Article Vegetation Determines Lake Sediment Carbon Accumulation during Holocene in the Forest–Steppe Ecotone in Northern China Qian Hao 1 , Shilei Yang 1 , Zhaoliang Song 1, *, Zhengang Wang 2 , Changxun Yu 3 and Hailong Wang 4,5   Citation: Hao, Q.; Yang, S.; Song, Z.; Wang, Z.; Yu, C.; Wang, H. Vegetation Determines Lake Sediment Carbon Accumulation during Holocene in the Forest–Steppe Ecotone in Northern China. Forests 2021, 12, 696. https:// doi.org/10.3390/f12060696 Academic Editor: Brian Tobin Received: 16 April 2021 Accepted: 27 May 2021 Published: 28 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Earth System Science, Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China; haoqian@tju.edu.cn (Q.H.); yangshilei@tju.edu.cn (S.Y.) 2 School of Geography and Planning, Sun Yat-Sen University, Guangzhou 510275, China; wangzhg33@mail.sysu.edu.cn 3 Department of Biology and Environmental Science, Linnaeus University, SE-39182 Kalmar, Sweden; changxun.yu@lnu.se 4 School of Environmental and Chemical Engineering, Foshan University, Foshan 528000, China; hailong.wang@fosu.edu.cn 5 School of Environmental and Resource Sciences, Zhejiang A&F University, Hangzhou 311300, China * Correspondence: zhaoliang.song@tju.edu.cn; Tel.: +86-152-0226-4081 Abstract: To understand the past carbon accumulation of forest–steppe ecotone and to identify the main drivers of the long-term carbon dynamics, we selected Huangqihai Lake and analyzed the sediment records. We measured the organic carbon content (TOC; %) of sedimentary samples and quantified the carbon accumulation rate (CAR; g C m −2 yr −1 ). Furthermore, the climate, soil erosion, and vegetation development of the past 6800 years were reconstructed using physicochemical parameters and pollen records. Human activities were also obtained from a 2200-year history record. Our results showed that the CAR was high during 5800~4100 cal yr BP (40~60 g C m −2 yr −1 ), which is mainly attributed to the high sediment accumulation rate (SAR) during this period. Pearson’s correlation, redundancy analysis and hierarchical variation partitioning analyses suggested that the CAR was influenced by the SAR and TOC, while vegetation dynamics (broadleaved tree percentage and vegetation coverage) and local soil erosion were the main drivers of the TOC and SAR. Especially when the vegetation was dominated by broadleaved forests, the CAR was significantly high due to the high gross primary productivity and carbon density of forest compared with steppe. Our study highlights the direct influence of local vegetation and soil erosion on the CAR, whereas climate might influence indirectly by changing local vegetation and soil conditions. Moreover, our results showed that human activities had positive influences on the carbon accumulation dynamics in this region since 2200 cal yr BP by influencing the SAR. Keywords: carbon accumulation rate (CAR); Huangqihai Lake; climate change; vegetation coverage; broadleaved forest; Holocene 1. Introduction Global climate change caused by the rapid emission of carbon dioxide (CO 2 ) and other greenhouse gases since the Industrial Revolution has attracted increasing public attention [1]. Although lakes cover only 2% of the Earth’s surface, they act as an im- portant sink for carbonaceous matter, which is only partly mineralized in the water col- umn [2–4], and thus play a significant role in regulating the global carbon cycle [5–7]. For instance, it is estimated that during the Holocene, lakes annually buried 42 Tg or- ganic carbon (OC), which is more than two-fifths of the amount of organic carbon buried in the ocean (100 Tg OC yr −1 )[8]. Furthermore, Dong et al. [9] suggested that lakes are 3.4 times more effective than terrestrial ecosystems in C burial. The lakes in China Forests 2021, 12, 696. https://doi.org/10.3390/f12060696 https://www.mdpi.com/journal/forests