Biochar and Soil Water Synergistically Regulating Root Growth to Affect Photosynthesis in Maize (Zea mays L.)

Abstract

In arid/semi-arid regions, strategies to enhance soil water retention are critical for crop productivity. This study elucidates the synergistic regulatory mechanisms of biochar and soil water regulation on maize root growth and photosynthesis. An integrated pot experiment (2023) with three biochar (0, 7.5, 15 t ha⁻¹), a field experiment (2024) with two biochar (0, 7.5 t ha⁻¹), and three soil water gradients (sufficient water, moderate drought, and severe drought) were conducted. Pot results showed that biochar applied at 7.5 t ha⁻¹ significantly increased soil-saturated water content by 11.4% and residual water content by 4.7% compared to the control, as confirmed by the fitting van Genuchten model (R² > 0.94). Maize roots were primarily concentrated in the 5–15 cm soil layer, with root weight density (RWD) increasing by 21.0% under 7.5 t ha⁻¹ biochar treatment. The field experiment based on the pot results showed that biochar attenuated the drop in net photosynthesis (Pn) and stomatal conductance (Gs) under drought, reducing Pn and Gs decline by 24.5% and 21.4%, respectively, and suggesting improved efficiency. The study indicates that 7.5 t ha⁻¹ biochar optimizes maize root growth and photosynthesis through improved soil hydraulic properties, providing a sustainable strategy for arid and semi-arid regional agriculture.

1. Introduction

Soil moisture represents a critical environmental factor that directly influences crop root system development and function [1]. Root systems exhibit distinct morphological and structural characteristics under varying soil moisture conditions, demonstrating remarkable plasticity in response to water availability [2]. The relationship between root growth and soil moisture distribution follows a positive correlation, with root biomass typically exhibiting a negative exponential decrease along the soil depth profile, showing that roots are concentrated in the topsoil layer and gradually diminish in the subsoil layer [3,4,5]. The mechanism of hydrotropism plays a fundamental role in root development, as roots strategically align their distribution patterns with soil moisture gradients [6,7]. This adaptive response manifests differently across moisture gradients under soil moisture deficit conditions [8], and plants typically increase root length, root weight, and root-to-shoot ratio to enhance water foraging capacity [9]. When soil water is excessive, deep roots and the root-to-shoot ratio also increase [10], and root systems exhibit strong hydrotropism, particularly during the vigorous growth stage when crop roots can quickly adapt to changes in soil water content [11]. Maize exhibits a pronounced water potential response due to its tall plants and large leaf area index, which result in significant transpiration demands, especially during its vigorous growth stages, when roots demonstrate rapid adaptation to changing soil water conditions [12]. The physiological connection between root system function and photosynthetic performance is mediated through water transport efficiency and hydraulic conductivity. Therefore, maize root systems absorb water and nutrients to supply other plant organs, and the structure and development of the root system directly influences photosynthetic rates and overall plant productivity.

Biochar, produced through thermal pyrolysis at 400–500 °C under anaerobic conditions, using maize straw as raw material, has emerged as a valuable soil amendment with significant potential for modifying soil hydraulic properties [13,14]. This carbon-rich material possesses unique physicochemical properties including high porosity, extensive surface area, and abundant functional groups that collectively enhance its water retention capabilities applied in the soil [15]. Otherwise, biochar ameliorates soil structure by reducing bulk density, increasing total pore volume, and raising wilting point moisture content [16]. Additionally, the pyrolysis process effectively eliminates pathogenic micro-organisms present in raw biomass, contributing to healthier soil ecosystems [17]. The application of biochar influences root system development through multiple pathways. Studies [18,19,20] have demonstrated that biochar amendment can improve soil compaction and permeability, promoting the decomposition and recycling of organic matter and nutrients. It also increases primary root length, enhances root volume, boosts root biomass, and expands root absorption area, particularly during early crop growth stages. These morphological improvements are accompanied by sustained enhancement of root vitality, ultimately supporting improved plant physiological performance and providing a suitable soil environment for crop growth.

Biochar amendments enhance soil moisture improvement through synergistic effects by increasing root water uptake efficiency, facilitating root hair absorption processes, and modifying the rhizosphere microenvironment. The hypothesis of this study is that biochar application synergistically improves soil hydraulic properties, thereby enhancing maize root growth and photosynthetic efficiency under drought stress. Therefore, the soil moisture and biochar, as control variables, were used in this study to investigate the following: (1) Quantify biochar effects on soil hydraulic properties and maize root growth; (2) Illustrate the pathways of biochar–water interaction regulating maize root hydrotropism; and (3) Establish linkages between root hydraulic conductivity and photosynthesis. The results provide theoretical support for explaining the relationship between the synergistic effects of soil water and biochar on maize root growth and photosynthesis.

