sustainability Article Sediment Organic Carbon Sequestration of Balkhash Lake in Central Asia Wen Liu 1,2,3 , Long Ma 1,2,3,* , Jilili Abuduwaili 1,2,3 , Gulnura Issanova 2,4 and Galymzhan Saparov 2,5 1 State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China; [email protected] (W.L.); [email protected] (J.A.) 2 Research Center for Ecology and Environment of Central Asia, Chinese Academy of Sciences, Urumqi 830011, China; [email protected] (G.I.); [email protected] (G.S.) 3 College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China 4 Faculty of Geography and Environmental Sciences, Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan 5 Kazakh Research Institute of Soil Science and Agrochemistry Named after U. U. Uspanov, Almaty 050060, Kazakhstan * Correspondence: [email protected] Abstract: As an important part of the global carbon pool, lake carbon is of great significance in the global carbon cycle. Based on a study of the sedimentary proxies of Balkhash Lake, Central Asia’s largest lake, changes in the organic carbon sequestration in the lake sediments and their possible influence over the past 150 years were studied. The results suggested that the organic carbon in the sediments of Lake Balkhash comes mainly from aquatic plants. The organic carbon burial rate fluctuated from 8.16 to 30.04 g·m−2·a−1 and the minimum appeared at the top of the core. The organic carbon burial rate continues to decline as it has over the past 150 years. Global warming, higher hydrodynamic force, and low terrestrial input have not been conducive to the Citation: Liu, W.; Ma, L.; improvement of organic carbon sequestration in Balkhash Lake; the construction of a large reservoir Abuduwaili, J.; Issanova, G.; had a greater impact on the sedimentary proxy of total organic carbon content, which could lead to a Saparov, G. Sediment Organic Carbon large deviation for environmental reconstruction. This is the first study to assess the sediment organic Sequestration of Balkhash Lake in carbon sequestration using the modern sediments of Central Asia’s largest lake, which is of great Central Asia. Sustainability 2021, 13, scientific significance. The results contribute to an understanding of organic carbon sequestration 9958. https://doi.org/10.3390/ in Central Asia and may provide a scientific basis for carbon balance assessment in regional and su13179958 global scales. Academic Editor: Jeroen Meersmans Keywords: organic carbon sequestration; climate change; human activities; environmental change; lacustrine sediments; Balkhash Lake; Central Asia Received: 9 August 2021 Accepted: 3 September 2021 Published: 5 September 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Over the past century, surface processes in Central Asia have been significantly published maps and institutional affil- affected by global warming and human activities [1–4]. The changes in lakes and the iations. ecological and environmental effects caused by climate fluctuations and human activities have had a profound impact on regional economic development [5–7]. Additionally, it is of great practical significance to study the ecological and environmental problems in Balkhash Lake caused by the combined influences of anthropogenic factors and climate change. With the increasing prominence of various climate and environmental issues, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. such as greenhouse gases and the greenhouse effect, and the objective need for carbon This article is an open access article source and carbon sink evaluation in international climate negotiations, the issue of the distributed under the terms and carbon cycle has received increasing attention from the public [8–10]. Although lake water conditions of the Creative Commons occupies a small surface area of the Earth, more and more studies have shown that lakes Attribution (CC BY) license (https:// are a carbon sink with a disproportionate effect on the global carbon cycle [11–13], and the creativecommons.org/licenses/by/ lake carbon storage plays an important role in mitigating climate change [14]. There are 4.0/). many lakes in Central Asia, but the research on lake sediments in Central Asia has mainly Sustainability 2021, 13, 9958. https://doi.org/10.3390/su13179958 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 9958 2 of 12 focused on the evolution and reconstruction of the climate environment [15–18]. Sediment organic carbon is an important global carbon sink; however, an in-depth understanding of the influences of climate change and anthropogenic factors on sediment organic carbon sequestration is still lacking, making this region one of the most uncertain regions in the assessment of the global carbon balance. Balkhash Lake, Central Asia’s largest lake and the tail of the Ili