Effect of soil profile structure on seasonal changes of soil temperature in urban forests

Field observations of soil temperature were carried out in an urban park to compare the characteristics of heat transfer in urban forest soils having different soil profile structures. The diurnal variation of soil temperature was significant down to 10 cm depth. Their patterns in the layers deeper than 30 cm differed by seasons and soil profile structures having contrasted soil properties. Temperature transmission to deeper layers was faster in the soil profile having stronger soil compaction and abundant artifacts than in the soil profile with weaker soil compaction and no artifacts. From February to April, the soil temperature was higher in the undisturbed profile, having lower soil pH (acidic), lower compaction, smaller bulk density, and larger carbon content, than in the lithological disturbed profile containing a large amount of concrete rubbles with higher soil pH (neutral to weak alkaline), higher compaction, larger bulk density, and smaller carbon content. The reverse trend appeared from mid-April to December. Moreover, the annual range of soil temperature was larger and occurred deeper in the lithological disturbed profile than in the undisturbed profile. Thermal diffusivity and thermal conductivity were 0.9-7.9 × 10^ cm^2 s^ and 0.20-1.85 Wm^K^ for the lithological disturbed profile, respectively. The values were smaller, 0.3-5.3 ×10^ cm^2 s^ and 0.02-0.45 Wm^K^, respectively for the undisturbed profile. Based on our two years observation, we conclude that the intensive soil compaction and lithological discontinuity regulate soil thermal properties of urban forests, by which soils may likely to be assigned to a higher soil temperature regime

Effect of structural modification on heat transfer through man-made soils in urban green areas

<div><p></p><p>This study examined the characteristics of heat and water transfer in structurally modified urban soils. To satisfy our goals, we measured the temperature and moisture content of anthropogenic soils to a depth of 50 cm. Field observations was carried out for three sets (each of two pedons) of soils in the Tokyo Metropolitan area. Each pedon had the same turf coverage but different profile modifications in the green areas. Soil temperature, soil moisture, and precipitation data were collected during the summer (July–Sept) and winter (Oct–Feb) every 10 min. From the results, we calculated the thermal diffusivity and thermal conductivity in each pedon. Soil temperature showed a clear daily variation down to 30 cm depth. Temperature transmission to deeper layers was faster in pedon having stronger soil compaction and more artificial fragments than in pedons with weaker soil compaction and fewer concrete fragments. This finding suggests that strongly compacted soil has a relatively high thermal conductivity, and easily transfers heat to deeper soil. In pedons composed of soft, organic-rich, and clay-rich soil, water retention impedes the increase in soil temperature during daytime, whereas nighttime cooling is prevented by the lower heat transmission due to the larger porosity. Throughout the observations, the water content ranged from 0.1 to 0.45 m³ m⁻³. The thermal diffusivity was obtained as 1.2–5.0 × 10⁻³ cm² s⁻¹ in pedons without artifacts, but was higher (2.5–7.3 × 10⁻³ cm² s⁻¹) in all pedons containing larger volumes of concrete artifacts. Although the directions of heat flow by time within the profiles having lithological disturbance was not much different with that of natural soils, the observation data revealed that heat flow per time differed by structural properties of the profiles. Furthermore, thermal properties such as thermal diffusivity and thermal conductivity of the soils characterized with lithological disturbance were significantly higher than those of natural soils and it was notable that they were not influenced by either volume fraction of water or air in the soil. The fact suggested that the anthropogenic soils containing a large amount of modifiers and concrete artifacts have small capacity of water retention due to specific macro pores. Artificial materials and compaction regulate the drainage and water retention of the soil, although the water transfer behavior in the studied pedons could be rather complicated.</p></div