Effects of sewage sludge addition to Norway spruce seedlings on nitrogen availability and soil fauna in clear-cut areas

Anaerobically digested and composted sewage sludge (CSS) has been suggested to be a slow-release fertilizer in forestry and an alternative to quick-release inorganic fertilizers. The effects of CSS with or without added carbohydrate on inorganic nitrogen availability and on soil animals were tested in two Norway spruce plantations. Half of the seedlings were individually fertilized with CSS, and the rest were left as controls. Solid sucrose was added to half of the fertilized and untreated seedlings. Soil samples were taken in the autumn in the first and the second year after the treatments. CSS increased soil NH4–N (2100%), the proportion of soil NO3–N, and the N concentration of spruce needles. CSS greatly reduced the abundances of enchytraeids, tardigrades and collembolans, but increased the proportion and abundance of bacterial-feeding nematodes irrespective of carbohydrate addition. A better stabilization method needs to be developed before CSS can be used as a forest fertilizer. •Spruces were fertilized with anaerobically digested and composted sewage sludge (CSS).•CSS increased soil N, proportion of NO3–N, and N concentration of spruce needles.•CSS reduced the abundances of enchytraeids, tardigrades and collembolans.•CSS increased the proportion and abundance of bacterial-feeding nematodes.•Sucrose did not reduce N pools or counteract negative CSS effects on soil animals. Composting and carbohydrate addition do not mitigate the harmful effects of anaerobically digested sewage sludge in boreal forest soil.