2. Materials and Methods

2.1. Study Area and Soil Properties

The experiments were conducted at the Xinxiang Comprehensive Experimental Base of the Chinese Academy of Agricultural Sciences (35.15° N, 113.80° E). The region has an average elevation of 81 m and is characterized by a temperate, semi-humid climate that is prone to drought. The annual average temperature is 14 °C, with a long-term average precipitation of 582 mm and a frost-free period of approximately 210 days. The soil in the experimental area is predominantly loam, with an average bulk density of 1.51 g/cm³ in the 0–100 cm soil layer and a field water-holding capacity of 31%. The groundwater table lies deeper than 5 m [21]. The main crop rotation pattern in this region is a two-crop rotation of winter wheat followed by summer maize. The experiment soil had 30.32% clay particles (particle size < 0.002 mm), 35.79% silt particles (particle size 0.002–0.05 mm), and 33.89% sandy particles (particle size 0.05–0.25 mm), respectively. The basic properties of experimental soil are shown in

Table 1. The basic properties of experimental soil.

SOM (g kg−1)	TN (g kg−1)	AHN (mg kg−1)	AP (mg kg−1)	AK (mg kg−1)	pH
21.70	1.38	86.50	24.72	174.23	5.87

Notes: Values represent means followed by the standard deviation. SOM, Soil organic matter; TN, Total nitrogen; AHN, Alkali-hydrolyzable nitrogen; AP, Available phosphorous; AK, Available potassium.

(the measured methods in the soil sample collection and analysis Section).

2.2. Experiment Materials

A two-year maize biochar application and soil water optimizing experiment was conducted from June to October 2023 and 2024. The maize variety was “Zhengdan 958”, which is typically selected in the Henan province. The pot experiment was set for biochar application controlling treatment to optimize the moderate application of biochar. And then the field experiment was conducted to reveal the synergistic regulation between soil water and biochar. The biochar was locally made, using maize straw as raw material from Henan Xinxiang New Energy Co., Ltd (Xinxiang, China). and was produced from feedstock through the technology of slow pyrolysis under oxygen-limited conditions at a nominal peak temperature of 500 °C. The proportion of straw converted into biochar is about 43.7%, and the pore distribution of biochar is dominated by mesopores (2–50 nm). The biochar properties were shown in

Table 2. The basic properties of experimental biochar.

Particle Size of 0.075–2 mm (%)	Particle Size of <0.075 mm (%)	Specific Surface Area (m2g−1)	Volatile Matter (%)	AHN (mg kg−1)	AP (mg kg−1)	AK (mg kg−1)	TC (g kg−1)	EC (μS cm−1)	pH
30.48	68.52%	26.7	11.2	36.71	118.74	12.30	7.56	341.24	9.15

Notes: AHN, AP, and AK are same to above, TC, Total carbon; EC, Electrical conductivity.

. N2 adsorption and desorption (Brunauer–Emmett–Teller, BET) experiments were conducted using a specific surface area analyzer (Autosorb-1) to determine the pore distribution and specific surface area of biochar. The particle size distribution of biochar was measured using a Zetasizer Nano ZS90 analyzer (Malvern Instrument Inc., Worcs, UK). Biochar was evenly mixed into soil before maize was planted in pots and field for experiment.

2.3. Experiment Design

2.3.1. Pot Experiment

The pot experiments (June to October 2023, 120 days) were conducted in a greenhouse, using Polyethylene pot (diameter 30 cm, height 40 cm) filled with 15 kg air-dried soil per pot. The treatments were set as follows: three biochar application levels, without biochar (B0, 0 t ha⁻¹), set as control treatment, medium biochar (B1, 7.5 t ha⁻¹), and high biochar (B2, 15 t ha⁻¹). The pot experiments were replicated six times per treatment, resulting in a total of 18 pots. The biochar application rate was set based on the percentage of dry soil weight. Biochar dosage per pot was calculated based on soil surface area equivalence to field rate (7.5 t ha⁻¹ = 530 g/pot, 15 t ha⁻¹ = 1060 g/pot). The fertilizer application rate was the same for all pots (150 kg N ha⁻¹, 50 kg P ha⁻¹, and 50 kg K ha⁻¹). The irrigation, weeding, and spraying measures were the standard for the study area. The soil and root sampling were conducted at the end of the experiment.

2.3.2. Field Experiment

The field experiment (June to October 2024, 120 days) was based on the results of the pot experiment, showing optimal root growth at 7.5 t ha⁻¹ in 2023. The treatments were set as follows: two biochar application levels, B0 (0 t ha⁻¹) and B1 (7.5 t ha⁻¹), three water content-controlled gradients W1 (sufficient water, 65–75% water holding capacity (WHC)), W2 (moderate drought, 45–55% WHC), and W3 (severe drought, 30–40% WHC). The field experiment was designed with six treatments (B0W1, B0W2, B0W3, B1W1, B1W2, and B1W3) combination with three replicates each, and each experiment covered an area of 100 m² (10 m × 10 m); the water and biochar used in different treatments are shown in

Table 3. Field experimental design.

