Mineralization of Sediment Organic Matter under Anoxic Conditions

Abstract

AbstractOrganic matter steadily accumulates in eutrophic lakes as a result of deposition of detrital tissue from algae and aquatic macrophytes and the slow rate of anaerobic decomposition. The rate of organic matter decomposition is affected by the nature of the sediment C (electron donor) and supply of electron acceptors. Batch incubation experiments were conducted to determine the rate and extent of organic matter decomposition in bottom sediments under anoxic conditions. Sediment samples from three distinct horizons were collected from a hypereutrophic lake (Lake Apopka, located in central Florida), which included the surface unconsolidated flocculent material (UCF), an underlying consolidated flocculent material (CF) and native peat sediment. Sediment samples were incubated in serum bottles at 15, 25, and 35°C in the dark for 534 d. Periodically, the serum bottle head space was analyzed for CO2 and CH4 produced during decomposition. At selected time intervals, the sediments were analyzed for water soluble organic C, pH, exchangeable NH4‐N, and soluble reactive P. First‐order rate constants for C mineralization (defined as the sum of gaseous C evolved and soluble C species produced) ranged from 6.7 × 10−4 d−1 for the UCF sediment incubated at 35°C to 8.2 × 10−6 d−1 for the peat sediments incubated at 15°C. Two phases of organic C decomposition were observed in the UCF and CF sediments while, decomposition in the peat sediments was characterized by only one phase. The UCF sediment, composed of recently deposited detrital organic matter, was found to be the most labile. However, under current experimental conditions, only 8.6% of the UCF sediment organic C was mineralized to CO2 and CH4. The biodegradability of sediment organic C ranked in the order UCF > CF > peat. Net mineralization of N and P was observed in all of the sediment samples (with the exception of the CF samples incubated at 15°C). The N and P mineralized during the decomposition of UCF organic matter may contribute to the nutrient load of the overlying water column.