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Variation of water uptake in degradation agroforestry shelterbelts on the North China Plain
Agriculture, Ecosystems and Environment 287 (2020) 106697
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Variation of water uptake in degradation agroforestry shelterbelts on the North China Plain ⁎
Ziqiang Liua, Guodong Jiab, , Xinxiao Yub, a b
T
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Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing, 210037, China Key Laboratory of Soil and Water Conservation and Desertification Combating of Ministry of Education, Beijing Forestry University, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Degradation Water use Water deficiency Isotopic composition
Agroforestry shelterbelts are important ecological barriers providing protection against wind and sand-related disasters in the North China Plain. However, large proportions of these agroforestry shelterbelts are degenerating or have died in recent years and the causes of degeneration are extremely complex. In this study, stable hydrogen and oxygen isotope techniques were used to quantify water sources for P. simonii Carr vegetation in four degrees of degradation (undegraded, U.D; slightly degraded, L.D; moderately degraded, M.D and seriously degraded, S.D), and to investigate how the vegetation can adapt to degradation. Results show that P. simonii Carr exhibited considerable plasticity in depth of water uptake; it prefers to utilize deeper water sources which have higher soil moisture. This vegetation predominantly absorbs water from depths below 160 cm and treatments with a relatively stable and high water content source, are characterized by U.D and L.D. The utilization of deep soil water has gradually decreased, while the utilization rate of surface soil water has gradually increased with an increase in the degree of degradation. Water and nutrient availability in depths of 0–40, 40–80, 80–120 and 120–160 cm were relatively low in M.D and S.D treatments. Therefore, P. simonii Carr degeneration in this region was attributed lack of available water and nutrients. The low precipitation levels and continuous decline in the groundwater level will be accelerating degradation and death of P. simonii Carr shelterbelts.
1. Introduction
on the regional level was mainly related to soil water content (SWC). Water deficiency, however, may be a major constraint on plant growth. As highlighted by Yang et al. (2015), trees have different water-use patterns and physiological responses to different intensities and duration of drought events during their growth. It has previously been shown that several tree species having a dimorphic root system which can adjust their water sources according to SWC (Liu et al., 2018a, 2018b), which an adaptive technique beneficial to overcoming drought events. However, a number of tree species are susceptible to drought or water deficiency as they absorb water from a fixed water source regardless of the water conditions (Moreno-Gutiérrez et al., 2012; David et al., 2013). In these tree species, drought or water shortage will not only affect the carbon assimilation rate of the trees, but also influence nitrogen absorption due to stomatal closure. A number of previous studies have investigated the mechanism of degradation and its influencing factors based on climatic conditions (Pellizzari et al., 2016), tree species distribution, human activities (Dickman et al., 2015) and tree physiology (Sun et al., 2019) in the arid and semi-area region. Sun et al. (2019) compared the water sources of non-dieback and dieback poplar; however, the depth of the water source did not exceed 150 cm even
The structure and function of natural forests have been destroyed by various human activities, which greatly influence the ecological protection function. Drought, wind and soil erosion have seriously restricted the economic and social development of ecologically fragile areas. Thus, countries around the world defend against ecological disasters by building artificial shelterbelts (Borrelli et al., 2016; Chang et al., 2019). In the 1970s, the Chinese government built a large area of artificial agroforestry shelterbelts using Populus simonii Carr in the North China Plain. These agroforestry shelterbelts play an important role in reducing the impact of wind, sand fixation, improving water conservation, and improving the regional microclimate (Qiao et al., 2016). However, large areas of the P. simonii Carr shelterbelts in this region have been extensively degraded or even died in recent years, due to the double impact of global climate change and human activities. Hypotheses for the degeneration and death of the agroforestry shelterbelts have included hydraulic failure, carbon starvation and biological attack (Rowland et al., 2015; Urrutia-Jalabert et al., 2015; Speckman et al., 2015). Previous studies suggest that tree degeneration
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Corresponding authors. E-mail addresses: [email protected] (G. Jia), [email protected] (X. Yu).
https://doi.org/10.1016/j.agee.2019.106697 Received 25 July 2019; Received in revised form 23 September 2019; Accepted 25 September 2019 0167-8809/ © 2019 Elsevier B.V. All rights reserved.
Agriculture, Ecosystems and Environment 287 (2020) 106697
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(392.7 mm) since 1957 (Fig. 3). About 65.4% of precipitation in the study area occurred from June–September. The annual mean temperature fluctuated between 1.9 °C and 5.1 °C, and increased annually (R2 = 0.53, p < 0.01).A large temperature difference occurs between day and night-time in this Bashang High Cold Region, with the perennial average temperature being 4.1 °C. In contrast to the temperature results, wind speed decreased annually (R2 = 0.73, p < 0.01) and fluctuated between 2.8 m s−1–5.8 m s−1. The annual relative humidity ranged from 50% to 66% mm over the last 60 years with a mean relative humidity of 57.1%. The dominant tree species in this region is P. simonii Carr, which planted in the 1970s to reduce the effects of wind and sand related disasters. P. simonii Carr has exhibited different degrees of degradation, with mortality occasionally occurring. Soil in this area is mainly classified as sandy, and soil depth can reach 450 cm.
though the poplar roots can reach 240 cm in depth. Therefore, it is necessary to further explore the water sources related to the degradation of P. simonii Carr and reveal the causes of P. simonii Carr shelterbelt degradation. Precipitation in the North China Plain is relatively low and exhibits obvious seasonal variation. Previous results by Nakayama et al. (2010) have shown that the water table depth in this area has undergone a yearly decrease, an occurrence which may result in the agroforestry shelterbelts experiencing drought conditions over a long period. Currently, it has been noticed that sections of the P. simonii Carr shelterbelts are degraded while other sections in the same area are perceived to be growing undisturbed. Reasons for these differences, and how P. simonii Carr can grow and adapt to drought conditions, are unclear. It is therefore important to understand the causes and factors for the degradation of P. simonii Carr shelterbelts in this area. We hypothesize that: 1) P. simonii Carr with different degrees of degradation have different water sources; and 2) severe shortage of water resources and nutrients in regional areas has led to degradation. To test these hypotheses, we investigated the water sources for P. simonii Carr vegetation in four degrees of degradation (undegraded, U.D; slightly degraded, L.D; moderately degraded, M.D and seriously degraded, S.D). In addition, the SWC and nutrient content of each stratum among the four treatments were compared to identify the cause of degeneration in the agroforestry shelterbelt. The results of this study provide a scientific basis for optimizing the stand structure of shelterbelts, maintaining the stability of ecosystems and maximizing the ecological protection function of shelterbelts.
2.2. Experimental design P. simonii Carr in agroforestry shelterbelts have the same initial planting density (900 trees ha−1) and can be divided into four degrees of degeneration based on growth status and spike top (U.D; L.D; M.D and S.D.). In order to clearly identify causes of degradation, sample surveys and sample collection were undertaken in accordance with the degree of degradation (Fig. 2). The slope gradient and slope position increase slowly with the degree of degradation. Five replicate treatments (20 m × 20 m) with a similar slope and the same slope direction were chosen in each treatment. The distance between the replicate treatments was approximately 60 m. For each treatment, 6–42 trees were identified and had an average age of approximately 30 years. The mean crown density for each treatment was 76.9% (U.D), 56.2% (L.D), 40.7% (M.D) and 19.0% (S.D) (Table 1). Populus simonii Carr growing in the U.D treatments (Fig. 2a) grew well and did not possess a spike on top. No dead or dying specimens were identified in this treatment. The average height and diameter at breast height (DBH) of the trees were 13.8 m were 22.1 cm, respectively, in this treatment. Trees in the L.D treatments had an average height and DBH of 11.2 m and 17.6 cm, respectively. There trees exhibited a slight spike on top (approximately 1 m in length) and had a withering ratio of no more than 10% (Fig. 2b).
2. Materials and methods 2.1. Site description This study was conducted in Zhangbei County (40°57′～41°34 N, 114°10′～115°27′E), northwestern Hebei Province, China (Fig. 1). This area is situated in a middle temperate zone, at an altitude of 1750 m, and experiences a continental monsoon climate. Precipitation in 2017 was 398.4 mm, which was 1.4% higher than the annual average rainfall
Fig. 1. The location of the study site and sample points. U.D–undegraded; L.D–slightly degraded; M.D–moderately degraded; S.D–seriously degraded. 2
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Fig. 2. Characteristic spikes on top of P. simonii Carr trees for the different categories of degradation. a, b, c and d reflect U.D, L.D, M.D and S.D, respectively.
