Biochar with Organic and Inorganic Fertilizers Improves Defenses, Nitrogen Use Efficiency, and Yield of Maize Plants Subjected to Water Deficit in an Alkaline Soil

Abstract

Nutrient management practices, such as optimum fertilizer addition rate and co-addition of inorganic fertilizers and organic amendments (compost and biochar), were investigated to enhance crop production and nitrogen (N) use efficiency. However, how these practices improve the defense system, N use efficiency, yield quantity and quality, and physio-biochemical constituents of maize (Zea mays L.) plants grown on an alkaline soil under different irrigation levels (well-watered conditions, mild drought, and severe drought) remains unclear. A two-year field trial was carried out in a split–split plot with a randomized complete block design with three irrigation levels (100% evapotranspiration (ET), 80% ET, and 60% ET) as the main plots, two amendments (5 Mg ha^–1 biochar or compost) as subplots, and three regimes of inorganic fertilization (119-16-69, 179-24-99, and 238-31-138 kg N-phosphorus (P)-potassium (K) ha⁻¹) as sub-subplots. We found that maize yield, oil, starch, protein, carbohydrates, and NPK uptake significantly decreased with increasing drought levels, but catalase, peroxidase, superoxide dismutase, and proline contents significantly increased. The addition of organic amendments (compost or biochar) with inorganic fertilizers was more effective in enhancing the above attributes than inorganic fertilizers alone, but these attributes were positively related to inorganic fertilizer rates. The treatment of compost or biochar plus 238-31-138 kg N-P-K ha⁻¹ was the best treatment. The agronomic efficiency of applied fertilizer N (NAE) significantly decreased with increasing drought levels by 28.4–34.7%. The addition of biochar with inorganic fertilizers significantly enhanced NAE by 11.6% compared to inorganic fertilizers alone. Comparing the effect of inorganic fertilizer rates across all irrigation levels, the treatment of 119-16-69 kg N-P-K ha⁻¹ gave the highest NAE. Thus, the addition of optimum NPK fertilizer rate with biochar or compost is suitable to enhance the yield quantity and quality of maize plants grown on alkaline soils by improving its defensive system and N use efficiency, especially under the expected increase in aridity response to climate change.

1. Introduction

Maize (Zea mays L.) is one of the essential cereals worldwide. It is desired for its multiple purposes as animal feed and human food and in industrial and pharmaceutical manufacturing. It grows throughout a wide range of climates. In Egypt, for example, maize is cultivated in all agro-climatic zones from north to south, and its cultivated area increased from 0.67 × 10⁶ to 1.46 × 10⁶ hectares during 1961–2020, with an average production of 7.50 × 10⁶ Mg (1 Mg = 10⁶ g) year⁻¹ in 2020 [1]. However, an approximate 45% disparity exists between the production and consumption of maize in Egypt [2]. Despite high water use efficiency, maize is more vulnerable to water stress than other crops, especially in the Middle East and North Africa. This is due to its unique floral structure, which features separate male and female floral organs, and the near-synchronous development of florets on a single ear that typically grows on each stem [3]. The Middle East and North Africa including Egypt are the most water-scarce regions in the world. The region’s water stress level is extremely high, with more than 70% of its gross domestic product being exposed to high or very high water stress. This is significantly more severe than the global average of 22%. Egypt, for example, suffers annually from a water shortage of about 7 × 10⁹ m³. Thus, water scarcity restricts the production of most crops, including maize in arid and semi-arid regions, including Egypt.

Maize cultivations in arid and semi-arid regions suffer not only from lack of water, but also from alkaline soil conditions and poor fertility due to their low content of organic matter [4]. The preferred soil for maize production should be rich in organic matter and have a moderate pH of 5.5–6.5, a condition that is not available in most Egyptian soil, especially newly reclaimed ones [5]. Egyptian soils are generally characterized by high pH (>8.0) and low organic matter, mainly due to the dry environment [5]. High soil pH affects maize health and performance by reducing nutrient availability, especially phosphorus (P), zinc (Zn), iron (Fe), and manganese (Mn), inhibiting plant growth [6]. The reduced biological activity resulting from slower decomposition enables crop residues to accumulate. Higher soil pH decreases soil enzyme activities that regulate gross nitrogen (N) mineralization [7]. For example, urease and β-N-acetylglucosaminidase activities decrease when soil pH elevates from 4.0 to 9.0 [8]. Furthermore, although Egypt is the largest consumer of fertilizers (~500 kg ha⁻¹) in Africa and >60% of the fertilizers are applied to cereal crops only (maize, rice, and wheat), nutrient (e.g., N and P) use efficiency of maize is mostly not more than 25% [9]. High soil pH, low organic matter content, and water stress were the most important factors affecting the efficiency of nutrient use in Egypt.

Nutrient management practices, such as the joint addition of inorganic fertilizers of N, P, and potassium (K), the use of optimum fertilizer addition rate, the use of biochar, and the co-addition of inorganic and organic fertilizers, were investigated to increase crop production while reducing nutrient loss to the environment [10]. The co-addition of NPK is key to maintaining soil health and sustainable crop production, enhancing the quantity and quality of agricultural crops [11,12]. In the Central-Eastern European regions, for example, the supply of N has not been balanced by K and P over the past decades, in turn seriously limiting cereals production [13]. In Sub-Saharan Africa, NPK rather than N-only fertilization doubled maize yield [14]. Furthermore, incorporating biochar, a recalcitrant carbonic product of biomass, into the soil resulted in several benefits such as increased moisture and nutrient retention, the attraction of beneficial microbes, improved cation exchangeable capacity, enhanced nutrient use efficiency, and reduced nutrient loss [15,16]. In addition, organic amendments improve soil’s physical and biochemical properties, enhancing soil productivity. Thus, these practices could be a good approach to increase the N use efficiency (NUE) and maize yield. To our knowledge, however, how these practices enhance the efficiency of N use and the quantity and quality of maize plants grown on alkaline soils under drought stress remains unclear.

To address this knowledge gap, a field trial was carried out for two years to investigate the impact of biochar with inorganic (NPK) and organic (e.g., compost) fertilizers on the antioxidant defense system, NUE, and yield quantity and quality of maize plants grown on an alkaline soil under different irrigation levels. We hypothesized that inorganic (NPK) fertilizers and biochar or compost could enhance the quantity and quality of maize plants grown on alkaline soils by enhancing the antioxidant defense system as well as the efficiency of N use. The results of this study are of great significance for ensuring sustainable agricultural development and national food security in arid and semi-arid regions.

2. Materials and Methods

A two-year field experiment was carried out during two successive summer seasons of 2020 and 2021 at the farm of El-Gemmiza Agricultural Research Station, El-Gharbia Governorate, Egypt (30°47′43.1″ N; 31°07′30.7″ E). The climate of the study area is characterized as semi-arid with mean annual precipitation and temperature of 58 mm and 19 °C, respectively. Before cultivation, topsoil samples (20 cm depth) were assembled, air-dried, and ground to pass through a 2 mm sieve to analyze their physical and chemical attributes according to the standard methods outlined by [17]. The soil (Entisols, US Soil Taxonomy) is clay, consisting of 44.2% clay, 30.1% silt, 19.0% fine sand, and 6.70% coarse sand. The soil has a pH of 8.22 in soil–water suspension (1:2.5), electric conductivity (EC) of 3.20 dS m⁻¹ in soil paste, organic matter content of 7.54 g kg⁻¹, calcium carbonate (CaCO₃) content of 12.5 g kg⁻¹, total available N of 44.5 mg kg⁻¹, available P of 4.92 mg kg⁻¹, available K of 175 mg kg⁻¹, available Fe of 2.31 mg kg⁻¹, available Mn of 1.28 mg kg⁻¹, and available Zn of 44.5 mg kg⁻¹. The soil has soluble sodium (Na⁺), K⁺, calcium (Ca²⁺), magnesium (Mg²⁺), chloride (Cl⁻), bicarbonate (HCO₃⁻), and sulfate (SO₄²⁻) of 44.2, 0.89, 10.5, 6.89, 11.0, 2.85, and 18.2 meq L⁻¹, respectively, on average for both seasons. Soil physical and chemical characteristics were estimated according to the standard methods outlined by [17,18].

