Arboriculture & Urban Forestry 45(6): November 2019 An additional ten unplanted containers of each soil pro- file were used to monitor moisture levels and oxygen lev- els. Volumetric water content was measured through holes in the side of the container using a time domain reflectom- eter (TDR). Between measurements, holes were covered to prevent soil and moisture loss. In loam-over-compacted- clay profiles, measurements at 5 cm from the bottom were in clay, at 13 cm were just above the clay, and at 19 cm were in loam. Measurements taken in loam profiles were in the same positions. Oxygen diffusion rate (ODR) was measured in the same ten unplanted containers of each soil profile. The ODR instrument is designed to average the measurements of ten probes taken at the same depth simultaneously, since conditions in the small pockets of soil around the 2 mm platinum needles can be so variable. It was not possible to get multiple sets of probes into each small container for measurements at different depths at the same time, so mea- surements were taken at 2.5 cm intervals as the probes were inserted progressively deeper. This could only be done once, and had to be done after the soil moisture measurements were complete, since it created holes for air and moisture to penetrate. At the conclusion of the study, the soil and roots were removed from each container and divided into six equal parts from top to bottom. The soil was washed from the roots. Length of fine roots (< 2 mm diameter) was mea- sured and converted to fine-root volume density with a WinRhizo system (Regent Instruments, Quebec, Canada). One-way ANOVA (P ≤ 0.05) with separation of means by the Tukey HSD method was used to compare soil mois- ture, soil oxygen, and fine-root density across soil depths. T-tests were used to compare fine root density between the two soil profiles at the same depth (JMP®, Version 14. SAS Institute Inc., Cary, NC, 1989-2017). RESULTS AND DISCUSSION Soil Conditions The soil conditions created in the containers were similar to the compacted poorly drained soils that exist on many urban sites. Soil moisture was greater deeper in the con- tainer, as would be expected with them standing in water (Figure 2). Moisture increased consistently with soil depth in the loam profile. Moisture at the deepest position in the container was at, or near, the saturation point, where all pore spaces are filled with water. In the loam-over- compacted-clay profile, moisture was highest in the loam just above the compacted clay layer, also very near the sat- uration point, and likely due to poor infiltration into the compacted clay. The moisture level in the compacted clay was somewhat lower, but also at or near saturation, given the reduced pore space resulting from the compaction (Brady and Weil 1996). 255 Figure 2. Soil moisture increased with depth and above the loam/compacted clay soil interface. ODR decreased with soil depth and increasing moisture levels in both soil profiles, as would be expected with fewer air-filled pores (Figure 3). The only significant dif- ference between the two soil profiles was higher ODR in the loam soil at 7.5 to 12.5 cm depth. The lower ODR in the loam-over-compacted-clay profile at this level may have been due to a perched water table over the loam/com- pacted clay interface. Root Growth Response Root growth generally decreased with soil depth (Figure 4). The increased moisture and reduced aeration conditions deeper in the container soil profiles are the factors most likely responsible. In the loam profile, there was a gradual reduction in root density with depth in Acer negundo, Catalpa speciosa, Gledetsia triacanthos, and Quercus Figure 3. Oxygen in the soil atmosphere decreased with soil depth and increasing moisture levels. ©2019 International Society of Arboriculture
November 2019
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