No. 21 Article 7/September 9, 2011

Can Nitrogen Applied in Fall Help Break Down Corn Stover?

As the growing season comes to an end and cornfields are harvested, farmers may be exploring their options for corn stover. New hybrids produce stronger stalks and relatively larger amounts of biomass, and as more corn-on-corn acres are planted and less tillage is done, corn stover has become a greater management concern over the years.

In terms of standability of the crop, stronger stalks are a desirable trait. For example, in 2009 when wet conditions prevented timely harvest in some fields, corn plants were still standing well into the winter, when they were finally harvested. Had those conditions occurred years earlier, most plants would have been lying on the ground by the time it was fit to go into the field.

While stronger stalks have benefits, the drawback is that they 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. Farmers thus may be looking for ways to reduce the amount of corn stover left in the field by planting time.

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 ratio of carbon to nitrogen. The basic concept behind applying N to corn residue is that the nitrogen makes it possible to reduce the C:N ratio and allow microbes to act on (start eating) the material more quickly.

While the concept makes sense, research at the University of Wisconsin has showed no benefit for fall application of nitrogen to increase microbial decomposition of corn residue (Table 2). The researchers observed that applying N did not change the C:N ratio. I suspect there was no change in the ratio because nitrogen can easily be washed off of residue by rain. This is important in 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 ratios are largely independent of temperature, so the fact that C:N ratios were unchanged in the Wisconsin study would indicate that the results are applicable to Illinois conditions.

Table 2. Corn stover residue remaining in April and June for fields receiving different rates and sources of nitrogen the previous fall.

Fall N (lb/acre)

% residue remaining

Apr 4, 1999

Jun 28, 1999

Apr 30, 2000

Jun 27, 2000

Apr 24, 2001

Jun 29, 2001

0

88

80

92 a

87

79

88 a

30 UAN

87

73

94 a

90

87

85 a

30 AMS

86

79

97 a

79

84

71 b

100 AMS

87

76

76 b

78

79

75 b

Adapted from L.G. Bundy and T.W. Andraski, 2002. Final Report to the Wisconsin Fertilizer Research Council. Project 175-99.
Within a column, values followed by the same letter are not statistically different.

Further, since nitrogen application made no difference in stover decomposition, the researchers also observed no difference in soil temperature due to treatment during the following spring. They also noted that applying nitrogen in the fall did not increase its availability through mineralization for the following crop compared with the untreated check. They concluded that applying nitrogen in the fall to help break down corn stover was not justified because the application did not contribute to residue breakdown and resulted in nitrogen loss.

The reason for the lack of response is that typically low temperature, and not nitrogen level, is the limiting factor for microbial decomposition of residue in the fall and early spring. I would also suspect that in dry falls, like last year and so far this year, microbial activity may be reduced not only because of the progressive decline in temperature that occurs in fall but also because of lack of moisture.

Additionally, because of some of the traits that hybrids have nowadays and because of the application of fungicides, it is common for there to be fields where plants reach the end of the season looking intact. There does not seem to be as much infection, stalk rot, or other problems as in the past. 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 the amount of pathogen is likely higher. Lower pathogen levels would also lower the chance to see greater stover decomposition as a result of added nitrogen.

Some people may argue that, in terms of potential for nitrogen loss, applying a small amount of AMS for residue breakdown in the fall differs little from applying an equivalent amount of nitrogen with diammonium phosphate (DAP). While this is conceptually true, keep the cost-benefit relationship in mind. In the case of DAP, the benefit of applying phosphorus in the fall instead of during the busy planting season, when soils also tend to be wetter and more prone to compaction, outweighs the risk of N loss from that fertilizer. In the case of AMS applications to break down residue, there is no benefit in terms of residue management and only a risk for nitrogen loss. Not only is nitrogen loss undesirable due to environmental degradation, it also reduces profitability.--Fabián G. Fernández

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