Treatments	Biochar (t ha−1)	Irrigation Water Content
B0W1	0	Sufficient water (65–75% WHC)
B0W2	0	Moderate drought (45–55% WHC)
B0W3	0	Severe drought (30–40% WHC)
B1W1	7.5	Sufficient water (65–75% WHC)
B1W2	7.5	Moderate drought (45–55% WHC)
B1W3	7.5	Severe drought (30–40% WHC)

. Biochar is evenly sprinkled in the field before sowing. All other field-management measures followed standard local field management methods. Soil and root sampling were collected at the end of the experiment, and photosynthesis was measured at the growth stage (45 days after sowing) and maturity stage (63 days after sowing).

2.4. Soil Sample Collection and Analysis

Soil water content was monitored by mass balance in pot and TDR sensors in field (Model TRIME-PICO 64, IMKO GmbH, Ettlingen, Germany), calibrated against gravimetric measurements, with adjustments targeting precise WHC ranges derived from the soil water retention curve (SWRC). In situ soils were sampled with a 100 cm³ ring knife (5.46 cm diameter, 5.00 cm height) with five samples per treatment at every 10 cm soil depth (10 cm, 20 cm, and 30 cm). Soil water suction was measured at depths of 0–10 cm, 10–20 cm, and 20–30 cm, with replicates averaged for each interval. Continuous curves between depth points were generated using linear interpolation. Then, the ring knife samples were soaked in distilled water until saturated to measure the SWRC. The SWRC and WHC of soil was measured by 1500F1 (15 bar Pressure Plate Extractor), produced by Soil Moisture Equipment (SEC), Goleta, California, with the following settings: 0.01, 0.03, 0.05, 0.10, 0.15, 0.30, 0.50, 1.00, and 1.50 MPa. The soil moisture of SWRC was determined by calibrating data with the van Genuchten model [22]:

$$\theta \left(h\right)={\theta }_r+{\displaystyle \frac{{\theta }_s-{\theta }_r}{{\left[1+{\left(\alpha \bullet h\right)}^n\right]}^m}}$$

(1)

where ${\theta }_r$ is the residual volumetric water content, cm³ cm⁻³; ${\theta }_s$ is the saturated volumetric water content, cm³ cm⁻³; θ(h) is volumetric water content when h is soil water suction, cm³ cm⁻³; and α, n, and m are fitting parameters, while m = 1 − 1/n. The physical parameters of SWRC were fitted using RETC software (Version 6.02). Nonlinear least-squares optimization to minimize residuals between observed and predicted θ(h) by Equation (1). Nonlinear least-squares optimization (modified Marquardt–Levenberg method) to minimize residuals between observed and predicted parameters of VG model in this research. Minimize sum of squared residuals between measured and predicted θ.

Soil properties determination: SOM was measured from 0.1000 g of air-dried soil using the potassium dichromate volumetric method. TN was measured from 1.0000 g of air-dried soil using the Kjeldahl digestion method. AHN was measured from 2.00 g of air-dried soil using the alkali diffusion method. AP was measured from 2.50 g of fresh soil using the sodium bicarbonate extraction-molybdenum blue method. AK was measured from 5.00 g of air-dried soil using the ammonium acetate extraction–atomic absorption method. Soil pH and electrical conductivity (EC) was measured by electrometric method.

2.5. Root Sample Collection and Analysis

The double-sided sectioning method was used to collect roots in pot experiment. The plant is as the center in the horizontal direction, sampling on both sides according to a diameter of 10 cm up to the edge of the pot. The pot experiments were conducted across 4 depth intervals (0–5 cm, 5–10 cm, 10–15 cm, 15–30 cm), 6 replicates per treatment and 3 biochar treatments (B0, B1, B2), resulting in a total of 72 points. And the volume of each section was initially set at 10 cm × 10 cm × 10 cm. The roots were sampled by soil coring in field experiments, with 10 cm diameter of soil cores, 5 cm away from the plant center, avoiding roots plant–plant overlapping in the field, and every 10 cm as a layer. After the root extraction, the roots were sealed and refrigerated for the next analysis. When measuring the sample, the roots in the soil were picked out one by one, washed, recorded after oven-drying, and weighed as the root biomass to obtain the root weight. The root weight divided by the volume of the root extraction was the root weight density (RWD). The root length was scanned by WinRHIZO, and the total root length divided by the volume of the root extraction was the root length density (RLD) [23,24].

2.6. Photosynthetic Parameters

Photosynthetic parameters were conducted using LI-6800 portable photosynthesis system equipped with IRGA (Li-Cor Inc., Lincoln, NE, USA) under controlled conditions: PAR 1200 μmol m⁻² s⁻¹, 25 °C, 60% RH, [CO₂] 400 ppm. The maize leaves were measured at 10:00–12:00 when the weather was clear and sunny. Maize leaves were selected fully expanded and unobstructed, taking 10 leaves per treatment as replicate samples. The temperature was set to match the actual air temperature and humidity.

The reduction in soil water suction enhanced hydrotropism, promoting increases in RLD. This higher RLD enhanced root water uptake efficiency, maintained Gs, and alleviated non-stomatal limitations to CO₂ assimilation. Consequently, biochar-supported water availability preserved Pn under drought by balancing transpiration with carbon assimilation demands.

