Abstract
This study compared the soil air composition in structural and conventional tree soils used for roadside plantings in the City of Helsinki, Finland. Soil air samples, collected from three depths during the growing seasons of 2012 and 2013, were analyzed for oxygen concentrations. The effect of soil cover on the soil–atmosphere gas flux was evaluated using relative gas diffusion coefficients as estimated from in situ measurements of CO2 fluxes and CO2 concentrations in soil air. The average O2 concentrations were higher in structural than in conventional soil air in both years. No obvious depth profile for the O2 concentration was detected. Structural soil aeration appears more favorable for root growth than aeration in conventional tree soil. However, soil aeration was also sufficient for healthy tree growth in the conventional soil. Cobblestone paving did not severely impair ground ventilation as compared with a cast-iron grate.
Street trees play an intrinsic role in creating a comfortable urban environment (Arnold 1993). In the urban context, their beneficial effects include cooling (King and Davis 2007), interception of noise, stormwater retention, and sequestration of carbon dioxide (McPherson et al. 1997; Becket et al. 2000; Nowak et al. 2006). Urban trees also provide numerous socioeconomic and psychological benefits (Schroeder and Cannon 1983; Sullivan and Kuo 1996).
In the built environment, tree roots frequently grow in limited volumes of soil and compete with infrastructure (Roberts et al. 2006). Previous research has demonstrated a lack of adequate rooting space (Krizek and Dubik 1987; Kopinga 1991) and an insufficient substrate volume (Schwets and Brown 2000) are common causes of problems in urban tree planting. In paved areas, trees often suffer from a lack of oxygen in the rooting zone (Hoff 2001; Sieghard et al. 2005; Roloff et al. 2009). Aboveground growing conditions are difficult to alter, but by improving belowground conditions, tree well-being and sustainability can be significantly increased (Bühler et al. 2007).
Soil aeration refers to the exchange of gases between the atmosphere and the soil. Atmospheric gases can enter the soil in several ways (see Hillel 1998). Rainfall, wind, fluctuations in air pressure, and plant root water uptake can cause mass gas flow between the soil and the atmosphere. Most importantly, however, gases enter the soil by molecular diffusion driven by concentration gradients. Gases mainly move in the soil by diffusion and convection, the former being the principal mechanism (Hillel 1998). Air fills the soil pores that are not filled with water, and continuous air-filled soil pores form the most rapid transport path for gases between the atmosphere and roots. If the continuity and volume of air-filled pores are reduced by excessive water, or compaction, the interrupted gas exchange rapidly leads to hypoxia in roots (Costello et al. 1991; Horn et al. 2007).
The biologically most significant gases in soil air are oxygen and carbon dioxide, the concentrations of which can provide information on soil aeration and structural properties. Near the surface, soil air contains approximately 79% N2, 20% O2, and 0.25% CO2 (Glinski and Stepniewski 1985). The O2 concentration in soil air is influenced by soil respiration (i.e., the biological oxidation of organic compounds by microbes and plant roots that is a function of soil water content and the quality and quantity of easily decomposable organic matter in soil). Soil respiration is usually most intense during the late summer months, and ceases toward the end of the growing season. In frozen soil, fluctuations in the soil oxygen concentration are minor (Glinski and Stepniewski 1985).
Depending on the tree species, roots perform best at oxygen levels above 10% (Kozlowski 1985). Minimum thresholds for tree root survival and growth are estimated to be 3% and 5%–10%, respectively (Kozlowski and Davies 1975). Poor aeration leading to inadequate O2 concentrations hampers root respiration (McDowell et al. 1999; Gaertig et al. 2002), diminishes water and mineral uptake, and retards protoplasm synthesis and cell membrane maintenance (Kozlowski 1999). In hypoxic conditions, the loss of fine roots (Kozlowski and Pallardy 2010) can restrict water and nutrient intake, resulting in poor tree growth and appearance (Krizek and Dubik 1987). While evidence on soil oxygen status effects on urban tree health remain unclear (Costello et al. 1991; Watson and Kelsey 2006), such effects are well-known from many other environments (e.g., Drew 1983; Glinski and Stepniewski 1985; Simojoki 2001).
