9:15 AM - 9:30 AM
[MTT42-02] Quantifying denitrification rate across water-sediment interface in aquatic areas by using triple oxygen isotopes as tracers
Keywords:denitrification rate, sediment, triple oxygen isotopic compositions
Nitrate is one of the most important nutrients in aquatic systems, however, increasing nutrients result in eutrophication. Denitrification, the microbial reduction of nitrate (NO3–) to gaseous products (N2 and/or N2O), is an important process removing excess loaded nitrate from aquatic systems. Denitrification primarily occurs in the sediment (Seitzinger, 1988). Methods to quantifying denitrification rates at water-sediment interface has been established, such as acetylene inhibition method (Sorensen, 1978). However, the rates quantified by using the method is not accurate, sediment slurries for assay of denitrification rates probably overestimate in situ rates because denitrification is severely limited by nitrate, and shaking forces nitrate into zones where it would not normally be present (Oremland et al., 1984).
Recently, triple oxygen isotopic composition has been used to quantify nitrogen dynamics in various ecosystems. While atmospheric nitrate shows anomalous 17O-enrichment with mean 17O-excess (Δ17O) of +26‰, remineralized nitrate shows Δ17O of 0‰ (Tsunogai et al., 2016). Besides, Δ17O of nitrate is constant during bacterial processes of denitrification and assimilation on nitrate. As a result, changes in Δ17O of nitrate occur only through addition of remineralized nitrate newly produced through nitrification. Therefore, both denitrification and nitrification rates can be independently quantified by using Δ17O nitrate as tracer.
The object of this study is to apply the Δ17O method to quantify both denitrification and nitrification rates at water-sediment interface in aquatic systems. Sediment samples were collected from Lake Biwa, in summer (31st, Aug) and winter (1st, Dec) of 2020. The cores contained 2 L ~ 3 L of sediment with 1.2 L ~ 2.5 L of overlying water, and were incubated in the dark at 7 ℃ for 48 hours, after the Δ17O nitrate tracer (Δ17O ≒ +19.2‰) was added to the overlying water. Water samples were collected at 12-hour interval with originally made sampler. Nitrate Δ17O measurements were determined by converting the nitrate in each sample to N2O using the chemical method (Nakagawa et al., 2013). Purified N2O was introduced into our original gold tube unit (Komatsu et al., 2008), which was held at 780℃ for the thermal decomposition of N2O to N2 and O2. The produced O2 purified from N2 was then subjected to a continuous-flow isotope ratio mass-spectrometry system to determine the δ33 and δ34 by simultaneous monitoring of O2+ isotopologues at m/z ratios of 32, 33 and 34.
Results showed that denitrification rates were about 20.0 µmol m2 h-1 and 3.0 µmol m2 h-1 in summer and winter respectively while the removal constant was stable at around 0.01h-1 between two seasons. Denitrification rate from the entire lake sediments was determined and compared with that from the entire water column of Lake Biwa in summer, at around 77.1 µmol m2 h-1. Denitrification rates in the water-sediment interface was about a quarter of that from the water column in summer.
Recently, triple oxygen isotopic composition has been used to quantify nitrogen dynamics in various ecosystems. While atmospheric nitrate shows anomalous 17O-enrichment with mean 17O-excess (Δ17O) of +26‰, remineralized nitrate shows Δ17O of 0‰ (Tsunogai et al., 2016). Besides, Δ17O of nitrate is constant during bacterial processes of denitrification and assimilation on nitrate. As a result, changes in Δ17O of nitrate occur only through addition of remineralized nitrate newly produced through nitrification. Therefore, both denitrification and nitrification rates can be independently quantified by using Δ17O nitrate as tracer.
The object of this study is to apply the Δ17O method to quantify both denitrification and nitrification rates at water-sediment interface in aquatic systems. Sediment samples were collected from Lake Biwa, in summer (31st, Aug) and winter (1st, Dec) of 2020. The cores contained 2 L ~ 3 L of sediment with 1.2 L ~ 2.5 L of overlying water, and were incubated in the dark at 7 ℃ for 48 hours, after the Δ17O nitrate tracer (Δ17O ≒ +19.2‰) was added to the overlying water. Water samples were collected at 12-hour interval with originally made sampler. Nitrate Δ17O measurements were determined by converting the nitrate in each sample to N2O using the chemical method (Nakagawa et al., 2013). Purified N2O was introduced into our original gold tube unit (Komatsu et al., 2008), which was held at 780℃ for the thermal decomposition of N2O to N2 and O2. The produced O2 purified from N2 was then subjected to a continuous-flow isotope ratio mass-spectrometry system to determine the δ33 and δ34 by simultaneous monitoring of O2+ isotopologues at m/z ratios of 32, 33 and 34.
Results showed that denitrification rates were about 20.0 µmol m2 h-1 and 3.0 µmol m2 h-1 in summer and winter respectively while the removal constant was stable at around 0.01h-1 between two seasons. Denitrification rate from the entire lake sediments was determined and compared with that from the entire water column of Lake Biwa in summer, at around 77.1 µmol m2 h-1. Denitrification rates in the water-sediment interface was about a quarter of that from the water column in summer.