Optimizing Nitrogen and Water Use Efficiency in Wheat Cropping Systems Through Integrated Application of Biochar and Bokashi Under Different Irrigation Regimes

Abstract

Addressing the challenge of reducing environmental pollution from agricultural practices by improving nitrogen use efficiency (NUE) and water use efficiency (WUE) while ensuring high crop yields is essential for sustainable agriculture. Using a controlled glasshouse experiment, we evaluated the combined effects of biochar and bokashi under different irrigation regimes on NUE, WUE, and yield-related parameters in a wheat cropping system. The experiment followed a completely randomized design with three replications with four treatments: (1) control (C), (2) bokashi only (B₀), (3) bokashi +1% biochar (B₁), and (4) bokashi +2% biochar (B₂). These treatments were evaluated at three irrigation levels—30% (IR₃₀), 50% (IR₅₀), and 60% (IR₆₀) of field capacity (FC), resulting in a total of twelve treatments. Co-application of bokashi–biochar significantly (p < 0.050) improved grain yield (GY), straw yield (SY), total biomass (TB), total nitrogen uptake (TNU), grain protein content (GPC), NUE, and WUE, with the most notable benefits observed at 1% biochar application compared to C and B₀ treatments. In addition, both types of treatment (bokashi and bokashi with biochar) and the level of irrigation had a significant impact on GY, SY, TB, TNU, GPC, NUE, and WUE. The B₁ and B₂ treatments further improved yield and efficiencies compared to bokashi alone. The positive correlation between grain yield and WUE underscores the importance of optimizing irrigation strategies alongside soil amendments for improved crop productivity. These enhancements in yield and efficiency are likely attributed to the increased soil fertility, nutrient availability, and water retention resulting from the combination of biochar and bokashi.

1. Introduction

Wheat (Triticum aestivum L.) is one of the major staple crops globally, significantly contributing to human dietary energy and protein intake. Regions with limited water resources and poor soil fertility face significant challenges in sustaining high wheat productivity. Moreover, intensive agricultural practices and removal of crop residues often lead to soil degradation, nutrient depletion, and reduced crop yields [1,2,3]. Therefore, there is an urgent need for sustainable agricultural practices that improve soil health and water use efficiency (WUE), and optimize nutrient utilization.

Nitrogen (N) is a vital macronutrient for wheat production due to its important role in growth, grain yield, protein synthesis and life processes [4]. While essential for global food security, excessive application of N fertilizer results in low nitrogen use efficiency (NUE) and environmental hazards, including soil degradation, water eutrophication, air pollution, groundwater contamination, and reduced carbon sequestration [5,6]. The application of synthetic N fertilizers along with organic amendments, such as animal manure, can enhance soil organic carbon, reduce N losses, and improve NUE by balancing soil N supply with crop demand [7]. However, the efficacy of these practices is dependent on soil properties, climate, and management strategies [8]. Precision fertilization approaches are essential to sustainably increase crop productivity, optimize NUE, and minimize environmental impacts.

Organic amendments have gained significant attention due to their ability to enhance soil fertility and crop yields [9]. Bokashi, a type of fermented organic matter, enhances soil microbial activity and nutrient levels [10,11]. Bokashi, derived from the Japanese term for “fermented organic matter”, is produced through the anaerobic fermentation of organic waste using a mix of beneficial microorganisms. This process rapidly decomposes organic materials and improves nutrient availability to plants [12,13,14,15,16]. The application of bokashi has been linked with increased soil microbial activity and enhanced plant growth, making it a viable alternative to conventional compost [7,17,18,19].

Biochar, a carbon-rich product derived from the pyrolysis of organic materials, facilitates soil structure, improves water retention, and enhances nutrient cycling [20,21,22]. Produced by pyrolyzing organic materials at high temperatures without oxygen, biochar has gained recognition for its numerous agronomic benefits. Studies show that biochar application can significantly enhance soil physical properties essential for plant growth, such as water-holding capacity and cation exchange capacity [21,23,24,25,26]. Additionally, the porous nature of biochar provides microhabitats for soil microbes, thereby enhancing microbial activity and nutrient cycling [27,28]. For instance, Tammeorg et al. reported that biochar application increased wheat yield and improved nitrogen retention in the soil [29].

Efficient management of irrigation is another key factor influencing wheat productivity, especially in water-limited conditions. Implementation of effective irrigation strategies can significantly improve WUE and crop yields [30,31]. Combining organic amendments with optimized irrigation regimes could potentially maximize the benefits of both practices, leading to sustainable increases in wheat productivity and nitrogen use efficiency [32].

The combination of bokashi with biochar is theorized to produce synergistic effects that exceed the benefits of applying each amendment individually, resulting in enhanced crop performance and soil health [7]. This synergistic potential arises from the complementary properties of bokashi, which rapidly enhances soil microbial activity and nutrient availability, while biochar provides long-term improvements in soil structure and nutrient retention [7,22,33]. Numerous studies have shown that the combined application of these amendments can lead to significant improvements in soil health and crop productivity [12,15,34,35,36,37,38,39]. Similarly, the combined use of biochar and compost improves soil nitrogen availability and reduces nitrogen losses [40,41,42].

