With the rapid urbanization of natural lands, researchers have begun to examine the capacity of urban soils to store carbon (C), with recent attention to residential yards. We performed a case study to examine four potential influences on soil C levels in residential yards. In 67 yards containing trees, we examined the relationship of soil C (kg m−2) to tree aboveground biomass, home age (3–87 years), yard maintenance (fertilization, irrigation, mulching or bagging lawn clippings), and soil texture (% clay, % sand, % silt), at three depths (0–15 cm, 15–30 cm, and 30–50 cm). Six tree aboveground biomass data sets were developed: 1) biomass, 2) biomass*(1/distance from tree), 3) biomass ≤ 15 m from sample site, 4) biomass ≤ 10 m, 5) biomass ≤ 5 m, and 6) biomass ≤ 4 m. Biomass ≤ 5 m and biomass ≤ 4 m had the greatest explanatory power for soil C at 30–50 cm depth (P = 0.001, R2 = 0.28; P = 0.05 R2 = 0.39, respectively). The relationship between soil C and home age was positive at 0–15 cm (P = 0.0003, R2 = 0.19), but constant at the two lower depths. Yard maintenance had no significant influence on soil C levels across home age. At 0–15 cm, soil C increased with % silt (P = 0.006, R2 = 0.12). Overall, trees in turfgrass yards may have a stabilizing effect on soil C levels below 15 cm but minimal influence above 15 cm.