regimes [62]. Our results indicate that the soil moisture content was highest in treatments with both biochar and bokashi, consistent with previous studies using a similar experimental setup, where the co-application of biochar and bokashi enhanced soil moisture retention by 15% to 17% compared to the bokashi-only treatment in [45].

The significant increase in PWUE with biochar application suggests that biochar improves soil water retention and enhances the efficiency of plant water use. This is particularly important under drought conditions, where efficient water use is crucial for maintaining crop yields [63,64]. Our results show that the combined application of biochar and bokashi increased PWUE by 3.57 to 3.90 times compared to the control. Furthermore, the PWUE was 36.86% higher under B₁ and 25.08% higher under B₂ than the bokashi-only treatment. These findings align with Sharma et al. [65], who reported a 25.9% increase in water use efficiency (WUE) with biochar application alone under field conditions, with further improvement when combined with farmyard manure (FYM) and vermicompost (VC), reaching a maximum WUE of 199.5 kg ha⁻¹ mm. Similarly, Ebrahimi et al. [66] found that adding biochar to vermicompost under field conditions and drought stress increased the PWUE by 44% compared to vermicompost alone in eggplant, which is comparable to the increase observed with 1% biochar under the IR₃₀ irrigation regime in our study, highlighting biochar’s potential to improve water use efficiency in water-limited environments and increase crop yields by increasing soil water retention and hydraulic conductivity. This result highlights the importance of optimizing biochar application rates and irrigation regimes to maximize the synergistic effects of biochar and bokashi composting, particularly in areas with water scarcity [45,67,68,69]. The observed improvements in soil moisture retention and plant water use efficiency from bokashi and biochar application suggest potential benefits under controlled conditions, but their effectiveness in diverse field environments with varying soil properties and moisture regimes requires validation through long-term trials.

4.3. Effect on Soil Chemical Properties

The combined application of biochar and organic manure significantly influenced soil chemical properties, including SOC, pH, EC, and available nutrients (Table 3). The increase in SOC with the co-application of biochar and bokashi by 61.26% compared to the control and 18.52% over bokashi alone in our study is consistent with findings by Agegnehu et al. [46], who reported a 34% increase in SOC in barley under field conditions, attributing it to the stable carbon forms that support soil fertility and plant growth. The peak SOC under B₁-IR₅₀ (10.8 g kg⁻¹) suggests that moderate irrigation (IR₅₀) optimizes microbial activity and carbon sequestration when combined with 1% biochar. However, the lack of further improvement with 2% biochar (B₂) implies a possible saturation effect or reduced biochar efficiency at higher doses. The combination of bokashi compost with biochar likely promoted organic matter breakdown and enhanced microbial activity, leading to greater SOC accumulation in the soil [33,44,45]. The positive correlation (Figure 3) between soil organic matter and yield indicates SOC’s role in influencing crop yields, although nutrient availability and water management are critical for crop growth and development [70].

Biochar–bokashi treatments caused slight increases in soil pH from 5.8 to 6.1, likely moderated by bokashi’s low pH. This aligns with previous research in maize where bokashi plus biochar co-composted increased soil pH from 6.5 to 6.7 in silty loam soil, using a similar experimental setup [45]. These findings suggest that adding biochar to bokashi improves soil nutrient content and enhances soil pH, creating a more favorable environment for plant growth. Soil EC increased with bokashi and biochar application, reflecting greater nutrient concentrations beneficial for growth; the EC levels remained within a non-toxic range (1.2 to 2.2 dS m⁻¹). The biochar and irrigation regimes jointly enhanced soil nutrient availability, with higher extractable potassium due to biochar [50,53] and a reduction in nitrogen likely due to microbial activity and nutrient cycling [14,71,72].

