Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 1 2007 10.5194/bg-4-125-2007 http://www.biogeosciences.net/4/125/2007/ http://www.biogeosciences.net/4/125/2007/bg-4-125-2007.html http://www.biogeosciences.net/4/125/2007/bg-4-125-2007.pdf 125 136 2007-02-15 Soils apart from equilibrium – consequences for soil carbon balance modelling T. Wutzler twutz@bgc-jena.mpg.de M. Reichstein Max Planck Institute for Biogeochemistry, Jena, Germany Many projections of the soil carbon sink or source are based on kinetically defined carbon pool models. Para\-meters of these models are often determined in a way that the steady state of the model matches observed carbon stocks. The underlying simplifying assumption is that observed carbon stocks are near equilibrium. This assumption is challenged by observations of very old soils that do still accumulate carbon. In this modelling study we explored the consequences of the case where soils are apart from equilibrium. Calculation of equilibrium states of soils that are currently accumulating small amounts of carbon were performed using the Yasso model. It was found that already very small current accumulation rates cause big changes in theoretical equilibrium stocks, which can virtually approach infinity. We conclude that soils that have been disturbed several centuries ago are not in equilibrium but in a transient state because of the slowly ongoing accumulation of the slowest pool. A first consequence is that model calibrations to current carbon stocks that assume equilibrium state, overestimate the decay rate of the slowest pool. A second consequence is that spin-up runs (simulations until equilibrium) overestimate stocks of recently disturbed sites. In order to account for these consequences, we propose a transient correction. 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