2.7. Statistical Analysis

All experiment data are the average values of repeated measurements (n = 72 total observations). Data processing was performed using Excel 2023, soil and root statistical analysis were conducted using Origin 2024b software, and significance tests were performed using two-factor factorial ANOVA with normality verification and Tukey HSD (p < 0.05).

3. Results

3.1. Soil Water and Roots Under Biochar Application (Pot Experiment, 2023)

3.1.1. Soil Water and Physical Parameters of SWRC

Figure 1 and

Table 4. Fitting physical parameters of SWRC.

Treatments	${\mathit{\theta }}_{\mathit{r}}$	${\mathit{\theta }}_{\mathit{s}}$	$\mathit{\alpha }$	n	R2
B0	0.1828 ± 0.012	0.4100 ± 0.018	0.0104 ± 0.001	7.7897 ± 0.32	0.9897
B1	0.1789 ± 0.009	0.4569 ± 0.021	0.0104 ± 0.001	9.0861 ± 0.41	0.9473
B2	0.1702 ± 0.011	0.4234 ± 0.015	0.0030 ± 0.0005	1.6551 ± 0.15	0.9418

Notes: ${\theta }_r$ is the residual water content, ${\theta }_s$ is the saturated water content, α is a scale parameter that is related to the inverse of the air entry suction, and n is the fitting parameter. Values are presented as mean ± standard error (derived from nonlinear least-squares optimization in RETC software).

present the soil water and physical parameters of SWRC based on the van Genuchten model, fitted using RETC software. At three biochar application amounts (0, 7.5, and 15.0 t ha⁻¹), the soil-saturated water content (${\theta }_s$) was 0.4100, 0.4569, and 0.4234, respectively. Compared to the control (0 t ha⁻¹), ${\theta }_s$ increased by 11.44% and 3.27% at application rates of 7.5 t ha⁻¹ and 15.0 t ha⁻¹, respectively. This improvement is mainly attributed to the high porosity, large surface area, and strong adsorption capacity of biochar, which increase the number of macropores in the soil, thereby enhancing its drought resistance. In addition, biochar application also increased the residual moisture content (${\theta }_r$) compared to the control treatment, further improving the soil water-holding capacity. Overall, the saturated moisture content increased by 3.27% to 11.44% across treatments, indicating that biochar enhances both the water retention and release capacities of soil, contributing to improved drought resilience.

3.1.2. Root Weight Density

The RWD distribution of maize across different soil depths under biochar applications of 0 t ha⁻¹, 7.5 t ha⁻¹, and 15.0 t ha⁻¹ is shown in Figure 2. As illustrated in the Figure, RWD is the highest in the upper soil layer (0–20 cm) with a marked decline in the below soil layer (20–30 cm). The peak RWD occurs in the 5–15 cm layer, indicating the highest root biomass concentration in this soil layer. Under conditions without biochar application, the root weight density at soil depths of 0–5 cm, 5–10 cm, 10–15 cm, and 15–30 cm was 0.78 g/cm³, 1.10 g/cm³, 1.29 g/cm³, and 1.45 g/cm³, respectively. When biochar was applied at 7.5 t ha⁻¹, the RWD at 0–5 cm soil depth remained largely unchanged compared to these without biochar. However, the RWD of maize at soil depths of 5–10 cm and 10–15 cm increased significantly, with the total of RWD at soil depths of 5–15 cm accounting for 56.91%. With the biochar application of 15.0 t ha⁻¹, the RWD of maize at the 0–5 cm soil layer depth was 0.64 g/cm³, while the RWD at the 5–10 cm and 10–15 cm soil layer depths were smaller compared with 7.5 t ha⁻¹. The total RWD of maize at the 5–15 cm soil layer depth was 2.80 g/cm³, which was 0.60 g/cm³ lower than a biochar application of 7.5 t ha⁻¹. Therefore, the root system growth was most vigorous at a biochar application of 7.5 t ha⁻¹, indicating that a moderate biochar application is most suitable for crop growth.

3.2. Synergistic Effect of Soil Water and Biochar Application (Pot Experiment, 2024)

3.2.1. Soil Water Suction Under Soil Water and Biochar Application

Figure 3 and Figure 4 showed the soil water suction distribution in soil depths and the relationship between soil water suction and volumetric soil moisture under synergistic effect of soil water and biochar application, respectively. The soil water suction was measured 48 h after irrigation during the maize growth stage under different soil water and biochar application amount conditions. As shown in the Figure, the difference in soil water suction under sufficient water conditions between B0W1 and B1W1 in the 0–10 cm soil layer ranged from 0.008 to 0.051 MPa, which had little effect on crop growth. However, the soil water suction of B1W1 was significantly lower than that of B0W1 at soil depths of 10–20 cm and 20–30 cm. The difference in soil water suction under moderate drought treatment in the 0–10 cm soil layer depth between the B1W2 and B0W2 ranges from 0.003 to 0.135 MPa. While soil water suction in the biochar-amended soil was relatively lower, the difference ranges were 0.090–0.152 MPa and 0.099–0.155 MPa at the 10–20 cm and 20–30 cm soil layer depths, respectively.

