Contrasting Pre- and Post-Pyrolysis Incorporation of Bentonite into Manure Biochar: Impacts on Nutrient Availability, Carbon Stability, and Physicochemical Properties

Abstract

Manure-derived biochar is a promising soil amendment, though its effectiveness is often constrained by limited structural stability and inconsistent nutrient retention. This study evaluated how the pyrolysis method (pre- vs. post-pyrolysis) and rate (5%, 10%, 20%, and 30% w/w) of bentonite incorporation influence the physicochemical properties, nutrient availability, and carbon stability of manure-derived biochar. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses revealed that pre-pyrolysis addition enhanced mineral integration, with silicon and aluminum contents increasing by up to 500% and 600%, respectively, while carbon content decreased by up to 34%. Water holding capacity (WHC) improved by approximately 102% with 5–10% bentonite, and carbon stability more than doubled (≥100% increase) at moderate application rates under pre-pyrolysis treatment. However, nitrate (NO₃⁻) and potassium (K) availability declined by up to 89% and 47%, respectively, in pre-pyrolysis treatments due to strong nutrient immobilization. In contrast, post-pyrolysis bentonite addition increased NO₃⁻ by ~44% and K by ~29%, while phosphorus (P) availability rose by 133% at 30% bentonite. Principal component analysis (PCA) showed a clear distinction between pre- and post-pyrolysis bentonite-treated biochar. Pre-pyrolysis treatments were linked to higher pH, WHC, and carbon stability, while post-pyrolysis treatments were associated with greater nutrient availability (e.g., NO₃⁻, and K levels) and higher EC. These findings underscore the importance of the pyrolysis method, showing that pre-pyrolysis bentonite incorporation strengthens biochar’s structural integrity and long-term carbon sequestration potential, whereas post-pyrolysis addition enhances immediate nutrient availability. This duality enables the development of targeted biochar formulations tailored to specific agronomic needs—whether for sustained soil improvement or rapid fertility enhancement in climate-smart and sustainable land management systems.

1. Introduction

Biochar, a carbon-rich material produced through the pyrolysis of organic biomass under limited oxygen conditions, has emerged as a promising tool for sustainable soil management and climate change mitigation due to its potential to improve soil properties and sequester carbon [1,2,3]. When derived from animal manure, biochar offers additional agronomic and environmental benefits by recycling nutrient-dense organic waste, transforming it into stable, carbon-rich soil amendments with enhanced physical and chemical properties that can improve soil structure, increase nutrient retention, stimulate beneficial microbial activity, and ultimately support sustainable agricultural productivity [4]. This valorization of livestock waste not only diverts it from environmentally harmful disposal routes but also enhances nutrient cycling in agricultural systems. Manure-derived biochar typically contains higher concentrations of essential macro- and micronutrients compared to plant-based biochar, making it particularly attractive for soil fertility improvement [5]. However, the agronomic efficiency of manure-derived biochar is often limited by its relatively low structural stability and variable nutrient retention capacity [6].

To address these limitations, researchers have investigated the integration of mineral additives such as bentonite clay—a naturally occurring aluminosilicate known for its high surface area, cation exchange capacity (CEC), and swelling properties [7,8]. Incorporating bentonite has demonstrated the potential to enhance the physicochemical characteristics of biochar, especially by improving water retention and nutrient sorption capacity [9]. However, a critical knowledge gap persists regarding the influence of bentonite addition—whether incorporated before pyrolysis (pre-pyrolysis) or after pyrolysis (post-pyrolysis)—on the structural and functional attributes of manure-derived biochar. While most existing studies have focused on co-pyrolysis or post-treatment methods, few have directly compared these approaches to determine their relative effects on biochar performance and environmental behavior [10,11,12,13,14]. This is particularly important as the high-temperature pyrolytic environment can induce mineralogical and chemical transformations in bentonite [15], potentially modifying its interaction with organic feedstocks and ultimately influencing the resulting biochar’s effectiveness as a soil amendment.

Additionally, bentonite has been shown to influence the availability of essential macronutrients—particularly nitrogen (N), phosphorus (P), and potassium (K)—a factor of particular importance for manure-derived biochar, which often contains labile and volatile nutrient fractions prone to leaching [16,17]. However, the effects of bentonite incorporation pre- or post-pyrolysis and application rate on nutrient retention and release dynamics in such biochar systems remain inadequately characterized. Beyond nutrient behavior, bentonite may also influence the carbon sequestration potential of biochar by altering the stability of organic carbon (OC) within the matrix. While biochar is widely recognized for its long-term carbon storage capacity, few studies have directly compared how pre- versus post-pyrolysis bentonite treatments impact OC stability [11,18]—a critical knowledge gap in the pursuit of climate-smart soil management.

Moreover, the rate at which bentonite is applied may significantly affect the overall performance of biochar. While moderate additions can improve mineral content, nutrient retention, and water-holding capacity [19], excessive application may result in diminishing returns or even adverse effects—such as reduced porosity and impaired nutrient release. Consequently, determining the optimal combination of the pyrolysis method and bentonite dosage is essential for maximizing the multifunctional benefits of bentonite-enhanced biochar in sustainable soil systems.

