Corn-soybean study: Researchers flag mixed outcomes for different crop rotation cycles

Scientists in the US are investigating questions about corn-soybean crop rotation and how different methods have varying environmental impacts, yield results, and cost implications.

As part of new research examining rotation of crops, the University of Illinois Urbana-Champaign scientists reveal the underlying mechanisms and impacts on crop productivity, planetary sustainability, and economic returns of these rotations.

Key findings include how rotation improves some sustainability metrics while compromising others.

Different crop rotation scenarios

According to the researchers, the majority of Midwestern farmers rotate corn and soybeans. However, many continuously plant corn year after year, driven by better yield and price results. 

Much foundational research has been done on the benefits of corn-soybean rotation, but US scientists wanted to unlock insights into the holistic questions that remain for farmers.

They used the sophisticated agroecosystem model ecosys to explain why corn yield is higher after soybeans at normal nitrogen fertilization rates. The ecosys model is a process-based ecosystem model that simulates the coupled cycles of water, energy, carbon, and nutrients within terrestrial ecosystems.

It also looked at how corn-soy rotation impacts soil greenhouse gas emissions and nitrogen leaching, as well as when corn-soy rotation is most economically advantageous. 

“We found that while corn-soy rotation can boost corn yields and reduce nitrogen fertilizer needs, the benefits come with nuanced environmental and soil carbon trade-offs,” says study leader Kaiyu Guan, professor in the Department of Natural Resources and Environmental Sciences at Illinois.

Higher corn harvests

The ecosys model was trained on ten years of Department of Crop Sciences field trials with varying rotations and nitrogen fertilizer rates.

The researchers were not surprised to find corn grown after soybeans consistently yielded about 6.4% more, on average, than continuous corn. This was at a standard nitrogen rate (151 kilograms of nitrogen per hectare).

They also found that lower biomass and faster breakdown of soybean residue left the soil surface more exposed when corn is planted in spring. 

Exposed soils warm more rapidly, which in turn causes soil microbes to mineralize more nitrogen from organic matter. This provides a source of available nitrogen for young corn plants, which can have the same effect as a farmer using common starter fertilizers. 

This led to greater corn yield by the season’s end but came with a downside. 

“The more nitrogen you add, the less yield benefit you get from rotation,” says the study’s first author, Ziyi Li, a research scientist at the university’s Agroecosystem Sustainability Center. “In some cases, the yield boost nearly disappears.”

Soil carbon and nitrogen loss

The fast-decomposing soybean residues led to an overall reduction in soil organic carbon compared to continuous corn. On the other hand, corn-soy rotation helped reduce nitrous oxide and ammonia emissions from soils.

Nitrogen leaching — the process where nitrogen, primarily in the form of nitrate (NO3-), is washed away from the soil by water, moving below the root zone and potentially into groundwater or surface water — was lower in soybean years due to the absence of fertilizer application. 

However, the research notes that the fact that nitrogen was still leaching from soybeans points to the importance of nitrogen mineralization from organic matter.

Leaching increased in subsequent corn years when decomposed soybean residues released nitrogen into the soil.

Prof Guan adds: “These results underscore a key trade-off. Rotation improves some sustainability metrics while compromising others, especially under typical fertilization practices.”

Using historical commodity prices, the researchers found that corn-soy rotation offered higher economic returns — up to US$458/acre (US$1,133/hectare) more than continuous corn — at low nitrogen fertilizer rates (45 pounds/acre) and under typical market conditions.

However, this advantage was significantly reduced or even reversed under high nitrogen inputs and market scenarios with elevated corn prices.

“The extent to which one cropping system outperforms the other in terms of net agronomic benefits depends on more than just corn yield and nitrogen fertilizer usage, but also needs to consider soybean yield and market-driven prices, including fertilizer, grain, and costs such as machinery,” adds Li.

“Such comprehensive economic assessments can help farmers make informed decisions about crop sequences, especially in response to market fluctuations, and inform insurance products and conservation initiatives accordingly.” 

Historically, research has pointed to the advantages of crop rotation in terms of yields, reduced fertilizer needs, and soil health. However, the new study provides “a robust scientific rationale for rotation and fills a previous knowledge gap by quantifying the agricultural outcomes of corn-soy rotation in detail.” 

The research paper is published in the international, peer-reviewed scientific journal Agriculture Ecosystems & Environment.