Many cities have abandoned industrial sites and other degraded areas that are prime candidates for ecological renewal, but it’s often hard to get restoration projects to take root – literally. Urban soils are frequently compacted, contaminated, or absent altogether.

The Lake Calumet Cluster Site, for example, is an 87-acre area on the southeast side of Chicago that was once home to five steel plants. The city would like to turn this Superfund site into a wetland lush with native vegetation that migratory birds could use as a stopover along their flyway. But 60 percent of the area lacks topsoil altogether, and much of it is covered by slag, a hard, glassy material that is a byproduct of steel manufacturing.

To remedy this, researchers from the Ohio State University and the Chicago Metropolitan Water Reclamation District turned to biosolids, a product made from municipal sewage sludge that undergoes a two-year process of cleaning, pathogen removal, and transformation into a nutrient-rich growing medium.

Biosolids are gaining popularity as a soil amendment. They’re an endlessly renewable resource, to say the least. But their effectiveness at improving ecosystem function has rarely been explicitly measured or compared to other soil amendments such as compost.

So the researchers established a series of 20 experimental plots near the proposed restoration site. They used a rototiller to incorporate either biosolids or compost made from decomposed woody material into the top six inches or so of the ground. The next year they scattered seeds, a blend of more than two-dozen native plant species, over the plots. They monitored soil quality, runoff, and the development of vegetation for three years.

Both compost and biosolids improved soil quality, helped native plant cover get established quickly, and aided earthworm populations, the researchers reported recently in a paper published as part of a special feature on urban soils in the Journal of Environmental Quality.

But the advantage goes to biosolids, which outperformed compost on most measures investigated in the study. Biosolids are richer in nutrients like nitrogen, phosphorus, and potassium compared to compost, and produced more vigorous plant growth. The biosolids plots provided more micronutrients like copper and zinc, better support for bacteria and fungi characteristic of healthy soils, and ultimately a greater diversity of native prairie vegetation.

In the end, the researchers’ strategy could help solve two problems at once, providing a way to deal with city sewage as well as a jump-start to ecological restoration. “You have to bring in the soil,” says Nick Basta, of the School of Environment and Natural Resources at Ohio State. “Why not connect the dots and bring in what’s available locally?” – Sarah DeWeerdt | December 15, 2015

Source: Basta N.T. et al. “Restoring ecosystem function in degraded urban soil using biosolids, biosolids blend, and compost.” Journal of Environmental Quality DOI: 10.2134/jeq2015.01.0009

Header image: The landscape outside an abandoned steel mill in Kentucky. Credit: Christopher Titzer via Flickr.