Four Swedish long-term field experiments with sewage sludge reveal a limited effect on soil microbes and on metal uptake by crops

Purpose This study aims to study the effect of sewage sludge amendment on crop yield and on microbial biomass and community structure in Swedish agricultural soils. Materials and methods Topsoil samples (0-0.20 m depth) from four sites where sewage sludge had been repeatedly applied during 14-53 years were analysed for total C, total N, pH and phospholipid fatty acids (PLFAs). Heavy metals were analysed in both soil and plant samples, and crop yields were recorded. Results and discussion At all four sites, sewage sludge application increased crop yield and soil organic carbon. Sludge addition also resulted in elevated concentrations of some heavy metals (mainly Cu and Zn) in soils, but high concentrations of metals (Ni and Zn) in plant materials were almost exclusively found in the oldest experiment, started in 1956. PLFA analysis showed that themicrobial community structure was strongly affected by changes in soil pH. At those sites where sewage sludge had caused low pH, Gram-positive bacteria were more abundant. However, differences in community structure were larger between sites than between the treatments. Conclusions At all four sites, long-term sewage sludge application increased the soil organic carbon and nitrogen content, microbial biomass and crop yield. Long-term sewage sludge application led to a decrease in soil pH. Concentrations of some metals had increased significantly with sewage sludge application at all sites, but the amounts of metals added to soil with sewage sludge were found not to be toxic for microbes at any site.