The Role of Synthetic Root Exudates in Modulating Soil Hydraulic Properties and Strengths Under Temperature Variations

Abstract

Root exudates play a crucial role in shaping rhizosphere soil structure, water dynamics, and adaptation to environmental stress. This study investigated the effects of environmental temperature (5 °C, 15 °C, and 25 °C) on water retention and soil strength in rhizosphere versus non-rhizosphere soils, simulated by adding glucose or deionized water to soil samples. Over a 10-day drying period, changes in soil water content, evaporation rate, water repellency, penetration resistance, and unconfined compressive strength were measured. The results showed that simulated root exudates significantly enhanced water retention at 15 °C (by 21.5%), but this effect diminished at 25 °C (to 8.3%) and was negative at 5 °C (by −8.9%). Additionally, root exudates improved soil mechanical stability, with the effect being more pronounced at higher temperatures. These changes were attributed to increased organic carbon decomposition and a higher proportion of micropores (<100 μm). These findings highlight the temperature-dependent role of root exudates in regulating soil properties, with implications for agricultural management and ecosystem resilience under climate change.

1. Introduction

The rhizosphere represents a critical interface between plant roots and soil, playing an essential role in plant water and nutrient uptake, regulation of soil microbial communities, and the maintenance of soil physical properties [1,2]. Through their roots, plants secrete a diverse range of compounds, including sugars, organic acids, and amino acids. These root exudates serve not only as carbon sources for soil microorganisms but also significantly alter the physical, chemical, and biological properties of the soil. Root exudates are involved in the formation and stabilization of soil aggregates, which enhances the soil water retention capacity and compressive strength, thereby improving its overall hydraulic and mechanical properties [3].

Temperature is a fundamental environmental factor that regulates both the release of root exudates and their interactions with the soil matrix. Fluctuations in temperature affect not only the quantity and composition of root exudates but also the activity of rhizosphere microorganisms and the dynamics of soil water conduction and evaporation. At lower temperatures, microbial metabolic activities are often suppressed, which may attenuate the effects of root exudates on soil properties. In contrast, under more favorable temperature conditions, root exudates can enhance microbial activity and significantly contribute to the improvement of soil structure [4,5]. However, the exact mechanisms by which root exudates influence soil hydraulic and mechanical properties under varying temperature regimes remain insufficiently explored. A deeper understanding of these processes is crucial to fully comprehending how temperature modulates rhizosphere soil functions and to informing practical agricultural strategies.

Root exudates are not only an important source of organic carbon in the rhizosphere but also a key factor influencing soil aggregation, water retention, and stability. Through their impact on the formation and stabilization of soil aggregates, root exudates help improve soil hydraulic conductivity and moisture retention [6]. These improvements are particularly noticeable under conditions of high temperature or drought stress [7]. However, the complex composition of natural root exudates and the difficulty in obtaining them have led researchers to simulate root exudate effects using synthetic compounds such as glucose and polygalacturonic acid. This approach allows for controlled experimental conditions and facilitates the simplification of variables [8]. Among these synthetic compounds, sugars—representing a significant portion of natural root exudates—have proven particularly effective in simulating microbial activity, enhancing soil hydraulic properties, and stabilizing soil structure [9]. This methodology thus provides a valuable tool for studying the intricate interactions that occur in the rhizosphere and furthering our understanding of its ecological dynamics.

Temperature exerts a significant nonlinear impact on soil water retention and mechanical properties, typically by altering evaporation rates, regulating microbial activity, and stabilizing soil structure. At lower temperatures, the slower evaporation rate contributes to greater water retention and enhanced soil structural integrity [10]. On the other hand, higher temperatures accelerate evaporation, causing the soil to dry and become more friable, which leads to a decrease in soil stability and compressive strength. The interaction between temperature and root exudates, however, is complex and not fully understood. For instance, while higher temperatures increase the release of root exudates, their ability to enhance moisture retention is often constrained by the elevated evaporation rates [11]. Despite a growing body of literature on the individual impacts of temperature and root exudates on soil properties, few studies have examined their combined effects on soil hydraulic and mechanical properties, particularly under drying conditions [12]. The influence of simulated root exudates on soil moisture retention, hydrophobicity, and mechanical properties during the drying process remains an underexplored area.