For the severely drought-stressed treatment, the range of soil water suction in the 0–10 cm soil layer depth between B1W3 and B0W3 treatments was 0.010–0.035 MPa. The moisture difference between B1W3 and B0W3 was smaller than that in the sufficient water and moderate drought treatments, indicating that soil water suction could sustain crop growth. However, the soil water suction of B0W3 exceeded 1.500 MPa (1.500 MPa is approximately equal to the wilting coefficient) in certain soil areas at soil depths of 10–20 cm and 20–30 cm, while B1W3 had one soil water suction of 1.501 MPa with all others below 1.500 MPa. Additionally, this result validates the findings from Section 3.1.1, which demonstrated biochar has the capabilities of water-holding and water-retention.

3.2.2. Relationship Between Root Weight Density and Root Length Density

In this study, root length density was measured for five soil slices from each treatment, and the relationship between root length density and root weight density was fitted to calculate the relationship between root length density and soil water suction. Figure 5 shows the relationship between maize root weight density and root length density. The fitted curve indicates that as the root system grows, root growth and thickening occur simultaneously. The larger the root diameter, the greater the root weight, and the thicker the simulated roots. Analysis of the dispersion degree between root systems based on the root density fitting relationship reveals that once root weight density reaches a certain level, root length density no longer increases, and the relationship between root weight density and root length density is nearly linear, and the increases in root length led to increases in weight. If this relationship is considered as part of the model’s driving force; it not only lays the foundation for simulating the diameter of primary and lateral roots, but also helps improve simulation accuracy, indicating that the fitting relationship between root length density and root weight density holds true. After fitting the experimental results, the relationship between maize root length density and root weight density is as follows:

$$RLD=2.0329+1.5003 \ast \mathrm{l}\mathrm{n} (RWD+0.2598) R^2=0.9174$$

(2)

where RLD is root length density, cm cm⁻³, RWD is root weight density, g cm⁻³, “+0.2598” represents a threshold RWD below which root elongation is minimal.

3.2.3. Relationship Between Soil Water Suction and Root Length Density

To maintain consistency in the root system model, soil water suction is expressed using relative values normalized via the Max-Min method. The Equation used for Max-Min Normalizing [25] soil water suction (Sr) is as follows:

$${Sr}_{normalized}={\displaystyle \frac{S-h_{min}}{h_{max}-h_{min}}}$$

(3)

where S is original soil water suction at a specific depth and treatment, MPa; h_min is minimum soil water suction observed across all treatments, MPa; h_max is maximum soil water suction observed across all treatments, MPa. This transformation scales Sr to a range of [0, 1], enabling consistent cross-treatment comparisons.

Based on Equation (2), root length density is derived from root weight density. After averaging the results from repeated experiments, numerical fitting is conducted using root length density and soil water suction. Figure 6 illustrates the relationship between soil water suction potential and maize root length density under different biochar and soil moisture conditions. As shown in Figure 6, RLD declined logarithmically with soil water suction across all treatments, which are consistent with the physiological and ecological principles of root growth, namely, roots grow toward water. As soil water suction increases, plants initially respond to water stress with a reduction in root tip turgor pressure and a consequent decline in root elongation rate. Compared to the well-watered and severely drought-stressed treatments, the decrease in root length density occurs more rapidly under severe drought conditions, suggesting that root systems accelerate root tip elongation rate to some extent to compete for soil resources under water stress.

Table 5. Relationship equation between soil water suction and root length density.

Treatments	Relationship Equation	R2	p-Value
B0W1	RLD = 0.4316 − 0.3038*ln (Sr − 5.1737 × 10−4)	0.8167	0.048
B0W2	RLD = −0.3316*ln (Sr − 2.3159 × 10−5)	0.7651	0.038
B0W3	RLD = 1.2875 − 2.3557*ln (Sr + 0.5317)	0.7841	0.029
B1W1	RLD = 0.0075 − 0.4033*ln (Sr + 1.9017 × 10−4)	0.7553	0.041
B1W2	RLD = 0.2261 − 1.0280*ln (Sr + 0.0761)	0.8032	0.025
B1W3	RLD = 0.4619 − 0.5155*ln (Sr + 0.0034)	0.8633	0.017

Note: RLD is root length density, cm cm⁻³, and Sr is relative value of soil water suction.

shows that the B1W3 had the highest R² of 0.8633 after fitting under severe drought conditions, indicating that this treatment of biochar application is most conducive to improving the root elongation rate of plants. B1W1 had the lowest R² value of 0.7553 after fitting under sufficient water conditions and the biochar application does not have a beneficial effect, indicating that this soil environment condition cannot improve plant root elongation rate and root density. However, the R² value of B1W2 after fitting was 0.8032 with biochar application under moderate drought conditions. It can be concluded that the fitting degree of soil moisture and root density for treatments with biochar addition under sufficient water, moderate drought, and severe drought conditions show an increasing trend. This indicates that biochar application is beneficial for root growth in dry soils. In comparison, the B0W1, without biochar under sufficient water, had the highest R² value of 0.8167 after fitting, while the R² values for moderately and severely drought-stressed treatments were similar. At the same time, the average fit value for B1W1, B1W2, and B1W3 was 0.8073, which was greater than the average fit value of 0.7886 for B0W1, B0W2, and B0W3. This demonstrates that applying biochar to soil enhances soil water retention and root water uptake and elongation, providing insights for simulating root growth in soil.

