Quality characterization of paddy straw biochar produced in open-kiln drum method and its potential for soil application

Introduction

Agricultural residue management, especially paddy straw, poses a major challenge across South Asia. Annually, India generates ~112-150 million tonnes of rice straw, of which a considerable portion is burned in open fields, contributing to severe air pollution, greenhouse gas emissions, and soil degradation (Gupta et al., 2021; Singh et al., 2020) [5,12] . Transforming crop residues into biochar through pyrolysis gives an environmentally sound and agronomically beneficial solution. Biochar is a stable, carbon-rich material produced by thermal decomposition of biomass under limited oxygen (Lehmann & Joseph, 2015) [7] . Biochar's structure enhances soil fertility by improving cation exchange capacity, nutrient retention, water holding capacity, and reducing soil acidity (Major et al., 2010; Mukherjee & Lal, 2013) [8,9] . Among various biomass resources, paddy straw is abundant, renewable, and easily available, making it ideal for decentralized biochar production. Due to limited access to costly pyrolysis units, farmers often use open-kiln drum methods, which are simple, lowcost, and viable at field scale (Pandit et al., 2020) [11] . However, the quality attributes of biochar produced using such traditional methods vary depending on feedstock, pyrolysis temperature, and process conditions (Cao & Harris, 2010) [1] . Hence, assessing the quality parameters of paddy straw biochar produced through field-level methods is essential for ensuring its effectiveness in soil application. This experiment emphasise on the quality characterization of paddy straw biochar produced in an open-kiln drum, evaluating its physicochemical properties and discussing its potential benefits for soil health, particularly in acidic and low-fertility conditions such as those prevalent in Eastern India.

Materials and Methods

Feedstock Collection

Paddy straw (Oryza sativa L.) was collected from rice fields, cleaned to remove soil and foreign materials, sun-dried, and stored in a dry environment until biochar production.

Biochar Production Using Open-Kiln Drum Method

The biochar was produced using a 200-L perforated metal drum to maintain controlled airflow under limited oxygen. The paddy straw was densely packed and ignited from the top to allow a downward charring process. Pyrolysis temperatures ranged between 350-450 °C monitored using a handheld infrared thermometer. Once charring reached twothirds depth, the drum was sealed to restrict oxygen entry and prevent complete combustion. The resulting char was quenched with water, dried, crushed, and sieved through a 2-mm mesh for laboratory analysis.

Biochar Characterization

The biochar samples were analyzed using established methods. pH and EC were determined in a 1:10 biocharwater suspension following Jackson (1973) [6] . Ash content, volatile matter, and fixed carbon were evaluated using proximate analysis (ASTM D1762-84). Bulk density was determined using the tapping method. Total carbon was estimated using the Walkley-Black wet oxidation method. Total nitrogen (N) was determined using the Kjeldahl method, phosphorus (P₂O₅) using the vanadomolybdate method, and potassium (K₂O), calcium (Ca), and magnesium (Mg) using flame photometry.

Results and Discussion

The physicochemical characterization of paddy straw biochar produced through the open-kiln drum method shows significant alterations in material quality compared to the original feedstock, confirming its suitability as a soil amendment. The biochar showed a strongly alkaline pH of 9.4, consistent with cereal-residue biochars produced under medium pyrolysis temperatures (350-450 °C). High pH values generally arise due to thermal degradation of acidic functional groups and concentration of basic cations such as Ca²⁺, Mg²⁺, and K⁺ (Lehmann & Joseph, 2015) [7] . Such alkalinity is particularly beneficial for ameliorating acidic soils, especially in regions where soil acidity constrains nutrient availability. Electrical conductivity (EC) measured 1.52 dS m⁻¹, indicates moderate salinity levels. This is within the acceptable threshold for agricultural application and suggests that the biochar has a substantial fraction of soluble mineral ions without posing toxicity risks. According to Chan et al. (2007) [2] , EC values between 1-3 dS m⁻¹ in biochar enhance nutrient availability while remaining safe for most crops. The bulk density of 0.28 g cm⁻³ indicate its lightweight and porous structure, which can improve soil porosity, decrease compaction, and enhance water infiltration when applied to heavy-textured soils. This finding aligns with earlier reports that low-density biochars improve soil aeration and root penetration (Downie et al., 2009) [3] . Proximate analysis showed 23.2% ash, 12.6% volatile matter, and 64.8% fixed carbon, indicating a stable carbonaceous structure with slow decomposition rates. High fixed carbon content is typical of biochars produced at midrange temperatures and contributes to long-term carbon sequestration (Spokas, 2010) [13] . The ash fraction, equivalent to 232 g kg⁻¹, is rich in mineral nutrients and contributes to the liming potential of the biochar. The total carbon content was 58.4% (584 g kg⁻¹), indicaing a strong carbonization efficiency. High carbon concentrations help enhance soil aggregation, microbial habitat formation, and overall soil organic carbon levels (Novak et al., 2009) [10] .

The nutrient analysis showed moderate concentrations of nitrogen (7.8 g kg⁻¹), available P₂O₅ (4.2 g kg⁻¹), and available K₂O (16.2 g kg⁻¹). Potassium content was especially high, typical for straw-derived biochars, and beneficial for improving crop stress resistance and enzymatic functioning. High K availability in cereal-residue biochars has been reported in similar studies (Gaskin et al., 2008) [4] . Calcium and magnesium contents, 12.8 g kg⁻¹ and 4.6 g kg⁻¹ respectively, contribute to the biochar's liming effect and enhancement of soil cation exchange capacity. Overall, the results indicate that paddy straw biochar produced through open-kiln pyrolysis holds potential for agricultural application. Its chemical characteristics suggest positive impacts on soil fertility, nutrient cycling, soil structure, and long-term carbon storage. Similar findings have been documented in studies evaluating the agronomic benefits of straw-derived biochars in tropical and subtropical soils (Yuan & Xu, 2011) [14] . The combination of alkalinity, nutrient enrichment, and high carbon stability positions this biochar as a promising amendment for sustainable soil management and climate-resilient farming.

Biochar-nutrient interactions:

Assessing the interaction of paddy straw biochar with mineral fertilizers, compost, and organic manures can help develop integrated nutrient management strategies suited for acidic and nutrient-deficient soils. 4. Impact on soil biological activity: Studies on microbial community structure, enzyme activities, and soil faunal response will provide deeper insight into how paddy straw biochar influences soil biological health. 5. Environmental and economic assessment: Life cycle assessment (LCA), carbon budgeting, and cost-benefit analysis should be conducted to evaluate the environmental benefits and economic feasibility of adopting biochar at large scale.

Development of farmer-friendly biochar units:

Engineering improvements in open-kiln systems or low-cost portable pyrolysis units can enhance efficiency, reduce emissions, and make biochar production more accessible to smallholder farmers. 7. Use in degraded land reclamation: Future studies may explore the application of paddy straw biochar for rehabilitating acidic, eroded, or metal-contaminated soils due to its liming effect and high carbon content.

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