MSU Extension

Improving the management of nitrogen fertilizer for field crops can improve nitrogen use efficiency (saving farmers money) and reduce nitrous oxide emissions (helping the climate).

What is nitrous oxide and why is it important?

Nitrous oxide (N₂O) is an important greenhouse gas that contributes to climate change. Because it has a long atmospheric lifetime (over 100 years) and is about 300 times better at trapping heat than is carbon dioxide, even small emissions of N₂ O affect the climate.

Nitrous oxide is produced by microbes in almost all soils. In agriculture, N₂O is emitted mainly from fertilized soils and animal wastes—wherever nitrogen (N) is readily available. In the United States, agriculture accounts for approximately 8 percent of all greenhouse gas emissions but contributes about 75 percent of all N₂O emissions linked to human activity. Of the three major greenhouse gases emitted naturally— carbon dioxide, methane and N₂O—N₂O is the most important in all field crops but rice.

This fact sheet explains how better management of N fertilizer can reduce N₂O emissions from crop fields.

Automated greenhouse gas sampling chambers in a wheat field on the KBS Long-term Ecological Research site. These chambers measure nitrous oxide, carbon dioxide and methane emissions multiple times every day throughout the year, allowing researchers to accurately estimate greenhouse gas emissions. Photo: J.E.Doll, Michigan State University

How does nitrogen fertilizer increase nitrous oxide emissions?

Farmers add new N to fields either as synthetic fertilizers such as urea or anhydrous ammonia, or as organic fertilizers such as manure. Most synthetic fertilizer N is readily available for uptake by plants; most of the N in organic fertilizer must be converted to inorganic N before its N is available for uptake. When not taken up by plants, most fertilizer N is mobile, hard to contain in the field and susceptible to loss. Nitrogen from fertilizer can be lost as nitrate to groundwater or as the gases N₂O, dinitrogen (N₂) or ammonia. Typically only about half of the fertilizer N applied to a crop is taken up by the crop during that growing season (Figure 1).

Nitrogen applied in excess of crop needs is particularly susceptible to loss. Though the amounts of carbon and oxygen available in soil also affect microbial N₂ O production, the presence of inorganic N usually matters most.

How can nitrogen fertilizer management decrease nitrous oxide emissions?

Because of the strong link between inorganic N in the soil and N₂O production, some emissions are unavoidable. But management that prevents the buildup of inorganic N reduces N₂O emissions. Numerous management strategies can keep soil N in check and minimize N₂O emissions. Many of these strategies also help to keep other forms of N from being lost, including nitrate and ammonia. In general, practices that reduce N₂O emissions increase N use efficiency (NUE), which keeps more of the added N in the crop.

The four main management factors that help reduce N₂O emissions from applied N fertilizer are commonly known as the 4R’s:

• Right N application rate
• Right formulation (fertilizer type)
• Right timing of application
• Right placement

Figure 1: This simplified nitrogen (N) cycle shows the typical fate of 100 pounds of N fertilizer applied to a corn field. The exact amounts vary with soil type, weather and crop. (Source: Ecologically Based Farming Systems, 20076 .)

Matching nitrogen fertilizer application rate to crop requirement

Nitrogen availability — the amount of inorganic N in soil at any given time — is the single best predictor of N₂O fluxes in cropped ecosystems. Michigan State University researchers have shown that N₂O emissions are especially high when N fertilizer is applied at rates greater than crop need. The emission rate grows exponentially with increases in fertilizer rate, so at higher rates of fertilizer application N₂O emissions increase disproportionately, particularly after crop N demands are met.

Recent fertilizer recommendations for Michigan corn crops provide farmers an improved capacity to predict crop N needs. These recommendations are based on dozens of field fertilizer response trials that define the maximum return to N rate (MRTN), which is the rate at which adding any additional N is not repaid by higher yields. This rate is typically a bit lower than the agronomically optimum N rate (AONR: the maximum level to which crops respond) by a margin that depends on the price of fertilizer vs. the price of grain. Typically, using the MRTN approach rather than the older yield-goal approach allows farmers to realize N fertilizer savings. Because both N₂O emissions and nitrate leaching increase exponentially when N fertilizer exceeds crop N demand, these N savings also can result in substantially lower losses of N₂O and nitrate.

Better estimating the amount of fertilizer N needed by a crop is an effective way to reduce N₂O emitted from cropped fields.

Improving nitrogen fertilizer formulation

Fertilizer formulations also can alter N₂O emissions in some cropping systems. For example, in corn-soybean rotations, emissions can be two to four times greater following anhydrous ammonia than following urea ammonium nitrate or broadcast urea. The trend toward using more urea in corn in the United States may help reduce N₂O emissions.

