Adaption of two grasses to soil thickness variation under different water treatments in a karst region

Acta Ecologica Sinica 37 (2017) 298–306

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Adaption of two grasses to soil thickness variation under different water treatments in a karst region Zhou Li, Jinchun Liu ⁎, Yajie Zhao, Haiyan Song, Qianhui Liang, Jianping Tao Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), Chongqing Key Laboratory of Plant Ecology and Resources Research in Three Gorges Reservoir Region, School of Life Sciences, Southwest University, Chongqing 400715, China

a r t i c l e

i n f o

Article history: Received 23 June 2016 Received in revised form 30 November 2016 Accepted 3 January 2017 Keywords: Karst Precipitation change Soil thickness Morphology Biomass accumulation and allocation

a b s t r a c t Properties of soil in karst regions are discontinuous and highly heterogeneous due to the adverse conditions of exposed rocks, tattered land form, steep slopes, and severe soil erosion. Uneven distribution of karst soil also leads to obvious spatial heterogeneity of moisture. Global precipitation changes might aggravate heterogeneity of soil moisture in soils with different thickness. Thus, exploring the responses of plants to water availability and soil heterogeneity in karst regions is necessary for the understanding of how precipitation changes might affect plant growth in soils with different thickness. Herbaceous plants especially grasses in karst regions are most easily to be affected by soil heterogeneity and water availability, considering they mainly utilize water and nutrients from the surface soil through their fibrous root system. Therefore, two graminaceous perennial grasses, Lolium perenne L. and Festuca arundinacea Schreb. were chosen for the present study. In addition, these two species are often chosen as pioneer plants for ecological restoration and reconstruction in karst regions because of their attributes of fast growth, strong adaptive ability, and high yield, which can effectively promote economic development and help to alleviate rural poverty in the harsh karst region. In our study, three water treatments (CK: 40 ml/day, D1: 20 ml/day and D2: 12 ml/day) were combined with three levels of soil thickness [shallow soil (SS; 5 cm), control (SCK; 15 cm) and deep soil (SD; 30 cm)] in a factorial randomized design and measurements were obtained of above- and below-ground growth, and biomass accumulation and allocation. The following results were obtained: (1) In CK water treatment, the total biomass, above-ground biomass, plant height and leaf area of both species were suppressed in SS as compared with those of SCK, and showed a decline to differing degrees, whereas these parameters were promoted in SD. The root biomass of L. perenne, and root length and surface area of both species in SS and SD were not significantly different to those of plants in SCK. The root biomass of F. arundinacea increased significantly in SS, but the observed values did not differ from those of the control (SCK). The specific root lengths of the two species decreased, and the ratio of root mass increased significantly in SS compared to SCK; but in SD, there was no difference compared to the control (SCK). (2) In D1 and D2 water treatments, there was a decrease in total biomass, above- and below-ground growth and biomass of both species in SS, and as water was reduced, the difference in plant height and leaf area between SS and SCK decreased in both species, and the difference in root length and root surface area between SS and SCK increased. In the SD treatment, apart from an increase in root length of F. arundinacea, there was no significant difference in other parameters of both species compared to SCK. The ratio of root mass of L. perenne in SS was still higher than SCK in D1 and D2 water treatments, but as water availability decreased from D1 to D2, the difference between SS and SCK decreased, and there was no significant difference between SD and SCK. There was no significant difference in the ratio root mass of F. arundinacea in both SS and SD compared to SCK under either D1 or D2 water treatments. The results of this study indicate that when water is sufficient, plant growth is restrained in shallow soil and promoted in deep soil, and as water decreases, plants in shallow and deep soil are both subjected to drought stress with resulting growth suppression, but the drought stress has a greater effect in shallow soil, and the drought conditions induced plant root depth increasing in deep soil. F. arundinacea, with a greater root depth, shows stronger adaptability to deep soil compared to L. perenne. © 2017 Ecological Society of China. Published by Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (J. Liu).

https://doi.org/10.1016/j.chnaes.2017.09.001 1872-2032/© 2017 Ecological Society of China. Published by Elsevier B.V. All rights reserved.

