The constant growth of the world’s population requires an increase in the pace and volume of production of agricultural food products, which, with increasing demand, have to be grown in extremely limited spaces. Due to the shortage of land, it is necessary to increase agricultural productivity in terms of yields per hectare in order to maintain sufficient global stocks of food, feed and bioenergy. Intensive crop production with economically and environmentally optimal use of fertilizers makes it possible to use the full potential of crop yields and is the most optimal option for increasing yields without resorting to extensive agriculture and expansion of arable land.
Like any human activity, intensive crop production and, in particular, the use of mineral fertilizers creates its own carbon footprint, leading to the release of large volumes of greenhouse gases. In this article, we will look at the causes of greenhouse gas emissions from fertilizers, the volume of these emissions and ways to compensate for them.
Greenhouse gas emissions produced by agriculture in different regions of the earth range from 1.5 to 50%. This is a very capacious industry in terms of greenhouse gas emissions, along with production, transport and buildings.
According to the Intergovernmental Panel on Climate Change (IPCC), agriculture accounts for more than 20% of total greenhouse gas emissions from human activities. And the percentage of these emissions is constantly growing every year. According to the latest analysis, synthetic nitrogen fertilizers alone account for about 2.4% of global greenhouse gas emissions.
Let’s start with such a fundamental concept as “soil carbon”, which occurs both during the growth and death of plant roots, and as a result of the transfer of carbon-enriched compounds from the roots into the soil layers.
Soil is the largest carbon storage. Globally, the carbon concentration in the soil is 2 times higher than in the atmosphere and 3 times higher than in living biomass (the total mass of plant and animal organisms). However, agricultural soil loses soil carbon due to decomposition and erosion, and its restoration is required to continue producing large volumes of crops. Just the same, fertilizer is used for these purposes.
For thousands of years, people have used mineral and organic fertilizers, such as manure or ground bones, to increase soil fertility. Mineral fertilizers are used to balance the gap between the constant depletion in the soil of nutrients consumed during crop cultivation.
Artificial fertilizers that appeared in the XX century significantly increased the volume of yields, allowing more products to be grown on less land. These measures contributed to the preservation of natural forests and pastures. But the consequence of the surge in the use of inorganic fertilizers was a multiple increase in greenhouse gas emissions that cause warming on the planet. According to various estimates, world agriculture has the second largest greenhouse footprint after industry, because both production and transportation and the use of mineral fertilizers entail large emissions of greenhouse gases, especially carbon dioxide (CO2) and nitrogen oxide (N2O).
On the other hand, fertilizers also increase agricultural productivity and stimulate CO2 uptake by crops. They increase yields and reduce the need for cultivation of new lands, thereby preventing greenhouse gas emissions as a result of land use change.
The total global production of mineral fertilizers is characterized by a slow but stable annual growth of 3-4%.
The production volumes of each type of fertilizer from the total output on average represent the following picture:
A significant source of greenhouse gas emissions is associated with the production of synthetic nitrogen (N) fertilizers used in crop production processes. The use of synthetic nitrogen fertilizers is recognized as the most important factor contributing to direct emissions of nitrogen oxide (N2O) from agricultural soils.
Nitrogen is one of the main nutrients needed by plants for growth. But plants cannot absorb nitrogen from the air the way they can absorb carbon dioxide or oxygen. In the early 1900s, scientists invented a process for the mass production of a nitrogen-containing compound, ammonia, which plants can absorb from the soil. Today, ammonia is the second most commonly produced chemical in the world, used in huge quantities as a very effective fertilizer.
This invention revolutionized agriculture by doubling the amount of agricultural products produced from one hectare of land. But the main problem is that ammonia must be produced under high pressure and at high temperatures, which means that it takes a lot of energy to produce it. Most of this energy comes from the burning of fossil fuels such as coal and methane, which in their carbon footprint emit carbon dioxide (carbon dioxide, CO2) – the main cause of climate change. Today, ammonia production accounts for 1 to 2% of global carbon dioxide emissions.
Next, fertilizers begin to produce greenhouse gases after entering the soil. Agricultural crops, on average, absorb only about half of the nitrogen they get from synthetic fertilizers. The remaining part is eroded by rainwater and goes into nearby reservoirs or decomposes by microbes in the soil, releasing nitrogen oxide into the atmosphere. And although nitrogen oxide accounts for only a small part of global emissions, the danger is that the greenhouse effect from one gram of nitrogen oxide is 300 times stronger than from the same amount of carbon dioxide.
Economically and environmentally optimal nitrogen application rate is about 195 kg per 1 hectare of agricultural land.
To understand the real volumes of greenhouse gas emissions in the agricultural sector, let’s take an agricultural farm with an area of about 20 hectares, on which wheat is grown with the use of fertilizers. In this case, annual CO2 emissions will be around 9.700 tons per year, which is approximately 30% of the total annual carbon footprint of the farm. For comparison, emissions from animal husbandry under these conditions will amount to about 11,500 tons per year or 36% of total emissions, and fuel combustion – 1,300 tons and 4%, respectively.
Scientists and engineers are working to transform the fertilizer production process in order to reduce the high temperatures and pressures currently required for ammonia production. The result of these works will be a reduction in the consumption of organic fuel by enterprises and, consequently, a reduction in the carbon footprint from the production of fertilizers. In parallel, the concept of transferring ammonia-producing plants to renewable energy sources or other environmentally friendly and safe sources for nature and climate is being developed. Simplification and greening of ammonia production in the future will allow private farms to independently produce ammonia fertilizers.
Also, the effective use of animal waste products can contribute to the reduction of the use of a certain amount of synthetic fertilizers. Some research laboratories are already conducting experiments to create a natural organic fertilizer. An important feature of such a fertilizer will be that it reduces emissions from the use of synthetic, as well as compensates for emissions produced in the process of animal husbandry.
Finally, in order to solve the problem of nitrogen oxide emissions, it is necessary to develop a methodology for reducing the amount of fertilizer used without sacrificing yield.
There are several ways to do this:
But in addition to the technical regulation of the use of fertilizers, it is necessary to take legislative measures to reduce the excessive use of fertilizers in order to maximize the effect to the detriment of the ecological quality of agricultural products. Some countries and regions are already starting to move in this direction. For example, Europe has already announced plans to reduce the use of fertilizers by a quarter over this decade as part of a program to increase the sustainability of local crop production.
Authors: Anna Zavaleeva, Vladislav Rossinsky