Carbon accumulation in a bare fallow Chernozem soil with high carbon input rates

A multitude of models for soil organic carbon (SOC) dynamics are available to predict management impacts on SOC dynamics in its interaction with environmental factors. Most models include one SOC pool that has no or a very slow, negligible turnover. For this pool of long-term stabilized (LTS) organic matter, we assumed a dynamic physical protection, whereby organic matter is protected in micropores and exposed to microbial access due to soil structure dynamics. Dynamics of the LTS pool is of special interest because of its very long turnover time. Therefore, it is of special importance whether the size of this pool is unlimited or if it reaches a saturation state. We investigated this topic by applying the CCB (CANDY Carbon Balance) model on a dataset from a long-term experiment that started in 1983 in Bad Lauchstadt, Germany on a Haplic Chernozem and includes bare fallow treatments with the application of farmyard manure at different rates from 5 to 20 kg m(-2) year(-1). Observations of SOC development over time supported our hypothesis about the existence of a saturation effect and were used to calibrate the CCB model with the hypothetical assumption of a limited LTS pool size. The model results suggest an upper limit of the LTS pool of 1.07 M% (percent of dry soil mass) SOC for the Bad Lauchstadt site. Furthermore, model calibration of LTS turnover revealed that organic matter in this pool is released from its protected state at a very low rate of 0.003 year(-1). Highlights Long-term bare fallow field experiment with FYM input rates up to 20 kg m(-2) Predicted long-term SOC dynamics reflect observations Proof of non-linear relationship between SOC storage and carbon input rate Evidence for limited protection capacity for SOC (saturation effect)