Very few studies have explored the combined effects of bokashi and biochar under different irrigation regimes on wheat productivity, WUE, and NUE. Therefore, this study presents the impact of bokashi and biochar on wheat productivity and NUE under different irrigation regimes. The study hypothesizes the following: (1) the combined application of bokashi and biochar will result in higher grain yield, straw yield, and total biomass compared to individual amendments; (2) these amendments will enhance nitrogen content and uptake in wheat; and (3) optimized irrigation regimes will amplify the benefits of bokashi and biochar on wheat productivity, WUE, and NUE. By addressing these hypotheses, this study aims to provide a comprehensive understanding of how organic amendments and irrigation management can be synergistically utilized to achieve sustainable wheat production and optimize NUE.

2. Materials and Methods

2.1. Pot Experimental Design

The experiment was conducted in a greenhouse at Saitama University, Japan (35°51′41″ N, 139°36′30″ E), to investigate the combined effects of biochar and bokashi under different irrigation regimes on NUE and WUE in a wheat cropping system. Each pot, with a capacity of 6 L, was filled with 5 kg of air-dried soil. The experiment utilized a completely randomized design (CRD) with a factorial arrangement, including three replications for each treatment. The treatment consisted of three biochar application rates: 0%, 1%, and 2% w/w of soil and three irrigation regimes: 30% (IR₃₀), 50% (IR₅₀), and 60% (IR₆₀) based on field capacity (FC). Given the soil’s bulk density of 1.51 t m⁻³, the 1% and 2% biochar rates equated to 20 and 40 t ha⁻¹, respectively. Soil moisture was maintained at 30%, 50%, and 60% of FC using daily tap water irrigation, with the gravimetric method ensuring consistent water availability for plant growth. The specific treatments, which consist of bokashi integrated with three levels of biochar, and the three irrigation regimes, are outlined 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

. The pots were kept in the glasshouse for 120 days, during which any weeds that appeared were manually removed. Wheat seeds of the cultivar “Satonosara” were sown by hand, and five plants per pot were maintained by thinning out extra seedlings 20 days after sowing.

2.2. Production and Characterization of Biochar and Bokashi

In this study, biochar was produced from corn stover through pyrolysis at 400 °C using a pilot-scale batch carbonizer (ECO500, Meiwa Co., Ltd., Kanazawa, Japan). After production, the biochar was cooled, ground, and sieved to 2 mm before undergoing analysis. Its key characteristics included a pH of 9.57, nitrogen content of 11.3 g/kg, phosphorus at 0.59 g/kg, and potassium at 0.97 g/kg. Additionally, bokashi fertilizer, obtained from Daiwa Fertilizer Company, Japan, had a pH of 5.7 and contained 6.0% total nitrogen, 4.2% total phosphorus, and 2.0% total potassium.

This experiment adhered to Saitama Prefecture’s crop fertilization guidelines, with bokashi applied at a rate of 20 t ha⁻¹ (44 g per pot), excluding the control group. Biochar was applied at three rates: 0% (0 g), 1% (50 g), and 2% (100 g) based on soil weight, using a bulk density of 1.51 t m⁻³ for calculation.

2.3. Sample Analysis, Calculations of NUE and WUE

Wheat was harvested when it reached physiological maturity, and the ears, along with the straw, were carefully separated and air-dried for a month in a glasshouse. Afterwards, the spikelets were counted, and the grains were threshed meticulously to assess the grain yield. The yields were adjusted and reported at a moisture content of 14% [43]. Next, the grains were soaked in tap water, and the number of floating versus sunken grains was counted to evaluate yield components, including the filled spikelet rate. A portion of both the grain and straw were oven-dried at 70 °C in a forced-air oven for three days, then grounded and analyzed for total nitrogen content using a C–N corder (Yanaco, MT-500, Yanagimoto Co., Ltd., Kyoto, Japan). Based on the nitrogen concentration in the grain and straw, grain nitrogen uptake (GNU) and straw nitrogen uptake (SNU) were calculated in grams of nitrogen in grain and straw, respectively. This involved multiplying the total grain yield (GY) by the grain nitrogen concentration and the total straw yield (SY) by the straw nitrogen concentration Thus, the total nitrogen uptake (TNU) can be calculated as:

$$\mathrm{T}\mathrm{N}\mathrm{U}=\mathrm{G}\mathrm{N}\mathrm{U}+\mathrm{T}\mathrm{N}\mathrm{U}$$

(1)

The total protein content of the grain (GPC) was calculated as follows [7]:

$$\mathrm{G}\mathrm{P}\mathrm{C}=\mathrm{N} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{i}\mathrm{n} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}\ast 5.83$$

(2)

The grain protein yield (GPY) was determined as [7]:

$$\mathrm{G}\mathrm{P}\mathrm{Y}(\mathrm{k}\mathrm{g} {\mathrm{h}\mathrm{a}}^{-1})=\mathrm{G}\mathrm{P}\mathrm{C}\ast \mathrm{G}\mathrm{Y}$$

(3)

The nitrogen use efficiency (NUE) metrics, such as agronomic efficiency (AE_N), apparent recovery efficiency (ARE_N), and physiological efficiency (PE_N), were calculated as follows [7]:

