.|  Baltimore Ecosystem Study
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).

Figure 1. 15N recovery, as a percentage of applied tracer, over time for forests and lawns (n = 4 plots each). Full bars represent total plot recovery, which includes recovery in six ecosystem pools: microbial biomass (MBN), exchangeable inorganic nitrogen (EIN), forest Oi-layer or lawn thatch (Oi/Th), fine roots (Root), aboveground biomass (AGB), and mineral soil organic matter (SOM). Data are means 6 standard deviation. Different lowercase letters indicate significant differences in total plot recovery within and across vegetation types at P < 0.05. From Raciti et al. (2008).
We were also keen to make measurements in actual residential parcels to determine if our long-term study plots were relevant to "real world" conditions. A first step in this effort was an assessment of homeowner practices through a detailed door-to-door survey in two of the main BES long-term study watersheds; one exurban area with large lots and expensive homes and one suburban area with older, smaller, less expensive homes. This survey produced surprising results in that fertilization was less common than we expected, ranging from approximately 50 - 75% and was higher in the older, denser, less wealthy neighborhood (Law et al. 2004).
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).

Figure 2. Comparison of carbon density, nitrogen density, carbon-to-nitrogen ratio, and bulk density between residential and forest soils at 0-100 cm depth (A, B, C, and D; n = 32 and n = 8 for residential and forest, respectively). *P < 0.05; **P < 0.01. From Raciti et al. (2011b)
These studies on actual residential parcels also produced some surprising mechanistic insights into lawn biogeochemistry. First, we were surprised that residential soil profiles were largely intact, with little evidence for compaction and/or alteration of soil material (Raciti et al. 2011b) (Figure 2d). Second, we were surprised that residential soils had higher carbon and nitrogen content than soils from the forested reference plots (6.95 vs. 5.44 kg C/m2 and 552 versus 403 g N/m2). We were particularly surprised that much of the carbon and nitrogen accumulation in the residential soils occurred at depth (30 - 100 cm) in the soil profile (Figure 3). We also observed strong relationships between carbon and nitrogen content and lawn age, but only at sites that were previously in agriculture (Figure 4). Rates of N accumulation at these sites were roughly equal to estimated fertilizer N inputs at the sites, confirming a high capacity for N retention.

Figure 3. Comparison of carbon and nitrogen density (A and B) and concentration (C and D) between residential and forest soils across four depth intervals (0-10, 10-30, 30-70, and 70 to 100 cm; n = 32 and n = 8 for residential and forest sites, respectively). *P < 0.05; **P < 0.01. From Raciti et al. (2011b).

Figure 4. Regression of housing age against soil carbon and nitrogen density at 0-100 cm depth for residential sites that were in agriculture prior to development (n = 9). The dashed line indicates the mean carbon (5.44 0.36 kg C/m2) concentrations measured in forested reference sites (n = 8). From Raciti et al. (2011b).
Detailed nitrogen cycle process results from the actual residential parcels also confirmed results from the long-term study plots and increased our understanding of lawn biogeochemistry. Consistent with the surprisingly low NO3- leaching that we observed in the long-term study plots, soil NO3- pools and internal NO3- production by nitrification were higher in residential parcels than in forested reference plots but they were not as high as expected, i.e. they were comparable to deciduous forest stands in other studies (Raciti et al. 2011a) (Figure 5). Also consistent with results from the long-term study plots was the observation that homeowner management practices (fertilization, irrigation) were not predictive of NO3- availability or production suggesting that active carbon and nitrogen cycling are key drivers of the environmental performance of lawns.

Figure 5. Comparison of soil parameters between residential lawn and forest soils across four depth intervals: 0-10 cm, 10-30 cm, 30-70 cm, and 70-100 cm (residential sites, n = 32; forest sites, n = 8). Error bars represent standard error. * P , 0.05; ** P , 0.01. From Raciti et al. (2011a).
Literature Cited:
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.