The experiment was carried out in a split–split plot with a randomized complete block design including three irrigation periods, which were 12 (I₁), 16 (I₂), and 20 (I₃) days as the main plots, two amendments (biochar, and compost) as subplots, and four rates of inorganic (NPK) fertilizers (0-0-0, 119-16-69, 179-24-99, and 238-31-138 kg N-P-K ha⁻¹) as sub-subplots. The details of the treatments under each irrigation interval were as follows: (1) Control (No additions), (2) R1 (119 kg N ha⁻¹ plus 15.7 kg P ha⁻¹ plus 69.0 kg K ha⁻¹), (3) R2 (179 kg N ha⁻¹ plus 23.5 kg P ha⁻¹ plus 99 kg K ha⁻¹), (4) R3 (238 kg N ha⁻¹ plus 31.4 kg P ha⁻¹ plus 138 kg K ha⁻¹), (5) Compost alone, (6) Compost plus R1, (7) Compost plus R2, (8) Compost plus R3, (9) Biochar alone, (10) Biochar plus R1, (11) Biochar plus R2, (12) Biochar plus R3. All these treatments were repeated three times each growing season. Utilizing the Penman–Monteith equation, maize irrigation requirements were assessed by estimating crop evapotranspiration. The well-watered irrigation requirements of maize were 725 and 765 mm in the first and second seasons, respectively (I₁). The irrigation level was diminished by 20% (I₂) and 40% (I₃), which represented mild and severe drought stress, respectively. Accordingly, the applied irrigation levels were 580 and 612 for the mild drought level and 435 and 459 mm for the severe drought level in the first and second seasons, respectively.

Grains of maize (Zea mays cv. Triple hybrid 310) obtained from the Maize Department, Filed Crop Res. Inst., ARC, were sown on 5 May 2020 and 10 May 2021. The plot size was 50 m² (5 × 10 m), having 14 ridges 5 m in length and 0.7 m in width and two plants hill⁻¹, with 20 cm between hills. The biochar and compost were mixed with soil at a rate of 5 Mg ha⁻¹ 20 days before planting. Three equal doses (on days 1, 30, and 50 of planting) of N fertilizer rates were applied as ammonium nitrate. Calcium superphosphate (67.6 g P kg⁻¹) was incorporated during seedbed preparation for P addition, and potassium sulfate (400 g K kg⁻¹) was added in two equal splits at 30 and 45 days after seeding for K addition. Inorganic NPK addition rates represent approximately 100%, 75%, and 50% of the recommended dose according to the Egyptian Ministry of Agriculture. Other recommended agricultural practices for growing maize by the Egyptian Ministry of Agriculture were followed. To create compost manure, a mixture of crop residues including rice straw, maize stover, and faba bean straw was dried in the open air and arranged in layers, each about 50 cm thick, with 300 kg of farmyard manure added to each pile to promote microorganism activity. The piles were then adequately moistened with approximately 60% water content. The piles were regularly turned over every 21 days until they were thoroughly decomposed, which took a total of 63 days. Once the compost was fully decomposed, it was deemed suitable for use. To produce the biochar, the residual wastes of rice straw, faba bean, cotton, and maize plants were heated at 350 °C in the absence of oxygen (pyrolysis conversion) according to the methods described by [19]. The final product was chemically analyzed according to [20]. The used biochar and compost had moisture content of 220 and 250 g kg⁻¹, pH of 7.95 and 7.60 in amendment–water suspension (1:2.5), EC of 3.80 and 3.20 dS m⁻¹ in amendment–water suspension (1:5), organic C of 334 and 329 g kg⁻¹, C/N ratio of 26.7 and 13.2, total N of 12.5 and 25.0 g kg⁻¹, total P of 8.51 and 9.01 g kg⁻¹, total K of 81.0 and 53.7 g kg⁻¹, total Fe of 350 and 320 mg kg⁻¹, total Mn of 120 and 110 mg kg⁻¹, and total Zn of 81.2 and 95.2 mg kg⁻¹, respectively.

Chlorophyll content was extracted from 0.1 g of fresh maize leaves using the pure acetone method [21] and measured based on [22]. Enzyme extraction was performed according to [23]. To evaluate the accumulation of proline in leaves, the approach described by [24] was employed. Total soluble sugar content was determined according to. Catalase (CAT) was assayed spectro-photochemically according to [25]. The method of [26] was used to estimate the activity of peroxidase (POD) in maize leaves. Superoxide dismutase (SOD) activity was determined by recording the decrease in absorbance of superoxide nitro blue tetrazolium complex by the enzyme [27].

At harvest (20 September 2020 and 27 September 2021), maize plants of each plot were randomly selected and labeled for yield evaluation. The following agronomic characters were estimated: plant height (cm), No. of ears per row, ear diameter (cm), 100-grain weight (g), and stover and grain yields (Mg ha⁻¹). The anthrone–sulfuric acid method was used to estimate the grain starch content. The oil concentration was measured using Soxhlet’s method [28]. Total soluble carbohydrates were estimated according to the methods of [29]. The macro-nutrient content of the plant samples was assessed by analyzing aliquots of the digested solutions. Plant samples were initially dried in an oven at 70 °C, and then digested using a mixture of H₂SO₄ and HClO₄ acids. The contents of N, P, and K were determined in the digestion of maize grains and stover using the methods described by [30]. Total N uptake by maize plants at harvest was determined by multiplying the N content and dry matter of the grains and stover for each plant. The resulting values for each plant part were added together to determine the total dry matter accumulation and total N uptake. The percentage of crude protein was calculated by multiplying the N concentration (%) by 6.25. Agronomic efficiency of applied fertilizer N (NAE, kg grain kg^–1 N applied) was computed as follows: NAE = [grain yield (kg ha⁻¹) of N added plots—grain yield (kg ha⁻¹) of control plots]/Total amount of N fertilizer applied (kg ha⁻¹). The physiological efficiency of applied N (NPE, kg kg⁻¹) was computed as follows: NPE = [(stover yield (kg ha⁻¹) of N added plots—stover yield of control plots)/(total N uptake in the fertilizer treatment—total N uptake in the control treatment)]. Soil pH and available NPK of the studied soil were determined after harvest according to the standard methods outlined by [17,18,30].

The data collected from both seasons of the study were subjected to statistical analysis, including analysis of variance (ANOVA), with the least significant difference (LSD) test applied at a 0.05 probability level to compare the treatment means. All results were expressed as mean ± standard error (SE).

3. Results

3.1. Maize Growth Traits and Yield

We found that plant length, No. of ears per row, ear diameter, 100-grain weight, grain yield, and stover yield were significantly decreased with increasing drought levels (

Table 1. Effect of irrigation, organic amendments, and/or inorganic fertilizers on maize growth and yield.

Treatment	Plant Length (cm)	No. of Ears per Raw	Ear Diameter (cm)	100-Grain Weight (g)	Grain Yield (Mg ha−1)	Stover Yield (Mg ha−1)
Irrigation
I1	208 a ± 2.79	15.8 a ± 0.17	7.16 a ± 0.16	36.7 a ± 1.45	2.32 a ± 0.08	3.18 a ± 0.09
I2	201 b ± 2.01	14.6 b ± 0.24	6.78 b ± 0.15	33.8 b ± 1.08	2.08 b ± 0.05	2.72 b ± 0.06
I3	195 c ± 1.61	14.5 c ± 0.20	6.36 c ± 0.15	30.6 c ± 0.97	1.99 c ± 0.05	2.58 c ± 0.08
Organic amendment
No amendment	187 c ± 1.21	14.4 c ± 0.27	5.76 c ± 0.12	25.9 b ± 0.60	1.92 c ± 0.04	2.65 c ± 0.08
Compost	211 a ± 2.05	15.3 a ± 0.21	7.35 a ± 0.10	37.9 a ± 0.65	2.28 a ± 0.07	2.95 a ± 0.09
Biochar	206 b ± 1.60	15.0 b ± 0.17	7.20 b ± 0.10	37.4 a ± 1.16	2.19 b ± 0.06	2.88 b ± 0.09
Inorganic fertilizer
No fertilizer	193 d ± 2.2	13.5 d ± 0.22	5.99 d ± 0.18	29.8 d ± 1.18	1.67 d ± 0.04	2.18 d ± 0.08
R1	198 c ± 1.75	14.4 c ± 0.14	6.64 c ± 0.16	31.6 c ± 1.10	2.14 c ± 0.03	2.85 c ± 0.03
R2	205 b ± 2.45	15.5 b ± 0.13	7.04 b ± 0.14	35.8 b ± 1.78	2.23 b ± 0.04	3.030 b ± 0.04
R3	210 a ± 3.20	16.4 a ± 0.11	7.40 a ± 0.15	37.7 a ± 1.10	2.48 a ± 0.07	3.25 a ± 0.1

I₁, I₂, and I₃ refer to well-watered, mild drought, and severe drought, respectively. R₁, R₂, and R₃ refer to 119-15.7-69, 179-23.5-99, and 238-31.4-138 kg NPK ha⁻¹, respectively. Data are the mean ± SE (n = 6). The same letters in each column indicate insignificant differences according to the LSD test (p ≤ 0.05).

