The GHG Emission Reduction Potential of Organic-Based Fertilizers
The agricultural sector is a huge contributor to global greenhouse gas (GHG) emissions. In fact, the global food system, comprising fertilizer production, food storage, and packaging, is responsible for about one-third of all human-caused GHG emissions. Most of these emissions come from agricultural production (86%), but a smaller amount (about 5%) comes from fertilizer production.
What are the most commonly used fertilizers?
Most fertilizers in use today are synthetic. Synthetic fertilizer products are derived from nitrogenous resources, potash, and phosphate (ground rock phosphate). According to the World Bank, the total amount of fertilizer consumed in the world has grown steadily from 115.6 kg per hectare in 2005 to 141.4 kg per hectare in 2012.
Organic fertilizers typically consist of peat, animal waste, agricultural waste, or sewage sludge. Peat is the most commonly used type of organic fertilizer, though it does not actually add nutritional value to the soil: instead it improves aeration and water absorption. Other organic fertilizers can be used to partly or completely replace synthetic fertilizers.
GHG Emissions from Fertilizers
While organic fertilizers usually contain fewer nutrients than synthetic fertilizers, they offer advantages in terms of reduced environmental impacts – including nitrate pollution of water bodies, soil acidification, soil contamination, changes in soil biology, and reduced greenhouse gas (GHG) emissions.
One of the reasons why the use of synthetic fertilizers produces more GHG emissions than organic fertilizers is their energy-intensive production process. In fact, 90 percent of the cost of producing ammonia, one of the main ingredients in synthetic nitrogen-based fertilizers, is the cost of natural gas. As a result, carbon dioxide, methane, and nitrous oxide are all byproducts of nitrogen fertilizer production. According to one study from the U.K., about 2.2 kg of carbon dioxide equivalent (CO2eq) are emitted for every kg of ammonium nitrate produced.
Moreover, once applied to crops, nitrogen fertilizer is converted by soil bacteria into nitrous oxide, a very potent GHG -- about 296-times the global warming potential (GWP) of carbon dioxide.
Quantifying CO2 Benefits
Determining the net CO2 benefits of organic fertilizer is difficult, mainly because of the vast number of variables in play, such as the type of fertilizer, the type of crop, and the type of soil. Studies have come to various conclusions on the CO2 benefits of organic fertilization.
One study looked at the GWP and GHG intensity benefits of substituting chemical nitrogen fertilizers with organic fertilization strategies in rice and wheat crops. They found that using a half-and-half combination of manure and nitrogen fertilizer could increase the level of carbon retained in the soil, while maintaining a similar overall GWP and GHG intensity. Interestingly, the use of straw waste on crops actually increased overall GHG emissions.
Another study used the Farm Energy Analysis Tool (FEAT) to compare energy use and GHG emissions from different crops and from the application of sustainable management practices. The results show that the integration of no tillage and a legume cover crop could reduce energy and GHG emissions from corn production by 37% and 42%, respectively.
The study also notes that crops with high nitrogen requirements have a higher GHG intensity – this is because of the extremely high GWP of nitrous oxide, which is emitted once the fertilizer is applied on crops. Any sustainable management practices or organic fertilizer application that reduces the requirement for nitrogen fertilizers will therefore have an appreciable impact on the GHG intensity of crop production.
In the same study, N2O emissions across all crop types accounted for the highest amount of GHG emissions (44% on average), followed by nitrogen production (16%). Total GHG emissions from crop inputs ranged from 847 (for soybeans) to 3,283 kg CO2e per ha per year (for corn).
The researchers also compared the GHG emissions of corn grain and silage production using three different techniques: tillage farming with synthetic fertilizer, no-till farming with synthetic fertilizer, and no-till farming with a legume cover crop to replace synthetic fertilizer use. No-till management resulted in a GHG emission reduction of 3%, while no-till with a cover crop to minimize nitrogen-based fertilizer use decreased emissions by 42%, from about 2,800 kg CO2e per ha per year down to about 1,600 kg CO2e per ha per year – a 1,200 kg CO2e per ha per year reduction.
The true emission reduction potential of organic fertilizer depends mainly on the type of organic fertilizer and the type of crop. The keys to achieving reductions in GHG emissions from fertilizer are reducing nitrous oxide emissions from the application of fertilizer and/or reducing the GHG intensity of fertilizer production.
Organic fertilizers can accomplish both: they can be produced sustainably, so that the net GHG emissions from fertilizer production are nil, and, unlike synthetic fertilizers, they can actually improve the soil’s ability to retain nitrogen and carbon.