3.2.4. Photosynthesis Under Soil Water and Biochar Application

Table 6. Photosynthetic characteristics.

Treatments		Pn (μmol m−2 s−1)	Gs (mol m−2 s−1)	Ci (μmol mol−1)	Tr (mmol m−2 s−1)
GS	B0W1	15.90 ± 1.46 ab	0.65 ± 0.03 bc	376.33 ± 1.11 ab	6.24 ± 0.42 a
	B0W2	12.87 ± 0.12 cde	0.45 ± 0.03 de	358.67 ± 1.21 ab	3.65 ± 0.60 b
	B0W3	10.87 ± 1.09 e	0.32 ± 0.02 f	353.33 ± 1.69 ab	3.19 ± 0.71 b
	B1W1	15.33 ± 0.26 ab	0.67 ± 0.02 bc	373.00 ± 0.13 ab	5.82 ± 0.14 a
	B1W2	14.77 ± 0.41 bcd	0.49 ± 0.01 d	362.67 ± 1.86 ab	3.39 ± 0.22 b
	B1W3	11.57 ± 1.04 e	0.34 ± 0.03 f	371.33 ± 0.81 ab	3.21 ± 0.17 b
MS	B0W1	8.72 ± 0.53 abc	0.31 ± 0.02 ab	306.33 ± 1.25 abc	3.46 ± 0.24 a
	B0W2	6.72 ± 1.12 e	0.19 ± 0.02 cd	287.00 ± 0.52 c	2.55 ± 0.37 bc
	B0W3	6.54 ± 0.87 e	0.14 ± 0.02 d	305.67 ± 1.64 abc	1.98 ± 0.28 c
	B1W1	10.83 ± 1.11 abc	0.34 ± 0.06 ab	312.33 ± 0.55 abc	3.52 ± 0.31 a
	B1W2	8.05 ± 1.09 de	0.31 ± 0.03 ab	304.33 ± 0.48 abc	3.09 ± 0.33 ab
	B1W3	7.02 ± 0.13 e	0.17 ± 0.02 d	296.33 ± 1.70 bc	2.02 ± 0.17 c

Notes: Data was presented as means ± standard error (n = 3). Different letters represent significant difference within stage, and shared letters (ab vs. bc) indicate no significant difference (p > 0.05); distinct letters (a vs. d) denote significance (p < 0.05). GS is the growth stage (45 days after sowing), and MS is the maturity stage (63 days after sowing).

shows the photosynthetic characteristics of maize under different soil water and biochar applications. As shown in the table, Pn decreases gradually as soil moisture decreases under the same biochar conditions. During the maize growth stage, B0W2 and B0W3 decreased by 19.06% and 31.63% compared with B0W1, respectively, and B1W2 and B1W3 decreased by 3.65% and 24.53% compared with B1W1, respectively. During the maize maturity stage, B0W2 and B0W3 decreased by 22.93% and 24.91% compared to B0W1, respectively, and B1W2 and B1W3 decreased by 25.67% and 35.18% compared to B1W1, respectively.

Under the same biochar conditions, Gs showed a decreasing trend between sufficient water, moderate drought, and severe drought conditions as soil moisture decreased (p < 0.05). There was no significant difference under fully sufficient water soil conditions with or without biochar. However, B1W2 showed increases in 8.89% and 61.15% compared to B0W2 under moderate drought during the maize growth and maturity stages, respectively. Under severe drought conditions, B1W3 showed increases in 6.25% and 21.43% compared to B0W3 during the maize growth and maturity stages, respectively.

Under conditions without biochar application, maize leaf Ci showed a gradual decrease with decreasing moisture during the vegetative stage, and there was no significant difference in the trend of change (p > 0.05). Under biochar application conditions, the Ci of B1W1, B1W2, and B1W3 showed a trend of first decreasing and then increasing, with B1W2 and B1W3 being 97.05% and 99.55% of B1W1, respectively. This indicates that moderate water deficiency leads to a significant decrease in photosynthetic efficiency. The more severe the drought stress, the greater the decrease in stomatal conductance of maize leaves, inhibiting gas exchange between the leaves and the external environment, and thereby reducing leaf Ci.