Karst is a distinctive ecological environment system in the geographical environment, with the slow soil forming rate leading to the

Z. Li et al. / Acta Ecologica Sinica 37 (2017) 298–306

congenital lack of soil resources in karst area, and the broken and undulating terrain resulting in discontinuous soil distribution and strong heterogeneity [1]. For karst depression, basins and valleys, the soil is thick and soil distribution is continuous. For hilly area and slopes, the soil is rock soil or missing [2]. Spatial heterogeneity of soil distribution also leads to a high degree of heterogeneity in water distribution [3]. Soil moisture is mainly determined by precipitation and soil water storage, and soil thickness is the key to soil moisture conservation [4]. In many shallow soil regions of karst, the lack of soil mass, poor soil quality and strong permeability lead to poor water storage capacity [5], resulted in karst region-specific karst drought. After sufficient rainfall, the field water holding capacity of soil conservation is only available for plant transpiration demand in 7–14 days [6,7]. Therefore, even in the rainy season, plants are often subjected to drought stress [8]. However, for many deep soil area, strong water storage capacity of the soil layer can be utilized to retain part of the water for plant growth from the evaporation and rock leakage process, and then the drought stress plant suffered can be relieved [9,10]. According to the IPCC forecast, with the global climate changing, rainfall in Chinese karst region (subtropical area) will be reduced, the frequency of rainstorm events will increase, and the interval between precipitations will be extended [11–13]. This prediction has also been confirmed by a lot of domestic scholars [14– 16]. A decrease in precipitation and rainfall frequency may increase the frequency and intensity of karst drought, which would lead to further deterioration of the karst habitat which is highly dependent and sensitive to the external environment, and the discontinuous soil distribution would exacerbate the complexity of the impact of rainfall changes on karst ecosystem. Faced with complex environmental variation, plants have the ability to adjust the allocation pattern, morphology and physiology [17], minimizing the impact of unfavorable factors on themselves [18]. Plant root growth is very sensitive to the size of root proliferation space [19]. The narrow underground space of shallow soil could stimulate roots secreting chemical substances to inhibit the growth, and with more restriction in confined space, the roots' auto-inhibitory effect will get larger [20]. While deep soil can provide more moisture, nutrients and available space for plant root growth, facilitating plant growth and producing more seed output [21]. Furthermore, in arid habitats, plants can adaptively alter the ratio of investment in shoots and roots to increase water and nutrient use efficiency [22,23]. For example, leaf area reduction is considered to be one of the initial responses of seedlings to drought stress [24], and root depth can also reflect the response of plants to arid habitats. In addition, plants can make compensation for soil moisture deficiencies by increasing root depth [25]. At present, harsh karst habitat and fragile ecosystems have received wide attention from academia, and it is generally accepted that water is the most critical limiting factor in the recovery process of karst ecosystem. Accordingly, a large number of studies on adaptability of karst plants to drought stress have been carried out. However, the root cause of karst drought is the lack of soil resources in karst regions. The collective effect of karst drought and the lack of soil resources (thickness) has a serious influence on plant growth, reproduction and distribution in the region. At present, the adaptability of plant to soil resources (thickness) under karst drought is seldom studied. Lolium perenne L. and Festuca arundinacea Schreb. are perennial grass of Poaceae family with shallow roots. They mainly utilize water and nutrients from the surface soil through their fibrous root system, unlike shrubs and trees with deeper roots, which can root deeply into the interstices and utilize the moisture and nutrients in the deep interstitial rocks [3]. Therefore, the growth of herbaceous plants especially grasses in karst area is most susceptible to the effects of soil thickness. In addition, both L. perenne and F. arundinacea have strong adaptability and can respond to environmental changes quickly, and they can grow well in karst harsh terrain and have a high yield, so they are widely used in karst ecological restoration [26,27].

