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How Much N Has Been Lost This Year?

By Steve Culman, Greg LaBarge
Rain. Rain. Rain. With excessive rain, chances are good most fields across this state have lost more N than in a typical year.  But how much have they lost? Everything applied? Is all of the N fertilizer gone? Although it’s difficult to estimate, it’s very unlikely that the majority of the N applied has been lost.
Nitrogen losses in Ohio fields occur by two main pathways: denitrification and leaching. Both pathways occur with nitrate (NO3-), a form of nitrogen that is readily available for plant uptake, but also susceptible to environmental loss. Denitrification is more prominent in heavy, poorly drained soils while leaching occurs more in lighter, well drained soils. Most soils will experience some N loss through both pathways, but the proportion from the two pathways can vary dramatically between soils.
Denitrification is the gaseous loss of soil nitrate. It is a microbially-driven process that occurs to some degree throughout the growing season, but is especially problematic when soils are saturated and oxygen is depleted. Saturated soil in late spring/early summer are especially prone to N losses by denitrification since it is a time when many fields have N fertilizer applied upfront and crop uptake of N is very low. This can create an environment when soils are N saturated and denitrification is rampant. Many factors influence denitrification, but the three most important are 1) saturated, anaerobic soil; 2) quantity of nitrate present; and 3) presence of crop residue on soil surface. Soil N losses from denitrification vary greatly year to year, but can range from 2-25% in well-drained soils and from 6-55% in poorly drained soils. Note that tiled fields would generally not be considered poorly drained.
Leaching is the other main pathway of N loss. Unlike denitrification, nitrate leaching is not a microbial reaction. It is the loss of soil nitrate below the rooting zone by water. Nitrate leaching is heavily influenced by soil water flowing through the profile, which in turn depends on total rainfall and crop uptake. Like denitrification, N leaching is very common in late spring/early summer, especially with saturated soils. Tiled fields may lose 30-40% of applied fertilizer annually, but again, precipitation patterns and crop cover strongly influence losses by leaching.
Farmers can reduce chances of N losses from denitrification and leaching in a variety of ways. Perhaps the most important is timely applications of N fertilizer via side dressing. Other practices include rotating with winter wheat, cover crops or forages, use of nitrification or urease inhibitors, and use of controlled release fertilizers. Although sound N management is incredibly important for crop nutrition, some years prove to be especially challenging, as this year is demonstrating.

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