Soil Carbon Content and d13 C in Residential Lawns Across Seven Major U.S. Cities.

An emerging theory in urban ecology is that anthropogenic drivers can dominate over natural drivers in controlling ecosystem responses.  Residential lawns are an ideal land cover type to study this since sociological drivers (e.g., management practices) can control ecosystem processes.  The overarching goal of this study is to identify social and natural drivers of soil C dynamics across multiple cities and to test whether the same drivers control soil C across these cities (i.e., homogenization hypothesis).  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.  Within each residential lawn, soil cores and lawn samples were collected and analyzed for δ¹³C and C content.  Soil C content was positively related to housing age at the soil surface and at 10-30 cm soil depth in Baltimore (r²=0.69, p<0.001 and r²=0.35, p<0.01, respectively) and Boston (r²=0.38, p<0.001 and r²=0.30, p<0.001, respectively).  In Baltimore, the surface soil (0-10cm) had significantly greater respiration rates in older yards (houses built before 1960) versus newer yards (p<0.05).  In Boston, the surface soil had significantly greater mineralization rates in the older versus newer yards (p<0.05).  In the deeper soil horizon (10-30 cm), older yards had greater nitrification rates than newer yards in both cities (p<0.05).  We found no significant differences in soil C based on lawn fertilization or irrigation practices in Baltimore or Boston.  In both cities, the surface soil δ¹³C was positively related to plant δ¹³C (p<0.05) as expected.  Soil C content appears to be driven by time and soil microbial processes and not directly by homeowner management practices.  Further analyses of the other cities will provide insight as to whether these patterns are consistent at the national scale, and can help identify mechanisms that may be controlling similar soil C patterns across multiple cities.