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Therefore, we chose these two species to study their responses to soil thickness under different water treatments by pot experiment to verify the following hypotheses: (1) Under well-watered conditions, the two species grow well in deep soil, while if soil thickness decreases, plant growth in shallow soil may be inhibited due to the restriction of nutrient and space; (2) When water is scarce, growth of the two plants is inhibited in different thickness of soil, but in deep soil, the drought conditions would induce the increase of plant root depth to obtain more water and nutrients, thus mitigating the negative impact of drought and the decrease of soil resources on plants. 2. Materials and methods 2.1. Test materials Lolium perenne L. and Festuca arundinacea Schreb. were used as the experimental materials. Experimental soil was yellow limestone soil, taken from Zhongliang mountain located in Shapingba, Chongqing, China. The physicochemical properties were as follows: pH was 7.4 ± 0.14, content of organic matter was 0.34 ± 0.02%, total nitrogen was 0.28 ± 0.03 g/kg, total phosphorus was 0.39 ± 0.02 g/kg and total potassium was 23.7 ± 3.22 g/kg. 2.2. Experiment design Three levels of soil thickness were set by three self-made rectangular containers with the same basal area and different heights. Five holes were drilled on the bottom of each container which could make the excess water out. Because L. perenne and F. arundinacea are with fibrous root system, their roots are mainly distributed in the topsoil layer within 15 cm [28]. Therefore, we defined soil thickness of 15 cm as control (SCK), 5 cm as shallow soil (SS) and 30 cm as deep soil (SD). Three containers had a same basal area of 0.01 m2, filled with 500 g, 1500 g and 3000 g of dry soil, respectively. On January 14, 2015, seeds of F. arundinacea and L. perenne were sowed in the ecological garden of Southwest University. On April 4, 2015, we selected the two species' seedlings with the uniform growth, and transferred them to the designed containers. Each container had two plants. All containers were placed and managed with same light condition in the ecological garden of Southwest University with the altitude being 245 m. After planting, keep the soil moist. Until all seedlings survived and adapted to grow after a period of time, we randomly selected five containers to measure the initial values of plant growth parameters. From each container, one plant was selected. On April 14, 2015, three water treatments were set. The water treatment levels were determined based on the monthly rainfall (119.58 mm/m2) from April to June over 30 years from 1981 to 2011 in Chongqing area. According to the basal area of the containers (0.01 m2), we calculated the average daily rainfall per 0.01 m2 area is 40 ml, and the height is 4 mm. We set 40 ml daily rainfall as the control (CK), while 20 ml (50% reduction) was water reduction group 1 (D1), 12 ml (70% reduction) was the water reduction group 2 (D2). Each container was watered once every three days and CK, D1 and D2 were watered 120 ml, 60 ml, and 30 ml every time, respectively. In addition, for each watering and soil thickness condition, three blank controls were set without plant independently. The same water treatments were performed synchronously with the above experiment to measure the soil water content by weighting method. The specific process of soil sample collection is as follows: before every water treatment, we collected mixed soil sample by five-point sampling with different soil depths on the four corners of the container and at an intermediate locations, put the mixed soil sample into an aluminum box, and took them back to lab. Soil moisture storage was calculated

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as soil gravimetric water contents multiply soil bulk density, and multiply soil volume. 2.3. Parameter measurement After 22 times (69 days) of water treatment, all plants were harvested under flowing water. The following parameters were measured. (1) Morphological parameters: Plant height and root length were measured with a ruler. The complete images of leaf and roots were obtained by using a digital scanner (STD1600Epson USA) and then leaf area, total root length, root surface area were quantitatively analyzed by Win.Rhizo (Version 410B, Regent Instrument Inc., Canada). (2) Biomass: After scanning, the root, stem, and leaf were put into kraft bags. All fresh plant parts were dried at 105 °C for 15 min to de-enzyme in oven, and then dried at 80 °C to constant weight and all parts' dry weights were weighed. Based on the values of leaf biomass, stem biomass and root biomass, we calculated above-ground biomass and total biomass. 2.4. Parameter calculation (1) Root mass ratio (RMR) was calculated as root mass divided by plant total biomass. (2) Specific root length (SRL) was calculated as total root length divided by root biomass. 2.5. Statistical analysis All data processing and statistical analysis were conducted using SPSS 17.0 and Microsoft Office Excel 2007. The effects of water, soil thickness and their interactions on soil water regime, plant morphology, biomass accumulation and allocation were evaluated by Two-way ANOVA. The difference between the different thicknesses of soil for each species under the same water treatment was analyzed by using One-way ANOVA. Significant difference at 0.05 level (P b 0.05) was determined. All figures were produced using Origin8.6. 3. Results 3.1. Soil water regime In CK water treatment, the soil moisture contents of shallow soil group (SS) and control group (SCK) were 21.3% and 20.28%, respectively, which belonged to slight humid habitat; while deep soil group (SD, 17.03%) was within the appropriate range [29]. The soil moisture content of SS was not significantly different with SCK, but for both SS and SCK, soil moisture contents were significantly higher than that in SD (Table 1). In D1 water treatment, the soil moisture contents of three thickness of soil were between 12.82% and 14.46%, all belonging to light drought level. However, the soil moisture content of SS was significantly higher than those in SCK and SD. In D2 water treatment, the soil moisture contents of three thicknesses of soil were between 7.29% and 9.05%, all belonging to middle drought level. However, the soil moisture contents of SCK and SD were significantly higher than that in SS. In

