Biochar is a carbon-rich material produced through the pyrolysis of organic biomass under limited oxygen conditions. It has gained considerable attention as a soil amendment due to its high surface area, porosity and stability, which enhance nutrient retention and adsorption capacity. The physicochemical properties of biochar are strongly influenced by the nature of the feedstock and pyrolysis temperature, with higher temperatures generally improving structural characteristics and adsorption efficiency. (Lehmann and Joseph, 2015; Sohi et al., 2016; Tripathi et al., 2016).
       
Guava (Psidium guajava) is an important tropical fruit crop cultivated widely in subtropical and tropical regions. Regular pruning is an essential cultural practice to maintain canopy structure, improve fruit quality and reduce pest and disease incidence. However, the disposal of guava pruning residues poses a significant challenge, as these wastes are often burned or discarded, leading to environmental concerns. Conversion of such biomass into biochar offers a sustainable approach for waste management while generating a value-added product for soil improvement (Ali et al., 2019; Tan et al., 2017).
       
Biochar generally exhibits alkaline pH and appreciable cation exchange capacity (CEC), which help ameliorate soil acidity, enhance nutrient retention and improve soil physical properties such as water-holding capacity. Its application has been reported to reduce nutrient leaching losses, improve soil organic carbon and stimulate microbial activity, thereby contributing to enhanced soil fertility and crop productivity (Coumaravel et al., 2019; Rajamani et al., 2022; Wang et al., 2021).
       
Despite extensive research on biochar derived from various agricultural residues, studies focusing on biochar produced from guava pruning waste remain limited. In particular, there is insufficient information on its proximate composition, structural characteristics and physicochemical properties and their implications for soil improvement in horticultural systems. (Devi and Saroha, 2013; Li et al., 2021; Pariyar et al., 2020).
       
Therefore, the present study aims to provide a comprehensive characterization of biochar derived from guava pruned cuttings, produced under controlled pyrolysis conditions (350-400°C), using standardized analytical procedures. The novelty of this study lies in the integrated evaluation of structural, functional and exchange properties of guava-derived biochar, which has not been adequately documented in earlier studies. The findings are expected to establish its suitability as a sustainable soil amendment and promote efficient utilization of pruning waste in tropical fruit production systems.
       
Overall, the conversion of guava pruning residues into biochar represents a promising strategy aligned with circular economy principles, facilitating waste valorization, improving soil health and supporting sustainable horticultural practices. (Sahoo et al., 2023; Tan et al., 2017).

This research experiment was conducted at SRM College of Agricultural Sciences, Baburayanpettai, Chengalpattu district, Tamil Nadu, from 2024 to 2025. The feedstock used for producing biochar was guava pruned shoots. The shoots were collected from the college orchard at SRMCAS.
 
Biochar production from cuttings guava
 
Guava pruned shoots were air-dried under shade for 7-14 days to reduce moisture content and then cut into uniform pieces. Biochar was produced using a closed pyrolysis unit under limited oxygen conditions.
       
The combustion chamber was preheated using dried charcoal. Pyrolysis was carried out at a temperature range of 350-400°C for approximately 2 h and the temperature was monitored using a digital thermocouple to ensure consistency. The unit was maintained in a closed condition with a chimney to regulate airflow.
       
Completion of pyrolysis was indicated by the cessation of smoke emission. The biochar was allowed to cool, collected, ground using a mortar and pestle and sieved to obtain uniform particle size. Approximately 10 kg of dry biomass yielded 1.98 kg of biochar. All measurements were carried out in triplicate (n = 3) and mean values were reported (Fig 1).


 
Structural and functional characterization of guava biochar
 
Physicochemical characterization
 
Conventional techniques were employed to evaluate pH, electrical conductivity (EC), moisture content, total ash, total nitrogen (Kjeldahl method), phosphorus (P₂O₅), potassium (K₂O), total organic carbon (TOC) and the carbon-to-nitrogen (C:N) ratio. Sieve analysis and mass-volume determination were used to determine the particle size and bulk density, respectively. The biofertilizers and organic fertilizers were assessed in accordance with the Fertilizer (Control) Order 1985 (amended up to March 2023), specifically Schedule IV - Part D and Schedule II - Part B-3 (V). The analysis methods for soils, plants, waters, fertilizers and organic manures were edited by H L S Tandon (FDCO) in 2009. The biofertilizers and organic fertilizers were evaluated in accordance with the Fertilizer (Control) Order 1985, as amended up to March 2023, Schedule V - Part D. The sample analysis adhered to the standards set by the Fertilizer (Control) Order 1985 and the procedures outlined by Tandon (2009).
 
