Observations préliminaires sur l'évolution de quelques propriétés d'un sol ferrallitique de Guadeloupe (FWI) après un apport de boues urbaines

An aerobic sewage sludge was buried (0-0.20 m depth) at 3 levels (0, 10 and 100 tons DW/ha) in a ferrallitic soil of Guadeloupe (French West Indies) in order to study the biodegradation of the sludge and changes in soil properties. The main characteristics of the soil and the sewage sludge are shown in table I. The study was conducted on plots (20 m2) in field over a period of 500 d. Total rainfall from July 1986 to December 1987 was 4 718 mm and the temperature varied between 22 and 27° C (fig 1). The soil was kept vegetation-free and cleared by hand. Samples were taken from the 0-0.20 m layer and sewage sludge aggregates (diameter > 5 mm) were removed by hand. Preliminary observations were reported according to the characterization of the average soil samples. Changes in the organic carbon and the total nitrogen concentrations in the soil (0-0.20 m depth) in relation to the sludge application rate are shown in figure 2. After 500 d, the increase in organic carbon content in the sludged soil (0-0.20 m depth) was equivalent to 0 and 34% of the carbon applied for 10 and 100 tons DW/ha (table I). In the same manner, for the total nitrogen content the values represented 0 and 27% of the nitrogen applied (table I). The cation exchange ability (CEC) (fig 3) and water content (fig 6) of the 100 t DW/ha sludged soil increased due to the rise in organic carbon content. After 500 d, the difference between the CEC of the control soil and that of the high rate sludged soil was 2 mEq/100g soil DW. Soil-pH decreased in all plots in relation to sludge application level (fig 4). At 100 t DW/ha, pH fell from 5.5 to 4.7. Al-exchangeable concentration increased up to 2 mEq/100g soil DW in the high rate sludged soil due to the extent of pH decrease (table I). Changes in exchangeable cation (Ca2+ , Mg2+ , K+ , Na+) concentrations in the soil (0-0.20 m depth) are shown in figure 5. The balance-sheet of exchangeable cations generally showed that losses were higher than inputs in the 0-0.20 m layer, both at the 10 and 100 t DW/ha level (table II). In previous works, the application of this aerobic sewage sludge appeared to be very useful, especially in relation to changes in inorganic nitrogen and phosphorus concentrations in sludged soil, and to its effect on maize crop yield. However, these preliminary observations demonstrated that the sludge application in a ferrallitic soil of Guadeloupe can display the same risks for the soil (ie, soil-pH decrease and rise in Al-exchangeab