addition, in the three water treatments, the soil water storage of SS was significantly lower than SCK, while SD was significantly higher than SCK. Two-way ANOVA indicated that significant water × soil thickness interaction on soil moisture content and soil water storage were found (Table 2). 3.2. Biomass accumulation In CK water treatment, the above-ground biomass and total biomass of L. perenne and F. arundinacea decreased in shallow soil (SS) in different degree compared with the control (SCK). The root biomass of L. perenne in SS was not significantly different to those of plants in SCK, while the root biomass of F. arundinacea increased 73.7% significantly compared to SCK (Fig. 1). In D1 water treatment, the above-ground biomass and total biomass of both species were significantly lower in SS than in SCK. The root biomass of L. perenne in SS was not significantly different to those of plants in SCK, while the root biomass of F. arundinacea decreased 34.8% significantly compared to SCK. In D2 water treatment, the above-ground biomass, root biomass and total biomass of both species were significantly lower in SS compared to SCK. For deep soil (SD), in CK water treatment, compared with SCK, the above-ground and total biomass of L. perenne increased significantly by 60.6% and 48.9%, respectively. The above-ground biomass and total biomass of F. arundinacea and root biomass of both species in SD were not significantly different to those of plants in SCK. No significant difference was found in aboveground biomass, root biomass and total biomass of two species between SD and SCK under D1 and D2 water treatments. Two-way ANOVA indicated that significant water × soil thickness interaction on aboveground and total biomass of two species, and root biomass of F. arundinacea were observed (Table 3). 3.3. Biomass allocation In CK water treatment, the root mass ratios (RMR) of both species were significantly higher in SS than in SCK, and there was no difference between SD and SCK (Fig. 2). In D1 water treatment, the RMR of L. perenne was still significantly higher in SS than in SCK, but for F. arundinacea, there was no difference between SS and SCK. No significant difference was found in RMR of both species between SD and SCK. In D2 water treatment, the RMR of L. perenne in SS and SD were not significantly different with SCK, but the RMR of L. perenne in SS was significantly higher than that in SD. No significant difference was observed in RMR of F. arundinacea among three thicknesses of soil. Two-way ANOVA indicated that significant soil thickness × water interaction on RMR of F. arundinacea was found, but no significant water × soil thickness interaction was observed in L. perenne (Table 3). 3.4. Plant morphology characteristics 3.4.1. Root morphology characteristics In CK water treatment, compared with the control (SCK), the specific root length (SRL) of L. perenne and F. arundinacea decreased significantly by 29.4% and 49.1% in shallow soil (SS), but the root length and root surface area of both species were not significantly different from those in SCK (Fig. 3). In D1 water treatment, the root lengths of L. perenne and F. arundinacea in SS decreased by 22.4% and 17% respectively compared

Table 1 The soil water regime of different thickness of soil under different water treatments (M ± SD). Water treatment

CK D1 D2

Soil moisture content (%)

Soil water storage (g)

SS

SCK

SD

SS

SCK

SD

21.3 ± 1.16a 14.46 ± 0.95a 7.29 ± 9.05b

20.28 ± 0.4a 13.89 ± 0.55b 9.05 ± 0.41a

17.03 ± 0.88b 12.82 ± 0.37b 9.03 ± 0.63a

136.33 ± 0.77c 123.01 ± 12.92c 46.63 ± 1.02c

389.34 ± 13.45b 264.95 ± 10.8b 173.8 ± 2.89b

654.14 ± 7.42a 492.35 ± 37.83a 346.65 ± 26.15a

Note: different lower letters indicate significant difference between the soil thickness under same water treatment, at P b 0.05 level.

