|Soil nitrogen biogeochemistry in urban grasslands (home lawns)|
Data from our long-term soil biogeochemistry plots motivated a series of more detailed analyses of the nitrogen biogeochemistry of urban grasslands (home lawns). The surprisingly high nitrogen retention that we observed motivated an isotope tracer study, where small amounts of fertilizer enriched with the stable isotope 15N were added to a series of forest and lawn plots and the movement of this tracer was followed into aboveground and belowground plant material and detritus, soil microbial biomass and soil organic matter (Raciti et al. 2008). These studies confirmed that lawns have a high potential for nitrogen retention. After one year, we were able to recover more of the tracer in the grass plots than in the forest plots suggesting that more of the N that we added was lost to the environment from the forests than the lawns (Figure 1).
The survey results motivated a more comprehensive analysis of variation in lawn carbon and nitrogen cycling. The High Ecological Resolution Classification for Urban Landscapes and Environmental Systems (HERCULES) system (Cadenasso et al. 2007) was used to produce an experimental design comparing 32 actual residential parcels with different tree density (driven largely by previous land use, i.e., forest versus agriculture) and structure density (i.e. larger versus smaller lawns). These studies, which included comparison with the 8 forested long-term study plots confirmed that lawns in the Baltimore region have high capacity for nitrogen retention, driven by active carbon cycling (Raciti et al. 2011a, Raciti et al. 2011b) and that our long-term study plots are generally representative of lawns in the region (Figure 2a,b,c).
Cadenasso, M. L., S. T. A. Pickett, and K. Schwarz. 2007. Spatial heterogeneity in urban ecosystems: reconceptualizing land cover and a framework for classification. Frontiers in Ecology and the Environment 5:80-88.
Law, L. N., E. L. Band, and J. M. Grove. 2004. Nitrogen input from residential lawn care practices in suburban watersheds in Baltimore County, MD. Journal of Environmental Planning and Management 47:737-755.
Raciti, S. M., P. M. Groffman, and T. J. Fahey. 2008. Nitrogen retention in urban lawns and forests. Ecological Applications 18:1615-1626.
Raciti, S. R., P. M. Groffman, J. C. Jenkins, R. V. Pouyat, and T. J. Fahey. 2011a. Nitrate production and availability in residential soils. Ecological Applications 21:2357-2366.
Raciti, S. R., P. M. Groffman, J. C. Jenkins, R. V. Pouyat, T. J. Fahey, M. L. Cadenasso, and S. T. A. Pickett. 2011b. Accumulation of carbon and nitrogen in residential soils with different land use histories. Ecosystems 14:287-297.