Effects of urban land development practices on soil greenhouse gas emissions

Background/Question/Methods

Carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) are three major greenhouse gases (GHG) contributing to global climate change. We asked how urban land development practices and also subsequent soil management can affect GHG efflux. Urbanization can degrade soils, potentially reducing ecosystem function and making vegetation establishment difficult. Soil rehabilitation through decompaction and organic amendment may restore some of these functions, but its effect on GHG emission is unknown. In 2007, four treatments were applied to soil pre-treated by topsoil stripping and grading to mimic typical land development practices: typical practice (10 cm topsoil replaced), enhanced practice (10 cm topsoil replaced plus tilling), rehabilitation via profile rebuilding (compost incorporated via subsoiling to 60-cm depth, 10 cm topsoil replaced plus tilling), and undisturbed (no pretreatment/no improvement). All plots were planted with five tree species. Plot measurements included microbial biomass carbon (MBC), root length. We measured CO₂ efflux with a Licor 6250 (closed dynamic chamber) throughout summer 2011. CO₂, N₂O, and CH₄ efflux were then measured via vented closed chambers seasonally beginning in September 2011. Samples were analyzed by gas chromatography and GHG emission rates and global warming potential calculated.

Results/Conclusions

Rehabilitated urban soils and undisturbed soils had greater CO₂ efflux (4.05 and 3.82 mmol m^-2 sec^-1, respectively) than soils subjected to typical and enhanced construction practices (both 1.70 mmol m^-2 sec^-1) during the growing season. Seasonal efflux of three GHGs (CO₂, N₂O, and CH₄) from fall 2011 up to summer 2012 will be presented in the context of MBC and root development and soil management practices. Rehabilitated soils (subjected to profile rebuilding) had greater MBC at the 15-30 cm depth (149.47 mg C/kg soil) than soils subjected to either typical or enhanced construction practices or undisturbed soil (ranging from 40.36 -90.53 mg C/kg soil), but there was little difference between management practices at shallower depths. Soil rehabilitation altered vertical root distribution with mean total root length increasing with soil depth (74.7, 108.5, and 380.2 cm at 0-5, 5-10, and 15-30 cm depths, respectively). Root length decreased with soil depth under typical construction practices. Overall, land development followed by typical management practices suppressed MBC, root development and CO₂ efflux compared to undisturbed soils. In contrast, land development followed by soil rehabilitation resulted in MBC, root development and CO₂ efflux similar to or exceeding levels in undisturbed soil.