Populus simonii Carr in the M.D treatments had an average tree height and DBH of 10.7 m and 16.8 cm, respectively. These trees exhibited a significant spike on top (2–3 m in length) and had a withering ratio of 10–40% (Fig. 2c). Finally, P. simonii Carr in the S.D treatments had an average tree height and DBH of 8.9 m and 13.7 cm, respectively. These trees exhibited a large spike on top (greater than 3 m) and had a withering ratio greater than 40% (Fig. 2d). Additionally, trees in this treatment had a high mortality. Soil conditions between the treatments were found to differ. The depth of the soil layer in the S.D treatments only attained a depth of 240 cm, after which a solid calcareous layer was present. Soil depth in the other treatments all reached a depth of 400 cm. Soil in the deep layers was mud–shaped and had a high water content.
Table 1 Basic information for the experimental plots. The degree of degradation
Average height (m)
Average DBH (m)
Crown density (%)
Rate of spike top (%)
Length of spike top (m)
U.D L.D M.D S.D
13.8 11.2 10.7 8.9
22.1 17.6 16.8 13.7
76.9 56.2 40.7 19.0
0 ≤10 10∼40 ＞40
0 ≤1 1∼3 ＞3
growing season of P. simonii Carr (Jul.–Aug.). Precipitation, temperature and wind speed data were collected every 15 min (Jan.–Dec., 2017) using a HOBO weather station (U30-NRC, Onset, USA) situated 4.5 km from the treatments. In addition, historical weather data from 1957 to 2017 and groundwater data from 1995 to 2017 were collated for this study.
2.3. Sample collection Soil and branch samples were collected eight times during the peak
Fig. 3. Variation of annual precipitation, temperature, wind speed and relative humidity from 1957 to 2017. The red and black lines indicate the linear regression equations of precipitation and temperature, respectively (a), and wind speed and relative humidity, respectively (b) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
3
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(2) and (3).
For each sampling time, in each treatment, three P. simonii Carr trees having average height and DBH (and not affected by artificial interference) were chosen. From each tree, three mature and suberized branches, having a length of 2–5 cm and a diameter of 0.3–0.5 cm, were selected. Branches from each tree specimen were selected from similar heights and orientation. Branch samples were placed in clean 50 mL polyethylene bottles, sealed with Parafilm M® and frozen at 4 °C. Soil samples were collected in close proximity to the sampled trees using a soil auger with an internal diameter of 3.5 cm. Soil samples were collected at intervals of 0–10, 10–20, 20–40, 40–60…360–380, 380–400 cm in the U.D, L.D and M.D treatments. Soil samples from S.D treatments only reached a depth of 240 cm, after which parent material or a calcic horizon was present. Three replicates were collected for each soil layer and soil samples were divided into three fractions for analysis. One fraction was encapsulated in an aluminum box and dried at 105 °C to calculate SWC. Another fraction was encapsulated in a sealed bag for the determination of soil nutrients, and the final fraction was place into clean 50 mL polyethylene bottles for isotope analysis.
δXT = c1δXS1 + c2δXS2 + c3δXS3 + c4δXS4 + c5δXS5+ c6δXS6+ c7δXS7 (2) c1 + c2 + c3 + c4 + c5 + c6 + c7 = 1
2.4. Investigation of the root system A soil cutting ring (50 cm × 20 cm × 40 cm) was used to investigate the root system of the sampled trees (undegraded). Soil samples at depths of 0–40, 40–80, 80–120, 120–160, 160–200, 200–240 and > 240 cm in the vertical direction, and 0–50, 50–100, 100–150 and 150–200 cm in the horizontal direction, were collected. Roots present in the soil were collected once the soil samples had been passed through a 0.01 mm sieve. Tree roots were washed and divided into size (diameter, d) using the categories of ≤ 0.2, 0.2–0.5, 0.5–1.0, 1.0–2.0, 2.0–5.0 and > 5.0 mm. By using the root system analysis program (WinRHIZO–EC, Canada), root length was then scanned for each diameter level. Total root length was calculated as the sum of root length in each scan.
3.1. The variation of soil water content and groundwater In general, SWC decreased as the degree of degradation increased (Fig. 4). Average SWC of the U.D treatment was 9.32%, which was 1.93 times greater than that of S.D. Although SWC results for the U.D, M.D and S.D treatments showed similar patterns of change, the point of change differed. SWC of U.D increased with soil depth from 0 to 160 cm before decreasing from 160 to 240 cm. After 240 cm, SWC in this treatment then increased and reached a maximum value (22.95%) at 400 cm. SWC for the M.D and S.D treatments increased with soil depth between 0–80 cm and 0–60 cm, respectively. SWC in these treatments peaked at 12.83% (at 80 cm) and 9.00% (at 60 cm) for the M.D and S.D treatments, respectively. SWC of the L.D treatment recorded a downward trend with soil depth from 0 to 120 cm. SWC between 120–400 cm increased, this being similar to the result recorded between 240–400 cm
Isotopic composition of soil and branch samples was conducted in the Laboratory of Eco hydrological Processes and Mechanisms at the Beijing Forestry University. For the branch samples, water samples were extracted using a low temperature vacuum extraction system on samples which had their bark and green sections cut but their xylem retained. The same extraction system was used to extract water from the soil samples. Extracted water samples were then filtered through a 0.22 μm filter to remove impurities. Subsequently, samples were analyzed for δ2H and δ18O using a liquid water isotope analyzer (DLT–100, LGR Inc., USA). Measurement accuracy was 0.3‰ for δ2H and 0.1‰ for δ18O. The isotopic ratio of δ2H and δ18O in the water samples was recorded in parts per thousand of the Vienna Standard Mean Ocean Water, this being express as: ⎜
18
3. Results
2.5. Sample determination
R sample ⎞ δ 2H / δ 18O = ⎛ × 1000‰ R ⎝ s tan dard ⎠
(3)
where δXT is either the δ H or O value in the xylem water. The subscripts S1–S7 are the water source from 0 to 40, 40–80, 80–120, 120–160, 160–200, 200–240 and > 240 cm, respectively. Additionally, c1-c7 represents the contribution ratios of S1–S7 for total xylem water absorption. Statistical analyses were performed using SPSS 16.0. The relationship between annual precipitation/annual mean temperature/annual mean wind speed/annual mean relative humidity and year, and the δ2H and δ18O values of soil water, were analyzed using the ordinary leastsquares linear regression model. The relationship among the proportion of water sources, water uptake depth and SWC were also analyzed using the multiple linear regression model. Two-way ANOVA was used to identify differences in soil water content and isotopic values of soil water and soil nutrients, using the factors of soil stratum and treatments. The LSD method was used to compare soil nutrients when necessary. All data was tested for normal distribution and homogeneity of variance analysis, all of which met the requirements. 2
⎟
(1)
where, Rsample and Rstandard represent the isotope ratio of the samples and the Vienna Standard Mean Ocean Water, respectively. 2.6. Data analysis The Iso-Source model was used to calculate the contribution percentage of water sources for tree species based on the isotopic mass conservation theory (Phillips and Gregg 2003). Soil layers with similar isotopic values were merged into the same water source. The δ2H and δ18O values of the 0–40, 40–80, 80–120, 120–160, 160–200, 200–240 and > 240 cm layers were calculated using the model to reduce experimental error and increase the accuracy of calculation. The equations for calculating the contribution ratio were expressed using Eqs.
Fig. 4. The variations of mean ( ± S.D.) soil water content under different categories of degradation in the study area. 4
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Fig. 5. Variation of water table depth from 1995 to 2017. Fig. 7. The relationship between isotopic compositions in U.D, L.D, M.D and S.D. The global meteoric water line (GMWL) is δ2H = 8.0δ18O + 10.0.
in the U.D treatment. The water table depth in the study area was less than 5 m before 1999, and decreased annually thereafter (R2 = 0.92, p < 0.01). The average descending depth was 0.74 m per year, and the average water table depth was 18 m since 2012 (Fig. 5).
