- Peter M. Groffman, Institute of Ecosystem Studies
- Candiss O. Williams, Tuskegee University (REU student)
- Richard V. Pouyat, United States Forest Service
- Ian Yesilonis, University of Maryland
Land use patterns are one of the major sources of patchiness in ecosystem function in the Baltimore area, in both time and space. The area was originally heavily forested (1700), was then almost entirely in agriculture (1850) and is now a mix of forest, agriculture and residential land use. The residential land uses have large areas of grass. Land use influences soil carbon and nitrogen cycling processes through its effects on fertilizer input, soil water, organic matter, pH and disturbance. Quantifying these influences is important for understanding patchiness in ecosystem functions related to soil fertility and the delivery of nitrate (a cause of eutrophication in salt water) to streams and estuaries. There is particular interest in nitrification, the microbial processes that leads to the production of nitrate. There is great uncertainty and interest in nitrogen cycling and loss is grass areas.
In this study, we sampled 14 forest, 10 row crop agriculture (corn) and 10 grass sites though out the Baltimore metropolitan area in summer 2000. Soils were assayed for microbial biomass carbon and nitrogen content, potential net N mineralization and nitrification, denitrification potential and nitrification potential. As expected, forests had low levels of soil nitrate and nitrification potential (Figures 1 and 2). Grass also had low levels of nitrate, very similar to forest, but had high rates of nitrification potential, similar to agriculture (Figures 1 and 2). Grass areas likely are able to support high levels of nitrification, but low levels of soil nitrate because levels of organic matter (Figure 3) and microbial biomass (Figure 4) are high under grass, creating a carbon-based "sink" for nitrate. Further monitoring on soil nitrate dynamics in grass areas, which are highly variable in time and space, is required to verify these patterns.
Figure 1. Soil nitrate in 14 forest, 10 row crop agriculture (corn) and 10 grass sites in the Baltimore metropolitan area in summer 2000. Values are mean with standard error.
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Figure 2. Nitrification potential in 14 forest, 10 row crop agriculture (corn) and 10 grass sites in the Baltimore metropolitan area in summer 2000. Values are mean with standard error.
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Figure 3. Soil organic matter in 14 forest, 10 row crop agriculture (corn) and 10 grass sites in the Baltimore metropolitan area in summer 2000. Values are mean with standard error.
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Figure 4. Microbial biomass carbon content in 14 forest, 10 row crop agriculture (corn) and 10 grass sites in the Baltimore metropolitan area in summer 2000. Values are mean with standard error.
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These results suggest that land use change has had important effects on nitrogen and carbon cycling and delivery of nitrate to Chesapeake Bay. There is a strong need for further research on grass areas. These areas are mechanistically more complex than we previously thought and their environmental performance, at least as far as
nitrate delivery to receiving waters, is unclear.
