Evaluation of Plant-Available Water in Degraded Alfisol Using Biomass Copyrolyzed with Plastic

Abstract

The exponential increase in global plastic production, reaching over 380 million tons in recent years, has exacerbated environmental problems, particularly in agriculture. Agricultural residues, such as hazel (Corylus avellana L.) pruning and plastic wastes, are underutilized resources that can be transformed via pyrolysis into biochar. This study focuses on copyrolyzed biochar produced from hazel biomass and polyethylene and aims to evaluate its effect on the water retention properties of degraded Alfisol. Van Genuchten’s hydrological model was used to analyze parameters such as rapid drainage pores, plant-available water pores, and air capacity (AC) under varying particle sizes (small and large) and application rates (1% and 5% w/w). The results revealed that fine particles at higher doses (5%), especially in P-5%-large and P-5%-small, considerably improved plant-available water retention, particularly within micropores and mesopores. Microstructural modifications induced during pyrolysis enhanced the water retention capabilities of biochar copyrolyzed with plastic. However, its effects on AC and pore connectivity warrant further investigation to assess long-term soil functionality. By integrating waste valorization with improved agricultural practices, this study underscores the potential of biochar copyrolyzed with plastic as an amendment for degraded soil. However, the long-term stability of this amendment requires further study.

1. Introduction

Recently, the global production of plastics has increased exponentially, exceeding 380 million tons in 2019, and is projected to triple by 2060 [1,2]. Owing to the lack of designated postdisposal uses, it is estimated that more than half of the plastics will end up in landfills, and less than one-fifth will be recycled [3]. This has led to the widespread practice of open burning as a disposal method, which contributes to air pollution and global warming [4]. For instance, when lignocellulosic biomass is combusted, it releases large amounts of greenhouse gases (GHGs), including carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O)—the latter two being considerably more potent in terms of the global warming potential than CO₂ [5]. The incorporation of plastics into the combustion processes further exacerbates emissions by elevating the release of atmospheric pollutants, including black carbon—a byproduct of incomplete combustion that absorbs solar radiation, thereby intensifying climate change and degrading air quality—and volatile organic compounds, such as benzene, toluene, and styrene, which are highly toxic and potentially carcinogenic to humans [6].

A considerable fraction of waste is created by agricultural residues, including greenhouse covers, packaging materials, and mulch films. These waste streams have notably increased with the intensity of modern agricultural practices [7,8]. The persistent accumulation of such waste in agricultural ecosystems has created a critical environmental challenge [9]. Moreover, agriculture generates large quantities of lignocellulosic biomass residues, such as hazel (Corylus avellana L.) pruning residues. In fact, pruning residues, particularly hazelnut shells, account for 47–61% of the total yield. Given an average yield of 3 t ha⁻¹, pruning residues can amount to over 1.5 t ha⁻¹, which can be further increased by pruning management techniques and the dynamic framework system. A typical framework entails planting double the number of trees during the establishment phase and subsequently removing them after the initial years, which generates abundant biomass residues [10]. This residual biomass presents a high potential for use as an organic amendment in degraded soils using valorization technologies, such as pyrolysis [11].

Biochar, a carbonaceous material produced from biomass pyrolysis, is characterized by a porous structure and high chemical stability. Biochar can enhance soil properties such as bulk density, water retention, and pore distribution [12]. However, the fraction of water truly available to plants, defined as the difference between the field capacity (FC) and permanent wilting point (PWP), is contingent upon the interaction between biochar and soil properties [13]. Recent studies have demonstrated that degraded Alfisols, which are characterized by their low organic matter content and limited water retention, can potentially benefit from the application of biochar because of its positive effects on aggregate stability and pore connectivity [14].

The copyrolysis of biomass and plastics is a promising strategy for waste management and energy production; however, it is not without controversy. These arise mainly from the variability in product yields, the differing behavior of various plastic types, and the environmental implications of the process. Synergistic effects such as improved product quality and yield can occur but are highly dependent on the type of biomass and plastic, mixing ratio, and operational conditions, making optimization a complex challenge [15,16]. Yields can vary considerably depending on the combination of feedstocks; for instance, the copyrolysis of rapeseed stalks with different plastics resulted in markedly different hydrocarbon profiles and biochar proportions, with some blends promoting certain compounds while others inhibited them [15]. In addition, plastics may lower the energy required for pyrolysis, as observed in bamboo/polystyrene blends that show strong positive synergistic effects [16]. Plastics also influence pyrolysis mechanisms: polypropylene (PP) tends to increase the formation of aromatic compounds, whereas polystyrene (PS) suppresses oil production and promotes biochar formation [17]. These differences are related to fundamental chemical mechanisms, such as the transfer of active hydrogen radicals, which can favor or inhibit specific reaction pathways [18]. Environmental concerns further complicate process assessment. While copyrolysis offers clear advantages in waste reduction and energy recovery, it may also generate hazardous emissions depending on reactor design, heating rate, and thermal conditions [18,19].

