Validation and Inversion of a Carbon Net Ecosystem Exchange Model for Croplands.

Croplands play an important role in the carbon budget of many regions. However, the estimation of their carbon balance remains difficult due to the diversity and the complexity of the processes involved.

We report the coupling of a one-dimensional soil water, heat and CO₂ flux model (SOILCO₂) with a pool concept of soil carbon turnover (RothC) and with a crop growth module (SUCROS) for the prediction of carbon net ecosystem exchange (NEE). The coupled model, further referred to as AgroC, was extended with routines for managed grassland as well as for root exudation and root death.

In a first step the coupled model was applied to two winter wheat sites and an upland grassland in Germany. The models were validated in terms of soil water content, soil temperature, soil CO₂ concentration, soil respiration, leaf area index, aboveground biomass and NEE measured with the Eddy covariance method. The overall model performance of AgroC was acceptable. Accounting for root exudation and root death improved model performance, which was tested with a dataset providing measured root and heterotrophic respiration.

In the second step the models were inverted and the effect of using either NEE measurements only or the combination of measured soil respiration and NEE was investigated. The estimated yearly carbon balance showed a distinct sensitivity towards the inclusion of soil respiration data in the inversion process. This was mainly attributed to the characteristics of the NEE measurements.