River, and its issue have attracted worldwide attention [19–21]. As the main means of surface material movement in a watershed, lake sediments carry information about watershed climate and environmental changes [22], and this information can provide a reliable way of assessing environmental evolution over a long time scale [19,23–26]. Sugai [23] analyzed the multi-environmental proxy indicators of core sediments in Balkhash Lake and concluded that the core sediments were deposited during the early Holocene. Based on the diatom combinations in the sediments, Endo et al. [24] and Chiba et al. [25] reconstructed lake water levels and pH changes in Balkhash Lake over the past 8000 and 2000 years, respectively; furthermore, Feng et al. [26] used the pollen–climate response to quantitatively reconstruct the climate change history of the Balkhash Lake basin over the past 2500 years. Mischeke et al. [19] used ostracod species assemblages and shell chemistry to estimate the lake level and lower salinity over the last 2900 years. Although there is still a lack of understanding of the dynamic changes in carbon burial in Balkhash Lake, studies have found that arid regions may have great potential for carbon sequestration and carbon storage [18,19]. In particular, the responses of organic carbon sequestration to climate change are still unclear in Central Asia. In the context of global warming, does the rate of carbon burial in lake sediments increase or decrease? Existing studies on the sediments of Small Aral Sea show that the organic carbon burial rate is increasing significantly [27]. The water area of Balkhash Lake is significantly larger than the Small Aral Sea, is there a similar change in Balkhash Lake? Focused on the burial pattern of organic carbon in modern sediments of Lake Balkhash, the largest lake in Central Asia, and its possible influencing factors, this paper examines the core sediments of Balkhash Lake as the main research object and discusses (1) the organic carbon burial information contained in the sediments of Balkhash Lake over the past about 150 years, (2) temporal variation of organic carbon sequestration and its relationship to environmental and main climatic factors. This research will provide the beneficial complement for a thorough understanding of organic carbon sequestration in Central Asia and will offer scientific data for carbon balance assessment in regional and global scales. 2. Regional Setting Balkhash Lake is in southeastern Kazakhstan, and the length of the lake is approx- imately 614 km from east to west (Figure1). The average width from north to south is approximately 30 km, the maximum width is approximately 74 km, and the water area is approximately 18,200 km2 [21]. The average annual water level of the lake is approximately 342 m above sea level, the maximum water depth is 26.5 m, the average depth is 5.8 m, and the water storage volume is 105.4 km3 [28]. The Uzynaral Strait (approximately 3.5 km) north of the Saryesik Peninsula divides Balkhash Lake into two parts: the eastern part and the western part. The eastern lake is infused by the Aksu, Karatal, Lepsy, and Ayaguz Rivers. The eastern part of Balkhash Lake is approximately 318 km long and has an average width of 24 km, a maximum width of 47.5 km, a surface area of 7580 km, an average depth of 7.6 m, a maximum depth of 26.5 m, and a water volume of 57.6 km3 [28]; furthermore, the salinity is approximately 4.6–5.8 g L−1 [29]. The western lake is 296 km long and has an average width of 36 km, a maximum width of 70 km, an average water depth of 4.6 m, a maximum water depth of 11 m, a surface area of 10,630 km2, and a water volume of 47.8 km3 [28]; furthermore, the salinity is 0.5–1.0 g L−1 [29]. The Ili River accounts for approximately 78% of the total inflow into Balkhash Lake [30]. Sustainability 2021, 13, x FOR PEER REVIEW 3 of 12 The area of the Balkhash Lake watershed is approximately 41.3 × 104 km2 [21], and it has sufficient light and heat resources, large temperature changes, strong evaporation, and low precipitation. The mean annual temperature of the Balkhash region is approxi‐ mately 5.8 °C, the total annual precipitation is 142 mm, the total annual evapotranspira‐ tion is 668 mm, the continentality index is 39.5, and the aridity index is 4.7 [21]. The estuary delta formed by the Ili River is approximately 8000 km2 [31]. The agricultural land in the river basin decreased from 1.8 × 106 ha in the 1970s to 1.4 × 106 ha in 2013 [32]. Basin water resources are mainly used for irrigation, rice crops, industry, and water supply in residen‐ tial areas and fisheries [28]. The proportion of water used for irrigation of agricultural land in the basin is also the largest among all water uses, exceeding 82% of the total water consumption [33].