). They decreased by 3.74, 7.22, 5.36, 7.94, 10.4, and 14.4%, respectively, under I₂, and by 6.38, 8.27, 11.2, 16.6, 14.1, 18.7%, respectively, under I₃ compared to the I₁ treatment. The addition of organic amendments (compost or biochar) with inorganic fertilization was more effective in enhancing the above attributes than inorganic fertilization alone (Table 1 and Figure 1 and Figure 2). Compared to inorganic fertilization alone, compost addition significantly increased plant length, No. of ears per row, ear diameter, 100-grain weight, grain yield, and stover yield by 12.4, 6.13, 27.6, 46.1, 19.0, and 11.2%, respectively, whereas biochar addition significantly increased them by 10.1, 4.19, 25.0, 44.2, 14.0, and 8.49%, respectively (Table 1). However, these previous traits had significant and positive responses to inorganic fertilizer addition rates (Table 1 and Figure 1 and Figure 2). Across all irrigation treatments, the previous traits enhanced by 2.60, 6.37, 10.8, 5.78, 27.9, and 30.7%, respectively, under R₁, by 6.09, 14.7, 17.4, 20.0, 33.5, and 39.3%, respectively, under R₂, and by 8.67, 21.4, 23.5, 26.3, 47.9, and 49.4%, respectively, under R₃ compared to the control treatment (no additions). The interactive effect of the irrigation × organic amendments, irrigation × inorganic fertilizer, organic amendments × inorganic fertilizer, and irrigation × organic amendments × inorganic fertilizer had a notable effect on the previous traits (Figure 1 and Figure 2). The treatment of compost plus R₃ was the best treatment for all traits under all irrigation treatments, which enhanced previous traits by 36.3, 22.7, 68.7, 106, and 97.1%, respectively, under I₁, by 21.1, 49.1, 52.1, 65.8, and 46.2%, respectively, under I₂, and by 16.7, 33.1, 71.7, 62.2, and 92.3%, respectively, under I₃ (Figure 1 and Figure 2).

3.2. Plant Physio-Biochemical Constituents

Our study showed that oil, starch, protein, carbohydrates, total N uptake, total P uptake, and total K uptake significantly decreased with increasing drought levels (

Table 2. Effect of irrigation, organic amendments, and/or inorganic fertilizers on physio-biochemical constituents of maize plants.

Treatment	Oil (%)	Starch (%)	Protein (%)	Carbohydrate (%)	Chlorophyll (mg g−1 FW)
Irrigation
I1	3.668 a ± 0.08	61.2 a ± 0.49	7.25 a ± 0.16	58.0 a ± 0.62	5.78 b ± 0.10
I2	3.539 b ± 0.09	59.9 b ± 0.32	6.84 b ± 0.13	56.7 b ± 0.61	5.92 a ± 0.24
I3	3.439 c ± 0.08	58.5 c ± 0.42	6.34 c ± 0.14	53.5 c ± 0.48	5.73 c ± 0.18
Organic amendment
No amendment	3.26 c ± 0.10	57.8 c ± 0.42	6.00 c ± 0.13	53.2 c ± 0.49	5.48 c ± 0.20
Compost	3.72 a ± 0.06	61.3 a ± 0.40	7.31 a ± 0.12	57.8 a ± 0.61	5.62 b ± 0.08
Biochar	3.67 b ± 0.05	60.6 b ± 0.29	7.12 b ± 0.11	57.1 b ± 0.59	6.33 a ± 0.21
Inorganic fertilizer
No fertilizer	2.88 d ± 0.08	57.3 d ± 0.53	5.90 d ± 0.16	52.5 d ± 0.52	4.88 d ± 0.1
R1	3.64 c ± 0.05	59.5 c ± 0.38	6.78 c ± 0.13	54.8 c ± 0.45	5.60 c ± 0.07
R2	3.79 b ± 0.03	61.1 b ± 0.36	7.13 b ± 0.14	57.7 b ± 0.7	6.47 a ± 0.27
R3	3.88 a ± 0.03	61.7 a ± 0.37	7.43 a ± 0.14	59.2 a ± 0.57	6.28 b ± 0.19

I₁, I₂, and I₃ refer to well-watered, mild drought, and severe drought, respectively. R₁, R₂, and R₃ refer to 119-15.7-69, 179-23.5-99, and 238-31.4-138 kg NPK ha⁻¹, respectively. FW is the fresh weight. Data are the mean ± SE (n = 6). The same letters in each column indicate insignificant differences according to the LSD test (p ≤ 0.05).

,

Table 3. The interaction effect of irrigation, organic amendments, and inorganic fertilizers on physio-biochemical constituents of maize plants.