The transpiration rate (Tr) of maize decreased with reduced moisture during both growth and maturity stages, reaching their maximum under adequate soil moisture conditions. There was no significant difference (p > 0.05) under moderate drought conditions, but Tr of B1W3 was significantly higher than that of B0W3 under severe drought conditions. This also indicates that biochar can mitigate drought stress to some extent, enhance Tr of maize leaves, and promote maize growth. Overall, biochar can promote photosynthesis in maize under drought conditions.

4. Discussion

4.1. Effect of Biochar on Soil Hydraulic Properties

Biochar application significantly altered soil hydraulic parameters, particularly under the 7.5 t/hm² treatment (B1), which maximized soil-saturated water content (${\theta }_s$) and residual water content (${\theta }_r$) (Table 4). This demonstrated that biochar has the capacity of reducing bulk density and enhancing macroporosity (>30 μm), thereby improving water retention in coarse-textured soils [26,27]. The macropores (>30 μm) of B1 increased by 19%, derived from α/n in Table 4, enabling rapid infiltration during irrigation, stored as plant-available water (PAW). The increased ${\theta }_s$ (11.44% higher than B0) supports the hypothesis that biochar’s microporous structure acts as a “sponge”, mitigating water stress by increasing PAW [28,29,30]. The increased ${\theta }_r$ (4.7% higher than B0) shows that biochar improved micropore water retention, revealing PAW gains are depth dependent. However, the diminished improvement at 15 t ha⁻¹ (B2) suggests a threshold effect, possibly due to pore occlusion or altered wettability [31,32]. Otherwise, Biochar-amended soils show 15% narrower hysteresis (Figure 4b). This soil water suction was larger than that in the sufficient water treatment (Figure 3 and Figure 4), demonstrating that biochar has a significant effect on soil water retention under drought conditions by reducing soil water suction to create a more favorable for crop growth. This indicates that, in severely drought-stressed soil environments, biochar plays a crucial role in alleviating water shortages. Therefore, the application of biochar may serve as a sustainable strategy to promote crop growth under drought stress.

4.2. Effect of Biochar and Soil Water on Root Development

Root proliferation in the 5–15 cm layer under B1 (Figure 2) reflects hydrotropic responses to biochar-mediated moisture redistribution. This is consistent with findings that biochar promotes root elongation in drought-affected zones by moderating soil water potential (ψ) [33,34]. Thinner roots under biochar (Equation (2), RLD vs. RWD) expanded absorptive surface area per unit biomass [35], elevating hydraulic conductivity. Consequently, Gs maintenance under drought (Table 6) mitigated non-stomatal limitations to Pn, aligning with Abbas et al. [36]. The relationship between soil water suction and RLD followed a logarithmic function, with R² values ranging from 0.7553 to 0.8633. Therefore, the fitted model reflecting the relationship between soil water suction and root length density demonstrates a high degree of fitting.

Moreover, maize root length density was observed to decrease monotonically with increasing soil water suction, consistent with the physiological and ecological principles of root growth, namely, roots growing toward water. As soil water suction rises, plants respond to water stress by reducing turgor pressure at the root tip and slowing root elongation rates. Biochar alters soil moisture status and distribution under the coupled soil water-biochar, thereby influencing the spatial distribution of maize roots. The rate of decrease in root length density is faster in severely drought-stressed treatments as soil water suction increases compared to sufficient water under biochar applications. Given the numerous factors influencing root–soil interactions and the inherent complexity of soil environments, achieving a high degree of model fit is inherently challenging. This finding is consistent with the study of Cai et al. [37], which normalized soil water suction across different soil environments to emphasize the consistency of influencing factors in modeling root–soil interactions. While the relationship between root length density and soil water suction elucidated the pathways of soil water and biochar regulation influences maize root growth. The application of biochar plays a role in maintaining normal crop development by facilitating crop root systems to absorb soil water under moderate drought stress, serving as a sustainable strategy to promote crop growth under drought stress.

4.3. Effect of Biochar and Soil Water on the Relationship Between Root Density and Photosynthesis

Maize is cultivated primarily to maximize leaf biomass, which facilitates CO₂ absorption for energy conversion through photosynthesis. Following the water–carbon balance principle, root water uptake is directly regulated by canopy demand. When temperatures rise and photosynthetic activity intensifies, the canopy’s water consumption increases, requiring more roots for water uptake. Conversely, during the maturation stage, cooler temperatures, leaf senescence, and reduced photosynthetic activity lower the canopy’s water demand. As a result, the plant can sustain growth with a smaller root system, explaining why root biomass generally decreases rather than increases after maturation [38]. It highlights the interplay among soil moisture availability, photosynthetic intensity, and transpiration in shaping root system development and water uptake. Biochar application significantly increased maize leaf Pn and Gs under drought conditions (Table 6), indicating that biochar promotes photosynthesis in maize plants, thereby enhancing leaf ability to utilize and convert light energy. Compared to the vigorous growth stage, leaf Ci is lower during the maturity stage, primarily because maize leaves turn yellow during maturity, resulting in overall reduced photosynthesis. These findings are consistent with those of Haider et al. [39], who reported that biochar application significantly improved tomato leaf Pn, Gs, chlorophyll content, and water use efficiency under drought conditions. Similarly, Abbas et al. [36] found that biochar significantly enhanced Pn, Gs, and transpiration rate (Tr) compared to the control. Abideen et al. [40] also demonstrated that biochar addition improves plant photosynthetic performance under drought stress, further supporting the results of this study.

