Nutrient Transformations in Sediments as Influenced by Oxygen Supply

Abstract

AbstractThe oxidation status of sediments greatly affects biogeochemical processes. The objective of this study was to determine the influence of various levels of O2 on nutrient transformations in lake sediments. Decomposition of sediment organic matter was evaluated under varying levels of O2. Bulk sediment samples obtained from a hypereutrophic lake in central Florida were incubated in stirred microcosms at 25°C for 100 d at four O2 levels. The O2 levels were accomplished by bubbling N2 containing 0, 0.2, 2.0, and 20.0% O2 through the stirred suspensions. Redox potential (Eh) measurements indicated that the two highest O2 levels (i.e., 2.0 and 20.0% O2) resulted in oxidizing conditions (Eh > 400 mV), whereas the lowest O2 levels (0 and 0.2% O2) resulted in reduced conditions (Eh < −300 mV). First‐order rate constants calculated based on CO2 evolution were 0.00010, 0.00027, 0.00062, and 0.00068 d−1 for the 0, 0.20, 2.0, and 20.0% O2 levels, respectively. Methane was detected in the sediments incubated under 0 and 0.2% O2 level, with CO2/CH4 ratios of 3.5 and 160, respectively. Accumulation of soluble organic C (SOC) was greater under reduced conditions than oxidized conditions. Ammonium concentrations decreased rapidly in the oxidized treatments, with concurrent increases in NO3‐N concentrations, whereas under reducing conditions NH4‐N concentrations slowly increased. Rates of N mineralization were slightly higher under oxidized conditions. Although the ratio of N mineralized to CO2 evolved was higher at low O2 levels, the amount of N mineralized vs. the sum of C metabolites (CO2 + CH4 + SOC) was relatively constant under all O2 levels (6.7:1). Reducing conditions resulted in a depletion of SO2−4, whereas under oxidizing conditions SO2−4 concentrations increased, probably due to sulfide oxidation. Soluble reactive P concentrations were lower under oxidizing conditions.