Enhancing Soil Environments and Wheat Production through Water Hyacinth Biochar under Deficit Irrigation in Ethiopian Acidic Silty Loam Soil

Abstract

The combined application of biochar and fertilizer has become increasingly popular for improving soil quality and crop productivity. However, the reported research results regarding the effects of biochar on soil properties and crop productivity have contradictory findings, indicating the requirement for further scientific research. Therefore, this study aimed to investigate the effects of a combined application of water hyacinth biochar (WHB) and NPS fertilizer on soil physicochemical properties and wheat yield under deficit irrigation conditions in acidic silty loam soil in Ethiopia. Four different biochar rates (0, 5, 10, and 20 t ha⁻¹), three fertilizer rates (0, 100, and 200 kg NPS ha⁻¹), and two irrigation regimes (50 and 100% of crop requirement) were evaluated to assess soil properties and wheat yields. The results showed that biochar amendment significantly reduced soil bulk density by 15.1–16.7%, and improved soil porosity by 6.8–8.6% and moisture content by 10.3–20.2%. Additionally, the combined application of biochar and fertilizer improved soil pH (0.26–0.87 units), NH₄⁺–N (73.7–144%), NO₃⁻–N (131–637%), and available phosphorus (85.8–427%), compared to the application of fertilizer alone. As a result, wheat dry biomass and grain yield increased by 260 and 173%, respectively. Furthermore, the combined application of WHB and fertilizer resulted in a comparable wheat dry biomass and grain yield even with a 50% reduction of irrigation water. Therefore, WHB has a significant potential to improve soil physicochemical properties and wheat yield when it is applied in combination with fertilizer, and it can reduce the water requirement for wheat production.

1. Introduction

The conversion of biomass ensures that crucial soil nutrients such as nitrogen, phosphorus, potassium, and micronutrients stay accessible in the soil for plant uptake. Rather than relying solely on external inputs like fertilizers, nutrient recycling replenishes soil nutrient levels by returning organic matter and decomposed plant residues back into the soil. Currently, most of the world’s soils are recognized as deficient in these nutrients, leading to a high demand for chemical fertilizers to address these deficiencies and the need to increase crop productivity [1]. Converting water hyacinth (Eichhornia crassipes) biomass into biochar for soil amendment is a locally available solution that plays a vital role in enhancing soil nutrients and reducing reliance on chemical fertilizers. Furthermore, moisture deficit is also a significant environmental factor that affects soil properties, crop growth, and productivity [2]. Moisture stress impedes plant growth and disrupts physiological processes [3] by affecting photosynthesis, nutrient transport, cellular functions, and enzyme activities. Additionally, soil acidity poses a serious problem in agricultural lands as it directly impacts the soil, crop production, and human health [4]. This issue is particularly severe in Ethiopia, where soil acidity is increasingly widespread, greatly limiting crop productivity [5]. In certain regions of the Ethiopian highlands where barley, wheat, and faba beans were grown, farmers are shifting towards cultivating other crops that are more tolerant to soil acidity compared to wheat and barley [5]. Lime is an important material for managing acidic soil, but its high transportation cost and limited availability restrict its widespread use, particularly in developing countries [4]. Hence, utilizing water hyacinth biomass for biochar production as a soil amendment offers a promising solution to address soil nutrient deficiencies and acidity, and alleviate moisture stress. Additionally, it helps reduce the costs and environmental impacts associated with chemical fertilizers, particularly benefiting resource-constrained farmers.

Biochar has gained recognition as a soil amendment that enhances soil’s physical and chemical properties, thereby increasing agricultural productivity through direct and indirect effects on soil quality and crop growth [6]. The effect of biochar on soil properties and soil microbes primarily depends on the biochar rate, initial soil pH, and soil textural properties [7], as well as the feedstock type and pyrolysis temperatures [8,9]. Water hyacinth, which is one of the most invasive aquatic weeds globally, including Ethiopia, affecting socioeconomic activities and watershed ecosystems, can be a good feedstock source for biochar production. Since 2011, water hyacinth has invaded Lake Tana, Ethiopia, causing significant damage to the lake’s biodiversity and livelihoods [10]. Currently, the Ethiopian government uses various control mechanisms, including mechanical and manual removal, biological control, and a combination of these measures to remove the weeds from the lake. The transportation and management of the removed biomass is labor-intensive and not economical. However, it could be a good locally available feedstock source of biochar for soil amendment to improve soil physical properties, such as soil bulk density, porosity, and moisture content, as well as soil nutrient availability for plants, including ammonium–nitrogen (NH₄⁺–N), nitrate–nitrogen (NO₃⁻–N), and available phosphorus, by reducing nutrient leaching, fixation, and nutrient recycling [11,12,13,14,15], thereby enhancing plant growth and yield.

According to a review by Joseph et al. [16], biochar amendment has been found to improve crop growth and production in several ways. It can lower soil acidity, increase dissolved and total carbon, enhance cation exchange capacity, improve nutrient availability, increase water retention, and enhance soil aggregate stability. The review also found that the yield improvement in biochar-amended soil was particularly significant in very acidic soils (pH ≤ 5) compared to other types of soils [17]. A study by Baiamonte et al. [18] focused on the benefits of biochar amendment for wheat crops under deficit irrigation. The study found that wheat crops experienced the greatest benefit from biochar amendment, as biochar increased water use efficiency and reduced irrigation frequency, especially when compared to sorghum and tomato crops. The study also found that the wheat crop yield in biochar-amended soil was not significantly impacted by water deficits. This is attributed to the ability of biochar to enhance both irrigation water use efficiency (IWUE) and crop water use efficiency (CWUE) [18]. Despite these positive findings, it is important to note that there have been inconsistent outcomes reported in various research studies regarding the combined use of biochar and fertilizer in soil amendment.