Irrigation	Amendment	Inorganic NPK Rate	Oil (%)	Starch (%)	Protein (%)	Carbohydrate (%)	Chlorophyll (mg g−1 FW)
I1	No amendment	No fertilizer	2.58 k ± 0.05	54.4 q ± 0.58	5.22 t ± 0.01	50.1 q ± 0.06	4.2 q ± 0.1
		R1	3.45 h ± 0.03	58.3 m ± 0.18	6.44 opq ± 0.25	55.9 i ± 0.03	5.32 lm ± 0.01
		R2	3.75 fg ± 0.03	60.2 ijk ± 0.11	6.56 nop ± 0.03	56.3 i ± 0.01	5.89 fg ± 0.02
		R3	3.93 b–f ± 0.02	60.7 ghi ± 0.03	6.84 k–n ± 0.02	57.1 h ± 0.03	6.07 de ± 0.01
	Compost	No fertilizer	3.22 ij ± 0.01	61.2 fg ± 0.08	7.11 jk ± 0.01	55.0 j ± 1.15	5.66 j ± 0.03
		R1	3.88 b–g ± 0.02	62.14 d ± 0.08	7.76 def ± 0.03	57.9 g ± 0.03	6.04 de ± 0.02
		R2	4.02 abc ± 0.01	64.6 b ± 0.05	8.17 bc ± 0.04	62.1 bc ± 0.02	6.15 d ± 0.03
		R3	4.13 a ± 0.02	66.3 a ± 0.18	8.51 a ± 0.01	62.9 a ± 0.02	6.35 c ± 0.03
	Biochar	No fertilizer	3.20 ij ± 0.12	60.1 jk ± 0.03	6.58 nop ± 0.05	56.3 i ± 0.03	5.24 m ± 0.02
		R1	3.85 b–g ± 0.03	61.5 ef ± 0.03	7.48 fgh ± 0.05	57.3 h ± 0.01	6.02 e ± 0.01
		R2	3.96 a–e ± 0.03	63.3 c ± 0.04	7.99 cd ± 0.05	62.1 bc ± 0.03	6.10 de ± 0.03
		R3	4.05 ab ± 0.03	61.8 de ± 0.03	8.34 ab ± 0.02	62.5 ab ± 0.03	6.29 c ± 0.02
I2	No amendment	No fertilizer	2.18 l ± 0.1	55.4 p ± 0.23	5.38 t ± 0.19	50.2 q ± 0.12	4.11 q ± 0.06
		R1	3.33 hi ± 0.19	58.0 m ± 0.04	6.21 qr ± 0.12	52.3 no ± 0.01	5.10 n ± 0.03
		R2	3.69 g ± 0.05	59.2 l ± 0.12	6.44 opq ± 0.25	54.6 jk ± 0.02	5.73 hij ± 0.02
		R3	3.85 b–g ± 0.03	60.1 jk ± 0.07	6.73 mno ± 0.02	57.1 h ± 0.02	8.82 b ± 0.01
	Compost	No fertilizer	3.12 j ± 0.07	59.3 l ± 0.17	6.18 qr ± 0.1	55.2 j ± 0.12	4.82 o ± 0.01
		R1	3.82 c–g ± 0.01	60.8 gh ± 0.03	7.22 hij ± 0.08	56.3 i ± 0.03	5.80 gh ± 0.03
		R2	3.95 a–f ± 0.03	61.2 fg ± 0.12	7.59 efg ± 0.05	61.9 c ± 0.02	5.12 n ± 0.01
		R3	3.99 a–d ± 0.05	63.2 c ± 0.12	7.88 cde ± 0.05	62.0 bc ± 0.02	5.25 lm ± 0.03
	Biochar	No fertilizer	3.13 j ± 0.02	59.2 l ± 0.12	6.18 qr ± 0.1	54.2 k ± 0.01	5.36 l ± 0.1
		R1	3.79 d–g ± 0.02	60.4 h–k ± 0.03	7.02 j–m ± 0.01	55.9 i ± 0.01	5.89 fg ± 0.02
		R2	3.77 efg ± 0.04	60.9 fgh ± 0.03	7.48 fgh ± 0.05	59.6 e ± 0.02	8.95 a ± 0.03
		R3	3.85 b–fg ± 0.03	61.3 efg ± 0.17	7.71 d–g ± 0.01	60.7 d ± 0.03	6.05 de ± 0.03
I3	No amendment	No fertilizer	2.20 l ± 0.12	53.0 r ± 0.58	4.32 u ± 0.24	48.3 r ± 0.17	4.33 p ± 0.02
		R1	3.20 ij ± 0.12	55.6 op ± 0.02	5.46 t ± 0.03	51.0 p ± 0.01	5.03 n ± 0.02
		R2	3.44 h ± 0.02	58.2 m ± 0.12	6.04 rs ± 0.02	52.3 no ± 0.01	5.55 k ± 0.03
		R3	3.49 h ± 0.05	59.9 jk ± 0.05	6.33 pqr ± 0.19	53.7 l ± 0.01	5.68 ij ± 0.02
	Compost	No fertilizer	3.20 ij ± 0.12	57.4 n ± 0.04	5.88 s ± 0.02	52.2 o ± 0.03	5.10 n ± 0.06
		R1	3.72 g ± 0.01	58.5 m ± 0.03	6.84 k–n ± 0.02	53.2 lm ± 0.01	5.33 lm ± 0.02
		R2	3.79 d–g ± 0.05	60.5 hij ± 0.04	7.13 ijk ± 0.02	55.9 i ± 0.03	5.79 ghi ± 0.02
		R3	3.82 c–g ± 0.01	60.8 gh ± 0.03	7.42 ghi ± 0.01	58.6 f ± 0.02	5.98 ef ± 0.02
	Biochar	No fertilizer	3.10 j ± 0.06	56.0 o ± 0.58	6.20 qr ± 0.12	51.1 p ± 0.08	5.11 n ± 0.06
		R1	3.74 fg ± 0.04	59.8 k ± 0.03	6.61 nop ± 0.01	52.8 mn ± 0.03	5.89 fg ± 0.02
		R2	3.75 fg ± 0.03	60.9 gh ± 0.05	6.79 lmn ± 0.05	54.4 k ± 0.01	8.95 a ± 0.03
		R3	3.82 c–g ± 0.01	61.2 fg ± 0.13	7.07 jkl ± 0.04	57.9 g ± 0.03	6.05 de ± 0.03

I₁, I₂, and I₃ refer to well-watered, mild drought, and severe drought, respectively. R₁, R₂, and R₃ refer to 119-15.7-69, 179-23.5-99, and 238-31.4-138 kg NPK ha⁻¹, respectively. FW is the fresh weight. Data are the mean ± SE (n = 6). The same letters in each column indicate insignificant differences according to the LSD test (p ≤ 0.05).

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Table 4. Effect of irrigation, organic amendments, and/or inorganic fertilizers on nutrient uptake (kg ha⁻¹) and N use efficiency (kg kg⁻¹) of maize plants.

Treatment	Total N Uptake	Total P Uptake	Total K Uptake	NAE	NPE
Irrigation
I1	88.1 a ± 4.80	20.7 a ± 1.69	129 a ± 5.93	4.24 a ± 0.16	25.9 b ± 2.22
I2	74.5 b ± 3.20	14.5 b ± 0.92	109 b ± 4.13	3.03 b ± 0.13	16.2 c ± 0.99
I3	64.5 c ± 2.99	13.5 c ± 0.79	98.4 c ± 4.21	2.77 c ± 0.11	35.6 a ± 3.07
Organic amendment
No amendment	64.9 c ± 2.99	12.3 c ± 0.61	99.0 c ± 4.38	3.17 b ± 0.15	26.1 a ± 1.28
Compost	83.6 a ± 4.43	17.5 b ± 1.15	121 a ± 5.47	3.29 b ± 0.19	26.0 a ± 3.27
Biochar	78.6 b ± 4.04	18.9 a ± 1.68	116 b ± 5.18	3.54 a ± 0.17	25.6 a ± 2.49
Inorganic fertilizer
No fertilizer	44.5 d ± 2.01	12.3 d ± 2.34	70.4 d ± 3.07	3.16 b ± 0.38	37.4 a ± 7.31
R1	76.2 c ± 2.01	14.3 c ± 0.55	114 c ± 2.36	3.76 a ± 0.13	24.2 b ± 1.45
R2	85.7 b ± 2.56	17.4 b ± 0.76	125 b ± 2.92	3.13 b ± 0.12	23.2 b ± 1.17
R3	96.5 a ± 4.20	21.0 a ± 1.15	139 a ± 5.04	3.27 b ± 0.19	22.6 b ± 0.97

I₁, I₂, and I₃ refer to well-watered, mild drought, and severe drought, respectively. R₁, R₂, and R₃ refer to 119-15.7-69, 179-23.5-99, and 238-31.4-138 kg NPK ha⁻¹, respectively. NAE and NPE refer to agronomic and physiological efficiencies of applied fertilizer N, respectively. Data are the mean ± SE (n = 6). The same letters in each column indicate insignificant differences according to the LSD test (p ≤ 0.05).

and

Table 5. The interaction effect of irrigation, organic amendments, and inorganic fertilizers on nutrient uptake (kg ha⁻¹) and N use efficiency (kg kg⁻¹) of maize plants.

