## 2.21.3. Environmental modifiers on decomposition rate[¶](#environmental-modifiers-on-decomposition-rate "Permalink to this headline")
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These base rates are modified on each timestep by functions of the current soil environment. For the single-level model, there are two rate modifiers, temperature (\\({r}\_{tsoil}\\), unitless) and moisture (\\({r}\_{water}\\), unitless), both of which are calculated using the average environmental conditions of the top five model levels (top 29 cm of soil column). For the vertically-resolved model, two additional environmental modifiers are calculated beyond the temperature and moisture limitations: an oxygen scalar (\\({r}\_{oxygen}\\), unitless), and a depth scalar (\\({r}\_{depth}\\), unitless).

The Temperature scalar \\({r}\_{tsoil}\\) is calculated in CLM using a \\({Q}\_{10}\\) approach, with \\({Q}\_{10} = 1.5\\).

(2.21.5)[¶](#equation-21-5 "Permalink to this equation")\\\[r\_{tsoil} =Q\_{10} ^{\\left(\\frac{T\_{soil,\\, j} -T\_{ref} }{10} \\right)}\\\]

where _j_ is the soil layer index, \\({T}\_{soil,j}\\) (K) is the temperature of soil level _j_. The reference temperature \\({T}\_{ref}\\) = 25C.

The rate scalar for soil water potential (\\({r}\_{water}\\), unitless) is calculated using a relationship from Andrén and Paustian (1987) and supported by additional data in Orchard and Cook (1983):

(2.21.6)[¶](#equation-21-6 "Permalink to this equation")\\\[\\begin{split}r\_{water} =\\sum \_{j=1}^{5}\\left\\{\\begin{array}{l} {0\\qquad {\\rm for\\; }\\Psi \_{j} <\\Psi \_{\\min } } \\\\ {\\frac{\\log \\left({\\Psi \_{\\min } \\mathord{\\left/ {\\vphantom {\\Psi \_{\\min } \\Psi \_{j} }} \\right.} \\Psi \_{j} } \\right)}{\\log \\left({\\Psi \_{\\min } \\mathord{\\left/ {\\vphantom {\\Psi \_{\\min } \\Psi \_{\\max } }} \\right.} \\Psi \_{\\max } } \\right)} w\_{soil,\\, j} \\qquad {\\rm for\\; }\\Psi \_{\\min } \\le \\Psi \_{j} \\le \\Psi \_{\\max } } \\\\ {1\\qquad {\\rm for\\; }\\Psi \_{j} >\\Psi \_{\\max } \\qquad \\qquad } \\end{array}\\right\\}\\end{split}\\\]

where \\({\\Psi}\_{j}\\) is the soil water potential in layer _j_, \\({\\Psi}\_{min}\\) is a lower limit for soil water potential control on decomposition rate (in CLM5, this was changed from a default value of -10 MPa used in CLM4.5 and earlier to a default value of -2.5 MPa). \\({\\Psi}\_{max,j}\\) (MPa) is the soil moisture at which decomposition proceeds at a moisture-unlimited rate. The default value of \\({\\Psi}\_{max,j}\\) for CLM5 is updated from a saturated value used in CLM4.5 and earlier, to a value nominally at field capacity, with a value of -0.002 MPa For frozen soils, the bulk of the rapid dropoff in decomposition with decreasing temperature is due to the moisture limitation, since matric potential is limited by temperature in the supercooled water formulation of Niu and Yang (2006),

(2.21.7)[¶](#equation-21-8 "Permalink to this equation")\\\[\\psi \\left(T\\right)=-\\frac{L\_{f} \\left(T-T\_{f} \\right)}{10^{3} T}\\\]

An additional frozen decomposition limitation can be specified using a ‘frozen Q10’ following [Koven et al. (2011)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#kovenetal2011), however the default value of this is the same as the unfrozen Q10 value, and therefore the basic hypothesis is that frozen respiration is limited by liquid water availability, and can be modeled following the same approach as thawed but dry soils.

An additional rate scalar, \\({r}\_{oxygen}\\) is enabled when the CH4 submodel is used (set equal to 1 for the single layer model or when the CH4 submodel is disabled). This limits decomposition when there is insufficient molecular oxygen to satisfy stoichiometric demand (1 mol O2 consumed per mol CO2 produced) from heterotrophic decomposers, and supply from diffusion through soil layers (unsaturated and saturated) or aerenchyma (Chapter 19). A minimum value of \\({r}\_{oxygen}\\) is set at 0.2, with the assumption that oxygen within organic tissues can supply the necessary stoichiometric demand at this rate. This value lies between estimates of 0.025–0.1 (Frolking et al. 2001), and 0.35 (Wania et al. 2009); the large range of these estimates poses a large unresolved uncertainty.

Lastly, a possible explicit depth dependence, \\({r}\_{depth}\\), (set equal to 1 for the single layer model) can be applied to soil C decomposition rates to account for processes other than temperature, moisture, and anoxia that can limit decomposition. This depth dependence of decomposition was shown by Jenkinson and Coleman (2008) to be an important term in fitting total C and 14C profiles, and implies that unresolved processes, such as priming effects, microscale anoxia, soil mineral surface and/or aggregate stabilization may be important in controlling the fate of carbon at depth [Koven et al. (2013)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#kovenetal2013). CLM includes these unresolved depth controls via an exponential decrease in the soil turnover time with depth:

(2.21.8)[¶](#equation-21-9 "Permalink to this equation")\\\[r\_{depth} =\\exp \\left(-\\frac{z}{z\_{\\tau } } \\right)\\\]

where \\({z}\_{\\tau}\\) is the e-folding depth for decomposition. For CLM4.5, the default value of this was 0.5m. For CLM5, this has been changed to a default value of 10m, which effectively means that intrinsic decomposition rates may proceed as quickly at depth as at the surface.

The combined decomposition rate scalar (\\({r}\_{total}\\),unitless) is:

(2.21.9)[¶](#equation-21-10 "Permalink to this equation")\\\[r\_{total} =r\_{tsoil} r\_{water} r\_{oxygen} r\_{depth} .\\\]

