Nitrous oxide production kinetics during nitrate reduction in river sediments.

Area of Science:

• Environmental Science
• Geochemistry
• Microbiology

Background:

• Riparian zones are crucial for denitrifying riverine nitrogen inputs.
• Understanding nitrate reduction kinetics and nitrous oxide emissions in river sediments is vital for nutrient cycling.
• Sediment homogenization techniques can influence measured biogeochemical rates.

Purpose of the Study:

• To evaluate the impact of nitrate and carbon concentrations on nitrate reduction kinetics in river sediments.
• To assess nitrous oxide emissions from river sediments under varying conditions.
• To compare rate measurements between intact sediment slices (flow-through reactors) and homogenized slurries.

Main Methods:

• Utilized flow-through reactors (FTRs) for intact sediment slices and slurry incubations for homogenized sediments.
• Measured maximum nitrate reduction rates (R(m)) and affinity constants (K(m)) under controlled conditions.
• Quantified nitrous oxide production rates and investigated the effect of carbon (acetate) addition.

Main Results:

• Nitrate reduction rates were higher in slurry incubations compared to FTRs, attributed to carbon release upon homogenization.
• Carbon addition did not limit nitrate reduction, suggesting it's not a limiting factor in these sediments.
• Nitrous oxide production rates were low, especially in slurries and with carbon addition, representing <0.01% of nitrate reduction.
• Nitrate reduction rates exhibited seasonality, peaking in fall/winter and declining in spring/summer.

Conclusions:

• Sediment homogenization significantly enhances measured nitrate reduction rates by increasing carbon availability.
• Nitrous oxide emissions are minimal under optimal conditions (anoxia, non-limiting nitrate and carbon) in river sediments.
• Flow-through reactors provide more realistic kinetic parameters for intact river sediments compared to slurry methods.