Discrepancies exist regarding the effects of biochar on soil nutrient dynamics, specifically concerning nutrient concentrations and nitrification rates. Contradictory findings are reported regarding the impact of biochar on NH₄⁺–N concentrations, with some studies demonstrating reductions and others showing increases post-amendment. In a study by Martí et al. [19], an incubation experiment showed a reduction of NH₄⁺–N after the soil was amended by biochar. On the other hand, in a study by Ginebra et al. [11], the application of biochar increased soil NH₄⁺–N concentrations compared to fertilizer alone. Moreover, while some studies suggest a decrease in nitrification processes and subsequent reductions in NO₃⁻–N concentrations with biochar application, others indicate an improvement in nitrification rates attributed to an increased abundance of ammonia-oxidizing bacteria. For example, according to Wang et al. [20] and Yao et al. [21], biochar soil amendment decreased ammonia-oxidizing bacteria, and the nitrification rate subsequently decreased the NO₃⁻–N concentration. Conversely, in a study by Liu et al. [12] and DeLuca et al. [22], biochar soil amendment showed that the NO₃⁻–N concentration was increased due to increasing the nitrification rate.

Additionally, discrepancies arise in the effects of biochar on crop yield, with conflicting results regarding optimal application rates and varying outcomes based on crop type and experimental conditions. According to Ye et al. [23], a meta-analysis indicated that an application of biochar greater than 10 t ha⁻¹ does not contribute to a greater crop yield. Conversely, the combined application of 20 t ha⁻¹ of biochar improved crop biomass and yield, as demonstrated by Faloye et al. [24]. A study by Sorensen and Lamb [25] on biochar soil amendment showed that the biochar rate did not exhibit either positive or negative effects on the performance of different crops. However, the effect of biochar was significant between the 5–10 t ha⁻¹ and 10–20 t ha⁻¹ groups on crop performance, as indicated by Ye et al. [23]. The effect of biochar amendment also varies for different crop types, and its impact differs between pot and field experiments, with the effect being three times higher for pot experiments compared to field experiments, as noted by Jeffery et al. [26]. This disparity in findings underscores the complexity of biochar’s interaction with soil and highlights the need for further research to elucidate its effects consistently across different contexts and conditions.

We hypothesized the following: (1) Locally produced biochar using a grounding system would have basic characteristics comparable to those of biochar produced in a furnace. (2) The application of biochar derived from water hyacinth biomass would effectively mitigate soil acidity in the acidic soils of the Ethiopian highlands, resulting in a notable increase in soil pH levels and substantial improvements in soil health. Consequently, this enhancement in soil conditions is anticipated to lead to a significant boost in crop productivity. (3) The synergistic combination of biochar with inorganic fertilizers soil amendment will result in a profound enhancement in soil nutrient availability and subsequent crop yield. (4) Biochar application would not only improve soil properties but also reduce the amount of irrigation water, thereby reducing the frequency of irrigation requirements. Therefore, this study aimed to ascertain the impact of the combined application of water hyacinth-derived biochar and NPS inorganic fertilizer on soil physicochemical properties and crop yield in acidic soils under conditions of deficit irrigation conditions.

2. Materials and Methods

2.1. Experimental Site Descriptions

The field experiment was conducted during the dry season of 2023, from January to May. The experiments were conducted at the Injibara University research station of the Awi Zone of the Amhara Region, Ethiopia (Figure 1). The site’s 33-year minimum and maximum temperatures were 10.3 and 22.5 °C, respectively. The mean annual rainfall was 1344 mm, with the main wet season occurring from June to September, followed by a less pronounced wet period extending until November [27]. The minimum and maximum temperatures, the amount of rainfall, and average solar radiation of the experimental site during the experiment period (January–May 2023) are provided in Figure 2.

2.2. Experimental Land/Plot Preparation

After land clearing, the experimental area was plowed five times using oxen. The plots were arranged with a width of 1.6 m and a length of 2 m, resulting in a total length of 29.5 m and a width of 7.8 m for the experimental site. The spacing between rows, plots, and replications was set at 0.2 m, 0.5 m, and 1.5 m, respectively. The site has been used for grazing for the past three years, with no addition of organic or inorganic fertilizers. Due to its high acidity, crop production has been challenging.

2.3. Biochar Production

The biochar was produced from water hyacinth biomass collected from Lake Tana, Ethiopia (12°72′78″ N and 37°52′02″ E). The water hyacinth biomass was gathered, and the stem parts were cleaned and sun-dried. To avoid high energy and cost requirements in producing biochar using an electric pyrolysis furnace, a local grounding system, which can be easily practiced by farmers having limited resources, was employed to produce water hyacinth biochar (WHB). After creating a pile of 0.4–0.5 kg water hyacinth biomass (stem parts only) with a diameter of 50–80 cm, it was covered with teff (Eragrostis tef) straw and a layer of soil to prevent the entrance of oxygen. It was ignited using matches readily available in local markets. The pyrolysis temperature and the residence time of the resulting WHB were estimated from the temperature and time-dependent basic properties of the biochar (such as pH, cation exchange capacity (CEC), and total carbon). The values of these basic properties were comparable with the values reported by Gezahegn et al. [28] for WHB, where water hyacinth biomass was collected from the same site (Lake Tana, Ethiopia) and produced using a laboratory pyrolysis furnace at temperatures of 450, 550, and 750 °C. Accordingly, the pyrolysis temperature of the WHB utilized in this study was presumed to be in the range of about 500 °C to 600 °C, with a residence time of 50–60 min. Water was sprayed to cool down the biochar, with subsequent sun-drying. It was then crushed by hand and sieved to <2 mm for laboratory characterization and <5 mm for field application.

