Parameterization and initialization of a soil organic matter decomposition model in an agricultural soil.

Organic matter decomposition and associated heterotrophic respiration fluxes are widely studied, as these processes could be modified under global warming. Many models have been built at different temporal and spatial scales to contribute to a better understanding of the mechanisms involved and to quantify soil carbon fluxes. Yet, agroecosystems have been less investigated so far, despite their considerable importance. In this study, a daily-time step ecosystem model derived from CENTURY is described, parameterized and initialized for the Carboeurope agricultural site of Lonzée in Belgium. At this stage, the model aims at describing soil heterotrophic respiration and carbon dynamics in the soil. Model parameterization was performed on the basis of a literature survey (biochemical parameters) and of data collected at the site itself (soil carbon content and soil texture). In order to set up the carbon repartition between the different pools of the model, an initialization phase was run until equilibrium was reached. For this phase, mean daily climatic data were used and the soil was cultivated with winter wheat, considering that all residues were brought to the soil at harvest. At the end, the repartition was found to be independent from the simulated soil carbon content. Simulations showed a very high sensitivity of the model to the amount of incorporated residues and allowed an estimation of the amount of residues that lead the soil to a stable state. It was compatible with field observations. The model was then run with 2007 climatic data and the above-mentioned carbon repartition to simulate heterotrophic respiration. A comparison between these simulated fluxes and automatic measurements of soil respiration, performed during a 3-month period in spring 2007 on a bare zone of the field, showed a reasonable good agreement. Most of the discrepancies between measured and simulated fluxes corresponded to dry events, attesting of a need to reconsider the relationship between soil heterotrophic respiration and soil moisture in the model.
To go further with the assessment of the model reliability, a calibration on data from the French Carboeurope site of Lamasquère will be achieved. Other sites may also be used. This heterotrophic soil respiration model is intended to be part of a more complete soil respiration model focused on agroecosystems and developed at the annual and ecosystem scales. In the end, autotrophic respiration, nitrogen mineralization and crop management would also be included.