Residential lawn and soil nitrogen isotopes in seven major cities across the U.S.
An emerging theory in urban ecology is that residential landscapes are more similar across cities than the native ecosystems they replace. Residential lawns are an ideal land cover type to study this since sociological drivers (e.g., management practices, N deposition) can homogenize ecosystem processes. We hypothesize that residential lawn δ15N is more similar across cities than native plant δ15N due to the homogenizing influence of urbanization and human management. Additionally, we expect more enriched δ15N in plants and soils with greater N deposition, N fertilization, and in older housing developments. We studied residential yards in seven cities across the U.S. that span major climatic regions: Baltimore, Boston, Los Angeles, Miami, Minneapolis-St. Paul, Phoenix, and Salt Lake City. We compared yard ecosystem function to native ecosystems surrounding each city. Within each city, yards were located along an urbanization gradient and ranged in management practices and housing age. The nitrogen (N) isotopic composition (δ15N) of plants and soils provides a unique means of integrating the effects of N sources, management practices, and soil N processes. Within each yard and native site two soil cores (1-m depth) and four lawn/plant samples (within 30-cm of each soil core) were collected and analyzed for δ15N.
Preliminary results for Baltimore and Los Angeles support our first hypothesis. Lawn δ15N in Los Angeles residential yards was 1.9‰ more enriched than Baltimore yards, whereas the plant δ15N in Los Angeles native sites was 5.0‰ more enriched than the plants in Baltimore native sites. Lawn δ15N became progressively depleted across an urbanization gradient in both Baltimore (δ15N: urban=1.5‰, suburban=-0.4‰, exurban=-0.8‰) and Los Angeles (δ15N: urban=2.9‰, suburban=1.6‰, exurban=1.1‰) suggesting that N sources across cities (e.g., NOx deposition) may be driving lawn δ15N. At the yard-scale, homeowner survey results suggested no significant differences in lawn δ15N with fertilization practices in Baltimore (p>0.10) or Los Angeles (p>0.05). However, as expected lawn δ15N significantly increased with increasing housing age in Baltimore (r2=0.31, p<0.05) and Los Angeles (r2=0.34, p<0.05) suggesting greater rates of soil N cycling. In fact, soil δ15N of residential yards in Baltimore demonstrated greater δ15N values in older homes (δ15N: 1936=4.7‰, 1979=4.2‰, 1996=3.2‰). The similarity in residential lawn δ15N between Baltimore and Los Angeles may be driven by similarity of N sources across cities and increased soil N cycling at the yard-scale. Further analysis will provide insight as to whether these drivers are important in determining ecosystem function across multiple cities.