Therefore, this study aimed to systematically evaluate the effects of bentonite incorporation before (pre-pyrolysis) and after (post-pyrolysis) thermal treatment at varying rates (5%, 10%, 20%, and 30% w/w) on the structural and chemical characteristics of manure-derived biochar. The specific objectives were to (i) examine how the pyrolysis method and amount of bentonite addition influence the structural and functional properties of biochar; (ii) assess the resulting impacts on water holding capacity, nutrient availability (N, P, K), and OC stability; and (iii) identify the optimal incorporation strategy that enhances biochar performance without triggering diminishing returns. It was hypothesized that (i) pre-pyrolysis bentonite incorporation would yield superior structural integrity, water retention, and carbon stability compared to post-pyrolysis application; (ii) pre-pyrolysis treatment would more effectively enhance nutrient retention, particularly for N, P, and K; (iii) this approach would lead to increased soil organic carbon (SOC) content and greater long-term carbon sequestration potential; and (iv) biochar performance would improve with increasing bentonite rates up to a threshold beyond which benefits plateau or decline.

2. Materials and Methods

2.1. Manure Biochar Preparation

The manure, sourced from a local farmer in Chiang Mai, Thailand, was crushed and ground prior to pyrolysis. The experiment was conducted in February 2025 and included three distinct pyrolysis processes: control, pre-pyrolysis, and post-pyrolysis (Figure 1). For the control treatment, the ground manure was pyrolyzed at 550 °C for 1 h. This pyrolysis temperature was selected based on previous studies indicating that this temperature range is optimal for maximizing biochar yield while ensuring sufficient carbonization and stability of the biochar produced from manure-based feedstocks [20,21]. The resulting manure biochar was oven-dried at 65 °C for 6–8 h to ensure consistency in moisture content across all treatments and to remove any residual moisture that might have been absorbed during sample handling, cooling, or storage. In the pre-pyrolysis process, the ground manure was mixed with bentonite before undergoing pyrolysis. The bentonite (World Chemical Group, Chiang Mai, Thailand) consisted primarily of montmorillonite, with minor components such as feldspar, gypsum, calcium carbonate, and quartz. It had a pH of 10.6 ± 0.03, electrical conductivity of 1042 ± 5.00 µS/cm, and total dissolved solids of 0.521 ± 0.003 g L⁻¹. Notably, its high swelling capacity, expanding to 5–20 times its dry volume, produces a gel-like consistency that enhances coating and adhesion to manure particles.

Bentonite was incorporated into the manure at concentrations of 5%, 10%, 20%, and 30% w/w. The specified amount of bentonite was first dissolved in 400 mL of deionized (DI) water and then mixed with 50 g of manure using a water-to-manure ratio of 8:1. This ratio was selected to facilitate uniform mixing while avoiding excessive moisture. The mixture was stirred thoroughly and allowed to sit for 30 min to ensure uniform contact between bentonite and feedstock before being oven-dried at 65 °C for 6–8 h, which was sufficient to reduce the moisture content for effective pyrolysis. Wet mixing was employed to promote better adhesion and distribution of bentonite particles on the feedstock surface, despite the additional drying step, as this approach has been reported to enhance the interaction between minerals and biomass during subsequent pyrolysis. The dried mixture was then subjected to pyrolysis at 550 °C for 1 h.

In contrast, the post-pyrolysis process followed the same initial steps, except that the ground manure was first pyrolyzed before being mixed with bentonite. The final biochar composites from both processes were ground and subsequently analyzed for their physicochemical properties before being applied to the soil. All chemicals and solvents used for analytical procedures were obtained from RCI Labscan (Bangkok, Thailand).

2.2. Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FT-IR)

The surface morphology of the biochar samples was analyzed using SEM (SU3800, Hitachi High-Tech Corporation, Hitachinaka, Japan). Elemental composition was assessed through energy-dispersive X-ray spectroscopy (EDS) (Ultim Max 40, Aztec Live Standard, Hitachinaka, Japan). EDS data were collected from specific sample positions and processed using Aztec software (version 5.0, Oxford Instruments, Abingdon, UK), which provided graphical representations along with the weight and atomic percentages of the detected elements.

FT-IR (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) was used to identify the functional groups and molecular structures in the biochar. Spectra were recorded in the range of 600–4000 cm⁻¹ and analyzed using OMNIC software version 9.8 for spectral interpretation.

2.3. Water Holding Capacity (WHC)

The WHC of the biochar was determined following the method described by Motsara and Roy [22]. Briefly, 5.0 g of oven-dried biochar was placed on filter paper inside a funnel and saturated with DI water. After 24 h, the biochar was weighed again, and WHC was calculated as the percentage of water retained relative to its initial dry weight.