3.3. Characteristics of soil nutrients in different degrees of degradation In general, soil C, N, P and K decreased with soil depth (Fig. 8). Average soil C of the U.D, L.D, M.D and S.D treatments were 11.26 g kg−1, 10.55 g kg−1, 9.67 g kg−1 and 10.94 gkg−1, respectively. Although soil C in the 0–40 cm layer in the U.D and L.D treatments were higher than that in the M.D and S.D treatments, no significant difference were recorded in the 40–80 or 120–160 cm soil layers among the U.D, L.D or M.D treatments (p > 0.05). However, soil C in the 80–120 cm layer of the S.D treatment was significantly higher than that in the L.D or M.D treatment (p < 0.05). Average concentrations of N, P and K in the soil decreased with deceased degradation; average N (31.12 mg kg−1), P (2.26 mg kg−1) and K (86.89 mg kg−1) in the U.D treatments were significantly higher than those recorded of the S.D treatments (15.11 mg kg−1, 1.58 mg kg−1 and 29.47 mg kg−1 for N, P and K, respectively) (p < 0.05). In the 0–40 cm layer, significant differences between soil N, P and K for the L.D and M.D treatments were recorded (p < 0.05). However, no significant differences were recorded for the majority of the other soil layers (p > 0.05).
3.2. Soil hydrogen and oxygen isotope characteristics in different degrees of degradation Average δ2H and δ18O values for the U.D, L.D, M.D and S.D treatments were –91.15‰ and –13.69‰; –79.61‰ and –11.57‰; –75.37‰ and –10.80‰; and –73.55‰ and –9.74‰, respectively, recording an increasing trend (Fig. 6). Although the isotopic values of soil water were distributed on both sides of the local meteoric water line (y = 2.76x –48.89, R2 = 0.41, P < 0.001) (Fig. 7), the variation range of isotopic composition increased as the rate of degradation increased. This indicated that the fractionation effect was more obvious as the rate of degradation increased. Results for δ2H and δ18O average values with soil depth recorded a decrease, a result which was evident in the 0–100 cm depth range. The isotopic composition in this soil layer exhibited a significant decrease compared to the other soil layers (p < 0.05). The δ2H and δ18O values in the 300–400 cm soil layer showed no significant variation among the U.D, L.D and M.D treatments (p > 0.05). However, the isotopic composition of the 150–240 cm depth in S.D recorded a change as great as that recorded in the 0–100 cm soil layer.
3.4. Root system distribution characteristics The root system of P. simonii Carr can spread 240 cm and 200 cm in the vertical and horizontal direction, respectively (Fig. 9). In general,
Fig. 6. The variation characteristics of mean ( ± S.D.) isotopic values of vertical soil profile under different degenerate sites in the study area. a and b are variation characteristics of δ2H and δ18O values in the profile, respectively. 5
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Fig. 8. The variation characteristics of mean ( ± S.D.) soil nutrients in vertical soil profiles under different degrees of degradation. a, b, c and d refer to organic matter, available nitrogen (N), rapidly available phosphorus (P) and available potassium (K), respectively.
decreased and the utilization rate of surface soil water gradually increased as the degree of degradation increased (Fig. 10). In the U.D treatments, P. simonii Carr primarily obtained water from the 200–240 cm and > 240 cm layers, with the average absorption being 28.69% and 35.63%, respectively. In the L.D treatments, water was predominantly absorbed from the 160–200 cm (30.86%) and 200–240 cm (27.1%) layers. In the M.D treatments, P. simonii Carr predominantly obtained water from the 160–200 cm (16.04%) layer, as well as the 80–120 cm (18.12%) and 120–160 cm (21.18%) layers. In the S.D treatments, the main water sources were from the 0–40 (22.84%), 40–80 (20.80%) and 80–120 cm (23.7%) layers; no water was sourced from deeper soil layers.
the root length of 0 < d < 0.2 mm and 0.2 < d < 0.5 mm was significantly longer than that of other diameters (p < 0.05). Roots were also found to increase with horizontal distance and depth (0–120 cm). The majority of roots were distributed in the soil depth from 120 to 200 cm and 0–50 cm horizontally. There was a dominant distribution of root lengths of 0 < d < 0.2 mm and 0.2 < d < 0.5 mm in the depths of 0–20 cm and 20–40 cm, and horizontally from 50 to 100 cm. 3.5. Water sources in different degrees of degradation P. simonii Carr preferred to use deeper water sources which have higher SWC (Fig. 11). The utilization of deep soil water gradually
Fig. 9. Vertical and horizontal root length distribution. The unit of d was mm. 6
Agriculture, Ecosystems and Environment 287 (2020) 106697
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Fig. 10. Mean ( ± S.D.) contribution percentages of the different water sources used by P. simonii Carr under different degrees of degradation. a, b, c and d indicate U.D, L.D, M.D and S.D, respectively.
in the four treatments (p < 0.05). Average SWC decreased from the U.D to S.D treatments (Fig. 4), a decline that may be largely attributed to different soil physical properties and groundwater recharge (Zheng et al., 2016a, 2016b). The SWC of U.D and L.D increased with soil depth; and soil in the 360–400 cm layer was mud–shaped. In the M.D and S.D treatments, SWC recorded a slight decrease with depth; a prominent calcium deposition layer at 380 cm and 220 cm was recorded, and the depth of the soil layer was no more than 400 cm and 240 cm, respectively. These findings suggest that groundwater is the main source of recharge for soil in the U.D and L.D treatments, but not for soils in L.D or S.D treatments. This may be attributed to large-scale farmland flood irrigation techniques used in the M.D and S.D treatments. Flood irrigation not only results in decreased ground water levels, it also leads to soil salinization and the formation of a calcic layer (Wang et al., 2014). Soil salinization results in nutrient loss and causes lower soil nutrients in degraded land (Minasny et al. 2016). The average soil N, P and K concentrations decreased with an increase in the degree of degradation (Fig. 8). This indicated that the M.D and S.D treatments face more serious water stress and nutrient stress than the U.D and L.D treatments. The calcic layer in the S.D treatments was formed due to prolonged deposition of calcium and caused higher hardness. The existence of calcic layers not only seriously affects the downward extension of P. simonii Carr roots, but also influence the absorption of deep soil water and nutrients (Eshel et al., 2014). Zheng et al. (2016a); (2016b) demonstrated that soil hardness in S.D treatments was significantly higher than that of U.D treatments in this area (p < 0.05). The variation of isotopic values in the soil profile also suggested that the water infiltration process or groundwater recharge in the four treatments were different. The isotopic value of soil in U.D and L.D treatments were more negative than those of M.D or S.D (Fig. 7). This indicated that the evaporative fractionation effect of infiltration process in M.D and S.D treatments was more obvious than that of U.D and L.D treatments. In addition, the variation range of isotopic composition and its isotopic values decreased with depth (Fig. 6), indicating that fractionation decreased with depth. These findings are in strong agreement with findings from Wen et al. (2010) and Griffis et al. (2011), who reported that
Fig. 11. The relationship among proportion of water sources, water uptake depth and soil moisture. The regression equation was z = 0.03x + 0.80y + 4.97 (R2 = 0.31, p = 0.015).
4. Discussion 4.1. Variation characteristics of potential water sources Soil water in the study area is predominantly derived from the combination of precipitation and groundwater, and it is affected by radiation, air humidity and soil physical properties (Bosco et al., 2016; Xie et al., 2016; Yu et al., 2018). Annual precipitation in the study area is less than 400 mm and, due to extraction and irrigation of agricultural land, water table depth has decreased in recent years (Fig. 4). This leads to lower average SWC in the study area. Although an equal amount of precipitation with the same isotopic compositions were identified in the sampling points, SWC and its isotopic values are significantly different 7
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strong evaporation effects generally occur in upper soil layers, and that this effect fades with depth. Isotopic values between 300–400 cm for U.D and L.D treatments recorded little change, indicating that water in these soil layers were not affected by evaporation fractionation. The reduction in variation of isotopic values in water in deeper soil layers were probably recharged by groundwater with relatively stable isotopes (Kumar et al., 2008; Stumpp and Hendry, 2012).
indicating that trees in the S.D treatments faced more severe water and nutrient stresses than the U.D treatments (Sun et al., 2019). Extreme drought can also lead to the failure of plant water transmission and water mechanics. Sustained drought and nutrient deficiency will influence vegetation growth, degradation, and even result in death (Grant et al., 2014). Further studies on the physiological factors of plant water sources will provide further insight to issues identified in this study.
