Straw keeps nitrogen down on the farm

PENN STATE (US)—When raising corn, straw left in the field after grain harvesting, along with legume cover crops, reduces nitrogen leaching into waterways, but may also lower economic return.

“In this study, three different quantities of straw residue were spread on research plots that were later planted with hairy vetch,” says Anna Starovoytov, a recent ecology master’s degree recipient at Penn State.

“A corn grain crop was later no-till planted into the vetch/straw residues. The type of residue present affected not only the magnitude of the peak of nitrogen in soil but also the timing of this peak, which is important when considering the synchrony of nitrogen availability to corn nitrogen demand.”

Agriculture is the largest source of nonpoint-source nitrogen pollution in US waterways entering the streams and rivers via erosion or leaching from the soil and minimizing it is a key environmental management goal.

Excess nitrogen in waterways leads to ecosystem degradation, including oxygen deprivation that causes fish kills and dead zones. Nitrates in drinking water are a human health concern linked to blue-baby syndrome, various cancers and birth defects.

Details of the two-year study were published in the May/June issue of Agronomy Journal.

“Legume cover crops, such as hairy vetch, have been considered as an alternative or supplement to synthetic nitrogen fertilizers that may improve the sustainability of agricultural systems,” explains Starovoytov.

“Such cover crops can contribute substantial amounts of nitrogen to subsequent crops, as well as protect soils from erosion and promote overall soil quality.

“Legumes tend to release nitrogen more slowly than synthetic fertilizers, possibly leading to more synchrony with crop demand. That does not mean that nitrogen from legumes cannot be lost from the system.”

One possible way to minimize these losses is to add crops with carbon-rich residues, such as cereal grains, into the rotation, notes Starovoytov.

Her study, under the supervision of Robert Gallagher, associate professor of cropping systems, revealed that adding straw residues to hairy vetch cover crops tended to lower soil inorganic nitrogen content compared to treatments with only legume residues. On average, soil inorganic nitrogen was 7.3 percent lower in the fields with straw residue.

However, the reduced availability of nitrogen in the soil negatively impacted corn yields, which fell 16 percent below the county average during one year of the study. Also, the straw residue left on the field is often sold, adding economic value to the overall grain crop. The study did not show definitively that retaining nitrogen with straw residue offset this loss of income.

The researchers concluded that leaving small-grain residues in the field prior to planting a hairy vetch cover crop can reduce legume nitrogen losses, but may result in reduced crop yields in some years.

“Further research is needed to help better predict legume nitrogen availability and how to best integrate legume cover crops within synthetic fertility management systems,” Starovoytov says.

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