Where does the nitrogen go: Tracing nitrogen through cover crops and soil

Commodities

Practices

One of the main reasons for the increase in cover crop use in the Upper Midwest (USA) is to improve soil health via nitrogen (N) management with a cash crop. Nutrient management is particularly relevant in dairy systems where manure application on fields risks overapplication of N. A key concern is the timing of nutrient availability following cover crop termination and establishing synchrony of plant-available N with the next cash crop. To date, ¹⁵N has been used as a tool to provide insights into N cycling with cover crops and other conservation agriculture practices. However, our knowledge of the fate of nutrients from cover crop biomass in soil organic matter (SOM) pools is still growing, especially in the SOM pools control nutrient cycling. This study seeks to determine cereal rye (CR) nitrogen partitioning in soil nitrogen pools, including mineral associated organic matter (MAOM) and particulate organic matter (POM) fractions of the SOM pool, using ¹⁵N enriched biomass as a tracer.

The objectives of this study are to

• Determine the partitioning of above-ground cereal rye cover crop N in soil N fractions
• Determine differences in cereal rye derived N as influenced by cover cropping, manure input, and their interaction.

Rye is the most commonly used cover crop, representing the dominant choice by farmers that use cover crops for soil health (CTIC and SARE 2023). Cereal rye fills a key niche in Upper Midwest cropping systems. It is winter hardy; able to germinate after corn is harvested in the late fall. The cover crop will start to grow, retaining nitrogen and keeping the soil covered, survive through the winter, then continue to grow in the spring. It is also known to be an excellent N scavenger, taking up soil N after the growing season. Rye continues to grow in the spring before the main cash crop is planted. At this point, rye is terminated and the biomass is either removed or left to decompose. Yet, despite the host of benefits to the soil, cover crops are under-represented on acres in the Upper Midwest. Farmers are aware that a rye cover crop retains nitrogen that is not immediately available to the next crop, but is that nitrogen stored in the soil, and if so, does it ever become plant-available? This project will inform folks interested in cover crop adoption about the nutrient cycling benefits of using rye in a dairy system.

Learning outcomes:

Change in Knowledge

Over the short-term farmers, crop consultants, government agencies and extension specialists will learn about the short-term fate of nitrogen after it is taken up by a cover crop. This in turn, will lead to a better understanding on how to manage N fertilizer in these systems, as N may be tied up in organic matter.

Change in Action

In the medium-term farmers, crop consultants, and agricultural professionals will utilize cover crops more frequently as they will better understand the pros and cons. They will also manage their N inputs more efficiently in these systems, stabilizing overall yield and profitability.

Chance in Condition

In the long-term, farmers will become more economically efficient while soil health and water quality improves in the region. The general public benefits from improved water quality.