Urban Tree Channeling of Soil Methane and Nitrous Oxide and Its Mitigation Using Biochar

Area of Science:

• Urban ecology
• Environmental science
• Soil science

Background:

• Urban forests are recognized carbon sinks, but their role in emitting other greenhouse gases (GHGs) like methane (CH4) and nitrous oxide (N2O) is understudied.
• Urban trees may transport CH4 and N2O from compacted, anaerobic soils to foliage, leading to emissions.

Purpose of the Study:

• To provide empirical evidence of foliar CH4 and N2O emissions from urban trees.
• To investigate the potential of soil biochar amendment in mitigating these foliar GHG emissions.
• To quantify the impact of biochar on urban tree GHG exchange and its implications for urban decarbonization.

Main Methods:

• In situ gas-exchange measurements were conducted on American Elm (Ulmus americana) and Hackberry (Celtis occidentalis) trees in Toronto, Canada.
• Foliar and soil CH4 and N2O fluxes were compared across different soil biochar application rates (0, 20, 40 t ha-1) and seasons.
• Structural equation modeling was used to analyze the relationship between soil and foliar gas fluxes.

Main Results:

• Urban tree foliage was found to emit CH4 and N2O.
• Biochar application shifted foliar CH4 exchange from emission to uptake and reduced N2O emissions by up to 70%.
• Similar mitigation patterns were observed for soil fluxes, with strong coupling between soil and foliar emissions.

Conclusions:

• Urban tree foliage is a source of CH4 and N2O, with emissions linked to soil conditions and potentially transported via xylem.
• Soil biochar amendment is an effective strategy to mitigate foliar and soil CH4 and N2O emissions from urban trees.
• Applying biochar to urban forests could transform them from minor GHG sources into net sinks, aiding urban decarbonization efforts.