Zhang Qingqing, Huang Jinghua, Hu Feinan, Huo Na, Shang Yingni, Chang Wenqian, Zhao Shiwei
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling, Shaanxi, China.
University of Chinese Academy of Sciences, Beijing, China.
PeerJ. 2018 Apr 13;6:e4611. doi: 10.7717/peerj.4611. eCollection 2018.
The loess-paleosol sequence on the Loess Plateau has been considered an important paleoclimatic archive to study global climatic and environmental changes in the Quaternary. So far, little attention has been paid to the characteristics of soil organic carbon fractions in loess-paleosol sequences, which may provide valuable information for exploring the evolution of climate and environment in the Quaternary on the Loess Plateau.
In order to explore the significance of mineral-associated organic carbon to total organic carbon (MOC/TOC) ratios in the loess-paleosol sequence for reconstructing paleoenvironmental and paleoclimatic evolution in the Quaternary on the Loess Plateau, we selected a typical loess-paleosol profile in Chunhua county, Xianyang city, Shaanxi province, as the research object. The content of total organic carbon (TOC) and MOC/TOC ratio in each loess and paleosol layers of the Chunhua loess-paleosol profile were analyzed, together with the paleoclimatic proxies, such as soil grain size, CaCO content and their correlations with organic carbon parameters.
The main results were as follows: (1) the total content of soil organic carbon and MOC/TOC ratios were generally higher in paleosol layers than in the underlying loess layers of the Chunhua loess-paleosol profile. Compared to total organic carbon content, MOC/TOC ratios changed more obviously in soil layers below a paleosol layer S8; (2) soil clay content and median grain size (Md ()) were higher in paleosol than in the underlying loess, while CaCO content showed an opposite tendency. In the Chunhua profile, the distribution characteristics of the three paleoclimatic proxies showed good indications of paleoclimate changes in the Quaternary; (3) in the Chunhua loess-paleosol profile, MOC/TOC ratios were positively correlated with clay content and median grain size (), while negatively correlated with CaCO content, and the correlations were more significant in soil layers below S8.
Our results indicated that MOC/TOC ratios in the Chunhua loess-paleosol profile correlated with the cold dry-warm wet paleoclimatic cycle in the Quaternary. The high MOC/TOC ratios in the loess-paleosol profile might reflect warm and humid climate, while lower ratios indicated relatively cold and dry climate. That is because when the climate changed from warm-humid to cold-dry, the vegetation coverage and pedogenesis intensity decreased, which increased soil CaCO content and decreased soil clay content and Md (), leading to decreased MOC/TOC ratios. Compared to TOC, MOC/TOC ratios had greater significance in indicating paleoenvironmental evolution in the Quaternary on the Loess Plateau. Therefore, investigating MOC/TOC ratios in loess-paleosol profile can offer new evidence to reconstructing paleoenvironmental changes, and also provide a basis for predicting responses of soil organic carbon pools to vegetation and climate changes in the future.
黄土高原的黄土-古土壤序列被认为是研究第四纪全球气候和环境变化的重要古气候档案。迄今为止，人们对黄土-古土壤序列中土壤有机碳组分的特征关注较少，而这些特征可能为探索黄土高原第四纪气候和环境演变提供有价值的信息。
为了探究黄土-古土壤序列中矿物结合有机碳与总有机碳（MOC/TOC）比值对重建黄土高原第四纪古环境和古气候演变的意义，我们选取了陕西省咸阳市淳化县的一个典型黄土-古土壤剖面作为研究对象。分析了淳化黄土-古土壤剖面各黄土层和古土壤层中总有机碳（TOC）含量和MOC/TOC比值，以及土壤粒度、CaCO₃含量等古气候指标及其与有机碳参数的相关性。
主要结果如下：（1）淳化黄土-古土壤剖面中古土壤层的土壤有机碳总量和MOC/TOC比值总体上高于下伏黄土层。与总有机碳含量相比，在古土壤层S8以下的土层中，MOC/TOC比值变化更明显；（2）古土壤中的土壤粘粒含量和中值粒径（Md（ø））高于下伏黄土，而CaCO₃含量则呈相反趋势。在淳化剖面中，这三个古气候指标的分布特征很好地指示了第四纪古气候变化；（3）在淳化黄土-古土壤剖面中，MOC/TOC比值与粘粒含量和中值粒径（ø）呈正相关，与CaCO₃含量呈负相关，且在S8以下土层中相关性更显著。
我们的结果表明，淳化黄土-古土壤剖面中的MOC/TOC比值与第四纪冷干-暖湿古气候旋回相关。黄土-古土壤剖面中较高的MOC/TOC比值可能反映温暖湿润的气候，而较低的比值则表明相对寒冷干燥的气候。这是因为当气候从暖湿变为冷干时，植被覆盖度和成土作用强度降低，导致土壤CaCO₃含量增加，土壤粘粒含量和Md（ø）减小，从而使MOC/TOC比值降低。与TOC相比，MOC/TOC比值在指示黄土高原第四纪古环境演变方面具有更大的意义。因此，研究黄土-古土壤剖面中的MOC/TOC比值可为重建古环境变化提供新的证据，也为预测未来土壤有机碳库对植被和气候变化的响应提供依据。
