Urban green space is purported to offset greenhouse‐gas (GHG) emissions, remove air and water pollutants, cool local climate, and improve public health. To use these services, municipalities have focused efforts on designing and implementing ecosystem‐services‐based “green infrastructure” in urban environments. In some cases the environmental benefits of this infrastructure have been well documented, but they are often unclear, unquantified, and/or outweighed by potential costs. Quantifying biogeochemical processes in urban green infrastructure can improve our understanding of urban ecosystem services and disservices (negative or unintended consequences) resulting from designed urban green spaces. Here we propose a framework to integrate biogeochemical processes into designing, implementing, and evaluating the net effectiveness of green infrastructure, and provide examples for GHG mitigation, stormwater runoff mitigation, and improvements in air quality and health.

Some natural ecosystems near industrialized and agricultural areas receive atmospheric nitrogen inputs that are an order of magnitude greater than those presumed for preindustrial times. Because nitrogen (N) often limits microbial growth on dead vegetation, increased N input can be expected to affect the ecosystem process of decomposition. We found that extracellular enzyme responses of a forest-floor microbial community to chronically applied aqueous NH 4 NO 3 can explain both increased and decreased litter decomposition rates caused by added N. Microbes responded to N by increasing cellulase activity in decaying leaf litter of flowering dogwood, red maple, and red oak, but in highlignin oak litter, the activity of lignin-degrading phenol oxidase declined substantially. We believe this is the first report of reduced ligninolytic enzyme activity caused by chronic N addition in an ecosystem. This result provides evidence that ligninolytic enzyme suppression can be an important mechanism explaining decreased decay rates of plant matter seen in this and other N-addition experiments. Since lignin and cellulose are the two most abundant organic resources on earth, these altered enzyme responses signal that atmospheric N deposition may be affecting the global carbon cycle by influencing the activities of microbes and their carbon-acquiring enzymes-especially the unique ligninolytic enzymes produced by white-rot fungi-over broad geographic areas.

In order to understand the effect of urban development on the functioning of forest ecosystems, during the past decade we have been studying red oak stands located on similar soil along an urban-rural gradient running from New York City ro rural Litchfield County, Connecticut. This paper summarizes the results of this work. Field measurements, controlled laboratory experiments, and reciprocal transplants documented soil pollution, soil hydrophobicity, litter decomposition rates, total soil carbon, potential nitrogen mineralization, nitrification, fungal biomass, and earthworm populations in forests along the 140 × 20 km study transect. The results revealed a complex urban-rural environmental gradient. The urban forests exhibit unique ecosystem structure and function in relation to the suburban and rural forest stands; these are likely linked to stresses of the urban environment such as air pollution, which has also resulted in elevated levels of heavy metals in the soil, the positive effects of the heat island phenomenon, and the presence of earthworms. The data suggest a working model to guide mechanistic work on the ecology of forests along urban-to-rural gradients, and for comparison of different metropolitan areas.

To determine the patterns of atmospheric deposition and throughfall in the vicinity of a large city, bulk deposition, oak forest throughfall, and particulate dust deposition were measured at sites along a transect within and to the north of New York City. Concentrations and fluxes of NO3 -, NH4 +, Ca2+, Mg2+, SO4 2-, and Cl- in throughfall all declined significantly with distance from the city, while hydrogen ion concentration and flux increased with distance from the city. Most of the change in concentrations and fluxes occurred within 45 km of the city. Throughfall deposition of inorganic N was twice as high in the urban sites as compared to the suburban and rural sites. Bulk deposition patterns were similar to those of throughfall, but changes along the transect were much less pronounced. The water-extractable component of dust deposition to Petri plates also was substantially higher in the urban sites for Ca2+, Mg2+, SO4 2-, NO3 -, and Cl-. The dust particles had little alkalinity, suggesting that alkaline aerosols were neutralized by acidic gases in the atmosphere. We propose that dust emissions from New York City act like an “urban scrubber”, removing acidic gases from the atmosphere and depositing them on the city as coarse particle dry deposition. Despite the urban scrubber effect, most of the dry deposition of nitrate was from gaseous nitrogen oxides, which were in much higher concentration in the city than in rural sites. Excess deposition of nutrients and pollutants could be important for the nutrient budgets of forests in and near urban areas.