This study aims to address this gap by investigating the effects of simulated root exudates on soil hydraulic and mechanical properties under different temperature gradients during the drying process. The findings will not only improve our understanding of these complex interactions but also provide empirical data to inform the development of temperature-responsive soil management strategies in agriculture, particularly under changing climatic conditions.

2. Materials and Methods

2.1. Soil and Synthetic Root Exudate

The soil samples used in this study were collected from the 0–20 cm soil layer of bare land at the Science and Technology Park of Inner Mongolia Agricultural University. The basic properties of the samples are presented in

Table 1. Basic physicochemical properties of the tested soil.

Particle Size Distribution (%)			Texture	Soil Organic Matter (g kg−1)	Electrical Conductivity (μS cm−1)	pH
Sand	Silt	Clay
57.92	40.54	1.54	Sandy Loam	14.42 ± 1.61	170 ± 6.96	8.6 ± 0.06

. Stones, crop residues, and root remnants were manually removed. The samples were air-dried and passed through a 2 mm sieve for subsequent use. Soil particle size distribution was measured using a laser particle size analyzer (Anton Paar, PSA1190L-D, Anton Paar GmbH., Graz, Austria). Soil texture classification followed the international soil texture classification standards. Soil organic carbon (SOC) content was determined using the potassium dichromate oxidation method with external heating. Soil electrical conductivity and pH were determined using a multiparameter analyzer (DZS-706F, Inesa Scientific Instrument Co., Ltd., Shanghai, China) after extracting the samples with a 2.5:1 water-to-soil ratio.

Glucose, one of the main components of root exudates, was used to simulate root exudates in this study. A glucose solution was prepared with a carbon concentration of 4.166% by dissolving it in deionized water. The synthetic root exudate was formulated by adding 6 mL of the solution per 100 g of dry soil, simulating rhizosphere soil (SRE) [8]. An equal amount of deionized water was added to the control samples to simulate non-rhizosphere soil (NRE).

2.2. Soil Core Preparation

The simulated rhizosphere and non-rhizosphere soils were thoroughly mixed and equilibrated in a dark environment at 4 °C for 48 h. After equilibration, the samples were evenly packed into PVC tubes with an inner diameter of 5 cm and a height of 5 cm for measuring hydraulic properties, and PVC tubes with an inner diameter of 2.5 cm and a height of 5 cm for measuring mechanical properties. The packing bulk density was maintained at 1.4 g cm⁻³, with three replicates for each treatment. Once fully saturated, the samples were placed in a dark, constant temperature and humidity incubator for drying experiments, set at three temperature gradients: 5 °C (T1), 15 °C (T2), and 25 °C (T3).

2.3. Measurement of Soil Hydraulic Properties

Soil water holding capacity: After saturation, 18 soil samples (both SRE and NRE) were subjected to the three temperature gradients to assess their water-holding characteristics, including volumetric water content, evaporation rate, and cumulative evaporation. The samples were weighed at 2 h intervals during continuous drying until their water content reached equilibrium. Subsequently, the samples were oven-dried at 105 °C to a constant mass, and volumetric water content, evaporation rate, and cumulative evaporation were then calculated for all the samples.

Soil water repellency: Upon completion of the drying process, water repellency was assessed using the water drop penetration time (WDPT) method. Five drops of distilled water (0.05 mL per drop) were applied to the sample surface using a micropipette, and the infiltration time of each drop was recorded with a stopwatch. The arithmetic mean of these five measurements was taken as the WDPT for each sample. Classification of soil water repellency was then performed according to the criteria proposed by Dekker [13], as detailed in

Table 2. Water repellency classification using the water drop penetration time method.

