Document Type : Research Paper
Authors
Department of soil science, Collage of Agriculture, University of Kurdistan, Sanandaj, Iran
Abstract
Keywords
Main Subjects
One of the modern approaches to improving soil properties is the use of biochar. However, previous studies have reported inconsistent findings regarding its influence on phosphorus (P) dynamics in soils. Given the critical role of P availability in plant nutrition, a deeper understanding of the phosphate sorption and release in the presence of biochar is essential. This study aimed to investigate the sorption and release behavior of phosphate in a sandy loam soil amended with different types of biochar.
Biochars were produced from two regionally common biomass feedstocks, winter wheat straw (Triticum aestivum) (WS) and pruned apple tree branches (Malus domestica) (AW), pyrolyzed at two temperatures (300 °C and 550 °C). The residues were first air-dried at ambient temperature and then ground. Pyrolysis was conducted in a laboratory electric furnace at temperatures of 300 °C and 550 °C for 15 minutes under limited oxygen availability, with a heating rate of approximately 10 °C per minute. The resulting biochars were crushed and passed through a 2 mm sieve.
Two laboratory experiments were conducted: the batch sorption tests to evaluate the phosphate adsorption capacity of biochars, and the soil column experiments to assess their effects on phosphate movement and leaching. In the batch experiments, 1 g of each biochar was mixed with 50 mL of a phosphate solution at a concentration of 30.8 mg L⁻¹ in 68 mL containers. Control treatments included containers without biochar containing only the phosphate solution. The mixtures were shaken at 55 rpm for 24 hours, after which the remaining phosphate in the solution was measured using a spectrophotometer.
In the soil column experiments, four columns were filled with sandy loam soil amended with 2% (w/w) of each biochar, and a fifth column containing only soil served as the control. The columns were first saturated with deionized water for 24 hours, followed by sequential leaching with 15 pore volumes of deionized water, 15 pore volumes of phosphate solution, and finally 10 pore volumes of deionized water. Leachate samples were collected from the column outlets, filtered through 22 µm filter paper, and analyzed for phosphate concentration.
Increasing the pyrolysis temperature led to a reduction in biochar yield for both biomass types (from 1.98% to 0.58% for AW biochar and from 5.57% to –1.12% for WS biochar), as higher temperatures drive off a greater proportion of volatile compounds from the feedstock, leaving less solid residue. Furthermore, apple wood produced a higher biochar yield than wheat straw, attributable to its more stable lignin-rich structure. The pyrolysis temperature also had a pronounced effect on the pH of both biochar types, with higher temperatures resulting in increased pH values. This can be explained by the degradation of acidic functional groups such as –COOH and –OH, along with the enrichment of alkaline compounds, including carbonates, oxides, and metal hydroxides, in the biochar.
Batch experiment results revealed that increasing the pyrolysis temperature from 300 to 550 °C reduced phosphate sorption capacity. Notably, biochar derived from wheat straw at 550 °C exhibited net phosphate release rather than sorption. Column experiments showed that phosphate sorption was lower in three treatments—apple wood biochar produced at 300 °C and 550 °C, and wheat straw biochar produced at 550 °C—compared with the control column, indicating phosphate desorption or release by the biochars. In contrast, the column containing wheat straw biochar produced at 300 °C demonstrated higher phosphate sorption relative to the control.
The decline in phosphate sorption capacity at higher pyrolysis temperatures can be attributed to the destruction of surface functional groups (–COOH and –OH) that play a key role in forming surface complexes with phosphate ions at lower temperatures. Additionally, the release of organic phosphorus compounds from the biochar matrix at elevated temperatures may increase soluble phosphorus in the aqueous phase. The batch experiment findings were consistent with the soil column results, which indicated that biochars showing greater phosphate sorption capacity in the batch tests also exhibiting reduced phosphate leaching in the column studies.
This study investigated the effects of different biochars on phosphate sorption and release in soil. The results demonstrated that both the feedstock type and the pyrolysis temperature are critical factors determining the ability of biochar to sorb or release phosphate in soil. These findings highlight the importance of selecting appropriate feedstock materials and production conditions for biochar intended for agricultural and environmental applications.
Conceptualization, M.A.M. and M.D.; methodology, M.A.M., A.M. and M.D.; software, M.A.M. and A.M.; validation, M.A.M., A.M., and M.D.; formal analysis, M.A.M., A.M., and M.D.; investigation, M.A.M. and M.D.; resources, M.A.M. and A.M.; data curation, A.M.; writing—original draft preparation, M.A.M.; writing—review and editing, M.A.M.; visualization, M.A.M. and A.M.; supervision, M.A.M. and M.D.; project administration, M.A.M. and A.M.; funding acquisition, M.A.M. and A.M., All authors have read and agreed to the published version of the manuscript.
Data are available on request from the authors.
The authors would like to express their sincere appreciation to the University of Kurdistan, Sanandaj, Iran, for providing laboratory facilities and supporting the implementation of this study.
The authors avoided data fabrication, falsification, plagiarism, and misconduct.
The author declares no conflict of interest.
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