The bokashi and biochar treatments generally lowered the available N levels but increased the P and K levels in our experiment (Table 3), suggesting biochar’s role in nutrient retention and, ultimately, plant growth and development [73]. The decline in available nitrogen (N) in the biochar- and bokashi-amended soils is attributed to microbial immobilization, as biochar enhances microbial biomass N, reducing mineral N availability through carbon-driven microbial activity [74,75,76]. Similarly, the incorporation of organic amendments, including manure, enhances microbial N immobilization, converting ammonium (NH₄⁺) into microbial biomass, thus reducing the available mineral nitrogen for plants [77]. The available N ranged from 5.9 to 13.8 mg kg⁻¹ (IR₆₀), 6.0 to 13.9 mg kg⁻¹ (IR₅₀) and 6.5 to 15.1 mg kg⁻¹ (IR₃₀), suggesting IR₃₀ may optimize N retention by reducing denitrification [78]. Such variations indicate that irrigation regimes significantly influence nitrogen dynamics, particularly the availability of nitrogen, which is crucial for crop productivity and soil fertility management. Despite an increase in total N, the available N declines due to microbial immobilization, as microbes sequester excess N for biomass growth under C limitation. This highlights how microbial activity prioritizes N retention when carbon availability is low, reducing the pool of available N in the soil [79]. Our study confirms the positive effect of bokashi–biochar mixtures on P and K availability, consistent with Pandit et al.’s [45] greenhouse pot experiment. We observed a 33–44% increase in available P and a 3.6 to 4.3-fold increase in extractable K with the bokashi–biochar treatments, similar to their findings of higher available P (50–173%) and K⁺ (1.67 fold) in bokashi–biochar compost, but unlike in their study, we did not observe significant effects on soil nitrogen. However, the decrease in available N is consistent with Agegnehu et al. [80], who reported a decrease in available N% by 5.56 after the addition of biochar in compost compared to compost treatment with a chemical fertilizer in a field experiment. Furthermore, our findings highlight that irrigation significantly influences phosphorous (P) availability, while potassium (K) remains largely unaffected. This suggests that although the combination of bokashi and biochar can enhance overall soil fertility, the availability of specific nutrients is complex and strongly influenced by irrigation practices. The co-application of biochar and bokashi likely fostered plant growth and increased PWUE (Figure 2d). Biochar’s high surface area and porosity, along with bokashi’s nutrients, improve microbial activity, soil structure and fertility [34,45,59] Additionally, a positive correlation was observed between bokashi application and soil moisture (Figure 3), underscoring its role in improving water retention under drought conditions. The co-application of bokashi and biochar significantly improved the soil chemical properties in the greenhouse experiment, but multi-season field trials are needed to evaluate its long-term effects and applicability across diverse soil types and climates.

5. Conclusions

The co-application of bokashi and biochar significantly enhances soil properties, nutrient availability, plant water use efficiency (PWUE) and crop productivity across various irrigation regimes, with the most pronounced benefits observed at a 1% biochar application rate. This study highlights the potential of biochar-amended bokashi as a sustainable, climate-resilient soil management practice for mitigating nutrient and water stress, particularly under water-limited conditions. Furthermore, our study emphasizes the significance of water-use efficiency rather than focusing solely on maximizing irrigation. The results suggest that a moderate irrigation level (IR₅₀) provides the optimal balance between yield enhancement and resource utilization, making it the most effective regime under water-limited conditions. By improving soil health, reducing reliance on synthetic fertilizers and fostering resilient agroecosystems, these organic amendments offer an innovative approach to sustainable agriculture.

Our findings underscore the synergistic effects of bokashi and biochar in enhancing crop productivity while minimizing environmental impacts, making them a viable solution for addressing soil degradation, water scarcity and food security challenges. However, further research is needed to optimize application rates and irrigation regimes across diverse crops and soil types. Future studies should integrate physiological, molecular and microbial analyses to elucidate plant responses under stress conditions while also assessing the long-term effects of bokashi–biochar mixtures on soil carbon sequestration, nutrient cycling and microbial community dynamics. A comprehensive understanding of these interactions will provide deeper insights into their environmental and agronomic benefits.

Additionally, on-farm trials across different agroecological regions are critical to validating these findings and facilitating large-scale adoption in real-world farming systems. By combining advanced analytical techniques with long-term field studies, future research can further reveal the mechanisms driving the benefits of bokashi–biochar co-application, ultimately contributing to the development of sustainable agricultural practices that enhance productivity, resilience, and environmental responsibility.