Urbanization effects on nitrogen cycling and plant growth

Background/Question/Methods

Urban areas are subject to increased temperatures and elevated amounts of CO₂, pollution, heavy metal deposition, earthworm activity, and nitrogen deposition. Urbanization in New York City and other cities has been associated with increased growth in some plant species. This increase in growth may reflect high amounts of nitrogen deposition, as plant growth in many terrestrial ecosystems historically has been limited by nitrogen availability. The introduction of historically limiting nutrients, such as nitrogen, into terrestrial ecosystems can have potentially significant effects which have cascading affects on the soil microbial communities which regulate the decomposition of organic matter, whereby regulating nutrient availability for plants. One way to better understand changes in decomposition and nutrient cycling is to examine the function of extracellular enzymes mediating these processes.

Results/Conclusions

We investigated whether urbanization affects plant growth through effects on the nitrogen cycle, complementary field and common-garden studies were conducted using ten sites varying in distance from the center of New York City, Central Park. In the common garden experiment, soils collected closer to New York City were associated with increased growth of red oak (Quercus rubra) seedlings, suggesting that below-ground factors contribute to increased productivity.  In situ measurements indicated that both nitrate concentrations and pH decreased with increasing distance from Central Park, yet N mineralization and nitrification rates did not vary among the sites. Leaf N was not correlated with distance to Central Park, nor was leaf N correlated with total soil N. Leaf N was significantly correlated with inorganic nitrogen, suggesting inorganic N is the favored form of N taken up by red oaks. Glycine aminopeptidase and phenol oxidase activity, two extracellular enzymes related to nutrient metabolism and assimilation, significantly increased with increasing distance from Central Park. These results suggest that urbanization has led to changes in soil N processes, and these alterations may partially account for increases in plant growth with increasing urbanization.