Organic-inorganic calcium lignosulfonate compounds for soil acidity amelioration

Soil acidification is a problem widely occurring worldwide, which severely threaten food security and agricultural sustainability. Calcium lignosulfonate (CaLS), a cheap and ecofriendly compound, is used for the first time to amend acid soil by utilizing its unique organic and inorganic functional moieties simultaneously. Both column leaching and incubation experiments were conducted to investigate the comparative effects of CaLS (four rates at 5, 10, 15, 20 g kg^-1) and compared with conventional amendments, including gypsum (5 g kg^-1), lignin (5 g kg^-1), L + G (each at 5 g kg^-1), and control. The soil pH, exchangeable acidity and base cations, organic carbon, and different Al fractions were determined to unravel the ameliorative performance and mechanism of the treatments. Regardless of application modes and dosages, the results demonstrated that CaLS incorporation significantly increased soil pH, exchangeable Ca²⁺, cation exchange capacity, and organic carbon and decreased the contents of exchangeable acidity, especially exchangeable Al³⁺. The ameliorative mechanism was that amendment material led to the displacement of H⁺ and Al³⁺ off soil colloids by Ca²⁺. These released H⁺ and Al³⁺ which complexed with lignosulfonate anions into soluble organo-Al were all quickly leached from the soil column. The CaLS addition enhanced the transformation of exchangeable Al³⁺ and low-to-medium organo-Al complexes into highly stable organically bound fractions and immobilized into the soil. The complexing of CaLS functional groups with Al³⁺ impeded Al³⁺ from undergoing hydrolysis to produce more H⁺. As an environmental-friendly material, CaLS can be a promising amendment for soil acidity and Al toxicity amelioration.