136 Morgenroth and Buchan: Soil Moisture and Aeration Beneath Pervious and Impervious Pavements 2) Impervious concrete pavement–a 10 cm (4 in) deep impervious concrete pad; 3) Pervious concrete pavement–a 10 cm deep per- vious concrete pad. As part of a larger experiment, referred to in the introduction, plane-tree (Platanus orientalis) seedlings were planted in the plot centers and, for pavement treatments, in a cir- cular cut-out (30 cm diameter). As trees contribute significantly to soil moisture dynamics, their presence is crucial for accurate- ly quantifying soil moisture in a simulated urban environment. Data Collection Soil volumetric moisture content (θsoil ) was measured every five minutes from December 2007–May 2008, using ECH2O EC-20 probes (Decagon Devices, Inc., Pullman, WA, U.S.) interfaced with a Campbell CR10X data logger (Campbell Scientific, Inc., Logan, UT, U.S.). Daily means were calculated and weekly means were used to compare differences amongst treatments. The mea- surement period coincided with early summer to late autumn. Fol- lowing previous authors (e.g., Baumhardt et al. 2000; Lane and Mackenzie 2001), rather than using the ECH2 O probe’s built-in calibration, the following soil-specific calibration was obtained, using methods recommended by the manufacturer (Cobos 2007): [1] Here, tent, and soil = 1.2447 · probe + 3.5422 By post-processing the data with this calibration, the accu- racy of soil (%) is the calibration-adjusted soil water con- probe (%) is the value predicted by the ECH2 O probe. soil is assured to ±2% (Decagon Devices Inc. 2006). In each plot, three probes were buried 5 cm (2 in), 10 cm, and 20 cm (7.9 in) beneath the soil surface, halfway between the seedling and the plot edge (45 probes in total). Each sensor was inserted parallel to the soil surface, with its flat surface vertical to minimize disturbance of soil moisture movement. The probes were installed in July 2007 and the first readings were collect- ed in December 2007 to allow sufficient time for equilibration. Four probes temporarily malfunctioned, during which time their readings were discarded. The readings from the remain- ing four probes, per treatment and depth combination, were used to calculate an average permanent wilting point (PWP) and field capacity (FC) of the soil were measured via pressure plate (Model 1500 15 bar ce- ramic plate extractor, Soil Moisture Equipment Corp., Santa Barbara, CA) and a soil moisture release curve. Their values are approximately 11.1% and 27.9% respectively, by volume. Aeration was determined using the steel rod technique (Car- soil for that combination. The nell and Anderson 1986). On December 4, 2007, one steel rod was allocated to each plot and inserted into soils following the method of Hodge et al. (1993). Rods were inserted halfway between the center and edge of each plot. On March 6, 2008, all rods were unearthed, cleaned, and swabbed in an ammonia solution to stop oxidation. Following Carnell and Anderson (1986), two corrosion categories were created: 1) red/brown rust or raised black corro- sion, which indicated well aerated soil; and 2) smooth black or matte gray corrosion indicative of anaerobic conditions, or shiny metal, both classed as inhospitable for root growth. Using these categories, the corrosion patterns were analyzed and scores reflect- ing the proportion of rust were assigned to each 12 cm (4.75 in) segment of rod based on the method of Hodge and Boswell (1993). ©2009 International Society of Arboriculture Figure 1. Variations of a) mean daily soil moisture in the upper- most 20 cm (7.9 in) of soil, and b) daily precipitation. hibiting higher process, whereby vapor diffuses towards, then condenses on, a cool surface. Soils gain heat energy and reach their maximum temperature later than maximum air temperature, with a delay be- tween c. 1 hour at the surface to c. 10 hours at 30 cm depth (Buch- an 2001; Celestian and Martin 2004). Following this, they release heat into the atmosphere. In the early evening, as the soil surface cools, water vapor is drawn upwards and condenses on the under- side of the pavement then drains back into the uppermost layer of Two compounding mechanisms likely result in paved soils ex- soil than unpaved soils. The first is a distillation Statistical Analyses Mean values of weekly volumetric soil moisture were contrast- ed using one-way analysis of variance (ANOVA), where treat- ment was the main factor. Two-way ANOVA was used to con- trast differences between treatments and depth classes for soil aeration data. Subsequent pairwise comparisons were computed by the Tukey-Kramer HSD test (R Development Core Team 2008). All statistical differences are reported at P value = 0.05. RESULTS AND DISCUSSION Soil Moisture The weekly mean soil moisture values for both pervious and imper- vious treatments were statistically similar throughout experiment save the final week (measurement days 127–133) when average soil moisture beneath impervious pavement (32.1%) was exceed- ed beneath pervious pavement (33.8%) (P < 0.001) (Table 1). Both pavement treatments exhibited significantly higher soil moisture values than controls throughout the experiment (P < 0.001). While the over 45% in early summer to c. 32% by late autumn (Figure 1). soil of unpaved (control) soils ranged from roughly 20%– 32%, soil under both pavement treatments steadily declined from θ θ θ θ θ θ θ θ θ
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