Can fall N break down corn stover?

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It's time to determine what to do with corn stover, said Fabian Fernandez, University of Illinois Extension soil and plant fertility specialist.

"As a new growing season comes to an end, growers are considering their options to determine what to do with corn stover," he said. "Corn stover has become more of a management concern over the years as new hybrids produce stronger stalks, relatively larger amounts of biomass, more corn-on-corn acres are planted, and less tillage is done."

Stronger stalks are a desirable trait to help with standability of the crop, he said. But the drawback is that these materials are more difficult to break down in time for the following growing season. Stalks, along with other crop residues, can interfere with planting in the spring. Large amounts of crop residue left on the soil surface can also delay planting or seed emergence by keeping soils cool and wet longer into the spring.

Fernandez said a practice that is increasingly being promoted is applying nitrogen, typically urea-ammonium nitrate (UAN) or ammonium sulfate (AMS), to increase microbial activity and induce residue decomposition.

"Microbial decomposition of corn stover is typically slow because the material has a high C:N ratio," Fernandez said. "The basic concept behind application of N to the residue is that by applying N, it is possible to reduce the C:N ratio and allow microbes to act on, or start eating, the material quicker."

While this concept makes sense, Fernandez said research conducted at the University of Wisconsin showed no benefit for fall application of nitrogen to increase microbial decomposition of corn residue. They observed that applying N did not change the C:N ratio.

"I suspect there was no change in the C:N ratio because nitrogen can easily be washed off from the residue with rain," he said. "This is important when considering applicability of these results to Illinois. It is possible that cooler temperatures that occur earlier in the fall in Wisconsin may not reflect Illinois conditions. However, C:N ratio measurements are largely independent of temperature; thus, the fact that C:N ratios were not changed in the study would indicate that these results are applicable to Illinois conditions."

Since nitrogen application made no difference in stover decomposition in this study, University of Wisconsin researchers also observed no difference in soil temperature due to treatment during the following spring. They also observed that applying nitrogen in the fall did not increase nitrogen availability through mineralization for the following crop compared to the untreated check.

They concluded that applying nitrogen in the fall to aid the breakdown of corn stover was not justified because it did not contribute to residue breakdown and resulted in nitrogen loss.

Fernandez said the reason for this lack of response is that typically low temperatures, and not nitrogen levels, are the limiting factor for microbial decomposition of residue in the fall and early spring. He also believes that dry falls such as 2010 and so far for 2011, reduce microbial activity because of the progressive decline in temperature that occurs during the fall and the lack of moisture.

"Because of improved hybrids, infection, stalk rot, or other problems to the cornstalk are less frequent," he said. "Since the amount of pathogens present in intact material is probably low, I suspect that stover decomposition progresses slower in those fields than in fields where the residue is already damaged and where the amount of pathogen is likely higher. Lower pathogen levels lower the chance to see greater stover decomposition with addition of nitrogen."

Some people may argue that applying a small amount of AMS for residue breakdown in the fall is not much different than applying an equivalent amount of nitrogen with diammonium phosphate (DAP) in terms of potential for nitrogen loss.

"While this is conceptually true, an important point to keep in mind to help us understand the difference is that of cost-benefit relationship," Fernandez said. "In the case of DAP, the benefit of applying phosphorus in the fall outweighs the risk of N loss from that fertilizer."

In the case of AMS applications to breakdown residue, there is no benefit in terms of residue management and only a risk for nitrogen loss, he added.

"Nitrogen loss is not only undesirable due to environmental degradation, but it reduces profitability," he said.


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