Influence of Land-Use Management on Isotopically Exchangeable Phosphate in Soils from Burkina Faso

Abstract

Little information exists on the effect of land use management on isotopically exchangeable phosphate (P) in agricultural soils of Burkina Faso. The 0–10 cm layer of 30 soils was sampled in February 1994 in 3 regions of Burkina Faso (in the east near Kouaré, in the south center near Thiougou and in the center in Saria). Four samples were taken under natural vegetation, 6 samples were taken in agricultural soils which had been left as fallow under natural re-growth, 7 samples were taken from soils cultivated without any P inputs and 13 samples were obtained from soils that had received P inputs in inorganic and/or organic forms. The phosphorus status of these soils was estimated by their total P (Pt), the Bray-1 extraction and by the isotopic exchange kinetic method (E1min the quantity of P isotopically exchangeable within 1 minute; E1min–1day the quantity of P isotopically exchangeable between 1 minute and 1 day; E1day–3m the quantity of P isotopically exchangeable between 1 day and three months; E>3m the quantity of P which cannot be isotopically exchanged within 3 months). The amounts of total P, Bray-1 P, and P isotopically exchangeable within three months were found to be very low in the non-cultivated soils (under natural vegetation or under fallow) (median values were for: Pt 145.1; Bray-1 P 1.7; E1min 0.16; E1min–1day 0.66; E1day–3m 1.34; E>3m 143.5 mg P kg−1 soil). Cropping these soils without any P input resulted in Kouaré in a slight decrease in isotopically exchangeable P compared to the non-cultivated soils, while no effect was observed in Thiougou. As the estimated P balances suggested that the amount of P removed annually from the non-fertilized systems was close or even larger than the amount of P isotopically exchangeable within the time span of growth of an annual crops (3 months) or than the Bray-1 P content of the soil, it appeared that crops derived a significant amount of their P from other sources. The mineralization of P associated to the soil organic matter, the release year after year of very slowly exchangeable P from the E>3m pool and the uptake of P from deeper layers could explain a significant fraction of the P uptake by crops in these non-fertilized systems. The amount of total P, Bray-1 P and of P isotopically exchangeable within 3 months increased when P was added to the soil as water-soluble P, phosphate rock or farmyard manure. There results show that natural fallow cannot be used as a practice to increase the P availability of soils to a level that would not be limiting for crop production and confirmed that to do so it is necessary to apply P fertilizers. Finally these results confirm that the Bray-1 P values are well correlated to the E1min values in the fertilized soils but that Bray-1 do not give a valuable information on soil P availability in non-fertilized soils.