Degree of Water Repellency	Wettable	Slightly Water Repellent	Strongly Water Repellent	Severely Water Repellent	Extremely Water Repellent
Infiltration Time (s)	<5	5–60	60–600	600–3600	>3600

.

2.4. Measurement of Soil Mechanical Properties

After saturation, the mechanical properties (specifically penetration resistance and unconfined compression) of both the SRE and NRE samples were measured under the 3 temperature gradients throughout the drying process. Measurements were conducted on the 1st, 2nd, 3rd, 5th, 7th, and 10th days of drying, totaling 216 soil samples.

Penetrometer resistance (PR) and unconfined compression (UC) were determined using an electronic universal testing machine (UTM6102 model, Shenzhen Sansizongheng Technology Co., Ltd., Shenzhen, China). For the PR measurements, a 2 mm diameter penetrometer probe with a 60° tip (Figure 1A) was used, and the probe was moved at a speed of 20 mm min⁻¹. The average resistance within the 0–40 mm penetration depth, recorded automatically, was used as the PR value. For the UC measurements, the fixture was replaced as shown in Figure 1B, and compression was applied at 5 mm min⁻¹ until failure occurred. The yield point, representing the UC, was defined as the point where the sample deformed or fractured during the test [14,15]. After testing, the samples were oven-dried at 105 °C to a constant weight, and the corresponding moisture content was calculated.

2.5. Measurement of Soil Organic Carbon and Statistical Analysis

Soil organic carbon (SOC) content was determined using the potassium dichromate oxidation method with external heating. Data calculation and organization were performed using Excel 2019, statistical analysis was conducted with Genstat (Version 24, 1.0.1041), and figures were generated using Origin Pro 2021.

3. Results

3.1. Soil Water Retention Capacity

The changes in soil water content during the drying process for simulated rhizosphere soil (SRE) and non-rhizosphere soil (NRE) under different temperatures (5 °C, 15 °C, and 25 °C) are shown in Figure 2. Overall, soil water content decreased significantly across all the treatments as drying time progressed, with notable differences observed between the SRE and NRE at different temperatures. At 5 °C, the water content of the NRE was consistently higher than that of the SRE throughout the drying process, with differences ranging from 0.009 to 0.049 cm³ cm⁻³. This disparity was greatest between 10 and 20 h, at 0.49 cm³ cm⁻³, and then gradually decreased over time. Over the entire drying period, the SRE experienced a total water loss of 0.230 cm³ cm⁻³, compared to 0.211 cm³ cm⁻³ for the NRE. At 15 °C, the trend was reversed, with the water content of the NRE being significantly lower than that of the SRE throughout the drying process. The difference ranged from 0.003 to 0.074 cm³ cm⁻³, peaking at 28 h before gradually decreasing. After 28 h, the rate of water loss for both treatments stabilized, although the NRE continued to lose water more quickly than the SRE. At 25 °C, the water content of the SRE remained slightly higher than that of the NRE for most of the drying process; however, the difference between the two treatments gradually diminished over time.

The changes in moisture content can be reflected in the evaporation rate, as shown in Figure 3. Under the 5 °C condition, the evaporation rate of the SRE (averaging around 0.22 mm h⁻¹) was significantly higher than that of the NRE (0.06 mm h⁻¹) during the first 0–12 h. After this period, the evaporation rate of the NRE slightly exceeded that of the SRE, but the difference was not significant, with both rates stabilizing around 0.03–0.04 mm h⁻¹. At 15 °C, for the first 28 h, the NRE maintained a relatively high and stable evaporation rate (approximately 0.44 mm h⁻¹), while the evaporation rate of the SRE decreased from 0.62 mm h⁻¹ to 0.23 mm h⁻¹. Following this period, the evaporation rates of both treatments became nearly equal, with a small difference of approximately 0.04–0.05 mm h⁻¹. At 25 °C, the SRE’s evaporation rate was consistently higher than the NRE’s during the first 36 h, with a difference ranging from 0.01 to 0.29 mm h⁻¹. However, between 36 and 81 h, the evaporation rate of the NRE surpassed that of the SRE by 0.02–0.09 mm h⁻¹. After 81 h, both evaporation rates decreased and stabilized, with the SRE’s rate remaining slightly higher (around 0.01 mm h⁻¹) than the NRE’s.

