Carbon recovered in degraded soils under conservation management is highly vulnerable to loss

Abstract. The recovery of soil organic carbon (SOC) lost due to human activities provides an important opportunity to restore degraded soils and contribute to climate change mitigation. However, there is much debate about how much and for how long carbon can be stored. In this work we took advantage of a long-term experiment established in 1963 in southern Uruguay to evaluate whether integrated crop-pasture systems can recover soil C in a previously degraded site, while also analyzing the persistence of the newly sequestered C. We designed and calibrated compartmental dynamic models that represent the trajectory of soil C and radiocarbon in a soil quality restoration treatment involving the implementation of a crop-pasture rotation following a prolonged period (20 years) of continuous cropping use. We found that it is possible to recover the SOC losses by prolonged agricultural use through the incorporation of perennial pastures in agricultural rotations. Moreover, this recovery occurred at high rates (∼ 0.65 Mg ha^-1 yr^-1). However, we found minimal allocation of sequestered SOC to compartments that cycle at low rates and behave as kinetically stable. Instead, most of this new C was incorporated into an intermediate-kinetic-pool that cycles in the order of years to decades. Once management changed (in 2008, after 24 years), this recovered C was susceptible to significant selective loss. The results of this study indicate that, although the compartments associated with greater stability are susceptible to loss over short time scales as a result of agricultural management, their recovery would be a much slower process. The rapid SOC recoveries that can be achieved through conservation management appear to be in forms that are highly susceptible to environmental or management changes.