Irrigation	Amendment	Inorganic NPK Rate	Total N Uptake	Total P Uptake	Total K Uptake	NAE	NPE
I1	No amendment	No fertilizer	39.8 qr ± 3.42	6.79 o ± 0.65	68.3 r ± 5.22	-	-
		R1	76.5 ij ± 1.63	12.78 k–n ± 0.42	117 ijk ± 0.99	4.28 b–e ± 0.25	23.4 d–g ± 2.25
		R2	79.2 ij ± 1.25	14.7 i–l ± 1.37	123 gh ± 0.92	3.23 f–j ± 0.17	22.6 d–g ± 1.81
		R3	85.3 fgh ± 1.14	16.8 ghi ± 0.76	131 f ± 0.57	2.69 ijk ± 0.13	23.1 d–g ± 1.57
	Compost	No fertilizer	65.4 m ± 1.68	11.7 lmn ± 0.38	104.4 no ± 3.23	5.50 a ± 0.76	35.1 cde ± 4.82
		R1	92.4 e ± 1.38	18.5 e–h ± 0.71	133.4 f ± 1.44	3.81 d–h ± 0.14	19.3 d–g ± 1.14
		R2	111 c ± 0.97	23.3 c ± 1.47	152 c ± 1.45	3.91 c–g ± 0.32	19.5 d–g ± 0.83
		R3	138 a ± 1.37	31.6 b ± 1.54	187 a ± 1.59	4.88 abc ± 0.09	20.9 d–g ± 0.72
	Biochar	No fertilizer	48.2 op ± 1.06	44.4 a ± 2.67	72.0 r ± 1.61	4.60 a–d ± 0.12	57.0 b ± 14.2
		R1	90.0 ef ± 1.07	17.2 f–i ± 0.73	131 f ± 1.36	4.69 a–d ± 0.17	19.9 d–g ± 1.21
		R2	102 d ± 1.24	21.7 cd ± 0.89	146 d ± 0.99	3.89 c–g ± 0.35	20.8 d–g ± 1.06
		R3	130 b ± 1.43	29.3 b ± 0.67	181 b ± 0.93	5.13 ab ± 0.1	22.7 d–g ± 0.86
I2	No amendment	No fertilizer	39.5 qr ± 3.51	4.89 o ± 0.4	59.4 s ± 2.94	-	-
		R1	69.4 lm ± 1.12	11.4 mn ± 1.37	104 no ± 0.57	3.89 c–g ± 0.39	21.4 d–g ± 2.11
		R2	75.3 jk ± 0.64	13.1 j–n ± 0.75	113 klm ± 0.74	2.83 h–k ± 0.25	21.7 d–g ± 1.51
		R3	79.2 ij ± 1.14	14.9 i–l ± 0.98	120 hij ± 1.41	2.45 i–l ± 0.19	21.2 d–g ± 1.6
	Compost	No fertilizer	49.4 o ± 3.82	10.0 n ± 0.47	73.4 r ± 3.69	1.67 l ± 0.17	9.59 fg ± 5.18
		R1	81.7 ghi ± 2.04	16.2 hij ± 0.89	118 h–k ± 1.49	3.14 f–j ± 0.21	17.4 efg ± 1.02
		R2	93.5 e ± 2.09	19.7 d–g ± 0.81	129 f ± 1.54	3.13 f–j ± 0.32	16.1 efg ± 1.08
		R3	100.4 d ± 0.93	22.8 c ± 1.11	139 e ± 1.8	2.97 g–k ± 0.14	16.0 efg ± 0.77
	Biochar	No fertilizer	48.6 op ± 4.77	7.13 o ± 0.42	78.7 q ± 1.41	2.93 g–k ± 0.52	4.94 g ± 0.76
		R1	77.8 ij ± 1.1	14.7 i–l ± 0.9	116 jk ± 0.68	4.03 c–f ± 0.27	16.8 efg ± 1.27
		R2	85.4 fgh ± 1.39	18.6 d–h ± 1.2	124 gh ± 1.29	3.10 f–j ± 0.36	16.8 efg ± 1.13
		R3	93.9 e ± 1.48	21.0 cde ± 0.47	133 f ± 1.48	3.21 f–j ± 0.16	16.0 efg ± 0.98
I3	No amendment	No fertilizer	30.8 s ± 1.28	15.6 h–k ± 0.67	46.8 t ± 1.35	-	-
		R1	59.4 n ± 1.14	10.4 n ± 0.57	94.1 p ± 1.57	3.78 d–h ± 0.53	38.1 cd ± 3.02
		R2	70.5 klm ± 1.47	12.1 lmn ± 0.29	103 o ± 1.4	2.82 h–k ± 0.27	32.7 cde ± 2.47
		R3	74.6 jkl ± 1.96	14.3 i–m ± 1.33	108 mno ± 1.44	2.56 i–l ± 0.21	30.7 cde ± 2.46
	Compost	No fertilizer	35.4 rs ± 0.28	4.96 o ± 0.21	58.7 s ± 0.83	2.27 jkl ± 0.49	73.4 a ± 26.02
		R1	70.5 klm ± 0.72	14.1 i–m ± 0.77	109 lmn ± 1.21	2.79 h–k ± 0.22	30.3 cde ± 1.72
		R2	79.9 hij ± 1.27	17.4 f–i ± 1.38	120 hij ± 0.85	2.67 ijk ± 0.23	28.6 c–f ± 1.65
		R3	85.7 fg ± 1.33	20.2 c–f ± 1.02	128 fg ± 0.86	2.71 ijk ± 0.15	26.1 c–f ± 1.38
	Biochar	No fertilizer	43.5 pq ± 1.59	5.43 o ± 0.27	71.5 r ± 1.73	2.00 kl ± 0.35	44.6 bc ± 3.53
		R1	68.2 m ± 1.87	13.1 j–n ± 0.98	106 no ± 1.84	3.42 e–i ± 0.28	30.9 cde ± 2.12
		R2	74.4 jkl ± 1.07	15.7 h–k ± 0.75	114 kl ± 1.39	2.56 i–l ± 0.34	30.0 cde ± 1.83
		R3	81.4 ghi ± 0.99	18.2 e–h ± 0.85	122 hi ± 1.34	2.88 g–k ± 0.17	26.6 c–f ± 1.35

I₁, I₂, and I₃ refer to well-watered, mild drought, and severe drought, respectively. R₁, R₂, and R₃ refer to 119-15.7-69, 179-23.5-99, and 238-31.4-138 kg NPK ha⁻¹, respectively. NAE and NAP refer to agronomic and physiological efficiencies of applied fertilizer, respectively. Data are the mean ± SE (n = 6). The same letters in each column indicate insignificant differences according to the LSD test (p ≤ 0.05).

). These attributes decreased by 3.52, 2.12, 5.71, 2.23, 15.5, 29.8, and 15.4%, respectively, under I₂ treatment, and by 6.24, 4.47, 12.5, 7.79, 26.8, 35.0, and 23.6%, respectively, under I₃ treatment as compared to I₁ treatment. However, the highest chlorophyll content was recorded under I₂ treatment, which enhanced chlorophyll content by 2.44 and 3.21% compared to I₁ and I₂ treatments, respectively. The addition of compost or biochar with inorganic fertilizers significantly enhanced the above attributes compared to inorganic fertilizers alone, as compost addition significantly increased oil, starch, protein, carbohydrates, N uptake, total P uptake, and total K uptake by 14.2, 6.20, 21.8, 8.51, 28.8, 42.3, and 22.3%, respectively, whereas biochar addition significantly increased them by 12.6, 4.84, 18.7, 7.20, 21.0, 53.2, and 17.5%, respectively (Table 2, Table 3, Table 4 and Table 5). The highest chlorophyll content was recorded under the addition of biochar. Inorganic fertilization rate had a significant and positive effect on oil, starch, protein, carbohydrates, N uptake, total P uptake, and total K uptake (Table 2, Table 3, Table 4 and Table 5). They enhanced by 26.5, 3.74, 15.1, 4.27, 71.2, 16.0, and 62.4%, respectively, under R₁ treatment, by 31.6, 6.39, 21.0, 9.89, 92.5, 41.0, and 77.4%, respectively, under R₂ treatment, and by 34.7, 7.65, 26.0, 12.7, 117, 70.6, and 97.4%, respectively, under R₃ treatment compared to the control treatment. Chlorophyll content also responded positively to fertilizer addition rates, but R₃ treatment significantly reduced it compared to R₂ treatment (Table 2, Table 3, Table 4 and Table 5). The interactive effect of the irrigation × organic amendments, irrigation × inorganic fertilizer, organic amendments × inorganic fertilizer, and irrigation × organic amendments × inorganic fertilizer had a notable effect on the previous traits (Table 2, Table 3, Table 4 and Table 5). The treatment of compost plus R₃ caused the highest contents of oil, starch, protein, and carbohydrates as well as N uptake, total P uptake, and total K uptake under all irrigation treatments, and the highest chlorophyll content under I₁ treatment. However, the treatment of biochar plus R₂ caused the highest chlorophyll content under I₂ and I₃ treatments (Table 2, Table 3, Table 4 and Table 5).