During the thermal degradation of polyethylene, one of the most widely used polyolefins in agricultural applications, a complex mixture of volatile organic compounds (VOCs) is released, including polycyclic aromatic hydrocarbons (PAHs), oxygenated aromatics, and heavy aliphatic hydrocarbons [17]. These emissions stem from the incomplete breakdown of long polymer chains and the interaction between plastic radicals and lignocellulosic components, which can produce recalcitrant and potentially toxic species. Recent studies have shown that copyrolysis with polyethylene significantly increases the aromatic content of bio-oil, including multi-ring PAHs known for their environmental persistence and toxicity to soil organisms [17]. Furthermore, polyethylene’s high hydrogen-donating capacity favors the formation of condensed hydrocarbons, which may pose additional environmental challenges if not properly captured or managed [18].

In this context, the copyrolysis of biomass with agricultural plastics emerges as a promising strategy not only for waste valorization but also for improving the functional performance of carbonaceous materials. The resulting products, such as plastichar, have demonstrated higher water retention capacity, structural stability, and the potential to enhance pore function in degraded soils [20]. However, these benefits must be interpreted within a sustainability framework that considers the full lifecycle of the residue. Polyethylene depolymerization during the process can lead to volatile emissions, which, if inadequately controlled, may pose environmental risks. While the Kon–Tiki flame cap kiln does not incorporate active gas emission control systems, its open, convective flame design promotes more complete combustion and thus may reduce the release of certain pollutants [21]. Nevertheless, advancing towards reactor systems with stricter emission control remains essential to ensure the environmental soundness of copyrolysis processes. Thereby positioning copyrolysis as a viable tool within circular agriculture strategies.

Within this framework, the present study contributes to the understanding of how variables such as particle size and application rate influence soil water availability, highlighting the agronomic potential of carbon-based materials in a circular economy context. However, there is still limited evidence on how the physical properties of plastichar, particularly its particle size and application rate, affect water availability in degraded soils such as Alfisols. The model presented in [22] is used to analyze key hydrological parameters, including pores of fast-draining (PFD), plant-available water pores (PAW), and air capacity (AC). By combining an experimental approach with advanced hydrological modeling, this research seeks to contribute to developing sustainable agriculture management practices by providing scientific evidence on the potential of plastichar to enhance water retention and pore distribution in degraded soils.

2. Materials and Methods

2.1. Obtaining Raw Materials

Agricultural residues were used as raw materials to produce carbonaceous amendments. The residues included European hazel (C. avellana) pruning, which was collected from a farm in the Mediterranean area of Central–South Chile. The biomass was dried at 105 °C and ground into particles smaller than 2 cm. Additionally, polyethylene (150–200 µm) was obtained from the local market. The plastic was thermally treated at 120 °C for 24 h prior to pyrolysis, following the protocol established by [18]. The thermal treatment partially melted the plastic, which increased its density and enabled better homogenization with the hazel biomass. The mixture in proportions of 90% biomass and 10% plastic was homogenized manually in a container to ensure proper interaction during pyrolysis.

2.2. Materials Preparation

A Nabertherm muffle furnace (LE/11/R7, Bremen, Germany) was used for pyrolysis. The furnace allows the closure of oxygen intake to create an inert atmosphere that facilitates slow pyrolysis. The heating ramp was set at 60 °C min⁻¹ and reached a final temperature of 600 °C, which was maintained for 90 min; the heating rate is consistent with the parameters established for slow pyrolysis, as detailed by [23], who indicated that rates ranging from 0.1 °C to 60 °C min⁻¹ effectively promote lignocellulosic biomass carbonization while preserving the structural integrity. Additionally, [24] observed that even higher heating rates (104–105 °C min⁻¹) maintained the properties of biochar, effectively demonstrating the robustness of the material during carbonization.

Following pyrolysis, distilled water was manually applied using a sprayer onto the biochar after removing it from the muffle furnace. This method was employed to reduce the temperature gradually, and the resulting biochars were ground using a blade mill before sieving to ensure homogeneity. The material was sieved into two particle size fractions: small particles (0–25 µm) and large particles (25–2000 µm).

2.3. Classification of the Different Amendments

The method described by [12] was used in this study for classification. The elemental compositions of carbon, nitrogen, and sulfur were determined, along with measurements of volatile matter, pH, electrical conductivity, moisture, calorific value, and ash content. The procedures used are detailed below:

Ash: Determined by following the UNE-EN ISO 18122 protocol. A 1 g sample was placed in a dish previously treated at 500 °C for 60 min, and its initial and final weights were recorded. The sample and the dish were then placed in a furnace at 550 °C for 60 min.

Elemental composition C-H-N: Conducted using an elemental analyzer (Leco^® CHN 628, Japan Corp, Tokyo, Japan). The calibration curve was established using a certified coal reference material.