Besides the supply of water, nutrients, and air to tree roots, urban soils are expected to fulfill numerous other functions. Urban soil characteristics important for root growth have been reviewed by Craul (1985) and Watson et al. (2014), among others. While serving as a suitable rooting space for urban trees, the same material may be required to sustain the weight of the aboveground structures, paving, and traffic. Increasing the topsoil strength by mechanical tamping decreases macro-porosity and hinders plant growth via restricted air and water movement (Kozlowski 1999). Moreover, soil compaction limits root penetration, as the mechanical resistance of the soil increases (Grabosky et al. 2002). Consequently, trees at paved sites often suffer from chronic nutrient deficiencies and water stress (Flueckiger and Brown 1999), both conditions being detrimental to tree growth (Kozlowski and Pallardy 2010).
Increased attention to the growing conditions and value of urban trees has contributed to the development of landscaping applications, such as structural soil (Roberts et al. 2006). In this paper, researchers use the term structural soil as a shorthand for the designed load-bearing urban soil, although the term structural soil as used in soil science has a wider meaning, referring more generally to the variability in the spatial arrangement of soil particles and pores. The first structural tree soil experiments were initiated in the 1990s (Grabosky and Bassuk 1995; Kristoffersen 1998). In structural soils, the gravel fraction provides a skeletal stone structure that transfers surface loads to the subsoil, whereas the fine material between the stones serves as the actual rooting space. Follow-up studies have demonstrated that roots exploit the rooting space expeditiously (Grabosky and Bassuk 1996; Kristoffersen 1999; Grabosky and Bassuk 2016). The total tree growth in structural soils has been demonstrated to be comparable with the growth in native topsoil (Kristoffersen 1999; Bühler et al. 2007), but some issues remain, such as the possibility of nutrient depletion (Loh et al. 2003; Smiley et al. 2006; Bühler et al. 2017).
This study compared the soil air O2 concentrations between conventional and structural tree soils at two sites in Helsinki, Finland, which is in the hemiboreal bioclimatic zone. Furthermore, the impacts of urban tree soil cover on soil aeration were evaluated by comparing the relative gas diffusion coefficients. Researchers hypothesized: 1) the average O2 concentration is higher in structural than in conventional soil, and 2) cobblestone paving impairs root-zone ventilation compared to cast-iron tree grating. The main objective was to gain empirical knowledge of fluctuations in soil O2 concentrations in different street tree establishments.
MATERIALS AND METHODS
Study Sites
Soil aeration was examined at two street tree sites in Helsinki, Finland. Soil air O2 concentrations were recorded during the growing seasons of 2012 and 2013. CO2 flux measurements were conducted in July and August 2014. The conventional-soil site (60°10’26”N, 24°57’40”E) was a main road (Pohjoisranta) planted in the early 1990s with a 700-m-long row of approximately 70 lime trees (Tilia × vulgaris Hayne). The trees were planted in rectangular baskets (110 cm × 110 cm × 440 cm) made of expanded metal fencing. The baskets were filled with conventional tree soil (i.e., a mixture of organic material, sand, and clay). The soils were covered with cast-iron tree grates, flush with the street surface. Six trees and their baskets were randomly chosen for the measurements from the north end of a 200-m-long stretch of road; the average diameter at breast height (DBH) of these trees, and that of the whole planting, was 17 cm in 2013.
