Efficiency is the name of the game
Excessive and inefficient fertiliser application not only leads to increased emissions, it can also cause pollution, leading to toxicity in organisms, oxygen depletion in aquatic ecosystems, and soil and water acidification, all of which can impact biodiversity. A high-impact intervention is therefore to improve nitrogen use efficiency, applying the fertiliser so that a higher proportion of it is taken up by the crops and less escapes as nitrous oxide via direct and indirect biological processes. Applying the majority of fertiliser a few weeks after, instead of at the point of, or before, planting can reduce the amount of fertiliser lost to leakage. This creates a win-win situation for both the farmer and the environment, increasing yield and profitability while decreasing the nitrogen surplus in the soil that may be lost as emissions and leaching. The Nutrient Stewardship provides a 4R framework to support these efforts, defining the best management practices to match nutrient supply with crop requirements.
Driving innovation in nitrogen products
Improvements in nitrogen efficiency can also take place at the level of the nitrogen-based fertiliser itself. Products such as urease and nitrification inhibitors are compounds that increase the time that the active nitrogen component remains in the soil, meaning that less fertiliser needs to be applied overall. Initial research and pilot projects suggest that these interventions can reduce nitrous oxide emissions by up to 40%-60%, although their efficacy is dependent on factors such as soil type, temperature, soil water content, and the amount of inhibitor applied. Slow-release or controlled-release fertilisers represent another opportunity for emission reduction potential as they release a gradual and steady stream of nutrients over time instead of releasing them too quickly for crops to absorb all of it.
Practising no-till agriculture
This is a sure-fire way to improve soil health by diminishing soil erosion, increasing soil organic matter, and recycling crop residues, thus reducing the fertiliser needed to meet the crops' nutrient requirements. One study shows this intervention has the potential to reduce nitrous oxide emissions by up to 50%-60% (although more data is needed to understand the optimum conditions for creating emission reductions). Planting cover crops, similarly, can have multiple co-benefits. In addition to protecting soil from wind and water erosion, these crops serve as sink for plant nutrients that could otherwise be lost to volatilisation. They can also act as a source of supplementary nitrogen (e.g. legumes) and slow-release nutrients, again reducing the amount of fertiliser required.
In organic fertilisers, the nitrogen is released much more slowly into the soil, again leading to efficiency gains. This fertiliser also has a much lower production-related carbon footprint compared to its synthetic counterpart, although it is not yet clear whether the emissions profile of each fertiliser is significantly different (it may be dependent on local factors such as crop, soil type, and agronomical conditions). A 2020 review which attempted to quantify the emissions from both organic and inorganic fertiliser use found a very high variability in emissions intensities and cautioned that emissions should be calculated on a case-by-case basis where possible. Elsewhere, research comparing urea and organic fertiliser shows that the use of organic fertiliser will produce higher indirect nitrous oxide emissions, but overall lower greenhouse gases. Instead of a simple switch, farmers should therefore develop a balanced fertiliser regime that uses a combination of organic and synthetic fertiliser to improve soil quality, increase crop yield, and reduce greenhouse gas emissions.
Agriculture is one of the most difficult sectors to decarbonise and the key to reducing its footprint undoubtedly lies in the more efficient, sustainable application of fertiliser, especially given the estimated increase in global usage by over 50% by 2050. There is no silver bullet that will solve the challenge of producing more food for a larger global population at a lower environmental impact… yet collaboration between farmers and FLAG-sector companies can drive meaningful reductions today and speed progress towards a net zero future.