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Water use characteristics of native and exotic shrub species in the semi-arid Loess Plateau using an isotope technique
Agriculture, Ecosystems and Environment 276 (2019) 55–63
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Water use characteristics of native and exotic shrub species in the semi-arid Loess Plateau using an isotope technique
T
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Jian Wanga,b, Bojie Fua,b, Nan Lua,b, , Shuai Wangc, Li Zhangd a
State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China University of Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China d State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Stable isotope Water use Native species Loess Plateau MixSIAR Root distribution
Knowledge of the processes underlying plant water use characteristics is critical for understanding soil–plant interactions and evaluating plant adaptability in water-limited ecosystems. The vegetation on the Loess Plateau has -dramatically changed due to the implementation of the Grain-for-Green Project from 1999. Despite this, water use characteristics of native and exotic shrub species remain poorly understood in this region. In this study, seasonal variations of water use characteristics of Spiraea pubescens (a native shrub) and Hippophae rhamnoides (an exotic shrub) in the Loess Plateau were investigated by examining the δ2H and δ18O of xylem and soil water from different soil layers within 300 cm of the surface, as well as the δ13C in plant leaves. Results revealed that H. rhamnoides and S. pubescens derived ˜80% of their water sources from the 0–120 cm soil layer during the growing seasons. However, H. rhamnoides absorbed higher proportions of deep soil water (120–300 cm) as the growing season progressed. H. rhamnoides flexibly converted its water source between shallow (0–40 cm) and deep soil layers. Moreover, H. rhamnoides had higher leaf-level water use efficiency than that of S. pubescens. These results suggest that H. rhamnoides has a greater degree of ecological plasticity in water use. Flexible water use characteristics are relevant to functionally dimorphic root systems as an adaptation strategy for the plants in water-limited environments. These findings indicate that water use characteristics of these plants should be considered when exotic species are introduced for revegetation in semiarid regions.
1. Introduction Vegetation restoration plays a crucial role in preventing desertification (Ma et al., 2017), controlling soil erosion (Zhu et al., 2015), and regulating hydrological regimes in semiarid regions (Yaseef et al., 2010). Revegetation using exotic and native species is encouraged worldwide due to the aforementioned advantages. However, inappropriate vegetation restoration strategies may lead to negative ecological consequences, such as soil desiccation (Chen et al., 2008; Wang et al., 2011), land degradation (Stokes et al., 2010), and loss of ecosystem functioning (e.g., soil conservation and water conservation) (Fu et al., 2017). Water use characteristics of plants used for revegetation provide critical insight into soil-plant interactions and adequate assessments of plant adaption in water-limited ecosystems (Grossiord et al., 2018). Water is the primary limiting factor of vegetation development in
arid and semi-arid regions (Porporato et al., 2004), and the temporal and spatial variations of plant water sources are called water use patterns (Wang et al., 2017). Water use efficiency (WUE) is another key indicator of plant water use characteristics and reflects the trade-off between plant productivity, water availability, and the ability of plants to adapt to and tolerate droughts (Saugier et al., 2012; Gao et al., 2017). The δ13C in plant leaves is an indicator of leaf-level intrinsic water use efficiency (WUEi) (Farquhar et al., 1989; Saugier et al., 2012). Previous studies have shown that δ13C in plant leaf has a positive relationship with WUEi (Farquhar et al., 1989; Saugier et al., 2012). As for the C3 photosynthesis plants, plants with higherplant leaf δ13C had higher leaflevel of WUEi (Farquhar et al., 1989). Moreover, water use characteristics vary among plant growth forms and species (Voltas et al., 2015; Antunes et al., 2018). The capacity to adjust water use characteristics is related to active root depth and distribution (Fort et al., 2017). Plants with functional dimorphic root systems have been observed to switch
⁎ Corresponding author at: State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. E-mail address: [email protected] (N. Lu).
https://doi.org/10.1016/j.agee.2019.02.015 Received 19 December 2018; Received in revised form 20 February 2019; Accepted 21 February 2019 0167-8809/ © 2019 Elsevier B.V. All rights reserved.
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and restored to natural or artificial vegetation due to the implement of the Grain-for-Green Project.
their utilization of water sources between different soil layer depths when coping with drought events (Dawson and Pate, 1996; Bargués Tobella et al., 2017). The processes by which plant species obtain water resources are critical for evaluating the adaptability of plants, which can affect the establishment and survival of restored ecosystems and their ecosystem functions. In the Loess Plateau due to the implementation of the Grain-forGreen Project that was implemented in 1999, dramatic vegetation changes have occurred (Chen et al., 2015; Fu et al., 2017). Some fast growth, short-lived species were introduced by practitioners and planted in high density for restoration purposes and short-term gains (Cao et al., 2009), which may have negatively affected ecosystem services and the sustainability of restored ecosystems. Several previous studies have revealed that inappropriately planted exotic plants consume excessive soil moisture and reduce deep soil water, causing plant dieback and death (Chen et al., 2015; Jia et al., 2017). Wang et al. (2011) reported that the soil moisture of native species was significantly higher than that of exotic species, and afforestation with the exotic plant, e.g., Robinia pseudoacacia, caused soil desiccation in the deep soil layer of the Loess Plateau (Jia et al., 2017). Exotic and native plants exhibit different physiological or phenotypic plasticity to drought stress and these responses are dependent on the species and environment (Arndt et al., 2015; Jian et al., 2015; Gao et al., 2018). Although soil moisture dynamics of vegetation restoration have been characterized previously, little is known about the seasonal variations in water use characteristics of exotic and native species in this region. Stable hydrogen and oxygen isotope techniques provide an effective and powerful method for identifying plant water use patterns (Rothfuss and Javaux, 2017). Isotopic discrimination does not occur during plant water absorption before transpiration for most terrestrial plants (Dawson et al., 2002) except for some mangrove and halophytic species (Lin and Sternber, 1993), as well as woody xerophytes (Ellsworth and Williams, 2007). Isotopic compositions of xylem water reflect the comprehensive isotopic signatures of different water sources utilized by plants. Therefore, it is feasible to use hydrogen and oxygen isotope techniques to trace plant water use patterns (Rothfuss and Javaux, 2017; Jespersen et al., 2018). In this study, we compared water use characteristics of Spiraea pubescens, a native shrub species, and Hippophae Rhamnoides, an exotic shrub species, in the Loess Plateau of China by using stable isotopes techniques (δ2H, δ18O, and δ13C). We investigated δ2H and δ18O of xylem and soil water within 300 cm of the surface soil in various layers to trace plant water sources, and measured δ13C in plant leaves to explore the interspecific differences in leaf-level WUEi. The root distributions of the plants and soil water content (SWC) were also investigated. The hypothesis is that the exotic and native plant had distinct water use characteristics. The main purposes of this study were: (i) to explore seasonal variations in water use characteristics for two species, and (ii) detect the differences in water use characteristics between the exotic and native plants.
2.2. Sampling design H. rhamnoides is a C3 deciduous shrub which was used for the revegetation and ecological restoration in the Loess Plateau, and it has become the primary shrub species in this region (Xiao et al., 2011; Zhang et al., 2015). S. pubescens is a C3 deciduous shrub with branches that spread from its base, and is a native species that is widely distributed in the hills and gullies of the Loess Plateau. Three plots (5 m × 5 m) dominated by H. rhamnoides and S. pubescens were selected during the study periods, respectively. The plots for each species had similar slope positions, aspects and slope gradients (˜13°). The mean stem diameter was 15.30 ± 4.24 mm for H. rhamnoides and 8.32 ± 2.53 mm for S. pubescens. Three individual plants were randomly selected as plant samples on each sampling date, once a month, from May to September 2016. At midday of each month, three plant xylem and leaf samples were collected for each species. A total of 30 xylem samples and 30 plant leaf samples were obtained. Suberized twigs from the different canopy directions were cut into 3–4 cm segments as a sample for each species, and the plant phloem tissue were removed to avoid contamination of xylem water by isotopic enriched water (Querejeta et al., 2007). These small plant segments were immediately placed into capped glass vials and sealed with polyethylene parafilm and kept frozen at −20 °C until water extraction. Three soil cores of 0–300 cm were obtained using a power auger around the sampling plants on the same day as xylem sampling for each species. Soil samples were collected monthly for each species at depths of 5, 10, 20, 40, 60, 80, 100, 120, 160, 200, 250, and 300 cm. In total, 360 soil samples were collected during the study period. One part of each soil sample was stored frozen at −20 °C for isotopic analysis, and the other part was used for determining SWC by drying the samples for 24 h at 105 ℃. A total of 46 rainwater samples were collected using a polyethylene bottle and funnel during the study periods. The root distributions of the two species were investigated by excavating soil layers at 10 cm intervals until no roots were found during the late growing seasons.