4.2. Water uptake for a shelter-forest subjected to degradation
5. Conclusions
Our results indicated that the P. simonii Carr reflected plasticity in regard to sources of water uptake. This phenomenon is similar to findings from other studies reporting that tree species with dimorphic root morphology could tap distinct zones of soil water (Brooks et al., 2010; Nie et al., 2011). The water use pattern of P. simonii Carr was not only related to its root distribution, but also to the soil physical properties. As this tree species has a root system that can spread 240 cm vertically (Fig. 7), it provides a prerequisite for this tree species to transfer its water sources between deep and shallow soil layers. The tree species can obtain more water from 120 to 200 cm since the majority of roots were distributed in these soil layers. In the U.D and L.D treatments, SWC at depths > 160 cm were higher than those < 160 cm (Fig. 4). In addition, SWC at depths < 160 cm were more vulnerable to the external environment than those > 160 cm. P. simonii Carr predominantly obtained water from soil layers > 160 cm (Fig. 10), thus it can have a stable water supply over a long time period (Fig. 11). This finding is in accordance with the findings of Kray et al. (2012), who reported that tree species with deep roots preferred to absorb water from deep soil layers. Fu et al. (2016) demonstrated that Populus trees in the study area have high transpiration rates, thus trees utilizing water from deep soil layers are more suited to meeting transpiration demand since SWC in deep layers is more stable. The SWC in the M.D and S.D treatments were lower than that recorded in the U.D and L.D treatments. In the M.D and S.D treatments, SWC and soil nutrients in layers < 120 cm were significantly higher than that in layers > 120 cm (Fig. 8). P. simonii Carr prefers to absorb more water from shallow soil layers, and the utilization of surface soil water increased with an increase in the degree of degradation. This finding is mainly attributed to two reasons. Firstly, the absorption of surface water can better satisfy transpiration demand and nutrient supply than that of deep water absorption. Secondly, energy consumption used to absorb surface water is lower than that used to absorb deep water. As annual precipitation was lower and unevenly distribution in the study area, this resulted in a long-term water shortage for this species. The water use pattern for P. simonii Carr may be the result of long-term adaptation to its environment. P. simonii Carr prefers to uptake water from soil layers with a higher moisture content, whether it is degenerated or not (Fig. 11). These results are consistent with previous findings (Yang et al., 2015; Liu et al., 2018a, 2018b) that showed that trees can change their water source according to soil moisture. Although P. simonii Carr prioritized absorbing water from surface soil layers that had a relatively higher moisture content in the M.D and S.D treatments, SWC in the surface soil layer varied greatly. The water and nutrient availability are relatively low and unstable, it may be difficult to meet the demands of this vegetation. As the root systems of P. simonii Carr are shown to have high hydrotropism (Xi et al., 2013), the absorption activity of the root system may be limited in conditions of low soil moisture, therefore increasing the difficulty of water absorption. Furthermore, the calcic layer in the M.D and S.D treatments was higher than that in the U.D and L.D treatments.. Water and nutrients available to be transported to the tree canopy may be limited, thus leaves on the canopy will therefore suffer serious water and nutrient loss, resulting in a spiked top. Drought episodes affecting P. simonii Carr were more serious due to the water table depth declining (Fig. 5). Water use efficiency of the degenerated P. simonii Carr was higher than that of trees with undegraded,
Results from this study highlight that the degradation of P. simonii Carr in the North China Plain region is closely related to water sources and available water. Our results show that this species prefers to use deeper water sources which have higher SWC. In the U.D and L.D treatments, P. simonii Carr primarily obtained water from soil layers > 160 cm as the SWC in deep soil layers was higher than that in shallow soil layers. The utilization of deep soil water gradually decreased, while the utilization rate of surface soil water gradually increased with an increase in degree of degradation. In the M.D and S.D treatments, although P. simonii Carr mainly absorbed water from soil layers < 160 cm, available water and nutrients may not be sufficient to meet the demand. The lack of available water and nutrients may therefore be the main reason for the degeneration of P. simonii Carr. The continuous decline in the groundwater level and low precipitation levels in this region has increased the intensity and duration of drought episodes, as well as accelerating degradation and death of P. simonii Carr shelterbelts. The protective benefits of agroforestry shelterbelts can be maintained by replacing P. simonii Carr with other tree species in the future. Acknowledgement This research was funded by National Natural Science Foundation of China (No. 41430747 & No. 41877152), National Science and Technology Support Project (2015BAD07B0302) and Beijing Municipal Education Commission (CEFF-PXM2019_014207_000099). References Borrelli, P., Lugato, E., Montanarella, L., Panagos, P., 2016. A new assessment of soil loss due to wind erosion in European agricultural soils using a quantitative spatially distributed modelling approach. Land Degrad. Dev. 28, 335–344. Bosco, T., Bertiller, M.B., Carrera, A.L., 2016. Combined effects of litter features, UV radiation, and soil water on litter decomposition in denuded areas of the arid Patagonian monte. Plant Soil 406 (1–2), 71–82. Brooks, J.R., Barnard, H.R., Coulombe, R., McDonnell, J.J., 2010. Ecohydrologic separation of water between trees and streams in a Mediterranean climate. Nat.Geosci 3 (2), 100–104. Chang, X.M., Sun, L.B., Yu, X.X., Jia, G.D., Liu, J.K., Liu, Z.Q., Zhu, X.H., Wang, Y.S., 2019. Effect of windbreaks on particle concentrations from agricultural fields under a variety of wind conditions in the farming-pastoral ecotone of northern China. Agric. Ecos. Environ. 281 (6), 16–24. David, T.S., Pinto, C.A., Nadezhdina, N., Kurz-Besson, C., Henriques, M.O., Quilhó, T., Cermak, J., Chaves, M.M., Pereira, J.S., David, J.S., 2013. Root functioning, tree water use and hydraulic redistribution in Quercus suber trees: a modeling approach based on root sap flow. For. Ecol. Manag 307, 136–146. Dickman, L.T., McDowell, N.G., Sevanto, S., Pangle, R.E., Pockman, W.T., 2015. Carbo hydrate dynamics and mortality in a piñon-juniper woodland under three future precipitation scenarios. Plant Cell Environ. 38 (4), 729–739. Eshel, G., Lifschitz, D., Bonfil, D.J., et al., 2014. Carbon exchange in rainfed wheat fields: effects of long-term tillage and fertilization under arid conditions. Agric. Ecosyst. Environ. 195, 112–119. https://doi.org/10.1016/j.agee.2014.05.007. Fu, S., Sun, L., Luo, Y., 2016. Combining sap flow measurements and modelling to assess water needs in an oasis farmland shelterbelt of Populus simonii Carr, in northwest China. Agric. Water Manag. 177 (177), 172–180. Grant, K., Kreyling, J., Dienstbach, L.F.H., Beierkuhnlein, C., Jentsch, A., 2014. Water stress due to increased intra-annual precipitation variability reduced forage yield but raised forage quality of a temperate grassland. Agric. Ecosyst. Environ. 186, 11–22. https://doi.org/10.1016/j.agee.2014.01.013. Griffis, T.J., Wood, J.D., Baker, J.M., et al., 2011. Investigating the source, transport, and isotope composition of water vapor in the planetary boundary layer. Atmos. Chem. Phys. 16, 1–36. Kray, J.A., Cooper, D.J., Sanderson, J.S., 2012. Groundwater use by native plants in response to changes in precipitation in an intermountain basin. J. Arid Environ. 83 (4),
