Anthropogenic additions of reactive nitrogen (N) to the atmosphere and deposition to the biosphere have dramatically altered the N cycle. While rates of N emissions and deposition have recently decreased across the U.S. in rural areas, they remain elevated in urban centers. High levels of oxidized N (NOx) emissions to the atmosphere can negatively impact vegetation through the production of tropospheric ozone, which damages plant tissues and reduces the capacity for tree carbon (C) sequestration. Although elevated N deposition and tropospheric ozone have been studied individually, much less is known about their combined effects on tree C storage throughout rural and urban ecosystems.
To examine the effects of urbanization and air quality, we selected 7 sites along a rural to urban gradient across Massachusetts. At all sites, we established a 90 meter transect from forest edge to interior to evaluate between-site urbanization and within-site forest edge effects on C and N dynamics. To assess air quality, we measured concentrations of ozone and NOx using Ogawa passive samplers and measured throughfall N deposition using mixed ion exchange resin collectors under the forest canopy. To characterize standing C stocks, we measured tree diameter and scaled these values to aboveground biomass using allometric equations.
Results/Conclusions
Results demonstrate that standing biomass C is higher at the forest edge than interior in rural areas, but that urban areas do not have differences between edge and interior biomass. Concentrations of ozone and NOx are higher at urban than rural sites and at the forest edge compared to forest interior. Rates of total atmospheric N inputs in throughfall are not significantly greater in urban than rural sites, but nitrate inputs in throughfall at forest edges are higher in urban areas. This work builds upon our understanding of the quantities and spatial heterogeneity of air pollutants in the greater Boston area to better understand their consequences for tree health and the terrestrial carbon sink. As carbon dioxide concentrations continue to rise, plants will continue to play an important role in removing carbon dioxide from the atmosphere through photosynthesis and plant growth; investigating their response to additional environmental factors such as elevated N deposition and tropospheric ozone is essential to understanding vegetation and global carbon dynamics.