178 Kämäräinen et al.: A Case Study of Street Tree Soil Aeration in Two Different Soil Types was measured three ing weeks 30 and 36 in the 2014 growing sea- son. Measurements were conducted aſter a dry weather period of at least three days. CO2 -flux times consecutively from the top of soil air samplers by placing the collar and the chamber on top of the paving. The soil water content was simultaneously measured, as explained below. The mean of the three consecu- tive gas flux measurements was considered as a single observation. The chamber was ventilated to reach CO2 concentrations close to 400 ppm at [1] the equation presented by Penttilä et al. (2013): [1] where J = gas flux (g m-2 dt J dc = [2] × M V0 × T H T 0 × h-1 𝑞𝑞숌 = −𝐷𝐷숌 ∆𝑐𝑐숌 conditions (m3 ), M = molar mass of the measured ), V0 ∆𝑥𝑥숌 → 𝐷𝐷숌 = − 𝑞𝑞숌 mol-1 ∆𝑐𝑐숌 /∆𝑥𝑥숌 = gas temperature un- ), dc/dt = slope of the linear regression of volumetric gas concentration on time (h-1 gas (g mol-1 der standard conditions (K), T = measured gas temperature (K), and H = chamber height (m). At a constant temperature and pressure, the dif- = gas volume under standard ), T0 1] size, shape, and continuity of air-filled pores in the soil (Hillel 1998). The gas diffusion coefficient in soil between the uppermost soil air sampler and the chamber was estimated using Fick’s first law: dt J dc = × V0 × T H T 0 × 2] [2] where q = gas flux (g m-2 𝑞𝑞숌 = −𝐷𝐷숌 ∆𝑐𝑐숌 (g m-3 ∆𝑥𝑥숌 → 𝐷𝐷숌 = − 𝑞𝑞숌 s-1 ∆𝑐𝑐숌 /∆𝑥𝑥숌 ), c = concentration ), and x = distance (m). According to Glinski and Stepniewski (1985), the aeration of topsoil can be conveniently described by a parameter called the relative gas diffusion coefficient (D/D0 ). In contrast to the effective gas ©2018 International Society of Arboriculture Statistical Analysis The differences in soil air oxygen concentra- tions and soil aeration between the two soil and paving types were examined by analysis of vari- ance (ANOVA). Prior to the statistical analysis, the O2 concentration data were log-transformed. As soil air oxygen concentrations did not differ between the three different structural-soil mix- tures, these mixtures were considered as replicates. fusive flux of a gas (q) is determined by the effective gas diffusion coefficient (D) and the concentra- tion gradient (Fick’s first law). In soil, the diffusion coefficient also depends not only on the diffu- sion coefficient in air (D0 M ) but also on the volume, the beginning of each repeated measurement. Soil air samples were extracted for chromatographic analysis within a few minutes of the gas flux mea- surements. The CO2 flux was estimated using diffusion coefficient (D), the relative gas diffu- rate at which the gas diffuses in air at a given tem- perature and pressure without impeding solids). by its D0 sion coefficient is a soil property that does not depend on air pressure, temperature, or the dif- fusing gas. The relative gas diffusion coefficient was calculated by dividing the measured diffu- sion coefficient of CO2 value (i.e., by the Soil Moisture and Temperature At the conventional-soil site, soil moisture and temperature were measured directly aſter each O2 and CO2 flux measurement. The soil volumetric water content was measured at the depths of 10, 20, 30, and 40 cm with a PR2 Profile Probe (Delta-T Devices Ltd, Cambridge, UK) using min- eral soil calibration. Access tubes for the mois- ture probe were installed in 2011 on the south side of each tree, approximately 100 cm from the trunk. Soil temperature was measured with a digital thermometer (433 MHz cable free; Weber, U.S.) at the depth of 10 cm from the soil surface. At the structural-soil site, soil moisture and temperature were recorded by the permanent measuring equipment installed during site con- struction in 2002 (Riikonen et al. 2011). Soil volumetric water content was measured with ML2x sensors (Delta-T Devices Ltd, Cam- bridge, UK) installed into the fine soil frac- tion of structural soil. Soil temperature was measured with resistors at depths of 10 and 30 cm from the soil–macadam interface at one location in each soil mixture (Riikonen et al. 2011), making altogether three measurements per depth per site. All measurements were col- lected at one- to ten-minute intervals using a data logger (Envic Ltd., Turku, Finland) and were transferred daily to mass data storage.
July 2018
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