Arboriculture & Urban Forestry 42(6): November 2016 amendments. Overlying bark mulch and surface- applied amendments were removed before sam- pling. The soil was too compact and rocky for collection with an auger, so samples were col- lected with a spade and trowel from the 0 to 15 cm depth below the mulch layer. Six samples were randomly collected per plot and composited. Soil bulk density was measured using an excava- tion method adapted from Grossman and Reinsch (2002), because the soil was too compact and rocky for standard cores. Holes were excavated with a trowel, and the hole lined with flexible plastic wrap. Water was measured into the hole to estimate the volume. The excavated soil (including gravel and rocks) was oven dried to determine mass. Three excavations were made and averaged per plot. Plant survival was determined in November 2007, September 2008, and September 2009 by counting live and dead plants of each species in each plot. Plant height (tallest shoot) and width (widest and narrowest) were mea- sured on all living plants in 2008 and 2009 at the same time as plant survival. A shoot- growth index was calculated as follows: [({wid- est width + narrowest width}/2) + height]/2. The experiment ended in February 2010, when the site was slated to be disturbed by pending WSDOT construction. Before the site was dis- turbed, the aboveground portions of all live plants were harvested, oven dried at 55°C, and weighed to determine dry biomass. In addition, root sys- tems of fragrant sumac were removed, washed, and lengths of the three longest roots measured. The entire root mass was oven dried and weighed. Data Analysis Soil C, N, and bulk density, and plant survival, shoot-growth index, biomass, and root measure- ments were analyzed with SAS version 9.4 (SAS Institute, Cary, North Carolina, U.S.) as a random- ized complete block design using PROC GLM. Shoot-growth index and root length were square root transformed, and aboveground biomass data were log transformed to meet assumptions of nor- mality. Means were back transformed for report- ing. Root biomass and survival were analyzed without transformation. Soil data showed bimodal distributions, and each mode of the distributions was analyzed separately without transformation. 421 Mean separations were done using LSD with the Tukey-Kramer adjustment at the P = 0.05 signifi- cance level. Orthogonal contrasts were used to compare the effects of surface application versus incorporation of amendments (biosolids blend and yard debris compost) on plant growth and survival. RESULTS AND DISCUSSION Soil C, N, and Bulk Density Soil differences between the surface (including the bark-mulch-only control) and incorporated- amendment treatments were so large, that the data for soil total C, total N, and bulk density all fell into bimodal distributions, as noted in the data analysis. Mean bulk density in the incor- porated treatments was less than half the bulk density in the surface-applied and control treat- ments, whereas mean total C was more than 12 times as great and mean total N, 10 times as great in the incorporated treatments as in the surface and bark mulch control treatments (Table 2). There were no significant differences for any of the soil properties within the surface and control treatments or within the incorpo- rated treatments (Table 2). The lack of differ- ence within the surface-applied and control treatments indicates that little if any mixing of the surface-applied amendments or the bark mulch of the control treatment and the under- lying native material occurred in the first year after application. In contrast, Cogger et al. (2008) observed evidence of mixing of surface- applied compost into uncompacted soil, as shown by reduced bulk density and visual pres- ence of organic matter in the underlying soil. Bulk densities measured in the native mate- rial in the control and surface-applied plots were unusually high, even for urban soils. Jim (1998b) measured an average bulk density of 1.65 Mg m-3 in 100 soil with a maximum of 2.63 Mg m-3 samples taken from 25 roadside locations in urban Hong Kong. Jim indicated that roadside urban soils are the most restric- tive of the urban soils for plant growth. Results from the current study reflect the high degree of compaction at the roadside site, but may also be influenced by the bulk density mea- surement method. For the excavation method ©2016 International Society of Arboriculture
November 2016
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