Modeling coupled enzymatic and solute transport controls on decomposition in drying soils

• An analytical equation is derived to describe respiration–moisture relations.
• The equation accounts for physical and biological processes during soil drying.
• Respiration is the product of a moisture function and a biochemical function.
• The equation predicts observed patters by calibrating only microbial deactivation.

Mechanistic descriptions of microbial processes are difficult to embed in ecosystem models because they require complex mathematical formulations. The interactions between microbes, soil carbon (C), and water availability are particularly complex, as they involve coupled physical (advection and diffusion in unsaturated media) and biochemical processes (enzymatic reactions, C uptake by microbes). Here we propose an approximated equation based on a quasi-equilibrium assumption that describes microbial uptake of soil C as a function of soil moisture and organic matter content during soil drying. The equation predicts that uptake depends on two terms, one dependent on soil organic C concentration and enzyme availability (analogous to a Michaelis–Menten equation) and one dependent on soil moisture via its effects on enzyme and solute mass transfer, and microbial uptake kinetics. Assuming that uptake is proportional to microbial respiration, model results are compared to measured respiration–water potential curves. Using independently estimated parameter values (except for the calibrated microbial uptake efficiency), the theoretical model captures well the respiration decline during drying and provides an explanation of respiration pulses at rewetting. Thus, this simple formulation could be employed in ecosystem models as an alternative to empirical respiration–moisture response functions.

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