The cumulative evaporation results are presented in Figure 4. Throughout the drying process, the cumulative evaporation of all the samples exhibited a continuous increase. At 5 °C, the SRE consistently had higher cumulative evaporation than the NRE, with differences ranging from 0.51 to 2.04 mm. However, due to the constraints imposed by the low temperature, the cumulative evaporation did not stabilize by the end of the experiment. Specifically, at the conclusion of the drying process, the SRE accumulated 11.73 mm of evaporation, while the NRE accumulated 10.77 mm. At 15 °C, the cumulative evaporation of the NRE was consistently higher than that of the SRE. By the end of the experiment, the cumulative evaporation for the NRE was 20.80 mm, while the SRE was 20.13 mm. Under the 25 °C condition, the SRE showed higher cumulative evaporation than the NRE during the first 57 h. However, in the later stages of drying, the NRE’s cumulative evaporation exceeded that of the SRE. At the end of the experiment, the SRE had accumulated 22.53 mm of evaporation, while the NRE reached 23.51 mm. When the cumulative evaporation under the NRE treatment reached 10.77, 10.55, and 10.36 mm (approximately 10 mm) at 5 °C, 15 °C, and 25 °C, respectively, the corresponding SRE treatment demonstrated water retention capacity improvements of −8.9%, 21.5%, and 8.3% compared to the NRE conditions.

At the end of the drying experiment, the hydrophobicity and corresponding moisture content of both the SRE and NRE samples were also measured, as shown in

Table 3. Water repellency of simulated rhizosphere soil samples (SREs) and non-rhizosphere soil samples (NREs) under different temperature gradients using the water drop penetration time method.

Root Exudate Treatment	Temperature Gradient	Water Drop Time (s)
SRE	5 °C	2.18 ± 0.42 Ac
	15 °C	11.72 ± 1.27 Ab
	25 °C	34.38 ± 0.80 Aa
NRE	5 °C	0.71 ± 0.29 Ba
	15 °C	0.04 ± 0.01 Bb
	25 °C	0.06 ± 0.01 Bb

Note: Lowercase letters represent significant differences between the SRE and NRE samples under different temperature gradients (p < 0.05), whereas uppercase letters denote significant differences between the SRE and NRE samples at the same temperature (p < 0.05).

. With increasing temperature, both the SRE and NRE displayed a decrease in moisture content and an increase in hydrophobicity. At identical temperatures, there was no significant difference in moisture content between the two, but the hydrophobicity of the SRE was significantly higher than that of the NRE. This difference became more pronounced at higher temperatures, with the disparities at 5 °C, 15 °C, and 25 °C being 1.47 s, 11.68 s, and 34.32 s, respectively.

3.2. Soil Mechanical Properties

The penetration resistance (PR) and unconfined compression (UC) of the SRE and NRE as a function of water content are shown in Figure 5 and Figure 6, respectively. Similar to the water retention results discussed earlier, the SRE generally demonstrates superior water retention capacity compared to the NRE, except at 5 °C. Higher temperatures accelerate the drying process, resulting in a faster increase in PR and UC.

