Implications of SOM Model Pool Parameterization Via Different Methods of Two Arable Soils of SW Germany.

While soil organic matter (SOM) models such as Century have been applied in a variety of environments and land uses to simulate measured SOM dynamics, the issue of how to parameterize the compartments or pools of these models is not straight forward.  Default pool sizes are not suited for all soils, equilibrium model runs of thousands of years may not well approximate the system if previous land-uses are not known or not at equilibrium.  Measured SOM fractions have been used to parameterize the pool sizes, but the intermediate and passive pool sizes have been difficult to relate to measureable fractions.  This study examined the use of a size/density fractionation (SOM fractions), mid-infrared spectroscopy derived pools (MIRS), and thermal evolved gas analysis derived pools (EGA) of two arable soils in SW Germany to parameterize the active, slow and passive model pools of the CENTURY SOM model implemented in the Land Use Change and Assessment tool (LUCIA).  Initial pool sizes were set via the various methods and measured soil organic carbon and crop residue inputs used for the beginning of the model run.  All other parameters were set to default.  Results were compared with measured data of field soil CO₂ fluxes, microbial biomass (SMB), and TOC after two growing seasons.  The EGA approach best approximated SMB, while the MIRS and SOM fractions parameterizations greatly overestimated SMB by more than two-fold.  The three different parameterizations did not greatly affect estimated CO₂ flux amounts during the two year study, but small differences in the sum CO₂ flux were evident.  This study demonstrated several approaches and possible implications of using these methods for model pool parameterization and the modifications that may need to be done in order to adapt these measureable fractions as model pools.