2.3. Isotopic analyses The cryogenic-vacuum distillation system was applied to extract soil and plant xylem water and the whole process lasted for 1.5–3 h based on the sample water content (West et al., 2006). The extracted proportion of water to samples was up to 98%. The δ2H and δ18O in xylem water and the δ13C in plant leaf were measured by an isotope ratio mass spectrometer (IRMS) (MAT253, Thermo Fisher Scientific, Bremen, Germany) with a measurement accuracy of ± 1‰ for δ2H, ± 0.2‰ for δ18O, and ± 0.15‰ for δ13C. Isotopic compositions of rainwater and soil water were measured by an isotopic ratio infrared spectroscopy (IRIS) system (DLT-100; Los Gatos Research, Mountain View, USA) and its analytical accuracy was ± 1.2‰ for δ2H and ± 0.3‰ for δ18O. The potential organic material in xylem water affected the measurement results with the IRIS method, but did not affect the analytical values with the IRMS method (West et al., 2010). No pronounced discrepancy was observed in soil water isotopic compositions analysed by both the IRMS and IRIS systems (Schultz et al., 2011; Wang et al., 2017). The isotopic compositions in water samples were expressed as follows:
2. Materials and methods 2.1. Study area The study sites are located in the Yangjuangou catchment in the central region of the Loess Plateau, China (36°42′45″ N, 109°31′45″). The landforms in this catchment are loess beam and loess gully with a total area of 2.02 km2. The average elevation is 1295 m with slope gradients ranging from 10° to 30°. The local climate is dominated by the semi-arid continental climate with a mean annual precipitation of 537 mm and a mean air temperature of 10 °C (Wang et al., 2017). Most of the precipitation falls between June and September, which is consistent with the growing seasons of plants. The soil type is loessial soil developed from loess, which is loose and vulnerable to corrosion. Most of the cultivated land on the steep slopes were gradually abandoned
δ X = Rsample / Rstandard -1 2
18
(1) 13
2
1
18
16
where X represents H, O, or C, and R is the H/ H, O/ O or 13 C/12C ratios. The Vienna Standard Mean Ocean Water (V-SMOW) is standard for 2H and 18O. The Vienna Pee Dee Belemnite (V-PDB) is standard for 13C. 56
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time. The SWC in 0–120 cm soil layers presented higher variation compared to deep soil layers (Fig. 1). Deep SWC had low variation through the seasons and had higher stability than shallow soil water. The average SWC of shallow, intermediate and deep soil layers in the H. rhamnoides plots were lower than the S. pubescens plots. The SWC of both shallow and deep soil layers between H. rhamnoides and S. pubescens plots exhibited a significant difference (p < 0.01). In contrast, the SWC of the intermediate soil layers from two species’ plots had no distinct difference. The SWC of H. rhamnoides and S. pubescens plots were significantly different during the sampling period (p < 0.001).
2.4. Water source partitioning The soil water in different soil layers was regarded as water sources pools utilized by the plants because the table depth of groundwater is buried deep (40–100 m) below the soil surface (Huang et al., 2013), and plant roots had difficulty accessing the groundwater. Additionally, there was no irrigation during the study period. We assumed that soil water isotopes were laterally homogeneous within the rooting area, and the time delays between the sampling and water transport time were not significant which makes stable isotopic analysis results reasonable.The Bayesian mixing model MixSIAR (version 3.1.7) was applied to identify the proportional contributions of each water source (Stock and Semmens, 2013). The raw isotopic ratios (δ2H and δ18O) in xylem water were input as the mixture data. The means and standard deviations of the isotopic values (δ2H and δ18O) from different soil layers were input as source data. The discrimination data were set to zero for both δ2H and δ18O. More details about the model settings have been documented by Wang et al. (2017). According to the temporal variability in the SWC, the isotopic compositions of soil water along the soil profile, and the impact of rainfall pulse, water sources were combined into three source endpoints: 0–40 cm for shallow soil layer, 40–120 cm for intermediate soil layer, and 120–300 cm for deep soil layer to facilitate subsequent analyses and comparisons (Eggemeyer et al., 2009; Wang et al., 2017).
3.2. Isotopic compositions in soil and xylem water The δ18O of soil water ranged from −3.37 to −11.97‰ (mean ± SD, −9.25 ± 1.73‰) for H. rhamnoides and ranged from −3.90 to −12.30‰ (mean ± 1SD, −9.33 ± 1.93‰) for S. pubescens. The mean δ2H of soil water in H. rhamnoides and S. pubescens plots were −68.0 ± 11.7‰ and −66.7 ± 14.0‰, respectively. The isotopic compositions of surface soil water were enriched compared with that of the deeper soil water except in July (Figs. 2, 3 ). There was no pronounced difference in soil water isotopic signatures among different seasons, but there were significant differences among soil depths (p < 0.001). The isotopic compositions of soil water from the H. rhamnoides and S. pubescens plots were not pronounced differences. The δ18O of shallow, intermediate, and deep soil water in two species’ plots had no significant differences. However, the δ2H of the soil water from intermediate soil layers in two species plots had significant differences (p < 0.001). The average isotopic compositions in xylem water from H. rhamnoides and S. pubescens plots were −7.99‰ and −7.56‰ for δ18O, respectively, and −62.2‰ and −60.0‰ for δ2H, respectively. Both δ2H and δ18O in the xylem water of different sampling dates exhibited significant variation (p < 0.05). However, the xylem water isotopic compositions of two studied species had no pronounced difference. The isotopic signatures in xylem water were within the range of isotopic ratios in soil water from the 0–120 cm soil layers (Fig. 4). The soil water evaporation line (SWL) was fitted based on the soil water isotopic values. The slope of SWL from the S. pubescens plots was higher than that of the H. rhamnoides plots (Fig. 4).
2.5. Leaf-level intrinsic WUEi The plant leaf δ13C values provided an estimate of leaf-level WUEi over a long integration time rather than an instantaneous WUE because the carbon of plant tissue accumulated over a period of time. The δ13C values of the plant leaves were related to intercellular CO2 concentrations (Ci) and decreased with the increase in Ci (Farquhar et al., 1989). The δ13C values in plant leaves had a strong positive relationship with leaf-level WUEi (Farquhar and Richards, 1984; Seibt et al., 2008), especially for C3 photosynthesis plants. In addition, the δ13C in atmospheric CO2 was almost consistent (-8‰) within a given year for the plants growing in the field (Michener and Lajtha, 2007). Sampling plots did not have a closed canopy, and the gradients in the atmospheric CO2 concentrations under the canopy were small and could be ignored. Thus, the δ13C values in the plant leaves were used to compare leaflevel WUEi over the long term for C3 plants. 2.6. Data analyses
3.3. Water use patterns across the growing season
The isotopic signatures in plant tissues and soil water met a normal distribution by the Kolmogorov–Smirnov (K–S) test. The difference of isotopic signatures and SWC among seasons and plant species were identified by two-way analysis of variance (ANOVA). Significant differences of the SWC and isotopic compositions of soil water from different soil layers per species was tested using one-way ANOVA combined with a post-hoc Tukey's least significant difference (LSD) test. A one-way ANOVA was applied to detect the differences in plant water source from shallow, intermediate and deep soil layers per species. The significance level of the statistical analyses was set at 0.05. These statistical analyses were performed using SPSS 20.0 (SPSS Inc., Chicago, IL, USA).
