Impacts of Pollutants (Phosphorus and Nitrogen) from Agricultural Activities on the Soils and Waters of Toho Lake (Benin)

Abstract

We evaluated the impacts of agricultural activities on the cultivated soils around Lake Toho as well as the waters from the streams through phosphorus, nitrogen and potassium pollution of the soils and waters from the streams heading towards Lake. In the soil, pHwater and pHKCl organic matter, moisture total phosphorus and its fractionation were determined. The various supernatants obtained are analyzed using a 1600PC UV spectrophotometer as well as the waters for the determination of ammonium, nitrite, nitrates and phosphate. In soils, the determination of ammonium, nitrite, nitrate, phosphate and potassium was carried out by DR 5000 MP-AES spectrometry method. Soils around this lake are weakly acidic with an average water pH of 6.91. The humidity increases from surface to depth (8% to 28%), while organic matter (11% on average) and organic carbon decrease from surface to depth. These soils are rich in phosphorus with an overall average concentration of 10.10 mg/g. The fractionation made it possibe to extract the following forms of phosphorus in soil in the order: P-residual > P-org&Al > P-Ca > P-Fe > P-Labile. The physicochemical analysis of the waters show that these waters are moderately loaded and present an increased risk of eutrophication with multifaceted consequences. In soils, nitrate is the dominant form of nitrogen. Cultivated soils provide an abundant source of nitrogen and phosphorus nutrients to Lake Toho via its recharge sources.