*Article* **Water Use Characteristics of Two Dominant Species in the Mega-Dunes of the Badain Jaran Desert**

**Jie Qin 1,2, Jianhua Si 1,\*, Bing Jia 1,2, Chunyan Zhao <sup>1</sup> , Dongmeng Zhou 1,2, Xiaohui He 1,2, Chunlin Wang 1,2 and Xinglin Zhu 1,2**

> <sup>1</sup> Key Laboratory of Eco-Hydrology of Inland River Basin, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China; qinjie18@lzb.ac.cn (J.Q.); jiab@lzb.ac.cn (B.J.); zhaochunyang@lzb.ac.cn (C.Z.); zhoudongmeng@nieer.ac.cn (D.Z.); hexiaohui@nieer.ac.cn (X.H.); wangchunlin@nieer.ac.cn (C.W.); zxinglin@yeah.net (X.Z.)

<sup>2</sup> Northwest Institute of Eco-Environment and Resources, University of Chinese Academy of Sciences, Beijing 100049, China

**\*** Correspondence: jianhuas@lzb.ac.cn

**Abstract:** The sparse natural vegetation develops special water use characteristics to adapt to inhospitable desert areas. The water use characteristics of such plants in desert areas are not yet completely understood. In this study, we compare the differences in water use characteristics between two dominant species of the Badain Jaran Desert mega-dunes—*Zygophyllum xanthoxylum* and *Artemisia ordosica*—by investigating δ <sup>2</sup>H and δ <sup>18</sup>O in plant xylem (the organization that transports water and inorganic salts in plant stems) and soil water, and δ <sup>13</sup>C in plant leaves. The results indicate that *Z. xanthoxylum* absorbed 86.5% of its water from soil layers below 90 cm during growing seasons, while *A. ordosica* derived 79.90% of its water from the 0–120 cm soil layers during growing seasons. Furthermore, the long-term leaf-level water use efficiency of *A. ordosica* (123.17 ± 2.13 µmol/mol) was higher than that of *Z. xanthoxylum* (97.36 ± 1.16 µmol/mol). The differences in water use between the two studied species were mainly found to relate to their root distribution characteristics. A better understanding of the water use characteristics of plants in desert habitats can provide a theoretical basis to assist in the selection of species for artificial vegetation restoration in arid areas.

**Keywords:** water sources; water use efficiency; stable isotopes; Iso-source model

#### **1. Introduction**

The Badain Jaran Desert is home to the highest mega-dunes in the world, with sparse rainfall, an arid climate, barren soil, and strong wind and sand activity. Despite the extremely arid environmental conditions, plants of the families Chenopodiaceae, Compositae, and Gramineae can be found widely distributed across the mega-dunes, as well as *Artemisia ordosica*, *Nitraria tangutorum*, *Zygophyllum xanthoxylum*, *Psammochloa villosa*, and other shrubs with tall obvious trunks and "arbor-like" growth. Among these, *Z. xanthoxylum* and *A*. *ordosica* represent the most dominant species and play an important role in preventing wind erosion and fixing sand. These plants are distributed 100–250 m above the lake surface; as such, it is difficult for them to use groundwater directly. Due to the extreme aridity of the desert—with an average annual rainfall of just 80 mm—the question of how these plants access and use water to survive in such an extreme environment remains. Answers to these questions are needed to clarify the water use characteristics of the plants in the Badain Jaran Desert.

Water—whether groundwater, surface water, or atmospheric precipitation—plays a highly important role in the desert ecosystem. It represents a dominant restrictive ecological factor affecting the growth of desert plants, restricting or determining the formation and development of desert vegetation [1,2]. Moreover, the distribution of desert vegetation can be analyzed to determine regional hydrological characteristics [3]. The degradation of desert vegetation is generally thought to be caused by a decline in the groundwater

**Citation:** Qin, J.; Si, J.; Jia, B.; Zhao, C.; Zhou, D.; He, X.; Wang, C.; Zhu, X. Water Use Characteristics of Two Dominant Species in the Mega-Dunes of the Badain Jaran Desert. *Water* **2022**, *14*, 53. https:// doi.org/10.3390/w14010053

Academic Editor: Carmen Teodosiu

Received: 19 November 2021 Accepted: 22 December 2021 Published: 28 December 2021

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level, making it impossible for plants to effectively access water [4]. However, studies show that not all desert plants use groundwater alone for their maintenance [5]. In addition to groundwater, many plants use seasonal precipitation, soil water, and condensation water (such as dew and fog) to maintain their survival [5–7] However, the relative contribution of various water sources to desert plants remains inconclusive. Many previous studies used the whole-root excavation method [8], the plant physiological and ecological index identification method [9], and stable hydrogen and oxygen isotope technology [9–12] to study differences in the water sources of desert plants between species (life forms) at different times and with various spatial distributions [2,5,12]. Few studies analyze the effect of time and spatial differences together on plant water sources, which can help to better understand plant water use. The effect of such temporal and spatial changes on plant water sources are known as water use patterns [13]. In the present study, we aimed to determine the water use patterns that allow desert plants to effectively avoid or overcome water stress, and thus survive in the extremely arid mega-dune area.