S and Cl content: Conducted according to the UNE-EN ISO 16967 and UNE-EN ISO 16994 methodologies. Digestion was carried out in a sealed container using 0.8 mL H₂O₂ per 1 g of sample, and the sulfate and chloride concentrations were subsequently determined via ion chromatography.

Oxygen: Oxygen values were calculated by difference using Formula (1).

Oxygen (% p/p) = 100 − % ash − % C − % N − % H − % S − % Cl,

(1)

Volatile matter: Conducted following the UNE-EN ISO 18123 protocol. A 0.1 g sample was placed in a furnace at 900 °C for 7 min. Subsequently, the percentage of volatile matter was calculated based on the difference in weight.

Electrical conductivity: The sieved and moistened sample was shaken with water in a 1:5 ratio. The suspension was centrifuged, and the electrical conductivity of the extract was measured [25].

pH: Conducted by following the TMECC04.111 protocol from test methods for compost, using the electrometric method [25].

2.4. Experiment

This study was conducted at the Laboratory of Carbonaceous Materials and Agriculture, University of Concepción, Chillán, Chile. The experimental trial was conducted between March and November 2023. An Alfisol soil was previously collected from the Cauquenes series from Quirihue (36°16′58″ S 72°32′27″ W) in the Ñuble region, Chile. The collected soil sample was dark brown, formed in situ, and was characterized by good porosity in noneroded horizons [26]. The soil was air-dried at 30 °C and sieved to less than 2 mm. The physical characterization determined a textural analysis of 36% silt, 34.3% sand, and 29.7% clay; 1.35 g cm⁻³ bulk density (cylinder method), 2.31 g cm⁻³ real density (pycnometer method), 18.43% FC expressed in dry soil base (dsb), and 10.35% PWP (% dsb) via water extraction using Richards’ pressure chamber [27].

The treatments were applied to the previously collected Alfisol, and the effects of the interactions among biochar type, application rate, and particle size on the different soil parameters studied were examined. A total of nine treatments, each with three replicates, were analyzed, resulting in twenty-seven experimental units. A detailed description of these treatments is provided in

Table 1. Treatments incorporated into the soil.

Acronym	Description	Doses (% p/p)	Size Particle (µm)
Control	Alfisol	100	-
B-1%-small	100% hazel biomass	1	<25
B-1%-large	100% hazel biomass	1	25–2000
B-5%-small	100% hazel biomass	5	<25
B-5%-large	100% hazel biomass	5	25–2000
P-1%-small	90% biomass + 10% polyethylene	1	<25
P-1%-large	90% biomass + 10% polyethylene	1	25–2000
P-5%-small	90% biomass + 10% polyethylene	5	<25
P-5%-large	90% biomass + 10% polyethylene	5	25–2000

.

The soil with the treatments applied was preincubated to ensure water impregnation. For this purpose, the samples were moistened with distilled water until saturation and then kept in open bags to allow the excess water to evaporate at 22 °C for 2 weeks. In Richards’ chamber plates (domestic manufacture), the mixture of samples incubated with the treatments was wet for 48 h until saturation was reached again. Subsequently, different pressures of 1/10, 1/3, 3, 5, and 15 atm were applied to obtain a water retention curve. The water released was recorded and expressed as a percentage over the dry base of the soil (% dsb) [27]. Data from the water retention curve were applied to the van Genuchten model [22], presented in Formula (3), according to the RETention Curve (RETC) V6.02 program. This allowed us to obtain the AC, PFD, and PAW.

2.5. Modeling of Water Retention Curves

The RETC v.6.02 program was used. The Pedotransfer functions included in the Rosetta V1.1 program, integrated into RETC, were used. These data integrations achieved the highest level of representativeness for the H5 soil (Formula (2)), and the data used in the physical characterization of the soil were applied.

H5 = ((% Textural sand, silt, clay) + Da + θ60 + θ1500)

(2)

The results of the analysis, once processed in the RETC Program, provided the following predicted values: θr (cm³ cm⁻³), θs, α (cm⁻¹), n, and Ks (cm day⁻¹). When these values were input into the RETC v.6.02 program, they were adjusted based on the parameters proposed by [22] and presented in Formula (3).

θ(ψm) = θr + [(θs − θr)/1 + [(α∙× ψm)n]m]; m = 1 − 1/n; n > 1

(3)

where θ denotes the volumetric water content, Ψm denotes the water tension (hPa), and θs and θr denote the volumetric water contents at saturation (0 hPa) and residual (>15,000 hPa), respectively. These data were converted to volumetric content by multiplying them by the bulk density (1.35 g cm⁻³) before being entered into the model.

Subsequently, a smoothed curve was obtained and mathematically fitted to the model in Formula (3), representing the variance in soil properties and parameters corresponding to the H5 level.