H. rhamnoides absorbed the largest proportion water (62.3 ± 13.2%) from the shallow layer in June. The proportional contributions of deep soil water absorbed by H. rhamnoides increased from 14.60% in June to 35.80% in September. The contribution fractions of soil water for H. rhamnoides from shallow, intermediate, and deep soil layers were significantly different (p < 0.01). S. pubescens absorbed most of its water (78.28%) from the shallow and intermediate soil layers during the sampling period (Fig. 5). S. pubescens derived the largest proportion (33.1 ± 7.03%) of water from the deep soil layers in August. The intermediate soil layer contributed the most water in July (33.90 ± 7.80%) and the shallow soil layers contributed the largest proportion (67.2 ± 14.5%) in June. The percentages of water sources used by S. pubescens from shallow, intermediate, and deep soil layers showed significant differences (p < 0.01). There were no significant differences in water absorption fractions from the different soil layers between H. rhamnoides and S. pubescens. However, the proportional contributions of shallow, intermediate, and deep soil water for two studied species had significant differences over time (p < 0.05). The water absorption fractions obtained from the shallow soil layers exhibited greater fluctuations between species than the other soil layers over time.
3. Results 3.1. Temporal and vertical variations of SWC The SWC in the two plant species plots varied with soil depths and seasons (Fig. 1). The SWC in H. rhamnoides plots displayed significant differences among depths (p < 0.01) and sampling dates (p < 0.01). The SWC in the S. pubescens plots showed a clear pattern along the vertical soil profile (p < 0.01), but remained relatively stable across 57
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Fig. 1. Vertical distribution of gravimetric soil water content (SWC, %) from (a) H. rhamnoides and (b) S. pubescens plots. Data are expressed as the means ± 1SD.
Fig. 2. Seasonal variations of δ2H in soil water and its vertical distribution within 0–300 cm soil layers. The vertical dash represents the δ2H of xylem water from H. rhamnoides and S. pubescens. Data are expressed as the means ± 1SD.
largest value in May (-28.05‰). The leaf δ13C value of S. pubescens decreased from May to August, but increased in September. The δ13C values of H. rhamnoides and S. pubescens were not significantly differences. However, the δ13C values of both H. rhamnoides and S. pubescens exhibited significant differences (p < 0.001) during the sampling period.
3.4. δ13C values in plant leaves The mean leaf δ13C value of H. rhamnoides was -28.49 ± 0.45‰, ranging from -28.03 to -29.28‰. H. rhamnoides had the smallest leaf δ13C value (-29.31‰) in August (Fig. 6). The mean leaf δ13C value of S. pubescens was -28.77 ± 0.46‰, ranging from -28.05 to -29.31‰. S. pubescens had the smallest leaf δ13C value in August (-29.31‰) and the 58
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Fig. 3. Seasonal variations of δ18O in soil water and its vertical distribution within 0–300 cm soil layers. The vertical dash represents the δ18O of xylem water from H. rhamnoides and S. pubescens. Data are expressed as the means ± 1SD.
4. Discussion 4.1. Difference in soil water isotopic signatures and SWC Isotopic compositions of soil water synthesized the processes associated with precipitation recharge, mixing with antecedent moisture, and evapotranspiration (Hsieh et al., 1998; Brooks et al., 2015). The isotopic signatures in surface soil water were highly variable and were depleted as soil depth increased (Figs. 2, 3). This result is likely attributed to intense evaporation causing isotopic enrichment (Gazis and Feng, 2004), as well as different precipitation events mixing with antecedent soil water isotopes (Tang and Feng, 2001). However, both δ2H and δ18O in the 60–300 cm soil water exhibited less variability and gradually stabilized from May to September (Figs. 2, 3). This pattern was consistent for exotic and native plants. Evaporation in shallow soil layers caused isotopic enrichment, and seasonal input of precipitation caused isotopic depletion that led to vertical gradients in soil water isotopes along the soil profiles. In this study, soil water isotopic compositions from exotic and native plant plots had no significant differences across months, indicating that soil water isotopic signatures had temporal stability. Moreover, no significant differences were found in the δ2H and δ18O of soil water between the two species’ plots. This is likely because the two plant species had the same micro-environment, which had similar evaporation, and no isotopic discrimination of water isotopes occurred during water absorption by the roots (Ehleringer and Dawson, 1992). A significant dissimilarity was also found in the SWC between S. pubescens and H. rhamnoides, especially in shallow and deep soil layers. Previous studies have found that exotic vegetation affected soil moisture deficit below the near-surface layers, and the SWC of exotic plants from intermediate and deep soil layers have been observed to have temporal stability in the semi-arid Loess Plateau (Yang et al., 2014). In this study, the SWC from deep soil layers in H. rhamnoides exhibited lower temporal stability, which was a lower degree of variation over time than that of S. pubescens (Fig. 1). The variations in SWC
Fig. 4. Values of δ2H as a function of δ18O from (a) H. rhamnoides and (b) S. pubescens during the sampling period. LMWL is the local meteoric line (y = 7.76 x + 5.14, R2 = 0.91, p < 0.01) based on the isotopic values of the precipitation (Wang et al., 2017). SWL is soil water evaporation line which is fitted based on the isotopic values of soil water. The isotopic compositions of the xylem water and soil water from different soil layers are displayed in the figure.
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Fig. 5. Seasonal variations in water uptake proportions for (a) H. rhamnoides and (b) S. pubescens from different soil layers. Error bar expressed as standard deviation.
ecological plasticity in water use. This result is consistent with the findings of a previous study in an alpine desert ecosystem in which H. rhamnoides switched its water sources from the 0–30 cm to 60–120 cm soil stratum, then switched back to a considerable proportion of its water sources from the 0–30 cm soil stratum during the growing seasons (Wu et al., 2016). In this study, H. rhamnoides absorbed the largest proportion (35.8%) of deeper soil moisture when surface soil moisture decreased in September (Fig. 1). In a recent study, similar findings were observed in the shrub, Vitex negundo which displayed strong ecological plasticity and absorbed a higher proportion of deep soil water as water stress increased (Wang et al., 2017). Compared to H. rhamnoides, the native plant S. pubescens mainly derived water from the 0–120 cm soil layer, and the relative proportions of the 0–120 cm soil water showed no significant difference among the months. Similarity, Yang et al. (2016) found that S. pubescens mainly relied on water from the 20–40 cm soil layers during the growing seasons in a semi-arid ecosystem of the Loess Plateau. Plant leaf δ13C is a validated indicator of long term leaf-level WUEi of plants. In arid and semi-arid regions, plants maintain a high WUE to reduce the impacts of water deficits and enhance their adaptability under drought conditions (Ogaya and Peñuelas, 2003). Since δ13C in plant leaves is positively correlated with leaf-level WUEi for C3 photosynthesis plants (Farquhar et al., 1989), we can use plant leaf δ13C in plant leaves to indirectly reflect leaf-level WUEi during a period of time. The δ13C assays indicated that H. rhamnoides had higher leaf-level WUEi than S. pubescens, suggesting that H. rhamnoides typically decreases transpiration, which induces water loss. A high WUE ensured normal physiological activity and growth of the plant (Farquhar et al., 1989). Some previous studies have shown that plants with low WUE had the advantage of water competition in seasons with sufficient water (Lucero et al., 2000; Robinson et al., 2001). In this study, the study period covered the entire rainy season from June to September. Thus, it is clear that S. pubescens utilized more surface water due to its lower leaf-level WUEi. No significant differences were found in the isotopic signatures of xylem water between H. rhamnoides and S. pubescens. This suggests that the two species had similar water sources. Nevertheless, the water use fractions of the two species had significant seasonal changes. The
Fig. 6. Temporal variation of leaf δ13C from H. rhamnoides and S. pubescens. Data are expressed as means ± 1SD. The different lowercase letters express pronounced difference of plant leaf δ13C among months at the level p < 0.05.