2.4. Field Experimentation

The plots were arranged in a randomized block design with three replications, resulting in a total of 36 experimental plots. Four different rates of water hyacinth biochar (WHB; 0, 5, 10, and 20 t ha⁻¹), three rates of NPS inorganic fertilizer (NPS; 0, 100, and 200 kg ha⁻¹), and two levels of irrigation water (50 and 100% of crop requirement) were employed in a partially factorial treatment arrangement (Table 1). We used a new compound fertilizer (NPS) containing nitrogen, phosphorus, and sulfur, at a ratio of 19% N, 38% P₂O₅, and 7% S, recently introduced by the Ministry of Agriculture of Ethiopia. This fertilizer has now replaced DAP as the primary source of phosphorus in the Ethiopian crop production system [29]. The experimental treatment was comprised of the full factorial arrangement for two levels of WHB rates (0 and 20 t ha⁻¹), two levels of NPS fertilizer rates (0 and 200 kg ha⁻¹), and two levels of irrigation rates (50 and 100%). Additionally, we included other rates of WHB (5 and 10 t ha⁻¹) and NPS fertilizer (100 kg ha⁻¹) as satellite treatments to minimize the number of combined treatments, to use the resources efficiently (Table 1).

The test crop was the “Kakaba” variety of bread wheat, planted at a seedling rate of 150 kg ha⁻¹. Biochar was incorporated into the soil at a depth of 20 cm for treatments that received biochar, two days before the planting date (28 December 2023) after well plowing, mixing, and leveling each plot similarly. For treatments that received inorganic fertilizer, all NPS and half of the urea were applied at the time of planting (30 December 2023), while the remaining half of the urea was applied at the tillering stage (9 March 2023) [30]. To ensure uniform germination of the crop, all plots were fully irrigated two days before sowing, and this continued up to one week after sowing. After one week, plots were daily irrigated to meet 50 and 100% of the crop’s water requirement, according to the treatment arrangement. For treatments that received 50 and 100% of full irrigation, five and ten liters of water was applied daily, respectively, until the booting stage of the crop. After the booting stage, 10 and 15 L of water was applied daily until the physiological maturity of the crop, using a water can. This is because the water requirement of the wheat crop is higher during the later stages compared to the early stages [31]. The amount of water applied was determined based on the irrigation water requirement of the wheat crop indicated by Desalegn et al. [32] and Tewabe [33].

2.5. Sampling and Characterization of Soil and Biochar Samples

A composite of 5 sub-samples was taken from the experimental land to characterize the experimental soil before preparing the experimental plots. For evaluating treatment effects, soil samples were taken from each plot on 7, 15, 30, 60, 90, and 130 days after sowing (DAS) at a depth of 0–20 cm. These samples were stored in a refrigerator until analysis.

The pH was determined according to Robinson et al. [34]. The pH of the soil was measured using 10 g of soil, and the pH of the biochar was measured using 2 g of biochar after the samples had been air-dried at 45 °C. For the soil sample, 25 mL of pure water was added to a 50 mL centrifuge tube, while 20 mL of pure water was added for the biochar sample. The tubes were then shaken horizontally at 160 strokes per minute for 1 h and allowed to stand for 30 min, and the pH was measured using a pH meter (LAQUA F-71, Horiba Scientific, Kyoto, Japan). The total carbon (C), hydrogen (H), and nitrogen (N) in the soil and biochar samples were measured using a CHN analyzer (Perkin Elmer, 2400 series II, Waltham, MA, USA). Five milligrams of soil and two milligrams of biochar samples was placed into tin capsules and analyzed according to the method described by Yeomans and Bremner [35]. The oxygen (O) content of the biochar was obtained by the calculation of 100% aa − (C + H + N + Ash) %. The Walkley–Black method was employed to determine organic carbon (OC) [36]. To determine the concentrations of NH₄⁺–N and NO₃⁻–N in the soil and biochar samples, 2.0 g of dry-weight-equivalent soil and 2.0 g of dried biochar (45 °C) was extracted with 20 mL of 2 mol L⁻¹ potassium chloride solution (KCl) in a centrifuge tube. The tube was shaken horizontally at 160 strokes per minute for 1 h. After filtration through a 0.45 μm filter membrane, the concentration of NH₄⁺–N and NO₃⁻–N in the extractant was determined at 670 nm and 540 nm, respectively, using a flow injection auto-analyzer 2000 (FIAlyzer-1000, FIAlab Instruments, Inc., Seattle, WA, USA) according to the methods described by Keeney and Nelson [37]. The available phosphorus (P) was determined by extracting 2.0 g of soil and 0.5 g of biochar that had been dried at 45 °C with 20 mL of Mehlich 3 extraction solution in a 50 mL centrifuge tube, according to the method described by Mehlich [38]. The tube was shaken horizontally at 200 strokes per minute for 5 min, and the mixture was then filtered through a 0.45 μm pore size membrane filter. The concentration of P was determined at 870 nm using the flow injection auto-analyzer 2000 (FIAlyzer-1000, FIAlab Instruments, Inc., Seattle, WA, USA). The cation exchange capacity (CEC) of the soil and biochar was determined by using 1 mol L⁻¹ ammonium acetate adjusted to pH 7. Ten grams of dry soil and 1 g of biochar was mixed with the ammonium acetate solution. The mixture was shaken at 160 strokes per minute for 5 min for the soil and 15 h for the biochar. After shaking, the mixture was filtered with Whatman filter paper, size 42, and the concentration of NH₄⁺ was measured using the auto-analyzer to calculate the CEC of the soil and biochar.

To measure the soil bulk density, we took soil samples three times—before preparing the experimental land, 70 DAS, and at the time of crop harvesting (end of the experiment)—from each experimental plot. We measured it by drying the core sampler soil in an oven at 105 °C until it reached a constant mass. Then, we calculated it as the mass of the core sample dried at 105 °C minus the mass of the core sample holder (g) divided by the volume of the core sample holder (cm³). Soil total porosity (S_t) was determined by the equation:

$${\mathrm{S}}_{\mathrm{t}}=1-(\frac{\rho \mathrm{b}}{\rho \mathrm{p}})$$

where ρb and ρp are soil bulk density and soil particle density respectively, by considering that the ρp for mineral soil is 2.65 g cm⁻³ as a rule of thumb [39]. To measure the moisture content of the soil sample, we took 3 g of fresh soil. We weighed the samples into an aluminum dish and then dried them in an oven at 105 °C for 24 h. We recorded the weight of the aluminum dish and soil and then determined the water content by subtracting the fresh soil mass from the dry soil mass and dividing it by the dry soil mass.

