Quantifying the stock of soil organic carbon (SOC) and evaluating its potential impact factors is important to evaluating global climate change. Human disturbances and past climate are known to influence the rates of carbon fixation, soil physiochemical properties, soil microbial diversity and plant functional traits, which ultimately affect the current SOC storage. However, whether and how the paleoclimate and human disturbances affect the distribution of SOC storage on the high-altitude Tibetan Plateau remain largely unknown. Here, we took the Qinghai Plateau, the main component of the Tibetan Plateau, as our study region and applied three machine learning models (random forest, gradient boosting machine and support vector machine) to estimate the spatial and vertical distributions of the SOC stock and then evaluated the effects of the paleoclimate during the Last Glacial Maximum and the mid-Holocene periods as well as the human footprint on SOC stock at 0 to 200 cm depth by synthesizing 827 soil observations and 71 environmental factors. Our results indicate that the vegetation and modern climate are the determinant factors of SOC stocks, while paleoclimate (i.e., paleotemperature and paleoprecipitation) is more important than modern temperature, modern precipitation and the human footprint in shaping current SOC stock distributions. Specifically, the SOC stock was deeply underestimated in near natural ecosystems and overestimated in the strongly human disturbance ecosystems if the model did not consider the paleoclimate. Overall, the total SOC stock of the Qinghai Plateau was underestimated by 4.69%, 12.25% and 6.67% at depths of 0 to 100 cm, 100 to 200 cm and 0 to 200 cm, respectively. In addition, the human footprint had a weak influence on the distributions of the SOC stock. We finally estimated that the total and mean SOC stock at 200 cm depth by including the paleoclimate effects was 11.36 Pg C and 16.31 kg C m−2, respectively, and nearly 40% SOC was distributed in the top 30 cm. The paleoclimate is relatively important for the accurate modeling of current SOC stocks. Overall, our study provides a benchmark for predicting SOC stock patterns at depth and emphasizes that terrestrial carbon cycle models should incorporate information on how the paleoclimate has influenced SOC stocks.

中文翻译：

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青海高原古气候和人类足迹对土壤有机碳储量估算的不确定性

量化土壤有机碳 (SOC) 的存量并评估其潜在影响因素对于评估全球气候变化非常重要。已知人为干扰和过去的气候会影响碳固定率、土壤理化特性、土壤微生物多样性和植物功能性状，最终影响当前的 SOC 储存。然而，古气候和人为干扰是否以及如何影响青藏高原高海拔地区 SOC 储存的分布仍然很大程度上未知。在这里，我们以青藏高原的主要组成部分青海高原为研究区域，应用了三种机器学习模型（随机森林、梯度增强机和支持向量机）来估计 SOC 储量的空间和垂直分布，然后评估末次盛冰期和全新世中期古气候以及人类足迹对 0 到 0 到 SOC 储量的影响通过综合 827 个土壤观测值和 71 个环境因素得出 200 cm 深度。我们的研究结果表明，植被和现代气候是 SOC 储量的决定因素，而古气候（即古温度和古降水）在塑造当前 SOC 储量分布方面比现代温度、现代降水和人类足迹更重要。具体来说，如果模型不考虑古气候，SOC 存量在近自然生态系统中被严重低估，而在强烈的人为干扰生态系统中被高估。总体而言，青海高原 0 至 100 厘米、100 至 200 厘米和 0 至 200 厘米深度的土壤有机碳总量分别被低估了 4.69%、12.25% 和 6.67%。此外，人类足迹对 SOC 库存分布的影响很小。我们最终估计，包括古气候效应在内的 200 cm 深度的总 SOC 储量和平均 SOC 储量分别为 11.36 Pg C 和 16.31 kg C m-2，近 40% 的 SOC 分布在顶部 30 cm。古气候对于当前 SOC 储量的准确建模相对重要。全面的，