13: Soil Temperature
- Page ID
- 38744
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In the prior chapter, we focused on the partitioning of energy at the soil surface between net radiation, latent heat flux, sensible heat flux, and soil heat flux. One of the key variables that controls or is controlled by that energy partitioning is soil temperature. The aim of this chapter is to explain how soil temperatures vary in space and time and how soil temperatures impact a wide variety of biological, chemical, and physical processes in the soil. This audio overview provides a helpful starting point for understanding soil temperature [website].
The global distribution of soil temperature regimes reflects the wide range of soil thermal environments, from soils with a megathermic temperature regime having mean annual temperatures at the 50-cm depth >28°C, to soils in the hypergelic temperature regime with mean annual temperatures <-10°C (Fig. 13-1).
Soil temperature strongly affects the global carbon cycle by influencing the soil respiration rate, which includes respiration of both soil microbes and plant roots. The soil respiration rate under boreal forest in Canada shows a strong logarithmic relationship with soil temperature (Fig. 13-2) [1], and similar relationships have been observed for many other locations and ecosystems around the world. If rising global temperatures cause increased soil respiration resulting in a net transfer of soil organic carbon to the atmosphere, then a positive feedback to climate change would occur [2].
Multiple plant growth processes can also be strongly influenced by soil temperature. For example, soil temperature is one of the primary factors controlling the germination and early growth of many crops. The time required for winter wheat seedlings to emerge decreases from >25 days at a soil temperature of 5°C to ~5 days at 20°C (Fig. 13-3) [3]. For com, warmer early season soil temperatures hasten plant development, increase leaf size in the upper half of the canopy, and linearly increase end-of-season grain yields [4]. In recent decades, long-term increases in soil temperature have been documented [5], and these increases may have significance for crop production. Perhaps in response to rising soil temperatures, there has been a trend toward earlier planting dates in the major com-producing region of the US, the Corn Belt, and these earlier planting dates have contributed significantly to yield increases, especially in the more northern parts of the Corn Belt [6].
- 13.1: Heat transfer in soil
- In the soil, heat transfer occurs primarily by conduction, although convective heat transfer can be important in some cases. Heat conduction is governed by Fourier’s Law, which was first documented in 1807 and published in 1822 in France and may have influenced the later development of Darcy’s Law (1856) [7].
- 13.2: Soil thermal properties
- The primary thermal properties of soil, or any substance, are the heat capacity and the thermal conductivity. The heat capacity can be defined per unit mass, in which case it is often called the specific heat, or per unit volume, in which case it is called the volumetric heat capacity. Sometimes it is useful to consider the ratio of the thermal conductivity to the volumetric heat capacity, and this ratio is called the thermal diffusivity. We will define and consider each of these in turn below.
- 13.3: Soil surface temperatures
- The highest and lowest soil temperatures occur at the soil surface. The highest recorded soil temperatures on Earth are near 700°C under an intense forest fire [11]. The lowest recorded soil temperatures on Earth are less extreme, reaching -20 °C in Arctic winters [12]. Regular oscillations in the temperature of the soil surface are driven by the daily cycle of the Earth spinning on its axis and the annual cycle of the Earth orbiting the sun.
- 13.4: Sub-surface soil temperatures
- We can also approximate the oscillations of subsurface soil temperatures as a sine wave if we assume: the surface temperature is (and has been) oscillating as a sine wave, the average soil temperature is the same for all depths, and deep in the soil the temperature is constant.
- 13.5: Measured soil temperatures
- In reality, soil temperature oscillations often bear little resemblance to a regular procession of sine waves. Figure 13-8 shows one week of measured soil temperatures for the 6-cm depth under perennial vegetation in Minnesota, USA. After two days of similar and near-sinusoidal oscillations, the temperatures on the third day reflect a much smaller amplitude and a more irregular temperature pattern.
- 13.6: Problem set
- The thermal diffusivity for a particular soil
- 13.7: References
- Gaumont-Guay, D., et al., Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand. Agricultural and Forest Meteorology, 2006. 140(1–4): p. 220-235. Davidson, E.A. and I.A. Janssens, Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature, 2006. 440(7081): p. 165-173. Lafond, G.P. and D.B. Fowler, Soil Temperature and Water Content, Seeding Depth, and Simulated Rainfall Effects on Winter Wheat Emergence. Ag