66 Fair et al.: Maple (Acer spp.) Response to Soil Compaction and Pre-plant Nitrogen densities. Bulk densities were plotted against percent volumetric water content to develop a Proctor density curve (not shown). The maximum practically-achievable bulk density, based on the physical and textural properties of the soil was 1.7 g·cm-3 volumetric water content of 0.31 cm·cm-3 at a . Results from this test allowed us to assess the effectiveness of compaction efforts. The Bouyoucos hydrometer method was used to analyze the field–soil texture, following methodology from Gee and Bauder (1986). Soil samples were taken, air-dried, and processed as for the Proctor test. In the eastern and central replicates the soil was clas- sified as a silty-clay (19% sand, 41% silt, and 40% clay) and in the western replicate a clay loam (23% sand, 39% silt, and 38% clay). Three weeks prior to planting, Roundup® (glyphosate, 41% a.i., Monsanto, St. Louis, Missouri, U.S.) was applied to the non-compacted (NC) plots at the recommended rate of 15.7 ml concentrate L-1 spray solution. Herbicide was applied to the NC plots initially to limit any potential increase in density that would result from the mechanical removal of vegetation as that used for the compacted plots. There is conflicting research regarding the effects of glyphosate on soil microorganisms. In their 2009 study, Zablotowicz et al. found that under no-till management (similar to the handling of the NC plots in this study), the use of glypho- sate would have a minimal impact on soil microbial populations. Additionally, Busse et al. (2001) found that using glyphosate at field rates had little or no effect on the soil microbial community in ponderosa pine plantations. One study showed a 50% decrease in linear growth of 20 species of fungal colonies after 30 days, when soil in a boreal forest was treated with 50 mg·L-1 glyphosate (Tan- ney and Hutchison 2010). Glyphosate use was continued through- out the study to control weeds in all whole treatment plots as need- ed. Wood chip mulch was placed on all treatment plots; therefore, its effects would have been similar across the entire study area. On October 22, 2001, 30 cm diameter holes were augured and the sides of the holes were scored with a shovel. Trees were planted between October 23 and October 25, 2001, 3 m on cen- ter and cultivars were randomly assigned within the sub-plots (pre-plant N). The roots were feathered-out, and trees were planted at grade depth. All cultivars were represented with at least two trees in each whole plot (soil treatments) with the ex- ception of ‘Frank’s Red,’ which was absent from plot C2, rep 1 (Figure 1). Additionally, each sub-plot (pre-plant N) had at least one tree of each cultivar, with the exception of plot C2 rep 1, in which ‘Frank’s Red’ was absent (as previously reported). Due to original availability of cuttings and mortality after field planting, there were an unequal number of trees of each culti- var (Figure 1). After planting, all treatment areas were mulched with un-composted wood chips to a depth of approximately 8 cm. Based on the results of the Proctor test, the study authors be- further, and therefore in July Figure 1. Layout of soil treatment whole plots, N treatment sub- plots, and the sub sub-plots showing the numbers of each cul- tivar (which were randomly located within the N treatment sub- plots) at Waterman Research and Education Facility in Columbus, Ohio, U.S. (not shown to scale). Mean bulk density for NC = 1.40 g·cm-3 density for C2 = 1.55 g·cm-3 , mean bulk density for C1 = 1.60 g·cm-3 . ©2012 International Society of Arboriculture , and mean bulk 2002, half of the previously compacted plots were randomly se- lected and an additional soil treatment applied. The wood chip mulch was raked from the areas by hand. Plots were irrigated for 24 hours prior to applying the treatment. Soil samples were taken just prior to compaction and gravimetric water content measured as previously. From those values, mean θw was calculated as 0.14 g·g-1 lieved they could increase the ρb of approximately 1.6 g·cm-3 . . A Bomag Model BW213PDH-3 vibrating, single drum, pad- foot roller (Hellerwald, Germany) was driven between the rows of trees to compact the soil. Plots compacted in 2002 will be referred to as C2; these plots had a mean ρb
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