Urban and suburban grasslands (home and institutional lawns) are a large, and increasingly common land cover type in the U.S. There is great uncertainty about nitrogen cycling and export to adjacent ecosystems in these ecosystems. While they can be heavily fertilized, lawns have an extended growing season and considerable potential for plant uptake and storage of nitrogen in soil organic matter. In the Baltimore Ecosystem Study, an urban component of the National Science Foundation long term ecological research (LTER) network, we have compared hydrologic losses of nitrogen using zero tension lysimeters and soil:atmosphere fluxes of nitrous oxide, methane and carbon dioxide using in situ flux chambers in forest and grass long-term study plots since 2001. Plots are also instrumented with probes to measure soil moisture and temperature. We also measured potential net N mineralization and nitrification, microbial biomass carbon and nitrogen content and soil organic matter content in a larger group of forest, grass and agricultural plots. Our objectives were to 1) compare hydrologic and gaseous losses of nitrogen from forest and grass plots and 2) evaluate the importance of soil temperature, moisture and microbial biomass and activity as controllers of differences between land cover types.
Results/Conclusions
Urban grasslands had consistently higher hydrologic losses of nitrogen than forests, but differences were not always statistically significant, especially in dry years. Heavily fertilized grass plots did not have consistently higher losses than other plots. There were no differences in soil:atmosphere fluxes of nitrous oxide and carbon dioxide losses between plots. However, while methane uptake occurred at high rates in forest plots, it was negligible in grass plots. Grass plots had high rates of nitrification relative to forest plots, but similar levels of soil organic matter and microbial biomass. The high organic matter, microbial biomass and soil carbon dioxide flux in the grass plots suggest that significant pools and fluxes of carbon foster retention of nitrogen such that hydrologic and gaseous losses from urban grasslands are not as high as might be expected given fertilizer input. These results suggest that if properly managed, lawns do not have to have negative effects of water and air quality in urban ecosystems.
