Organic matter components, aggregate stability and biological activity in a horticultural soil fertilized with di€erent rates of two sewage sludges during ten years

The e€ects of the application as fertilizer during ten years of two sewage sludges (aerobically and anaerobically digested, at rates of 400, 800, and 1200 kg of N/ha yr), on the aggregate stability and contents of related organic matter components, microbial biomass and levels of ®ve enzymatic activities (alkaline phosphomonoesterase, phosphodiesterase, urease, arylsulphatase and dehydrogenase) were investigated. The application of both sludges at mid and high rates gave rise to signi®cant increases of organic matter, humi®ed substances and humic acids, but no e€ects on carbohydrates, microbial gums and aggregate stability were observed. As for biological activity in soils, the high variability of data led to a general absence of statistical signi®cance despite the large di€erences between treatments observed. Signi®cant increases of phosphodiesterase activity were nevertheless produced by the high rate of aerobic sludge and the mid and high rates of the anaerobic sludge