Optimizing nutrient management for reduced on-farm emissions
Companies striving to reduce emissions derived from mineral fertilizers should adopt fertilizer best management practices and Nutrient Use Efficiency (NUE)
Nitrogen, phosphorus, and potassium (or NPK) are fundamental nutrients for plant growth and play an essential role in agriculture. Nitrogen is the most important nutrient, as plants absorb more nitrogen than any other element.
Fertilizers are vital for global food production. For most of human history, food production was limited by the amount of reactive nutrients that were available for crops. In the 20th century, the Haber-Bosch process made this an issue of the past, allowing industry to produce its own nitrogen rather than relying on the scarce nitrogen that exists naturally within the world’s soils.
The use of mineral fertilizers has led to a boost in agricultural yields; about half of the world’s population is dependent on food produced using mineral fertilizer inputs (i), but it comes with a cost. The additions are often used inefficiently, leading to nitrogen losses via leaching, volatilization, and emissions to the atmosphere. Mineral nitrogen fertilizer use is associated with annual GHG emissions of around 0.7 billion tons of carbon dioxide equivalent (Gt CO2e), alongside other forms of nitrogen pollution (1). These emissions are often directly related to nutrients added to the soil in the form of mineral fertilizers and animal manure. Nitrous oxide (N2O) emissions from soils are responsible for about 3% of GHG emissions (2).
Maximizing crop-nitrogen uptake through improved nutrient management is an efficient and cost-effective way to mitigate GHG emissions from agriculture. Nutrient management can also help reduce methane (CH4) emissions from rice production and increase carbon sequestration in agricultural soils.
Source: Systemiq (2022) (3)
Mitigation strategies for mineral fertilizer use
Mitigation strategies focus on fertilizer best management practices and Nutrient Use Efficiency (NUE).
Best practice for fertilizer use is dependent on the agroecosystem under consideration, its management regime, and environmental factors. Thus, management plans need to reflect local conditions.
Mineral nitrogen fertilizer applications should synchronize nutrient supply with crop requirements and so maximize the share of nutrients taken up by the plant, thereby reducing nutrient losses to the environment. These practices are based on the principle of the 4R’s: right source, at the right rate, at the right time, and with the right placement. Farmers and agronomists use the 4R Framework to select practices that are most suitable for their site and crop.
The 4R Framework
Application type – right source: The best type of fertilizer will depend on crop needs. Balancing nitrogen applications with other required nutrients, including phosphorus, potassium, and sulphur is a major way of improving NUE. Not all fertilizers emit the same number of emissions, with urea-based fertilizers (the most concentrated solid nitrogen fertilizer) leading to higher N2O emissions than ammonia or nitrate fertilizers. Environmental factors such as soil conditions and climate, and management factors such as tillage, also play a role in determining the proportion of applied nitrogen lost as N2O (4).
Application rates – right rate: Appropriate nitrogen application rates are required to limit the buildup of nitrates in soil, which can accumulate when more nitrogen is applied than the crop demands at that time. Studies have shown that when an agronomic nitrogen threshold level is exceeded, N2O emissions increase dramatically. Managing nitrogen fertilizer levels is challenging because appropriate application rates will differ for each agroecosystem and growing season (i).
Application timing – right time: Applications should be avoided prior to planting and, instead, concentrated in the initial crop development phase at the time of and shortly after planting to maximize crop uptake and minimize nitrogen loss from the system.
Application placement – right placement: Greater proportions of applied nitrogen are generally lost if fertilizer is applied at the surface, although this may be in the form of ammonia-volatilization rather than N2O emissions. The optimal application depth will be influenced by the tillage regime, among other factors (i).
Another possible mitigation strategy is increasing the use of inhibitors and controlled-release fertilizers, which work by controlling the speed at which fertilizer, or a coating applied to it, dissolves in soil water. By affecting the timing of nitrogen release from fertilizer, these compounds have the potential to reduce the loss of nitrogen and thus improve nitrogen use efficiency.