The adjusted parameters were used to simulate new water tension points and enable the analysis of pore distribution in three categories:

Air capacity (AC) was calculated as the difference between total porosity and water content at −10 kPa. This parameter is used to evaluate the soil’s aeration potential and its capacity to retain air after gravitational drainage has occurred. The total soil porosity was estimated by assuming it to be homogeneous throughout the soil when integrating the treatments [13,28].

AC (% vol) = PT − θ₆₀

(4)

where PT (% vol) = total porosity calculated as in Formula (5), and θ₆₀ indicates the volumetric water content at a pressure of 60 hPa. PT is calculated considering the actual density (rd) and the apparent density (bd):

PT (% vol) = [(rd − bd) rd−1] 100

(5)

Preferential flow domain (PFD) was estimated as the water released between saturation and −10 kPa (i.e., pores draining rapidly under low tension). This indicator represents the fraction of macropores contributing to rapid water flow and gravitational drainage. Related to low tensions (<1/3 atm) [29], and it is defined as follows:

PFD = θ₀ hPa − θ₆₀ hPa

(6)

where θ₀ denotes the volumetric water content at a pressure of 0 hPa, and θ₆₀ denotes the volumetric water content at a pressure of 60 hPa.

Pore-available water (PAW) is defined as the pore volumetric water content retained between field capacity (−33 kPa) and permanent wilting point (−1500 kPa). This parameter is strongly influenced by the presence of meso- and micropores [28] and was calculated as follows:

PAW = θ₆₀ hPa − θ_15,000 hPa

(7)

where θ₆₀ denotes the volumetric water content at a pressure defined as FC, and θ_15,000 denotes the volumetric water content at the PWP for plants [13].

2.6. Data Analysis and Experimental Design

A completely randomized design with a split-plot arrangement was used. The results were analyzed using the Infostat^® 2020 program, with normality tests conducted using Shapiro–Wilks and homogeneity tests using Levene’s test, in addition to an analysis of the variance in which the modeled physical parameters were analyzed using Fisher’s LSD test with three levels of significance corresponding to α = 0.05, α = 0.01, and α = 0.001.

Additionally, the measured and predicted parameters at all pressure points were correlated. Finally, the modeled curves were presented in graphs using the GraphPad Prism^® program.

3. Results

3.1. Properties of the Different Carbonaceous Amendments

To classify the different amendments, the criteria proposal by [12] were used according to their % C (>50%) and O/C ratio (<0.4). According to the classification, the 100% hazel biomass used in this study corresponds to biochar, given its C content of 78.19% and an O/C ratio of 0.16, and its pH and electrical conductivity (EC) levels are within the ranges accepted by the standard. However, the PA amendment is not classified as biochar because the O/C ratio is 0.51 (

Table 2. Characteristics of the amendments.

Parameter	Unit	1 B	1 P
Ashes	%	5.28	6.52
pH	-	9.86	9.8
EC	dS m−1	0.98	0.88
2 VM	%	12.8	13.1
N	%	<2	<2
C	%	78.19	61.08
H	%	2.64	2.49
S	%	<2	<2
O	%	12.62	31.13

¹ Biochar (B), Plastichar (P); ² Volatile material (VM).

). This is directly attributed to the amount of aromatic groups produced during the interaction between the biomass composition and polyethylene, leaving more residual oxygen in its structure as oxygenated or aliphatic groups that have high reactivity but very low resistance to degradation, potentially classifying it as unstable [30,31].

Table 2 indicates that the amendments produced based on hazel pruning have a C content of 61–78%, which is attributed to the high lignin content that produces stabilized carbon following pyrolysis. An increase in the amount of O and a decrease in the C content are evident upon incorporating polyethylene; this is attributed to the addition of 10% p/p of plastic in the pyrolysis process, the lower proportion of organic biomass in the amendments coprolyzed with plastic, and to the fact that the plastic depolymerizing in O and H allows increasing the O/C ratio of the plastichar [18].

3.2. Water Released at Different Pressure Points

Table 3. Water released at different pressure points to form the retention curve.