Volatile matter
 
The organic carbon content of the media was determined by the loss-on-ignition method. For this, 10 g of oven-dried sample was placed in a muffle furnace and ignited at a temperature of 650-700°C for 6-8 hours. The percentage of total organic matter was calculated using the formula:
 

  

 
The total organic carbon content was then estimated by dividing the organic matter value by the Van Bemmelen factor (1.724), as follows:
 

  

 
Cation exchange capacity (cmol/Kg)
 
The cation exchange capacity (CEC) was measured by saturating the biochar with 1 M ammonium acetate, washing with ethanol and extracting with 1 M KCl. The NH4⁺ retained was analyzed using spectrophotometry.
 
Anion exchange capacity (AEC) (cmol/Kg)
 
The anion exchange capacity (AEC) was similarly measured using KNO₃/KCl saturation and K₂SO₄ extraction, with NO₃^- and Cl^-  concentrations quantified using ion chromatography. CEC and AEC were reported in cmol/kg units.
 
Water holding capacity (WHC) (%)
 
The water holding capacity (WHC) was determined to evaluate the ability of the biochar to retain moisture, which influences its agronomic performance in soil. A known weight of oven-dried sample (typically 5 g) was saturated with distilled water and allowed to drain under gravity on Whatman No. 1 filter paper for 24 hours. The sample was then reweighed after removing the surface moisture. The WHC was calculated as the difference between the wet and dry weights, expressed as a percentage of the dry weight. This method is widely adopted in organic amendment studies and aligns with the procedures outlined in Tandon (2009) and the Fertilizer Control Order (FCO, 1985) for organic fertilizers and soil conditioners.
 
Structural and morphological characterization
 
Scanning electron microscopy (SEM)
 
Surface morphology was examined using a scanning electron microscope (FEI Nova® 400 Nano SEM) operated at 20 kV.
 
Brunauer-emmett-teller (BET) analysis
 
Specific surface area was determined using the nitrogen adsorption method.
 
X-ray diffraction (XRD) analysis
 
Crystalline structure and mineral phases were analyzed using a Bruker® D8 X-ray diffractometer with Cu Kα radiation. Scanning was performed over a 2θ range of 10-90° with a step size of 0.02°.
 
Fourier transform infrared (FTIR) and raman spectroscopy
 
FTIR analysis was carried out using Shimadzu IR Tracer-100 and PerkinElmer® Spectrum 100 instruments over the range of 4000-400 cm^-1 using the KBr pellet method.
       
Raman spectroscopy was performed using a Bruker Senterra II Raman microscope with 532 nm laser excitation. Measurements were recorded with 15 replicates (n = 15) to ensure reproducibility, using synthetic graphite as a reference.

Physicochemical properties
 
The physicochemical characteristics of the guava (Psidium guajava) derived biochar are presented in Table 1. The biochar exhibited an alkaline pH of 8.40, while the electrical conductivity (EC) was 3.30 dS m^-1, indicating a moderate level of soluble salts. The bulk density was 0.49 g cm^-3, reflecting its lightweight nature. The alkaline pH (8.40) of the biochar indicates its potential to ameliorate acidic soils, which is consistent with previous reports on lignocellulosic biochars. The moderate EC value suggests the presence of soluble mineral constituents without posing a salinity risk under controlled application rates (Lehmann and Joseph, 2015; Sohi et al., 2016).


       
The cation exchange capacity (CEC) was 30 cmol/ kg and the anion exchange capacity (AEC) was 5 cmol/ kg. The water holding capacity (WHC) was 25% and the total organic carbon (TOC) content was 50.40%. The relatively high CEC reflects the ability of the biochar to retain nutrient cations, while the Anion Exchange Capacity indicates limited but functional anion adsorption capacity. The WHC (25%) suggests improved moisture retention potential, which is beneficial under water-limited conditions. The high TOC content confirms the carbon-rich nature of the biochar and its potential contribution to soil organic carbon pools. (Rajamani et al., 2022; Wang et al., 2021; Zhang et al., 2020).
 
