Citrate vs. malate: an assessment of plant cost-adjusted micronutrient solubilisation efficiency in three soils

Abstract Background and aims Solubilisation of poorly available micronutrients in the rhizosphere by root-derived carboxylates is crucial for plant uptake. Citrate and malate are especially important because they can solubilise Fe, Zn, Mn, and Cu. We quantified micronutrient solubilisation by citrate and malate at high but realistic rhizosphere concentrations and tested whether citrate—despite its higher metabolic cost—provides greater plant-cost–adjusted solubilisation efficiency (PACE) than malate. Methods Batch extractions were performed on three soils using 500 µmol L⁻ 1 citrate, malate, or their combination in 10 mmol L⁻ 1 KNO₃ or NaN₃ (biocide). Controls contained only water or KNO₃/NaN₃. Carboxylates were measured by HPLC–DAD, total soil elements by XRF, and solubilised micronutrients by ICP–OES. We then developed and applied a new framework to calculate PACE, linking solubilisation outcomes to plant metabolic investment. Results Citrate and malate solubilised only per-mille fractions of total soil Fe, Zn, Mn, and Cu, with soil organic matter strongly influencing release. Citrate generally solubilised more micronutrients than malate, reflecting its higher complexation capacity, particularly when microbial degradation was prevented by NaN₃. When normalized to plant energetic and carbon costs, citrate remained the most efficient ligand for all four micronutrients—as long as it was not rapidly degraded. In one soil where citrate was fully decomposed, its advantage disappeared, showing that microbial turnover can negate its effectiveness. Conclusion Overall, both soil properties and microbial degradation determine whether a carboxylate benefits micronutrient acquisition. PACE provides the first quantitative framework linking carboxylate efficiency to metabolic cost.