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. Land use conversions can strongly impact soil organic matter (SOM) storage,which creates paramount opportunities for sequestering atmospheric carboninto the soil. It is known that land uses such as annual cropping andafforestation can decrease and increase SOM, respectively; however, therates of these changes over time remain elusive. This study focused onextracting the kinetics (k) of turnover rates that describe these long-termchanges in soil C storage and also quantifying the sources of soil C. Weused topsoil organic carbon density and δ13C isotopiccomposition data from multiple chronosequences and paired sites in Russiaand United States. Reconstruction of soil C storage trajectory over 250 years following conversion from native grassland to continual annualcropland revealed a C depletion rate of 0.010 yr−1 (first-order krate constant), which translates into a mean residence time (MRT) of 100years (R2≥0.90). Conversely, soil C accretion was observed over70 years following afforestation of annual croplands at a much faster k rateof 0.055 yr−1. The corresponding MRT was only 18 years (R2=0.997) after a lag phase of 5 years. Over these 23 years of afforestation,trees contributed 14 Mg C ha−1 to soil C accrual in the 0 to 15 cmdepth increment. This tree-C contribution reached 22 Mg C ha−1 at 70 years after tree planting. Over these 70 years of afforestation, theproportion of tree C to whole-soil C increased to reach a sizable 79 %.Furthermore, assuming steady state of soil C in the adjacent croplands, wealso estimated that 45 % of the prairie C existent at the time of treeplanting was still present in the afforested soils 70 years later. Asan intrinsic property of k modeling, the derived turnover rates that represent soil Cchanges over time are nonlinear. Soil C changes were much more dynamicduring the first decades following a land use conversion than afterwardswhen the new land use system approached equilibrium. Collectively, resultssubstantiated that C sequestration in afforested lands is a suitable meansto proactively mitigate escalating climate change within a typical person'slifetime, as indicated by MRTs of a few decades.

Nonlinear turnover rates of soil carbon following cultivation of native grasslands and subsequent afforestation of croplands