An emerging theory in urban ecology is that anthropogenic drivers can dominate natural drivers controlling ecosystem responses, leading to convergence of ecological structure and function. Residential lawns are an ideal venue for studying convergence since sociological drivers (e.g., management practices) can control ecosystem processes. The overarching goal of this study was to examine soil C and N dynamics at the continental scale (i.e., convergence hypothesis), and to determine whether spatial (i.e., urbanization influence) and temporal scales (i.e., housing age) within cities control soil C and N dynamics. To address our objective, we measured soil δ13C, soil organic carbon (SOC), soil total C (TC), soil δ15N, and total soil N content in residential lawns and paired reference sites (i.e., natural unmanaged soils) in six cities across the U.S. (Baltimore, Boston, Los Angeles, Miami, Minneapolis-St. Paul, and Phoenix) spanning multiple ecological biomes and climatic regions. Residential lawns in each city were located along an urbanization gradient and ranged in housing age. Within each residential lawn and natural site, two 1-m deep soil cores were collected and separated into four depth profiles. We report data for surface (0-10 cm) and subsurface soil (10-30 cm) layers.
Results/Conclusions
Across six cities, SOC, organic soil δ13C, and δ15N were more variable within reference sites than residential lawns, supporting the convergence hypothesis. Additionally, SOC and TC and soil-N were greater in residential lawns compared to natural sites. This suggests greater SOC in residential lawns results in convergence of SOC, whereas greater soil-N leads to greater variability in lawns than natural landscapes. Furthermore, greater SOC in urban versus suburban/exurban residential yards supports greater inputs with intense urbanization. Similarly, soil-N and δ15N declined across the urbanization gradient in most cities. SOC and TC increased with housing age especially within colder/drier climates (i.e. Boston, Los Angeles, Phoenix). Total soil δ13C followed similar pattern to TC, increasing with housing age in Boston, yet decreasing in both Los Angles and Phoenix indicating that in warm arid climates inorganic-C inputs are important. Soil-N and δ15N significantly increased with increasing housing age in most cities. Soil-C dynamics strongly support the convergence hypothesis, whereas soil-N dynamics are more greatly influenced by urbanization and housing age. Our findings address a critical gap in our knowledge regarding how urban inputs affect soil-C and soil-N dynamics at continental and regional scales.