Z. Li et al. / Acta Ecologica Sinica 37 (2017) 298–306 Table 2 Results of Two-way ANOVA test for the effects of soil thickness and water treatment on soil moisture content and storage. Sources of variation

F-value Soil thickness

Water treatment

Water treatment ∗ soil thickness

df Soil moisture content Soil water storage

2 9.31⁎⁎

2 131.54⁎⁎⁎

4 7.87⁎⁎

401.54⁎⁎⁎

106.47⁎⁎⁎

10.92⁎⁎⁎

Note: significance levels are: ns, not significant (P N 0.05); df, degrees of freedom. ⁎⁎ P b 0.01. ⁎⁎⁎ P b 0.001.

with SCK. No significant difference was found in two species' root surface area and SRL between SS and SCK. In D2 water treatment, the root length and root surface area of both species in SS decreased significantly compared to that in SCK. The SRL of L. perenne in SS was not significantly different from SCK, but for F. arundinacea, the SRL increased significantly. For deep soil (SD), in CK and D1 water treatments, no significant difference was found in root length, root surface area and SRL between in SD

Total biomass(g)

1.8

3.4.2. Above-ground morphology characteristics In CK water treatment, the plant height and leaf area of both species decreased significantly in shallow soil (SS) compared with the control (SCK) (Fig. 4). In D1 water treatment, compared with SCK, the leaf area of L. perenne and F. arundinacea in SS decreased by 64.5% and 53%, respectively, but there was no significant difference in plant height of both species between in SS and SCK. In D2 water treatment, compared to SCK, the leaf area of L. perenne in SS decreased significantly by 65.5%. No significant difference was found in the leaf area of F. arundinacea and the plant height of both species between in SS and SCK. In CK water treatment, the plant height of L. perenne and F. arundinacea and leaf area of L. perenne in SD increased significantly by 27.6%, 12.2% and 58.9%, respectively, and there was no significant difference in leaf area

a

1.2

a

b c

B

SS SCK SD

a ab

ab a

0.6

and in SCK. In D2 water treatment, the root surface area of L. perenne decreased significantly by 30%, while the root length of F. arundinacea increased by 21.7% in SD compared with the control. And there was no significant difference in any other root growth parameter of both species between in SD and in SCK. Two-way ANOVA indicated that significant water × soil thickness interaction on root surface area of L. perenne and SRL of F. arundinacea was found (Table 4).

2.2

A

301

a

a ab

1.1

b

a a

b

b

b b 0.0

0.0 CK

Above-ground biomass(g)

1.5

D1

CK

D2 1.8

C

1.0

a

ab

b a

a a

b

b

a

b 0.0

CK 0.32

ab 0.6

b

a

a

0.5

c

a

1.2

D2

a

a

0.0 D1

D2

CK 0.45

E a

Root biomass(g)

D

D1

a

a

D1

D2

F a

0.24

a

a

0.30

a

a 0.16

b

a

b

a

a ab

b 0.15

b

b

0.08

a

0.00

0.00 CK

D1

D2

CK

D1

Water treatment

Water treatment

L.perenne

F.arundinacea

D2

Fig. 1. The biomass accumulation of L. perenne and F. arundinacea under different water and soil thickness treatments (M ± SD). Note: Different lowercase letters above bars indicate significant difference at 0.05 level between the soil thickness under same water treatment. The same following Figures.

