Nitrogen and Carbon Biogeochemistry in Soil and Vegetation Along an Indirect Urban-Rural Gradient in Southeastern Michigan

Abstract

Within an urbanizing landscape, land is frequently set aside for the preservation of native communities and species. While land protection generally restricts immediate disturbances, subtle long-term effects are less well regulated. To evaluate the biogeochemical impacts which urbanization in the surrounding landscape has upon protected areas, 12 protected forest sites in Southeastern Michigan USA were stratified in an indirect urban-rural gradient and evaluated for overstory composition, ground-cover, and nutrient content in soil and foliage. Within each site, overstory species composition and diameter at breast height were recorded, as were ground-cover and soil texture. Overstory basal area, total tree biomass, and foliar biomass were calculated. Soil and foliar carbon and nitrogen concentration and C/N ratio were sampled. Findings included a) a non-linear relationship between housing density and urban ecological impacts, b) an increase in housing density in the surrounding landscape positively correlated with an increased nitrogen concentration in forest soil, c) an increase in housing density positively correlated with an increase in overstory biomass. d) the region was highly nitrogen-rich, with a mean soil C/N ratio of 13.4, e) overstory biomass was a strong control on ground-cover, far more so than was the nitrogen content in soil. Results indicate that the halo of urban-ecological impacts exists in landscapes other than linear urban-rural gradients, and that the surrounding landscape has long-term impacts on soil and plant community composition in a protected area. Further exploration is required to determine the precise scale at which surroundings remain impactful.