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Long-term experiments show low levels of uranium in soil supplied with phosphorus fertilizers

A research group with the participation of Aarhus University has, with the help of the long-term fertilizer experiment at Askov Research Station and two field trials in Germany, shown that a low and harmless concentration of uranium in agricultural soil can be maintained, but only if phosphorus fertilizers from raw phosphate with a low content of uranium are used.

2020.04.18 | Camilla Brodam

Photo: AU Askov

Phosphorus fertilizers are produced from phosphate-containing rocks, commonly referred to as crude phosphate. Crude phosphate contains varying amounts of uranium, with crude phosphate of volcanic origin generally having a low uranium content. In contrast, crude phosphate from sediments on the seabed or seabed (sediments) has a very variable, but usually substantially higher uranium content. When the raw phosphate is processed into fertilizer, most of the uranium enters the finished phosphorus fertilizer that is released into the fields. In general, the accumulation of heavy metals, and thus also uranium, in cultivated soil is undesirable as it represents an irreversible deterioration in the quality of the soil. But what happens to the released uranium? It has a team of researchers with the participation of the Institute of Agroecology investigated by the involvement of soil samples taken regularly during the period 1923 to 2016 in the long-term fertilizer experiment at Askov Research Station.

Risk assessment of uranium in European agricultural soil

The uranium supplied is expected to be retained in the plow layer, where it binds to clay minerals and organic matter. The crop does not absorb uranium from the soil, but soil that hangs on the roots and stems of vegetables can become part of the food we eat. In addition, uranium may also be transported to groundwater. If the amount of uranium in the soil is high, it could potentially pose a health threat.

"Unlike a number of other heavy metals, there are currently no restrictions on the amount of uranium in fertilizers, and the new revised EU fertilizer regulations also do not provide limit values ​​for uranium," says Professor Bent Tolstrup Christensen of the Department of Agroecology at Aarhus University.

"But we clearly need better knowledge about the accumulation of uranium in cultivated soil under European conditions and thus also in Danish agricultural land."

Long term fertilizer experiments were the key to new knowledge

In total, three different lengthy trials have been under the researchers' eye. In addition to the experiment in Askov, which was built in 1894, two German field trials were involved, one in the Rengen plant in 1941 and one in Thyrow in 1937. The annual change in the soil content of uranium is very small. Therefore, in order to quantify the change, it is essential to have access to land where a known fertilizer supply has been maintained over a very long period of years.

The experiment in Rengen has been supplied with phosphorus with Thomas slag (a by-product of the steel industry), while phosphorus based on volcanic raw phosphate has been used in Askov and Thyrow. A very special feature of the Askov experiment was that access to soil samples collected regularly during the period 1923 to 2016. This enabled the historical change in the soil's uranium content to be analyzed. In addition, this experiment could also include samples of soil which, since 1894, had received only livestock manure or had been kept completely manure-free. These soil samples provided reference values ​​that are very important in assessing changes in uranium in the phosphorous soil.

“It shows the strength of the long-running field experiments and archives with soil samples, like what we have in the Askov experiment. Without the long time series, we would not be able to establish such compelling documentation for the evolution of the earth's content of uranium,” explains Bent Tolstrup Christensen.

Slight increase

To their surprise, the researchers found that the amount of uranium accumulated in all three experiments was more than ten times lower than previously reported from a number of other experiments around the world. But most startling was the result from Askov, which showed a slight increase in the amount of accumulated uranium, whether or not phosphorus was fertilized.

“The very slight increase has occurred, whether the soil has been fertilized or has been supplied with commercial or livestock manure. This is an increase we cannot explain,” explains Bent Tolstrup Christensen. "We can only guess at uranium supplied by liming or from atmospheric deposition."

If the results from Askov were grouped into periods, a slightly higher content could be recognized in the NPK-fertilized soil in the period 1985-2016 than in 1938-1976. This may be attributed to a slightly higher uranium content in the manure released in recent years. But there is still a very small difference.

Not alarming

The study shows that a low uranium content can be maintained even for a very long time if phosphorus fertilizers produced from low phosphorus uranium content are used.

“Elsewhere in the world where phosphorus fertilizers are based on sedimentary crude phosphate, we see much larger and worrisome increases in the uranium content of the cultivated soil. But the situation is not alarming in Denmark as long as we continue to phosphorus fertilization based on crude phosphate of volcanic origin,” explains Bent Tolstrup Christensen.

However, researchers are concerned about the accumulation of heavy metals such as uranium in the soil. Once the heavy metals are there, they cannot be removed again if they are found to have a negative impact on the soil microbial community or the soil fauna.

"We therefore hope that our results can raise awareness of the importance of using phosphorus fertilizers with a low concentration of uranium," explains Bent Tolstrup Christensen.

Collaboration partners: Department of Agroecology at Aarhus Universitet, Institute of Bio- and Geosciences at Forschungszentrum Jülich GmbH, Institute of Crop Science and Resource Conservation ved University of Bonn, and Albrecht-Daniel-Thaer Institute of Agricultural and Horticultural Sciences at Humboldt University.

Funding:The Danish contribution was financially supported by funding from the Ministry for Environment, Food and Agriculture as a part of the public support research preparedness. The German contribuation was supported by the German Federa lMinistry of Education and Research (BMBF) in the framework of the funding initiative “Soil as a Sustainable Resource for the Bioeconomy – BonaRes”,  project “BonaRes (Module A): Sustainable SubsoilManagement - Soil3 [grant numbers 031B0026A and 031B0026C, 2015]

Conflict of interest: None

More information: You can read the article here: ”Non-critical uranium accumulation in soils of German and Danish long-term fertilizer experiments”. It is published in Geoderma (Bind 370; 2020) and is written by Y. Sun, B. Wu, W. Amelung, B.T. Christensen, S. Pätzold, S.L. Bauke, K. Schweitzer, M. Baumecker and R. Bol.

Contact: Professor Bent Tolstrup Christensen, Department of Agroecologi, Aarhus Universitet. Email: bent.t.christensen@agro.au.dk. Phone: +45 8715 7764

Research, Agro, DCA