The nitrogen harvest index (NHI) was calculated in % using Equation (1), defined as the portion of TNU contributed by the N uptake in grain (GNU) as follows:

$$\mathrm{N}\mathrm{H}\mathrm{I}\left(\%\right)={\displaystyle \frac{\mathrm{G}\mathrm{N}\mathrm{U}}{\mathrm{T}\mathrm{N}\mathrm{U}}}\ast 100$$

(4)

The agronomic nitrogen efficiency (AE_N) is defined by unit weight increase in grain yield per N applied and calculated using Equation (2) as follows:

$$\mathrm{A}{\mathrm{E}}_{\mathrm{N}}(\mathrm{k}\mathrm{g} {\mathrm{k}\mathrm{g}}^{-1})={\displaystyle \frac{{\mathrm{G}\mathrm{Y}}_{\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}-{\mathrm{G}\mathrm{Y}}_{\mathrm{C}}}{{\mathrm{N}}_{\mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}{\mathrm{d}}_{\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}}}}$$

(5)

where

GY_treatment = grain yield in the treatment (B₀, B₁ and B₂ in kg ha⁻¹),

G_C = grain yield (kg ha⁻¹) in the control treatment for each replication, and

N_{applied treatment} = amount of nitrogen applied in the treatment (B₀, B₁ and B₂ in kg ha⁻¹).

The apparent recovery efficiency (ARE_N), which is also called applied N uptake efficiency, is the % of N applied in aboveground biomass and calculated from Equation (3).

$${\mathrm{A}\mathrm{R}\mathrm{E}}_{\mathrm{N}}(\%)={\displaystyle \frac{TN{U}_{treatment}-{\mathrm{T}\mathrm{N}\mathrm{U}}_{\mathrm{C}}}{{{\mathrm{N}}_{\mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}}_{\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}}}\ast 100$$

(6)

where

TNU_treatment = Total nitrogen uptake in the treatment (B₀, B₁ and B₂ in kg ha⁻¹), and

TNU_C = Total nitrogen uptake in the control treatment (kg ha⁻¹) for each treatment.

The physiological nitrogen efficiency (PE_N, the unit weight increases in grain yield per unit weight increase in N uptake from N fertilizer), was calculated using Equation (4),

$$\mathrm{P}{\mathrm{E}}_{\mathrm{N}}(\mathrm{k}\mathrm{g} {\mathrm{k}\mathrm{g}}^{-1})={\displaystyle \frac{{\mathrm{G}\mathrm{Y}}_{\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}-{\mathrm{G}\mathrm{Y}}_{\mathrm{C}}}{\mathrm{T}\mathrm{N}{\mathrm{U}}_{\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}-{\mathrm{T}\mathrm{N}\mathrm{U}}_{\mathrm{C}}}}$$

(7)

where

GY_treatment = Grain yield in the treatment (B₀, B₁ and B₂ in kg ha⁻¹), and

GY_C = Grain yield in control treatment (kg ha⁻¹).

Water use efficiency (WUE) is the ratio of yield to evapotranspiration (ET) of wheat. The water balance method was used to calculate the ET [44] using Equation (5) as follows:

$$\mathrm{E}\mathrm{T}=\mathrm{P}+\mathrm{U}+\mathrm{I}-\mathrm{D}-\mathrm{R}-\mathsf{\Delta }\mathrm{W}$$

(8)

where ET = amount of water requirement by the crop (mm), P = rainfall (mm), U = groundwater recharge (mm), I = Irrigation water applied (mm), D = drainage (mm), and ΔW = change in soil moisture (mm) during experiment.

The experiment was carried out in the greenhouse, ensuring the absence of precipitation (P = 0). Additionally, the precision irrigation system used in the study prevented surface runoff (R = 0) and drainage (D = 0), and groundwater recharge (U = 0) was also effectively zero. Thus,

$$\mathrm{E}\mathrm{T}=\mathrm{I}-\mathsf{\Delta }\mathrm{W}$$

(9)

The water use efficiency (WUE) was determined using Equation (6) [45]:

$$\mathrm{W}\mathrm{U}\mathrm{E}={\displaystyle \frac{\mathrm{G}\mathrm{Y}}{10\ast \mathrm{E}\mathrm{T}}}$$

(10)

2.4. Statistical Analysis

The data were analyzed using two-way analysis of variance (ANOVA) in R statistical software (version 3.6.3) to evaluate both the main and interactive effects of treatments and irrigation regimes on grain yield, biomass production, total nitrogen uptake, and nitrogen and water use efficiency parameters. An average, along with least significant difference (LSD) test at a 5% significance level, was used to compare the overall effect of all treatments at different irrigation regimes as well as the overall effect of all irrigation regimes for each treatment. To assess the relationship between variables, linear and non-linear regression analysis was conducted to explore the relationships between GY, WUE, NUE, and TNU as dependent variables and biochar/bokashi levels and irrigation regimes as independent variables, providing insights into the model. Additionally, a correlation matrix was performed to determine the relationship between yield, water use efficiency, and nitrogen use efficiency parameters, providing insights into the degree of association between these variables using R.