3.3. Nitrogen Use Efficiency

The agronomic efficiency of applied fertilizer N (NAE) was significantly decreased with increasing drought levels. It decreased by 28.4 and 34.7% under I₂ and I₃ treatments, respectively, compared to the treatment of I₁ (Table 4). The interactive effect of the irrigation × organic amendments, irrigation × inorganic fertilizer, organic amendments × inorganic fertilizer, and irrigation × organic amendments × inorganic fertilizer had a notable effect on NAE (Table 4 and Table 5). The addition of biochar with inorganic fertilizers significantly enhanced NAE by 11.6 and 7.57% compared to the inorganic fertilizers alone or in combination with compost, respectively, without any significant differences between the last two treatments (Table 4 and Table 5). When comparing the effect of inorganic fertilization rates on NAE across all irrigation treatments, the R₁ treatment gave the highest NAE compared to other rates (Table 4). Under the treatment of I₁, the highest NAE was observed when compost was added alone (Table 4 and Table 5). However, under I₂ treatment, the highest NAE was observed when biochar was added in combination with inorganic fertilizers (R₁), while the lowest inorganic fertilization rate (R1) was the best treatment for NAE under I₃ treatment (Table 4). The lowest NAE was observed under the application of compost and biochar alone under I₂ and I₃ treatments, respectively, but it was under the highest application rate of inorganic fertilizers alone under I₁ treatment (Table 4 and Table 5). In contrast, the treatment of I₃ significantly enhanced the physiological efficiency of applied N (NPE) by 37.5 and 120% compared to the treatments of I₁ and I₂, respectively (Table 4). The highest NPE was recorded when biochar was added alone under the treatment of I₁, but was recorded for the treatment with R₁ under I₂ and with the addition of compost alone under I₃ (Table 4 and Table 5).

3.4. Maize Defense Systems

Our study showed that CAT, POD, SOD, proline, and soluble sugars significantly increased with increasing drought levels (

Table 6. Effect of irrigation, organic amendments, and/or inorganic fertilizers on catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), proline, and soluble sugar of maize plants.

Treatment	CAT (Unit mg−1 Protein)	POD (Unit mg−1 Protein)	SOD (Unit mg−1 Protein)	Proline (µmol g−1 DW)	Soluble Sugar (mg g−1 DW)
Irrigation
I1	4.96 c ± 0.17	8.57 c ± 0.22	3.36 c ± 0.17	64.3 c ± 0.54	19.6 c ± 0.28
I2	8.95 b ± 0.26	17.2 b ± 0.43	7.18 b ± 0.28	138 b ± 1.02	36.4 b ± 0.68
I3	14.0 a ± 0.29	30.2 a ± 0.58	11.7 a ± 0.26	173 a ± 1.58	50.0 a ± 0.78
Organic amendment
No amendment	8.71 c ± 0.24	17.5 c ± 0.61	6.77 b ± 0.33	122 b ± 1.63	33.3 c ± 0.70
Compost	9.65 a ± 0.49	19.2 a ± 0.58	7.71 a ± 0.43	127 a ± 1.15	36.1 b ± 0.61
Biochar	9.58 a ± 0.34	19.3 a ± 0.61	7.76 a ± 030	127 a ± 1.26	36.5 a ± 0.80
Inorganic fertilizer
No fertilizer	8.55 d ± 0.26	17.2 d ± 1.95	6.67 d ± 0.17	122 c ± 5.6	33.7 d ± 4.22
R1	9.01 c ± 0.73	18.4 c ± 1.18	7.08 c ± 0.16	124 c ± 5.8	34.8 c ± 4.27
R2	9.51 b ± 0.64	19.1 b ± 1.24	7.67 b ± 0.25	126 b ± 6.1	35.8 b ± 4.53
R3	10.2 a ± 0.81	19.9 a ± 1.25	8.24 a ± 0.29	128 a ± 6.5	36.9 a ± 4.69

I₁, I₂, and I₃ refer to well-watered, mild drought, and severe drought, respectively. R₁, R₂, and R₃ refer to 119-15.7-69, 179-23.5-99, and 238-31.4-138 kg NPK ha⁻¹, respectively. DW is the dry weight. Data are the mean ± SE (n = 6). The same letters in each column indicate insignificant differences according to the LSD test (p ≤ 0.05).

). These parameters increased by 98.2, 96.4, 133, 116, and 74.4%, respectively, under I₂ treatment, and by 229, 269, 349, 167, and 157%, respectively, under I₃ treatment as compared to the treatment of I₁. The addition of compost or biochar with inorganic fertilization significantly enhanced CAT, POD, SOD, proline, and soluble sugars compared to inorganic fertilization alone (

Table 7. The interaction effect of irrigation, organic amendments, and inorganic fertilizers on catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), proline, and soluble sugar content of maize plants.

Irrigation	Amendment	Inorganic NPK Rate	CAT (Unit mg−1 Protein)	POD (Unit mg−1 Protein)	SOD (Unit mg−1 Protein)	Proline (µmol g−1 DW)	Soluble Sugar (mg g−1 DW)
I1	No amendment	No fertilizer	3.92 u ± 0.32	7.23 u ± 0.28	2.34 z ± 0.11	60.9 z ± 3.2	18.0 v ± 1.2
		R 1	4.33 t ± 0.26	7.92 t ± 0.2525	2.62 y ± 0.13	62.0 yz ± 2.9	18.5 uv ± 1.4
		R 2	4.74 s ± 0.58	8.10 t ± 0.26	2.86 x ± 0.16	63.1 xy ± 3.6	18.7 uv ± 1.3
		R 3	5.29 r ± 0.55	8.91 rs ± 0.71	3.34 vw ± 0.18	63.3 x ± 2.9	19.1 tu ± 1.7
	Compost	No fertilizer	4.26 t ± 0.41	8.06 t ± 0.33	3.22 w ± 0.16	63.6 wx ± 4.1	19.2 stu ± 1.3
		R 1	4.87 s ± 0.47	8.43 t ± 0.53 s	3.51 uv ± 0.16	64.6 vw ± 2.8	19.9 rst ± 1.6
		R 2	5.23 r ± 0.39	8.95 rs ± 0.36	3.89 st ± 0.11	65.9 u ± 3.1	20.2 r ± 1.7
		R 3	5.88 q ± 0.52	9.86 q ± 0.66	3.93 s ± 0.15	65.9 u ± 3.2	20.6 qr ± 1.6
	Biochar	No fertilizer	4.99 u ± 0.44	8.18 t ± 0.33	2.99 x ± 0.17	64.5 vw ± 3.2	19.3 stu ± 1.6
		R 1	4.90 s ± 0.63	8.38 rst ± 0.33	3.69 tu ± 0.17	64.6 vw ± 2.9	19.9 rs ± 1.4
		R 2	5.40 r ± 0.58	9.03 r ± 0.55	3.75 st ± 0.17	65.4 uv ± 2.8	20.5 qr ± 1.1
		R 3	5.73 q ± 0.39	9.78 q ± 0.45	4.22 r ± 0.19	67.7 t ± 2.5	21.2 q ± 1.1
I2	No amendment	No fertilizer	7.77 p ± 0.66	14.2 p ± 0.85	5.46 q ± 0.33	132 s ± 4.1	31.4 p ± 2.4
		R 1	7.79 p ± 0.85	15.7 o ± 0.95	6.17 p ± 0.37	134 r ± 4.6	34.2 o ± 1.7
		R 2	8.18 o ± 0.75	16.0 n ± 0.99	6.51 o ± 0.27	135 qr ± 2.6	34.3 no ± 1.6
		R 3	8.69 mn ± 0.52	17.4 kl ± 1.1	7.58 m ± 0.48	140 mn ± 5.2	35.1 mn ± 1.2
	Compost	No fertilizer	8.51 mn ± 0.55	16.8 lm ± 1.2	6.23 p ± 0.39	137 p ± 4.4	35.7 m ± 1.7
		R 1	9.04 l ± 0.85	17.7 k ± 1.9	6.65 o ± 0.25	138 o ± 4.7	37.1 kl ± 2.1
		R 2	9.94 jk ± 0.68	18.7 ij ± 1.3	8.09 k ± 0.41	142 kl ± 4.8	38.3 ij ± 2.6
		R 3	10.1 j ± 0.81	19.1 i ± 1.1	8.27 k ± 0.38	142 jk ± 4.9	39.4 h ± 3.1
	Biochar	No fertilizer	8.43 no ± 0.96	16.2 mn ± 1.4	7.21 n ± 0.63	136 q ± 4.9	36.7 l ± 1.6
		R 1	8.77 m ± 0.75	17.9 k ± 1.4	7.44 m ± 0.56	140 n ± 4.6	37.7 jk ± 2.7
		R 2	9.72 k ± 0.95	18.1 jk ± 1.6	7.84 l ± 0.56	141 m ± 5.2 l	37.9 jk ± 2.6
		R 3	10.5 i ± 0.56	18.9 i ± 1.6	8.70 j ± 0.36	143 j ± 3.6	39.0 hi ± 2.5
I3	No amendment	No fertilizer	12.9 h ± 0.85	26.7 h ± 1.8	10.5 i ± 0.62	162 hi ± 4.8	46.3 g ± 2.7
		R 1	13.3 g ± 0.88	27.4 g ± 2.1	10.7 h ± 0.59	164 g ± 5.6	46.4 g ± 2.8
		R 2	13.7 f ± 0.87	29.9 e ± 2.4	11.2 f ± 0.68	168 f ± 7.2	47.2 g ± 3.6
		R 3	13.9 ef ± 0.99	29.9 e ± 2.8	12.0 d ± 0.95	174 d ± 6.3	50.8 d ± 1.9
	Compost	No fertilizer	13.0 gh ± 0.86	28.3 f ± 1.9	11.0 g ± 0.48	173 e ± 7.2	48.8 ef ± 2.1
		R 1	14.0 ef ± 0.82	30.9 d ± 2.9	11.4 e ± 0.77	174 d ± 6.3	50.4 d ± 3.1
		R 2	14.4 c ± 0.91	31.1 cd ± 3.2	12.7 b ± 1.3	177 b ± 4.1	51.9 c ± 2.8
		R 3	16.6 a ± 0.93	32.2 b ± 2.1	13.6 a ± 1.1	180 a ± 3.6	52.0 c ± 2.4
	Biochar	No fertilizer	13.2 g ± 0.83	28.9 f ± 2.1	11.1 fg ± 0.63	171 e ± 6.7	48.3 f ± 2.9
		R 1	14.1 de ± 0.93	31.6 bc ± 2.8	11.5 e ± 0.84	173 de ± 4.9	49.4 e ± 2.7
		R 2	14.3 cd ± 1.2	31.7 bc ± 2.6	12.2 d ± 1.1	176 c ± 5.2	53.3 b ± 2.9
		R 3	14.9 b ± 1.3	33.2 a ± 1.9	12.5 c ± 1.4	178 b ± 3.9	54.7 a ± 2.3