2.4. Organic Carbon (OC) and Normalized Organic Carbon Content

Loss on ignition (LOI) was used to estimate organic matter (OM) and OC content in biochar, assuming OM contains ~58% OC [22]. Briefly, 0.5 g of <2 mm biochar was weighed in a pre-weighed crucible (W₁), dried at 105 °C for 4 h, cooled in a desiccator, and reweighed (W₂). It was then ignited at 550 °C for 4 h, cooled, and weighed again (W₃). OM, OC, and normalized OC (or OC content per unit of biochar, independent of the bentonite content) were calculated using Equations (1)–(3):

$$Organicmatter\left(OM,\%\right)={\displaystyle \frac{\left(W_2-W_1\right)-(W_3-W_1)}{W_2-W_1}}$$

(1)

$$Organiccarbon\left(OC,\%\right)=OM(\%)\times 0.58$$

(2)

$$NormalizedOC={\displaystyle \frac{OC(\%)}{Biocharfraction}}$$

(3)

2.5. Carbon Stability

Carbon stability in biochar was assessed using hydrogen peroxide (H₂O₂) oxidation, following the method of Cross and Sohi [23], which estimates minimum stability by removing structural protection. Biochar samples were milled, oven-dried at 80 °C, and cooled in a desiccator. A subsample containing 0.1 g of carbon (BtC, based on LOI) was weighed into a test tube (Bt). A solution of 0.01 mol H₂O₂ (30% AR grade, Q RëCTM, Chonburi, Thailand) in 7 mL of DI water was added to each tube. The samples were heated at 80 °C for 2 days with intermittent agitation (2–3 times per day). After the reaction, the tubes were dried at 105 °C overnight, cooled, and reweighed to determine mass loss (Br). The remaining carbon content after oxidation (BrC) was determined using the LOI method. Biochar carbon stability (Æ) was then calculated using Equation (4).

$$\mathrm{\AE }\left(\%\right)={\displaystyle \frac{Br\times BrC}{Bt\times BtC}}\times 100$$

(4)

2.6. Soil Nutrient Analysis

Soil concentrations of nitrate (NO₃⁻), available P, and available K were assessed using methods modified from Motsara and Roy [24]. For NO₃⁻ extraction, 10 g of soil was combined with 20 mL of 0.5 M K₂SO₄ and agitated for 30 min. The resulting suspension was centrifuged at 4000 rpm for 5 min, and the supernatant was filtered through Whatman No. 5 filter paper (Cat No. 1005-110, Cytiva, Shanghai, China). NO₃⁻ levels in the filtrate were analyzed by reacting with 5% salicylic acid to form a colorimetric compound, which was quantified at 410 nm using a UV–vis double-beam spectrophotometer (Lambda 365, PerkinElmer Ltd., Bangkok, Thailand).

Available P was measured using the Olsen extraction method, in which 1.0 g of soil was mixed with 20 mL of 0.5 M NaHCO₃ and shaken for 30 min on an orbital shaker (MS-NOR-30, Major Science, Saratoga, NY, USA). The solution was then centrifuged at 4000 rpm for 5 min and filtered with the P concentration determined at 882 nm using a UV–vis spectrophotometer.

For available K analysis, 2.5 g of soil was extracted with 25 mL of 1 M ammonium acetate (NH₄C₂H₃O₂) and shaken for 30 min. After filtration, K levels were determined using a flame photometer (AE-FP8201, A&E Lab (UK) Co., Ltd., London, UK) at a detection wavelength of 383 nm.

2.7. Statistical Analyses

Statistical analyses were performed using R software (version.4.3.1, R Core Team, Vienna, Austria). The normality of residuals was assessed using the Shapiro–Wilk test, and the data were transformed where necessary. Homogeneity of variance was tested with Levene’s test. A two-way ANOVA was conducted to evaluate the main and interaction effects of the pyrolysis method and bentonite application rate, with the control group analyzed as a separate factor. Tukey’s post hoc test was applied to identify significant differences among treatment means (α = 0.05). Pearson correlation coefficients were calculated using the cor() function and tested for significance with cor.mtest() at a 95% confidence level. Significant correlations (p < 0.05) were visualized using the corrplot package v.0.92 (R software). Principal component analysis (PCA) was performed with the FactoMineR package v.2.9 and visualized using the factoextra package v.1.0.7, with biplots incorporating ellipses and color coding to illustrate the effects of different pyrolysis treatments.

3. Results and Discussion

3.1. Surface Morphology of Biochar Composites

SEM images (Figure 2) show marked structural alterations in manure-derived biochar modified with 10% and 30% bentonite, both pre- and post-pyrolysis. In the control samples, the biochar displayed a typical highly porous structure—an inherent feature of pyrolyzed biomass—which facilitates nutrient retention and adsorption [25]. In the pre-pyrolysis treatments, 10% bentonite visibly adhered to the biochar matrix, imparting a granular appearance and partially filling the pores, while 30% bentonite incorporation led to a denser and rougher surface morphology. These changes are attributed to thermal interactions between bentonite and OM during pyrolysis, which may enhance structural integrity but reduce accessible pore space [26]. In the post-pyrolysis treatments, bentonite appeared to coat the external surfaces of the already formed biochar. At 10%, bentonite retained the porous framework while visibly embedding within the matrix, possibly buffering against pore collapse during thermal exposure. However, at 30%, the bentonite heavily coated or occluded the pores, leading to a notable decrease in visible porosity. This pore-filling effect may alter the biochar’s water retention and nutrient exchange potential, as excessive bentonite can limit internal surface area and hinder permeability [27,28].