Throughout the drying process, the SRE consistently exhibits higher PR and UC than the NRE at both 15 °C and 25 °C. The disparity between the SRE and NRE becomes more pronounced at higher temperatures and lower water contents. For instance, at 15 °C on day 3 of drying, the SRE had a water content of 0.22 cm³ cm⁻³, while the NRE was at 0.16 cm³ cm⁻³. The corresponding PR values were 571.22 kPa for the SRE and 378.52 kPa for the NRE. By day 10, both samples had reached a water content of approximately 0.08 cm³ cm⁻³, with the PR values rising to 1727.07 kPa for the SRE and 1409.48 kPa for the NRE. At 25 °C, on day 1, both the SRE and NRE had a water content of 0.25 cm³ cm⁻³, with PR values of 715.95 kPa and 322.68 kPa, respectively. By day 10, the water content had dropped to around 0.01 cm³ cm⁻³, and the PR values increased to 2503.72 kPa for the SRE and 1846.85 kPa for the NRE. Similarly, when the water content is approximately 0.25 cm³ cm⁻³, the UC of the SRE exceeds the NRE by 4.33 kN m⁻² at 15 °C and 0.70 kN m⁻² at 25 °C. At a water content of approximately 0.13 cm³ cm⁻³, the UC of the SRE is 17.11 kN m⁻² and 24.15 kN m⁻² greater than the NRE at 15 °C and 25 °C, respectively. In contrast, under 5 °C conditions, the PR of the SRE and NRE shows minimal differences at similar water contents, while the UC consistently displays a trend where the NRE exceeds the SRE throughout the drying process.

Both properties exhibit an exponential growth trend during the drying process, with coefficients of determination (R²) ranging from 0.889 to 0.998 (Figure 7). At lower temperatures, the fitting accuracy for the PR improves, while the UC shows better fitting at higher temperatures. In general, lower water content and temperature lead to a faster increase in PR during the drying process. When water content drops below 0.2 cm³ cm⁻³, the PR decreases sharply across all treatments. At 25 °C, the PR change rate is nearly identical for both the SRE and NRE treatments. However, as the temperature decreases, the discrepancy between the two treatments becomes more pronounced, with the SRE demonstrating a greater capacity to mitigate the rapid rise in PR. The rate of change in the UN follows an inverse trend to that of the PR; as the temperature decreases, the rate at which the UN increases with decreasing water content slows. Under 5 °C, the rate of UN change is slower for the SRE than for the NRE, while at both 15 °C and 25 °C, the UN increases faster under the SRE as water content decreases.

3.3. Soil Organic Carbon Changes

We quantified changes in soil organic carbon (SOC) content in SRE and NRE samples before and after the drying process at different temperatures (Figure 8). Prior to drying, the SOC levels were comparable between the samples, and, as expected, higher temperatures resulted in greater SOC losses. After drying, the SOC content in the SRE and NRE decreased by 0.076%/0.044% at 5 °C, 0.193%/0.106% at 15 °C, and 0.361%/0.206% at 25 °C. The SOC loss rates for the SRE were 4.42%, 11.28%, and 22.21% at 5 °C, 15 °C, and 25 °C, respectively, while those for the NRE were 2.57%, 6.27%, and 12.19%. The SOC loss was consistently more pronounced in the SRE than in the NRE, with the discrepancy becoming increasingly significant at elevated temperatures.

4. Discussion

Our findings demonstrate that both synthetic root exudates (SREs) and temperature play significant roles in influencing soil water retention. SREs notably enhance water retention, with the most pronounced effect observed at 15 °C, suggesting that this temperature is optimal for maximizing the benefits of exudates on soil moisture. However, as the temperature rises to 25 °C, the water retention effect diminishes, while at 5 °C, a reduction in moisture retention is observed. Upon rewetting the dried samples, hydrophobicity was found to increase with temperature, with the SRE samples demonstrating significantly stronger hydrophobicity than the NRE samples.

