Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidation by MnO 2(birnessite), yielding 3-ketoglutarate, acetoacetate, and Mn II. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concentration decreases the induction period. The maximum rate ( r max) along the reaction coordinate follows a Langmuir–Hinshelwood dependence on citrate concentration. Increases in pH decrease r max and increase the induction time. Adding Mn II, Zn II, orthophosphate, or pyrophosphate at the onset of reaction decreases r max. Mn II addition eliminates the induction period, while orthophosphate and pyrophosphate addition increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidation of free citrate by surface-bound Mn III,IV, yielding Mn II and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from Mn II–citrate complexes to surface-bound Mn III,IV, generating Mn III–citrate and Mn II. Subsequent intramolecular electron transfer converts Mn III–citrate into Mn II and citrate oxidation products.