Spatial variability of urban forest topsoil properties: towards representative and robust sampling design

Area of Science:

• Urban Soil Science
• Environmental Science
• Forest Ecology

Background:

• Soil spatial variability is critical for representative topsoil sampling, especially in urban forests with complex disturbances.
• Existing agricultural sampling designs are insufficient for urban soils, which exhibit significant horizontal, vertical, and anthropogenic variability.
• Understanding urban soil properties is crucial for managing urban forest health and ecosystem services.

Purpose of the Study:

• To evaluate the spatial variability of topsoil properties under urban trees (Acer rubrum).
• To define a statistically robust sampling design for basal soil respiration (BSR).
• To assess the utility of electrical conductivity (EC) and bulk density (BD) as indicators of spatial variability.

Main Methods:

• Thirteen sampling sites with park and street trees in Cambridge, MA, were selected.
• Topsoil properties including basal soil respiration (BSR), electrical conductivity (EC), and bulk density (BD) were measured.
• Statistical analysis was used to determine the number of samples needed for reliable characterization of BSR.

Main Results:

• Street tree topsoil exhibited approximately twice the variability of park tree topsoil, necessitating more intensive sampling.
• Variability of BSR was similar to EC, while BD showed the least variation.
• A high number of samples were required for high statistical reliability (e.g., 44.4 for BSR), but fewer were needed for lower certainty (e.g., 6.8 for BSR).

Conclusions:

• Electrical conductivity (EC) testing is proposed as a cost-effective baseline measure to assess topsoil spatial variation.
• This approach can inform sampling strategies for basal soil respiration (BSR), reducing analytical costs.
• Factors like soil disturbance and access restrictions influence sampling design and generalizability in urban settings.