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<p>Low phosphorus (P) use efficiency in calcareous soils and the increasing pressure on finite mineral P resources require alternative fertilization strategies based on nutrient recycling. This study investigated iron-modified, P-loaded biochars as circular fertilizers capable of recovering phosphorus from wastewater and supplying it gradually to soil while contributing to soil carbon management. Biochars produced from four woody feedstocks using two slow-pyrolysis technologies were modified with iron to produce iron-oxides and enriched with P recovered from sludge-derived wastewater. Structural and chemical changes were assessed using FTIR, XRD, and SEM-EDS. The P-adsorption capacity was evaluated through batch experiments, while agronomic performance was examined using soil incubation and soil P desorption tests, comparing biochar treatments with mineral P fertilizers and a reference compost. Soil organic carbon dynamics were evaluated through the assessment of the total organic carbon and the KMnO4 oxidizable fraction beside to the δ¹³C natural abundance. Iron modification markedly enhanced phosphate adsorption through the formation of Fe-P inner-sphere complexes. During soil incubation, pre-loaded Fe-modified biochars showed moderate initial Olsen-P levels but maintained stable P availability over time, indicating a controlled-release behavior. Conversely, Fe-modified biochars co-applied with soluble P displayed release patterns similar to mineral fertilizers. Phosphorus desorption kinetics confirmed slower and more sustained P release from pre-loaded biochars. Biochar application increased soil total organic carbon by 10–20%, mainly within the non-labile carbon pool, with δ¹³C signatures confirming the persistence of biochar-derived carbon in soil. Pre-loaded Fe-modified biochars act as effective slow-release phosphorus fertilizers and enhance KMnO4- resistant soil carbon fraction, representing a promising circular approach for sustainable phosphorus management in P-fixing soils.</p> Graphical Abstract <p></p>

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Iron-Modified Biochar as a Carrier for Wastewater-Derived Phosphorus: Slow-Release Fertilization and Implications for Soil Carbon Persistence

  • 期刊论文

  • 发表日期：

    2026年7月18日

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  • Aleksandra Shulga,
  • Elisa Boanini,
  • Claudio Ratti,
  • David Chiaramonti,
  • David Casini,
  • Giulia Lotti,
  • Marco Grigatti

摘要

Low phosphorus (P) use efficiency in calcareous soils and the increasing pressure on finite mineral P resources require alternative fertilization strategies based on nutrient recycling. This study investigated iron-modified, P-loaded biochars as circular fertilizers capable of recovering phosphorus from wastewater and supplying it gradually to soil while contributing to soil carbon management. Biochars produced from four woody feedstocks using two slow-pyrolysis technologies were modified with iron to produce iron-oxides and enriched with P recovered from sludge-derived wastewater. Structural and chemical changes were assessed using FTIR, XRD, and SEM-EDS. The P-adsorption capacity was evaluated through batch experiments, while agronomic performance was examined using soil incubation and soil P desorption tests, comparing biochar treatments with mineral P fertilizers and a reference compost. Soil organic carbon dynamics were evaluated through the assessment of the total organic carbon and the KMnO4 oxidizable fraction beside to the δ¹³C natural abundance. Iron modification markedly enhanced phosphate adsorption through the formation of Fe-P inner-sphere complexes. During soil incubation, pre-loaded Fe-modified biochars showed moderate initial Olsen-P levels but maintained stable P availability over time, indicating a controlled-release behavior. Conversely, Fe-modified biochars co-applied with soluble P displayed release patterns similar to mineral fertilizers. Phosphorus desorption kinetics confirmed slower and more sustained P release from pre-loaded biochars. Biochar application increased soil total organic carbon by 10–20%, mainly within the non-labile carbon pool, with δ¹³C signatures confirming the persistence of biochar-derived carbon in soil. Pre-loaded Fe-modified biochars act as effective slow-release phosphorus fertilizers and enhance KMnO4- resistant soil carbon fraction, representing a promising circular approach for sustainable phosphorus management in P-fixing soils.

Graphical Abstract