The yield of biochar was calculated by dividing the weight of the produced biochar by the dried biomass used as a feedstock and expressing the result as a percentage. The biochar-specific surface area, pore volume, and pore size were determined using N₂ adsorption–desorption performed at 77 K with a (Micromeritics ASAP 2020, Shimadzu, Tokyo, Japan). The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method [40]. The fixed carbon, volatile matter, and ash content of the biochar were determined by thermal gravimetric analysis (TGA) using simultaneous differential thermogravimetry (SDT Q600, TA Instruments, Lukens Drive, New Castel, DE, USA) in the range of a 10 °C min⁻¹ heating rate up to 750 °C, then to 950 °C.

2.6. Plant Data Collection

Dry biomass refers to the quantity of dry plant material above ground, including stems, leaves, and the grain yield of the wheat crop. To minimize any border effects, one row on each side was left untouched during harvesting. The biomass was measured from the six central rows after it had been sun-dried. The entire six central rows were harvested to measure grain yield. The grain yield was weighed after it had been sun-dried, threshed, and separated from the wheat straw.

2.7. Statistical Analysis

The One-Way Analysis of Variance (ANOVA) was computed to compare the means of each combined treatment on soil physical and chemical parameters, as well as wheat crop biomass and grain yield. A two-way ANOVA was performed to test for the interaction effects between biochar and fertilizer, biochar and irrigation, and fertilizer and irrigation, and additionally, a three-way ANOVA was conducted to examine the interactive effects of biochar, fertilizer, and irrigation amount on soil physicochemical properties and crop performance. All analyses of variance were carried out using the R-software program, specifically, the version R-4.3.0 packages including stats, emmeans, and ANOVA test [41]. Data normality was assessed using the Shapiro–Wilk procedure [42], and the difference between treatment means was determined using Tukey’s Highly Significant Difference (HSD) at a 5% probability level (p < 0.05).

3. Results

3.1. Characteristics of Soil and Biochar

The field experiments were conducted on Nitisol soil according to the Schad [43] soil classification, belonging to the silt loam texture class. The soil at the experimental site was strongly acidic (pH of 4.42). The concentrations of NH₄⁺–N, NO₃⁻–N, and available P in the soil were 1.52, 15.7, and 0.392 mg kg⁻¹, respectively (Table 2). According to Beyene et al. [44], the soils in the northwestern and western parts of Ethiopia are predominantly acidic, with pH levels ranging from 4.6 to 4.75. This encompasses Injibara, our experimental site, as well as nearby locations such as the Burie district. The soils in these areas are predominantly nitisols, similar to those at our experimental site, indicating that our site is representative of the broader region.

The biochar used was alkaline (pH of 10.7). The concentrations of NH₄⁺–N, NO₃⁻–N, and available P in the soil were 0.748, 0.676, and 837 mg kg⁻¹ respectively. The cation exchange capacity of the biochar was 33.4 cmol_c kg⁻¹. The total carbon and organic carbon were 35.2 and 16.8%, respectively. The biochar had fixed C, volatile matter, and ash contents of 20.3, 59.2, and 20.5%, respectively. Additionally, the biochar exhibited a specific surface area of 53.2 m² g⁻¹, a pore volume of 0.059 cm³ g⁻¹, and an average pore size of 4.45 nm (Table 3).

3.2. Effects of Combined Application of Biochar and Fertilizer on Soil Physical Properties

3.2.1. Bulk Density

The bulk density of the soil generally decreased at the end of the experiment, regardless of treatments. The main effect of biochar significantly (p < 0.001) impacted the bulk density on both 70 and 130 DAS, but this was not so for fertilizer and irrigation. The interaction did not significantly affect the bulk density of the soil (Table A1). The ANOVA results showed that the bulk density in biochar-amended plots significantly (p < 0.001) decreased from 0.838 and 0.750 g cm⁻³ (0 t ha⁻¹ WHB) to 0.698 and 0.637 g cm⁻³ (20 t ha⁻¹ WHB) on 70 and 130 DAS, respectively (Figure 3). The ANOVA results also revealed that the bulk density reduction was higher in the plots that received higher rates of WHB (20 t ha⁻¹) compared to those that received lower rates (5 and 10 t ha⁻¹) of WHB.

3.2.2. Total Porosity

The total porosity at the end of the experiment (130 DAS) was higher than in the middle of the experiment period (70 DAS), regardless of the treatment. The application of biochar significantly affected soil total porosity (p < 0.001), while fertilizer and irrigation did not have a significant impact (Table A1). The total porosity showed an increasing trend with the increase in biochar rate. It increased from 68.4% (0 t ha⁻¹ WHB) to 73.6% (20 t ha⁻¹ WHB) on 70 DAS and remained higher on 130 DAS (76.0%; 20 t ha⁻¹ WHB) (Figure 4).

3.2.3. Moisture Content

The ANOVA results revealed that soil moisture content was positively influenced by the application of WHB compared to unamended plots. The moisture content was generally higher at the beginning and end of the experiment. Soil moisture was affected only by the main effect of biochar. The main effect of both fertilizer and irrigation and interaction effects did not affect it. The moisture content on 15, 30, and 130 DAS was significantly (p < 0.01) affected by biochar application. It increased from 26.7% (WHB₀F₀I₁₀₀) to 29.5% (WHB₂₀F₁₀₀I₁₀₀) on 15 DAS and continued to be higher on 30 (32.6%; WHB₂₀F₂₀₀I₁₀₀) and 130 DAS (33.9%; WHB₂₀F₂₀₀I₁₀₀) (Table 4).

