What conditions promote denitrification in wetlands, and why is this process significant at watershed scale?

• A. Aerobic, carbon-poor soils with rapid drainage promote denitrification.
• B. Anoxic, carbon-rich, submerged soils with available nitrate promote denitrification; wetlands remove substantial fractions of watershed nitrate.
• C. Dry, exposed soils maximize denitrification.
• D. Cold, leaf-litter–free soils promote denitrification.

Answer: (B)

Denitrification in wetlands happens when the environment becomes oxygen-poor, with plenty of organic carbon and available nitrate. In submerged or waterlogged soils, oxygen is depleted, and denitrifying bacteria use nitrate as the electron acceptor, converting it to nitrogen gas (mostly) or some nitrous oxide. The organic carbon supplies the energy for these microbes, so carbon-rich conditions with anoxic soils maximize the process. Under these conditions, nitrate introduced from upstream can be reduced and removed from the water column as gas, effectively pulling nitrogen out of the watershed system.

This is significant at watershed scale because wetlands act as widespread, natural buffers that intercept runoff before it reaches streams and rivers. When the wetlands have the right combination of saturation, carbon, and nitrate, they can remove substantial fractions of watershed nitrate, helping to reduce downstream nutrient loading, eutrophication, and related water-quality problems.

Other scenarios don’t fit as well because aerobic, carbon-poor soils with rapid drainage stay oxic and can't support denitrification; dry exposed soils lack the saturated conditions needed; and cold, carbon-poor soils limit microbial activity and energy available for the denitrification process.