Microbial extracellular enzyme function and nutrient cycling along the New York City urban-to-rural gradient

Background/Question/Methods

Elevated CO₂, nitrogen deposition, and changes in global and local climates are factors which will continue to influence ecosystem productivity and functioning. Models predict that warming temperatures associated with global change will stimulate soil decomposition, thereby increasing efflux of CO₂ and creating a positive feedback to increasing global temperatures. Elevated CO₂, nitrogen deposition, and increased temperatures are also factors associated with urbanization. Urban areas in the United States have greatly increased over the past 100 years, and the growth rate of urban land is exceeding that of preserved land in parks of conservation areas. Accordingly, urban forests are becoming an increasing proportion of the world’s ecosystems, thus highlighting the significance of understanding urbanization effects on ecosystem function. A two-year field study was conducted at 10 sites along the urban-to-rural gradient located on a north-south transect ranging from 0m to 128 km from Central Park, New York City. Specifically, I tested the following hypotheses: 1) urbanization will be associated with changes in soil microbial community function, and these changes will vary seasonally, 2) inorganic nitrogen pools of NO₃^- and NH₄⁺ vary along an urbanization gradient, 3) urbanization will be associated with changes in pools of total soil C and N. 

Results/Conclusions

Soil nitrogen concentration decreased with increasing distance from Central Park (P = 0.030) in the spring 2011 collection, yet increased with increasing distance from Central Park (P = 0.042) in the spring 2010 collection. Soil pH significantly decreased with increasing distance from Central Park (P = 0.036) in the summer 2010 collection. Extractable ammonium concentrations significantly increased with urbanization (P <0.001) in the spring 2011 collection. Extractable nitrate concentrations significantly decreased with increasing urbanization (P <0.001) in the spring 2011 collection. Glycine aminopeptidase significantly increased with increasing distance from Central Park (P ˂ 0.001) in the fall 2010 collection. Though acid phosphatase, peroxidase, and β- 1, 4- N- acytylglucoseaminadase activities did not vary with urbanization, their activity level varied relative to one another. Additionally, activity levels varied seasonally. These data suggest seasonality may affect soil processes more strongly than urbanization.