Microbial turnover of low molecular weight organic acids during leaf litter decomposition

To assess the production of water-soluble organic compounds during litter decomposition, solution was collected with small ceramic suction plates placed in a beech forest floor (i.e., between leaves) of soil columns that were irrigated periodically. Low molecular weight organic acids (LMWOA), indicative of anaerobic microbial processes, were detected in the solution; LMWOA concentrations displayed a high spatial and temporal Variation that ranged from 0 to 3.5 mM. Whereas formate and lactate were transient initial products in the forest floor solution, the concentration of acetate increased over time; maximum amounts of acetate-C approximated 25% of the total dissolved organic C. In microcosm studies, O-2 was rapidly consumed when O-2 was added to leaf litter that had been incubated anaerobically; O-2 consumption was concomitant with an increase in the production of CO2 and the disappearance of anaerobically-formed LMWOA. The rate at which acetate was consumed aerobically exceeded the rate at which it was formed anaerobically. Independent of the degree of decomposition and humification of the parent litter material, acetate was the dominant organic product when litter was subjected to long-term anaerobic incubation. Leaf litter collected in summer yielded the highest initial rate of formation of acetate, as well as the highest amount of acetate formed. More humified horizons did not display a seasonal variation and had a small, but continuous, capacity to form acetate. Methane was formed in litter microcosms only after extended incubation; the shortest lag phase before the onset of CH4 formation occurred in horizons of aged leaf litter collected in summer. These results demonstrate that (i) the forest floor harbors a high capacity to form LMWOA when anaerobic processes are initiated by rainfall, and (ii) the successful detection of LMWOA as a reactive pool of the dissolved organic C in forest floor solution is dependent on combined anaerobic-aerobic turnover dynamics, as well as on the sampling procedure utilized. (C) 1998 Elsevier Science Ltd. All rights reserved.