I₁, I₂, and I₃ refer to well-watered, mild drought, and severe drought, respectively. R₁, R₂, and R₃ refer to 119-15.7-69, 179-23.5-99, and 238-31.4-138 kg NPK ha⁻¹, respectively. DW is the dry weight. Data are the mean ± SE (n = 6). The same letters in each column indicate insignificant differences according to the LSD test (p ≤ 0.05).

). Inorganic fertilization rate had also a significant and positive effect on CAT, POD, SOD, proline, and soluble sugars. The interactive effect of the irrigation × organic amendments, irrigation × inorganic fertilizer, organic amendments × inorganic fertilizer, and irrigation × organic amendments × inorganic fertilizer had a notable effect on the previous traits (Table 7). Under the treatment of I₁, biochar addition significantly increased CAT by 17.1% but decreased SOD by 7.69% compared to compost addition (Table 7). Under the treatment of I₂, in contrast, biochar addition significantly increased SOD by 15.7% compared to compost addition. The co-addition of biochar and R₃ caused the highest contents of CAT, POD, SOD, proline, and soluble sugars under I₁ treatment. Under the treatment of I₂, the highest contents of CAT, SOD, and proline were also observed when biochar was co-added with R₃; however, the highest contents of SOD and soluble sugars were recorded when compost was added with R₃. Under the treatment of I₃, in contrast, the highest contents of CAT, SOD, and proline were observed when compost was added with R₃, but the highest contents of SOD and soluble sugars were recorded when biochar was added with R₃ (Table 7).

3.5. Soil Properties and Nutrient Contents

Our study revealed that soil-available NPK contents were significantly higher under the irrigation treatment of I₂ than those under I₁ and I₃, and the lowest NPK content was recorded under the treatment of I₁ (Table S1). The addition of inorganic fertilization alone significantly enhanced soil-available NPK contents compared to the treatments of compost and biochar (Table S1). Soil-available NPK contents were higher in the treatment of compost than those in the treatment of biochar (Table S1). Both irrigation and organic/inorganic additions did not affect soil pH (Table S1). We also found that available NPK contents increased significantly with increasing inorganic fertilization rates under all irrigation treatments, but soil pH significantly decreased (Tables S1 and S2). The highest soil-available NPK contents were recorded under the highest addition rate of inorganic NPK (R₃) under all irrigation treatments (Table S2). The highest soil pH was observed when biochar was added alone under the irrigation treatments of I₁ and I₃, while the addition of compost alone resulted in the highest soil pH under the treatment of I₂ (Table S2). The lowest soil pH was noted under the treatment of compost plus R₃ at the irrigation treatment of I₁ (Table S2).

3.6. Principal Component Analysis (PCA)

The relationship among different soil and plant indices was examined using principal component analysis (PCA). The PCA ordination plot displayed the correlations among the studied variables (Figure 3). Smaller angles between arrows represented a higher correlation between variables, and the direction of the arrows indicated positive or negative correlations. The first (PCA-1) and second (PCA-2) ordination axes explained 82.0% and 7.04% of the total variations. Grain yield and plant length showed a positive correlation with plant NPK uptake and soil pH, chlorophyll, carbohydrates, and NAE, but a negative correlation with POD, CAT, SOD, proline, and soluble sugar. Our analysis also showed that increasing soil nutrient availability (NPK) stimulated total NPK uptake.

4. Discussion

Our study showed that plant length, No. of ears per row, ear diameter, 100-grain weight, grain yield, stover yield, oil, starch, protein, and carbohydrates decreased significantly with increasing drought levels. These findings are consistent with the previous findings of Desoky et al. [31]. Water stress has been reported as a key yield-reducing factor for maize [32,33]. Compared to other cereals, the size of maize grain is greater. Thus, the water requirements of maize are high to maintain the osmotic potential and germination [34]. The activation of metabolic enzymatic is inhibited by reducing water availability which inhibited the germination of maize grain [31]. Water stress also reduces the water potential of the leaves, which reduces the production of plant pigments such as chlorophyll, which ultimately reduces the carbohydrates produced and grain yield [35]. Under irrigation water deficiency conditions, the leaves’ stomata are closed, resulting in a decrease in the fixation of carbon dioxide (CO₂), but the light reaction and electron transfer remain normal. As a consequence, electron acceptance by NADP is restricted; thus, oxygen is utilized as an electron acceptor, leading to an overproduction of reactive oxygen species (e.g., O₂^•− and H₂O₂) [36,37]. The excessive generation of reactive oxygen species leads to the impairment of plasma membranes and a decline in the chlorophyll levels in leaves.

We also noticed that increasing drought levels caused a gradual and significant reduction in the content of total chlorophyll, which is a main early plant response to drought stress, significantly reducing metabolite accumulation and plant production [31,35]. When irrigation water is limited, the hydraulic conductivity of maize roots is reduced, impeding the flow of water from the roots to the shoots. This leads to a decrease in leaf water content and induces stomatal closure as a mechanism to maintain adequate leaf hydration [38]. This decrease in the water content of maize leaves leads to metabolic and physiological changes and ultimately inhibits maize growth. Furthermore, our study showed that increasing drought levels also have a significant and negative effect on plant nutrient (NPK) uptake (Table 4), in line with the meta-analysis of [39] which reported that drought stress reduces N and P contents in plant tissue. Drought conditions reduce nutrient supply through mineralization and nutrient mass flow and by influencing nutrient uptake kinetics by roots [31,35,40]. This reduction in nutrient uptake by plants is likely to be one of the main reasons for reducing maize production and NUE when the drought level was increased.

