154 Fair et. al: Characterization of Compacted Subsoil and its Effects on Two Maples ues for the three-year study were 1.64, 1.78, and 1.77 g·cm-3 , respectively. For the most part, compaction efforts did result in bulk densities very close to the targeted values with the ex- ception of the lowest target of 1.4 g·cm-3 . The inability to ob- tain this lower density may have been due to soil textural type or unavoidable changes in gravimetric water content during the compaction process. Mechanical properties of clay soil, in par- ticular sheer strength and pore space discontinuity, may prevent homogeneous soil compaction, despite use of consistent com- paction techniques (Cook et al. 1996; Krümmelbein et al. 2010). No statistical differences were found between the 1.78 g·cm-3 and 1.77 g·cm-3 these values were combined and are presented as the “HIGH- density” treatment. Henceforth, the 1.64 g·cm-3 treatments for any soil parameter. Therefore, treatment will be referred to as “MODERATE-density.” This combination means there are twice as many observations for the HIGH-density” treatment than the MODERATE-density treatment (Table 1). Table 1. Dry bulk density (ρb), aeration porosity, log-aeration porosity, total porosity, void ratio, saturated hydraulic con- ductivity (Ks), natural log-normalized saturated hydraulic conductivity [nLog(Ks)], and % (v/v) volumetric water con- tent for compacted soils averaged over three-year study. 2002–2004 mean soil variables ρb (g·cm-3 ) Aeration porosity (%) Log-aeration porosity Total porosity (%) Void ratio (m3 Ks (cm·hr-1 nlog- Ks ·m-3 ) y Volumetric water content (%, v/v) x (n = 56 samples). y ) Soil treatmentz MODERATE 1.64 ± 0.02 b 13 ± 1.2 b -0.94 ± 0.04 b 38 ± 0.7 a 0.62 ± 0.02 a 330.0 ± 113.8 a 3.47 ± 0.58 a 24 ± 0.02 a HIGH 1.78 ± 0.01 a 16 ± 0.7 a -0.84 ± 0.02 a 33 ± 0.3 b 0.50 ± 0.01 b 21.5 ± 6.9 b 0.98 ± 0.30 b 16 ± 0.01 b z MODERATE = mean bulk density for the three-year study of 1.64 g·cm-3 soil samples), HIGH = mean bulk density for the three-year study of 1.77 g·cm-3 Saturated hydraulic conductivity (Ks) was measured in 2002 and 2003. x Volumetric water content was determined for each sample from gravimetric water content and bulk density measures; means ± mean standard errors followed by different letters indicate a significant difference between soil treatments, at P ≤ 0.05 (Tukey’s honestly significant difference test, HSD). Mean ρb for the MODERATE-density treatment for the three-year study was significantly lower than the HIGH-den- sity (Table 1). As would be expected, total porosity and void ratio values were highest for the MODERATE-density soil samples (Table 1). There was 25% more void space in the MODERATE-density treatment, with a concomitant greater potential for water and gas movement, and root extension. To- tal porosities were 38% and 33% for MODERATE and HIGH- density treatments, respectively. These values are low, but with- in the 30%–60% range typical of mineral soils (Hillel 1998). In the present study, air-filled porosity was 13% for MOD- ERATE-density and 16% for HIGH-density soils (Table 1). The MODERATE-density treatment held 50% more volumetric water content than the HIGH-density (Table 1), which may explain the lower air-filled porosity for the MODERATE-density soil. For both treatments, air-filled porosity was within the acceptable range for this textural type, indicating sufficient space for gas diffusion through the soil matrix (Greenwood 1971; Brady and Weil 2002). Soil concentrations of CO2 were significantly higher in the MODERATE-density soils than the HIGH-density soils (Table ©2012 International Society of Arboriculture (n = 28 than atmospheric concentrations in the MODERATE and HIGH- density treatments, respectively. Oxygen and N2 2). As expected, CO2 average similar to atmospheric levels (Table 2), despite higher O2 12%–20% by volume in soil gases, at soil depths of 30 to 150 cm, are typical during the growing season in most mineral soils. Although there is no established critical limit for O2 tions ≤10% cause many tree roots to lose vigor (Kozlowski et al. 1991). Oxygen concentrations <19% were not recorded for either compaction treatment in this study. Although O2 likely a result of significantly higher volumetric water content in these treatments when compared to the HIGH-density soil (Table 1). According to Scott (2000), O2 levels in the HIGH-density treatment. The lower O2 higher CO2 concentrations in the MODERATE-density soil were concentrations from , concentra- concen- fusion rate are limiting to plants growing in highly compacted soils, although O2 content, but there was no clear correlation between soil strength (resistance to root growth) and diffusion rate. Therefore, aeration porosity alone may not sufficiently describe the aeration status of a compacted soil, or the movement of O2 through that system. Concentrations of ethylene in the samples were below detect- able limits (data not shown); therefore, ethylene was not consid- ered a factor in tree response. Additionally, there were no ethyl- ene-induced architectural differences (Kays et al. 1974; Morgan et al. 1993) between trees growing in different soil treatments. Both Ks and nlog-Ks values were 93% higher in the MODER- ATE-density soil compared to the HIGH-density treatment (Table 1). This mirrors a field study also conducted in a clay loam soil where 98% difference in Ks was noted between compaction treat- ments (Fair et al. 2012). The differences found between compac- tion treatments were greater than those found by Coutadeur et al. (2002) at 40% or Gebhardt et al. (2009) at 60% in a clay loam. Table 2. Comparison of mean percent CO2 Soil treatmenty MODERATE-density HIGH-density Soil gasz CO2 0.66 ± 0.05 a 0.21 ± 0.03 b mean for the three sample ports per pot. y samples taken biweekly in 2002 and 2003 from MODERATE- and HIGH-density soil treatments. , O2 , and N2 O2 19.0 ± 0.48 b 21.2 ± 0.10 a N2 80.4 ± 0.56 a 78.7 ± 0.09 b z Gases were sampled during growing season of 2002 and 2003 and represent the MODERATE= mean bulk density for three-year study of 1.64 g·cm-3 samples), and HIGH= mean bulk density for the three-year study of 1.77 g·cm-3 (n = 24 gas samples); means ± mean standard error followed by different letters indicate a significant difference between soil treatments, at P ≤ 0.05. Tree Transpiration and Growth Table 3 summarizes the analysis of variance done on the follow- ing data. In 2002, trees growing in MODERATE-density soils had a 395% greater daily transpiration rate than trees growing in HIGH-density soils (Table 4). In 2003, the difference was 221%, (n = 21 gas . gas tion increases with depth and density of the soil matrix (Shierlaw and Alston 1984; Day et al. 2000). Day et al. (2000) indicated a reduction in O2 concentration decreases and CO2 diffusion rate was weakly correlated to soil water trations were somewhat lower for measurements taken at the lowest port location, these values were not significantly different from values sampled from other port locations (data not shown). Recent research suggests that neither O2 concentration nor dif- concentra- concentrations were 18 and 5 times higher levels were on and
July 2012
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