Arboriculture & Urban Forestry 44(4): July 2018 concentrations. 177 Table 1. Composition of the three structural soils used at the structural-soil site. The data for these three soil types were pooled, as no significant differences were detected between the O2 Structural soil 1 2 3 Matrix stone size (mm) 32–64 30–120 64–150 Stone matrix / Fine material volume (%) 65/35 70/30 70/30 tree soil and adjacent (approx. 1 m distance) to 11 trees at the structural-soil site. The gas sam- plers in the conventional soil were installed in August 2011 at depths of 10, 30, and 90 cm from the soil–atmosphere interfaces. The gas samplers in the structural soil were installed during struc- tural soil-construction in 2002 at depths of 10, 30, and 60 cm from the structural soil–fine macadam interface (see Figure 1). With the cobblestone coating and the macadam bed, the actual distance from the sampling tubes to the atmosphere–soil interface was 40, 60, and 90 cm, respectively. In general, soil air samples were collected dur- ing the growing season at intervals of 14 days. Toward the end of the growing season in 2012, conventional soil was sampled less frequently. Air samples from structural soil were taken four times in 2012: in weeks 21, 27, 37, and 43. Soil air samples were collected through perma- nently installed 300 mm long aluminum tubes with an inside diameter of 6 mm and the holes on the first 50 mm from tip were covered with GORE- TEX® fabric (porous polytetrafluoroethylene coat- ing). The gas samplers were connected to ground level sampling heads (butyl rubber and neoprene septa) with 3 mm diameter Nylon tubing. Soil air samples (volume approx. 5–20 ml) were drawn into polypropylene syringes (20 ml) with a hypo- dermic needle. In 2012–2013, the samples were analyzed with two interconnected gas chromato- graphs equipped with flame ionization, thermal conductivity, and electron capture detectors (HP 5890 Series II, Hewlett Packard, Bothell, Wash- ington, U.S.), followed by manual injection to gas sampling valves. In 2014, the samples taken into syringes were then injected through a double- wadded septum (VC329) into He-flushed and pre- evacuated 3-ml soda glass vials (Exetainer®, Labco Ltd., Buckinghamshire, UK). An autosampler and Fine material constituents Peat, clay, coarse, and fine sand Sewage sludge, bark, and peat compost Clay, gravel, sand, leaf compost, and bark a gas sampling valve facilitated the injection of samples into the gas chromatograph Agilent 7890B Series Custom GC GHG-Analyzer.) equipped with flame ionization, thermal conductivity, and electron capture detectors (Agilent Technologies, Inc., Santa Clara, California, U.S). One-point cali- bration for each measured gas (CH4 N2 mixture as described by Penttilä et al. (2013). O, O2 , N2 Ground Ventilation To assess the effect of different soil sealing meth- ods on the ventilation of the root zone, CO2 flux measurements were conducted with a portable closed-chamber CO2 exchange measuring sys- -flux measurements were carried out two times at five points (five different trees) dur- ©2018 International Society of Arboriculture chamber by measuring the infrared radiation ab- sorbance level at 15-second intervals for 5 minutes. Relative humidity (RH) and temperature (T) were simultaneously measured with a top-mounted probe (HMP75B; Vaisala Ltd, Vantaa, Finland). An approximately 4-cm-diameter fan was integrated in the chamber to ensure air mixing. A cylinder- shaped plastic collar of 70 mm (height) × 211 mm (inner diameter) was randomly placed on top of the soil cover at a maximum horizontal distance of 50 cm from the uppermost soil air sampler. The gaps between the soil cover and the collar were sealed using wet quartz sand from the exterior side of the collar before placing the chamber on it. The data were processed and stored with an M170 mea- surement indicator (Vaisala Ltd, Vantaa, Finland). CO2 tem described in Kolari et al. (2005). The cham- ber was an open-bottomed and wide-edged plastic cylinder of 250 mm (height) × 200 mm (inner diameter), with a GMP 343 carbon dioxide probe (Vaisala Ltd, Vantaa, Finland) mounted on top. The device recorded the accumulation of CO2 in the ) was carried out with a certified gas , C2 H4 , CO2 ,
July 2018
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