Water Logged Soils – How Can There Be Any N Left? (2016)

Some areas in NE North Dakota and NW Minnesota were water logged from midsummer through harvest. We have had several questions from customers wondering how there can be any nitrate left in the 0-24” profile on the heavy clay soils with the excessive rainfall the area has received.

Nitrogen losses from poorly drained, fine textured, water logged soils is mostly cause by denitrification. Denitrification occurs when soils become saturated and many types of bacteria use oxygen from the nitrate (NO3) for respiration. The nitrate is transformed mostly to N2 gas which is lost to the atmosphere (see figure)

For denitrification to occur you need four things:

  • Nitrate in the soil profile
  • Warm soil temperatures
    (80-100° F is best for denitrification)
  • Water logged soil
  • Dissolved carbon (high organic
    matter soil)

Denitrification is greatest in the topsoil where the organic matter is the highest, where the soil is the warmest, most of the bacteria are located and where the highest amount of nitrate usually is. So in these water logged soils in our region, it makes sense that most of the N was denitrified from the topsoil, but there is substantial nitrate remaining in the subsoil in some fields. Denitrification of N from the subsoil is lower due to cooler temperatures, lack of bacteria and much lower organic matter. This is a good thing as it will reduce the N fertilizer needs for farmers next season. Hopefully the areas with water logged soils will have a chance to dry out this fall and winter and be able to be planted next spring.