Biochemical Thresholds to Differentiate Mineralizing and Stabilizing Organic Waste Amendments for Soil Carbon Management

Sirine Ridene¹Lotfi Khiari^2*Haithem Bahri³Mohamed Annebi³Safia Menasseri⁴Rim Benjannet¹Chiraz Abbes⁵

• ¹Universite Laval, Québec City, Canada
• ²Laval University, Quebec, Canada
• ³Institut National de Recherche Agronomique de Tunis, Ariana, Tunisia
• ⁴Universite de Rennes Institut universitaire de technologie de Rennes, Rennes, France
• ⁵Universite de Carthage Faculte des Sciences de Bizerte, Bizerte, Tunisia

Identifying organic amendments that can retain stable organic carbon is essential for improving soil health and mitigating greenhouse gas emissions. This study assessed the carbon stability of 104 fertilizing residual materials (including manure) using a biochemical fractionation method (Van Soest) and a standardized 91-day incubation protocol. Two kinetic parameters, residual organic carbon and the mineralization rate constant, were derived from first-order kinetic modeling. Eight biochemical indicators were tested for their diagnostic performance using the Cate-Nelson partitioning method to distinguish between amendments with predominantly mineralizing or stabilizing behavior. Three indicators showed strong discriminatory power and consistent performance: lignin content, the Biological Stability Index, and the lignin-to-polysaccharide ratio LIC SOL+HEM+CEL. For each, a critical interval was identified beyond which amendments shifted toward carbon stabilization. These intervals ranged from 27.3 to 32.8 g lignin per 100 g dry matter, 0.77 to 0.99 g stability index per gram, and 0.38 to 0.49 for the lignin-to-polysaccharide ratio. Amendments exceeding these ranges were associated with high residual carbon content (up to 71.8 g per 100 g) and low mineralization rates (as low as 0.067 day⁻¹), indicating enhanced carbon persistence. This work led to the development of a robust diagnostic framework for classifying and recommending organic amendments based on their potential for carbon retention. The approach offers practical value for selecting materials suited to long-term soil improvement and climate mitigation strategies. Further validation under field conditions is recommended to support the implementation of this approach in sustainable agricultural systems.