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25–34. Kumar, U.S., Sharma, S., Navada, S.V., 2008. Recent studies on surface water–groundwater relationships at hydro-projects in India using environmental isotopes. Hydrol. Process. 22, 4543–4553. Liu, Z.Q., Jia, G.D., Yu, X.X., Lu, W.W., Zhang, J.M., 2018a. Water use by broadleaved tree species in response to changes in precipitation in a mountainous area of Beijing. Agric. Ecosyst. Environ. 251 (10), 132–140. Liu, Z.Q., Yu, X.X., Jia, G.D., Zhang, J.M., Zhang, Z.Y., 2018b. Water consumption by an agroecosystem with shelter forests of corn and Populus in the North China Plain. Agric. Ecosyst. Environ. 265 (10), 178–189. Minasny, B., Hong, S.Y., Hartemink, A.E., Kim, Y.H., Kang, S.S., 2016. Soil PH increase under paddy in South Korea between 2000 and 2012. Agric. Ecosyst. Environ. 221, 205–213. Moreno-Gutiérrez, C., Dawson, T.E., Nicolás, E., Querejeta, J.I., 2012. Isotopes reveal contrasting water use strategies among coexisting plant species in a Mediterranean ecosystem. New Phytol. 196 (2), 489–496. Nakayama, T., Yang, Y., Watanabe, M., Zhang, X., 2010. Simulation of groundwater dynamics in the North China plain by coupled hydrology and agricultural models. Hydrol. Process. 20 (16), 3441–3466. Nie, Y.P., Chen, H.S., Wang, K.L., Tan, W., Deng, P.Y., Yang, J., 2011. Seasonal water use patterns of woody species growing on the continuous dolostone out cropsand nearby thin soils in subtropical China. Plant Soil 341 (1–2), 399–412. Pellizzari, J.J., Camarero, A., Gazol, G., Sangüesa-Barreda, M., 2016. Wood anatomy and Carbon isotope discrimination support long term hydraulic deterioration as a major cause of drought induced dieback. Glob. Change Biol. Bioenergy 22 (6), 2125–2137. Qiao, Y.Q., Fan, J., Wang, Q.J., 2016. Effects of farmland shelterbelts on accumulation of soil nitrate in agro-ecosystems of an oasis in the Heihe river basin, China. Agric. Ecos. Environ. 235, 182–192. Rowland, L., Da Costa, A., Galbraith, D., Oliveira, R., Binks, O., Oliveira, A., Pullen, A., Doughty, C., Metcalfe, D., Vasconcelos, S., 2015. Death from drought in tropical forests is triggered by hydraulics not carbon starvation. Nature 528 (7580), 119–122. Speckman, H.N., Frank, J.M., Bradford, J.B., Miles, B.L., Massman, W.J., Parton, W.J., Ryan, M.G., 2015. Forest ecosystem respiration estimated from eddy covariance and chamber measurements under high turbulence and substantial tree mortality from
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Variation of water uptake in degradation agroforestry shelterbelts on the North China Plain ⁎
Ziqiang Liua, Guodong Jiab, , Xinxiao Yub, a b
T
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Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing, 210037, China Key Laboratory of Soil and Water Conservation and Desertification Combating of Ministry of Education, Beijing Forestry University, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Degradation Water use Water deficiency Isotopic composition
Agroforestry shelterbelts are important ecological barriers providing protection against wind and sand-related disasters in the North China Plain. However, large proportions of these agroforestry shelterbelts are degenerating or have died in recent years and the causes of degeneration are extremely complex. In this study, stable hydrogen and oxygen isotope techniques were used to quantify water sources for P. simonii Carr vegetation in four degrees of degradation (undegraded, U.D; slightly degraded, L.D; moderately degraded, M.D and seriously degraded, S.D), and to investigate how the vegetation can adapt to degradation. Results show that P. simonii Carr exhibited considerable plasticity in depth of water uptake; it prefers to utilize deeper water sources which have higher soil moisture. This vegetation predominantly absorbs water from depths below 160 cm and treatments with a relatively stable and high water content source, are characterized by U.D and L.D. The utilization of deep soil water has gradually decreased, while the utilization rate of surface soil water has gradually increased with an increase in the degree of degradation. Water and nutrient availability in depths of 0–40, 40–80, 80–120 and 120–160 cm were relatively low in M.D and S.D treatments. Therefore, P. simonii Carr degeneration in this region was attributed lack of available water and nutrients. The low precipitation levels and continuous decline in the groundwater level will be accelerating degradation and death of P. simonii Carr shelterbelts.
1. Introduction
on the regional level was mainly related to soil water content (SWC). Water deficiency, however, may be a major constraint on plant growth. As highlighted by Yang et al. (2015), trees have different water-use patterns and physiological responses to different intensities and duration of drought events during their growth. It has previously been shown that several tree species having a dimorphic root system which can adjust their water sources according to SWC (Liu et al., 2018a, 2018b), which an adaptive technique beneficial to overcoming drought events. However, a number of tree species are susceptible to drought or water deficiency as they absorb water from a fixed water source regardless of the water conditions (Moreno-Gutiérrez et al., 2012; David et al., 2013). In these tree species, drought or water shortage will not only affect the carbon assimilation rate of the trees, but also influence nitrogen absorption due to stomatal closure. A number of previous studies have investigated the mechanism of degradation and its influencing factors based on climatic conditions (Pellizzari et al., 2016), tree species distribution, human activities (Dickman et al., 2015) and tree physiology (Sun et al., 2019) in the arid and semi-area region. Sun et al. (2019) compared the water sources of non-dieback and dieback poplar; however, the depth of the water source did not exceed 150 cm even
The structure and function of natural forests have been destroyed by various human activities, which greatly influence the ecological protection function. Drought, wind and soil erosion have seriously restricted the economic and social development of ecologically fragile areas. Thus, countries around the world defend against ecological disasters by building artificial shelterbelts (Borrelli et al., 2016; Chang et al., 2019). In the 1970s, the Chinese government built a large area of artificial agroforestry shelterbelts using Populus simonii Carr in the North China Plain. These agroforestry shelterbelts play an important role in reducing the impact of wind, sand fixation, improving water conservation, and improving the regional microclimate (Qiao et al., 2016). However, large areas of the P. simonii Carr shelterbelts in this region have been extensively degraded or even died in recent years, due to the double impact of global climate change and human activities. Hypotheses for the degeneration and death of the agroforestry shelterbelts have included hydraulic failure, carbon starvation and biological attack (Rowland et al., 2015; Urrutia-Jalabert et al., 2015; Speckman et al., 2015). Previous studies suggest that tree degeneration
⁎
Corresponding authors. E-mail addresses: [email protected] (G. Jia), [email protected] (X. Yu).
https://doi.org/10.1016/j.agee.2019.106697 Received 25 July 2019; Received in revised form 23 September 2019; Accepted 25 September 2019 0167-8809/ © 2019 Elsevier B.V. All rights reserved.
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(392.7 mm) since 1957 (Fig. 3). About 65.4% of precipitation in the study area occurred from June–September. The annual mean temperature fluctuated between 1.9 °C and 5.1 °C, and increased annually (R2 = 0.53, p < 0.01).A large temperature difference occurs between day and night-time in this Bashang High Cold Region, with the perennial average temperature being 4.1 °C. In contrast to the temperature results, wind speed decreased annually (R2 = 0.73, p < 0.01) and fluctuated between 2.8 m s−1–5.8 m s−1. The annual relative humidity ranged from 50% to 66% mm over the last 60 years with a mean relative humidity of 57.1%. The dominant tree species in this region is P. simonii Carr, which planted in the 1970s to reduce the effects of wind and sand related disasters. P. simonii Carr has exhibited different degrees of degradation, with mortality occasionally occurring. Soil in this area is mainly classified as sandy, and soil depth can reach 450 cm.
though the poplar roots can reach 240 cm in depth. Therefore, it is necessary to further explore the water sources related to the degradation of P. simonii Carr and reveal the causes of P. simonii Carr shelterbelt degradation. Precipitation in the North China Plain is relatively low and exhibits obvious seasonal variation. Previous results by Nakayama et al. (2010) have shown that the water table depth in this area has undergone a yearly decrease, an occurrence which may result in the agroforestry shelterbelts experiencing drought conditions over a long period. Currently, it has been noticed that sections of the P. simonii Carr shelterbelts are degraded while other sections in the same area are perceived to be growing undisturbed. Reasons for these differences, and how P. simonii Carr can grow and adapt to drought conditions, are unclear. It is therefore important to understand the causes and factors for the degradation of P. simonii Carr shelterbelts in this area. We hypothesize that: 1) P. simonii Carr with different degrees of degradation have different water sources; and 2) severe shortage of water resources and nutrients in regional areas has led to degradation. To test these hypotheses, we investigated the water sources for P. simonii Carr vegetation in four degrees of degradation (undegraded, U.D; slightly degraded, L.D; moderately degraded, M.D and seriously degraded, S.D). In addition, the SWC and nutrient content of each stratum among the four treatments were compared to identify the cause of degeneration in the agroforestry shelterbelt. The results of this study provide a scientific basis for optimizing the stand structure of shelterbelts, maintaining the stability of ecosystems and maximizing the ecological protection function of shelterbelts.
