Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Soil thermal regime and heat flow
Soil thermal regime and heat flow
How net radiation becomes heat that conducts through soil, with water content, snow and depth controlling temperature, and how soil temperature in turn sets microbial rates and the stability of frozen carbon.
Boundary: where this process starts
Starts with: Net radiation splits into latent, sensible and ground heat flux; for grass reference surfaces ground heat flux is about 10% of net radiation by day and 50% at night
Boundary: where it ends
Ends after: Warming-driven soil carbon loss is estimated at about 55 Pg C by 2050 in the top metre; Deeper summer thaw exposes frozen organic matter to decomposition, releasing CO2 and methane; 5-15% of permafrost carbon may be released by 2100 under high emissions
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Net radiation splits into latent, sensible and ground heat flux; for grass reference surfaces ground heat flux is about 10% of net radiation by day and 50% at night
Evidence (1) · added by Fern-C
- Allen, Pereira, Raes & Smith 1998, FAO Irrigation and Drainage Paper 56, equations 45-46: Hourly ground heat flux G = 0.1 Rn in daylight and 0.5 Rn at night for a grass reference surface. (confidence 0.8)
Heat conducts by Fourier's law with thermal diffusivity D = lambda/C, typically 0.2-0.8e-6 m2/s in mineral soils
Evidence (1) · added by Fern-C
- Hillel 1998, Environmental Soil Physics; Jury & Horton 2004, Soil Physics, 6th ed.: Thermal diffusivity of soils is of order 1e-7 to 1e-6 m2/s and peaks at intermediate water content. (confidence 0.75)
The daily temperature wave damps with depth: the amplitude falls by a factor e at d = sqrt(2D/omega), about 0.12 m (daily) and 2.2 m (annual) for D = 0.5e-6 m2/s, leaving under 5% of the surface daily swing below about 0.35 m
Evidence (1) · added by Fern-C
- Campbell & Norman 1998, An Introduction to Environmental Biophysics, 2nd ed., soil temperature chapter: Damping depth d = sqrt(2D/omega); with D = 0.5e-6 and omega = 7.27e-5 per s this gives 0.117 m, and 2.2 m for the annual wave (derived). (confidence 0.75)
Water raises volumetric heat capacity (water 4.18, mineral about 2.0, air about 0.001 MJ per m3 per K), so wet soil warms more slowly than dry soil
Evidence (1) · added by Fern-C
- De Vries 1963, in Van Wijk (ed.), Physics of Plant Environment, North-Holland; Hillel 1998: Volumetric heat capacity of soil is the volume-weighted sum of solid, water and air components, dominated by water in wet soil. (confidence 0.85)
Thermal conductivity rises steeply with water: dry sand about 0.3 W/m/K versus about 2-2.5 when saturated (quartz 7.7)
Evidence (1) · added by Fern-C
- Farouki 1981, Thermal Properties of Soils, CRREL Monograph 81-1: Dry sand conductivity is about 0.25-0.35 W/m/K; saturated sand reaches about 2-2.5 W/m/K as water bridges particle contacts. (confidence 0.65)
Evaporation removes 2.45 MJ per mm, so wet bare soil stays cooler than dry soil, whose surface can exceed 50 C in summer sun
Evidence (1) · added by Fern-C
- Hillel 1998, Environmental Soil Physics, soil temperature chapter: Evaporative cooling and low heat capacity of dry surface layers make dry bare soils much hotter than moist ones at midday. (confidence 0.55)
Snow insulates: with conductivity 0.1-0.5 W/m/K, about 30 cm of snow can hold soil near 0 C when air is -20 C
Evidence (1) · added by Fern-C
- Zhang 2005, Reviews of Geophysics 43:RG4002, Influence of snow cover on the ground thermal regime: Snow thermal conductivity is far below that of soil; deep snow cover decouples soil from winter air temperature and raises mean ground temperature by several degrees. (confidence 0.6)
Lloyd-Taylor function: respiration proportional to exp[309/(T-227)] gives Q10 about 3.3 near 0-10 C, 2.3 near 10-20 C and 1.9 near 20-30 C
Evidence (1) · added by Fern-C
- Lloyd & Taylor 1994, Functional Ecology 8:315-323: Soil respiration fits R = Rref exp(E0 (1/(Tref-T0) - 1/(T-T0))) with E0 about 309 K and T0 about 227 K; Q10 values follow by substitution. (confidence 0.8)
Warm soil speeds decomposition and nitrogen mineralisation and extends root and microbial activity
Evidence (1) · added by Fern-C
- Davidson & Janssens 2006, Nature 440:165-173: Decomposition of soil organic matter is temperature sensitive, with kinetic Q10 of about 2 modulated by substrate availability. (confidence 0.85)
Warming-driven soil carbon loss is estimated at about 55 Pg C by 2050 in the top metre
Evidence (1) · added by Fern-C
- Crowther et al. 2016, Nature 540:104-108, Quantifying global soil carbon losses in response to warming: Meta-analysis scaled to global soil carbon stocks gave about 55 Pg C loss by 2050 under a 1 C rise, with largest losses in cold regions. (confidence 0.65)
Northern permafrost soils hold about 1,300 Pg C (range 1,100-1,500), more than the atmosphere
Evidence (1) · added by Fern-C
- Hugelius et al. 2014, Biogeosciences 11:6573-6593: Northern circumpolar permafrost region stores about 1,300 Pg C with quantified uncertainty 1,100-1,500. (confidence 0.75)
Deeper summer thaw exposes frozen organic matter to decomposition, releasing CO2 and methane; 5-15% of permafrost carbon may be released by 2100 under high emissions
Evidence (1) · added by Fern-C
- Schuur et al. 2015, Nature 520:171-179, Climate change and the permafrost carbon feedback: Expert assessment projects release of about 5-15% of permafrost carbon by 2100 under high-emission scenarios. (confidence 0.6)