EDS analysis (Figure 2,

Table 1. Elemental analysis results determined by energy-dispersive X-ray spectroscopy (EDS) of pre- and post-pyrolysis manure biochar without and with bentonite.

Pyrolysis Type	Bentonite (%)	Element (% w/w)
		C	O	Si	K	Al	Ca	Mg	Fe	Na
Control	0	79.53	16.22	1.70	1.22	0.50	0.65	0.18	-	-
Pre-pyrolysis	10	64.15	22.94	6.33	2.42	0.86	2.19	0.54	-	0.57
	30	52.41	23.43	10.98	1.75	3.14	4.69	1.07	2.18	0.35
Post-pyrolysis	10	66.18	21.77	4.15	2.62	0.93	2.84	0.95	-	0.56
	30	60.22	22.89	9.85	1.17	1.67	2.24	0.83	2.76-	1.12

) revealed clear trends in elemental composition associated with bentonite addition and pyrolysis mode. Carbon (C) content declined progressively with increasing bentonite levels, more sharply in the pre-pyrolysis samples (−19% at 10%, −34% at 30%) than in post-pyrolysis (−9% at 10%, −17% at 30%). This reduction likely results from the enhanced volatilization of organic carbon due to bentonite’s thermal stability and adsorption capacity during pyrolysis. These findings support previous reports [29], which noted that minerals such as montmorillonite—rich in CEC and with low interlayer bonding energy—facilitate C oxidation during high-temperature processing.

In contrast, levels of silicon (Si) and aluminum (Al)—the primary constituents of bentonite—significantly increased with bentonite incorporation. Si content rose more than threefold at 10% and up to sixfold at 30%, while Al increased by 70–600%, indicating successful thermal integration of bentonite into the biochar matrix [30,31]. Notably, pre-pyrolysis treatments showed slightly higher increases in Al than post-pyrolysis, suggesting greater retention and possibly better mineral–biochar fusion during co-pyrolysis.

K showed a non-linear trend: initial increases at 10% bentonite were followed by declines of ~25–38% at 30% bentonite in both pyrolysis treatments. This may reflect a saturation threshold at which excess bentonite binds K more tightly, limiting its bioavailability [32]. In contrast, calcium (Ca), magnesium (Mg), and iron (Fe) increased steadily with bentonite content across both pyrolysis strategies, further supporting the mineral enrichment effect of bentonite [33].

Together, the SEM and EDS results confirm that both the amount and method of bentonite incorporation influence the physicochemical profile of biochar. Pre-pyrolysis incorporation tends to promote deeper mineral integration and greater modification of the biochar matrix, with implications for its structural stability, nutrient composition, and soil amendment performance [26].

3.2. Functional Group of Biochar Composites

FTIR analysis (Figure 3) revealed consistent functional groups across all manure biochar treatments, with varying peak intensities depending on the pyrolysis process and bentonite concentration. While the pyrolysis process itself did not substantially alter the types of functional groups present, the addition of bentonite significantly influenced their intensity and distribution, reflecting changes in biochar chemistry and surface interactions. Post-pyrolysis samples generally exhibited higher transmittance, indicating lower functional group intensity due to more extensive thermal degradation. In contrast, pre-pyrolysis incorporation appeared to preserve more functional groups, likely due to bentonite’s buffering effect, which also helped maintain structural integrity—consistent with SEM observations.

Characteristic absorption peaks observed across all samples included Si–O–Al (~650 cm⁻¹) and C–H bending (600–800 cm⁻¹), corresponding to aluminosilicate structures and aliphatic hydrocarbons, respectively [34,35]. These peaks became increasingly prominent with higher bentonite levels, particularly in pre-pyrolysis samples, indicating effective mineral incorporation and surface complexation. The Si–O stretching region (1000–1100 cm⁻¹) showed enhanced intensity with increasing bentonite content, which was more pronounced in pre-pyrolysis samples, suggesting efficient integration of silicate structures. Additionally, stronger peaks for C–O (~1100 cm⁻¹) and C–NH–C (~1125 cm⁻¹) bonds suggest an enrichment of oxygenated and nitrogenous functional groups with bentonite addition [27].

