Urbanization and fragmentation have opposing effects on soil nitrogen availability in temperate forest ecosystems

Area of Science:

• Ecology
• Environmental Science
• Forest Science

Background:

• Terrestrial ecosystems globally face declining nitrogen (N) availability, termed N oligotrophication.
• Northeastern U.S. temperate forests show a sharp decrease in bioavailable N, worsened by climate change.
• Urban expansion and forest fragmentation in this region raise questions about their combined impact on N cycling and tree uptake.

Purpose of the Study:

• To investigate the combined effects of urbanization and forest fragmentation on nitrogen (N) availability and uptake in oak-dominated forests.
• To compare N cycling and tree N status along an urban-to-rural gradient and between forest edges and interiors.

Main Methods:

• Studied eight oak forest sites along an urban-to-rural gradient in Massachusetts.
• Measured plant and soil properties along 90m transects from developed edges to forest interiors.
• Assessed rates of net ammonification, net mineralization, net nitrification, and foliar N and C:N concentrations.

Main Results:

• Urban forests exhibited higher net ammonification, net mineralization, and foliar N concentrations than rural forests.
• Urban forest interiors showed greater N availability and uptake than their edges.
• Rural forest edges had higher net nitrification and foliar C:N than interiors, contrasting with urban sites.

Conclusions:

• Urbanization in the northeastern U.S. enhances soil N availability and tree N uptake, counteracting widespread N oligotrophication.
• Forest fragmentation, particularly at urban edges, diminishes these N benefits.
• The study highlights contrasting impacts of urbanization and fragmentation on coupled N availability and tree demand.