from different soil layers may be caused by the effect of rainfall pulse, vegetation transpiration, and soil evaporation (Seneviratne et al., 2010). Some previous studies have also found that SWC in deep soil layers displayed lower temporal variability than shallow soil layers in some semi-arid regions (Penna et al., 2013; Yang et al., 2014). 4.2. Differences in water use characteristics The isotopic compositions in xylem water from the two species were within that of water from the 0–120 cm soil layers (Fig. 4), implying that H. rhamnoides and S. pubescens mainly absorbed water sources from 0 to 120 cm soil. The contribution fractions were ˜80% as predicted by the MixSIAR model during the sampling period (Fig. 5). Previous studies have noted that non-phreatophyte shrubs mainly used upper soil water derived from precipitation for survival in the Central Asia desert (Xu and Li, 2006). However, H. rhamnoides progressively increased its contribution proportion of deep soil water as the growing season progressed (Fig. 5), implying that H. rhamnoides had a higher degree of 60
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Fig. 7. Vertical profiles of fine root density (cm3/m3) for H. rhamnoides and S. pubescens.
shallow water sources. H. rhamnoides derived more deep soil water when shallow soil water was limited. Moreover, H. rhamnoides had higher leaf-level WUEi than S. pubescens. These findings suggest that H. rhamnoides had a greater degree of ecological plasticity in water use. Previous studies have demonstrated that H. rhamnoides was suitable for afforestation in the semi-arid Loess Plateau with high levels of rainfall (> 70 mm) (Jian et al., 2015). However, it should be noted that H. rhamnoides may cause soil desiccation due to higher uptake of deep soil moisture. Appropriate management measures, such as reasonable planting density and prescribed disturbance, should be implemented to maintain ecosystem sustainability. Plant water use characteristics provide important information for screening exotic species for revegetation and ecological management. Plants with flexible water use patterns can change their water sources between shallow and stable water sources (deep soil water or groundwater) depending on soil water availability. In addition, some plants that continuously absorb shallow and deep soil water or groundwater may not be preferred because long-term continuous utilization of deep soil water or groundwater may cause soil desiccation and the degradation of ecosystem services (Chen et al., 2008). Moreover, plants that continuously derive water from shallow soil layers may have difficulty surviving in extreme drought conditions. The combination of using plant species with different water use characteristics may form a good community configuration that complements the inadequate use of water resources by each species.
difference in water use patterns among the species was closely linked with fine root (< 2 mm) distribution (Dawson et al., 2002). Fine roots are an important active physiological component in water uptake (Schenk, 2008). Field investigation found a fine root density in the 0–120 cm soil layers from the S. pubescens plots accounted for 88.49% of the total fine root density, which allowed S. pubescens to derive its water sources primarily from the 0–120 cm soil layers (Fig. 7). However, the fine root density of H. rhamnoides was distributed in deeper layers, within a range of 250 cm. The fine root density in the 0–200 cm layer of the soil of H. rhamnoides plots accounted for 89.12% of the total fine root density. Therefore, H. rhamnoides could obtain higher percentages of deep soil moisture compared to S. pubescens. Many perennial plants in arid and semi-arid ecosystems have been observed to have dimorphic root systems (Dawson and Pate, 1996; Wang et al., 2017). The lateral root zone potentially obtains surface water during the wet season, and the penetrating taproots zone derives deep soil water when shallow water sources are insufficient (Dawson and Pate, 1996; McCole and Stern, 2007). H. rhamnoides could flexibly adjust water sources between different soil stratums due to its dimorphic root system and these field investigation suggest that H. rhamnoides had dimorphic root systems with lateral roots distributed in the shallow soil and a deep tap root extension into the deep soil (Fig. 7). A previous study also showed that H. rhamnoides displayed flexible water use patterns and switched water sources among different soil layers following a decrease in precipitation (Tang et al., 2018). Additionally, plants adjusted their water use characteristics by changing photosynthesis, gas exchange, hydraulic and morphology at the leaf level during water source deficits (Lazaro-Nogal et al., 2013).
5. Conclusion Seasonal water use characteristics of the exotic H. rhamnoides and native plant S. pubescens in the Loess Plateau were investigated using δ2H, δ18O, and δ13C combined with the MixSIAR model. The MixSIAR model predicted that H. rhamnoides and S. pubescens obtained 78.80% and 78.28% of their water from the 0–120 cm soil layer during the growing season, respectively. However, H. rhamnoides progressively utilized more fractions of deeper soil water as the season progressed and could flexibly convert its water sources between shallow and deep soil layers due to its dimorphic root system. Additionally, the leaf δ13C implied that H. rhamnoides had higher leaf-level WUEi than that of S. pubescens during the growing season. These results suggest that H. rhamnoides has a greater degree of ecological plasticity in water use. However, H. rhamnoides may cause negative effects due to its capability to derive more water from deep soil layers. This study provides valuable
4.3. Implications for ecological restoration S. pubescens utilized the most fractions of shallow soil water and was more sensitive to recent precipitation during the growing season. However, H. rhamnoides displayed flexible water use patterns which shifted its water sources among different soil stratum as the growing season progressed (Fig. 5). Deciduous plant species in seasonal arid ecosystem respond to drought stress with different strategies. Shifting water sources among different soil layers, depending on water availability, is an important strategy for deciduous plants to cope with drought in water limited ecosystem (Hasselquist et al., 2010). Plant species that can shift water sources can maximize their soil water use and avoid interspecific competition with plants that consistently utilize 61
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information for screening exotic species for revegetation and ecological management, and forming more effective restoration strategies for revegetation in semiarid ecosystems.
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Acknowledgements This work was supported by National Natural Science Foundation of China (NO. 41390462) and the National Key Research and Development Program of China (NO. 2016YFC0501602). We are grateful to Jianye Li, Weiwei Fang, Weiliang Chen, and Jianbo, Liu for the assistance in the field experiments. References Antunes, C., Díaz-Barradas, M.C., Zunzunegui, M., Vieira, S., Máguas, C., Paruelo, J., 2018. Water source partitioning among plant functional types in a semi-arid dune ecosystem. J. Veg. Sci. 29, 671–683. Arndt, S.K., Sanders, G.J., Bristow, M., Hutley, L.B., Beringer, J., Livesley, S.J., 2015. Vulnerability of native savanna trees and exotic Khaya senegalensis to seasonal drought. Tree Physiol. 35, 783–791. Bargués Tobella, A., Hasselquist, N.J., Bazié, H.R., Nyberg, G., Laudon, H., Bayala, J., Ilstedt, U., 2017. Strategies trees use to overcome seasonal water limitation in an agroforestry system in semiarid West Africa. Ecohydrology 10. Brooks, P.D., Chorover, J., Fan, Y., Godsey, S.E., Maxwell, R.M., McNamara, J.P., Tague, C., 2015. Hydrological partitioning in the critical zone: Recent advances and opportunities for developing transferable understanding of water cycle dynamics. Water Resour. Res. 51, 6973–6987. Cao, S., Chen, L., Yu, X., 2009. Impact of China’s Grain for Green Project on the landscape of vulnerable arid and semi-arid agricultural regions: a case study in northern Shaanxi Province. J. Appl. Ecol. 46, 536–543. Chen, H., Shao, M., Li, Y., 2008. Soil desiccation in the Loess Plateau of China. Geoderma 143, 91–100. Chen, Y., Wang, K., Lin, Y., Shi, W., Song, Y., He, X., 2015. Balancing green and grain trade. Nat. Geosci. 