The C–O–C band (~1250 cm⁻¹), associated with ether linkages, and C=O stretching (~1750 cm⁻¹), indicative of carbonyl groups, were more prominent in pre-pyrolysis treatments [36,37]. These findings imply that early bentonite incorporation may help retain labile functional groups during pyrolysis, thereby enhancing the biochar’s reactivity and sorption capacity. The aromatic C=C band (~1600 cm⁻¹) remained consistent across treatments, indicating that the core aromatic structure of biochar was preserved, which is important for long-term carbon stability [38,39]. Lastly, a broad O–H stretching band (~3300 cm⁻¹) appeared in all samples but was slightly reduced in post-pyrolysis treatments, reflecting decreased hydrophilicity due to thermal degradation [40].

Overall, these FTIR results emphasize how bentonite concentration and method of incorporation modulate biochar’s surface chemistry and functional group retention. Pre-pyrolysis mixing of bentonite enhances the stability of key functional groups and contributes to a more chemically reactive and structurally robust biochar, supporting its multifunctional potential in soil improvement and environmental applications.

3.3. Physicochemical Properties of Biochar Composites

3.3.1. pH and Electrical Conductivity (EC)

The pH and EC of manure-derived biochar were significantly influenced by both the pyrolysis method and bentonite addition rate, with strong interaction effects (p < 0.001;

Table 2. Summary of two-way analysis of variance (ANOVA) for biochar properties as affected by pyrolysis processes and bentonite addition rates.

Parameters	Pyrolysis Processes (PP)	Bentonite Addition Rate (BR)	PP × BR
pH (-)	***	***	***
EC (µm/cm)	***	***	***
WHC (%)	ns	*	ns
OC (%)	***	***	***
Normalized OC (%)	***	***	***
C stability (%)	***	***	***
NO3− (mg/kg)	***	***	***
Available P (mg/kg)	***	*	**
Available K (mg/kg)	***	***	***
DF	1	4	3

ns: not significant at p = 0.05; *, **, *** indicate p < 0.05, p < 0.01, and p < 0.001, respectively.

). Biochar produced through pre-pyrolysis incorporation of bentonite consistently exhibited higher pH values than both the control and post-pyrolysis treatments (Figure 4a). This increase is likely attributed to the presence of alkaline minerals in bentonite, particularly montmorillonite—a smectite clay characterized by high CEC, a layered structure, and an expansive surface area [30,41]. The bentonite used in this study was characterized by high intrinsic alkalinity and moderate soluble salt content, as described in Section 2.1. These properties contribute to pH buffering and enhance the retention of basic cations such as K⁺ and Na⁺ during pyrolysis, especially above 300 °C [42]. Conversely, post-pyrolysis samples exhibited a decreasing pH trend with increasing bentonite concentration, showing an overall reduction of approximately 5% from the control to the 30% bentonite treatment. This decline may be explained by the formation of organic acids, thermal loss of alkaline components, and degradation of pH-buffering functional groups (e.g., hydroxyl and carboxyl groups), as evidenced by reduced O–H stretching near 3300 cm⁻¹ in FTIR spectra (Figure 3) [43]. Nevertheless, the pH of all biochar samples remained in the basic range, indicating that even the most acidic treatments are suitable for soil improvement applications.

EC patterns also varied significantly among treatments (p < 0.001; Table 2). In pre-pyrolysis biochar, EC declined markedly, decreasing by approximately threefold across all bentonite concentrations compared to the control (Figure 4b). This trend is consistent with bentonite’s relatively low soluble salt content, and strong ion-binding capacity, which reduces the availability of free salts during pyrolysis [44]. In contrast, EC in post-pyrolysis biochar remained comparable to the control at bentonite levels of 5–20%, with a noticeable reduction only at 30%. The higher EC observed in post-pyrolysis treatments is likely due to the concentration of soluble salts during pyrolysis-driven OM decomposition and the partial activation of bentonite’s ion exchange surfaces [45,46]. However, the limited effect of post-pyrolysis bentonite on EC suggests minimal chemical interaction with the biochar matrix after thermal treatment [27].

3.3.2. Water Holding Capacity

The WHC of the biochar composites was significantly influenced by the rate of bentonite addition (p < 0.05), although the interaction between pyrolysis method and bentonite rate was not statistically significant (Table 2). WHC increased substantially—by approximately 102%—with 5% and 10% bentonite relative to the control, whereas higher rates (20% and 30%) did not further enhance WHC and maintained levels similar to those of the control. Among all treatments, the control biochar showed the lowest WHC, while biochar produced via pre-pyrolysis bentonite incorporation consistently exhibited greater WHC than its post-pyrolysis counterparts. This enhancement is attributed to the improved integration of hydrophilic functional groups (e.g., –OH, –COOH) and preservation of the porous structure during co-pyrolysis, as evidenced by SEM and FTIR analyses (Figure 2 and Figure 3). In contrast, post-pyrolysis treatments likely underwent structural condensation and porosity loss due to volatile matter release and increased aromatic condensation [47,48].

The pronounced WHC improvement at 5% and 10% bentonite during pre-pyrolysis suggests a synergistic effect, leveraging bentonite’s high surface area, cation exchange capacity, and swelling properties [49,50]. However, a decline of ~14% in WHC was observed when bentonite content increased to 20–30%, pointing to a potential saturation threshold. At excessive levels, bentonite may occlude biochar pores [26], thereby reducing the available pore space for water retention. This is consistent with earlier findings that high bentonite loading can induce structural compaction, limiting water-accessible porosity despite strengthening the composite matrix [51,52].

