Restoration Techniques for Landscape Soils Damaged by Construction

Land development is accompanied by construction activities that damage soil structure, remove organic matter, and subsequently can reduce tree survival and establishment, slow growth rates and reduce ultimate canopy cover. Soil rehabilitation protocols are needed to assist arborists, landscape contractors, landscape architects, developers and planners to recommend and implement effective soil improvement methods that allow maximum root development and growth for urban and landscape trees. This study evaluates several specific soil rehabilitation protocols to determine their effects on soil physical properties, tree establishment, root development, and other growth parameters for five tree species on a graded and heavily compacted site. This represents the 2^nd year of a long-term study that will evaluate whether increasing access of roots to lower soil regions will enhance carbon sequestration in the soil and contribute to soil biogenesis. Twenty-four 4.6 by 18.3 meter plots were installed in 2007 on a 1,980 m² field site comprised of Shottower and Slabtown loam soils. Control plots were left undisturbed while remediation plots were scraped and graded according to common construction protocols. Subsoil compaction to an average bulk density of 2.0 g/cm³ was initially achieved following topsoil removal with 8 passes of a 4,800 kg sheep’s foot vibrating riding compactor.   Protocols under evaluation consist of combinations of topsoil replacement, amendments and mechanical loosening techniques: Undisturbed (no topsoil removal, no compaction, no amendments), Minimum Effort (topsoil surface application), Enhanced Topsoil (topsoil application and rototilling), and Profile Rebuilding (compost amendment, subsoiling with excavator to a depth of 60 cm, topsoil application and rototilling). Six replications of each treatment are installed in a completely random design with five deciduous tree species in each experimental unit. Bulk densities throughout the soil profile 9 months after treatment installation are characterized. After one growing season, Profile Rebuilding resulted in a 112-223% greater increase in trunk cross-sectional area than the average of the other treatments. Height increase, photosynthesis rates, and rooting depth are also described. This long-term study site will allow assessment of the effects of mechanical loosening and incorporation of organic matter on the soils’ ability to provide ecosystem services (e.g. support vegetation, intercept rainfall, promote groundwater recharge, sequester carbon, etc.) over time.