Which of the following has the greatest impact on nitrogen losses from field soils?

Tile drainage has a significant impact on nitrogen losses from field soils because it alters the hydrology of the land. When tile drainage is used, excess water is removed from the soil profile more efficiently. While this is beneficial for improving field access and reducing crop stress during wet conditions, it can lead to increased nitrogen loss through leaching and denitrification processes.

In situations where water is moved through the soil rapidly, nitrogen in the form of nitrates can dissolve in that water and be carried away from the root zone before plants have a chance to use it. This can result in both economic losses for farmers and environmental concerns, such as water quality issues in nearby streams and lakes due to nitrogen runoff.

Effective management practices can help mitigate these nitrogen losses, but the presence of tile drainage fundamentally changes the way nitrogen behaves in the soil, making it a primary factor in nitrogen loss relative to the other options listed.