Cover crops have been a key tool for protecting water quality in the Chesapeake Bay region and helping farmers improve soil tilth and health. Many growers would also like cover crops to contribute to yield stability and increases when possible.

However, regulating nitrogen (N) with covers has proven difficult because most species either sequester N and reduce leaching, or fix N for the following cash crop. Rarely do they excel at both.

Penn State University researchers have been studying this issue for the past few years, testing mixed-species cover-crop stands to see if they could balance the N-fixing and N-scavenging capabilities of individual species.

They tested six cover-crop monocultures and four mixtures in an organically managed maize-soybean-wheat feed-grain rotation at Penn State’s Russell E. Larson Agricultural Research Center.

For 3 years, researchers used a suite of integrated approaches to quantify soil nitrogen dynamics and measure plant nitrogen uptake.

Here are some interesting things they found:

  • All cover-crop species — including legume monocultures — reduced nitrogen leaching compared to fallow plots.
  • Cereal rye monocultures reduced nitrogen leaching by 90% relative to fallow plots. Notably, mixtures with a low-seeding rate of rye did almost as well.
  • Austrian winter-pea monocultures increased N uptake in cash crops the most, relative to fallow. Conversely, rye monocultures decreased N uptake, relative to fallow.

The research results, published this month in PLOS One, show cover-crop species selection and mixture design can substantially mitigate tradeoffs between nitrogen retention and nitrogen supply to cash crops.

“The mixtures were all very good at reducing pollution, but their impact on yields was limited,” said Jason Kaye, professor of soil biogeochemistry at Penn State. “Mixtures that performed best are the ones that had a higher proportion of legumes and mixed in a little bit of the grasses such as rye.

“Mixtures that include cereal rye dramatically decreased pollution because grasses establish root biomass that holds nitrogen and provides microbial activity in the soil.”