These results provide the first quantitative evidence that pre-pyrolysis mixing of bentonite with manure-derived biochar at optimized concentrations (5–10%) can significantly enhance water retention properties. This insight offers a tailored approach for improving soil moisture management in water-scarce agricultural systems through mineral–biochar composite engineering.

3.4. Biochar Carbon Characteristics

The pyrolysis method and bentonite application rate significantly affected the OC content, normalized OC, and carbon stability of biochar (p < 0.001), with significant interaction effects (Table 2). Increasing bentonite levels led to a progressive decrease in OC content in both pyrolysis methods (Figure 5a), likely due to the dilution effect of the added inorganic fraction. In the pre-pyrolysis treatments, OC content declined by approximately 26% from 0% to 30% bentonite, while post-pyrolysis treatments showed a 19% decrease.

Interestingly, normalized OC exhibited distinct trends between treatments (Figure 5b). In pre-pyrolysis biochar, it declined slightly (by ~9%) at 5–20% bentonite and stabilized at 30%. Conversely, post-pyrolysis samples showed an increase in normalized OC—up to 13%—at higher bentonite levels (20% and 30%). This may indicate that bentonite, when applied after pyrolysis, contributes to relative carbon enrichment, possibly by adsorbing carbonaceous residues or stabilizing surface-bound carbon via mineral–organic interactions. Bentonite’s high surface area and CEC are key properties that facilitate such stabilization effects [53,54,55].

Carbon stability increased markedly with bentonite addition (5–20%) in pre-pyrolysis composites, more than doubling compared to the control (Figure 5c). However, stability decreased at the 30% bentonite level, suggesting that excessive bentonite may disrupt carbonization efficiency or alter structural integrity. In contrast, carbon stability declined by 20% across increasing bentonite rates in post-pyrolysis biochar, highlighting the importance of the pyrolysis method. These findings underscore that only pre-pyrolysis application promotes effective carbon stabilization, likely due to enhanced organo-mineral bonding and the formation of more recalcitrant aromatic structures during pyrolysis. Previous research supports this mechanism, indicating that clay minerals enhance aromatic condensation and the development of stable C–Si–C or C–Al–C linkages during pyrolysis [11,29,56].

Optimal outcomes were observed at bentonite rates of 5–20%, aligning with earlier studies emphasizing the need for controlled mineral loading to avoid diminishing returns [14,57]. Notably, this study goes further by identifying a critical threshold beyond which bentonite ceases to provide stabilization benefits and may instead hinder carbon retention. It also provides clear evidence that only co-pyrolysis facilitates meaningful interactions between bentonite and OM, as post-treatment does not significantly alter the internal carbon matrix [58]. Together, these results highlight the potential of pre-pyrolyzed biochar–bentonite composites as a viable strategy for enhancing long-term carbon sequestration in climate-resilient agricultural systems.

3.5. Nutrient Availability

3.5.1. Nitrate Concentration

The concentrations of NO₃⁻ in the biochar were significantly influenced by pyrolysis process, bentonite addition rate, and their interaction (p < 0.001) (Table 2). Post-pyrolysis biochar exhibited a striking ~412% increase in NO₃⁻, rising from 3.13 to 16.04 mg/kg (Figure 6a). However, the effect of bentonite varied substantially depending on the pyrolysis method. Pre-pyrolysis addition of bentonite markedly reduced NO₃⁻ levels by up to 89% at 20% bentonite, whereas post-pyrolysis incorporation resulted in a ~44% increase compared to the control.

This contrast highlights the importance of application timing. In the pre-pyrolysis process, bentonite’s high CEC, large surface area, and layered structure likely facilitated the retention of N in non- NO₃⁻ forms, thereby suppressing NO₃⁻ accumulation [59,60]. These findings align with Viglašová et al. [61], who demonstrated enhanced NO₃⁻ adsorption using bentonite-amended bamboo biochar. Additionally, Liu et al. [27] reported that bentonite can slow moisture diffusion and nutrient loss, further supporting our observed NO₃⁻ reduction in pre-pyrolysis composites. The electrostatic repulsion often seen in unmodified biochar, which limits NO₃⁻ adsorption [62], appears to be mitigated by bentonite incorporation prior to pyrolysis.

In contrast, post-pyrolysis bentonite treatments led to increased NO₃⁻ levels. This may result from the thermal decomposition of OM during pyrolysis [63,64], which releases N-containing volatiles, and from reduced CEC due to structural densification, limiting bentonite’s adsorption capacity [11,65]. SEM analysis (Figure 2) supports this, showing more compact pore structures in post-pyrolysis composites, potentially reducing NO₃⁻ adsorption. Together, these results suggest that bentonite incorporation before pyrolysis enhances N immobilization and reduces leaching risk, while post-pyrolysis treatments may increase NO₃⁻ availability. This differentiation offers a strategic pathway for tailoring N dynamics in biochar-amended soils depending on environmental or agronomic objectives.