Fertilizer additives can also reduce N₂O emissions. Nitrification inhibitors such as nitrapyrin, which delay the microbial transformation of soil ammonium to nitrate, can delay the formation of nitrate until closer to the time that plants can use it. Likewise, urease inhibitors can delay urea fertilizer’s dissolving in soil water. Slow-release formulations such as polymer coatings can have the same effect. For example, in irrigated no-till corn, N₂O emissions can be reduced by using polymer-coated urea or a combined nitrification and urease inhibitor with urea ammonium nitrate, compared with using either urea or urea ammonium nitrate alone. As yet, however, there have been too few field studies to fully judge the benefit of additives or fertilizer formulations for N₂O emissions.

Improving nitrogen fertilizer timing

Applying N fertilizer when it is most needed by plants can also help reduce N₂O emissions. Applying the majority of fertilizer a few weeks after planting rather than at or before planting increases the likelihood that the N will end up in the crop rather than be lost to groundwater or the atmosphere. Sidedressing N fertilizer at the V-6 stage in corn, for example, can increase N use efficiency— especially if application is preceded by a pre-sidedress-nitrate test (PSNT) to allow residual N to be taken into account.

Adding N fertilizer in the fall or spreading manure on frozen fields often leads to especially large nitrate and N₂O losses. In such cases, fertilizer applications are way out of sync with the timing of crop needs.

Improving nitrogen fertilizer placement

Placing N fertilizer close to plant roots also can reduce N₂O emissions. For example, applying urea in narrow bands next to the plants rather than broadcasting across the field can reduce N₂O emissions. Likewise, emissions are lower when canola and wheat are side-banded rather than banded midrow. In corn, shallow rather than deep placement of ammonium nitrate or anhydrous ammonia has led to reduced N₂ O emissions.

Precision fertilizer application can also improve NUE by tailoring N application to soil spatial variability. Adding less N to those parts of a field with low yield potential, as measured by yield monitoring, will avoid wasting N on locations in the field that are not as likely to respond to N fertilizer. In one study, precision fertilizer application reduced the average N fertilizer rate by 22 lb N per acre (25 kg N per hectare), substantially reducing N₂O emissions.

Figure 2: Data from Michigan corn fields12 showing how nitrous oxide (N2 O) emissions increase exponentially with increasing fertilizer N rate. By more precisely estimating crop fertilizer N needs, farmers can greatly reduce N2 O emissions from their field

How can we best reduce nitrous oxide emissions from field crop agriculture?

An integrated approach is best suited to reduce N₂ O emissions from field crop agriculture. The same principles of N fertilizer best management practices for increased NUE hold true for reducing emissions:

• Apply fertilizer at the economically optimum rate
• Use an appropriate fertilizer formulation
• Apply as close to the time of crop need as possible
• Apply as close to the crops root zone as possible

Following these practices will, in general, result in more N in the crop and less lost to the environment.

Earning Carbon Credits for Nitrous Oxide Reductions

As previously mentioned, even small amounts of N₂O in the atmosphere can greatly affect the climate. Because of this, there is great interest in reducing emissions of N₂O from various economic sectors, including field crop agriculture. By using the N management practices described in this bulletin, farmers can reduce N₂O emissions from their fields without reducing crop yield or economic return. This is the basis for programs offered through carbon credit organizations in the United States that use the marketplace to pay farmers for these reductions.

Aerial view of the KBS Long-term Ecological Research experiment showing corn’s response to varying levels of nitrogen fertilizer rates. Data from this and other experiments across Michigan showed how nitrogen rates can be reduced, resulting in lower nitrous oxide emissions without harming crop yield. Photo: K.Stepnitz, Michigan State University

Most straightforward and accessible programs use a methodology that estimates N₂O emissions reductions on the basis of the reduction of N fertilizer rate. This methodology is based on data collected on commercial Michigan farms and was developed primarily by Michigan State University scientists. It allows farmers to convert their N₂O emissions reductions to equivalent units of carbon dioxide. These can then be traded as carbon credits on environmental markets to generate income.

Reductions in N fertilizer input without crop yield loss can best be achieved through the use of an integrated approach that uses corn and fertilizer prices to estimate recommended N rates, and improves management of the formulation, timing and placement of N fertilizer.

These changes in management practice, in combination with programs that pay for the environmental benefits they deliver, help to ensure the long-term sustainability of field crop agriculture, N use, and a stable climate.

Table 1. Proposed and potential nitrous oxide (N2O) mitigation technologies and practices for croplands. Adapted from Cavigelli et al., 201224.

Notes: The effectiveness of many mitigation options is influenced by soil type and climate, and there are major uncertainties about the effectiveness of most mitigation strategies.