Evaluating Nitrogen Loss After Heavy Rainfall

2016

  • Carrie Laboski
  • UW-Madison Soil Science
Project Media

The purpose of this paper is to explain how to evaluate the potential for N loss after heavy rainfall and determine corrective measures that may be taken.

Denitrification

Denitrification is the process whereby nitrate is converted to the gases dinitrogen or nitrous oxide and subsequently released to the atmosphere. This conversion is carried out by soil bacteria. Denitrification can be a significant mechanism for N loss on medium- and fine-textured soil. It is generally not an issue on coarse-textured soils because they do not remain saturated for any length of time. There are several environmental factors that determine if denitrification occurs and to what extent.

1. Nitrate. Nitrate must be present for denitrification to occur. If nitrate is not present or is in low concentrations, denitrifiaction losses will be minimal.

2. Soil water content and aeration. Denitrification occurs in wet soils with low oxygen concentrations. Denitrification increase with the length of time the soil is saturated. Standing water may result in a greater percentage of nitrate being denitrified.

3. Temperature. Denitrification proceeds faster on warmer soils, particularly when soil temperature is greater than 75°F.

4. Organic matter. Denitrification occurs because soil bacteria are breaking down organic matter under low oxygen conditions and the bacteria use nitrate in a biochemical process. Soils with low soluble organic carbon will have less potential for denitrification than soils with high soluble organic carbon. Thus, nitrate that resides deeper in the soil profile (e.g., below 12 inches) where there is less organic matter will have a greatly reduced or minimal probability of being denitrified.

5. Soil pH. Denitrification is negligible in soils with a pH < 5.0. Thus, pH likely does not limit denitrification on most of our cropland in Wisconsin.