3.5.2. Available Phosphorus Concentration

Available P was significantly influenced by the pyrolysis process (p < 0.001), bentonite addition rate (p < 0.05), and their interaction (p < 0.01) (Table 2), underscoring the combined effects of thermal treatment and mineral amendment on nutrient availability. In pre-pyrolysis treatments, P availability increased by approximately 81%—from 306.38 mg/kg in the control to an average of 555.84 mg/kg with bentonite addition (Figure 6b). Post-pyrolysis samples showed similar trends, with available P rising by ~55% at 5–20% bentonite and peaking at 697.84 mg/kg (+133%) with 30% bentonite.

These consistent increases in P availability across both pyrolysis strategies highlight bentonite’s central role in nutrient retention and stabilization. Bentonite’s high CEC and expansive surface area—attributed to its montmorillonite-rich composition—facilitate P adsorption and reduce nutrient leaching [66]. During pyrolysis, organic matter decomposition may release organically bound P, while bentonite simultaneously stabilizes these forms, minimizing thermal losses and enhancing plant-available P fractions [67]. Additionally, the elevated Al content in bentonite (Table 1) may contribute to the formation of stable Al–phosphate complexes [68], which enhance exchangeable and slowly available P pools in the biochar matrix. This mechanism could explain the superior P retention observed, particularly at higher bentonite concentrations.

Collectively, these findings demonstrate that bentonite-amended biochar—especially at elevated application rates—offers a robust strategy to enhance P availability, providing promising implications for nutrient-enriched soil amendments in sustainable agriculture.

3.5.3. Available Potassium Concentration

Available K dynamics were significantly influenced by pyrolysis mode, bentonite application rate, and their interaction (p < 0.001) (Table 2), emphasizing the pivotal role of both thermal treatment and mineral loading in K behavior. As shown in Figure 6c, pre-pyrolysis bentonite addition reduced K availability by ~36.7% at 5% and by ~47.5% at higher rates (10–30%), while post-pyrolysis treatments enhanced K levels by ~29% at 5–20% bentonite and by ~18% at 30%, reaching maximum concentrations of 9.2 and 8.4 µg/kg, respectively.

These contrasting effects suggest that the timing of bentonite incorporation critically alters K retention and release. In pre-pyrolysis composites, K may form thermally stable K–aluminosilicate phases due to high-temperature reactions with bentonite’s layered structure [69]. Furthermore, encapsulation within biochar micropores and possible volatilization as KCl or KOH [70] during pyrolysis may reduce bioavailable K fractions. Co-pyrolysis studies have shown that bentonite can enhance nutrient retention [10], though high doses may immobilize K within biochar matrices [71].

In contrast, bentonite added post-pyrolysis appears to facilitate K availability via surface sorption or ion exchange processes, especially at moderate application rates (5–20%). At 30%, however, slight declines in available K may occur due to pore occlusion or stronger electrostatic binding, which can limit ion desorption [72]. An et al. [73] demonstrated that moderate bentonite levels optimize nutrient availability by balancing retention and release. Collectively, these findings reveal divergent K release profiles depending on the timing of bentonite addition: pre-pyrolysis bentonite favors gradual K release and reduced leaching—suitable for slow-release formulations—while post-pyrolysis treatments enhance immediate availability, making them more applicable to K-deficient soils or short-cycle crops.

A limitation of this study is that only plant-available fractions of N (as NO₃⁻), K, and P were measured. Other important forms of N, such as ammonium (NH₄⁺) and nitrite (NO₂⁻), as well as total nutrient contents, were not assessed. The exclusion of NH₄⁺ and NO₂⁻ was based on the fact that, in well-aerated, neutral agricultural soils such as those in this experiment, NH₄⁺ is typically immobilized by microbes, adsorbed onto soil or biochar surfaces, or rapidly converted to nitrate, while NO₂⁻ is a short-lived intermediate in nitrification and rarely accumulates under normal field conditions [74,75]. These forms generally provide less direct information on short-term plant nutrition compared with nitrate. Including these parameters in future research would provide a more comprehensive understanding of nutrient dynamics and long-term soil fertility. Addressing these additional nutrient forms alongside the observed differences in NO₃⁻, K, and P could further refine bentonite–biochar–soil management strategies tailored to specific agroecosystems and nutrient management goals.

3.6. PCA and Correlation Analysis of Pre- and Post-Pyrolysis Bentonite Biochar

PCA revealed a clear separation between biochar samples amended with bentonite before pyrolysis (pre-pyrolysis) and those amended after pyrolysis (post-pyrolysis) (Figure 7a). Variables such as pH, WHC, and carbon stability were strongly aligned with pre-pyrolysis samples, whereas EC, NO₃⁻, normalized OC, and available K clustered with post-pyrolysis samples. This distinct partitioning underscores how the timing of bentonite addition fundamentally shapes the physicochemical characteristics of biochar.