The structural-soil site (60°13’36”N, 25°01’43”E) was installed in 2002 in a residential street (Norkkokuja) and planted with a columnar form of black alder (Alnus glutinosa f. pyramidalis Dippel ‘Sakari’); the trees averaged 17 cm in DBH in 2013. The 1-m-deep pre-mixed structural soils were installed in three layers of equal thickness, each compacted with a vibratory tamper. Three different soil mixtures were used (Riikonen et al. 2011), each in two to four planting pockets. From two to four balled-and-burlapped trees were planted in each pocket directly into the structural soil, resulting in 22 trees in 8 planting pockets at the site. Cast-iron gratings (150 cm × 150 cm) were installed around the alder stems, and the rest of each planting pocket was paved with granite cobblestones (160 mm × 160 mm × 160 mm) with a joint width of approximately 13 mm. The paving was laid on the top of an approximately 140-mm-thick layer of fine macadam (Figure 1), which was also used for filling the joints. The paving rose approximately 120 mm above the street surface.
Schematic illustration of the different material layers at the conventional (on the left side) and structural soil (on the right) sites. The plastic collar and the chamber used for CO2-flux measurements were placed and sealed on the grate at the conventional-soil site and on the cobblestones at the structural-soil site.
Experimental Soils
The tree soils at the conventional-soil site were sampled to determine the particle-size distribution in August 2011. Samples were taken with a soil corer from six tree baskets from depths of 0 to 30 cm and 30 to 90 cm. Particle-size analysis was conducted by dry sieving. A subsample from each sample was sieved (2 mm, rectangular) and analyzed for organic matter content by loss on ignition (LOI, 550°C, 2 hours). For soil type analysis, the six samples from both depths were combined to form two pooled samples per depth.
At the structural-soil site, three different mixtures of structural soils were used (Table 1). Three of the sampled trees were planted in Mixture 1, three in Mixture 2, and five in Mixture 3. The organic matter content of the mixtures was analyzed by LOI in 2011.
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 concentrations.
Soil Air O2 Concentrations
Soil air samples were collected from the planting baskets of 6 trees at the conventional tree soil and adjacent (approx. 1 m distance) to 11 trees at the structural-soil site. The gas samplers 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 structural 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 during 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 permanently 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 GORETEX® fabric (porous polytetrafluoroethylene coating). 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 hypodermic needle. In 2012–2013, the samples were analyzed with two interconnected gas chromatographs equipped with flame ionization, thermal conductivity, and electron capture detectors (HP 5890 Series II, Hewlett Packard, Bothell, Washington, 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 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 calibration for each measured gas (CH4, C2H4, CO2, N2O, O2, N2) was carried out with a certified gas mixture as described by Penttilä et al. (2013).
Ground Ventilation
To assess the effect of different soil sealing methods on the ventilation of the root zone, CO2 flux measurements were conducted with a portable closed-chamber CO2 exchange measuring system described in Kolari et al. (2005). The chamber 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 chamber by measuring the infrared radiation absorbance 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 measurement indicator (Vaisala Ltd, Vantaa, Finland).
CO2-flux measurements were carried out two times at five points (five different trees) during weeks 30 and 36 in the 2014 growing season. Measurements were conducted after a dry weather period of at least three days. CO2-flux was measured three 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 consecutive gas flux measurements was considered as a single observation. The chamber was ventilated to reach CO2 concentrations close to 400 ppm at the beginning of each repeated measurement. Soil air samples were extracted for chromatographic analysis within a few minutes of the gas flux measurements. The CO2 flux was estimated using the equation presented by Penttilä et al. (2013):
1
where J = gas flux (g m−2 h−1), dc/dt = slope of the linear regression of volumetric gas concentration on time (h−1), M = molar mass of the measured gas (g mol−1), V0 = gas volume under standard conditions (m3 mol−1), T0 = gas temperature under standard conditions (K), T = measured gas temperature (K), and H = chamber height (m).