Water use patterns of desert plants determine, to a certain extent, the response of the ecosystem to changes in environmental water conditions [12]. Cyclical changes in precipitation and groundwater, climate, and groundwater level in desert systems [10] lead to temporal and spatial fluctuations in the habitat conditions of desert plants, which require desert plants to adapt to changing water availability through multiple strategies such as changing their physiological characteristics and reproduction methods [14], thereby forming special water use characteristics. Water use efficiency (WUE) is another key indicator of plant water use [12,15], which can be used to characterize the ability of plants to fix organic matter under the same water consumption conditions [16]. Current mainstream research is primarily focused on instantaneous and long-term water use efficiency at the plant leaf scale [17], the latter of which can reflect the physiological conditions of plants over long-term timescales such as months and years—typically through the use of carbon stabilization isotope ratios (δ <sup>13</sup>C) [18]. Numerous studies show that δ <sup>13</sup>C is significantly positively correlated with water use efficiency [18,19]. Therefore, carbon stable isotope technology is currently the most common method for the analysis of the long-term water use efficiency of plants.

In this study, we compare the water use characteristics of two dominant species— *Zygophyllum xanthoxylum*, a shrub species, and *Artemisia ordosica*, a semi-shrub species—in the mega-dunes of the Badain Jaran Desert using stable isotope techniques (δ <sup>2</sup>H, δ <sup>18</sup>O and δ <sup>13</sup>C). We investigated δ <sup>2</sup>H and δ <sup>18</sup>O in plant xylem and soil water from various layers up to 240 cm under the dry sand layer to analyze plant water sources. Additionally, we measured δ <sup>13</sup>C in plant leaves to explore the interspecific differences in leaf-level WUE. The soil water content was also investigated. We hypothesized that the two species had completely different water use characteristics. The main aims of this study were: (1) to clarify the water sources of two species and determine the seasonal variation in their characteristics, (2) to analyze seasonal variations in water use efficiency (WUE) for the two species, and (3) to compare the differences in water use characteristics between the two plants. This study comprehensively analyzed the water use characteristics of different life-type plants from the perspective of time and space, which is conducive to further exploring the survival mechanism of plants in desert habitats.

#### **2. Materials and Methods**

#### *2.1. Study Area*

The Badain Jaran Desert (39◦04015"–42◦12023" N, 99◦23018"–104◦34002" E) is located in the west of the Alxa Plateau in Inner Mongolia. It is the second largest desert in China, with a total area of 5.22 <sup>×</sup> <sup>10</sup><sup>4</sup> km<sup>2</sup> , an east-to-west length of 442 km, and a north-to-south distance of 354 km [20]. The region has an extremely arid temperate continental climate with sparse precipitation, strong evaporation, and abundant sunshine all year round. It is very hot in summer and windy in winter and spring. The average annual precipitation is 78.1 mm [21], which is mainly concentrated in the period of June to September, with the

majority occurring in July and August. The air temperature ranges from a mean of −9.1 ◦C in winter to a mean of 25.3 ◦C in summer, with a mean annual temperature of 7–8 ◦C. The vegetation is dominated by xerophyte and super-xerophyte shrubs and semi-shrubs with low coverage. Representative plants include *Nitraria tangutorum*, *Zygophyllum xanthoxylum*, *Reaumuria soongorica*, *Artemisia ordosica*, *Agriophyllum squarrosum*, and *Psammochloa villosa* [22]. The landform is dominated by mobile sand dunes. The soil is mostly sandy soil with weak water storage capacity. The soil texture is mainly fine sand, followed by medium sand and very fine sand. The soil is alkaline, with high salinity, low organic matter and nutrient elements (Table 1). There are about 144 lakes in the Badain Jaran Desert. The depth of the lakes is relatively shallow, and the water level fluctuates less than 0.5 m. The groundwater level in this area is between 1000–1300 m [23].

**Table 1.** Basic soil properties of Badain Jaran Desert.


#### *2.2. Materials*

The mega-dunes near Bataan Lake (102◦3701.74" E, 39◦43019.31" N) were selected as the test site. The test was conducted from May to August 2020, and samples were collected from 6:00 a.m. to 9:00 a.m. on each sampling date (all typical sunny days) in the middle of each month. Considering the height of the mega-dunes and the distribution of vegetation, the entire mega-dunes were divided into three regions: the upper part (180–250 m from the base of the mega-dunes), the middle part (from 90–160 m), and the lower part (from 0–70 m). Sampling was performed in 3 sampling regions on 5 mega-dunes, for a total of 15 sampling areas (Figure 1). *Zygophyllum xanthoxylum* and *Artemisia ordosica* (Figure 2), as plants typical of the mega-dune vegetation, were selected as the research objects. *Z. xanthoxylum* is a sand shrub growing in a "tree-like" shape, with fleshy leaves and an obvious main stem. The average plant is about 70 cm tall, with average main root length and diameter of 138 cm and 2.8 cm, respectively, and average lateral root length and diameter of 136 cm and 0.8 cm, respectively. *A. ordosica* is a semi-shrub, with slightly fleshy leaves, inconspicuous main stems, and many branches. The average plant is about 50 cm tall with an average root length of approximately 70 cm. In each area, three individual plants were randomly selected as representative samples for each species, and three parallel samples were collected for each individual plant. After sample collection was completed in the middle of each month, the indoor experimental analysis was carried out.

**Figure 1.** Location of the study area and sampling points.

**Figure 2.** Photographs of Zygophyllum xanthoxylum and Artemisia ordosica.
