Encouraging urban tree growth with organic soil amendments


Urban trees provide many benefits for city-dwellers. They filter air, manage stormwater, store carbon, and buffer temperatures while providing a taste of nature. But a lot of the benefits of trees come when they reach maturity, and with the pressures of urbanization, urban trees often don’t live very long. So researchers are looking for amendments that will help urban trees be happier and healthier.

headshot of Bryant Scharenbroch

Chicago, IL is one of the most studied urban forests. A recent report from 2011 found that there about 157 million trees in the seven-county region of Chicago, and trees occupy about 21% of the land area. Trees, then, form an important part of the urban ecosystem.

But the soils in which urban trees grow are often mistreated and of poor quality. These soils are often compacted, and they may have little organic matter or high levels of harmful contaminants. Poor quality soil means trees don’t grow well and the benefits they provide are minimized.

“To prepare for infrastructure, top soil is typically scraped off and subsoil is compacted,” said Bryant Scharenbroch of the Morton Arboretum at the ASA, CSSA, SSSA Annual Meetings in November. “As a result, it’s a very challenging environment for trees.”

To maximize the benefits of urban forests and trees, scientists are looking for organic soil amendments that help trees grow and thrive. In a paper recently published in Journal of Environmental Quality, Scharenbroch and his co-authors characterize various materials for their impact on soil quality and tree growth.

“We are trying to find materials that are effective and efficient,” said Scharenbroch. “By efficient I mean that they are locally produced and they can provide a relative bang for the buck. So they’re cheap and they increase tree establishment, growth, and longevity.”

cleaning course roots for measurements

The researchers set up an 18-month experiment in a greenhouse to look at the effects of several soil amendments on two different tree types in three different soils. The soils were a high-quality silt loam, a compacted clay soil typical in urban areas, and a well-drained sand soil often found in tree cutouts along streets.

The amendments used included wood chips, compost, compost tea, biochar, and biosolids. Biochars are produced by pyrolysis or gasification, processes that heat biomass in the absence of oxygen. In this study, biochar was created from pine feedstock. Biosolids are nutrient-rich organic materials created during the treatment of sewage.

The two trees types used in the study, sugar maple and honey locust, were grown for 18 months. At the end of the experiment, soil samples were gathered to measure carbon, plant-available nitrogen, and microbial respiration. The trees were also collected to determine the biomass of the course roots, fine roots, stems, leaves, and total tree.

As expected, soil type affected tree growth. “Soil type definitely influenced our tree root biomass. We had a lot more biomass with the silt loam compared to the relatively poor urban soils,” explained Scharenbroch.

The soil amendments also affected tree growth. Total tree biomass of both tree types was greater in soils treated with biochar and biosolids compared to untreated soils. Biosolids treatment caused the greatest increase. The addition of compost and wood chips, however, did not result in increased tree biomass.

researchers pull respiration traps in a nursery

The soils were also affected by the biosolids and biochar treatments. When compared to other organic amendments, biosolids treatment increased nitrogen available to plants and microbial respiration. Biochar addition also increased microbial respiration, and it increased carbon levels in the soil as well.

Overall, biosolids and biochars had the greatest impact on tree growth and soil quality compared to other organic treatments. “Biosolids and biochar did well compared to our controls,” said Scharenbroch. “We’ve had some improvements with other amendments, but, in general, these are the strongest treatments.”

Going forward, the researchers have begun to scale up their studies and move their experiments into the urban setting itself. They are currently conducting experiments in downtown Chicago, in Bolingbrook, IL (a suburb of Chicago), and in a tree nursery at The Morton Arboretum.

If the results of these larger studies also show that biochar and biosolids promote the growth of healthy, long-lasting trees, another benefit may become clear. Both biochar and biosolids are generated from waste. So these products could potentially provide the added benefit of reusing, and therefore reducing, waste in populated urban areas.

“Around 7 to 12 percent of municipal solid waste is actually urban trees,” explained Scharenbroch. “There is good potential for taking that urban tree waste, chipping it up, and locally producing biochar that is then applied to these urban soils. We think these are great opportunities.”



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