8, 739–741. Dawson, T.E., Pate, J.S., 1996. Seasonal water uptake and movement in root systems of Australian phraeatophytic plants of dimorphic root morphology: a stable isotope investigation. Oecologia 107, 13–20. Dawson, T.E., Mambelli, S., Plamboeck, A.H., Templer, P.H., Tu, K.P., 2002. Stable Isotopes in Plant Ecology. Annual review of ecology and systematics, pp. 507–559. Eggemeyer, K.D., Awada, T., Harvey, F.E., Wedin, D.A., Zhou, X., Zanner, C.W., 2009. Seasonal changes in depth of water uptake for encroaching trees Juniperus virginiana and Pinus ponderosa and two dominant C4 grasses in a semiarid grassland. Tree Physiol. 29, 157–169. Ehleringer, J., Dawson, T., 1992. Water uptake by plants: perspectives from stable isotope composition. Plant Cell Environ. 15, 1073–1082. Ellsworth, P.Z., Williams, D.G., 2007. Hydrogen isotope fractionation during water uptake by woody xerophytes. Plant Soil 291, 93–107. Farquhar, G., Richards, R., 1984. Isotopic composition of plant carbon correlates with water-use efficiency of wheat genotypes. Funct. Plant Biol. 11, 539–552. Farquhar, G.D., Ehleringer, J.R., Hubick, K.T., 1989. Carbon isotope discrimination and photosynthesis. Annu. Rev. Plant Biol. 40, 503–537. Fort, F., Volaire, F., Guilioni, L., Barkaoui, K., Navas, M.-L., Roumet, C., Watling, J., 2017. Root traits are related to plant water-use among rangeland Mediterranean species. Funct. Ecol. 31, 1700–1709. Fu, B., Wang, S., Liu, Y., Liu, J., Liang, W., Miao, C., 2017. Hydrogeomorphic ecosystem responses to natural and anthropogenic changes in the Loess Plateau of China. Annu. Rev. Earth Planet. Sci. 45, 223–243. Gao, Y., Markkanen, T., Aurela, M., Mammarella, I., Thum, T., Tsuruta, A., Yang, H., Aalto, T., 2017. Response of water use efficiency to summer drought in a boreal Scots pine forest in Finland. Biogeosciences 14, 4409–4422. Gao, X., Zhao, X., Li, H., Guo, L., Lv, T., Wu, P., 2018. Exotic shrub species (Caragana korshinskii) is more resistant to extreme natural drought than native species (Artemisia gmelinii) in a semiarid revegetated ecosystem. Agric. For. Meteorol. 263, 207–216. Gazis, C., Feng, X., 2004. A stable isotope study of soil water: evidence for mixing and preferential flow paths. Geoderma 119, 97–111. Grossiord, C., Sevanto, S., Bonal, D., Borrego, I., Dawson, T.E., Ryan, M., Wang, W., McDowell, N.G., 2018. Prolonged warming and drought modify belowground interactions for water among coexisting plants. Tree Physiol tpy080-tpy080. Hasselquist, N.J., Allen, M.F., Santiago, L.S., 2010. Water relations of evergreen and drought-deciduous trees along a seasonally dry tropical forest chronosequence. Oecologia 164, 881–890. Hsieh, J.C., Chadwick, O.A., Kelly, E.F., Savin, S.M., 1998. Oxygen isotopic composition of soil water: quantifying evaporation and transpiration. Geoderma 82, 269–293. Huang, T., Pang, Z., Edmunds, W.M., 2013. Soil profile evolution following land-use change: implications for groundwater quantity and quality. Hydrol. Process. 27, 1238–1252. Jespersen, R.G., Leffler, A.J., Oberbauer, S.F., Welker, J.M., 2018. Arctic plant ecophysiology and water source utilization in response to altered snow: isotopic (delta(18)O and delta(2)H) evidence for meltwater subsidies to deciduous shrubs. Oecologia 187, 1009.
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Water use characteristics of native and exotic shrub species in the semi-arid Loess Plateau using an isotope technique
T
⁎
Jian Wanga,b, Bojie Fua,b, Nan Lua,b, , Shuai Wangc, Li Zhangd a
State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China University of Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China d State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Stable isotope Water use Native species Loess Plateau MixSIAR Root distribution
Knowledge of the processes underlying plant water use characteristics is critical for understanding soil–plant interactions and evaluating plant adaptability in water-limited ecosystems. The vegetation on the Loess Plateau has -dramatically changed due to the implementation of the Grain-for-Green Project from 1999. Despite this, water use characteristics of native and exotic shrub species remain poorly understood in this region. In this study, seasonal variations of water use characteristics of Spiraea pubescens (a native shrub) and Hippophae rhamnoides (an exotic shrub) in the Loess Plateau were investigated by examining the δ2H and δ18O of xylem and soil water from different soil layers within 300 cm of the surface, as well as the δ13C in plant leaves. Results revealed that H. rhamnoides and S. pubescens derived ˜80% of their water sources from the 0–120 cm soil layer during the growing seasons. However, H. rhamnoides absorbed higher proportions of deep soil water (120–300 cm) as the growing season progressed. H. rhamnoides flexibly converted its water source between shallow (0–40 cm) and deep soil layers. Moreover, H. rhamnoides had higher leaf-level water use efficiency than that of S. pubescens. These results suggest that H. rhamnoides has a greater degree of ecological plasticity in water use. Flexible water use characteristics are relevant to functionally dimorphic root systems as an adaptation strategy for the plants in water-limited environments. These findings indicate that water use characteristics of these plants should be considered when exotic species are introduced for revegetation in semiarid regions.
1. Introduction Vegetation restoration plays a crucial role in preventing desertification (Ma et al., 2017), controlling soil erosion (Zhu et al., 2015), and regulating hydrological regimes in semiarid regions (Yaseef et al., 2010). Revegetation using exotic and native species is encouraged worldwide due to the aforementioned advantages. However, inappropriate vegetation restoration strategies may lead to negative ecological consequences, such as soil desiccation (Chen et al., 2008; Wang et al., 2011), land degradation (Stokes et al., 2010), and loss of ecosystem functioning (e.g., soil conservation and water conservation) (Fu et al., 2017). Water use characteristics of plants used for revegetation provide critical insight into soil-plant interactions and adequate assessments of plant adaption in water-limited ecosystems (Grossiord et al., 2018). Water is the primary limiting factor of vegetation development in
arid and semi-arid regions (Porporato et al., 2004), and the temporal and spatial variations of plant water sources are called water use patterns (Wang et al., 2017). Water use efficiency (WUE) is another key indicator of plant water use characteristics and reflects the trade-off between plant productivity, water availability, and the ability of plants to adapt to and tolerate droughts (Saugier et al., 2012; Gao et al., 2017). The δ13C in plant leaves is an indicator of leaf-level intrinsic water use efficiency (WUEi) (Farquhar et al., 1989; Saugier et al., 2012). Previous studies have shown that δ13C in plant leaf has a positive relationship with WUEi (Farquhar et al., 1989; Saugier et al., 2012). As for the C3 photosynthesis plants, plants with higherplant leaf δ13C had higher leaflevel of WUEi (Farquhar et al., 1989). Moreover, water use characteristics vary among plant growth forms and species (Voltas et al., 2015; Antunes et al., 2018). The capacity to adjust water use characteristics is related to active root depth and distribution (Fort et al., 2017). Plants with functional dimorphic root systems have been observed to switch
⁎ Corresponding author at: State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. E-mail address: [email protected] (N. Lu).
https://doi.org/10.1016/j.agee.2019.02.015 Received 19 December 2018; Received in revised form 20 February 2019; Accepted 21 February 2019 0167-8809/ © 2019 Elsevier B.V. All rights reserved.
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and restored to natural or artificial vegetation due to the implement of the Grain-for-Green Project.
