World Soil Day: How Can Nuclear Techniques Be the Solution to Soil Pollution and Increased Productivity?

“There are more mouths to feed worldwide than ever before, but the answer is not more fertilizer — the overuse of fertilizer is a big part of why the agriculture sector has gradually become one of the major sources of greenhouse gases over the last 70 years,” said Christoph Müller, a soil and plant expert at the Institute of Plant Ecology, Justus Liebig University Giessen in Germany and at the School of Biology and Environmental Science at University College Dublin. In 2014, the agriculture sector, including forestry and other land use, accounted for 24% of global greenhouse gas emissions, according to the Food and Agriculture Organization of the United Nations (FAO).

“We need to protect the environment while helping farmers, but to do that, we first need a detailed understanding of how fertilizers interact with soil and crops, and at what point they release greenhouse gases,” said Müller. “Nuclear techniques can help us get those details and find sustainable ways to grow more food while minimizing the environmental impact.”

As plants and soil convert fertilizer into useful nutrients, some of the by-products are greenhouse gases: carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). With the right amount of fertilizer, plants thrive and minimal greenhouse gases are released. However, when there is too much fertilizer  for plants to process and a surplus is left in the soil, it causes an exponential increase in emissions.

Müller and scientists from nine countries along with experts from the IAEA, in partnership with the FAO, are tracking isotopes to understand the link between fertilizer, crops, soil and greenhouse gas emissions (see Stable isotope techniques). These techniques are also being used as part of a Free-Air CO₂ Enrichment (FACE) experiment, which is helping scientists to study how crop quality and fertilizer needs can be affected by the higher levels of CO₂ in the atmosphere associated with climate change. The findings of their isotopic studies will be used to develop guidelines to help reduce fertilizer use in agriculture, without compromising crop quality and yield.

Their research results have already revealed ways to optimize fertilizer use on an area of over 100 hectares with pasture and rice, maize and wheat crops: greenhouse gas emissions were reduced by 50% and crop yields increased by 10%.

“We have also seen in our FACE experiment that plants are growing more, but their quality is changing,” said Müller. FACE is a large-scale climate change facility under natural conditions. The test site in Giessen, Germany is one of the longest running studies of this kind simulating the atmospheric CO₂ conditions over typical grassland expected by the middle of this century.

Plants grown in these high CO₂ conditions become tougher and their protein content drops. As cows graze on these plants, their stomachs have to work harder and they have to eat more to extract enough nutrients to produce milk. This not only jeopardizes milk production but also causes the cows to emit more methane — a greenhouse gas 34 times more potent than CO₂.

Alongside contributing to greenhouse gas emissions, excess fertilizer is often washed away into rivers and streams by rain or melting snow, ending up in the ocean and drinking water supplies.

“Agro-contaminants can make water undrinkable and harm aquatic ecosystems and biodiversity,” said Lee Heng, Head of the Soil and Water Management and Crop Nutrition Section at the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. “Fertilizer, for example, can constitute a main source of heavy metals and radionuclides, which can be taken up by plants, consumed by human and animals. The nutrients in fertilizer can also encourage algae growth, which lowers oxygen levels in water and harms fish and aquatic life.”

Fertilizers are one of several agricultural chemicals that contaminate the environment. Others include pesticides, salt from irrigation, sediments and drug residues from livestock. The use of these substances is rising as food producers seek ways to increase food production while combating the effects of climate change, said Heng.

Scientists from 15 countries are working with experts from the Joint FAO/IAEA Division to track multiple stable isotopes to analyse agro-contaminants, their origins, and movement (see Stable isotope techniques).These techniques will form a toolkit for identifying agro-contaminant sources and developing innovative sustainable practices to counteract their overuse and impact on the environment.

For over 20 years, scientists have used single isotopes to identify agro-contaminants, but using one isotope at a time does not provide enough information to distinguish between different contaminants and their distinctive isotopic signatures.

“Analysing multiple isotopes allows for a more complete picture of the relative contribution of each chemical from each source, so scientists can know which approach to take to deal with contaminants in fields and across landscapes,” Heng said.