Could Organic Acids Be an Alternative to Sulfuric Acid for Improving the Fertilizer Value of Digestate Solid Fraction?

ABSTRACT Phosphorus (P) recovery from biowaste is essential for improving nutrient recycling in agriculture. Acidification with sulfuric acid has proven effective in increasing the P availability of digestate solid fraction (DSF), but its use poses safety and regulatory concerns, especially in organic farming. This study evaluated the potential of five organic acids (acetic, formic, lactic, citric, and boric acids) as alternatives to sulfuric acid to enhance the P fertilizer value of DSF. In the first experiment, titration curves were established to assess the pH reduction and P solubilization capacity of each acid. Sulfuric and citric acids were the most effective, achieving the lowest pH and highest water‐extractable P (WEP). In the second experiment, selected acidified DSFs were applied in rhizoboxes with maize to assess effects on plant growth, root architecture, and P dynamics. Citric acid–treated DSF showed moderate improvements in WEP and root proliferation but did not enhance P uptake or biomass compared to untreated DSF. In contrast, acetic, formic, and lactic acid treatments significantly reduced root growth and P uptake, likely due to phytotoxic effects. All organic acids showed limited pH stability, with significant pH increases and WEP reductions during storage. While sulfuric acid remained the most effective and stable treatment, citric acid emerged as the most promising organic acid for further optimization. Overall, organic acids showed limited potential as substitutes for sulfuric acid in DSF acidification, mainly because of their lower effectiveness, higher costs, and risk of phytotoxic effects.