At a constant temperature and pressure, the diffusive flux of a gas (q) is determined by the effective gas diffusion coefficient (D) and the concentration gradient (Fick’s first law). In soil, the diffusion coefficient also depends not only on the diffusion coefficient in air (D0) but also on the volume, 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:
2
where q = gas flux (g m−2 s−1), c = concentration (g m−3), 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 diffusion coefficient (D), the relative gas diffusion coefficient is a soil property that does not depend on air pressure, temperature, or the diffusing gas. The relative gas diffusion coefficient was calculated by dividing the measured diffusion coefficient of CO2 by its D0 value (i.e., by the rate at which the gas diffuses in air at a given temperature and pressure without impeding solids).
Soil Moisture and Temperature
At the conventional-soil site, soil moisture and temperature were measured directly after 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 mineral soil calibration. Access tubes for the moisture 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 construction in 2002 (Riikonen et al. 2011). Soil volumetric water content was measured with ML2x sensors (Delta-T Devices Ltd, Cambridge, UK) installed into the fine soil fraction 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 collected at one- to ten-minute intervals using a data logger (Envic Ltd., Turku, Finland) and were transferred daily to mass data storage.
Statistical Analysis
The differences in soil air oxygen concentrations and soil aeration between the two soil and paving types were examined by analysis of variance (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 mixtures, these mixtures were considered as replicates.
To analyze differences in the O2 concentration, the data were grouped according to soil type (two types, conventional and structural) and measuring depth (three depths for both soil types). The average O2 concentrations over two growing seasons were compared among these six groups. The means calculated from each measuring point were used for ANOVA to satisfy the assumption of independency between observations. The average water content was similarly compared between soils (depths of 10 and 30 cm). When ANOVA results were significant, pairwise comparisons were implemented using Fischer’s least significance (LSD) tests.
Furthermore, the differences between CO2 diffusion coefficients were tested with ANOVA by comparing the means of 10 measurements obtained from both experimental sites. The data were analyzed with SPSS Statistic 22 software (IBM®, U.S.). The P-value required for a significant difference was set at 0.05.
RESULTS
Soil Air O2 Concentrations
Near atmospheric O2 concentrations in soil air were detected at both street tree sites. When the data were combined over both growing seasons, the mean soil air O2 concentration was higher in structural soil at both depths (10 cm and 30 cm) when compared between sites (P10cm < 0.001, P30cm < 0.001). Similar results were obtained when the two growing seasons were analyzed separately. The deviation from the atmospheric concentration and the differences between experimental sites increased as a function of increasing depth (Figure 2).
The average oxygen concentration of soil air in conventional (Pohjoisranta) and structural (Norkkokuja) street tree soils. The O2 values are means of two growing seasons from six measuring points. Different letters above the bars indicate significant differences between the means (P < 0.05). Error bars represent SD. Pohjoisranta n10cm = 164, n30cm = 168, n90cm = 162; Norkkokuja n10cm = 110, n30cm = 173, n60cm = 131.
In comparison with structural soil, the O2 concentration fluctuated more in conventional tree soil (Figure 3). The O2 concentration was generally higher in structural than in conventional soil. Contrasting measurements were recorded only two times at the depth of 10 cm in 2013: in weeks 23 and 27 (Figure 3b). In 2012, there were no differences in the average O2 concentrations of conventional tree soil between the three soil horizons measured (P = 0.122). In 2013, the average O2 concentration was highest at the depth of 10 cm (P10cm/30cm = 0.027, P10cm/90cm < 0.001). In structural soil, the average O2 concentration exceeded 20% at each depth, in both years, with no difference between the soil horizons in either year (P2012 = 0.324, P2013 = 0.304).
The average oxygen concentration (±SD) in conventional and structural soil air in a) 2012 and b) 2013. Conventional soil n = 6, structural soil n = 8–11.