2.2. Experimental design P. simonii Carr in agroforestry shelterbelts have the same initial planting density (900 trees ha−1) and can be divided into four degrees of degeneration based on growth status and spike top (U.D; L.D; M.D and S.D.). In order to clearly identify causes of degradation, sample surveys and sample collection were undertaken in accordance with the degree of degradation (Fig. 2). The slope gradient and slope position increase slowly with the degree of degradation. Five replicate treatments (20 m × 20 m) with a similar slope and the same slope direction were chosen in each treatment. The distance between the replicate treatments was approximately 60 m. For each treatment, 6–42 trees were identified and had an average age of approximately 30 years. The mean crown density for each treatment was 76.9% (U.D), 56.2% (L.D), 40.7% (M.D) and 19.0% (S.D) (Table 1). Populus simonii Carr growing in the U.D treatments (Fig. 2a) grew well and did not possess a spike on top. No dead or dying specimens were identified in this treatment. The average height and diameter at breast height (DBH) of the trees were 13.8 m were 22.1 cm, respectively, in this treatment. Trees in the L.D treatments had an average height and DBH of 11.2 m and 17.6 cm, respectively. There trees exhibited a slight spike on top (approximately 1 m in length) and had a withering ratio of no more than 10% (Fig. 2b).
2. Materials and methods 2.1. Site description This study was conducted in Zhangbei County (40°57′～41°34 N, 114°10′～115°27′E), northwestern Hebei Province, China (Fig. 1). This area is situated in a middle temperate zone, at an altitude of 1750 m, and experiences a continental monsoon climate. Precipitation in 2017 was 398.4 mm, which was 1.4% higher than the annual average rainfall
Fig. 1. The location of the study site and sample points. U.D–undegraded; L.D–slightly degraded; M.D–moderately degraded; S.D–seriously degraded. 2
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Fig. 2. Characteristic spikes on top of P. simonii Carr trees for the different categories of degradation. a, b, c and d reflect U.D, L.D, M.D and S.D, respectively.
Populus simonii Carr in the M.D treatments had an average tree height and DBH of 10.7 m and 16.8 cm, respectively. These trees exhibited a significant spike on top (2–3 m in length) and had a withering ratio of 10–40% (Fig. 2c). Finally, P. simonii Carr in the S.D treatments had an average tree height and DBH of 8.9 m and 13.7 cm, respectively. These trees exhibited a large spike on top (greater than 3 m) and had a withering ratio greater than 40% (Fig. 2d). Additionally, trees in this treatment had a high mortality. Soil conditions between the treatments were found to differ. The depth of the soil layer in the S.D treatments only attained a depth of 240 cm, after which a solid calcareous layer was present. Soil depth in the other treatments all reached a depth of 400 cm. Soil in the deep layers was mud–shaped and had a high water content.
Table 1 Basic information for the experimental plots. The degree of degradation
Average height (m)
Average DBH (m)
Crown density (%)
Rate of spike top (%)
Length of spike top (m)
U.D L.D M.D S.D
13.8 11.2 10.7 8.9
22.1 17.6 16.8 13.7
76.9 56.2 40.7 19.0
0 ≤10 10∼40 ＞40
0 ≤1 1∼3 ＞3
growing season of P. simonii Carr (Jul.–Aug.). Precipitation, temperature and wind speed data were collected every 15 min (Jan.–Dec., 2017) using a HOBO weather station (U30-NRC, Onset, USA) situated 4.5 km from the treatments. In addition, historical weather data from 1957 to 2017 and groundwater data from 1995 to 2017 were collated for this study.
2.3. Sample collection Soil and branch samples were collected eight times during the peak
Fig. 3. Variation of annual precipitation, temperature, wind speed and relative humidity from 1957 to 2017. The red and black lines indicate the linear regression equations of precipitation and temperature, respectively (a), and wind speed and relative humidity, respectively (b) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
3
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(2) and (3).
For each sampling time, in each treatment, three P. simonii Carr trees having average height and DBH (and not affected by artificial interference) were chosen. From each tree, three mature and suberized branches, having a length of 2–5 cm and a diameter of 0.3–0.5 cm, were selected. Branches from each tree specimen were selected from similar heights and orientation. Branch samples were placed in clean 50 mL polyethylene bottles, sealed with Parafilm M® and frozen at 4 °C. Soil samples were collected in close proximity to the sampled trees using a soil auger with an internal diameter of 3.5 cm. Soil samples were collected at intervals of 0–10, 10–20, 20–40, 40–60…360–380, 380–400 cm in the U.D, L.D and M.D treatments. Soil samples from S.D treatments only reached a depth of 240 cm, after which parent material or a calcic horizon was present. Three replicates were collected for each soil layer and soil samples were divided into three fractions for analysis. One fraction was encapsulated in an aluminum box and dried at 105 °C to calculate SWC. Another fraction was encapsulated in a sealed bag for the determination of soil nutrients, and the final fraction was place into clean 50 mL polyethylene bottles for isotope analysis.
δXT = c1δXS1 + c2δXS2 + c3δXS3 + c4δXS4 + c5δXS5+ c6δXS6+ c7δXS7 (2) c1 + c2 + c3 + c4 + c5 + c6 + c7 = 1
2.4. Investigation of the root system A soil cutting ring (50 cm × 20 cm × 40 cm) was used to investigate the root system of the sampled trees (undegraded). Soil samples at depths of 0–40, 40–80, 80–120, 120–160, 160–200, 200–240 and > 240 cm in the vertical direction, and 0–50, 50–100, 100–150 and 150–200 cm in the horizontal direction, were collected. Roots present in the soil were collected once the soil samples had been passed through a 0.01 mm sieve. Tree roots were washed and divided into size (diameter, d) using the categories of ≤ 0.2, 0.2–0.5, 0.5–1.0, 1.0–2.0, 2.0–5.0 and > 5.0 mm. By using the root system analysis program (WinRHIZO–EC, Canada), root length was then scanned for each diameter level. Total root length was calculated as the sum of root length in each scan.
3.1. The variation of soil water content and groundwater In general, SWC decreased as the degree of degradation increased (Fig. 4). Average SWC of the U.D treatment was 9.32%, which was 1.93 times greater than that of S.D. Although SWC results for the U.D, M.D and S.D treatments showed similar patterns of change, the point of change differed. SWC of U.D increased with soil depth from 0 to 160 cm before decreasing from 160 to 240 cm. After 240 cm, SWC in this treatment then increased and reached a maximum value (22.95%) at 400 cm. SWC for the M.D and S.D treatments increased with soil depth between 0–80 cm and 0–60 cm, respectively. SWC in these treatments peaked at 12.83% (at 80 cm) and 9.00% (at 60 cm) for the M.D and S.D treatments, respectively. SWC of the L.D treatment recorded a downward trend with soil depth from 0 to 120 cm. SWC between 120–400 cm increased, this being similar to the result recorded between 240–400 cm
Isotopic composition of soil and branch samples was conducted in the Laboratory of Eco hydrological Processes and Mechanisms at the Beijing Forestry University. For the branch samples, water samples were extracted using a low temperature vacuum extraction system on samples which had their bark and green sections cut but their xylem retained. The same extraction system was used to extract water from the soil samples. Extracted water samples were then filtered through a 0.22 μm filter to remove impurities. Subsequently, samples were analyzed for δ2H and δ18O using a liquid water isotope analyzer (DLT–100, LGR Inc., USA). Measurement accuracy was 0.3‰ for δ2H and 0.1‰ for δ18O. The isotopic ratio of δ2H and δ18O in the water samples was recorded in parts per thousand of the Vienna Standard Mean Ocean Water, this being express as: ⎜
18
3. Results
2.5. Sample determination
R sample ⎞ δ 2H / δ 18O = ⎛ × 1000‰ R ⎝ s tan dard ⎠
(3)
where δXT is either the δ H or O value in the xylem water. The subscripts S1–S7 are the water source from 0 to 40, 40–80, 80–120, 120–160, 160–200, 200–240 and > 240 cm, respectively. Additionally, c1-c7 represents the contribution ratios of S1–S7 for total xylem water absorption. Statistical analyses were performed using SPSS 16.0. The relationship between annual precipitation/annual mean temperature/annual mean wind speed/annual mean relative humidity and year, and the δ2H and δ18O values of soil water, were analyzed using the ordinary leastsquares linear regression model. The relationship among the proportion of water sources, water uptake depth and SWC were also analyzed using the multiple linear regression model. Two-way ANOVA was used to identify differences in soil water content and isotopic values of soil water and soil nutrients, using the factors of soil stratum and treatments. The LSD method was used to compare soil nutrients when necessary. All data was tested for normal distribution and homogeneity of variance analysis, all of which met the requirements. 2
⎟
(1)
where, Rsample and Rstandard represent the isotope ratio of the samples and the Vienna Standard Mean Ocean Water, respectively. 2.6. Data analysis The Iso-Source model was used to calculate the contribution percentage of water sources for tree species based on the isotopic mass conservation theory (Phillips and Gregg 2003). Soil layers with similar isotopic values were merged into the same water source. The δ2H and δ18O values of the 0–40, 40–80, 80–120, 120–160, 160–200, 200–240 and > 240 cm layers were calculated using the model to reduce experimental error and increase the accuracy of calculation. The equations for calculating the contribution ratio were expressed using Eqs.
