TheDualArrhenius andMichaelis–Menten kinetics model for decomposition of soil organic matter at hourly to seasonal time scales

Abstract

Decomposition of soil carbon stocks is one of the largest potential biotic feedbacks to climate change. Models of decomposition of soil organic matter and of soil respiration rely on empirical functions that relate variation in temperature and soil water content to rates of microbial metabolism using soil-C substrates. Here, we describe a unifying modeling framework to combine the effects of temperature, soil water content, and soluble substrate supply on decomposition of soluble soil-C substrates using simple functions based on process concepts. The model's backbone is the Michaelis-Menten equation, which describes the relationship between reaction velocity and soluble organic-C and O 2 substrate concentrations at an enzyme's reactive site, which are determined by diffusivity functions based on soil water content. Temperature sensitivity is simulated by allowing the maximum velocity of the reaction (V max ) to vary according to Arrhenius function. The Dual Arrhenius and Michaelis-Menten kinetics (DAMM) model core was able to predict effectively observations from of laboratory enzyme assays of b-glucosidase and phenol-oxidase across a range of substrate concentrations and incubation temperatures. The model also functioned as well or better than purely empirical models for simulating hourly and seasonal soil respiration data from a trenched plot in a deciduous forest at the Harvard Forest, in northeastern United States. The DAMM model demonstrates that enzymatic processes can be intrinsically temperature sensitive, but environmental constrains of substrate supply under soil moisture extremes can prevent that response to temperature from being observed. We discuss how DAMM could serve as a core module that is informed by other modules regarding microbial dynamics and supply of soluble-C substrates from plant inputs and from desorption of physically stabilized soil-C pools. Most importantly, it presents a way forward from purely empirical representation of temperature and moisture responses and integrates temperature-sensitive enzymatic processes with constraints of substrate supply.

Pore-scale investigation on the response of heterotrophic respiration to moisture conditions in heterogeneous soils

“…It affects soil organic carbon (SOC) bioavailability and the rate of oxygen delivery that affect microbial metabolism in regulating heterotrophic SOC decomposition (Moyano et al 2012;Rodríguez-Iturbe and Porporato 2005). Low moisture content restricts pore-water connectivity and decreases SOC mass transport, and thus reduces SOC bioavailability (Davidson et al 2012). Full saturation and inundation decreases the effective rate of oxygen diffusion and thus aerobic respiration in soils (Franzluebbers 1999;Skopp et al 1990).…”

“…The prediction of carbon cycling in soils is, however, highly dependent on the empirical representations of the HR processes (Davidson et al 2012), and contains large uncertainty (Bauer et al 2008;Rodrigo et al 1997;Sierra et al 2015). Reactive transport processes including moisture-dependent diffusion for describing substrate transport and Michaelis-Menten kinetics for describing microbial respiration have been considered in the process-based models (Davidson et al 2012).…”

Microbial dormancy improves development and experimental validation of ecosystem model

“…However, these feedbacks are not yet predictable because we lack both mechanistic understanding and appropriate model structure. Theoretical analyses have been developed to account for the roles of microbes and/ or exoenzymes in decomposition of litter and soil organic matter (SOM; for example, Schimel and Weintraub, 2003;Lawrence et al, 2009;Allison et al, 2010;Davidson et al, 2012;Moorhead et al, 2012;. Observations suggest that a critical parameter, the microbial carbon use efficiency (CUE) or microbial growth efficiency, is likely to decrease as a function of warming (Devevre and Horwath, 2000;Manzoni et al, 2008;Steinweg et al, 2008), which may result in lower microbial biomass, lower enzyme production and reduced heterotrophic respiration rates (Bradford et al, 2008;Hartley and Ineson, 2008;Tucker et al, 2013;.…”

Diffusion based modelling of temperature and moisture interactive effects on carbon fluxes of mineral soils

“…In our 15 model we used a combination, simulating a diffusion flux between enzyme pools and calculating the how much CD is available for uptake at each time step. We did not assume a diffusion regulation of available particulate C, an approach that is closer to empirical functions scaling the decomposition flux directly and that has been implemented in other microbial models (Davidson et al, 2012).…”