MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Drought and hail storms have inflicted low and variable corn yields on farmers throughout Michigan. This brings up the perennial question, what happens to nitrogen after corn is harvested? Significant amounts of nitrogen may remain behind in a low production situation. Understanding the fate of nitrogen from fertilizer, manure, crop residues and from the soil is challenging. The complexity of the nitrogen cycle is an active area of research where new lessons are being learned all the time and added to proven knowledge.

Research findings are highlighted here from a 15 year corn-based rotation trial located at MSU’s Kellogg Biological Station, in southwest Michigan. The bottom line from this long-term trial is that nitrogen is a moving target. Nitrogen credits need to be adjusted over time, depending on proven yields, on management objectives, and on the history of organic matter amendments such as cover crops, manure and crop residues.

Loss pathways

Nitrogen remaining in the soil after a crop is harvested can be lost or captured. The primary loss pathway is by leaching, although denitrification and volatilization are sizeable loss pathways in specific situations.

Leaching happens when excess rainfall or irrigation causes rapid movement of water below the rooting zone. Nitrate is the main inorganic form of nitrogen that moves with water and is leached. The ammonium ion is quite different than the negative ion nitrate; it is a positively charged form of nitrogen that it is held tightly in soil by negatively charged organic matter and clay particles. This tightly held nitrogen, in the form of ammonium, is much less likely to be leached than nitrate. It is important to bear in mind that nitrogen is readily transformed from ammonium to nitrate by soil microorganisms through the nitrification pathway.

Denitrification is the process by which nitrate ions are transformed into gaseous forms of nitrogen by anaerobic microorganisms. This occurs generally under waterlogged soil conditions, and the gaseous forms of nitrogen produced are subsequently lost to the atmosphere. In specific sites in the soil, such as within the center of soil aggregates, denitrification can also occur even when the soil is not flooded. For substantial losses to occur, however, nitrogen application must be in excess of plant demand. If excess nitrate is in the soil, it is vulnerable to being denitrified whenever the conditions are right. Matching soil supply and plant demand for nitrogen, by using the proven yield for a given field to evaluate the amount of nitrogen to apply, is the foundation to reducing losses from denitrification and leaching.

Volatilization of ammonia is another pathway of gaseous loss of nitrogen from the soil, often when manure or urea-containing fertilizers are left on the surface and not incorporated. To minimize volatilization and loss of nitrogen from this pathway, fertilizer applied as urea or as manure should be incorporated into the soil.

Improving nitrogen efficiency

Limiting the amount of inorganic nitrogen that is available to “leak” from the system is the key to limiting losses from leaching or denitrification. This poses a challenge, as nitrogen availability must also be sufficient to support optimum yields. A slow release source of nitrogen is an ideal way to improve both yields and limit losses. Slow release fertilizers are available, but can be quite expensive. Cover crops and manure can act as a form of slow release fertilizer, and have the advantage of improving the soil’s capacity to release nitrogen in a “just in time” fashion over time. Results from a long-term trial at Kellogg Biological Station are being used to reevaluate nitrogen credits for cover crops and composted dairy manure. Current recommendations are to use a nitrogen credit of 30 to 50 percent of the nitrogen applied in an organic form; yet our findings indicate this may be a significant underestimate of the nitrogen that is available over the long-term, particularly from manure applied in combination with cover crops. Improving our ability to estimate nitrogen contributions from organic inputs is our goal. This will help improve nitrogen efficiency while maintaining profitable yields.

Recycle nitrogen. Growing a cover crop or a forage crop is another way to improve nitrogen efficiency. The nitrogen remaining in the soil after harvest, especially following a poor growing season, is susceptible to leaching during the next winter and spring, however, it can be recycled. A winter cover crop can capture and release this nitrogen, thereby reducing the rate of nitrogen fertilizer needed on that field next year. Note: it is important to manage this recycled nitrogen so that residues are incorporated or killed with an herbicide, so that the residues decompose completely before a crop is grown. Residues provide a mineralizable, slow release nitrogen form, supported by microorganisms activity, to make nitrogen available, and support growth of the next crop.

Check your variety and your soil. There is some evidence that new crop genetics may have changed nitrogen efficiency compared to some historic varieties. In the past, some varieties may not have been able to use nitrogen as effectively to produce grain, compared to today’s high yielding hybrids. The take home message is that both plant nitrogen demand, and soil sources of nitrogen, are moving targets and require careful consideration in this time of increasing fertilizer costs. A presidedress soil nitrate test PSNT measurement is one of the best ways to evaluate your soil nitrogen supply capacity, which may be changing over time. Check with the MSU Soil and Plant Nutrient Laboratory to learn more about PSNT, see: http://www.css.msu.edu/SoilTesting.cfm.

Dr. Snapp's work is funded in part by MSU's AgBioResearch.