Design and test of a generic cohort model of soil organic matter decomposition: the SOMKO model

1 SOMKO is a new simulation model of soil organic matter (SOM) dynamics aimed at predicting long-term and short-term SOM dynamics based on a mechanistic approach focusing on microbes as the key agents of decomposition. 2 SOM is partitioned into cohorts and chemical quality pools (classified by age and chemical composition), the microbial community processes are explicitly represented, and the C : N stoichiometric constraints are accounted for through a new mechanism of offer and demand. 3 The analysis of model equations shows that: (1) SOM C : N cannot decrease below microbial C : N; and (2) the nitrogen limitation of decomposition depends on SOM C : N, microbial biomass and soil mineral nitrogen. First tests of the: model show good qualitative behaviour for simulating the dynamics of short-term litter-bag type decomposition, longterm SOM increase, pulsed mineral nitrogen production, the priming effect due to easily decomposable carbon addition, and the effects of vegetation clearance and climate change on SOM. Simulations are in good agreement with long-term experimental data. 4 SOMKO is an integrated component of the coupled soil-vegetation models within the ETEMA (European Terrestrial Ecosystem Modelling Activity) framework. Future extensions of this work include: (1) estimating microbial parameters from specific experiments; (2) spatial distribution of SOMKO in multistrata models; and (3) implementing nitrification/denitrification processes, phosphorus limitation and microfaunal activity.