3. Results

3.1. Grain Yield

The interactive effect of treatment and irrigation regimes significantly increased grain yield (p ≤ 0.001) with higher levels of both biochar and irrigation compared to the C and B₀ treatments (

Table 2. Significance of the effects of biochar (B), irrigation regimes (IR), and their interaction on yield, nitrogen content, and nitrogen use efficiency parameters.

Parameter	B	IR	B×IR	CV	Root MSE
GY	***	***	***	5.10	305
SY	***	***	***	4.22	452
GN	***	***	***	2.96	494
SN	***	***	***	2.51	0.06
GNU	***	***	***	21.23	0.10
SNU	***	***	***	6.98	10.37
TNU	***	***	***	26.03	13.84
NHI	***	***	***	7.80	15.74
GPC	**	***	***	4.94	3.63
AEN	***	***	***	2.51	0.34
AREN	***	*	***	4.54	4.20
PEN	***	***	***	7.34	0.19

Within each column, means followed by different letters are significantly different at (* p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001), as determined by two-way ANOVA and Tukey’s test, respectively; B: Biochar; IR: irrigation regime; GY: grain yield; SY: straw yield; GN: grain nitrogen; SN: straw nitrogen; GNU: grain nitrogen uptake; SNU: straw nitrogen uptake; TNU: total nitrogen uptake; NHI: nitrogen harvesting index; GPC: grain protein content; AE: agronomic efficiency; ARE: apparent N recovery efficiency; PE: physiological efficiency; CV: coefficient of variation; MSE: mean square error.

), with B₁ consistently achieving superior yield across all moisture conditions. Wheat yield varied from 1551 ± 4.58 kg ha⁻¹ to 9988.33 ± 233.39 kg ha⁻¹, with the highest yield (8310.11 kg ha⁻¹) in the group with B₁ treatment and the lowest (2275.78 kg ha⁻¹) in the control group (Figure 1a). Averaged across the irrigation levels, the highest grain yield was observed with IR₆₀ (7250.67 kg ha⁻¹), followed by IR₅₀ (6020.67 kg ha⁻¹), while the lowest yield was recorded in IR₃₀ (4637.58 kg ha⁻¹), though these were not significantly different between IR₅₀ and IR₃₀ treatments (p > 0.05). Overall, the yield trend follows this order: B₁ > B₂ > B₀ > C.

3.2. Straw Yield

Straw yield showed a notable increase with the addition of biochar and improved irrigation regimes compared to C and B₀ treatment (Figure 1b). Furthermore, B₁ and B₂ led to a significant increase in straw yield compared to B₀. The highest straw yield was recorded at 14,448.78 kg ha ⁻¹ for the application of B₁, 1,413,601.11 kg ha⁻¹ for B₂, followed by 13,601.11 kg ha⁻¹ for B_0, and the lowest yield of 3770.11 kg ha⁻¹ with C treatment. Across irrigation regimes, the straw yield peaked at IR60 (11,987.08 kg ha⁻¹), followed by IR50 (10,330.0 kg ha⁻¹), with the minimum mean straw yield recorded at IR30 (9764.25 kg ha⁻¹). The statistically significant (p ≤ 0.001) interactive effect of treatment and irrigation regimes is illustrated in Table 2, demonstrating that straw yield increased notably with higher levels of biochar and improved irrigation regimes.

3.3. N Content % (Grain and Straw)

Biochar and irrigation regimes have a notable influence on the nitrogen content found in both grain and straw (Table 2). ANOVA results showed that both treatments and irrigation regimes had significant effects on GN% and SN% (p ≤ 0.001), with a significant interaction between the two factors. The highest grain N content (3.09%) was observed under B₁ with IR_50, while the lowest (1.53%) was in control under IR₆₀ treatment. Similarly, straw N content was highest (0.82%) in B₂ with IR₃₀ and lowest (0.22%) in B₁ under IR₆₀. Additionally, with the addition of biochar 1% and 2%, grain nitrogen increased by 59–73% compared to C and by 14–24% compared to B₀, with the highest increase in B₁ (

Table 3. Means of main effects of organic amendments and water deficit on wheat agronomic parameters.