Switching to organic sources of nitrogen
With synthetic fertilizers inducing N2O emissions from soils, requiring energy, and producing GHG emissions during their manufacture, another key mitigation strategy is to make better use of existing organic sources of nutrients, including animal manure, crop residues, and nitrogen-fixing crops such as legumes. Such organic nitrogen sources may also contribute to increasing carbon sequestration in soils.
Animal manure: As well as reducing mineral fertilizer requirements and the GHG emissions associated with their manufacture, using animal waste may also reduce soil N2O emissions. Research also suggests that organic manures do not cause spikes in emissions that occur with mineral fertilizers if there is heavy rainfall around the time of application (5).
Nitrogen-fixing crop rotations: Legume crops such as beans, peanuts, and soy can fix nitrogen from the air, and flourish on nitrogen-deficient soils. Using these plants as part of a crop rotation schedule will allow more biological nitrogen fixation, and further reduce nitrogen fertilizer use. While this tends to raise background emissions of N2O from the soil, it can mitigate total emissions over a longer term (accounting for reductions in N2O emissions from mineral fertilizers applied to the other crops in rotation) (6).
Spotlight on Nutrien
With a direct relationship with 500.000 growers globally, Nutrien is in a unique position to increase productivity while protecting natural resources and enhancing grower resilience. As an upstream actor in the agri-food value chain, the company approaches its downstream impact by addressing the nature pressures of greatest concern. For example, regarding marine habitat degradation in the Gulf of Mexico due to nutrient pollution in the Midwest, Nutrien's best opportunity for impact is to promote good agronomy practices that support productive, profitable and resilient farms in the Mississippi River Basin, reducing nutrient runoff at the source. To achieve its 2030 commitment to "enable growers to adopt sustainable and productive agricultural products and practices on 75 million acres globally", Nutrien provides growers with "whole-acre" solutions including environmental farm plans, leveraging 4R Nutrient Stewardship and IPM planning, conservation tillage or no-till, cover crops and crop rotation. Nutrien's Sustainable Nitrogen Outcomes program launched in 2022 offers growers a financial incentive to reduce nitrogen-based carbon emissions at the field-level. The program aims to reduce nitrogen-drive GHG emissions through farm solutions that enable growers to reduce application rates while managing for increased yields. In addition to lowering emissions, the program enables improved soil and water outcomes, increasing resilience across landscapes.
Targeted emissions sources
For input providers:
Scope 3, Category 11: Use of Sold Products
Will go down, as fertilizers are used in an optimal way by farmers, reducing N2O
Scope 1: Direct emissions from agricultural soils
Will go down, as NUE is reached and N2O is emitted at a lesser rate
For actors downstream of the farm (food processors, retailers, etc.):
Scope 3, Category 1: Purchased goods and services
Will go down, as the embedded carbon in goods purchased is lower
Improving average global nitrogen use efficiency could save 190-370 Mt CO2e in nitrous oxide emissions and 30-50 Mt of carbon dioxide in 2050, relative to a business-as-usual scenario (6).
Reduce input costs: Increasing NUE reduces farmer input costs, as fertilizers are used more sparingly.
Generate additional income: Farmers can also generate income from soil carbon sequestration, as soil health improves through the application of the 4R principles.
Avoid fines: Farmers may avoid fines due to water pollution, as fertilizers run off fields and into streams.
The investment required when implementing mitigation strategies for mineral fertilizer use is minimal; the only costs associated is if there is a need to obtain professional advice from an agronomist around implementing the 4R principles to the needs of the site and target crop.
An increase in costs will occur if switching to organic fertilizers, as they are generally more expensive than chemical fertilizers, mostly because chemical fertilizers have more concentrated levels of nutrients per weight of product than organic fertilizers do. However, organic fertilizers are becoming more cost competitive as new products enter the market.
