Differences in aggregate stability due to various sewage sludge treatments on a Mediterranean calcareous soil

Three types of sewage sludge from the same wastewater treatment plant but processed in different ways (composted, fresh and thermally dried) were applied in doses equivalent to approximately 10 Mg of dry matter per hectare to the surface of a loamy soil in order to improve its physical condition. The effect of this organic amendment on the structural stability of the soil was measured using a procedure which explores three types of disaggregation mechanisms: slaking, mechanical breakdown by raindrop impact and disaggregation induced by differential swelling. Two years after the surface application of the sludge, the treated soils have a higher organic carbon and hydrolyzable carbohydrate content and greater structural stability than the control treatment. It was observed that while all three types of sludge reduced soil disaggregation due to differential swelling and splash, only the treatments with composted and thermally dried sludge were effective in reducing slaking.