Short-term and long-term changes in urban forest soils

Photo Credit

Photo courtesy of Dr. Kathy Szlavecz

Ian Yesilonis sampling earthworms in a Baltimore City urban forest.

Because of human activities, urban soils are different from soils in less developed areas in many ways. In general, healthy soil helps cycle important elements like carbon, nitrogen, calcium, and phosphorous. Humans affect these cycles by disturbing the land and adding pollutants to the air, water, and ground.

Scientists have studied long-term changes in agricultural soils, but very little is known about long-term changes in urban forest soils. NRS soil scientist Ian Yesilonis has been working with a team of scientists to track changes in the soil at rural, suburban, and urban sites in and around Baltimore, Maryland.

The research team is also interested in how earthworm populations change over time in urban soils – and how earthworms change the soil. Earthworms generally increase the amount of available nitrogen, calcium, phosphorus, and potassium in soil, which is good for growing plants. However, earthworms negatively affect forest ecosystems by eating the top layer of soil that is made of decayed leaves on the ground. This can change soil properties like moisture retention, temperature, and nutrient cycling. The top layer of soil is also important for plant germination and the development of fine roots in plants.

Key Findings 

Soil properties

• Urbanization changes soil properties like pH, calcium, carbon, and carbon dioxide over time.
• Suburban sites had more soil organic matter after 17 years than rural or urban sites and the scientists are not sure why.
• While other studies have found a decrease in soil calcium and pH over time, this research found an increase in both calcium and pH, especially in urban soils. This may affect which plant seedlings can establish and thrive in these soils.

Impacts of earthworms

• As the number of earthworms increases, soil pH, bulk density, and calcium also increase.
• The number of Asian jumping worms went up in rural soils over 17 years. These worms change the soil in a number of ways including making it more likely to erode.

Approach

This research was conducted in forested areas of Baltimore City and Baltimore County. Study plots were located on rural, suburban, and urban land so that the differences between them could be analyzed.

The research team took soil samples in 2001 and again in 2018, checking for nutrients, pH levels, and carbon. They expected to find higher pH and higher levels of nitrogen, calcium, and phosphorous in the urban soils compared to the suburban and rural soils in 2018 because of phosphorus accumulation in cities. They also thought sulfur levels would drop between 2001 and 2018 because of decreased sulfur pollution in the region, and that soil carbon would increase more in the urban sites (compared to the suburban and rural sites) because of more plant growth.

The scientists also sampled earthworms at all of the study plots to figure out how many earthworms there are in the different kinds of soil and how earthworm abundance has changed over time. They expected to see increased amounts of non-native invasive earthworms along the urban-rural gradient.

Outcomes

This research provides important information about how urbanization changes soils over time. The scientists are continuing to process data on soil properties and earthworms from the research plots.