their utilization of water sources between different soil layer depths when coping with drought events (Dawson and Pate, 1996; Bargués Tobella et al., 2017). The processes by which plant species obtain water resources are critical for evaluating the adaptability of plants, which can affect the establishment and survival of restored ecosystems and their ecosystem functions. In the Loess Plateau due to the implementation of the Grain-forGreen Project that was implemented in 1999, dramatic vegetation changes have occurred (Chen et al., 2015; Fu et al., 2017). Some fast growth, short-lived species were introduced by practitioners and planted in high density for restoration purposes and short-term gains (Cao et al., 2009), which may have negatively affected ecosystem services and the sustainability of restored ecosystems. Several previous studies have revealed that inappropriately planted exotic plants consume excessive soil moisture and reduce deep soil water, causing plant dieback and death (Chen et al., 2015; Jia et al., 2017). Wang et al. (2011) reported that the soil moisture of native species was significantly higher than that of exotic species, and afforestation with the exotic plant, e.g., Robinia pseudoacacia, caused soil desiccation in the deep soil layer of the Loess Plateau (Jia et al., 2017). Exotic and native plants exhibit different physiological or phenotypic plasticity to drought stress and these responses are dependent on the species and environment (Arndt et al., 2015; Jian et al., 2015; Gao et al., 2018). Although soil moisture dynamics of vegetation restoration have been characterized previously, little is known about the seasonal variations in water use characteristics of exotic and native species in this region. Stable hydrogen and oxygen isotope techniques provide an effective and powerful method for identifying plant water use patterns (Rothfuss and Javaux, 2017). Isotopic discrimination does not occur during plant water absorption before transpiration for most terrestrial plants (Dawson et al., 2002) except for some mangrove and halophytic species (Lin and Sternber, 1993), as well as woody xerophytes (Ellsworth and Williams, 2007). Isotopic compositions of xylem water reflect the comprehensive isotopic signatures of different water sources utilized by plants. Therefore, it is feasible to use hydrogen and oxygen isotope techniques to trace plant water use patterns (Rothfuss and Javaux, 2017; Jespersen et al., 2018). In this study, we compared water use characteristics of Spiraea pubescens, a native shrub species, and Hippophae Rhamnoides, an exotic shrub species, in the Loess Plateau of China by using stable isotopes techniques (δ2H, δ18O, and δ13C). We investigated δ2H and δ18O of xylem and soil water within 300 cm of the surface soil in various layers to trace plant water sources, and measured δ13C in plant leaves to explore the interspecific differences in leaf-level WUEi. The root distributions of the plants and soil water content (SWC) were also investigated. The hypothesis is that the exotic and native plant had distinct water use characteristics. The main purposes of this study were: (i) to explore seasonal variations in water use characteristics for two species, and (ii) detect the differences in water use characteristics between the exotic and native plants.
2.2. Sampling design H. rhamnoides is a C3 deciduous shrub which was used for the revegetation and ecological restoration in the Loess Plateau, and it has become the primary shrub species in this region (Xiao et al., 2011; Zhang et al., 2015). S. pubescens is a C3 deciduous shrub with branches that spread from its base, and is a native species that is widely distributed in the hills and gullies of the Loess Plateau. Three plots (5 m × 5 m) dominated by H. rhamnoides and S. pubescens were selected during the study periods, respectively. The plots for each species had similar slope positions, aspects and slope gradients (˜13°). The mean stem diameter was 15.30 ± 4.24 mm for H. rhamnoides and 8.32 ± 2.53 mm for S. pubescens. Three individual plants were randomly selected as plant samples on each sampling date, once a month, from May to September 2016. At midday of each month, three plant xylem and leaf samples were collected for each species. A total of 30 xylem samples and 30 plant leaf samples were obtained. Suberized twigs from the different canopy directions were cut into 3–4 cm segments as a sample for each species, and the plant phloem tissue were removed to avoid contamination of xylem water by isotopic enriched water (Querejeta et al., 2007). These small plant segments were immediately placed into capped glass vials and sealed with polyethylene parafilm and kept frozen at −20 °C until water extraction. Three soil cores of 0–300 cm were obtained using a power auger around the sampling plants on the same day as xylem sampling for each species. Soil samples were collected monthly for each species at depths of 5, 10, 20, 40, 60, 80, 100, 120, 160, 200, 250, and 300 cm. In total, 360 soil samples were collected during the study period. One part of each soil sample was stored frozen at −20 °C for isotopic analysis, and the other part was used for determining SWC by drying the samples for 24 h at 105 ℃. A total of 46 rainwater samples were collected using a polyethylene bottle and funnel during the study periods. The root distributions of the two species were investigated by excavating soil layers at 10 cm intervals until no roots were found during the late growing seasons.
2.3. Isotopic analyses The cryogenic-vacuum distillation system was applied to extract soil and plant xylem water and the whole process lasted for 1.5–3 h based on the sample water content (West et al., 2006). The extracted proportion of water to samples was up to 98%. The δ2H and δ18O in xylem water and the δ13C in plant leaf were measured by an isotope ratio mass spectrometer (IRMS) (MAT253, Thermo Fisher Scientific, Bremen, Germany) with a measurement accuracy of ± 1‰ for δ2H, ± 0.2‰ for δ18O, and ± 0.15‰ for δ13C. Isotopic compositions of rainwater and soil water were measured by an isotopic ratio infrared spectroscopy (IRIS) system (DLT-100; Los Gatos Research, Mountain View, USA) and its analytical accuracy was ± 1.2‰ for δ2H and ± 0.3‰ for δ18O. The potential organic material in xylem water affected the measurement results with the IRIS method, but did not affect the analytical values with the IRMS method (West et al., 2010). No pronounced discrepancy was observed in soil water isotopic compositions analysed by both the IRMS and IRIS systems (Schultz et al., 2011; Wang et al., 2017). The isotopic compositions in water samples were expressed as follows:
2. Materials and methods 2.1. Study area The study sites are located in the Yangjuangou catchment in the central region of the Loess Plateau, China (36°42′45″ N, 109°31′45″). The landforms in this catchment are loess beam and loess gully with a total area of 2.02 km2. The average elevation is 1295 m with slope gradients ranging from 10° to 30°. The local climate is dominated by the semi-arid continental climate with a mean annual precipitation of 537 mm and a mean air temperature of 10 °C (Wang et al., 2017). Most of the precipitation falls between June and September, which is consistent with the growing seasons of plants. The soil type is loessial soil developed from loess, which is loose and vulnerable to corrosion. Most of the cultivated land on the steep slopes were gradually abandoned
δ X = Rsample / Rstandard -1 2
18
(1) 13
2
1
18
16
where X represents H, O, or C, and R is the H/ H, O/ O or 13 C/12C ratios. The Vienna Standard Mean Ocean Water (V-SMOW) is standard for 2H and 18O. The Vienna Pee Dee Belemnite (V-PDB) is standard for 13C. 56
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time. The SWC in 0–120 cm soil layers presented higher variation compared to deep soil layers (Fig. 1). Deep SWC had low variation through the seasons and had higher stability than shallow soil water. The average SWC of shallow, intermediate and deep soil layers in the H. rhamnoides plots were lower than the S. pubescens plots. The SWC of both shallow and deep soil layers between H. rhamnoides and S. pubescens plots exhibited a significant difference (p < 0.01). In contrast, the SWC of the intermediate soil layers from two species’ plots had no distinct difference. The SWC of H. rhamnoides and S. pubescens plots were significantly different during the sampling period (p < 0.001).
2.4. Water source partitioning The soil water in different soil layers was regarded as water sources pools utilized by the plants because the table depth of groundwater is buried deep (40–100 m) below the soil surface (Huang et al., 2013), and plant roots had difficulty accessing the groundwater. Additionally, there was no irrigation during the study period. We assumed that soil water isotopes were laterally homogeneous within the rooting area, and the time delays between the sampling and water transport time were not significant which makes stable isotopic analysis results reasonable.The Bayesian mixing model MixSIAR (version 3.1.7) was applied to identify the proportional contributions of each water source (Stock and Semmens, 2013). The raw isotopic ratios (δ2H and δ18O) in xylem water were input as the mixture data. The means and standard deviations of the isotopic values (δ2H and δ18O) from different soil layers were input as source data. The discrimination data were set to zero for both δ2H and δ18O. More details about the model settings have been documented by Wang et al. (2017). According to the temporal variability in the SWC, the isotopic compositions of soil water along the soil profile, and the impact of rainfall pulse, water sources were combined into three source endpoints: 0–40 cm for shallow soil layer, 40–120 cm for intermediate soil layer, and 120–300 cm for deep soil layer to facilitate subsequent analyses and comparisons (Eggemeyer et al., 2009; Wang et al., 2017).