Treatments	1/10 Atm	1/3 Atm	3 Atm	5 Atm	15 Atm	WA 1
Control	21.49 ± 0.52 e	18.43 ± 0.83 d	14.14 ± 0.39 e	12.03 ± 0.31 d	10.35 ± 0.42 e	8.08 ± 0.58 b
B-1%-large	25.03 ± 0.82 cd	21.87 ± 0.27 bc	16.88 ± 0.43 d	15.00 ± 0.11 c	13.37 ± 0.61 c	8.50 ± 0.40 ab
B-1%-small	26.23 ± 0.54 b	22.51 ± 0.29 b	16.57 ± 0.44 d	15.00 ± 0.29 c	13.88 ± 0.29 c	8.63 ± 0.38 ab
B-5%-large	26.37 ± 0.6 b	22.72 ± 0.36 b	18.00 ± 0.3 c	15.15 ± 0.26 c	13.89 ± 0.44 c	8.83 ± 0.59 ab
B-5%-small	27.77 ± 0.26 a	24.56 ± 0.41 a	18.98 ± 0.81 b	16.00 ± 0.36 b	14.89 ± 0.13 b	9.66 ± 0.44 a
P-1%-large	25.62 ± 0.55 bc	21.97 ± 0.72 b	17.00 ± 0.77 d	15.00 ± 0.73 c	13.36 ± 0.36 c	8.61 ± 1.02 ab
P-1%-small	24.24 ± 0.57 d	21.01 ± 0.53 c	17.37 ± 0.32 cd	15.01 ± 0.12 c	12.12 ± 0.49 d	8.89 ± 0.96 ab
P-5%-large	28.56 ± 0.65 a	25.14 ± 0.27 a	20.12 ± 0.79 a	17.55 ± 0.68 a	15.33 ± 1.09 ab	9.80 ± 1.13 a
P-5%-small	28.39 ± 0.82 a	25.25 ± 0.79 a	20.00 ± 0.52 a	18.00 ± 0.63 a	15.64 ± 0.8 a	9.61 ± 1.29 a
CV (%)	2.37	2.39	3.19	2.9	4.24	9.23
p value	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05

Lowercase letters indicate significant difference between treatments. Significances calculated using the LSD Fisher test between treatments at each of the different pressure points. ¹ WA = Water availability (FC – PWP).

presents the different pressure points. The parameters θ60 + 15,000 were extracted, which represent the saturation point at FC (0.33 atm) and the residual point that represents the PWP (15 atm).

While there exists a tendency to decrease the water that is extracted in relation to the control in different ranges of significance, if we evaluate each pressure point separately as an increase in usable moisture, it is crucial to ensure that this increase occurs over a range rather than at a single point. Therefore, these data were modeled according to Formula (3), which includes saturation and residual range.

3.3. Data Insertion in Program Rosetta v 1.1

The prediction of soil hydraulic properties was performed using Rosetta v1.1, a Pedotransfer function model developed by the USDA Salinity Laboratory. The model was run under hierarchical level 5, which includes input data such as soil texture (sand, silt, and clay), bulk density, and volumetric water content at field capacity and wilting point. These values were obtained experimentally for each treatment. The predicted parameters included residual water content (θr), saturated water content (θs), α (inverse of the air entry suction), and n (pore-size distribution index) for the van Genuchten model. The outputs were used as initial estimates for further fitting in RETC.

The properties of the physical soil analysis used in the modeling of parameters by Rosetta are shown in the soil description. The textural percentages of silt, sand, and clay, along with the soil bulk density, were estimated as constants for modeling with the integration of the rest of the treatments. However, within the range of usable water measured by treatments, all the parameters estimated by Rosetta v.1.1 remained consistent (

Table 4. Predicted values estimated by Rosetta v.1.1.

RETC Calculated	Θr (cm3 cm−3)	θs (cm3 cm−3)	α (cm−1)	N	Ks (cm d−1)
Predicted Alfisol	0.0483	0.457	0.0336	4.85	65.5

).

3.4. Water Retention Curves by RETc v 6.02

The distinct curves adjusted to the van Genuchten M = 1−1/n model were created using the RETC v.6.02 software by entering the data required by the program and adding the five data points from the three repetitions obtained from the five pressure points of the treatments under the Richards chamber. Soil water retention data (θ vs. h) were collected through pressure plates (0, −10, −33, −100, −300, and −1500 hPa) and fitted using the RETC v6.02 program [22]. Assuming a residual water content (θr) equal to the water content at −1500 hPa, and a saturated water content (θs) equal to the value at saturation. The fitting was conducted with default iteration settings (maximum 100 iterations, tolerance 10⁻⁶) to obtain parameters α, n, and the shape of the curve for each treatment. They are presented in Figure 1 and Figure 2.

3.5. Analysis of the Dynamics of Water Flow

The parameters PT, AC, PFD, and PAW were calculated to analyze the dynamics of water flow in the various treatments presented in Figure 1 and Figure 2. The slopes between the various pressures exerted by the Richards chamber were evaluated to assess the dynamics of water flow in soils amended with carbonaceous materials.

In

Table 5. Parameters of the soil analyzed after modeling and with the integration of different treatments.

