Nitrogen Retention in Urban Lawns and Forests
Home lawns are a dominant cover type in urban and suburban ecosystems and there is concern about their impacts on water quality. However, recent watershed-level studies suggest that these pervious areas might be net sinks, rather than sources, for nitrogen in the urban environment. A 15N pulse-labeling experiment was performed on lawn and forest plots in the Baltimore metropolitan area to test the hypothesis that lawns are a net sink for nitrogen and to compare and contrast mechanisms of N retention in these vegetation types. A pulse of 15N-NO3-, simulating a precipitation event, was followed through soils, roots, Oi-layer/thatch, aboveground biomass, microbial biomass, inorganic nitrogen and evolved N2 gas over a one-year period. Gross rates of production and consumption of NO3- and NH4+ were measured to assess differences in internal nitrogen cycling under the two vegetation types. Rates of nitrogen retention in forests and lawns were similar during the first 5 days of the experiment, with lawns showing higher nitrogen retention than forests after 10, 70, and 365 days. Lawns had larger pools of available NO3- and NH4+; however, gross rates of mineralization and nitrification were also higher, leading to no net differences in NO3- and NH4+ turnover times between the two systems. Levels of 15N remained steady in forest soils from days 70 to 365 (at 23% of applied 15N), but continued to accumulate in lawn soil organic matter (SOM) over this same time period, increasing from 20% to 33% of applied 15N. The dominant sink for nitrogen in lawn plots changed over time; abiotic immobilization in soils dominated immediately (1 day) after tracer application (42% of recovered 15N), plant biomass dominated the short (10 days) term (51%), thatch and SOM pools together dominated the medium (70 days) term (28% and 36% respectively), while the SOM pool alone dominated long (1 year) term retention (70% of recovered 15N). These findings illustrate the mechanisms whereby urban and suburban lawns under low to moderate management intensities are an important sink for nitrogen.
Committee members: Timothy J. Fahey, Peter M. Groffman, David R. Lee
National Science Foundation funded IGERT in Biogeochemistry and Environmental Biocomplexity at Cornell University, National Science Foundation funded Baltimore Ecosystem Study Long Term Ecological Research project (DEB-0423476)
15N; nitrogen; soil; forests; lawns; turfgrass; mineralization; nitrification; urban; development
dissertation or thesis