3.2. Isotopic compositions in soil and xylem water The δ18O of soil water ranged from −3.37 to −11.97‰ (mean ± SD, −9.25 ± 1.73‰) for H. rhamnoides and ranged from −3.90 to −12.30‰ (mean ± 1SD, −9.33 ± 1.93‰) for S. pubescens. The mean δ2H of soil water in H. rhamnoides and S. pubescens plots were −68.0 ± 11.7‰ and −66.7 ± 14.0‰, respectively. The isotopic compositions of surface soil water were enriched compared with that of the deeper soil water except in July (Figs. 2, 3 ). There was no pronounced difference in soil water isotopic signatures among different seasons, but there were significant differences among soil depths (p < 0.001). The isotopic compositions of soil water from the H. rhamnoides and S. pubescens plots were not pronounced differences. The δ18O of shallow, intermediate, and deep soil water in two species’ plots had no significant differences. However, the δ2H of the soil water from intermediate soil layers in two species plots had significant differences (p < 0.001). The average isotopic compositions in xylem water from H. rhamnoides and S. pubescens plots were −7.99‰ and −7.56‰ for δ18O, respectively, and −62.2‰ and −60.0‰ for δ2H, respectively. Both δ2H and δ18O in the xylem water of different sampling dates exhibited significant variation (p < 0.05). However, the xylem water isotopic compositions of two studied species had no pronounced difference. The isotopic signatures in xylem water were within the range of isotopic ratios in soil water from the 0–120 cm soil layers (Fig. 4). The soil water evaporation line (SWL) was fitted based on the soil water isotopic values. The slope of SWL from the S. pubescens plots was higher than that of the H. rhamnoides plots (Fig. 4).
2.5. Leaf-level intrinsic WUEi The plant leaf δ13C values provided an estimate of leaf-level WUEi over a long integration time rather than an instantaneous WUE because the carbon of plant tissue accumulated over a period of time. The δ13C values of the plant leaves were related to intercellular CO2 concentrations (Ci) and decreased with the increase in Ci (Farquhar et al., 1989). The δ13C values in plant leaves had a strong positive relationship with leaf-level WUEi (Farquhar and Richards, 1984; Seibt et al., 2008), especially for C3 photosynthesis plants. In addition, the δ13C in atmospheric CO2 was almost consistent (-8‰) within a given year for the plants growing in the field (Michener and Lajtha, 2007). Sampling plots did not have a closed canopy, and the gradients in the atmospheric CO2 concentrations under the canopy were small and could be ignored. Thus, the δ13C values in the plant leaves were used to compare leaflevel WUEi over the long term for C3 plants. 2.6. Data analyses
3.3. Water use patterns across the growing season
The isotopic signatures in plant tissues and soil water met a normal distribution by the Kolmogorov–Smirnov (K–S) test. The difference of isotopic signatures and SWC among seasons and plant species were identified by two-way analysis of variance (ANOVA). Significant differences of the SWC and isotopic compositions of soil water from different soil layers per species was tested using one-way ANOVA combined with a post-hoc Tukey's least significant difference (LSD) test. A one-way ANOVA was applied to detect the differences in plant water source from shallow, intermediate and deep soil layers per species. The significance level of the statistical analyses was set at 0.05. These statistical analyses were performed using SPSS 20.0 (SPSS Inc., Chicago, IL, USA).
H. rhamnoides absorbed the largest proportion water (62.3 ± 13.2%) from the shallow layer in June. The proportional contributions of deep soil water absorbed by H. rhamnoides increased from 14.60% in June to 35.80% in September. The contribution fractions of soil water for H. rhamnoides from shallow, intermediate, and deep soil layers were significantly different (p < 0.01). S. pubescens absorbed most of its water (78.28%) from the shallow and intermediate soil layers during the sampling period (Fig. 5). S. pubescens derived the largest proportion (33.1 ± 7.03%) of water from the deep soil layers in August. The intermediate soil layer contributed the most water in July (33.90 ± 7.80%) and the shallow soil layers contributed the largest proportion (67.2 ± 14.5%) in June. The percentages of water sources used by S. pubescens from shallow, intermediate, and deep soil layers showed significant differences (p < 0.01). There were no significant differences in water absorption fractions from the different soil layers between H. rhamnoides and S. pubescens. However, the proportional contributions of shallow, intermediate, and deep soil water for two studied species had significant differences over time (p < 0.05). The water absorption fractions obtained from the shallow soil layers exhibited greater fluctuations between species than the other soil layers over time.
3. Results 3.1. Temporal and vertical variations of SWC The SWC in the two plant species plots varied with soil depths and seasons (Fig. 1). The SWC in H. rhamnoides plots displayed significant differences among depths (p < 0.01) and sampling dates (p < 0.01). The SWC in the S. pubescens plots showed a clear pattern along the vertical soil profile (p < 0.01), but remained relatively stable across 57
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Fig. 1. Vertical distribution of gravimetric soil water content (SWC, %) from (a) H. rhamnoides and (b) S. pubescens plots. Data are expressed as the means ± 1SD.
Fig. 2. Seasonal variations of δ2H in soil water and its vertical distribution within 0–300 cm soil layers. The vertical dash represents the δ2H of xylem water from H. rhamnoides and S. pubescens. Data are expressed as the means ± 1SD.
largest value in May (-28.05‰). The leaf δ13C value of S. pubescens decreased from May to August, but increased in September. The δ13C values of H. rhamnoides and S. pubescens were not significantly differences. However, the δ13C values of both H. rhamnoides and S. pubescens exhibited significant differences (p < 0.001) during the sampling period.
3.4. δ13C values in plant leaves The mean leaf δ13C value of H. rhamnoides was -28.49 ± 0.45‰, ranging from -28.03 to -29.28‰. H. rhamnoides had the smallest leaf δ13C value (-29.31‰) in August (Fig. 6). The mean leaf δ13C value of S. pubescens was -28.77 ± 0.46‰, ranging from -28.05 to -29.31‰. S. pubescens had the smallest leaf δ13C value in August (-29.31‰) and the 58
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Fig. 3. Seasonal variations of δ18O in soil water and its vertical distribution within 0–300 cm soil layers. The vertical dash represents the δ18O of xylem water from H. rhamnoides and S. pubescens. Data are expressed as the means ± 1SD.
4. Discussion 4.1. Difference in soil water isotopic signatures and SWC Isotopic compositions of soil water synthesized the processes associated with precipitation recharge, mixing with antecedent moisture, and evapotranspiration (Hsieh et al., 1998; Brooks et al., 2015). The isotopic signatures in surface soil water were highly variable and were depleted as soil depth increased (Figs. 2, 3). This result is likely attributed to intense evaporation causing isotopic enrichment (Gazis and Feng, 2004), as well as different precipitation events mixing with antecedent soil water isotopes (Tang and Feng, 2001). However, both δ2H and δ18O in the 60–300 cm soil water exhibited less variability and gradually stabilized from May to September (Figs. 2, 3). This pattern was consistent for exotic and native plants. Evaporation in shallow soil layers caused isotopic enrichment, and seasonal input of precipitation caused isotopic depletion that led to vertical gradients in soil water isotopes along the soil profiles. In this study, soil water isotopic compositions from exotic and native plant plots had no significant differences across months, indicating that soil water isotopic signatures had temporal stability. Moreover, no significant differences were found in the δ2H and δ18O of soil water between the two species’ plots. This is likely because the two plant species had the same micro-environment, which had similar evaporation, and no isotopic discrimination of water isotopes occurred during water absorption by the roots (Ehleringer and Dawson, 1992). A significant dissimilarity was also found in the SWC between S. pubescens and H. rhamnoides, especially in shallow and deep soil layers. Previous studies have found that exotic vegetation affected soil moisture deficit below the near-surface layers, and the SWC of exotic plants from intermediate and deep soil layers have been observed to have temporal stability in the semi-arid Loess Plateau (Yang et al., 2014). In this study, the SWC from deep soil layers in H. rhamnoides exhibited lower temporal stability, which was a lower degree of variation over time than that of S. pubescens (Fig. 1). The variations in SWC
Fig. 4. Values of δ2H as a function of δ18O from (a) H. rhamnoides and (b) S. pubescens during the sampling period. LMWL is the local meteoric line (y = 7.76 x + 5.14, R2 = 0.91, p < 0.01) based on the isotopic values of the precipitation (Wang et al., 2017). SWL is soil water evaporation line which is fitted based on the isotopic values of soil water. The isotopic compositions of the xylem water and soil water from different soil layers are displayed in the figure.