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Table 3 Results of Two-way ANOVA test for the effects of soil thickness and water treatment on biomass accumulation and allocation of L. perenne and F. arundinacea. Species

Source of variation

df

(L. perenne)

Soil thickness Water Soil thickness ∗ water Soil thickness Water Soil thickness ∗ water

2 2 4 2 2 4

(F. arundinacea)

F-value Total biomass

Above-ground biomass

Root biomass

Root mass ratio

46.66⁎⁎⁎ 18.58⁎⁎⁎ 4.92⁎⁎ 9.01⁎⁎ 13.43⁎⁎⁎ 2.80⁎

87.08⁎⁎⁎ 6.40⁎⁎ 4.30⁎⁎ 8.81⁎⁎ 9.92⁎⁎⁎ 3.08⁎

ns 22.38⁎⁎⁎ ns ns 9.72⁎⁎⁎ 5.37⁎⁎⁎

30.37⁎⁎⁎ 5.06⁎ ns 3.77⁎ ns 3.62⁎

Note: significance levels are: ns, not significant (P N 0.05); df, degrees of freedom. ⁎ P b 0.05. ⁎⁎ P b 0.01. ⁎⁎⁎ P b 0.001.

of F. arundinacea between in SD and SCK. In D1 and D2 water treatments, no significant difference was found in plant height and leaf area of both species between in SD and SCK. Two-way ANOVA indicated that significant water × soil thickness interaction on leaf area of both species and plant height of L. perenne was observed (Table 4). 4. Discussion Soil thickness is considered to be a complex factor, which may exert its influence on plants growth and distribution through the availability of space, nutrition and water [19,21,30]. Thicker soil has a larger soil volume, and can store more moisture and nutrients and provide greater space available for plants root system growth, which is helpful for plant growth and dry matter accumulation. The present study indicated that with the increase of soil thickness, soil moisture content showed an upward trend [31], and plant height, leaf area, above-ground biomass and total biomass of plant also increased [31–33]. However, under different water conditions, soil moisture in different thicknesses of soil cannot always follow this rule, especially in karst area with numerous small-scale habitats. In some concave regions with very limited soil, excessive rainfall cannot form surface runoff in time, which may make the area be flooded, becoming a temporary water-wet habitat [3]. In CK water treatment of this study, with the increase of soil thickness, soil water content decreased significantly. Since the external water supply exceeded the storage capacity of the shallow soil (SS), it became a water-wet habitat. The soil water content of the control group (SCK) also slightly exceeded the upper limit of soil moisture for suitable plant growth, while the deep soil (SD) effectively accommodated the

70

A

external water supply through larger soil volume, and its soil moisture condition was suitable. In D1 water treatment, due to the reduction of external water supply, soils with three thicknesses all became light dry habitats. Compared with SCK, unit soil volume got more water in SS. Although the soil transpiration in SS was strong, the external water supply was continuous, which resulted in that soil water content of SS was still significantly higher than that of SCK. And there was no significant difference in soil water content between SD and SCK. As water further reduced, soil transpiration led to the rapid loss of limited water in SS. Whereas per unit volume of soil got less moisture in SCK and SD, but the deep soil layer with weaker transpiration was able to retain moisture effectively. Therefore, the soil water content of SCK and SD were significantly higher than SS under D2 water treatment. In addition to soil texture and structure, the water storage capacity of soil is largely dependent on soil volume. In the present study, due to the disparity of the soil volume of three thickness soil, the soil water storage showed a significant increasing trend with the increase of soil thickness under three water treatments. In CK treatment, root morphological parameters and biomass of two species in SS were not inhibited significantly compared with SCK, which did not match the assumptions mentioned previously. Despite SS had a higher soil moisture content, but the soil water content and nutrient availability were very limited, which would make the water and nutrient conditions of SS not fully meet the growth demand of two species. Although the underground space of SS was small, the plant still expanded the absorption range by increasing the root depth and root surface area to increase the contact area with soil [34], which was conducive to plants roots absorbing more water and nutrients for plant growth

50

SS SCK SD

B a

Root mass ratio(%)

a a

a 35

a

a b

b

a

25

b b

a

ab

0

b

b

a

b

0 CK

D1

D2

CK

D1

Water treatment

Water treatment

L.perenne

F.arundinacea

D2

Fig. 2. The root mass ratio of L. perenne and F. arundinacea under different water and soil thickness treatments (M ± SD).