Treatments	PT	AC	PFD	PAW
	% by volume ± SD
Control	41.55	16.47 ± 1.20	3.91 ± 1.24	10.85 ± 1.39
B-1%-large	41.55	11.89 ± 0.39 **	4.06 ± 0.81	11.68 ± 0.55
B-1%-small	41.55	11.42 ± 0.46 **	5.35 ± 0.29	11.83 ± 0.43
B-5%-large	41.55	10.58 ± 0.29 **	4.52 ± 1.04	12.31 ± 0.78
B-5%-small	41.55	8.40 ± 0.69 **	4.47 ± 1.15	13.48 ± 0.67 *
P-1%-large	41.55	11.63 ± 1.11 **	4.55 ± 1.14	11.90 ± 1.40
P-1%-small	41.55	12.09 ± 0.55 **	2.68 ± 1.03 *	12.61 ± 1.34
P-5%-large	41.55	7.10 ± 0.63 **	3.83 ± 1.53	13.73 ± 1.96 *
P-5%-small	41.55	7.34 ± 0.87 **	4.08 ± 1.83	13.12 ± 1.82 *

PT: total porosity; AC: air capacity; PFD: rapid drainage pores; and PAW: plant-available water pores. Average values ± standard deviation (n = 3). The asterisk indicates significance at p ≤ 0.05 (*) and p ≤ 0.001 (**) compared with the control treatments.

, it is observed that in all treatments, AC decreases substantially compared with the control, where the P-5%-small and P-5%-large treatments show the greatest reductions of 55.4% and 56.9%, respectively, relative to the control soil. This reduction in AC could represent a decrease in soil macroporosity, which could imply a risk of waterlogging by exhibiting values below 10%, thereby affecting root aeration and soil biogeochemical processes. The values of the other treatments remain within the range considered good for Alfisols, classified as having moderate aeration capacity in which plants achieve optimal water uptake with nonexcessive drainage [32].

4. Discussion

4.1. Parameters Evaluated in the Amendments

EC is a crucial parameter that reflects the ability of biochar to influence soil salinity. The materials produced from hazel residues are rich in lignin and cellulose, which results in relatively low ECs of 0.98 dS m⁻¹ (B) and 0.88 dS m⁻¹ (P) because of the greater stability provided by the aromatic compounds in the lignin derived from hazel biomass. Study [33] compared the EC of lignin and cellulose and found that the latter generally exhibited higher EC at pyrolysis temperatures >600 °C, which produces fewer volatile and soluble compounds owing to the stability of the aromatic groups and induces lower interaction with the depolymerization of polyethylene, maintaining the low EC [18,34].

Volatile matter indicates the stability of biochar and its capacity to release organic compounds under heat conditions [35]. Hazel pruning biochar showed a volatile matter content of 12.89%, which increased to 13.19% after the biochar was copyrolyzed with polyethylene. These values indicate high thermal stability, corroborating the findings of [36], who highlighted that the pyrolysis of lignocellulosic materials results in biochars with low volatile matter contents. Those derived from lignin amendments are primarily aromatic and influence the reactivity of biochar, but as the pyrolysis temperature increases, the stability of these compounds on the surface tends to increase, which coincides with the findings [37,38].

The ash content is a critical parameter that reflects the amount of inorganic minerals present. Hazel pruning biochar had an ash content of 5.28%, which is considered low and is explained by the fact that biomass with a higher lignin content tends to generate fewer mineral residues because of the efficiency of combustion [39] and the lower volatilization of inorganic compounds during pyrolysis [40]. In the copyrolyzed treatments, the ash content increased slightly (6.52%); this brief increase following the addition of polyethylene can be explained by the mineral residues of plastic after pyrolysis, as observed by [41].

Hazel biochar obtained from hazel pruning biomass exhibited a high C content, surpassing its copyrolyzed treatment (78.9% and 63.5%, respectively), which is directly attributed to its composition. However, the addition of polyethylene during copyrolysis decreased the C content and consequently increased the O/C ratio and EC, which reflects a greater presence of oxygenated compounds and soluble salts that may have critical implications for its application in agricultural soils. High EC could negatively influence soil salinity; furthermore, volatile matter and ash varied among the different amendments and were higher in the copyrolyzed samples, which suggests lower thermal stability and higher mineral residue [12,18].

4.2. Pore Structure Induced by Plastic in Biochar

Several studies have demonstrated that the copyrolysis of biomass with plastic residues, particularly polyethylene and polypropylene, induces significant modifications in the pore structure of biochar, especially within the micro- and mesopore ranges. During pyrolysis, the thermal depolymerization of plastics generates a large number of volatile compounds that act as pore-forming agents, promoting structural expansion of the carbon matrix and increasing specific surface area [42]. This effect is enhanced at higher temperatures, where the release of gases creates localized pressure that facilitates the collapse of biomass cell walls, resulting in the formation of interconnected pores [43,44].

For instance, ref. [42] reported that a mixture containing 75% polypropylene and 25% bamboo waste produced biochar with a specific surface area of 7.70 m² g⁻¹, highlighting the potential of plastics to induce a more porous structure. Similarly, Ref. [43] observed that the incorporation of 20% plastic residues into biomass led to biochars with greater pore volumes and higher mass yields. These physical changes are closely linked to synergistic effects arising from the interaction between biomass-derived free radicals and hydrogen donors from plastics, which help stabilize the carbon matrix [16,18]. Overall, the increase in total pore volume in biochar produced via copyrolysis is generally associated with a higher number of smaller-diameter pores, primarily within the micro- and mesoporous ranges. This pore distribution enhances both water adsorption and nutrient retention, key properties for improving the hydrological and chemical functions of amended soils [42,44].