3.3. Effects of Combined Application of Biochar and Fertilizer on Soil Chemical Properties

3.3.1. Soil pH

The soil pH at the beginning of the experiment (7 DAS) was generally lower, and it was higher at the end of the experiment (130 DAS), regardless of the treatments. The main effect of biochar significantly (p < 0.001) affected soil pH, but this was not so for fertilizer and irrigation, as well as interaction effects (Table A1). The application of WHB significantly (p < 0.001) affected soil pH on 7 DAS, 60 DAS, 90 DAS, and 130 DAS. The pH increased from 4.65 (WHB₀F₀I₁₀₀) to 5.29 (WHB₂₀F₁₀₀I₁₀₀) on 7 DAS and continued to be significantly higher in WHB₂₀F₁₀₀I₁₀₀ (5.47) on 60 DAS, WHB₂₀F₂₀₀I₁₀₀ (5.33) on 90 DAS, and WHB₂₀F₁₀₀I₁₀₀ (5.90) on 130 DAS (Table 5). The soil pH increased with the increase in biochar rate without significant differences. Treatments with a higher rate of biochar (20 t ha⁻¹) showed higher pH values compared to treatments with lower rates (5 and 10 t ha⁻¹).

3.3.2. Ammonium–Nitrogen

The concentration of soil NH₄⁺–N generally increased until 60 DAS and then decreased regardless of treatments. The NH₄⁺–N was significantly affected (p < 0.001) by the combined application of WHB and fertilizer throughout the crop’s growth periods. It was influenced by the main effects of the biochar and fertilizer on all DAS (except fertilizer on 15 DAS) and irrigation on 60 and 90 DAS (Table A1). The interaction effects of biochar and fertilizer were also significant on 60, 90, and 130 DAS, while the interaction of biochar and irrigation positively affected NH₄⁺–N on 60 and 90 DAS. The three-way interaction effect was only significant on 60 DAS. The NH₄⁺–N concentration was higher in the plots that received a higher rate of biochar and fertilizer under full irrigation until 60 DAS. However, after that, the plots without biochar had a higher NH₄⁺–N concentration. The higher NH₄⁺–N concentrations were 52.2 and 194 mg kg⁻¹ in WHB₂₀F₁₀₀I₁₀₀ on 7 and 60 DAS, respectively, and 43.3 and 61.1 mg kg⁻¹ in WHB₂₀F₂₀₀I₁₀₀ on 15 and 30 DAS, respectively, compared to controls (WHB₀F₀I₁₀₀ and WHB₀F₂₀₀I₁₀₀) (Table 6). However, after 60 DAS, the controls had higher NH₄⁺–N concentrations: WHB₀F₀I₁₀₀ (58.3 mg kg⁻¹) and WHB₀F₂₀₀I₁₀₀ (11.0 mg kg⁻¹) on 90 and 130 DAS, respectively.

3.3.3. Nitrate–Nitrogen

The concentration of soil NO₃⁻–N increased until 90 DAS and decreased afterward regardless of treatments. The combined application of biochar and fertilizer significantly affected NO₃⁻–N. Both the biochar and fertilizer main effects had a significant (p < 0.001) effect on all DAS, and irrigation had significant effects on 7, 15, and 90 DAS (Table A1). The interaction between biochar and fertilizer significantly affected NO₃⁻–N on all DAS except on 130 DAS. Biochar and irrigation had a significant interaction effect on 7, 15, and 90 DAS. However, the three-way interaction was only significant on 7 and 90 DAS. The concentration of soil NO₃⁻–N was higher in plots that received the highest rate of biochar and fertilizer until 60 DAS, and then, it was higher in plots without biochar. On 7 DAS, NO₃⁻–N increased from 1.36 mg kg⁻¹ (WHB₀F₀I₁₀₀) to 10.7 mg kg⁻¹ (WHB₂₀F₂₀₀I₅₀) and remained higher in WHB₂₀F₂₀I₁₀₀ on 15 DAS (11.7 mg kg⁻¹) and 60 DAS (46.4 mg kg⁻¹) and in WHB₂₀F₂₀I₁₀₀ on 30 DAS (16.3 mg kg⁻¹) compared to WHB₀F₀I₁₀₀ and WHB₀F₂₀₀I₁₀₀ (Table 7). However, on 90 and 130 DAS, NO₃⁻–N was higher in WHB₀F₀I₁₀₀ (58.3 and 13.4 mg kg⁻¹, respectively) compared to WHB₂₀F₂₀₀I₁₀₀ (21.1 and 4.31 mg kg⁻¹, respectively). Although the concentration of soil NO₃⁻–N was higher in the treatments with full irrigation (100%), it did not significantly (p > 0.05) affect NO₃⁻–N.

3.3.4. Available Phosphorus

The combined application of WHB and fertilizer significantly (p < 0.001) increased the available phosphorus (P) during the crop’s growth stage. Both the biochar and fertilizer main effects had significant effects on available P. The interaction between biochar and fertilizer also had a significant effect on the concentration of available P in the soil, particularly on 7, 60, and 130 DAS (Table A1). On 7 DAS, the WHB₂₀F₂₀₀I₁₀₀ treatment had a higher available P concentration (2.71 mg kg⁻¹) compared to the WHB₀F₀I₁₀₀ treatment (0.316 mg kg⁻¹) and the WHB₀F₂₀₀I₁₀₀ treatment (0.327 mg kg⁻¹) (Table 8). This higher concentration was maintained on 15, 30, 60, 90, and 130 DAS. The application of higher rates of WHB and fertilizer led to an increase in the available P concentration in the soil. Furthermore, reducing the irrigation amount to 50% did not significantly affect the availability of P in the soil.