a

Z. Li et al. / Acta Ecologica Sinica 37 (2017) 298–306

45

60

A

Root length(cm)

ab a 30

a

b

a

a

a

15

a

400

a a

a b c

20

0 CK

Root surface area(cm2)

a

b

0

C

D1

D2

CK 600

a

D

a

a

400

a

a

D1

a

a

a

a

a

a

a

a

b 200

D2

a

a

b

200

c 0

0 CK

300

Specific root length(m/g)

a 40

b

SS SCK SD

B

b

600

303

D1

D2

CK 250

E a

a

a 200

ab

b

a

200

100

a a a

150

100

D2

a a

a

a

a

F

D1

a

b

D1

D2

ab

b

50

0

0 CK

D1

D2

CK

Water treatment

Water treatment

L.perenne

F.arundinacea

Fig. 3. The root morphology characteristics of L. perenne and F. arundinacea under different water and soil thickness treatments (M ± SD).

in the limited soil resources. At the same time, because of the great disparity of soil resources, the differences in water and nutrient supply made the plant height, leaf area and above-ground biomass of two species in SS significantly lower than those in SCK. In addition, the two species invested more energy in roots during their growth in SS, which would also make the above-ground growth of two species limited in

SS. The ratio of root length is considered to be related to soil resource availability [35], and its value could indicate the degree of root physiological activity [36]. In the current research, compared with SCK, the specific root length of two species in SS decreased significantly, indicating that plants had to weigh the absorption efficiency of water and nutrients in nutritional poor barren water environment. In the early period

Table 4 Results of Two-way ANOVA test for the effects of soil thickness and water treatment on plant morphology of L. perenne and F. arundinacea. Species

Sources of variation

df

(L. perenne)

Soil thickness Water Soil thickness ∗ water Soil thickness Water Soil thickness ∗ water

2 2 4 2 2 4

(F. arundinacea)

F-value Root length

Root surface area

Specific root length

Plant height

Leaf area

17.46⁎⁎⁎ 3.21⁎

8.9⁎⁎⁎ 23.49⁎⁎⁎ 4.26⁎⁎ 4.94⁎ 6.44⁎⁎

6.23⁎⁎ ns ns 7.55⁎⁎

9.93⁎⁎⁎ ns 3.32⁎

93.05⁎⁎⁎ 12.05⁎⁎⁎ 3.07⁎ 28.91⁎⁎⁎ 14.82⁎⁎⁎ 4.88⁎⁎

ns 3.42⁎ 9.13⁎⁎⁎ ns

Note: significance levels are: ns, not significant (P N 0.05); df, degrees of freedom. ⁎ P b 0.05. ⁎⁎ P b 0.01. ⁎⁎⁎ P b 0.001.

ns

ns 8.02⁎⁎⁎

ns ns ns

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Z. Li et al. / Acta Ecologica Sinica 37 (2017) 298–306

Plant height(cm)

50

75

A

a

40

a

b

a

c

a

a

c

0 CK

D1

CK

a a

210

D2 350

C

280

D

a

D1

a a

210

b a

a

D2

a

280

140

a a

140

b

b

c

a

a

b 70

a

15

0

Leaf area(cm2)

b

45 30

10

350

ab

a

a 20

SS SCK SD

a

a b

60

a

a

30

B

70

b

0

0 CK

D1

D2

CK

D1

Water treatment

Water treatment

L.perenne

F.arundinacea

D2

Fig. 4. The plant height and leaf area of L. perenne and F. arundinacea under different water and soil thickness treatments (M ± SD).