Treatments	Irrigation Regimes (%)	N Content (%)		N Uptake (kg ha−1)		(%)
		Grain N	Straw N	GNU	SNU	NHI	GPC
Control	IR60	1.53 ± 0 h	0.46 ± 0 cd	46.35 ± 0.06 g	21 ± 0.09 cd	68.82 ± 0.11 cde	8.92 ± 0 h
	IR50	1.56 ± 0 h	0.31 ± 0 d	35.05 ± 0 g	11.48 ± 0.05 d	75.33 ± 0.08 bcd	9.09 ± 0 h
	IR30	1.77 ± 0 g	0.28 ± 0 d	27.45 ± 0.08 g	8.34 ± 0.04 d	76.7 ± 0.03 bc	10.32 ± 0 g
Bokashi only	IR60	2.43 ± 0.09 d	0.26 ± 0.08 d	185.86 ± 6.66 cd	32.17 ± 9.83 cd	85.42 ± 3.43 ab	14.14 ± 0.52 d
	IR50	2.24 ± 0.03 e	0.42 ± 0.01 cd	147.08 ± 1.89 e	47.23 ± 0.67 bcd	75.69 ± 0.47 bcd	13.06 ± 0.18 e
	IR30	2.07 ± 0.03 f	0.5 ± 0.02 bcd	86.69 ± 1.06 f	45.09 ± 1.44 bcd	65.79 ± 0.97 de	12.05 ± 0.16 f
Bokashi + 1% biochar	IR60	2.89 ± 0.16 b	0.22 ± 0.04 d	288.45 ± 21.45 a	35.41 ± 6.24 cd	88.99 ± 2.44 a	16.83 ± 0.95 b
	IR50	3.09 ± 0 a	0.61 ± 0.16 abc	254.09 ± 11.98 b	78.69 ± 23.74 ab	76.54 ± 6.21 bc	17.99 ± 0 a
	IR30	2.43 ± 0 d	0.79 ± 0.02 ab	163.01 ± 8.22 de	110.71 ± 4.89 a	59.54 ± 2.27 e	14.17 ± 0 d
Bokashi + 2% biochar	IR60	2.37 ± 0.04 de	0.42 ± 0.07 cd	197.71 ± 7.93 c	60.45 ± 9.04 bc	76.6 ± 3.31 bc	13.84 ± 0.21 de
	IR50	2.62 ± 0 c	0.62 ± 0.18 abc	184.05 ± 22.44 cd	82.39 ± 24.4 ab	69.35 ± 6.82 cde	15.25 ± 0.03 c
	IR30	2.74 ± 0.05 bc	0.82 ± 0.23 a	166.85 ± 1.99 de	104.89 ± 29.92 a	61.88 ± 6.13 e	15.96 ± 0.31 bc
CV		4.91	24	6.76	26.62	5.07	4.91
LSD (p ≤ 0.05)		0.19	0.19	16.79	23.67	6.23	1.11

Within each column, means followed by different letters are significantly different at p ≤ 0.05. GNU: grain nitrogen uptake; SNU: straw nitrogen uptake; NHI: nitrogen harvesting index; GPC: grain protein content; LSD: least significant difference at 5% level of significance.

).

3.4. Grain Nitrogen Uptake (GNU)

A significant effect (p ≤ 0.001) of bokashi and biochar application on GNU was observed under varying irrigation regimes (Table 2), and it followed the trend of C < B₀ < B₁ > B₂, with the interaction between these factors also having a notable impact on GNU. Among the treatments, the highest GNU (288.45 kg ha⁻¹) was recorded for B₁ under IR₆₀, followed by B₁ at IR₅₀ (254.09 kg ha⁻¹) and B₂ at IR₆₀ (197.71 kg ha⁻¹). The control treatment (C) exhibited the lowest GNU, with values of 46.35, 35.05, and 27.45 kg ha⁻¹ under IR₆₀, IR₅₀, and IR₃₀, respectively.

3.5. Straw Nitrogen Uptake (SNU)

SNU also showed significant variation (p < 0.001) in response to bokashi and biochar application under different irrigation regimes (Table 2) and followed the trend of C < B₀ < B₁ < B₂. The interaction between these factors significantly influenced SNU. The highest SNU (110.71 kg ha⁻¹) was recorded in B₁ under IR₃₀, followed by B₂ at IR₃₀ (104.89 kg ha⁻¹), and B₂ at IR₅₀ (82.39 kg ha⁻¹). In contrast, the lowest SNU was observed in the control treatment (C), with values of 21.0, 11.48, and 8.34 kg ha⁻¹ under IR₆₀, IR₅₀, and IR₃₀, respectively.

3.6. Total Nitrogen Uptake (TNU)

The TNU varied significantly across treatments and irrigation regimes (Table 2). B₁ treatment achieved the highest TNU (310.12 kg ha⁻¹), a 521.6% increase over C (49.89 kg ha⁻¹) and an increase by 70.99% compared to B₀ (181.37 kg ha⁻¹)_, while B₂ led to a decrease in TNU by 14.4% compared to B₁ treatment (Figure 2). Among irrigation regimes, IR₆₀ (216.85 kg ha⁻¹) and IR₅₀ (210.02 kg ha⁻¹) performed similarly, while IR₃₀ (178.26 kg ha⁻¹) showed a decline. The combination of B₁ with IR₆₀ resulted in the highest TNU (332.78 kg ha⁻¹), while the control under IR₃₀ recorded the lowest TNU (35.79 kg ha⁻¹).

3.7. Nitrogen Harvesting Index (NHI) and Grain Protein Content (GPC)

Significant differences in the NHI and GPC across the different treatments, as well as different irrigation regimes, were observed (Table 2). NHI and GPC were lower in the control treatment as compared to other treatments. For B₀, the highest levels of NHI and GPC were found at IR₆₀ (85.42 ± 3.43% and 14.14 ± 0.52%). Similarly, for B₁, a significant increase in NHI (88.99 ± 2.44%) was found under IR_60, and an increase in GPD (17.99 ± 0%) was found under IR₅₀. B₂ did not appear to benefit NHI and GPC when compared to B₁ (e.g., NHI: 76.6 ± 3.31%; GPC: 13.84 ± 0.21%) at IR_60, but it was still higher than C. Interestingly, B₂ outperformed B₁ under IR₃₀, showing a higher NHI (by 3.9%) and GPC (by 12.6%).

