Reviewers: Lee H2, Reinsch S3
Measurement unit: oC; Measurement scale: plot; Equipment costs: €€; Running costs: €; Installation effort: medium; Maintenance effort: medium; Knowledge need: medium; Measurement mode: data logger
Soil temperature is a measure of the intensity of heat present in soil (Buchan, 2001), which impacts critical processes taking place, including the germination of seeds, microbial activity (Hanson et al., 2000; Fierer et al., 2006), chemical reactions, carbon sequestration (Kirschbaum, 1995), gas production and emissions (Schaufler et al., 2010), the growth and maintenance of plant roots (Atkin et al., 2000), soil evaporation (Kalma et al., 2008), plant transpiration, and freeze/thaw cycling.
Atmospheric conditions drive near-surface soil temperature, which is directly affected by climate change and position in the landscape (e.g. along gradients). Soil temperature is perhaps one of the most common soil measurements and reliable long-term monitoring stations (e.g. Rothamsted, UK; 1931–present) provide valuable information on temperature trends associated with climate change and other global-change drivers. An initiative building a global database of soil temperatures SOILTEMP may help to provide global soil temperature records in the future.
3.5.1 What and how to measure?
Soil temperature measurement – direct contact measurements
Subsurface temperatures are more commonly measured using a thermocouple or a thermistor, and increasingly are incorporated into other sensors, such as soil moisture sensors. Thermocouples employ a bi-metal junction where the gradient in voltage is directly proportional to the gradient in temperature (Seebeck effect) and must be read using an advanced circuit common in multi-meter and data-logging devices. A thermistor is a type of negative coefficient resistor, whose resistance is dependent on temperature and which can be read by a simpler voltage reading circuit. Today’s thermistors and thermocouples can resolve 0.01 oC but absolute temperature calibration can be costly to achieve at this resolution. More accurate temperature measurements are available using a resistance temperature detector (RTD), which uses a more expensive metal-based temperature-dependent resistance circuit. Subsurface temperature sensors are generally epoxy-coated and may be embedded in stainless steel tubing for protection from corrosion and water damage. Advances in microelectronics have facilitated heat pulse probes (Campbell et al., 1991) that employ temperature rise measurements in soil to infer soil thermal properties as well as other processes of interest (e.g. soil heat- and water-flux density; Yang et al., 2013). Because of the low cost of temperature-sensing circuits, most environmental sensors today include a temperature measurement in addition to other properties (e.g. soil moisture, electrical conductivity).
Soil temperature measurement – non-contact measurements
Measurement of a bare soil surface using contact thermometry is prone to serious errors (i.e. contact loss, surface disturbance). Remotely measured soil temperature relies on surface-emitted infrared radiation sensed by a distant thermometer (infrared-spot thermometer, pyrometer, or detector array). Early instruments employed fine-wire thermocouples but advances in microelectronics have produced faster response and more accurate detectors for infrared thermometry applications. Infrared radiation emitted from a surface is a function of the surface emissivity as well as surface temperature. Although most surfaces including soils have high emissivity of 0.9 to 0.97 (Fuchs & Tanner, 1968), which is a key assumption of today’s low-cost radiometers, measurement errors arise when lower emissivity surfaces (e.g. highly reflective) are detected without adjusting for the assumed high emissivity value.
Soil sensors are often used to monitor seasonal changes in the soil environment. Daily changes occur, but can often be considered noise against the slower seasonal signal. Rather than measuring at fixed depths, it is of interest to know the temperature near the soil surface, to obtain the upper boundary condition for modelling temperature, and in the upper horizon (e.g. within 5–15 cm) at a point corresponding with moisture measurements, such as at maximum root density (see Figure 1.5.1 in protocol 1.5 Meteorological measurements). In the best of all worlds, a second set of sensors would be placed at greater depth, perhaps near the bottom of the root system. Such positioning would capture the rare drought or snowmelt events that deplete or refill the whole soil profile. In boreal forests, these deeper sensors are often placed at 50 cm below the surface. As much as we would like to standardise these depths, the variation in diurnal/seasonal cycles and in root water depletion depths prevents convergence on a single recommendation.
When installing sensors in a plot it is generally best to install them horizontally (to measure temperature at the desired height) which can be achieved by excavating a small trench from outside the plot into the plot (see Figure 1.5.1 in protocol 1.5 Meteorological measurements). This prevents preferential flow of water along the cables.
Generally, in areas with rodents, it can be useful to protect the wire of a sensor with PVC tubes to prevent damage. In alpine areas, where there is a lot of snow in spring, it can be advisable to protect the wire higher up, because rodents can climb up on the snow.
For each of the sensors, the optimal sampling interval is every minute and should be reported in the form of half-hourly to hourly averages. Modelling of ecosystem gas exchange will require half-hourly to hourly data input.
Where to start
Buchan (2001), Deryng et al. (2014), IPCC (2014)
3.5.2 Special cases, emerging issues, and challenges
Since most soils support plant growth naturally, it is important to understand the impact of plant canopies, which can insulate and alter temperature gradients (important in modelling) between the soil surface and the atmosphere. This suggests that both canopy and soil surface temperature measurements are important to consider.