Soil Aeration
The estimated CO2 diffusion coefficients revealed differences in tree soil ventilation between the two establishment types. The average CO2 diffusion coefficient was lower (P = 0.017) at the conventional soil site (4.17 × 10−7 m2 s−1) than at the structural-soil site (1.08 × 10−6 m2 s−1). Gas movement appeared to depend on the type of soil and/or on the type of soil coverage; the experimental design did not allow the separation of these from each other. When the CO2 diffusion coefficients were converted to relative diffusion coefficients, the average values were 0.03 and 0.02 for the conventional-soil site and 0.07 and 0.06 for the structural-soil site. The volumetric water content at the depth of 10 cm, recorded simultaneously with CO2 flux measurements, was 6% and 8% at the conventional-soil site and 11% and 21% at the structural-soil site, respectively.
Soil Moisture and Temperature
The mean water content was highest in conventional soil at the depth of 30 cm when the soil water content data were combined over both growing seasons (Figure 4). At 10 cm, the average water content was higher in structural than in conventional soil (P < 0.001). At 30 cm, the structural soil was drier on average than the conventional soil (P < 0.001). In conventional soil, the average water content was greater at the depth of 30 cm than at 10 cm (P < 0.001). In structural soil, the reverse was observed (P < 0.001).
The volumetric water content in conventional and structural soils averaged over the growing seasons of 2012 and 2013. Different letters above the bars indicate significant differences between the means (P < 0.05), and error bars represent SD. Conventional soil n10cm = 129, n30cm = 142; structural soil n10cm = 114, n30cm = 179.
Compared to structural soil, the water content of conventional soil fluctuated more and remained continuously higher at the depth of 30 cm through both growing seasons. The water content appeared to remain more stable in the upper horizons of the conventional soils (Figure 5). The structural-soil water content was continuously highest at the depth of 10 cm. The soil temperature was only measured at both sites in 2013. The temperature remained higher and fluctuated more in conventional than in structural soil from weeks 23 to 31. The sudden drop in the temperature of conventional soils in week 33 in 2013 was related to a heavy rain event, which was also seen in the soil water content measurements. Temperature and soil water content measurements were conducted during the morning hours (8 am to 10 am), when the conventional-soil site was exposed to direct sunlight, whereas the structural-soil site was mostly shaded by buildings.
Average water content and temperature measured in conventional and structural soil during the study period. Soil temperature was not measured at the conventional-soil site in 2012.
Tree Soils
The conventional-soil type was sandy till, containing modest amounts of organic matter. The bulk density was 1.37 g cm−1 in the 0 to 30 cm layer and 1.23 g cm−1 in the 30 to 90 cm layer. The organic matter content averaged 3.7% and 5.6%, respectively. The proportion of fine matter appeared to increase from the top to the bottom (Figure 6).
The volumetric particle-size distribution of conventional soil (analyzed by dry sieving) at depths 0–30 cm and 30–60 cm (n = 6). Error bars represent SD.
The structural soil Mixture 1 contained 2.3% and 2.4% organic matter in the fine soil fraction at the depths of 0–30 cm and 30–60 cm, respectively. In Mixture 2, the organic matter content was 7.8% and 9.5% in the top and deeper layers, respectively. The corresponding figures for Mixture 3 were 1.25% and 1.5%, respectively.
DISCUSSION
The results supported the first hypothesis: soil air O2 concentration tends to remain at a higher level in structural soil than in the conventional type of tree soil. These results are not surprising, since O2 concentrations are known to decrease more slowly as a function of increasing depth in coarse soil (Hillel 1998). Moreover, the results parallel the findings of Grabosky et al. (1998), who reported higher O2 levels in structural soil under pavement than in compacted, unpaved conventional soil. It is known that the soil air O2 concentration can reach near atmospheric levels in the upper or surface horizon, and decreases with increasing depth, as registered on both research sites. Gaertig et al. (2002) found that on unpaved urban sites, tree soil can be aerated as well as in natural forest sites, and they demonstrated the collapse of topsoil O2 levels caused by sealing or compaction. Yelenosky (1963) reported O2 levels of 18% in soil air at the depth of 100 cm in forest stands. In this study, O2 concentrations in conventional soil air appeared similar to those of forest soils, whereas concentrations in structural soil air appeared higher. Even if the interpretation of these findings is confounded by the greater distance of the structural soil air samplers from the pavement–atmosphere interface than in conventional soil (Figure 1), this only underscores the good aeration status of structural soil.