3.8. AE_N, ARE_N and PE_N

The NUE parameters (AE_N, ARE_N and PE_N) declined with a decrease in irrigation levels and increased with the addition of biochar to the soil (Figure 2). Across treatments, B₁ resulted in significantly better AE_N and RE_N. Agronomic efficiency varied significantly among the different treatments and irrigation regimes (Table 2). The AE_N ranged from 2.24 to 3.59 kg kg⁻¹, 2.84 to 4.17 kg kg⁻¹, and 3.14 to 4.85 kg kg⁻¹ in IR₃₀, IR_50, and IR _60, and from 2.24 to 3.92 kg kg⁻¹, 3.59 to 4.85 kg kg⁻¹, and 2.69 to 3.14 kg kg⁻¹ in B₀, B_1, and B₂ treatment, respectively. The highest AE_N (4.85 kg kg⁻¹) was observed in B₁ under IR₆₀, while the lowest AE_N (2.24 kg kg⁻¹) was recorded in B₀ under IR₃₀, following the trend B₁ > B₀ > B₂, indicating that the B₁ treatment exhibited the highest agronomic efficiency, while across irrigation regimes AE_N followed the trend IR₆₀ > IR₅₀ > IR₃₀.

Similarly, ARE_N ranged from 8.12% to 16.58%, 12.5% to 19.94%, and 11.3% to 17.87% in IR₃₀, IR₅₀, and IR₆₀, and from 8.12% to 12.75%, 16.58% to 19.94%, and 11.3% to 13.98% in B₀, B₁, and B₂ treatment, respectively. The highest ARE_N (19.94%) was recorded in B₁ under IR₅₀, while the lowest ARE_N (8.12%) was observed in B₀ under IR₃₀, following the trend B₁ > B₂ > B₀ and IR₅₀ > IR₆₀ > IR₃₀, indicating that B₁ treatment under IR₅₀ irrigation regime had the most effective nitrogen.

Across the treatments, PE ranged from 27.55 to 30.99 kg kg⁻¹ in B₀, 20.98 to 27.16 kg kg⁻¹, in B₁, and 19.5 to 27.75 kg kg⁻¹ in B₂. The highest PE (30.99 kg kg⁻¹) was found in the B₀ treatment under IR₆₀, while the lowest PE (19.5 kg kg⁻¹) was observed in B₂ under IR₃₀, following the trend of B₀ > B₁ > B₂, indicating that B₀ treatment is the most efficient in nitrogen utilization. PE_N ranged from 19.5 to 27.55 kg kg⁻¹ in IR₃₀, 20.98 to 29.21 kg kg⁻¹ in IR₅₀, and 27.16 to 30.99 kg kg⁻¹ in IR₆₀. Across irrigation regimes, PE followed the following trend: IR₆₀ > IR₅₀ > IR₃₀.

3.9. Relation Between TNU and TB

The results indicate that different irrigation regimes significantly influence the relationship between TNU and TB in wheat (Figure 3). A quadratic model indicates a strong positive correlation in the IR₃₀ (R² = 0.981) and a consistently increasing TNU as TB increased. IR₅₀ also exhibited a strong correlation (R² = 0.963), but with signs of diminishing returns at higher biomass levels. In contrast, IR₆₀ had the weakest correlation (R² = 0.9412), with TNU plateauing at higher TB values and showing a plateauing effect. Overall, lower irrigation levels (IR₃₀) facilitated more efficient nitrogen uptake relative to biomass production, while higher irrigation levels (IR₆₀) resulted in weaker nitrogen absorption.

3.10. Relation Between GY and WUE

The relationship between grain yield (GY) and biomass water use efficiency (WUE) varied across all irrigation regimes, with all exhibiting strong positive linear correlations (R² = 0.95) (Figure 4). IR₆₀ showed the highest correlation (R² = 0.9932), indicating a strong linear increase in GY with increasing WUE, following the equation GY₆₀ = 5.2429 (WUE) + 0.6726. This suggests that for every 1 kg ha⁻¹ mm⁻¹ increase in WUE, GY increased by 5.2429 kg ha⁻¹. Similarly, IR₅₀ demonstrated a strong correlation (R² = 0.958) with a slightly lower yield response, where a unit increase in WUE resulted in a 4.4646 kg ha⁻¹ rise in GY, following the equation GY₅₀ = 4.4646 (WUE) − 0.0063. IR₃₀, while still maintaining a strong correlation (R² = 0.9922), exhibited the lowest response, with GY₃₀ = 2.3034 (WUE) + 0.1479, indicating a 2.3034 kg ha⁻¹ increase in GY per unit rise in WUE. This also illustrates that higher levels of irrigation (IR₆₀) lead to increased grain yield with per unit increment in water use efficiency. This is especially evident in the steeper slopes of the regression lines for the IR₅₀ and IR₆₀ irrigation regimes when compared to IR₃₀. These results highlight that higher irrigation levels (IR₆₀) led to the most substantial grain yield response per unit increase in WUE, while lower irrigation levels (IR₃₀) resulted in comparatively weaker yield responses.