4.3. Water Retention Curves in Alfisol

The modeled retention curve exhibits unimodal behavior, characteristic of Alfisols, which indicates that water flow is predominantly regulated by a porous phase—in this case by micro- and mesopores with a lesser contribution from macropores. This porous distribution pattern is generally a consequence of erosion and compaction processes induced by continuous tillage and local edaphoclimatic conditions [45]. Consequently, the soil shows a progressive and uniform decrease in plant-available water as the matric potential increases, a behavior that conforms to the water retention model from study [22]. It describes how water availability in unimodal soils follows a pattern of gradual release among porous fractions; in the soil with 1% amendment, especially in fine particles, the curve tends to smooth at higher tensions, which demonstrates the range of action of meso- and micropores in these amendments. A similar behavior was observed in the soil treated with 5% fine particle; however, amendments sieved as large particles introduce a bimodal behavior in the soil, partially causing a rapid drop in moisture at tensions of 2–2.5 hPa, caused by the incorporation of coarser particles [46], and generating a second phase of effective release dominated by micro- and mesopores at tensions between 3.5 and 4.2 hPa. This differential behavior is crucial as it demonstrates how the physical properties of biochar, particularly the distribution of pore sizes, can influence soil water retention based on the dominant porous phase [47].

The P-5%-small and B-5%-small treatments showed the highest overall water retention capacity, with a predominant effect in the mesopores. The inclusion of plastic in copyrolysis presented a differential impact as the P-5%-small treatment showed a slight advantage in water retention in the micropores, which suggests that copyrolysis with plastics modifies the structure of the biochar, generating a higher proportion of micropores, which increases the water retention capacity in specific spaces [48]. Recent studies have demonstrated that the pyrolysis temperature and the raw material used affect the pore distribution, favoring water retention when biochar is produced at temperatures above 400 °C and with lignocellulosic residues combined with plastics [49]. It has been observed that this type of biochar increases the density and connectivity among micropores, which in turn improves soil moisture stability in moderate to high suction ranges [50]. Moreover, the structural alteration induced by copyrolysis promotes a redistribution of water within the biochar and the soil, which generates a progressive release of the moisture retained in the micropores without abruptly emptying the macropores [32]. This is consistent with previous studies, which suggest that biochar obtained via copyrolysis can generate additional microstructures that act as water reservoirs in the soil, prolonging its availability for plants without substantially modifying drainage in larger pores [49]. The influence of meso- and micropores is verified in Table 3, where an increase in available water is reflected in the B-5%-small, P-5%-small, and P-5%-large treatments.

4.4. Air Capacity, Rapid Drainage Pores, and Plant-Available Water Pores

The reduction in AC is mainly due to the redistribution of porous space: macropores decrease while micro- and mesopores increase after biochar addition. According to [32], AC can drop by up to 25% in the 0–5 cm layer and 17% in the 5–10 cm layer, reflecting higher water retention at the expense of air-filled pores. This trend has been observed in various biochar-amended soils, where the total soil porosity increases by up to 40%, but with a reconfiguration of the porous structure that favors water storage in micropores rather than in macropores [51]. Likewise, the analysis of the water retention curves confirms that the greatest influence of the treatments is found in the meso- and micropore range, which is reflected in a water response at lower tension. However, when a uniform total porosity is assumed, neither the inter- nor intraparticle porosity of the biochar nor its interaction with the soil matrix is considered an aspect that could modify pore connectivity and the dynamics of water redistribution in the soil. Therefore, it is suggested to carry out more in-depth analyses to determine how this porous redistribution affects the macroporosity of the treatments and its impact on water retention and soil aeration, which would explain the controversy expressed by other authors regarding biochar derived from lignocellulosic biomass, such as that from hazel pruning, which generates pores ranging from 1 to 100 µm [41] in a highly stable structure following pyrolysis [52].

In the case of PFD, no considerable differences were observed in the treatments, which indicates that the applied carbonaceous amendments did not affect the pores responsible for the rapid release of water [50]. The exception is the P-1%-small treatment, which shows a considerable reduction in the PFD volume. This effect can be explained by the combined influence of the reduced particle size (<25 μm) and the high microporosity induced by copyrolysis with plastic. This mechanism has been described by the authors of [42], who observed that copyrolysis with polyethylene increases microporosity and decreases the connectivity of large pores, directly affecting fast flow channels, which is consistent with a decrease in gravitational flow in favor of capillary storage. However, in the P-5%-small treatment, the combined effect of copyrolysis and particle size appears to be diluted because of possible structural saturation. Although fine plastichar has the potential to modify preferential flow, the high dose may have led to excessive occupation of the pore space, limiting the effective reconfiguration of the macropore system. This phenomenon suggests the existence of a structural efficiency threshold that depends not only on the quality of the biochar but also on its relative proportion in the soil system [42,43].

