Residential lawn and soil nitrogen isotopes in seven major cities across the U.S.

Background/Question/Methods

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.    

Results/Conclusions

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., NO_x 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.