3.11. Correlation Matrix Analysis

GY showed strong positive correlations with SY (r = 0.95), GN (r = 0.87), and GNU (r = 0.98), all significant at p < 0.001 (Figure 5). This indicates that higher GY is closely associated with increased SY, GN, and GNU. SY also exhibited strong correlations with GN (r = 0.87), GNU (r = 0.92), and TNU (r = 0.95), all significant at p < 0.001, suggesting that greater SY is linked to higher nitrogen content and uptake in both grains and the entire plant. GN and SN had a positive correlation (r = 0.33, p < 0.05). GN was significantly correlated with GNU (r = 0.93) and TNU (r = 0.95), while SN correlated with SNU (r = 0.91) significant at p < 0.001 and TNU (r = 0.41) significant at p < 0.05. GNU demonstrated strong correlations with SNU (r = 0.48) significant at p < 0.01 and TNU (r = 0.95) significant at p < 0.001, indicating that higher grain nitrogen uptake is associated with increased overall plant nitrogen uptake. AE_N and ARE_N showed significant correlations with GY (r = 0.95 for AE_N, r = 0.90 for ARE_N) and TNU (r = 0.91 for AE_N, r = 0.99 for ARE_N), suggesting that higher GY and TNU contribute to AE_N and ARE_N. PE_N was positively correlated with AE_N (r = 0.89) and ARE_N (r = 0.78), both significant at p ≤ 0.001, indicating that enhancements in AE_N and ARE_N are linked to better PE_N (Figure 5).

4. Discussion

The synergistic effects of combining biochar and bokashi under varying irrigation regimes demonstrated their potential for wheat productivity, NUE, and WUE. The combined application of biochar and bokashi significantly increased grain and straw yields compared to the control and bokashi only (Figure 1a,b), indicating that the synergistic effects of biochar and bokashi are maximized under optimal irrigation conditions, resulting in enhancing soil water retention and nutrient availability [20,21,22]. Biochar has been extensively documented for its ability to enhance soil physical properties, such as porosity, water retention, and nutrient retention [28,42]. This creates a conducive environment for microbial activities, and nutrient cycling and availability, thereby promoting plant growth and yield in the presence of organic fertilizers like bokashi [38,46]. The observed increase in grain yield in this study aligns with previous research, where biochar and organic amendments significantly improve crop yields [47,48]. For example, Agegnehu et al. [7] found that co-application of biochar and organic manure significantly increased grain yield due to enhanced nitrogen and phosphorus availability in the soil. Moreover, the present study showed a significant increase in grain and straw yields along with the addition of biochar, particularly at the 1% biochar application (Figure 1a,b). These findings support our hypothesis 1, indicating that the synergistic effects of bokashi and biochar enhance grain yield, straw yield, and total biomass more effectively compared to individual amendments. This underscores biochar’s potential to improve soil fertility, water retention, and nutrient availability [49,50].

The significant increase in yield with 1% biochar and optimal irrigation level highlights the critical importance of integrated soil fertility and water management practices for sustainable wheat production [4,51,52]. This study demonstrated notable improvements in nitrogen content and uptake in both grain and straw through the combined application of biochar and bokashi (Table 3) with the highest values observed in B₁ treatment. The improved nitrogen retention and uptake indicate that these amendments enhance nutrient availability, particularly under optimized moisture conditions, thereby validating hypothesis 2, as both amendments contributed to higher nitrogen efficiency. These findings are consistent with the previous research indicating that biochar enhances soil nitrogen retention and availability, thereby improving plant nitrogen uptake [29,53,54]. The increase in total nitrogen uptake (TNU) following biochar application can be attributed to improved nitrogen availability and enhanced root growth resulting from better soil structure and microbial activity [28,55]. Conversely, the decrease in TNU (Figure 2) with the 2% biochar application suggests that there is an optimal biochar concentration beyond which the beneficial effects do not proportionally increase, and this phenomenon could be due to nutrient immobilization or reduced microbial activity [33,56]. The relationship between TNU and TB in our findings is important for advancing irrigation strategies to enhance nitrogen use efficiency and wheat cropping system productivity (Figure 3). The strong polynomial relationships observed, especially under IR₃₀ (R² = 0.98), suggest that moderate water stress can enhance nitrogen uptake efficiency, likely due to improved root activity and nutrient mobilization under mild stress conditions [57].

Nitrogen content in both grain and straw showed significant variation across treatments, with the highest levels recorded in the 1% biochar treatment (Table 3). This aligns with previous reports that biochar-amended soils enhance nitrogen retention, reducing leaching losses and improving plant nitrogen assimilation [58]. Similarly, 1% biochar treatment maximized wheat productivity, NUE, and WUE, whereas 2% was less effective, likely due to nutrient immobilization, reduced aeration, microbial shifts, and altered soil pH and moisture dynamics, limiting nutrient availability [7,56,58]. However, the irrigation regime alone did not significantly affect nitrogen percentage in grains, suggesting that nitrogen accumulation is more dependent on soil amendments than water availability alone [59]. The nitrogen harvesting index (NHI) and grain protein content (GPC) also exhibited significant improvements with biochar addition. The highest NHI and GPC were observed under 1% biochar and 60% irrigation conditions (Table 3), supporting previous findings that biochar can enhance nitrogen use efficiency by improving microbial interactions and nutrient cycling [28,60]. The decline in NHI and GPC with 2% biochar application, relative to 1%, may be attributed to excessive nitrogen immobilization or altered microbial dynamics, which reduce nitrogen availability to plants [58].