3.4. Effects of Combined Application of Biochar and Fertilizer on Crop Dry Biomass and Grain Yield

3.4.1. Wheat Dry Biomass

The main effects of the biochar and fertilizer had a significant impact on dry biomass (p < 0.001) (Table A1). For instance, the dry biomass in WHB₂₀F₂₀₀I₁₀₀ (19.1 t ha⁻¹) was significantly higher (p < 0.001) compared to WHB₀F₀I₁₀₀ (2.01 t ha⁻¹) and WHB₀F₂₀₀I₁₀₀ (5.31 t h⁻¹) (Figure 5). The dry biomass did not show a significant difference between the treatments that received different rates of biochar (5, 10, and 20 t ha⁻¹) under full-crop-requirement irrigation water. However, there was an increasing trend observed with the biochar rate. It increased from 15.8 t ha⁻¹ (5 t ha⁻¹) to 18.3 t ha⁻¹ (10 t ha⁻¹), and finally to 19.1 t ha⁻¹ (20 t ha⁻¹). Furthermore, reducing irrigation water to 50% of the crop water requirement did not exert a significant impact on the crop dry biomass. Despite a 22.6% decrease in dry biomass observed under 50% crop water requirement conditions (WHB₂₀F₂₀₀I₅₀) compared to those under 100% crop water requirement conditions (WHB₂₀F₂₀₀I₁₀₀), this disparity was not deemed statistically significant.

3.4.2. Wheat Grain Yield

The main effect of biochar and fertilizer, as well as the interaction of biochar and fertilizer, showed a significant (p < 0.001) effect on wheat crop grain yield (Table A1). Although there was not a significant difference among treatments that received different amounts of water (50 and 100% of crop requirement), the grain yield was higher (3.92–33.1%) in treatments with a 100% crop water amount compared to those with a 50% crop water amount, regardless of the biochar and fertilizer amount. Grain yield significantly increased (p < 0.001) from 0.881 t ha⁻¹ (WHB₀F₀I₁₀₀; without amendment) and 1.50 t ha⁻¹ (WHB₀F₂₀₀I₁₀₀; fertilizer alone) to 4.10 t ha⁻¹ (WHB₂₀F₂₀₀I₁₀₀) (Figure 6).

4. Discussion

4.1. Effects of Combined Application of Biochar and Fertilizer on Soil Physical Properties

Incorporating biochar into soils is expected to improve soil physical and hydraulic characteristics. This is because biochar has unique attributes such as high concentrations of organic carbon, significant porosity, extensive surface area, and the presence of micropores. As a result, we can anticipate improvements in soil bulk density, porosity, and water-holding capacity [15].

4.1.1. Bulk Density

The density of biochar is lower than that of soil particles, allowing it to decrease the overall density of the soil. When biochar is incorporated into the soil, it forms aggregates that further decrease the bulk density of the soil [8,45]. The surfaces of biochar particles provide sites for microbial colonization and organic matter binding, which can lead to the formation of soil aggregates. These aggregates improve soil structure and decrease bulk density [46]. In our study, the application of biochar resulted in a 16.7% decrease in soil bulk density on 70 DAS and a 15.1% decrease at 130 DAS, compared to the treatments without biochar. Similarly, other studies have shown that biochar amendment can improve soil bulk density by 18% [8], 14.8% [47], and 7.41% [24] compared to unamended soil. In our study, on 70 DAS, the bulk density decreased from 0.807 g cm⁻³ (5 t ha⁻¹) to 0.731 g cm⁻³ (10 t ha⁻¹) and further decreased to 0.698 g cm⁻³ (20 t ha⁻¹). This trend continued on 130 DAS. Our findings are consistent with a study by Zhang et al. [47], which also showed a decrease in soil bulk density with increasing biochar rates. However, different levels of irrigation did not have a significant effect on bulk density, which aligns with the results of our study. According to a review study by Blanco-Canqui [45], biochar application lowers soil bulk density through several mechanisms. Firstly, biochar’s lower density and higher porosity compared to soil particles result in dilution upon mixing, reducing the overall density. Secondly, the increased concentration of organic carbon from biochar, particularly labile carbon, enhances biological activity and soil aggregation, leading to the formation of larger pores and a decrease in bulk density. Additionally, the high cation exchange capacity and specific surface area of biochar facilitate bonding with organic matter and clay particles, thereby altering the distribution of soil pore sizes.

4.1.2. Total Porosity

Biochar is highly porous, containing numerous small and large pores. When incorporated into the soil, it creates a matrix of channels and spaces, which enhances overall soil porosity [46]. Microorganisms fostered by biochar application, such as fungi and bacteria, create networks of filaments and excrete substances that help create and maintain pore spaces within the soil structure [48]. In our study, the application of biochar significantly improved total soil porosity compared to the treatment without biochar. Specifically, in the WHB₂₀ treatments, the soil porosity was improved by 7.60 and 5.56% on 70 DAS and 130 DAS, respectively, compared to the WHB₀ treatment. Similarly, Omondi et al. [49] demonstrated that biochar amendment significantly improved soil porosity by 8.4% compared to unamended soil, by directly increasing the total pore volume. Additionally, other studies have shown that the application of biochar improved soil porosity by 12% [50] and 14–64% [51] compared to the treatment without biochar.

4.1.3. Moisture Content

Biochar has a high water-holding capacity due to its porous structure and large surface area. When biochar is added to the soil, it can retain water within its pores, increasing the overall water retention capacity of the soil [52]. The moisture content of the soil was significantly improved in soil amended with biochar compared to treatments without biochar. In the higher biochar application rate (20 t ha⁻¹), the soil moisture content was higher than in the lower rates (5 and 10 t ha⁻¹) and in the treatments without biochar. This is likely due to the high surface area and hydrophilic functional groups of biochar, which enable it to improve soil moisture content [53]. Although there was no significant difference among the treatments with 20 t ha⁻¹ of biochar, the highest water content was recorded in the WHB₂₀F₂₀₀I₁₀₀ treatment (12.3–31.5%) compared to the treatments without biochar (WHB₀F₀I₁₀₀). Similarly, the field experiment conducted by Faloye et al. [24] showed that the water-holding capacity of the soil was improved by 3.58–8.70% at different soil water retentions due to biochar amendment compared to unamended soil. The study by Pandit et al. [54] also demonstrated that biochar application increased water retention at field capacity from 29.9% (without biochar) to 35.3% (2% biochar). The lower soil bulk density and higher soil porosity observed in the treatment with 20 t ha⁻¹ of biochar in this study likely contributed to the soil’s ability to retain higher soil moisture.

