Percolation and leachate composition in a disturbed soil layer mulched with sewage biosolids

The effect of sludge and sludge compost as a mulch for dissipating raindrop impact and maintaining a relatively high percolation rate was studied using a Paleudalf from South Africa. Three kg soil were packed in perforated boxes in a making a two cm layer, and subjected to five consecutive simulated rainstorms during a period of 26 days. A 4-day incubation was allowed between the first 4 storms and 10 days before the 5th storm. The soil was amended with either a dry waste activated sludge or a sludge compost at a rate equivalent to 45 Mg ha(-1). The amendments (< 2-mm) were applied either on the surface or mixed with the whole soil. The application of both sludge and sludge compost reduced the final percolation rate (FPR) compared with the unamended control. The effect of the consecutive rainstorms on the average FPR from all the treatments was as follows: FPR after Ist storm >> 2nd congruent to 3rd congruent to 4th > 5th. The effect of amendment on average FPR from all storms was: control > sludge mixed with the soil > surface-applied sludge > sludge compost mixed with the soil greater than or equal to surface-applied sludge compost. The decrease of FPR was not related to the electrical conductivity or clay concentration of the leachates. The main chemical species that appeared in the leachates from the treated soils, and persisted throughout the five rainstorms, were ammonium and sulfate. Other ions, such as calcium and magnesium, were present in high concentrations in the leachate during the first storm but at low concentrations in subsequent storms. Nitrate concentrations were high during the first and fifth storms. It is suggested that the adverse effect of the amendments resulted from mechanical and microbial clogging of soil pores. (C) 1998 Elsevier Science B.V. All rights reserved.