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Fig. 5. Seasonal variations in water uptake proportions for (a) H. rhamnoides and (b) S. pubescens from different soil layers. Error bar expressed as standard deviation.
ecological plasticity in water use. This result is consistent with the findings of a previous study in an alpine desert ecosystem in which H. rhamnoides switched its water sources from the 0–30 cm to 60–120 cm soil stratum, then switched back to a considerable proportion of its water sources from the 0–30 cm soil stratum during the growing seasons (Wu et al., 2016). In this study, H. rhamnoides absorbed the largest proportion (35.8%) of deeper soil moisture when surface soil moisture decreased in September (Fig. 1). In a recent study, similar findings were observed in the shrub, Vitex negundo which displayed strong ecological plasticity and absorbed a higher proportion of deep soil water as water stress increased (Wang et al., 2017). Compared to H. rhamnoides, the native plant S. pubescens mainly derived water from the 0–120 cm soil layer, and the relative proportions of the 0–120 cm soil water showed no significant difference among the months. Similarity, Yang et al. (2016) found that S. pubescens mainly relied on water from the 20–40 cm soil layers during the growing seasons in a semi-arid ecosystem of the Loess Plateau. Plant leaf δ13C is a validated indicator of long term leaf-level WUEi of plants. In arid and semi-arid regions, plants maintain a high WUE to reduce the impacts of water deficits and enhance their adaptability under drought conditions (Ogaya and Peñuelas, 2003). Since δ13C in plant leaves is positively correlated with leaf-level WUEi for C3 photosynthesis plants (Farquhar et al., 1989), we can use plant leaf δ13C in plant leaves to indirectly reflect leaf-level WUEi during a period of time. The δ13C assays indicated that H. rhamnoides had higher leaf-level WUEi than S. pubescens, suggesting that H. rhamnoides typically decreases transpiration, which induces water loss. A high WUE ensured normal physiological activity and growth of the plant (Farquhar et al., 1989). Some previous studies have shown that plants with low WUE had the advantage of water competition in seasons with sufficient water (Lucero et al., 2000; Robinson et al., 2001). In this study, the study period covered the entire rainy season from June to September. Thus, it is clear that S. pubescens utilized more surface water due to its lower leaf-level WUEi. No significant differences were found in the isotopic signatures of xylem water between H. rhamnoides and S. pubescens. This suggests that the two species had similar water sources. Nevertheless, the water use fractions of the two species had significant seasonal changes. The
Fig. 6. Temporal variation of leaf δ13C from H. rhamnoides and S. pubescens. Data are expressed as means ± 1SD. The different lowercase letters express pronounced difference of plant leaf δ13C among months at the level p < 0.05.
from different soil layers may be caused by the effect of rainfall pulse, vegetation transpiration, and soil evaporation (Seneviratne et al., 2010). Some previous studies have also found that SWC in deep soil layers displayed lower temporal variability than shallow soil layers in some semi-arid regions (Penna et al., 2013; Yang et al., 2014). 4.2. Differences in water use characteristics The isotopic compositions in xylem water from the two species were within that of water from the 0–120 cm soil layers (Fig. 4), implying that H. rhamnoides and S. pubescens mainly absorbed water sources from 0 to 120 cm soil. The contribution fractions were ˜80% as predicted by the MixSIAR model during the sampling period (Fig. 5). Previous studies have noted that non-phreatophyte shrubs mainly used upper soil water derived from precipitation for survival in the Central Asia desert (Xu and Li, 2006). However, H. rhamnoides progressively increased its contribution proportion of deep soil water as the growing season progressed (Fig. 5), implying that H. rhamnoides had a higher degree of 60
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Fig. 7. Vertical profiles of fine root density (cm3/m3) for H. rhamnoides and S. pubescens.
shallow water sources. H. rhamnoides derived more deep soil water when shallow soil water was limited. Moreover, H. rhamnoides had higher leaf-level WUEi than S. pubescens. These findings suggest that H. rhamnoides had a greater degree of ecological plasticity in water use. Previous studies have demonstrated that H. rhamnoides was suitable for afforestation in the semi-arid Loess Plateau with high levels of rainfall (> 70 mm) (Jian et al., 2015). However, it should be noted that H. rhamnoides may cause soil desiccation due to higher uptake of deep soil moisture. Appropriate management measures, such as reasonable planting density and prescribed disturbance, should be implemented to maintain ecosystem sustainability. Plant water use characteristics provide important information for screening exotic species for revegetation and ecological management. Plants with flexible water use patterns can change their water sources between shallow and stable water sources (deep soil water or groundwater) depending on soil water availability. In addition, some plants that continuously absorb shallow and deep soil water or groundwater may not be preferred because long-term continuous utilization of deep soil water or groundwater may cause soil desiccation and the degradation of ecosystem services (Chen et al., 2008). Moreover, plants that continuously derive water from shallow soil layers may have difficulty surviving in extreme drought conditions. The combination of using plant species with different water use characteristics may form a good community configuration that complements the inadequate use of water resources by each species.
difference in water use patterns among the species was closely linked with fine root (< 2 mm) distribution (Dawson et al., 2002). Fine roots are an important active physiological component in water uptake (Schenk, 2008). Field investigation found a fine root density in the 0–120 cm soil layers from the S. pubescens plots accounted for 88.49% of the total fine root density, which allowed S. pubescens to derive its water sources primarily from the 0–120 cm soil layers (Fig. 7). However, the fine root density of H. rhamnoides was distributed in deeper layers, within a range of 250 cm. The fine root density in the 0–200 cm layer of the soil of H. rhamnoides plots accounted for 89.12% of the total fine root density. Therefore, H. rhamnoides could obtain higher percentages of deep soil moisture compared to S. pubescens. Many perennial plants in arid and semi-arid ecosystems have been observed to have dimorphic root systems (Dawson and Pate, 1996; Wang et al., 2017). The lateral root zone potentially obtains surface water during the wet season, and the penetrating taproots zone derives deep soil water when shallow water sources are insufficient (Dawson and Pate, 1996; McCole and Stern, 2007). H. rhamnoides could flexibly adjust water sources between different soil stratums due to its dimorphic root system and these field investigation suggest that H. rhamnoides had dimorphic root systems with lateral roots distributed in the shallow soil and a deep tap root extension into the deep soil (Fig. 7). A previous study also showed that H. rhamnoides displayed flexible water use patterns and switched water sources among different soil layers following a decrease in precipitation (Tang et al., 2018). Additionally, plants adjusted their water use characteristics by changing photosynthesis, gas exchange, hydraulic and morphology at the leaf level during water source deficits (Lazaro-Nogal et al., 2013).
5. Conclusion Seasonal water use characteristics of the exotic H. rhamnoides and native plant S. pubescens in the Loess Plateau were investigated using δ2H, δ18O, and δ13C combined with the MixSIAR model. The MixSIAR model predicted that H. rhamnoides and S. pubescens obtained 78.80% and 78.28% of their water from the 0–120 cm soil layer during the growing season, respectively. However, H. rhamnoides progressively utilized more fractions of deeper soil water as the season progressed and could flexibly convert its water sources between shallow and deep soil layers due to its dimorphic root system. Additionally, the leaf δ13C implied that H. rhamnoides had higher leaf-level WUEi than that of S. pubescens during the growing season. These results suggest that H. rhamnoides has a greater degree of ecological plasticity in water use. However, H. rhamnoides may cause negative effects due to its capability to derive more water from deep soil layers. This study provides valuable
4.3. Implications for ecological restoration S. pubescens utilized the most fractions of shallow soil water and was more sensitive to recent precipitation during the growing season. However, H. rhamnoides displayed flexible water use patterns which shifted its water sources among different soil stratum as the growing season progressed (Fig. 5). Deciduous plant species in seasonal arid ecosystem respond to drought stress with different strategies. Shifting water sources among different soil layers, depending on water availability, is an important strategy for deciduous plants to cope with drought in water limited ecosystem (Hasselquist et al., 2010). Plant species that can shift water sources can maximize their soil water use and avoid interspecific competition with plants that consistently utilize 61
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information for screening exotic species for revegetation and ecological management, and forming more effective restoration strategies for revegetation in semiarid ecosystems.
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