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Long-term biogeochemical study plots
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The Baltimore Ecosystem Study (BES) has established a network of long-term permanent biogeochemical study plots. These plots provide long-term data on vegetation, soil and hydrologic processes in the key ecosystem types within the urban ecosystem. Plots are currently instrumented with lysimeters (drainage and tension) to sample soil solution chemistry, time domain reflectometry probes to measure soil moisture and trace gas flux chambers to measure the flux of carbon dioxide, nitrous oxide and methane from soil to the atmosphere. Measurements of in situ nitrogen mineralization, nitrification and denitrification were made at approximately monthly intervals from fall 1998 to fall 2000. Detailed vegetation characterization (all layers) was done in summer 1998 and 2003
The current network of study plots includes eight forest plots, chosen to represent the range of forest conditions in the area, and four grass plots. Four of the forest plots are located close to the urban core of Baltimore City, and four are located approximately 30 km into the rural/suburban fringe of Baltimore County. In addition to evaluating effects of an urban land use matrix on forest ecosystems, these plots also allow us to compare the major natural soil/vegetation controls in this region as three of the plots are located on relatively fine-textured, fertile soils while five are located on less fertile, more coarse-textured soils. The four grass plots are located in suburban areas, 10 – 20 km from the urban core.
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Urban forests
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(Results from: Groffman, P.M., R.V. Pouyat, M.L. Cadenasso, W. C. Zipperer, K. Szlavecz, I.C. Yesilonis, L.E. Band and G. S. Brush. Land use context and natural soil controls on plant community composition and soil nitrogen and carbon dynamics in urban and rural forests. Forest Ecology and Management. In press.)
Figure 1. Mean in situ net N mineralization (top) and nitrification (bottom) rates in urban and rural forest sites measured at 15 dates from fall 1998 – summer 2000. Values are mean (standard error). Hillsdale plots 1 and 2 and Leakin plot 1 are located on high fertility soil types while the other plots are located on low fertility soil types. Bars with different superscripts are significantly different at p < 0.05 in a one-way analysis of variance with "plot" as the main effect. A two-way analysis of variance with "land use" and "soil fertility" as main effects found no significant land use effect, but soil fertility was significant for both variables.
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Data to date suggest that forest productivity and N cycling vary more with soil type than with proximity to urban land use, while forest composition and soil:atmosphere fluxes of CO2 and CH4 are more strongly influenced by exposure to an urban land use matrix and atmosphere. There was no difference in net N mineralization and nitrification rates (Figure 1) nor in soil NO3- levels (data not shown) between urban and rural forest sites. However, all three variables were significantly (p < 0.05) higher on sites with high fertility soil types than on sites with low fertility soil types (Figure 1). Soil solution concentrations of NO3- were consistent with patterns of soil N cycling, with higher (p < 0.05) concentrations in plots on high fertility soil types and no land use effect (Figure 2). Soil:atmosphere fluxes of CO2 showed marked seasonal patterns, with higher rates during the warmer seasons than the colder seasons, but fluxes of N2O and CH4 did not (Figure 3). Production of CO2 was higher (p < 0.05), and consumption of CH4 was lower (p < 0.05) in the urban than the rural plots (Figure 4).
Figure 2. Volume-weighted nitrate concentrations in tension lysimeters at 10 (top) and 50 (cm) depth in eight urban and rural forest stands from fall 1999 – fall 2002). Note the separate y-axis that applies only to the Urban (HD) – 2 site. Values are the mean of four replicate lysimeters at each depth that produced samples from 1 – 10 times each year. Hillsdale (HD) plots 1 and 2 and Leakin (LEA) plot 1 are located on high fertility soil types while the other plots are located on low fertility soil types. A two-way analysis of variance with "land use" and "soil fertility" as main effects found no significant land use effect, but soil fertility was significant at 10 cm depth.
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Annual N mineralization ranged from 6 – 7 g N m-2 y-1 and nitrification ranged from 0.5 to 1.5 g N m-2 y-1. Annual leaching losses of NO3- were low relative to mineralization and nitrification (< 0.1 g N m-2 y-1).Annual fluxes of N2O ranged from 0.03 to 0.14 g N m-2 y-1. The data show that natural nutrient cycling processes are important in the context of urban and suburban landscapes, i.e. production of, and annual variation in, inorganic N in forest patches is large relative to watershed-scale atmospheric deposition, fertilizer use and food/sewage fluxes that have been measured in other studies in these study landscapes.
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Urban forests versus urban grasslands
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Somewhat surprisingly, urban grasslands do not have markedly higher nitrate leaching than urban forests (Figure 5). Results varied by year, with higher losses from grasses in wet years (2001, 2003) than in dry years (2002). Losses were not strongly driven by fertilizer application, i.e. plot UMBC 2, the most heavily fertilized plot did not have the highest losses.
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