An assessment of cumulative CO₂ efflux as proxy of organic matter turnover time in the soil priming effect: Analysis using the lumped parameter approach

Abstract

Soil microbes respond to fresh organic matter in soil by altering their respiration rate and hence the cumulative CO₂ efflux (henceforth called efflux) from soil. Parallelly, perturbed microbial respiration alters organic matter recalcitrance. Consequently, the turnover time of two carbon pools, namely recalcitrant soil organic matter and active microbial biomass, changes. The former outcome is the real, and the latter outcome the apparent soil priming effect (PE). Either or both PEs can occur. If pool turnover time changes, then the pool efflux necessarily changes. The PE is therefore widely measured using the pool efflux as proxy of pool turnover time since the latter cannot be measured directly. Typically, labelled experiments are performed to identify the carbon source in the efflux. We identify a caveat in this procedure. If soil PEs in multiple locations are compared, the initial soil carbon content per unit soil mass varies across locations, becoming a confounding variable. We propose a correction for this confounding effect. We also propose a simulation methodology to identify the two PEs from the total efflux without the need of expensive and onerous labelled experiments. A lumped parameter multi-carbon pool model is developed, in which the turnover time of the two carbon pools corresponds to two distinct model parameters. This model structure allows defining the two PEs by the change in the relevant model parameter value in treatment relative to control. Applying the definition to experimental data arising from glucose addition to loamy Haplic Luvisol soil gives results consistent with microbial ecology.