When it comes to fertiliser, balance is key: if we apply the right amount at the right time, crops will grow and help feed the world's growing population; too much fertiliser, however, can be harmful to plants, pollute soil and water, and perpetuate global warming. How do we achieve this balance? For example, with the help of isotope techniques to optimise fertiliser use and combat its effects as an agro-pollutant and source of greenhouse gas emissions.
As plants and soil convert fertiliser into useful nutrients, some by-products are generated that are greenhouse gases: carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). With the right amount of fertiliser, plants grow and the amount of greenhouse gases emitted is minimal. However, when there is so much fertiliser that plants are not able to process it and there is leftover fertiliser stored in the soil, emissions increase exponentially.
Over the last decades, the agricultural sector has gradually become a major source of greenhouse gases mainly due to the massive use of fertiliser. Therefore, to protect the environment and farmers, it is necessary to understand in detail how fertilisers interact with soil and crops, and at what point they emit greenhouse gases. Nuclear techniques can be used for this.
According to data collected by the IAEA and FAGO, they have shown ways to optimise fertiliser use on an area of over 100 hectares of land used for grazing and growing rice, maize, and wheat: greenhouse gas emissions were reduced by 50 percent and crop yields increased by 10 percent. In addition, the plants grow more and their quality improves.
It has also been shown that plants grown under conditions of high CO2 concentration harden and their protein content decreases. In addition to having to put extra effort into eating these plants, cows have to consume more to get enough nutrients to produce milk. This situation not only threatens milk production but also causes cows to emit more methane, a greenhouse gas 34 times more potent than CO2.
In addition to contributing to greenhouse gas emissions, excess fertiliser often becomes an ‘agro-pollutant’, through rain or snowmelt, into rivers and streams, and from there into oceans and drinking water supplies, causing the nutrients in the fertiliser, for example, to promote algae growth, which reduces oxygen levels in the water and is harmful to fish and aquatic life.
Fertilisers are one of several chemicals used in agriculture that pollute the environment. Other examples include pesticides, salt from irrigation water, sediments, and residues of veterinary drugs. These substances are increasingly being used as food producers look for ways to increase food production while combating the effects of climate change.
What nuclear techniques are used?
Stable isotope techniques
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, resulting in different atomic weights. For example, nitrogen 15 has the same chemical behaviour as nitrogen 14, but has one more neutron, making it heavier. Scientists can use this information to track and understand how isotopes are transformed, as well as their flux pathways and their exchanges with plants, soil, and water bodies.
Scientists use nitrogen 15 and carbon 13 to track the movement and origin of nitrous oxide, methane, and carbon dioxide emissions in agriculture. Using fertilisers labelled with the isotope nitrogen 15, scientists can track the isotope and determine how efficiently crops absorb the fertiliser, as well as how much is left over. Carbon 13 is tracked to determine the movement and origin of carbon dioxide and methane.
Multi-isotope analysis
Scientists use the stable isotopes of carbon, hydrogen, nitrogen, oxygen and sulphur to track agro-pollutants, including their origin and their movement from soil to water bodies. Why use these isotopes? Because fertilisers and pesticides contain nitrogen, sulphur and carbon, elements that water, which contains isotopes of oxygen and hydrogen, dissolve and transports. The isotopes are measured simultaneously to distinguish the water cycle from the pollution cycle and to better understand the source and fate of pollutants.
While scientists have used individual isotopes to detect agro-pollutants for more than 20 years, using one isotope at a time does not provide sufficient information to distinguish between different pollutants and their characteristic isotopic signatures. Analysing multiple isotopes provides a more complete picture of the relative contribution of each chemical from each of the different sources. This allows scientists to know which approach to take to deal with contaminants present in fields and different areas.
