Could cover crops replace offset in-season corn fertilizer?

Over-application of nitrogen fertilizer on crop fields often leads to increased groundwater contamination, as well as a reduction in net financial returns for farmers. With the volatile costs of fertilizer and the large input demand of corn, the optimization of nitrogen use is crucial to both environmental and economic vitality.

DWFI supported student Tauana Almeida and her DWFI Faculty Fellow advisor Andrea Basche are exploring whether nitrogen captured in the soil or biologically fixed by cover crops - plants used to cover the soil in the off-season rather than for the purpose of being harvested - can offset part of the nitrogen fertilizer that should be applied in corn fields during the growing season.

Although prior research exists about nitrogen uptake and release by cover crops, the availability of this nitrogen in-season for cash crops is not well understood and can vary significantly season to season. Almeida and Basche are quantifying the nitrogen released from cover crops decomposition, and subsequently, the amount of nitrogen taken up by corn.

The practice continues to grow around the U.S. and the globe and research could be critical to taking full advantage of cover crops and their potential to supply nitrogen, reducing the amount of nitrogen fertilizer required to be applied. Results will help enhance local and global groundwater health and support DWFI's mission of minimizing negative agricultural impacts on water quality.

As cover crops die off and decompose, they release nitrogen and other nutrients into the soil, making it available for future profitable crops. To study how much nitrogen is being released in this process, Almeida and Bache planted cover crop treatments of cereal rye; hairy vetch; and a mixture of cereal rye and hairy vetch; and compared the nitrogen generated to those fields in which no cover crops were planted in order to find a baseline of corn yield measurements.

To capture each cover crops' nitrogen release, the team sampled biomass (the organic matter above ground) from each one of the cover crop treatments at different points in the growing season.

Researchers dried the samples, measured them and then ground the residue to analyze for the total nitrogen concentration. Almeida and Basche were then able to find the decomposition rate of each cover crop and how much nitrogen was contained in the residue.

They also determined the carbon-to-nitrogen ratio of the cover crops each time the mesh bags were collected, which is important because the carbon-to-nitrogen ratio is one of the drivers of cover crop decomposition. Researchers were able to also determine the nitrogen release rate of the cover crops, which is important for nutrient timing and crop productivity. They were also able to measure how quickly the cover crops grow, and its correlation to the growth of corn.

Almeida and Basche are in the third year of field data collection and evaluation, but current findings suggest that the initial carbon-to-nitrogen ratio of the cover crops plays a significant role in their decomposition and subsequent nitrogen release, which can affect the growth and yield of the corn crop.

They have also found that hairy vetch - a cover crop and pollinator plant - has released up to 75 lbs per acre of nitrogen (or about 84 kilograms per hectare), a significant amount considering an average soybean crop is assumed to release approximately 45 pounds per acre (or about 50.5 kilograms per hectare).

"The incorporation of cover crops can offset a third of a corn field's nitrogen needs the season," said Almeida.

Almeida and Basche's work shows promise in using cover crops to reduce the amount of fertilizer farmers need to apply to corn fields and thus, reduce the potential for groundwater contamination from leaching.