Greenhouse gas emissions and fertilizer use

According to the Canadian Crop Nutrients Council:

“Plants are one of our greatest assets in reducing greenhouse gases (GHGs). As crops grow they absorb CO₂ (carbon dioxide) from the atmosphere to obtain the carbon they need to produce leaves, stems, roots and grain. Nutrients enhance this process by increasing crop growth and therefore the amount of atmospheric carbon absorbed and stored in plant material. When this plant material is worked into the soil it decomposes with a portion released back to the atmosphere and a portion used to build carbon-rich soil organic matter. This process not only improves our atmosphere but enhances the quality of our agricultural land by increasing soil organic matter.”

“Nitrous oxide (N₂O) is one of the primary GHGs targeted by scientists as it is estimated to have 310 times⁵ the impact of carbon dioxide. Emissions of nitrous oxide from soil are an unavoidable consequence of naturally occurring soil-microbiological processes. Nitrogen (N) in the soil, whether it is naturally occurring, derived from fertilizer, manure or crop residues, can contribute to nitrous oxide emissions." ⁶

The use of nitrogen fertilizers therefore results in both the generation and the removal of GHGs.

The Intergovernmental Panel on Climate Change (IPCC) has set an emission factor of 1.25% of the fertilizer applied being lost as N₂O. However, research from a number of sources suggests that a lower factor is appropriate: 1.0% in the FAO/IFA report on nitrogenous gas emissions from agricultural land⁷, and as low as 0.5% in some other studies⁸. It is becoming abundantly clear that emission rates are dependent on a host of regional factors. Certain management practices to reduce emissions may apply generally from one region to another.

It is well established that various agricultural management practices can be used to increase carbon sequestration in soils, thereby reducing GHG emissions. It is worth noting that CO₂ used for urea production is released in the field after hydrolysis of urea, giving no benefit to urea versus other N fertilizers in terms of CO₂ emissions from a life-cycle perspective.

There are three major situations related to nutrients that can trigger greenhouse gas emissions from agricultural lands: insufficient nutrients, unbalanced nutrient applications and excess nutrients. Each of these requires a different approach, although the latter two are closely related, and these approaches can all be summarized by the concept of Fertilizer Best Management Practices (FBMPs).

When properly used, fertilizers help plants capture more carbon than is emitted during the production, transport and application of fertilizers⁹. Fertilizers foster higher yields, and thus
increasing “the amount of carbon that is sequestered by the plant and released into the soil during growth, or when incorporating plant residues into the soil.” ¹⁰  Appropriate fertilizer use can also slow the decline of soil organic matter. In contrast, inadequate fertilization limits crop biomass production and can result in lower soil organic matter and, potentially, impaired long-term soil productivity¹¹. When nutrient-depleted land is poor in soil organic matter, the additional carbon generated through fertilizer use is particularly beneficial. Greater fertilizer use will therefore have a positive effect in extremely nutrient deficient regions like much of Africa¹².

While it is logical that excessive nutrient applications (regardless of whether the source is fertilizers) trigger losses to the environment, it is less well known that losses can also arise from unbalanced use. This is because plants can only make full use of the nutrients provided if all of their nutritional needs are met. A deficiency of any one nutrient (or other growth factor) could lead to a suboptimal uptake of the nutrients that are present. This means that losses can occur at recommended rates if other nutrients are lacking. Good nutrient management therefore requires an integrated approach.

The fertilizer industry is helping reduce emissions through improved products and Fertilizer Best Management Practices

A number of fertilizer companies produce enhanced-efficiency fertilizers (slow- and controlled-release fertilizers and stabilized N fertilizers). These products minimize the potential of nutrient losses to the environment, as compared to “reference soluble” fertilizers. Urease and nitrification inhibitors have shown good potential in increasing soil retention and plant recovery of applied fertilizer N, but less is known about their impacts on reductions in total N₂O emissions. Slow-release, controlled-release and stabilized fertilizers have been shown to enhance crop recovery and reduce losses of nitrogen via drainage or atmospheric emissions. Their benefits in reducing N₂O emissions have been explored to a lesser degree. Recent evidence suggests they can be effective in reducing short-term emissions, but the effect on long-term losses is less clear. Studies are underway to better quantify these emissions and potential benefits¹³.