Structural properties
 
SEM analysis and determination
 
The SEM image taken at 500x magnification provides a macroscopic view of the biochar particle morphology. The sample shows a well-defined, fibrous and elongated structure, suggesting that the original plant cell wall architecture was partially preserved during the pyrolysis process. The presence of distinct longitudinal striations on the surface is indicative of vascular tissue origin (e.g., xylem or phloem), confirming the lignocellulosic nature of the feedstock material (Fig 2). The primary particle in the image measures approximately 100 μm in width (as per the scale bar), placing it in the fine particle range, which facilitates better mixing and interaction with soil particles. The structure appears compact and rigid, characteristic of biochar produced at relatively high pyrolysis temperatures, contributing to its structural stability and long-term persistence in soil. Surrounding the main particle, there is visible amorphous particulate matter, possibly representing ash residues or finer biochar fragments. This diversity in particle size and structure contributes to improved soil aeration, water retention and microbial colonization capacity. The microstructural morphology of the biochar was examined using Scanning Electron Microscopy (SEM) at a magnification of 5000x. The SEM image reveals a porous, heterogeneous surface structure, which is a characteristic feature of well-pyrolyzed biochar. Multiple pores of varying sizes are visible across the sample surface. The pore diameters, as annotated in the image, range from approximately 1.051 μm to 2.789 μm. These micropores and mesopores are essential for enhancing the surface area, which plays a crucial role in the adsorption of nutrients, retention of water and colonization by soil microorganisms. Some key pore measurements observed in the SEM image include: 1.051 μm, 1.480 μm, 2.178 μm, 2.251 μm and 2.789 μm. The visible honeycomb-like structure is indicative of the preservation of the original plant tissue anatomy during pyrolysis. This porous architecture increases the cation exchange surface and facilitates better interaction with soil particles and microbial communities. The presence of fine particulate matter and structural residues on the surface also suggests that the biochar may retain some ash or mineral content, which can contribute to nutrient availability when applied to soils. The SEM image, captured at 2,500x magnification, displays a fractured biochar particle with a distinct and well-preserved layered and porous structure. The longitudinal and cross-sectional striations across the particle surface suggest retention of original plant vascular tissue, which is common in biochars derived from lignocellulosic biomass. The surface shows large open channels and elongated pores, with diameters roughly in the tens of micrometers, as indicated by the 30 μm scale bar. These structural features facilitate air and water movement and the exposed internal surfaces provide abundant sites for nutrient adsorption and microbial colonization. The relatively smooth internal walls and the clean fracture lines indicate that the biochar underwent thermochemical transformation at high pyrolysis temperatures, which results in greater structural rigidity and carbon stability. These characteristics are advantageous for long-term soil amendment applications, as they contribute to both physical stability and chemical reactivity. There is also evidence of fragmented material and fine particulates on the surface and surroundings, possibly representing ash residues or mineralized carbon, which can contribute to the nutrient pool when incorporated into soils. (Liu et al., 2017; Pariyar et al., 2020).


 
XRD analysis and determination
 
The XRD pattern provides insights into the crystalline and amorphous nature of the biochar sample (Fig 3). The spectrum displays a broad hump centered around 2θ ≈ 20°-30°, which is typical of amorphous carbon structures. This broad peak suggests that the carbon in the biochar is largely disordered and non-crystalline, a common feature in biochars produced from lignocellulosic biomass through pyrolysis (Devi and Saroha, 2013; Li et al., 2021; Pariyar et al., 2020).

​
       
Calcite (CaCO₃), the primary phase, was present in samples with a dominant peak at 29.2° 2θ (d-spacing 3.05 Å). The highest intensity with the sharpest peaks was observed, suggesting an ordered structure. Minor Peaks were observed at 22.7°, 24.3°, 35.8° and 42.9° (silica, potassium salts and biomass ash minerals, including calcite), which supply Ca, K and micronutrients and improve cation exchange. Root handling, nutrient storage and microbial presence all improve with stable biochar. It promotes media structure, root penetration and water and nutrient retention, thereby supporting the media structure. Minerals and carbon structures create microbial habitats in the microbiome. Hydroxyl groups enhance water retention, which shortens the time required for sprouting and increases sprout length, stem thickness and root mass. Calcite supplies calcium, aiding in the development of cell walls and enzyme function. Aromatic and hydroxyl groups can affect nutrient availability, potentially boosting the activity of hormones (IBA). The porous nature of biochar, along with its aromatic carbon and calcite content, facilitates microbial colonization (Lehmann and Joseph, 2015; Sahoo et al., 2023).
 