4.2. Effects of Combined Application of Biochar and Fertilizer on Soil Chemical Properties

4.2.1. Soil pH

Amending soil with biochar has recently emerged as a method for improving soil pH. This is because biochar is alkaline in nature and has the ability to enhance soil physical, chemical, and biological properties [4]. In our study, the pH significantly increased with biochar amendment. The pH was improved by 5.36–17.3% (WHB₂₀F₁₀₀I₁₀₀) compared to the control (WHB₀F₀I₁₀₀) during the crop’s growth stages. Similarly, biochar soil amendments improved soil pH by 13% [55] compared to no amendment. The application of different biomass-derived biochar also increased soil pH by 3.38–14.9% [11]. Mbabazize et al.’s [56] study also showed that pH improved by 20.8% due to the combined application of 5 t ha⁻¹ biochar and 500 kg ha⁻¹ DAP fertilizer compared to fertilizer alone (only 500 kg ha⁻¹). Similarly, in our study, pH was improved by 8.04–15.2% in the combined application of biochar and fertilizer (WHB₂₀F₁₀₀I₁₀₀) compared to treatments amended with fertilizer alone (WHB₀F₂₀₀I₁₀₀). The pH improvement is mainly due to biochar providing cations such as Ca, which plays a role in soil aggregate stability. Additionally, the OH⁻ produced from biochar neutralizes H⁺ ions, thus affecting the mobility and bioavailability of Fe³⁺ and Mn²⁺ [4]. In our study, the observed improvement in soil pH is likely to have facilitated the availability of essential plant nutrients, such as nitrogen and phosphorus. This enhancement in soil pH conditions is expected to have positively influenced the solubility and accessibility of these nutrients, thereby promoting their uptake by plants.

4.2.2. Ammonium–Nitrogen

Biochar has a high CEC due to its porous structure and large surface area, which allows it to adsorb and retain NH₄⁺ [57]. This prevents ammonium from being leached away and improves soil ammonium concentrations, thus making it available for plant uptake [58,59]. In our study, the concentration of NH₄⁺–N in the soil generally increased until 60 DAS and then decreased, regardless of the treatments. This finding is consistent with the study by Yao et al. [21], which investigated the combined application of biochar and nitrogen fertilizer and observed an initial increase, followed by a decrease in NH₄⁺–N concentrations, as the incubation time increased. This pattern may be attributed to various processes of NH₄⁺–N such as nitrification, microbial fixation, and volatilization, which affect the conversion and migration of NH₄⁺–N. The NH₄⁺–N concentration reached its peak at 60 DAS following the supplemental application of nitrogen in the form of urea, and then decreased. Similarly, Chen et al. [60] observed a sharp increase in NH₄⁺–N concentration in the soil after the supplemental application of nitrogen fertilizer (on 90 DAS), followed by a decrease to low concentrations, which continued until the end of the experiment (150 DAS). The application of biochar increased the soil NH₄⁺–N concentration by 10.5–65.1% compared to NPK fertilizer alone [11]. In the current study, the NH₄⁺–N concentration was significantly improved by 73.7–144% in the WHB₂₀F₁₀₀I₁₀₀ treatment compared to the fertilizer alone (WHB₀F₂₀₀I₁₀₀) until 60 DAS. However, after 90 DAS, the opposite trend was observed. The NH₄⁺–N concentration on 90 DAS and 130 DAS was higher (by 139 and 136%, respectively) in the control treatment (WHB₀F₂₀₀I₁₀₀) compared to the biochar-amended treatment (WHB₂₀F₁₀₀I₁₀₀). This is likely due to reduced nutrient absorption by the plants, since the plant biomass in the control treatment (WHB₀F₂₀₀I₁₀₀) was 211% lower than in the WHB₂₀F₁₀₀I₁₀₀ treatment (Figure 5). The observed improvement in NH₄⁺–N concentration in the soil likely contributed to an increase in NO₃⁻–N levels, as NH₄⁺–N serves as a substrate for nitrification processes.

4.2.3. Nitrate–Nitrogen

The concentration of NO₃⁻–N in this study was initially low and then increased until 60 DAS, and then decreased. After 90 DAS, it decreased regardless of the treatments. Similarly, the concentration of NO₃⁻–N in the soil peaked at 100 DAS and then decreased, remaining relatively low until the end of the experiment [60]. According to a meta-analysis by Liu et al. [12], the application of biochar increased the abundance of ammonia-oxidizing bacteria (AOB) by 37% and the nitrification rate by 57%, particularly in acidic soil (pH ≤ 5), which resulted in a higher NO₃⁻–N concentration in the soil. In our study, the combined application of biochar and fertilizer under full irrigation increased the NO₃⁻–N concentration by 131–637% in WHB₂₀F₂₀₀I₁₀₀ compared to the control (WHB₀F₂₀₀I₁₀₀) until 60 DAS. Similarly, the soil NO₃⁻–N concentration increased by 109 and 158% due to the application of 0.5 and 2% biochar on silty loam soil [54]. Ginebra et al. [11] also demonstrated that the application of biochar increased soil NO₃⁻–N concentrations by 21.7–139% compared to NPK fertilizer alone. However, in our study, after 90 DAS, the NO₃⁻–N concentration in the control (WHB₀F₂₀₀I₁₀₀) was higher by 173 and 139% compared to the combined biochar and fertilizer treatments under full irrigation (WHB₂₀F₂₀₀I₁₀₀). This is likely due to the higher nutrient absorption by plants in the higher biochar-amended treatments, as the biomass was higher in the amended treatments than in the control. The application of biochar increased the concentration of NO₃⁻–N in the soil by increasing the soil pH, which promotes the conversion of NH₄⁺ to NH₃ as a direct substrate for ammonia monooxygenase catalysis, thereby increasing the nitrification rate of the soil [61]. Additionally, biochar increased the soil nitrification rate by adsorbing nitrification-inhibiting compounds such as soluble phenols and terpenes [22] and decreasing the leaching of nitrate ions [58].