The fertilizer industry fosters the use of its products within the framework of good nutrient management. Through the International Fertilizer Industry Association (IFA), the industry is currently developing a framework¹⁴ to significantly increase the elaboration of locally adapted fertilizer management practices and to facilitate the measurement of their effectiveness. Originally defined primarily in terms of optimizing agricultural production, best management practices today aim to achieve a balance of agronomic, economic, social and environmental objectives. Until recently, climate change mitigation was not directly addressed, but is more and more often the subject of specific codes of good practice. In some cases, there are tradeoffs between impacts (e.g. greenhouse gas emissions and nitrogen losses through leaching may be inversely related). Therefore, it is important that best management practices be site-specific and crop-specific in order to achieve greatest the net benefit with regard to a specific set of desired outcomes.

All Fertilizer Best Management Practices can be summarized by a simple principle: the right product(s) should be used at the right rate, time and place. This implies that manufactured fertilizers should be managed conjointly with all other sources of plant nutrients.

Right product(s): Match the fertilizer source and product to crop need and soil properties. Be aware of nutrient interactions and balance nitrogen, phosphorus, potassium and other nutrients according to soil analysis and crop needs. Balanced fertilization is one of the keys to increasing nutrient use efficiency. Adapt the form of nitrogen (speed of uptake, susceptibility to leaching or volatilization, acidification potential, etc.) to the site and crop specificities.

Right rate: Match the amount of fertilizer applied to the crop needs. Too much fertilizer leads to leaching and other losses to the environment and too little results in lower yields and crop quality and less residue to protect and improve the soil structure. Realistic yield goals, soil testing, omission plots, crop nutrient budgets, tissue testing, plant analysis, applicator calibration, variable rate technology, crop scouting, record keeping and nutrient management planning are elements of FBMPs that help determine the right rate of fertilizer to apply.

Right time: Make nutrients available when the crop needs them. Nutrients are used most efficiently when their availability is synchronized with crop demand. Application timing (pre-plant or split applications), controlled-release technologies, stabilizers and product choice are aspects of FBMPs that influence the timing of nutrient availability.

Right place: Place and keep nutrients where crops can use them. An appropriate application method is critical for efficient fertilizer use. The cultivar, cropping system and soil properties dictate the most appropriate method of application, but incorporation is usually the best option to keep nutrients in place and increase their efficiency. Conservation tillage, buffer strips, cover crops and irrigation management are other measures that will help keep fertilizer nutrients where they were placed and accessible to growing crops.

What Do Farmers Need to Adopt FBMPs?

The definition of FBMPs makes it clear that farmers must have access to a range of appropriate fertilizers and knowledge about how to adapt them to local conditions. This implies that resources should be invested in training farmers and keeping them abreast of latest developments. Studies have shown that FBMPs must be profitable (or supported by financial incentives) and culturally appropriate in order for farmers to implement them. It is therefore helpful for farmers to be involved in the development of FBMPs.

Predictable policies on climate change are vital, in addition to policies that affect the availability of fertilizer products, increase knowledge about nutrition management and strengthen farmers’ capacity to implement site-specific management. Farmers must be confident that they will receive due credit for early adoption of products and management practices that improve their performance, with regard to greenhouse gas emissions and carbon sequestration.

Where Is More Research Needed?

A wider range of enhanced fertilizer products, adapted to additional agro-climatic contexts, should be developed.

Fertilizer Best Management Practices should specifically address climate-change objectives without compromising other sustainability goals.

Few studies have simultaneously measured and analyzed the role of CO₂, N₂O and CH₄ over extended time periods. Such knowledge would provide valuable input into improved management practices.

Greater collaboration is required between agronomists and environmental scientists to consider issues such as nutrient management for biofuel crops and long-term evaluation of nutrient losses via leaching/drainage/run-off, among others.