Regarding the PAW within the range available for plants, the P-5%-large, B-5%-small, and P-5%-small treatments showed considerable differences with increases of 26.5%, 24.23%, and 21%, respectively, compared with the control. These findings suggest greater water availability in the soil for absorption within the tension range necessary for optimal root uptake (2.5–4.2 hPa) [53]. Although copyrolysis with plastic in large- and small-sized particles represents a substantial increase over the control, it is not statistically different from that of the noncopyrolyzed treatments, which suggests that the doses of amendment applied and the soil pore redistribution play a predominant role than the specific composition of the material [32]. Treatments with a 1% dose showed no significant differences in PAW compared with the control, confirming previous findings [54,55] and the split-plot analysis. These results highlight the importance of 5% doses and biochar composition as key factors in water retention.

In the analyzed curves, the correlations (R2) between data entered into the model and the values predicted by the model were between 0.967 and 0.996, which is quite high. When comparing the plant-available water (calculated in Table 5) prior to and after modeling, we obtain a coefficient value of 0.9897 (Figure 3), which indicates a strong correlation between the measured and predicted parameters and represents good model performance for these amendments.

The copyrolysis of biomass and plastic waste presents both opportunities and challenges in the context of sustainable waste management and soil amendment strategies. One of the main advantages is its ability to valorize heterogeneous and contaminated plastic waste streams, which are often unsuitable for conventional mechanical recycling processes [43,55,56]. However, achieving uniform pyrolysis may require extensive sorting and preprocessing of feedstocks, potentially increasing operational complexity and costs. In terms of material characteristics [16,18,42,43], the inclusion of plastics during pyrolysis has been shown to enhance microporosity and increase specific surface area, likely because of the release of volatiles during thermal degradation. Nonetheless, excessive plastic content may compromise the uniformity of pore structures and reduce overall char quality, highlighting the need for optimized feedstock ratios.

Regarding soil improvement, plastichar exhibits beneficial effects such as improved water retention and modification of pore size distribution, attributed to the formation of synergistic carbon structures. Despite these benefits, the long-term stability of plastichar in field conditions remains uncertain, particularly when the oxygen-to-carbon ratio (O/C) exceeds 0.5, which could limit its persistence and efficacy in soil systems [42,44].

From an environmental standpoint, copyrolysis typically results in lower pollutant emissions than incineration and can contribute to reducing landfill pressure [55,57]. However, the potential formation of volatile and toxic compounds during the process remains a concern, underscoring the need for adequate emission control strategies. At the mechanistic level, radical interactions between biomass and plastics during pyrolysis have been reported to improve carbon matrix stability and promote pore development. These synergistic effects may also contribute to increased crop productivity. Nevertheless, such outcomes are highly dependent on the pyrolysis temperature and the specific biomass-to-plastic ratio used, which necessitates precise process control to achieve consistent and reproducible results [16,18,20].

This comparison allows us to contextualize the novelty of our study and to acknowledge both the benefits (e.g., enhanced water retention and microporosity) and limitations (e.g., stability and environmental concerns) of plastichar-based amendments. In this framework, our research provides new insights by systematically evaluating the effect of copyrolyzed biomass–plastic char on key soil physicochemical properties under controlled conditions, with a specific focus on water retention and pore architecture. Furthermore, our study addresses the gap related to the structural stability of plastichar in soil-like environments by analyzing its composition and oxygen functionalization, providing relevant data to inform its long-term behavior.

5. Conclusions

This study demonstrated that the application of hazel pruning biochar and its copyrolysis with polyethylene substantially influences soil water retention. Its effects are directly related to the dose and particle size. Treatments with fine particles and high doses (5%) showed an improvement in plant-available water, particularly in the micropore and mesopore ranges, thereby optimizing the distribution of water available to plants in degraded soils.

Furthermore, biochar copyrolyzed with plastics presented higher water retention in the micropores owing to the generation of additional porous structures during the depolymerization of polyethylene in the copyrolysis process. However, these results also suggest that their impact on the AC and pore connectivity may require further evaluation to determine their implications for the long-term functionality of the soil.

As a recommendation, future studies should focus on a deeper understanding of the intra- and interparticle organization of biochar and its interaction with the soil, integrating experimental methodologies with advanced modeling to evaluate its effects on soil water dynamics. Additionally, a comprehensive and integrated assessment of the impacts on both the biological and chemical properties of the soil will be essential to fully elucidate the implications of producing amendments through copyrolysis with plastic waste, thereby optimizing their application across different edaphic contexts. This work contributes to the scientific evidence regarding the use of biomass copyrolyzed with plastic as a strategy to integrate the valorization of plastic and biomass wastes; however, its effects on the stability of carbon and soil functionality require extensive studies.