Seven‐year effects of sewage sludge biochar on soil organic carbon pools and yield: Understanding the role of biochar on carbon sequestration and productivity

Abstract The increasing need for sustainable agricultural practices and climate change mitigation has driven research into biochar's role in enhancing soil carbon (C) sequestration and fertility. This study aimed to investigate the long‐term effects of sewage sludge biochar (SSB), prepared at different temperatures (300 and 500°C), combined or not with mineral fertilizer application, on soil C stocks and organic matter fractions, addressing whether SSB can improve soil C sequestration. In the first two growing seasons (2015–2016), SSB was applied at 15 t ha −1 (dry weight) per crop and incorporated into the top 0.2 m of soil using a rotary hoe before planting. Over 7 years, soil samples were collected from the 0–0.2 m depth post‐harvest each season to analyse total C, total N, and various organic matter fractions, including easily oxidizable organic C, permanganate‐oxidizable C, non‐oxidizable C, humic substances, particulate C, and mineral‐associated C. Results demonstrate that SSB application enhances soil total C (up to 9.5%) and total N levels (up to 28.8%), indicating improved soil fertility and C sequestration potential. Notably, SSB amendments increased the non‐oxidizable organic C pool (up to 11.9%), contributing to soil organic matter stabilization. While the easily oxidizable organic C pool was increased under SSB at 300°C (up to 7.7%), the permanganate oxidizable C pool was not affected by treatments, suggesting that SSB primarily affects more recalcitrant C fractions, essential for long‐term C sequestration. Additionally, SSB application substantially increased crop yield, with higher grain yield (up to 105.0%) and shoot biomass (up to 75.1%) observed over multiple growing seasons. However, a decline in corn yield from the fourth season onwards in SSB‐only treatments highlights a limited capacity of biochar to sustain long‐term productivity. These findings underscore the effectiveness of SSB in enhancing soil C pools and its potential role in sustainable agricultural practices. Future research should focus on long‐term field studies under various environmental conditions and explore the potential of co‐pyrolysis of sewage sludge with other feedstocks to enhance C stability. The broader adoption of biochar technology could play a vital role in mitigating climate change and promoting sustainable agricultural development.