4.2.4. Available Phosphorus

Biochar application to the soil can decrease P fixation by iron and aluminum cations (Fe³⁺ and Al³⁺) and enhance P availability in P-fertilized soils [13,14]. In this study, the concentration of available P increased with an increase in biochar rate, regardless of fertilizer and irrigation water amount, throughout the experimental period. The application of 20 t ha⁻¹ biochar combined with inorganic fertilizer (WHB₂₀F₂₀₀I₁₀₀) improved available P by 85.8–427% compared to fertilizer alone (WHB₀F₂₀₀I₁₀₀) during different times of the crop-growing period. Similarly, the application of biochar improved soil available P by 111 and 658% (0.5 and 2% biochar, respectively) compared to those without biochar treatment [54]. Ginebra et al.’s [10] experiment also showed that available P was increased by 21.6–219% (compared to unamended soil) and 102–116% (compared to NPK alone) due to the amendment of different-biomass biochar. Furthermore, the observed improvement in soil pH in our study likely contributed to an enhancement in available P levels. Optimal soil pH conditions help decrease phosphorus fixation, which occurs more prominently in acidic soils, increase its mobility within the soil matrix, and increase the availability of phosphorus, thus promoting enhanced nutrient uptake [62] and potentially boosting crop productivity.

4.3. Effects of Combined Application of Biochar and Fertilizer on Crop Biomass and Grain Yield

According to Hussain et al. [63], biochar soil amendment improves crop productivity mainly by increasing nutrient use efficiency and water-holding capacity. Moreover, as stated by Yan et al. [64] biochar amendment improves nutrient availability, photosynthesis, and plant water availability, which increases crop growth and development, particularly in acidic soil.

4.3.1. Wheat Dry Biomass

The dry biomass of the wheat crop significantly increased with the combined application of biochar and fertilizer (WHB₂₀F₂₀₀I₁₀₀) by 850% (compared to no amendment) and 260% (compared to fertilizer alone) under the full crop water requirement. Similarly, the application of biochar improved wheat crop biomass by 26.2–33.9% compared to no amendment [65]. In Cong et al.’s [66] study, the application of biochar under deficit irrigation showed that wheat biomass was improved by 45.2% in 20 t ha⁻¹ biochar under 0.8ETc compared to without biochar. Similarly, in our study, the application of biochar only (WHB₂₀F₀I₅₀) under a 50% crop water requirement increased dry biomass by 391% compared to no biochar treatment (WHB₀F₀I₁₀₀). This result showed that, although a reduction in crop performance under deficit irrigation is a common response of most crops, amendment of the soil with organic materials such as biochar can mitigate the determinantal effect of moisture deficit [67].

4.3.2. Wheat Grain Yield

Amendment of the soil with biochar can decrease Fe³⁺, Al³⁺, and H⁺ toxicity effects on plant root growth, cell division, and nutrient availability, resulting in low crop productivity, particularly in acidic soil [4]. In our study, the combined application of biochar and fertilizer under a full crop water requirement (WHB₂₀F₂₀₀I₁₀₀) improved grain yield by 173% compared to fertilizer alone (WHB₀F₂₀₀I₁₀₀). Similarly, Hu et al.’s [68] study showed that the combined application of biochar and NP inorganic fertilizer increased the wheat crop yield by 81.7% compared to treatments without biochar amendments. Although reducing the irrigation water to 50% of the crop requirement (WHB₂₀F₂₀₀I₅₀) reduced the grain yield of the crop by 20%, it was not statistically significantly different from the 100% crop water requirement (WHB₂₀F₂₀₀I₁₀₀). This result is consistent with Singh et al. [69], which showed that a reduction of water use to 70% of ETc maintained plant physiology, growth, and yield similar to 100% ETc due to the amendment of biochar. This is probably related to the ability of biochar to improve water use efficiency, as indicated by the study of Mohsen et al. [67] where water use efficiency was higher in the treatments amended by biochar under a 50% plant water requirement, and the total crop yield under all deficit irrigation levels with biochar-amended treatments was higher than unamended treatments. Moreover, in our study, the reduction in soil bulk density and increase in soil porosity due to biochar amendment allows for adequate water infiltration, drainage, and air movement, promoting healthy plant growth, and pH and plant nutrients (NH₄⁺–N, NO₃⁻–N, and available P) were also enhanced in the combined application of biochar and fertilizer treatments, which contributed to the crop yield.

5. Conclusions

Water hyacinth biochar significantly improved soil bulk density, porosity, moisture content, pH, available nitrogen, and phosphorus when it was applied in combination with chemical fertilizer, as compared to the sole application of fertilizer. Eventually, the dry biomass and grain yield of the wheat crop were significantly increased.

Due to the positive effects of water hyacinth biochar, a 50% reduction of the irrigation water required for wheat production resulted in a wheat dry biomass and grain yield comparable with the 100% irrigation water requirement of wheat production. Therefore, it can be concluded that water hyacinth biochar combined with NPS fertilizer can improve soil physicochemical properties as well as wheat productivity in acidic silty loam soils. By demonstrating the effectiveness of biochar and fertilizer in improving soil quality and crop yield under the specific acidic conditions of our experimental site, we anticipate that similar positive outcomes can be expected in other regions with comparable soil characteristics.