Theory, significance, and large datasets
Bond-Lamberty & Thomson (2010), Chapin et al. (1979), Dai et al. (2004), Davidson & Janssens (2006), Hay & Wilson (1982), Jury & Horton (2004), Parlange et al. (1998)
More on methods and existing protocols
ASTM (2017), McInnes (2002)
ASTM Subcommittee E20.02 on Radiation Thermometry. (2017). Annual Book of ASTM Standards, Volume 14.03. https://www.astm.org/COMMIT/SUBCOMMIT/E2002.htm.
Atkin, O. K., Edwards, E. J., & Loveys, B. R. (2000). Response of root respiration to changes in temperature and its relevance to global warming. New Phytologist, 147(1), 141-154.
Bond-Lamberty, B., & Thomson, A. (2010). Temperature-associated increases in the global soil respiration record. Nature, 464(7288), 579-582.
Buchan, G. D. (2001). Soil temperature regime. In K. A. Smith, & C. R. Mullins (Eds.), Soil and Environmental Analysis: Physical Methods (pp. 539-594). New York: Marcel Dekker.
Campbell, G. S., Calissendorff, C., & Williams, J. H. (1991). Probe for measuring soil specific heat using a heat-pulse method. Soil Science Society of America Journal, 55(1), 291-293.
Chapin, F. S., van Cleve, K., & Chapin, M. C. (1979). Soil temperature and nutrient cycling in the tussock growth form of Eriophorum vaginatum. Journal of Ecology, 67(1), 169-189.
Dai, A., Trenberth, K. E., & Qian, T. (2004). A global dataset of Palmer Drought Severity Index for 1870–2002: relationship with soil moisture and effects of surface warming. Journal of Hydrometeorology, 5(6), 1117-1130.
Davidson, E. A., & Janssens, I. A. (2006). Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature, 440(7081), 165-173.
Deryng, D., Conway, D., Ramankutty, N., Price, J., & Warren, R., (2014). Global crop yield response to extreme heat stress under multiple climate change futures. Environmental Research Letters, 9(3), 034011.
Fierer, N., Colman, B. P., Schimel, J. P., & Jackson, R. B. (2006). Predicting the temperature dependence of microbial respiration in soil: A continental‐scale analysis. Global Biogeochemical Cycles, 20(3), GB002644.
Fuchs, M., & Tanner, C. B. (1968). Surface temperature measurements of bare soils. Journal of Applied Meteorology, 7(2), 303-305.
Hanson, P. J., Edwards, N. T., Garten, C. T., & Andrews, J. A. (2000). Separating root and soil microbial contributions to soil respiration: a review of methods and observations. Biogeochemistry, 48(1), 115-146.
Hay, R. K. M., & Wilson, G. T. (1982). Leaf appearance and extension in field-grown winter wheat plants: the importance of soil temperature during vegetative growth. Journal of Agricultural Science, 99(02), 403-410.
IPCC. (2014). Summary for Policymakers. In C. B. Field, V. R. Barros, D. J. Dokken, K. J. Mach, M. D. Mastrandrea, T. E. Bilir, … L. L. White (Eds.), Impacts, Adaptation and Vulnerability. Part A: Contributions of Working Group II to the Intergovernmental Panel on Climate Change Fifth Assessment Report (pp. 1-32). Cambridge: Cambridge University Press.
Jury, W. A., & Horton, R. (2004). Soil Physics. John Wiley & Sons.
Kalma, J. D., McVicar, T. R., & McCabe, M. F. (2008). Estimating land surface evaporation: a review of methods using remotely sensed surface temperature data. Surveys in Geophysics, 29(4-5), 421-469.
Kirschbaum, M. U. F. (1995). The temperature dependence of soil organic matter decomposition, and the effect of global warming on soil organic C storage. Soil Biology & Biochemistry, 27, 753-760.
McInnes, K. J. 2002. Temperature. In J. H. Dane, & C. G. Topp (Eds.), Methods of Soil Analysis: Part 4—Physical Methods (pp. 1183-1199). Madison: Soil Science Society of America.
Parlange, M. B., Cahill, A. T., Nielsen, D. R., Hopmans, J. W., & Wendroth, O. (1998). Review of heat and water movement in field soils. Soil and Tillage Research, 47(1), 5-10.
Schaufler, G., Kitzler, B., Schindlbacher, A., Skiba, U., Sutton, M. A., & Zechmeister‐Boltenstern, S. (2010). Greenhouse gas emissions from European soils under different land use: effects of soil moisture and temperature. European Journal of Soil Science, 61(5), 683-696.
Yang, C., Sakai, M., & Jones, S. B. (2013). Inverse method for simultaneous determination of soil water flux density and thermal properties with a penta‐needle heat pulse probe. Water Resources Research, 49(9), 5851-5864.
Author: Jones SB1
Reviewers: Lee H2, Reinsch S3
1 Department of Plants, Soils and Climate, Utah State University, Logan, USA
2 NORCE Norwegian Research Centre and Bjerknes Centre for Climate Research, Bergen, Norway
3 Centre for Ecology & Hydrology, Environment Centre Wales, Bangor, UK