Diffusion limitations and Michaelis–Menten kinetics as drivers of combined temperature and moisture effects on carbon fluxes of mineral soils

Fernando Esteban Moyano,Lorenzo Menichetti,Nadezda Vasilyeva

doi:10.5194/bg-15-5031-2018

Abstract

Abstract. CO2 production in soils responds strongly to changes in temperature and moisture, but the magnitude of such responses at different timescales remains difficult to predict. Knowledge of the mechanisms leading to the often observed interactions in the effects of these drivers on soil CO2 emissions is especially limited. Here we test the ability of different soil carbon models to simulate responses measured in soils incubated at a range of moisture levels and cycled through 5, 20, and 35 ∘C. We applied parameter optimization methods while modifying two structural components of models: (1) the reaction kinetics of decomposition and uptake and (2) the functions relating fluxes to soil moisture. We found that the observed interactive patterns were best simulated by a model using Michaelis–Menten decomposition kinetics combined with diffusion of dissolved carbon (C) and enzymes. In contrast, conventional empirical functions that scale decomposition rates directly were unable to properly simulate the main observed interactions. Our best model was able to explain 87 % of the variation in the data. Model simulations revealed a central role of Michaelis–Menten kinetics as a driver of temperature sensitivity variations as well as a decoupling of decomposition and respiration C fluxes in the short and mid-term, with general sensitivities to temperature and moisture being more pronounced for respiration. Sensitivity to different model parameters was highest for those affecting diffusion limitations, followed by activation energies, the Michaelis–Menten constant, and carbon use efficiency. Testing against independent data strongly validated the model (R2=0.99) and highlighted the importance of initial soil C pool conditions. Our results demonstrate the importance of model structure and the central role of diffusion and reaction kinetics for simulating and understanding complex dynamics in soil C.

Highlights

Soils are a main component of the global carbon (C) cycle, storing ca. 2200 Pg of C in the top 100 cm according to recent estimates (Batjes, 2014)
Using different reaction kinetics resulted in variations in model performance as measured by root mean square error (RMSE) (Table 1)
Changes in RMSE were more sensitive to the kinetics of decomposition (FPD), with models using M and Mr decomposition kinetics resulting in lower RMSE values compared to first- and second-order kinetics

Summary

Introduction

Soils are a main component of the global carbon (C) cycle, storing ca. 2200 Pg of C in the top 100 cm according to recent estimates (Batjes, 2014). 2200 Pg of C in the top 100 cm according to recent estimates (Batjes, 2014) This soil C pool is dynamic and often exists in a non-equilibrium state as the result of an imbalance between input and output C fluxes, in which case it will act either as a C sink or as a C source over time. Changes in the speed at which soil organisms decompose soil organic matter (SOM) and mineralize soil organic carbon (SOC) into CO2 are one way in which an imbalance can occur, producing a net sink or source of atmospheric CO2. Feedback effects, either positive or negative, are expected to occur from the interaction between climate change and global soil C stocks (Crowther et al, 2016; Davidson and Janssens, 2006; Kirschbaum, 2006). Increased soil respiration with a resulting net loss of soil C, and a positive climate feed-

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