Transformation of organic matter under anoxic conditions in soils

The transformation of organic matter under anoxic conditions is mediated by hydrolysis and fermentation processes resulting in products such as acetate and hydrogen which are then utilized by microorganisms in respiration. Respiring microorganisms employ an array of electron acceptors in soils, including nitrate, manganese(IV), iron(III), and sulfate, which are consumed depending on availability and decreasing Gibbs free energy yield. The classical view is that respiration is more rapid than fermentation and these two processes do not co-occur, however, evidence has mounted to challenge this view. In addition, it is unclear how the production of ammonium during ammonification of soil organic nitrogen is intertwined with fermentation and respiration. Accordingly, stirred-batch microcosms were incubated to quantify relevant chemical species over time (acetate, nitrate, iron(II), manganese(II), and ammonium) using native terminal electron acceptors (TEAs) and soil organic matter in four soils varying in drainage status under anoxic conditions. The net rate of acetate production in one of the moderately well-drained (Sadler) soils was 1.1 ± 0.07 μmol g⁻¹ d⁻¹, which was similar to Mn(II) accumulation rates (0.95 ± 0.3 μmol g⁻¹ d⁻¹, P = 0.57). A similar trend was observed in the well-drained (Feliciana) soil, indicating that Mn(IV) respiration and fermentation can co-occur in certain soils. In the other moderately well drained and the poorly drained soil, acetate production was suppressed due in part to elevated native nitrate levels, which raised the redox potential and acted as a competitive electron acceptor. Across all four soils, ammonification rates were positively correlated with acetate formation rates (r = 0.88, P < 0.001), suggesting the possibility of amino acid fermentation during these anoxic incubations. These results challenge the current paradigm that the fermentation step in anoxic organic matter decomposition is slow and Mn(IV) respiration is rapid, with implications for organic matter transformations and nutrient cycling.