A concentration of approximately 10% O2 in the soil atmosphere is generally considered adequate for healthy root growth (Tackett and Pearson 1964; Stolzy 1974; Gilman et al. 1987). Regarding healthy tree growth, researchers measured high and adequate O2 concentrations through both growing seasons in both street tree planting designs. However, the O2 concentration in soil air does not equivocally indicate O2 availability to tree roots, which can experience lower O2 levels than measured from soil air due to factors such as restricted diffusion through water films surrounding them (Glinski and Stepniewski 1985; Simojoki 2000; Simojoki 2001; Bartholomeus et al. 2008).
In contradiction with the second hypothesis, soil aeration was not seriously interrupted or considerably slowed down by the cobblestone paving laid on macadam with wide joints. The study results differ from those of Weltecke and Gaertig (2012), who reported no significant differences between relative diffusion coefficients obtained for typical urban soil cover types: asphalt, flagstones, and cobblestone-coating. However, the numerical D/D0 values determined in this study cannot be compared with the absolute values reported by Weltecke and Gaertig (2012). Vertical and horizontal gas movement through washed macadam, used in laying the cobblestones and in the wide joints between them, could explain the differences between the findings of the current study and previous research. Koolen et al. (2000) presented gas diffusion coefficient values for a range of paving materials and concrete elements. They estimated the gas exchange through jointed paving by measuring the O2 diffusion coefficient of the sand that was used for the joints. The results of Koolen et al. (2000) suggested that brick (20 cm × 5 cm × 10 cm) paving with a 7.7% relative area of joints caused several magnitudes lower resistance to belowground gas exchange than asphalt. The D/D0 is an index of soil aeration and usually varies between 0.003 and 0.2, reaching 0.5 in loose soils (Glinski and Stepniewski 1985). Researchers measured D/D0 values in this range at both studied sites.
The diffusion of O2 in soil is retarded by increasing water content (Hillel 1998). During weeks 21 and 33 of the latter growing season, decreases in the conventional soil O2 concentration were detected, with simultaneous upsurges in the soil water content. These weeks were associated with heavy rainfall. Because the surrounding pavements conducted storm water toward the planting baskets, it is possible that the conventional soil reached a water-saturated state on these occasions. Similar low O2 concentrations were not observed at the structural-soil site during these periods, indicating that structural soils may be better suited to sites prone to water-logging, such as stormwater infiltration areas.
The growth and oxygen consumption of tree roots increase with rising temperature (Glinski and Stepniewski 1985). Hence, it is probable that the lower temperature of the structural soil promoted its higher O2 concentrations in comparison to conventional tree soil at the study sites. However, the general effect of the temperature differences appears somewhat irrelevant due to the similar O2 differences during the cooler periods. Additionally, oxygen consumption in the soil depends on soil organic matter content, which was relatively low on both sites overall, but in soils with high organic matter, soil O2 consumption may be very significant (Glinski and Stepniewski 1985).
The intrinsic limitation of this study arises from measurements being executed at only two sites. Hence, the results should only be extrapolated with caution. Further research is needed to evaluate the effects of environmental factors on the air composition in structural soil. Nevertheless, researchers conclude that structural soil is able to maintain similar or higher soil air oxygen concentrations under a pavement than an unsealed conventional tree soil, and recommend considering the use of structural soils for hypoxy-sensitive species.
Acknowledgments
The research sites were established in collaboration with the City of Helsinki Public Works Department. This work was partly financed by the Academy of Finland’s Centre of Excellence (grant no. 272041). We thank Janne Järvinen and Mari Mäki for their help with field data collection, technical assistance, and instrument maintenance.
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