Where are most nitrogen losses observed in agricultural fields?

• A. Evaporation from soil
• B. Tile drainage system
• C. Surface runoff
• D. Plant uptake inefficiencies

Answer: (B)

Most nitrogen losses from agricultural fields occur with drainage water, especially in soils that are drained with subsurface tile systems. When fertilizer or soil organic matter mineralizes, nitrogen often becomes nitrate, which is highly soluble and moves readily with water. In fields with tile drainage, this nitrate-laden water is quickly removed through the tiles and exported to waterways, making the tile drainage system the dominant pathway for nitrogen loss. Evaporation from the soil and surface runoff can carry some nitrogen, but they generally remove far less nitrate than the rapid, subsurface drainage. Plant uptake inefficiencies aren’t a loss pathway in themselves; they reflect how much nitrogen isn’t used by the crop, whereas the observed field losses are the water-borne nitrate leaving via the drainage network.