The influence of aboveground tree biomass, home age, yard maintenance and soil texture on soil carbon levels in residential yards

Abstract

Urban land area is expanding worldwide; however, little is known about the dynamics of urban soil C. In natural forests and grasslands, soil C levels increase after disturbances in part due to organic C contributions from roots and possible protection from oxidation by the binding of organic matter to clay particles. In urban areas, home lawns can provide stable chronosequences for studying the response of urban soil C to environmental variables. Residential yards experience the passage of time since home construction, contain roots from turfgrasses and trees, and encompass a range of soil textures. Homeowners may also apply fertilizers, irrigation, and mulched turfgrass clippings to lawns, which may influence soil C levels over time. An observational case study was performed in 102 residential yards in Auburn, AL, and examined the linear regression relationships of soil C to home age, aboveground tree biomass, soil texture, and yard maintenance, within 0-15 cm, 15-30 cm, and 30-50 cm soil depths. The yards were divided into three vegetation categories. Twenty-three ‘pure lawns’ (PL) contained only turfgrass with home ages 1-51 yrs. Sixty-seven ‘lawns with trees’ (LwT) had a mix of turfgrass and trees with home ages 3-87 yrs, and twelve yards composed of ‘unmanaged forest stands ’ (pure woods, PW) contained trees with forest floor instead of turfgrass. Home age was not obtained for PW given that the home was built within a preexisting forest stand and the history of recent forest disturbance was unknown. In PL and LwT, soil C levels at 0-15 cm increased slightly across home age, with greater accumulation in the first three decades for LwT. When compared across a 50 yr home age range, soil C at 15-30 cm had a significantly more positive slope in LwT than PL. Yard maintenance and % clay had no significant influence on soil C levels. In LwT, the explanatory power of significant positive relationships between aboveground tree biomass and soil C increased with the closeness of the trees to the sample site and was greatest at the two lower soil depths, though the R2 values were consistently less than 41%. Further analyses of mean soil C levels across groups of home age and tree biomass showed prevalence for soil C stabilities before and after a significant change in mean soil C levels. Overall, the results indicate that most of the increases in soil C levels occur in the initial few decades and in the top soil layer. The low explanatory power of aboveground tree biomass suggests that measurement of belowground biomass, including fine roots, may provide a better biomass variable to analyze the influence of trees on soil C levels over time.