Indicative abatement cost
Figure 1: Total abatement cost of 11 key measures that can reduce nitrogen losses from croplands to air and water
Impact beyond climate and business
The build-up of nitrogen in the environment is one of the main threats to global biodiversity. Most plants cannot tolerate synthetic fertilizers or high levels of nitrogen. Nitrogen pollution causes nitrogen-tolerant species to thrive and outcompete more sensitive wild plants and fungi. This reduces wildlife diversity and damages plant health. Excessive application of synthetic fertilizers has been shown to acidify soils too, damaging soil health and reducing its productivity. Nitrogen pollution can also impact fish and other aquatic life. In some areas, like the Gulf of Mexico, the effects of nitrogen pollution from farming are so severe, it’s resulted in a marine ‘dead zone’ the size of Wales. This comes with disastrous consequences for biodiversity and local livelihoods.
Optimizing fertilizer use would limit these negative impacts on the environment.
There are concerns that simply reducing the amount of fertilizer used will lead to a decline in production. This may not only have a negative impact on food security, but may also lead to more land use change, as farmers turn to converting land for agricultural use to compensate for the lower yields.
We should not think of reducing fertilizer use as a blanket solution for the whole world. Many poorer countries need more fertilizers; improvements in crop yields have been slow, and large yield gaps could be closed through more and better management of inputs.
Typical business profile
Implementation of a NUE program requires, at a minimum, collaboration between farmers, agronomy experts, and fertilizer and agri-business industry leaders. Companies striving to implement a reduction in fertilizer nutrient use should train their producers in appropriate nutrient use.
All those involved in the application of crop nutrients, from making the applications to making recommendations, need to consider the comprehensive impact of their practices on sustainability.
To start implementing the 4R Nutrient Stewardship, Michigan State University Nutrient Stewardship 4R Pocket Guide (9) offers the following starting points:
To determine the right source, consider:
Are the fertilizer nutrients being used (commercial or manure) available for immediate or delayed crop uptake?
Is there a combination of fertilizers that can be used best?
What nutrients are already available in the soil?
Match the amount of fertilizer applied to the crop nutrient uptake
What is the crop nutrient demand?
Perform a soil analysis (manure analysis as well, if using this as the fertilizer source) to appropriately match the amount of fertilizer needed for crops based on individual field fertility
Make sure equipment being used to spread the fertilizer or manure is calibrated properly for appropriate distribution
Consider crop yield goals
Consider the law of diminishing returns: the unit of nutrient applied = crop yield increase generated
Plan for fertilizer nutrients to be available during crop demand – many times this is close to planting
Consider the weather and seasonal conditions:
Potentially more nutrient runoff during the winter
Saturated fields are unable to retain nutrients effectively
Application of fertilizer immediately before a large rainfall could contribute to nutrient runoff
The MI EnviroImpact Tool is a decision support tool to help with short-term planning of nutrient applications
Include mitigation of potential odors, mainly with manure
Place fertilizer in the root zone, where crops can successfully access the nutrients
Consider the management practices for each field based on the following:
Distance to surface waters
Soil characteristics (can differ throughout the field), like nutrient supply capacity and the vulnerability to nutrient loss
Phosphorus or P-Index
Potentially incorporate GPS and variable rate seeding data
Buy-in from an array of internal and external stakeholders is needed to optimize the on-farm use of nutrients, for example:
Executive Management: To approve the development of a corporate-wide nutrient use efficiency strategy; approve budget for in-house agronomist to support the roll-out of the plan
Sustainability and Procurement team: To develop a roadmap for engaging and training producers on the company’s Nutrient Use Efficiency strategy
Farmers: To roll out Nutrient Use Efficiency strategies, and monitor efficacy on an ongoing basis
Key parameters to consider
The implementation of nutrient use efficiency improvements has long been used by producers to ensure cost-effectiveness of measures at farm and regional levels. However, nutrient management plans such as the 4R Nutrient Stewardship are now seen as both a cost-saving and an opportunity to reduce on-farm emissions through optimized nutrient management.
Implementation and operations tips
Achieving optimal nutrient use efficiency to reduce emissions requires ongoing management. Optimal nutrient use is likely to change based on time of year, crop, weather, and location. Ongoing collaboration between agronomists and producers is therefore crucial to ensure NUE is achieved and emissions are reduced, while yield is kept at an optimal level.
(i) Note: ibid