Urban areas have experienced significant expansion worldwide in recent decades, with further growth expected in the coming years. This has led to an increasing demand for tree planting in urban environments to mitigate rising temperatures and air pollution. However, urban tree planting faces challenges such as high tree mortality rates8, partly due to the loss of mutualism between urban trees and their essential symbiont, ectomycorrhizal (ECM) fungi. ECM fungi provide nutrients and protect trees from pathogens.
The existence of an "urban suite" of mycorrhizal fungi has been suggested, as urban environments create unique conditions with high temperatures and chemical depositions. Heat tolerance and nutrient preferences depend on mycorrhizal taxa. The soils commonly used in urban areas exhibit considerable variation in moisture and organic matter content, which can also influence mycorrhizal communities. ECM fungal application could mitigate climate change feedback from urban soils, as ECM fungi have shown a negative correlation with denitrifying and fast-decomposing bacteria, which accelerate CO2 and N2O emissions. Nevertheless, the effects of mycorrhizal community variation on tree health and soil biogeochemistry in different urban soil types remain unknown.
In this study, we hypothesize that (1) applying forest soil to urban soil will improve plant health, growth, and soil health and sustainability by reducing pathogenic microbes and the potential for climate change feedback (e.g., low copiotroph and N-cycling bacterial abundance). This improvement will be further enhanced by (2) the urban suite of ECM fungi compared to rural forest ECM fungal communities, and (3) in urban soil with mulch application. To test these hypotheses, we propose establishing a pot experiment to grow trees on three types of typical urban soil in Boston, treated with either urban or rural forest soils.