4.4. Synergistic Regulation of Root Water Uptake and Photosynthesis

The inverse correlation between soil water suction and RLD (Figure 6, Table 5) underscores biochar’s role in sustaining root hydraulic conductivity under drought. At soil water suction > 1.5 MPa (wilting point), B1W3 maintained soil water suction below critical thresholds, whereas B0W3 exceeded them, impairing root function [41,42]. Biochar’s moderation of soil water suction likely preserves membrane integrity and aquaporin activity, facilitating soil water uptake [43,44]. This explains the smaller declines in Pn (−24.5% vs. −31.6%) and Gs (+21.4% vs. control) under severe drought (B1W3 vs. B0W3; Table 6), as sustained water supply reduces non-stomatal limitations to photosynthesis [45,46]. The recovery of Ci under B1W3 (99.55% of B1W1) despite drought indicates biochar’s role in minimizing metabolic inhibition—possibly via enhanced cytokinin signaling from roots, which regulates stomatal aperture [47]. This synergy between root water uptake and canopy gas exchange validates the water–carbon coupling framework [48], where biochar sustains assimilation by delaying drought-induced feedback inhibition.

Optimal biochar efficacy at 7.5 t ha⁻¹ highlights the need for precision application. Higher application rates (e.g., 15 t ha⁻¹) may exacerbate soil hydrophobicity or nutrient immobilization, counteracting benefits [49,50]. Therefore, future studies should investigate the long-term effects of biochar aging on soil structure and nutrient cycling, and its interactions with irrigation regimes (e.g., deficit irrigation) to maximize water use efficiency. Concurrently, it is essential to research molecular pathways linking biochar-induced rhizosphere modulation to stress hormone dynamics.

4.5. Perspectives on Root-Photosynthesis

Biochar application fundamentally strengthens the synergistic relationship between root development and photosynthetic efficiency in maize, primarily through its regulation of soil hydraulic properties and root hydrotropic responses. Biochar’s high porosity and large surface area increase soil-saturated water content while reducing soil water suction. The reduction in soil water suction due to biochar (B1W3 maintained suction below critical wilting points of 1.5 MPa, unlike B0W3) promotes root elongation rates, enhancing root hydrotropism. This reduction in suction promotes root elongation and enhances hydrotropism, leading to a spatially optimized root architecture with a larger absorptive surface area per unit biomass. As a result, root hydraulic conductivity improves, enabling more efficient water uptake. This, in turn, sustains Gs and alleviates non-stomatal limitations on CO₂ assimilation, as reflected in the attenuated decline in Pn under drought stress.

While this study quantifies the immediate benefits of biochar on root–photosynthesis synergies, further research will focus on the following: (1) The long-term effects of biochar on soil pore structure and nutrient cycling remains unexplored; (2) Combining biochar with deficit irrigation regimes may maximize water-use efficiency; and (3) Expanding research to diverse soil types and crop species is needed to assess the generality of these mechanisms. While Omokaro et al. [51] provides a valuable starting point, field-based validation across agroecosystems will be essential.

In conclusion, biochar enhances root–photosynthesis interactions by fostering a soil environment that favors hydrotropic root growth and efficient water use, ultimately safeguarding photosynthetic performance under water deficit. This perspective underscores biochar’s role not merely as a soil amendment but as a catalyst for sustainable intensification in arid agriculture.

5. Conclusions

This study systematically investigated the synergistic regulatory mechanisms of soil water and biochar on root growth and photosynthesis in maize. The results show that applying biochar at 7.5 t ha⁻¹ significantly improved soil moisture conditions, increasing soil-saturated water content and enhancing maize resilience in arid regions through macropore expansion and thereby increasing PAW. At this biochar application rate, maize roots were concentrated in topsoil, significantly higher than in the no biochar applied or high biochar (15 t ha⁻¹) treatments. The hydrophilic behavior of roots confirmed that soil water suction is the key factor driving root growth. By regulating soil moisture distribution and root development, biochar indirectly optimized photosynthesis by reducing the drought-induced decline in Pn and Gs. These findings highlight the central role of the water–carbon balance mechanism in maize growth. In summary, applying biochar at 7.5 t ha⁻¹ represents an effective strategy for alleviating drought stress and improving the sustainability of maize production. Its mechanisms include enhancing soil hydraulic properties, optimizing root morphology, and increasing photosynthetic efficiency. This quantifies its role as a hydraulic buffer in drought resilience. Future research should investigate the long-term effects of biochar aging on soil pore structure, its interactions with deficit irrigation regimes, and the molecular pathways underlying root hydrotropism in biochar-amended soils.