Soil Nitrogen Isotope Composition in Residential Lawns Across Six U.S. Cities.

Urban residential landscapes and associated lawns are widespread across the U.S. Lawns often require large inputs of water and nutrients for establishment and optimal growth. The goal of this study was to quantify the soil nitrogen isotope composition of lawns and identify drivers of soil N dynamics across multiple U.S. cities. We expected more enriched soil δ¹⁵N with N fertilization and in older lawns vs. newly established lawns. We studied residential yards in six cities across the U.S. that span major climatic regions:  Baltimore, Boston, Los Angeles, Miami, Minneapolis-St. Paul, and Phoenix, and compared lawn soil N dynamics to native ecosystems surrounding each city. Soil δ¹⁵N was positively related to housing age (a proxy for lawn age) at the soil surface (0-10 cm) in Baltimore (r²=0.62, p<0.001), Boston (r²=0.29, p<0.01), LA (r²=0.34, p<0.01), Minneapolis-St. Paul (r²=0.25, p<0.05), and Phoenix (r²=0.18, p<0.05). There was a weaker but still positive relationship in Miami (r²=0.16, p=0.08). We found no significant differences in soil δ¹⁵N based on lawn fertilization practices except in Boston, where fertilized yards were depleted (mean δ¹⁵N = 4.3±0.25‰) relative to unfertilized yards (mean δ¹⁵N = 5.1±0.19‰). Across all cities, soil δ¹⁵N was enriched in residential lawns (mean δ¹⁵N = 4.5±0.37‰) compared to the native ecosystems (mean δ¹⁵N = 3.0±0.58‰; p<0.10). Soil δ¹⁵N appears to be driven strongly by time since development and was not correlated with our metrics of lawn management, leading to significantly enriched soil δ¹⁵N in older lawns across the U.S. Future analyses will provide more insight as to whether these patterns are related to other metrics of lawn management or ecological processes such as soil microbial activity, which may further identify mechanisms controlling soil N patterns across multiple cities.