Earth and Environmental Sciences
Soil Microbial Biomass and Activity in Afforested Urban Ecosystems
Gisselle Mejía is a Ph.D. student and Graduate Center Fellow at the CUNY Graduate Center in the Earth and Environmental Sciences PhD Program. She holds a B.A. in environmental studies and political science from Case Western Reserve University and a M.S in Forest Management from Michigan Technological University. She has served as a Peace Corps agroforestry extensionist in El Salvador and, prior to embarking on a Ph.D., was an urban forester for the City of New York. Today, Gisselle studies soil microbial activity to better understand the natural and human-driven factors that alter nutrient cycling in urban forest ecosystems.
Urban forest remnants are significant natural features of the urban landscape that provide important ecosystem services, such as climate regulation (e.g. carbon sequestration) and cultural benefits (e.g. recreation). Urban forests have also been exposed to altered environmental conditions (e.g. elevated temperatures, atmospheric pollution, non-native species invasion) that impact natural processes and patterns. These anthropogenic disturbances can alter plant and microbial community development, affecting soil nutrient cycling integral to forest biophysical functions. Carbon and Nitrogen are critical to forest productivity and have been well studied in rural landscapes; however, these nutrient dynamics are poorly understood in urban areas. Urban land-use changes impact valuable ecological functions of urban forests; therefore, understanding the plant-soil interactions that support their development will inform restoration strategies to enhance ecosystem services. This summer, samples were collected across New York City parklands to measure soil microbial activity and determine nutrient regulation in response to restoration techniques implemented through the New York City MillionTrees (NYCMT) afforestation program.
During the summer, I conducted a field campaign involving vegetation surveys and soil sampling at eight parks across the five boroughs as part of a collaborative effort to better understand the co-development of soil microbial processes and plant communities in urban forest ecosystems. This research experience provided me with a unique opportunity to connect with our city’s ecologically diverse natural areas. I now see the city as a vibrant ecosystem.
This research is part of a long-term monitoring experiment developed in partnership with the New York City Department of Parks and Recreation (NYC DPR). In 2007, the City of New York launched the New York City MillionTrees (NYCMT) initiative, a large afforestation plan to expand the urban canopy by 2,000 acres between 2007-2017, increasing biodiversity and restoring the native habitat and vegetation of NYC. These efforts are important because by looking at altered environmental conditions (e.g. high temperatures) in urban forests we can project how non-urban ecosystems are likely to respond to climate change in the future.
Urban land-use change, or the conversion of natural ecosystems to human-dominated ones, is an important component of global environmental change because it has the potential to affect landscape and regional scale storage and soil-to-atmosphere fluxes of Carbon and Nitrogen. Understanding how urban land-use change alters natural ecological processes is best studied through microbial regulation of nutrients (e.g. carbon and nitrogen) because soil microbial processes exhibit signs of change faster than plant communities. For example, high rates of Nitrogen turnover can lead to losses from soils to the atmosphere in the form of nitrous oxide (N2O), a potent greenhouse gas with a longer residence time than carbon dioxide (CO2), and nitrate (NO3-), a pollutant in freshwater and marine ecosystems. In addition, losses of this key nutrient inhibits forest productivity and health, limiting critical ecosystems functions (e.g. carbon storage). However, there is limited research on how these dynamics take place in NYC’s forest ecosystems. My research will provide insight about these plant-soil feedbacks in our urban natural areas.
In my research, I collected soil samples throughout NYC Parklands – Pelham Bay , Marine Park, Fort Totten Park, Alley Pond, Canarsie, Clearview, Conference House and Clove Lake – which were then analyzed for potential microbial activity at each site. These analyses help determine the microbial abundance (biomass), how much carbon and nitrogen is available for organisms to utilize (pools), and the potential for soils to store and release (fluxes) these nutrients either to the atmosphere (i.e. CO2 and N2O) or groundwater (i.e. NO3-). Samples were collected up to one meter to document how nutrients are transformed at different depths. In addition, deep coring will elucidate the impact of historical disturbance on nutrient cycling. The next steps in my research are to analyze soil texture, bulk density (compaction), and other soil properties to identify if these natural factors affect nutrient cycling differently than anthropogenic factors. The final results will provide recommendations for effective restoration strategies that support natural ecosystem functions across different types of urban soils.