As urbanization increases worldwide, so too are investments in nature-based solutions that aim to mitigate urban stressors and counter the impacts of global climate change. Tree planting on degraded urban lands—or afforestation—is one form of nature-based solution that has been increasingly implemented in cities around the world. The benefits of afforestation are, however, contingent on the capacity of soils to support the growth of planted trees, which poses a challenge in some urban settings where unfavorable soil conditions limit tree performance. Soil-focused site treatments could help urban areas overcome impediments to afforestation, yet few studies have examined the long-term (> 5 years) effects of site treatments on soils and other management objectives. We analyzed the impacts of compost amendments, interplanting with shrubs, and tree species composition (six species vs. two species) on soil conditions and associated tree growth in 54 experimental afforestation plots in New York City, USA. We compared baseline soil conditions to conditions after six years and examined changes in the treatment effects from one to six years. Site treatments and tree planting increased soil microbial biomass, water holding capacity, and total carbon and nitrogen and reduced soil pH and bulk density relative to baseline conditions. These changes were most pronounced in compost-amended plots, and the effects of the shrub and species composition treatments were minimal. In fact, compost was key to sustaining long-term changes in soil carbon stocks, which increased by 17% in compost-amended plots but declined in unamended plots. Plots amended with compost also had 59% more nitrogen than unamended plots, which was associated with a 20% increase in the basal area of planted trees. Improvements in soil conditions after six years departed from the initial trends observed after one year, highlighting the importance of longer-term studies to quantify restoration success. Altogether, our results show that site treatments and tree planting can have long-lasting impacts on soil conditions and that these changes can support multiple urban land management objectives.

This study was conducted within long-term research plots (NY-CAP) located in Kissena Corridor Park, a 40-ha urban park in Queens, New York, USA. The NY-CAP experimental plots test the impacts of three treatments on planted tree growth and soil conditions over time. Treatments include: tree species composition (two species or six species), compost addition (compost-amended or unamended), and interplanting with shrubs (presence or absence). We sampled soils four times over the course of the experiment in 2009, 2010, 2011 and 2016. In 2011 and 2016 we measured DRC and DBH, respectively, for all planted trees in experimental plots. Soils were processed and analyzed at Yale University. 

This dataset is comprised of three tabs in a single excel file. See the "MetaData" tab for information pertaining to the variables measured and analyzed. The "SoilData" tab includes values for all the soil variables analyzed and reported on and "TreeData" includes values for all the tree growth variables analyzed and reported on in the manuscript.