The Science Behind Nitrate Transport

Contributed by: Michael Castellano, an associate professor in the Department of Agronomy and William T. Frankenberger Professor in Soil Science, and Matthew Helmers, a professor in the Department of Agricultural and Biosystems Engineering and the Dean’s Professor in the College of Agriculture and Life Sciences. 

Nitrate is both a critical nutrient for crop production and a substance that can impair water quality. We would like to clarify the science of when, where and how nitrate is transferred from Iowa crop fields to waterways. This science underpins strategies for Iowans to improve water quality.

Corn and soybean fields are the primary cause of nitrate in Iowa waterways. In an average year, one acre planted to corn or soybeans loses about 30 pounds of nitrogen in the form of nitrate to Iowa waterways. In 2014, Iowa planted more than 23 million acres of corn and soybeans.

Most nitrate is lost from corn and soybean fields before they are planted and start to vigorously grow. During these times, if the soil is warm and wet, microbes naturally produce nitrate from the soil. Microbial nitrate production exceeds nitrate uptake by corn and soybeans in all but two or three months of the year. Without corn or soybeans to use it, nitrate is transported by rain from soil to waterways.

In contrast to annual corn and soybeans, perennial crops, pastures and prairie lose very little nitrate. When soils are warm and wet, these perennial plants are using the nitrate produced by microbes.

Strategies that create plant demand for nitrate during times when corn and soybeans are not growing can significantly reduce nitrate loss. Cover crops are one example. Cover crops are planted primarily to reduce nitrate loss and soil erosion. In the late summer, cover crops can be aerially seeded into standing corn and soybeans. Cover crops reduce nitrate loss by taking up the nitrate and releasing it back to the soil when the crop is killed before corn and soybean planting in the spring.

Although corn receives nitrogen fertilizer and soybeans do not, nitrate loss from fields planted to both crops is similar. Why? Because most nitrate loss is due to the absence of plant nitrate uptake rather than nitrogen fertilizer inputs. That is why, although important, improved nitrogen fertilizer management is insufficient to meet Iowa’s water quality goals.

Regardless of whether it is derived from soil organic matter or fertilizer, nitrate not taken up by plants is susceptible to loss from the crop root zone when rainfall exceeds the capacity for plants to use the rain and the soil to hold the rain.

About half of Iowa corn and soybean fields have tile drainage pipes installed under the soil to manage water levels in the fields and support economically viable crop production. In corn and soybean fields with tile drains, nitrate is routed to streams. However, nitrate is also lost in fields that are not tile-drained. In these areas, nitrate can move to streams through subsurface water flow.

Strategies to reduce nitrate movement from tile-drained and non-drained crop fields have some similarities. In both cases, improved nitrogen fertilizer management and changes in land use practices can reduce the concentration of nitrate in the soil when corn and soybeans are not vigorously growing. These strategies include nitrogen fertilizer management, cover crops, diversified crop rotations and perennial vegetation such as crops dedicated to biomass energy production and pasture.

In areas where nitrate is transported through tile drainage systems, there are also edge-of-field practices including wetlands, denitrification bioreactors and saturated stream buffers that reduce nitrate after it leaves the field.

It is important to note that while nitrogen fertilizer management can reduce nitrate loss, it will not solve the problem. There is still the lack of demand for nitrate when corn and soybeans are not growing. Edge-of-field practices, cover crops and perennials will be more effective than nitrogen fertilizer management.

There is no single strategy to effectively reduce nitrate loss. Individual farms will require different combinations of nitrate strategies.

The Iowa Nutrient Reduction Strategy Nonpoint Source Science Assessment presents several scenarios of what it might take to achieve the nitrate reduction goals set by the Gulf of Mexico Hypoxia Task Force.

One thing is clear: All the scenarios require extremely high levels of implementation of a wide variety of methods. That is why all landowners in Iowa will need to engage in nutrient reduction strategies if we are to reach our water quality goals.