To achieve optimal growth and yield traits, the maize crop must uptake enough NPK during its growth stages. Inorganic NPK fertilizers have been widely used to enhance the performance and production of maize plants. We found that all physiochemical and yield attributes of the normal and stressed plants enhanced significantly with increasing inorganic NPK dose (Table 1, Table 2, Table 3, Table 4 and Table 5). Inorganic NPK fertilizers are readily soluble and thus can supply NPK to maize plants within a short time following application [41]. Our study revealed that soil-available NPK enhanced significantly with increasing inorganic NPK dose (Tables S1 and S2), which was in line with previous studies [42]. The enhanced physiochemical and yield attributes of maize might be due to the enhanced availability of nutrients from the increased dose of inorganic NPK applied to the crop. The mineral nutrient status of plants also plays a key role in enhancing plant resistance to drought stress [43], as evidenced by the current study (Table 6 and Table 7). For example, N improves soluble sugars and starch accumulation, in turn enhancing leaf growth [44]. During water deficit conditions, the deficiency of N caused by drought leads to growth inhibition, particularly affecting the size of the leaves by reducing both cell size and number [45].

It is generally reported that P uptake by plants decreased in dry soils. For instance, P translocation to plant shoots is restricted under drought stress [46]. Phosphorus deficiency is one of the earliest impacts of mild to moderate drought stress on plants. However, P fertilizer addition enhances plant growth under such conditions, mainly due to enhanced drought tolerance, stomatal conductance, cell membrane stability, and photosynthesis [47,48]. Furthermore, K nutrition increases crop tolerance to water stress by increasing the efficiency of soil water use. For instance, Martineau et al. [49] found that K fertilizer addition enhanced grain yield and water use efficiency of maize under water stress conditions by enhancing stomatal sensitivity to drought. Recently, Xu et al. [50] stated that K fertilization alleviated drought stress on rapeseed plant growth by regulating the secretion and morphology of roots and soil ecosystems. Under water stress conditions, K addition maintains the osmotic potential, regulates the stomatal functioning, and increases the photosynthetic rate and plant growth and production [51]. When comparing the effect of inorganic fertilization rates on NAE across all irrigation treatments, we found that the NAE decreased significantly with the increasing inorganic NPK fertilizer rate (Table 4 and Table 5). Large additions of inorganic NPK fertilizers under the use of traditional farming methods reduce nutrient recovery and enhance nutrient surplus [52]. Thus, an adequate supply of nutrients must be ensured to optimize the efficiency of applied fertilizers.

Our study showed that the addition of organic amendments (compost or biochar) with inorganic fertilization was more effective in enhancing growth and yield traits, physio-biochemical constituents, nutrient uptake, and NUE of maize plants grown on the alkaline soil than inorganic fertilization alone (Table 1, Table 2, Table 3, Table 4 and Table 5). Our study showed that compost or biochar additions increased soil P contents, which ultimately enhanced maize growth and production. Organic amendments increased P availability by decreasing its fixation, promoting efficient P utilization by plants for greater production [53]. Similarly, previous studies have demonstrated an increase in P availability and uptake after the addition of biochar and manure to calcareous soil [54]. The release of organic acid compounds after applying organic amendments (compost or biochar) to alkaline soils may compete for soil exchange sites with P ions, influencing the precipitation–dissolution of P minerals as well as the adsorption–desorption processes of P in soil solution [54,55]. Improvement in growth and yield traits was also linked to improved soil characteristics and soil total N under combined organic and inorganic N fertilizers [56]. In two Mediterranean agricultural soils, Manolikaki and Diamadopoulos [57] found that biochar and/or compost addition increased the above-ground dry weight and nutrient content of maize plants.

Furthermore, our study showed that the addition of biochar with inorganic fertilization significantly enhanced NAE by 11.6% compared to the addition of inorganic fertilizers alone. This was partially in line with previous studies that reported that compost and biochar with inorganic fertilizers enhanced fertilizer use efficiency [58]. The increased NAE in response to biochar application could be due to the improved soil water-holding capacity, nutrient availability, and physiochemical properties within the biochar [59]. Our findings also agree with the results of Sun et al. [60], who found that the addition of 5–20 Mg ha⁻¹ biochar enhanced the NUE of plants grown under saline conditions by 5.2–37.9%. Similarly, Agegnehu et al. [61] reported that the apparent recovery efficiency and agronomic efficiency responded significantly to biochar with N fertilizer. However, our study showed that adding compost with inorganic NPK did not show any significant effect on NAE compared to adding inorganic NPK alone. Our study also showed that the biochar or compost addition increased plant growth, biomass, physio-biochemical constituents, and nutrient uptake of drought-stressed maize plants (Table 1, Table 2, Table 3, Table 4 and Table 5). The beneficial impact of compost and biochar additions under water deficit conditions has been also reported in previous studies [62]. Our results were inconsistent with the findings of some studies that reported no effect of biochar on plant growth [63] and chlorophyll content [64] under drought stress, indicating that the response to biochar addition under drought stress may be affected by some factors such as soil and biochar type and plant species. Compost also contains a considerable quantity of humic substances, which are natural organic polyelectrolytes [65]. Among the various functional actions of humic substances, their ability to enhance plant growth and production and nutrient uptake under drought stress conditions has been previously demonstrated [65].

Plants create many antioxidant defense systems including non-enzymatic (e.g., proline, etc.) and enzymatic antioxidants (e.g., SOD, CAT, POD, etc.) to avoid reactive oxygen species damage. Our study showed that proline was enhanced by the addition of compost or biochar and inorganic fertilizer (Table 6 and Table 7). Proline plays an important role in the osmotic adjustment of cells, and plants accumulate proline under stress conditions [34]. The accumulation of proline is likely to decrease the damage caused by reactive oxygen species and thus improve plant tolerance to drought by removing toxins from an overproduction of reactive oxygen species. The increase in proline resulted in the improvement of the antioxidant system in maize to counteract the degradation caused by drought stress [66]. Previous studies reported that biochar addition increased the proline content of plants [67]. Additionally, Bokobana et al. [68] noted that compost application resulted in the accumulation of proline and an increase in enzymatic activity in plants subjected to water deficit. The elevated rates of proline accumulation observed in plants grown with compost provide evidence that organic matter can enhance plants’ tolerance to water deficit [69].

Our study also revealed that the addition of compost or biochar as well as inorganic fertilizer activated POD, CAT, and SOD enzymes in stressed maize plants, which likely reduced oxidative damage (Table 6 and Table 7). CAT is the main scavenger of reactive oxygen species in leaves, playing an important role in eliminating H₂O₂. CAT is exclusively found in glyoxysomes and peroxisomes and may inhibit hydroxyl radical formation, which is accountable for the lipid peroxidation of cell membranes and many effects on plant growth [70]. In the current study, treatment with compost or biochar as well as inorganic fertilizer activated CAT in drought-stressed plants, thus preventing oxidative damage in plant tissues. Moreover, SOD converts the superoxide (O₂^•−) into H₂O₂ and water [71]. The activity of SOD was increased in maize leaves under drought stress and further increased by compost or biochar as well as inorganic fertilizer application, proving the defensive job of SOD for biological systems.

5. Conclusions

In conclusion, this two-year field trial investigated the effectiveness of nutrient management practices, including co-addition of inorganic NPK fertilizers, organic amendments (compost and biochar), and co-addition of inorganic fertilizers and organic amendments, to enhance crop production and NUE of water-stressed maize plants grown in alkaline soil. The study showed that increasing drought levels significantly decreased maize yield and nutrient uptake but increased catalase, peroxidase, superoxide dismutase, and proline contents. The addition of organic amendments with inorganic fertilizers was found to be more effective in enhancing the above attributes than inorganic fertilizers alone, and the treatment of compost or biochar plus 238-31-138 kg N-P-K ha⁻¹ was the best treatment. The agronomic efficiency of applied fertilizer N significantly decreased with increasing drought levels, but the addition of biochar with inorganic fertilizers significantly enhanced it. The results suggest that the co-addition of inorganic fertilizers and organic amendments improves the yield quantity and quality of water-stressed maize plants grown in alkaline soils by enhancing their defensive system and NAE.