Root exudates and similar substances, such as extracellular polymeric substances (EPSs) produced by microorganisms, significantly affect soil water retention and evaporation. Generally, the addition of root exudates or EPSs increases soil water retention and reduces evaporation rates [16,17]. This effect is largely attributed to the higher water retention capacity of these substances, which alter soil structure, affect pore connectivity, and modify the surface tension and viscosity of soil water [18,19]. Our findings are consistent with these studies. Except under the 5 °C, the addition of simulated root exudates (glucose) significantly reduced cumulative evaporation. In addition to improving soil water retention, numerous studies have shown that re-wetting dry soils can lead to hydrophobicity, evidenced by an increased contact angle of water droplets or delayed wetting. Benard et al. [20] suggested that during soil drying, root exudates move toward the voids between soil particles, and once their concentration reaches a critical threshold, they occlude enough pores to induce hydrophobicity. Carminati et al. [21] also found that rhizosphere soils retained more water than non-rhizosphere soils during drying, with the latter rapidly re-wetting, while rhizosphere soils remained dry until their water content eventually surpassed that of non-rhizosphere soils. Similarly, Zarebanadkouki et al. [22] showed that chia mucilage reduced evaporation in coarse soils, with stronger effects at higher concentrations. In line with these findings, our results show that both elevated temperatures (low water content) and the presence of simulated root exudates increased the hydrophobicity of the samples.

The addition of simulated root exudates can enhance microbial activity and modify soil physical properties by altering soil structure and pore distribution. Root exudates, particularly low-molecular-weight carbon compounds, provide a primary source of carbon and energy for heterotrophic microorganisms in the rhizosphere [23]. Soil microbial activity is typically stimulated by carbon released from root exudates, whether in the form of exuded carbon from the roots themselves [24] or synthetic root exudates [17]. Our findings on soil organic carbon (SOC) support this view: higher temperatures accelerate SOC decomposition, and the presence of simulated root exudates further increases this decomposition, indicating that microbial activity is heightened under varying temperature conditions. At lower temperatures (5 °C), the microbial activity is limited, and root exudates have a minimal effect on improving soil structure, resulting in relatively poor moisture retention by simulated root exudates (SREs) [25]. At 15 °C, microbial activity increases, and root exudates become more effective in enhancing soil moisture retention, improving soil aggregation and pore structure. This leads to better moisture retention by SREs. However, at 25 °C, the moisture retention capacity of SRE decreases compared to 15 °C, likely due to excessive microbial activity and accelerated decomposition of organic carbon at higher temperatures. The elevated temperature diminishes the influence of root exudates, as heightened microbial activity and accelerated carbon decomposition reduce their effectiveness.

The observed differences in evaporation rates further corroborate these changes. Higher temperatures accelerated water evaporation, but the presence of simulated root exudates partially mitigated the evaporation rate [26], particularly during the initial hours of evaporation at 15 °C. This effect is likely due to the formation of a protective film on the soil surface induced by root exudates, which reduced evaporation losses. However, as the temperature rose and the drying process progressed, this effect diminished, and the differences in evaporation rates became less pronounced. Moreover, changes in soil hydrophobicity are closely linked to temperature. At higher temperatures, the decomposition of organic matter on the soil surface likely produces more hydrophobic compounds [27], thus increasing soil hydrophobicity. Simulated root exudates (SREs) exhibit stronger hydrophobicity than non-root exudates (NREs), which explains why SREs display different moisture retention and evaporation rates under varying temperature conditions. Root exudates enhance soil hydrophobicity, and their influence on soil water dynamics becomes more complex as temperatures rise.

The mechanical properties analysis in this study was performed under consistent moisture conditions, evaluating the penetration resistance (PR) and unconfined compressive strength (UC) of simulated root exudates (SREs) and non-root exudates (NREs) at various moisture contents. The observed differences in the mechanical properties of SREs and NREs under identical moisture conditions highlight the substantial influence of simulated root exudates on soil structure. At elevated temperatures, the SREs exhibited higher PR and UC, suggesting that root exudates enhance soil aggregation and moisture retention [28]. At lower moisture contents, the simulated rhizosphere soils displayed greater resistance, indicating that root exudates improve the soil’s mechanical strength and stability. The primary intrinsic changes underlying these differences are improvements in soil structure. Root exudates likely promote the aggregation of soil particles, enhancing both soil compactness and porosity [29]. These structural changes increase the soil’s mechanical strength, which enables SRE to exhibit greater resistance during the drying process. In contrast, the soil structure of NRE remains unaffected by root exudates, leading to lower PR and UC values.

