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^<−3> cm^2 s^<−1> and 0.20-1.85 Wm^<-1>K^<-1> for the lithological disturbed profile, respectively. The values were smaller, 0.3-5.3 ×10^<−3> cm^2 s^<−1> and 0.02-0.45 Wm^<-1>K^<-1>, 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.