Harmless traces from nuclear testing more than half a century ago are helping researchers assess soil erosion rates. In Africa, about 65 percent of the continent's farm land is affected by erosion-induced losses of topsoil and soil nutrients, according to the Food and Agriculture Organization of the United Nations (FAO). Benin is among those countries severely impacted by soil erosion, which poses a major problem for economic development since agriculture represents approximately 35 percent of the country's GDP and 80 percent of its export income. A recent study applied a nuclear technique to assess rates of soil erosion and support land conservation in Benin.
"Evidence shows that over 90% of soils in Benin have a high level of degradation," said Pascal Houngnandan, Director of the Laboratory of Soil Microbiology and Microbial Ecology at the National University of Agriculture in Benin. "The study used tools and means that guide the action for the preservation and regeneration of agricultural land, which is a particularly acute problem because any agricultural production is dependent on the soil."
Most of Benin's arable land is used by small family farms, based on manual labour and great crop variability. These small plots with multiple and irregular physical boundaries pose a challenge to conventional measurements of soil erosion to produce representative and reliable results.
Techniques based on radionuclides are much more suitable: the Cs-137 radionuclides that were released into the atmosphere by nuclear weapon tests and deposited in soils worldwide in the 1950s and 1960s can be used to measure soil erosion. If the amount of Cs-137 in the soil at the studied site is lower than the amount found in selected reference areas unaffected by erosion, it means soil at the studied site has been lost to erosion. This difference can be quantified, so scientists can measure Cs-137 in order to assess the rate of erosion and its impact on soil fertility.
Cassava and oil palms - an example of complex land use in Benin. (Photo: E. Fulajtar/IAEA)
Using this technique, scientists have estimated soil erosion rates in southern Benin in three agro-ecological zones, which are areas of land defined by a combination of soil, landform and climatic characteristics. They found that the annual mean soil erosion rate was 11.8 tons/hectare in Govie, 18.8 tons/hectare in Linsinlin and 22.4 tons/hectare in Lokogba. Based on the estimated rates, 48 mm, 77 mm and 91 mm of soil, respectively, has been lost at the studied sites since 1954, when Cs-137 deposition began. Since soil formation does not exceed 0.1 mm per year, it was determined that approximately 6 mm of soil layer was formed during this period, while 48-91 mm of soil was lost, leading to severe consequences for soil fertility and thus crop yields, said Houngnandan.
The research team also measured the loss of soil nutrients and organic matter. Nutrients can be lost to erosion as well -- washed off from what were once fertile lands into rivers and lakes. As a result of intensive farming and water erosion, the total soil nitrogen was reduced by 27 percent in Govie, 6 percent in Lokogba and 34 percent in Linsinlin. The team also noted that soil phosphorus decreased by 35 percent in Lokogba, and soil organic matter declined by 2.5 percent in Govie and 3 percent in Lokogba.
"Based on these results, soil conservation measures are required," Houngnandan said. "The next step of our research will be to test several conservation practices, and to identify the most efficient practice, which will then be shared with farmers. The results of the IAEA technical cooperation project will guide and foster this intended conservation effort and improve the national soil conservation programme."
The Benin erosion study, with technical support from the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, in collaboration with the National Agricultural Research Institute of Benin, is a good example of how nuclear techniques can contribute to soil conservation, particularly in environmental conditions characterized by tropical climate and long, smooth slopes of denudation plateaus, said Lee Heng, Head of the Soil and Water Management and Crop Nutrition Section at the FAO/IAEA Joint Division.
Long smooth slopes of African denudation plateaus with complex land use are prone to soil erosion. (Photo: E. Fulajtar/IAEA)