Which feature of wetlands enhances nitrate removal at watershed scales?

• A. Aerobic, well-drained soils.
• B. Anoxic microhabitats and carbon sources for denitrifiers.
• C. Absence of microbial activity.
• D. Nitrate removal occurs only by plant uptake.

Answer: (B)

Nitrate removal in wetlands at watershed scales mainly comes from denitrification, which needs anoxic microhabitats and carbon sources for denitrifying bacteria. In waterlogged, saturated sediments, oxygen is scarce, creating anaerobic pockets where microbes use nitrate as an electron acceptor and organic carbon as fuel, converting nitrate to nitrogen gas and releasing it to the atmosphere. Wetlands naturally provide both conditions: the saturated soils create the low-oxygen zones, and plant material plus dissolved organic carbon supply the carbon needed for denitrification. Without these conditions, nitrate would not be removed as effectively. Aerobic, well-drained soils promote nitrification rather than denitrification; absence of microbial activity would halt nitrate processing; and relying solely on plant uptake misses the majority of nitrate removal that happens through microbial denitrification in the sediments.