Fig. 4. The variations of mean ( ± S.D.) soil water content under different categories of degradation in the study area. 4
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Fig. 5. Variation of water table depth from 1995 to 2017. Fig. 7. The relationship between isotopic compositions in U.D, L.D, M.D and S.D. The global meteoric water line (GMWL) is δ2H = 8.0δ18O + 10.0.
in the U.D treatment. The water table depth in the study area was less than 5 m before 1999, and decreased annually thereafter (R2 = 0.92, p < 0.01). The average descending depth was 0.74 m per year, and the average water table depth was 18 m since 2012 (Fig. 5).
3.3. Characteristics of soil nutrients in different degrees of degradation In general, soil C, N, P and K decreased with soil depth (Fig. 8). Average soil C of the U.D, L.D, M.D and S.D treatments were 11.26 g kg−1, 10.55 g kg−1, 9.67 g kg−1 and 10.94 gkg−1, respectively. Although soil C in the 0–40 cm layer in the U.D and L.D treatments were higher than that in the M.D and S.D treatments, no significant difference were recorded in the 40–80 or 120–160 cm soil layers among the U.D, L.D or M.D treatments (p > 0.05). However, soil C in the 80–120 cm layer of the S.D treatment was significantly higher than that in the L.D or M.D treatment (p < 0.05). Average concentrations of N, P and K in the soil decreased with deceased degradation; average N (31.12 mg kg−1), P (2.26 mg kg−1) and K (86.89 mg kg−1) in the U.D treatments were significantly higher than those recorded of the S.D treatments (15.11 mg kg−1, 1.58 mg kg−1 and 29.47 mg kg−1 for N, P and K, respectively) (p < 0.05). In the 0–40 cm layer, significant differences between soil N, P and K for the L.D and M.D treatments were recorded (p < 0.05). However, no significant differences were recorded for the majority of the other soil layers (p > 0.05).
3.2. Soil hydrogen and oxygen isotope characteristics in different degrees of degradation Average δ2H and δ18O values for the U.D, L.D, M.D and S.D treatments were –91.15‰ and –13.69‰; –79.61‰ and –11.57‰; –75.37‰ and –10.80‰; and –73.55‰ and –9.74‰, respectively, recording an increasing trend (Fig. 6). Although the isotopic values of soil water were distributed on both sides of the local meteoric water line (y = 2.76x –48.89, R2 = 0.41, P < 0.001) (Fig. 7), the variation range of isotopic composition increased as the rate of degradation increased. This indicated that the fractionation effect was more obvious as the rate of degradation increased. Results for δ2H and δ18O average values with soil depth recorded a decrease, a result which was evident in the 0–100 cm depth range. The isotopic composition in this soil layer exhibited a significant decrease compared to the other soil layers (p < 0.05). The δ2H and δ18O values in the 300–400 cm soil layer showed no significant variation among the U.D, L.D and M.D treatments (p > 0.05). However, the isotopic composition of the 150–240 cm depth in S.D recorded a change as great as that recorded in the 0–100 cm soil layer.
3.4. Root system distribution characteristics The root system of P. simonii Carr can spread 240 cm and 200 cm in the vertical and horizontal direction, respectively (Fig. 9). In general,
Fig. 6. The variation characteristics of mean ( ± S.D.) isotopic values of vertical soil profile under different degenerate sites in the study area. a and b are variation characteristics of δ2H and δ18O values in the profile, respectively. 5
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Fig. 8. The variation characteristics of mean ( ± S.D.) soil nutrients in vertical soil profiles under different degrees of degradation. a, b, c and d refer to organic matter, available nitrogen (N), rapidly available phosphorus (P) and available potassium (K), respectively.
decreased and the utilization rate of surface soil water gradually increased as the degree of degradation increased (Fig. 10). In the U.D treatments, P. simonii Carr primarily obtained water from the 200–240 cm and > 240 cm layers, with the average absorption being 28.69% and 35.63%, respectively. In the L.D treatments, water was predominantly absorbed from the 160–200 cm (30.86%) and 200–240 cm (27.1%) layers. In the M.D treatments, P. simonii Carr predominantly obtained water from the 160–200 cm (16.04%) layer, as well as the 80–120 cm (18.12%) and 120–160 cm (21.18%) layers. In the S.D treatments, the main water sources were from the 0–40 (22.84%), 40–80 (20.80%) and 80–120 cm (23.7%) layers; no water was sourced from deeper soil layers.
the root length of 0 < d < 0.2 mm and 0.2 < d < 0.5 mm was significantly longer than that of other diameters (p < 0.05). Roots were also found to increase with horizontal distance and depth (0–120 cm). The majority of roots were distributed in the soil depth from 120 to 200 cm and 0–50 cm horizontally. There was a dominant distribution of root lengths of 0 < d < 0.2 mm and 0.2 < d < 0.5 mm in the depths of 0–20 cm and 20–40 cm, and horizontally from 50 to 100 cm. 3.5. Water sources in different degrees of degradation P. simonii Carr preferred to use deeper water sources which have higher SWC (Fig. 11). The utilization of deep soil water gradually
Fig. 9. Vertical and horizontal root length distribution. The unit of d was mm. 6
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Fig. 10. Mean ( ± S.D.) contribution percentages of the different water sources used by P. simonii Carr under different degrees of degradation. a, b, c and d indicate U.D, L.D, M.D and S.D, respectively.
in the four treatments (p < 0.05). Average SWC decreased from the U.D to S.D treatments (Fig. 4), a decline that may be largely attributed to different soil physical properties and groundwater recharge (Zheng et al., 2016a, 2016b). The SWC of U.D and L.D increased with soil depth; and soil in the 360–400 cm layer was mud–shaped. In the M.D and S.D treatments, SWC recorded a slight decrease with depth; a prominent calcium deposition layer at 380 cm and 220 cm was recorded, and the depth of the soil layer was no more than 400 cm and 240 cm, respectively. These findings suggest that groundwater is the main source of recharge for soil in the U.D and L.D treatments, but not for soils in L.D or S.D treatments. This may be attributed to large-scale farmland flood irrigation techniques used in the M.D and S.D treatments. Flood irrigation not only results in decreased ground water levels, it also leads to soil salinization and the formation of a calcic layer (Wang et al., 2014). Soil salinization results in nutrient loss and causes lower soil nutrients in degraded land (Minasny et al. 2016). The average soil N, P and K concentrations decreased with an increase in the degree of degradation (Fig. 8). This indicated that the M.D and S.D treatments face more serious water stress and nutrient stress than the U.D and L.D treatments. The calcic layer in the S.D treatments was formed due to prolonged deposition of calcium and caused higher hardness. The existence of calcic layers not only seriously affects the downward extension of P. simonii Carr roots, but also influence the absorption of deep soil water and nutrients (Eshel et al., 2014). Zheng et al. (2016a); (2016b) demonstrated that soil hardness in S.D treatments was significantly higher than that of U.D treatments in this area (p < 0.05). The variation of isotopic values in the soil profile also suggested that the water infiltration process or groundwater recharge in the four treatments were different. The isotopic value of soil in U.D and L.D treatments were more negative than those of M.D or S.D (Fig. 7). This indicated that the evaporative fractionation effect of infiltration process in M.D and S.D treatments was more obvious than that of U.D and L.D treatments. In addition, the variation range of isotopic composition and its isotopic values decreased with depth (Fig. 6), indicating that fractionation decreased with depth. These findings are in strong agreement with findings from Wen et al. (2010) and Griffis et al. (2011), who reported that
Fig. 11. The relationship among proportion of water sources, water uptake depth and soil moisture. The regression equation was z = 0.03x + 0.80y + 4.97 (R2 = 0.31, p = 0.015).