The rate of organic carbon decomposition is crucial for understanding changes in soil moisture, mechanical properties, and microbial activity. Previous studies have shown that organic carbon decomposes more rapidly under high-temperature conditions [30], a result corroborated by our findings. At elevated temperatures, SRE exhibited a more rapid loss of organic carbon, particularly at 25 °C, where the SOC loss was significantly higher in the SREs than in NRE. This is likely because root exudates enhance microbial activity, thereby promoting organic carbon decomposition [31]. The rate of organic carbon decomposition is closely related to changes in soil moisture and mechanical properties. As soil moisture decreases, SOC decomposition accelerates, further driving changes in the soil’s mechanical properties. Notably, at 25 °C, the SRE exhibited a more rapid decomposition of organic carbon, which aligns with increased microbial activity and alterations in soil structure at higher temperatures. The accelerated loss of SOC enhanced microbial activity both on the surface and within the soil, thereby exacerbating moisture loss and altering the soil’s mechanical strength [32].

To further elucidate the impact of different temperatures on soil structure in both simulated rhizosphere and non-rhizosphere samples, we randomly selected one sample from each treatment for CT scanning and pore distribution analysis. The scans were conducted using an industrial micro-CT with a resolution of 10 µm, and pore distribution was calculated using the “Thickness” plugin in ImageJ (ver. 1.51, Rasband, 1997–2011). [8]. Due to budget limitations, only one replicate per treatment was conducted; nevertheless, these results provide valuable data to support our findings. The grayscale images of the CT-scanned samples and the corresponding pore distribution maps are shown in Figure 9. Visual inspection of the grayscale images reveals that the NRE contains more large pores than the SRE. Across all temperature conditions, the total porosity of SRE and NRE was comparable. At 5 °C, SRE exhibited a greater proportion of both smaller pores (100–180 µm) and larger pores (400 µm) compared to NRE. At 15 °C, the NRE displayed a higher number of larger pores (>140 µm) and fewer smaller pores (<60 µm) than the SRE. At 25 °C, no distinct pattern in pore distribution was observed between the two treatments. Overall, the addition of simulated root exudates promoted structural densification of the samples, while elevated drying temperatures resulted in an increased proportion of small-scale pores, primarily within the <100 μm pore size range. This finding is consistent with the evaporation behavior of each treatment, where the primary water loss occurred through larger pores initially, followed by smaller pores.

This study mainly used glucose as an artificial root exudate for lab experiments. However, there are several limitations regarding experimental conditions, methods, and interpretation of results, which need further improvement:

(1) In natural soils, root exudates are highly complex and unevenly distributed, and the roles of microbes living in the root zone are not yet fully understood. Therefore, using glucose alone and performing experiments only in lab conditions differ greatly from real field situations. In future studies, we should try to conduct experiments using natural root-zone soils and include tests under various temperature and environmental conditions.

(2) Regarding the effects of soil texture, our initial findings showed that glucose (simulated root exudate) had different effects on the hydraulic properties of sandy loam soil. These differences may be due to glucose itself, microbial activities, or simply because of soil texture differences. Therefore, future studies should carefully analyze soils of different textures to better understand how root exudates and microbes interact with different soil types and identify the mechanisms involved.

5. Conclusions

This study utilizes glucose as a model for root exudates to explore their impact on soil water retention and mechanical properties during the drying process under various temperature conditions. The potential mechanisms behind these effects were also analyzed. Our results show that the simulated root exudates enhance soil moisture retention, mechanical stability, and compressive strength. However, these effects are strongly temperature-dependent, with the most pronounced improvements observed at 15 °C. At both higher and lower temperatures, the benefits are significantly reduced. Future studies should further investigate the mechanisms of different root exudates in various soil types, particularly under extreme temperature conditions. Moreover, exploring the interaction between root exudates and soil microbial communities, and their subsequent effects on soil mechanical properties, represents an important avenue for future research.