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.
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 Govié, 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.