4. Discussion 4.1. Variation characteristics of potential water sources Soil water in the study area is predominantly derived from the combination of precipitation and groundwater, and it is affected by radiation, air humidity and soil physical properties (Bosco et al., 2016; Xie et al., 2016; Yu et al., 2018). Annual precipitation in the study area is less than 400 mm and, due to extraction and irrigation of agricultural land, water table depth has decreased in recent years (Fig. 4). This leads to lower average SWC in the study area. Although an equal amount of precipitation with the same isotopic compositions were identified in the sampling points, SWC and its isotopic values are significantly different 7
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strong evaporation effects generally occur in upper soil layers, and that this effect fades with depth. Isotopic values between 300–400 cm for U.D and L.D treatments recorded little change, indicating that water in these soil layers were not affected by evaporation fractionation. The reduction in variation of isotopic values in water in deeper soil layers were probably recharged by groundwater with relatively stable isotopes (Kumar et al., 2008; Stumpp and Hendry, 2012).
indicating that trees in the S.D treatments faced more severe water and nutrient stresses than the U.D treatments (Sun et al., 2019). Extreme drought can also lead to the failure of plant water transmission and water mechanics. Sustained drought and nutrient deficiency will influence vegetation growth, degradation, and even result in death (Grant et al., 2014). Further studies on the physiological factors of plant water sources will provide further insight to issues identified in this study.
4.2. Water uptake for a shelter-forest subjected to degradation
5. Conclusions
Our results indicated that the P. simonii Carr reflected plasticity in regard to sources of water uptake. This phenomenon is similar to findings from other studies reporting that tree species with dimorphic root morphology could tap distinct zones of soil water (Brooks et al., 2010; Nie et al., 2011). The water use pattern of P. simonii Carr was not only related to its root distribution, but also to the soil physical properties. As this tree species has a root system that can spread 240 cm vertically (Fig. 7), it provides a prerequisite for this tree species to transfer its water sources between deep and shallow soil layers. The tree species can obtain more water from 120 to 200 cm since the majority of roots were distributed in these soil layers. In the U.D and L.D treatments, SWC at depths > 160 cm were higher than those < 160 cm (Fig. 4). In addition, SWC at depths < 160 cm were more vulnerable to the external environment than those > 160 cm. P. simonii Carr predominantly obtained water from soil layers > 160 cm (Fig. 10), thus it can have a stable water supply over a long time period (Fig. 11). This finding is in accordance with the findings of Kray et al. (2012), who reported that tree species with deep roots preferred to absorb water from deep soil layers. Fu et al. (2016) demonstrated that Populus trees in the study area have high transpiration rates, thus trees utilizing water from deep soil layers are more suited to meeting transpiration demand since SWC in deep layers is more stable. The SWC in the M.D and S.D treatments were lower than that recorded in the U.D and L.D treatments. In the M.D and S.D treatments, SWC and soil nutrients in layers < 120 cm were significantly higher than that in layers > 120 cm (Fig. 8). P. simonii Carr prefers to absorb more water from shallow soil layers, and the utilization of surface soil water increased with an increase in the degree of degradation. This finding is mainly attributed to two reasons. Firstly, the absorption of surface water can better satisfy transpiration demand and nutrient supply than that of deep water absorption. Secondly, energy consumption used to absorb surface water is lower than that used to absorb deep water. As annual precipitation was lower and unevenly distribution in the study area, this resulted in a long-term water shortage for this species. The water use pattern for P. simonii Carr may be the result of long-term adaptation to its environment. P. simonii Carr prefers to uptake water from soil layers with a higher moisture content, whether it is degenerated or not (Fig. 11). These results are consistent with previous findings (Yang et al., 2015; Liu et al., 2018a, 2018b) that showed that trees can change their water source according to soil moisture. Although P. simonii Carr prioritized absorbing water from surface soil layers that had a relatively higher moisture content in the M.D and S.D treatments, SWC in the surface soil layer varied greatly. The water and nutrient availability are relatively low and unstable, it may be difficult to meet the demands of this vegetation. As the root systems of P. simonii Carr are shown to have high hydrotropism (Xi et al., 2013), the absorption activity of the root system may be limited in conditions of low soil moisture, therefore increasing the difficulty of water absorption. Furthermore, the calcic layer in the M.D and S.D treatments was higher than that in the U.D and L.D treatments.. Water and nutrients available to be transported to the tree canopy may be limited, thus leaves on the canopy will therefore suffer serious water and nutrient loss, resulting in a spiked top. Drought episodes affecting P. simonii Carr were more serious due to the water table depth declining (Fig. 5). Water use efficiency of the degenerated P. simonii Carr was higher than that of trees with undegraded,
Results from this study highlight that the degradation of P. simonii Carr in the North China Plain region is closely related to water sources and available water. Our results show that this species prefers to use deeper water sources which have higher SWC. In the U.D and L.D treatments, P. simonii Carr primarily obtained water from soil layers > 160 cm as the SWC in deep soil layers was higher than that in shallow soil layers. The utilization of deep soil water gradually decreased, while the utilization rate of surface soil water gradually increased with an increase in degree of degradation. In the M.D and S.D treatments, although P. simonii Carr mainly absorbed water from soil layers < 160 cm, available water and nutrients may not be sufficient to meet the demand. The lack of available water and nutrients may therefore be the main reason for the degeneration of P. simonii Carr. The continuous decline in the groundwater level and low precipitation levels in this region has increased the intensity and duration of drought episodes, as well as accelerating degradation and death of P. simonii Carr shelterbelts. The protective benefits of agroforestry shelterbelts can be maintained by replacing P. simonii Carr with other tree species in the future. Acknowledgement This research was funded by National Natural Science Foundation of China (No. 41430747 & No. 41877152), National Science and Technology Support Project (2015BAD07B0302) and Beijing Municipal Education Commission (CEFF-PXM2019_014207_000099). References Borrelli, P., Lugato, E., Montanarella, L., Panagos, P., 2016. A new assessment of soil loss due to wind erosion in European agricultural soils using a quantitative spatially distributed modelling approach. Land Degrad. Dev. 28, 335–344. Bosco, T., Bertiller, M.B., Carrera, A.L., 2016. Combined effects of litter features, UV radiation, and soil water on litter decomposition in denuded areas of the arid Patagonian monte. Plant Soil 406 (1–2), 71–82. Brooks, J.R., Barnard, H.R., Coulombe, R., McDonnell, J.J., 2010. Ecohydrologic separation of water between trees and streams in a Mediterranean climate. Nat.Geosci 3 (2), 100–104. Chang, X.M., Sun, L.B., Yu, X.X., Jia, G.D., Liu, J.K., Liu, Z.Q., Zhu, X.H., Wang, Y.S., 2019. Effect of windbreaks on particle concentrations from agricultural fields under a variety of wind conditions in the farming-pastoral ecotone of northern China. Agric. Ecos. Environ. 281 (6), 16–24. David, T.S., Pinto, C.A., Nadezhdina, N., Kurz-Besson, C., Henriques, M.O., Quilhó, T., Cermak, J., Chaves, M.M., Pereira, J.S., David, J.S., 2013. Root functioning, tree water use and hydraulic redistribution in Quercus suber trees: a modeling approach based on root sap flow. For. Ecol. Manag 307, 136–146. Dickman, L.T., McDowell, N.G., Sevanto, S., Pangle, R.E., Pockman, W.T., 2015. Carbo hydrate dynamics and mortality in a piñon-juniper woodland under three future precipitation scenarios. Plant Cell Environ. 38 (4), 729–739. Eshel, G., Lifschitz, D., Bonfil, D.J., et al., 2014. Carbon exchange in rainfed wheat fields: effects of long-term tillage and fertilization under arid conditions. Agric. Ecosyst. Environ. 195, 112–119. https://doi.org/10.1016/j.agee.2014.05.007. Fu, S., Sun, L., Luo, Y., 2016. Combining sap flow measurements and modelling to assess water needs in an oasis farmland shelterbelt of Populus simonii Carr, in northwest China. Agric. Water Manag. 177 (177), 172–180. Grant, K., Kreyling, J., Dienstbach, L.F.H., Beierkuhnlein, C., Jentsch, A., 2014. Water stress due to increased intra-annual precipitation variability reduced forage yield but raised forage quality of a temperate grassland. Agric. Ecosyst. Environ. 186, 11–22. https://doi.org/10.1016/j.agee.2014.01.013. Griffis, T.J., Wood, J.D., Baker, J.M., et al., 2011. Investigating the source, transport, and isotope composition of water vapor in the planetary boundary layer. Atmos. Chem. Phys. 16, 1–36. Kray, J.A., Cooper, D.J., Sanderson, J.S., 2012. Groundwater use by native plants in response to changes in precipitation in an intermountain basin. J. Arid Environ. 83 (4),
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