Land use conversions can strongly impact soil organic matter (SOM) storage, which creates paramount opportunities for sequestering atmospheric carbon into the soil. It is known that land uses such as annual cropping and afforestation can decrease and increase SOM, respectively; however, the rates of these changes over time remain elusive. This study focused on extracting the kinetics (k) of turnover rates that describe these long-term changes in soil C storage and also quantifying the sources of soil C. We used topsoil organic carbon density and δ¹³C isotopic composition data from multiple chronosequences and paired sites in Russia and United States. Reconstruction of soil C storage trajectory over 250 years following conversion from native grassland to continual annual cropland revealed a C depletion rate of 0.010 yr⁻¹ (first-order k rate constant), which translates into a mean residence time (MRT) of 100 years (R²≥0.90). Conversely, soil C accretion was observed over 70 years following afforestation of annual croplands at a much faster k rate of 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 cm depth increment. This tree-C contribution reached 22 Mg C ha⁻¹ at 70 years after tree planting. Over these 70 years of afforestation, the proportion of tree C to whole-soil C increased to reach a sizable 79 %. Furthermore, assuming steady state of soil C in the adjacent croplands, we also estimated that 45 % of the prairie C existent at the time of tree planting was still present in the afforested soils 70 years later. As an intrinsic property of k modeling, the derived turnover rates that represent soil C changes over time are nonlinear. Soil C changes were much more dynamic during the first decades following a land use conversion than afterwards when the new land use system approached equilibrium. Collectively, results substantiated that C sequestration in afforested lands is a suitable means to proactively mitigate escalating climate change within a typical person's lifetime, as indicated by MRTs of a few decades.

中文翻译：

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天然草地耕种和农田植树造林后土壤碳的非线性周转率

土地利用转换会强烈影响土壤有机质 (SOM) 的储存，这为将大气碳封存到土壤中创造了至关重要的机会。众所周知，一年生作物和植树造林等土地利用可以分别减少和增加 SOM；然而，随着时间的推移，这些变化的速度仍然难以捉摸。本研究的重点是提取描述土壤碳储存长期变化的周转率动力学 ( k ) 并量化土壤碳的来源。我们使用表土有机碳密度和δ ¹³来自俄罗斯和美国多个年代序列和配对位点的 C 同位素组成数据。从原生草地转变为连续一年生农田后 250 年的土壤碳储存轨迹重建显示碳消耗率为 0.010 yr ^-1（一阶k 速率常数），这意味着平均停留时间（MRT）为 100 年( R ² ≥0.90 )。相反，在一年生农田植树造林后的 70 年中观察到土壤 C 增加，k速率快得多，为 0.055 yr ^-1。相应的 MRT 仅为 18 年（R ² =0.997) 经过 5 年的滞后期。在这 23 年的植树造林中，树木在 0 至 15 厘米深度增量中为土壤 C 累积贡献了 14 Mg C ha ^-1。在植树后 70 年，这种树 C 贡献达到 22 Mg C ha ^-1。在这 70 年的造林过程中，树木 C 占全土 C 的比例增加到了 79%。此外，假设邻近农田的土壤碳处于稳定状态，我们还估计植树时存在的 45% 的草原碳在 70 年后仍存在于造林土壤中。作为k的内在属性模型中，代表土壤碳随时间变化的衍生周转率是非线性的。在土地利用转变后的最初几十年中，土壤碳变化比新土地利用系统接近平衡后的动态要大得多。总体而言，结果证实，如几十年的 MRT 所表明的那样，在造林土地中封存碳是在典型人的一生中主动缓解气候变化不断升级的合适手段。