There is a strong annual cycle in air temperature and soil temperature follows it with some delay. There is stronger annual variation at the surface soil and more stable temperature in the deeper layers. The surface temperature in soil is higher than the deeper layers in winter and colder in winter since snow cover isolates soil and balance the differences between winter months. The difference is usually about 10 to 15 oC degrees in winter when surface temperature is 15 to 20 oC degrees and 10 oC degrees in the deeper layers.
Soil temperature regulates soil processes through enzyme activity. The increase in temperature will increase synthesis rate of different processes in soil and this affects to the VOC emissions from soil and roots. Increasing temperature can cause compound consumption in the soil since microbes can use VOCs as an energy source. Soil temperature has also an effect to the diffusion transport of VOC emissions from soil into air, described as a diffusion coefficient, since volatility of different gas depends on temperature (van Roon et al., 2005, Asensio et al., 2007).
Uneven structure increases the temperature variation in soil and for this reason soil temperature measurements represent only individual measurement points. Soil temperature varies greatly at the different depths so the location of the temperature measurement sensor impacts significantly to the measured value. For this reason, we usually measure temperature at the same time for different soil layers.