Key observations
 
Amorphous peak
 
The broad, intense hump around 2θ ≈ 22°-26° corresponds to disordered carbon structures, such as turbostratic carbon or amorphous graphite-like material. Sharp peaks superimposed on the pattern at various 2θ positions indicate the presence of crystalline mineral phases, which are typically derived from the inorganic components of the biomass feedstock or ash residues. Notable crystalline peaks appear between 2θ ≈ 28°-60°, which may be attributed to minerals such as: Quartz (SiO₂), Calcite (CaCO₃), Sylvite (KCl), Kaolinite or other clay minerals.
 
Interpretation
 
The presence of a dominant amorphous phase indicates that the carbon is in a stable, recalcitrant form, making the biochar effective for long-term carbon sequestration in soils. The crystalline mineral phases contribute to the nutrient value of the biochar, providing essential elements such as potassium (K), calcium (Ca) and silicon (Si), which are beneficial for plant and soil health. The combination of amorphous and crystalline components enhances the biochar’s performance as a soil amendment by improving nutrient adsorption, water retention and cation exchange capacity (CEC) (Chen et al., 2021; Sohi et al., 2016).
 
FTIR spectroscopy and determination
 
FTIR spectroscopy was employed to identify the functional groups present on the surface of the biochar sample. These functional groups play a crucial role in determining the reactivity, nutrient adsorption capacity, cation exchange behaviour and microbial interactions of the biochar when applied as a soil amendment (Fig 4) (Lehmann and Joseph, 2015; Ok et al., 2015).


       
The FTIR spectrum spans the range from 4000 to 500 cm^-1, which encompasses most of the vibrational frequencies of organic and inorganic functional groups. The spectrum reveals multiple absorption peaks, each corresponding to a specific chemical bond or molecular vibration.
 
Functional implications of identified groups
 
1. Aromatic Carbon Structures: Peaks at 1415.75 cm^-1 and 871.82 cm^-1 clearly indicate that the biochar contains highly condensed aromatic structures, derived mainly from the pyrolysis of lignin and cellulose components of plant biomass. These structures are chemically stable, making the biochar resistant to microbial decomposition and thus suitable for long-term carbon sequestration in soils (Sohi et al., 2016; Tripathi et al., 2016).
2. Hydroxyl and Carboxyl Groups (-OH, -COOH): The broad and distinct peaks in the 3600-3635 cm^-1 region denote surface hydroxyl groups, which are critical for: Water retention due to hydrogen bonding. Enhancing cation exchange capacity (CEC), allowing for better nutrient retention. Providing active sites for microbial colonization. The peak at 2357 cm^-1 may reflect carboxyl functionalities or surface adsorbed CO₂, both of which suggest that the biochar has undergone surface oxidation, increasing its reactivity and ability to form complexes with metal ions or nutrients (Lehmann and Joseph, 2015; Sahoo et al., 2023).
3. Nitrogenous and Unsaturated Groups: Peaks in the 2027-1977 cm^-1 range indicate the presence of minor nitrogenous or unsaturated compounds, such as nitriles or alkynes.  These may arise from partial decomposition of proteins or lignin derivatives and can contribute to nutrient availability or slow-release nitrogen sources in soils.

This study presents a comprehensive characterization of a biochar sample derived from guava-pruned cuttings, focusing on its physicochemical properties, structural attributes and functional implications for soil amendment. The biochar exhibited an alkaline pH, moderate electrical conductivity, high total organic carbon and a high C:N ratio, indicating a stable carbon source and a high capacity for nutrient retention. SEM analysis revealed a highly porous structure with cracks, crevices and flaky surfaces, facilitating root access, water retention and microbial colonization. XRD analysis identified calcite as the dominant mineral phase, with minor peaks indicating the presence of silica, potassium salts and ash minerals, which contribute to nutrient buffering and cation exchange capacity. FTIR spectroscopy detected functional groups, including hydroxyls, aromatic rings, aliphatic hydrocarbons and oxygenated structures, which play crucial roles in water adsorption, nutrient retention and interactions with soil microorganisms. The structural and chemical properties of biochar support its potential as a valuable soil amendment, promoting root development, microbial activity and overall soil health.

We acknowledge the Nanotechnology Research Centre (NRC), SRMIST for providing the research facilities. We would also like to express our sincere gratitude to National Agro Foundation (NAF), Taramani, Chennai, for providing necessary facilities and services; Specialized Testing Services (STS), Coimbatore, for their technical expertise and assistance; Nano Science Technology (NST), Tamil Nadu Agricultural University (TNAU), Coimbatore, for their support and contributions to this research.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.
 
Informed consent
 
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The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.