The grouping of WHC, pH, and carbon stability with pre-pyrolysis treatments suggests that high-temperature interactions between bentonite and organic precursors during pyrolysis likely promote enhanced porosity, stronger organo-mineral bonding, and more stable carbon structures. These structural benefits contribute to greater moisture retention and long-term stability, in line with previous work by An et al. [66], who found that co-pyrolysis with minerals enhances the slow-release fertilizer potential of biochar. Conversely, the post-pyrolysis group’s association with elevated EC, NO₃⁻, and available K points to enhanced short-term nutrient availability. Bentonite added after pyrolysis may primarily act on the biochar surface through adsorption or ion-exchange interactions, without significantly altering the internal carbon framework or porosity. This mechanism favors the release of soluble nutrients rather than long-term retention.

The correlation analysis (Figure 7b) supports these patterns. WHC positively correlated with pH and carbon stability, suggesting that alkaline, structurally stable biochars retain water better. Meanwhile, carbon stability showed negative correlations with OC, OM, and normalized OC, indicating that more stable biochar contains less labile or volatile organic fractions—consistent with Spokas [76] and Singh et al. [77]. Additionally, pH and carbon stability were inversely related to EC, NO₃⁻, and K, implying that more stable biochar may provide slow-release nutrient delivery due to tighter chemical binding or reduced solubility [78].

These multivariate analyses collectively reveal that incorporating bentonite before pyrolysis enhances the structural integrity, water retention, and carbon stability of biochar, making it highly suitable for long-term soil conditioning. In contrast, post-pyrolysis bentonite addition improves the availability of key nutrients—particularly NO₃⁻ and K—supporting applications aimed at rapid soil fertility enhancement. This clear functional divergence underscores the importance of strategically selecting the timing of bentonite incorporation to tailor biochar properties for specific agronomic or environmental goals.

The implications of these findings are far-reaching. Pre-pyrolysis bentonite integration produced biochar with superior porosity, chemical stability, and moisture-holding capacity. These characteristics are especially advantageous for use in degraded or drought-prone soils, where long-term improvements in soil health and carbon sequestration are critical. The strong organo-mineral interactions formed during high-temperature co-pyrolysis likely contribute to the enhanced structural and functional resilience of the biochar, offering a pathway toward climate-resilient and low-input agricultural systems.

In contrast, the addition of bentonite after pyrolysis was more effective in enhancing the immediate availability of nutrients. This is particularly relevant for nutrient-deficient or intensively managed cropping systems, where rapid nutrient release is essential. Such biochar can act as short-term fertility booster, making it ideal for fast-growing or nutrient-demanding crops.

Furthermore, the use of nutrient-rich manure as a biochar feedstock, combined with a low-cost mineral such as bentonite, represents a circular and sustainable approach to organic waste valorization. These tailored biochar composites not only reduce environmental waste but also offer multifunctional benefits as either slow-release conditioners or fast-acting fertilizers. Additionally, their high sorption capacity and improved structural properties suggest potential for environmental remediation applications, such as nutrient retention and contaminant immobilization in polluted or at-risk soils.

Overall, this study introduces a novel framework for optimizing biochar functionality through controlled amendment timing. By aligning biochar production strategies with specific soil and crop needs, this research advances the precision design of biochar-based inputs for sustainable agriculture and environmental stewardship. The statistical summary in Table 2 further reinforces these conclusions, showing that both pyrolysis process and bentonite addition rate significantly influenced nearly all measured biochar properties, with notable interactive effects. While nutrient-related traits such as NO₃⁻ and available K were particularly enhanced by post-pyrolysis bentonite addition, structural and stability-related properties—pH, OC, C stability, and normalized OC—were maximized through pre-pyrolysis incorporation. These patterns quantitatively confirm the functional divergence described above and highlight the importance of jointly optimizing pyrolysis conditions and additive rates for targeted agronomic outcomes.

4. Conclusions

This study provides key insights into how the timing and rate of bentonite incorporation influence the physicochemical properties of manure-derived biochar, including its structure, water holding capacity, nutrient availability, and carbon stability. SEM and EDS analyses reveal that pre-pyrolysis bentonite addition enhances mineral integration but may also accelerate carbon loss and cause pore collapse, while FTIR results show that this method more effectively preserves functional groups and structural features. Pre-pyrolysis incorporation significantly improves carbon stability and water retention, making the resulting biochar more suitable for long-term soil enhancement and carbon sequestration. However, it also promotes stronger binding of NO₃⁻ and K, reducing their immediate availability. In contrast, post-pyrolysis bentonite addition improves the availability of these nutrients, supporting short-term fertility goals. Although higher bentonite rates enhance P availability, moisture retention, and carbon stability, the benefits decline at elevated concentrations, indicating diminishing returns. Together, these findings offer an effective strategy for tailoring biochar functionalities to specific agronomic or environmental objectives—whether enhancing long-term soil health or improving immediate nutrient delivery.