Wet, poorly drained soils throughout North America and Europe are often artificially drained
with subsurface tile systems to remove excess (gravitational) water from the upper 1 to 1.2-m
soil profile. Improved crop production that often results from drainage is in large part due to
better physical conditions for field operations and a deeper unrestricted root zone for greater crop
rooting and yields. Removal of excess water by drainage lessens the potential for anaerobic
conditions and consequently reduces the potential for nitrate to be lost from the soil profile by
the process of denitrification. The combination of less N lost via denitrification and increased
transport of subsurface water results in higher nitrate concentrations in the receiving surface
water bodies. Watersheds containing similar production systems and soils without subsurface
drainage generate lower nitrate concentrations because anaerobic conditions exist more
frequently. Under anaerobic conditions, denitrification predominates resulting in nitrate losses to
the atmosphere.
Factors influencing nitrate content in subsurface waters draining from agricultural production
landscapes can be divided into two categories - - noncontrollable and controllable. Precipitation,
including variation in annual amount, temporal distribution within a year, and extreme daily
events, is a noncontrollable factor having the greatest impact on nitrate loss.
Controllable factors are those management practices that crop producers use to improve yield
and profitability of their enterprise. Time of N application, N fertilizer product, and nitrification
inhibitors play a significant role in minimizing nitrate loss, especially under wetter and warmer
fall, winter, and spring conditions (Dinnes et al., 2002).