The agronomic efficiency (AEN), recovery efficiency (REN), and physiological efficiency (PEN) of nitrogen all increased with the addition of biochar, with the most favorable impact at 1% (w/w) biochar application rate (Figure 2). Our results are in line with other studies that have demonstrated the effectiveness of biochar in enhancing NUE by reducing nitrogen losses through leaching and volatilization [7,33,41]. The improvement in NUE parameters highlights the ability of biochar and bokashi to maximize nitrogen utilization in wheat cropping systems and can further reduce the need for synthetic fertilizers, minimizing environmental impacts [38,46,50]. The significant interactive effects between biochar, bokashi, and irrigation regimes on crop productivity suggest that under optimal irrigation conditions, co-application of biochar and bokashi boosts wheat yield (Figure 1) and NUE (Figure 2). Our findings indicate that an equilibrium of biochar application in irrigation regimes maximizes nutrient availability and water use efficiency (WUE), highlighting the importance of integrated soil fertility and water management practices for sustainable wheat production and resource use efficiency. The co-application of biochar and bokashi demonstrated synergistic effects which were probably due to the complementary properties of these amendments. Bokashi quickly amplifies microbial activity and nutrient availability in soil, while biochar provides long-term improvements in soil structure and nutrient retention [33,37,61]. This combination leads to significant improvements in soil health and crop productivity, as demonstrated by the increased yields, nitrogen content, and NUE parameters observed in this study [12,62].

The positive linear correlation between grain yield and WUE across all irrigation regimes (Figure 4) highlights the significant potential of efficient irrigation management in enhancing wheat productivity [63,64,65,66]. The highest WUE under the 60% irrigation regime indicates that adequate water availability is essential for maximizing the optimal benefits of biochar and bokashi applications [32]. These findings are similar to those in previous studies, which have shown that improved soil water retention due to biochar application can enhance WUE, particularly under water-limited conditions [21,25]. Moreover, our study highlights the importance of water-use efficiency (WUE) rather than just maximizing irrigation. The findings indicate that moderate irrigation (IR₅₀ = 50% FC) strikes the best balance between yield improvement and resource efficiency, making it the most efficient regime under water-deficit conditions. With a strong correlation (R² = 0.958) and a yield increase of 4.4646 kg ha⁻¹ per unit WUE, IR₅₀ optimizes water use while avoiding excessive input, making it a sustainable choice for drought-prone areas. In contrast, excessive irrigation may not be practical in water-limited environments, highlighting the need for strategic water management. Interestingly, the relationship remained robust under IR₃₀ (R² = 0.99), indicating that even in a water-limited environment, efficient water use can sustain grain production by alleviating water stress and increasing plant available water during critical crop growth period (Figure 4), aligning with previous research [67]. The interaction between irrigation and organic amendments showed that optimized irrigation enhanced the benefits of bokashi and biochar on wheat productivity, WUE, and NUE. However, under severe drought (IR₃₀ = 30% FC), 2% biochar was less effective, suggesting diminishing returns under extreme water deficit. Despite this, hypothesis 3 is largely supported, highlighting irrigation’s key role in maximizing soil amendment benefits. This aligns with the findings of Forooq et al., [68] who highlighted the importance of drought-adaptive strategies in maintaining yield under stress conditions. The strong correlation matrix between grain yield, straw yield, nitrogen content, and nitrogen uptake (Figure 5) demonstrates how closely these factors interact in determining overall wheat productivity. This analysis further emphasizes the critical importance of integrated soil fertility and water management practices in enhancing crop performance [69]. Additionally, the significant correlations between AEN, REN, and PEN emphasize the importance of targeted strategies to improve NUE and WUE to achieve sustainable wheat production.

5. Conclusions

The significant interactive effects of biochar, bokashi, and irrigation methods on crop productivity suggest that the co-application of biochar and bokashi, along with optimized irrigation, can effectively enhance wheat productivity, NUE, and WUE, validating study hypotheses. These soil amendments significantly increased grain and straw yields, improved nitrogen retention and uptake, and enhanced NUE, particularly with 1% biochar. In contrast, 2% biochar showed no further benefits, likely due to nitrogen immobilization or microbial shifts reducing nutrient availability. These results highlight the necessity of integrated soil fertility and water management strategies to encourage sustainable wheat production. This can also minimize the environmental impact caused by excessive fertilization and maximize crop production. Further on-farm trials and economic analyses are needed to assess the broader applicability of bokashi–biochar amendments on wheat yield and efficiency across different agroecological conditions, while future research should explore long-term applications and crop diversity to enhance generalizability. The importance of resource-use efficiency and ameliorating soil health in wheat-based cropping systems is one of the objectives of receiving attention in wheat research.