of roots growth and development, wet soil environment stimulated root system to decrease the root physiological activity by the low specific root length. While in the subsequent roots growth and development process, the lack of water and nutrient resources gradually revealing, at this point, plants were mainly through adjusting root length and root surface area to increase absorption range and area, thereby improving their own nutrient uptake efficiency. In CK water treatment, soil water content was significantly lower in SD than that in SCK, but the larger soil volume of SD could provide more nutrients, space and water for plants growth. The results of this experiment indicated that there were no significant differences in root length, root volume, specific root length or root biomass of two species between SD and SCK, probably because the soil conditions provided by SCK for plants root system were capable to meet the requirements of normal growth of two species. Wang [37] also thought that within a certain range, plant growth and biomass accumulation increase significantly with soil thickness, but when a threshold is reached, the increase may gradually flatten. Therefore, in CK water treatment, the root growth of two species in SD cannot be promoted significantly compared to SCK. Additionally, the two species had similar biomass allocation strategy in SD and SCK, which also indicated that SCK and SD provided similar soil water conditions for two species. Good soil moisture conditions allowed plants to develop a small amount of root to get enough water for plants growth and transpiration, so the two species both reduced their investment in the roots, and more energy allocated to the above-ground, increasing photosynthetic area, and promoting the accumulation of organic matter. However, compared with SCK, SD provided more nutrients for plants growth, promoting the plant height and leaf area of both species and the above-ground biomass and total biomass of L. perenne. In D1 water treatment, the total biomass of L. perenne and F. arundinacea in SS were significantly lower than that in SCK, which indicated that the growth of two species was inhibited in SS. In SS, narrow

soil space and the serious shortage of water and nutrient limited plants root growth to a certain extent [19], the root length and root biomass of two species were significantly lower than SCK. Secondly, compared with SCK, the limited soil resource in SS increased the difficulty of obtaining nutrition, resulting in that two species still had higher root investment in SS than SCK. While the root growth of two kinds of plant were inhibited in SS, the reduction of nutrient and water uptake would inevitably affect the normal growth activities of plants leaves, resulting in the decline of photosynthesis and the reduction of photosynthate production, and the accumulation of biomass was inhibited obviously, which reduced above-ground biomass accumulation, but in turn limited the growth and biomass accumulation of plants roots. In D1 water treatment, there was no significant difference in the growth and biomass accumulation of each organ of two species between SD and SCK , which confirmed the point previously mentioned that there was a threshold value for the promotion effect of soil thickness. When soil thickness was no longer a limiting factor for plant growth, in other words, when the saturation point was reached, the continuous increase of soil thickness could not promote plants growth any longer. In D2 water treatment, although SS and SCK belonged to moderate arid habitat, soil water storage was significantly lower in SS than that in SCK. In other words, the drought stress plants suffered in SS was more serious. When plants are subjected to mild drought stress, they would alleviate the drought stress by increasing the investment on roots [38], while under severe drought stress, roots are the first vanguard organ to receive water stress, and their growth and regeneration are affected firstly, the biomass of plants is preferentially distributed to stems and leaves and the ratio of root mass decreases with stress [39]. In this work, the root mass ratio of F. arundinacea in SCK increased significantly with the decrease of water, and reached the maximum in D2 water treatment, indicating that F. arundinacea was subjected to mild

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drought stress during this process, and relieved the effects of drought on its growth by altering the root investment ratio to obtain more moisture. While compared to the F. arundinacea, L. perenne was not so sensitive to drought stress, and it was not able to adjust the biomass allocation in time to adapt to drought habitats, and the root mass ratio was hardly influenced by the water treatment [40]. However, the root mass ratio of two species decreased gradually with the decrease of water in SS and reached the minimum value under D2 water treatment, which indicated that two species were subjected to severe drought stress in D2 water treatment, root growth was severely damaged, and the biomass of plants was distributed to stems and leaves preferentially. Two species' plant height, the leaf area and shoot biomass of F. arundinacea were not significantly different from SCK, but the root length, root surface area and root biomass of two species were significantly decreased. Studies have shown that the root with greater specific root length had higher N content, respiration rate and root activity [36]. Wang [41] found that under severe drought stress, Betula maximowicziana Regel. was with greater specific root length, showing that drought stress led to an increase in root activity. In the D2 water treatment of this study, the specific root length of F. arundinacea in SS was significantly higher than that in SCK, indicating that F. arundinacea had higher root physiological activity in SS. Although L. perenne had no significant difference in specific root length between SS and SCK, with the gradual reduction of external water supply, the specific root length of L. perenne increased gradually in SS, and the difference between SS and SCK gradually decreased. Therefore, in SS two species both actively enhanced the absorption efficiency of nutrients and water to adapt to the drought arid harsh environment in SS. In D2 water treatment, SCK and SD belonged to the middle drought habitat, and the soil effective water content was at a lower level. Although there were some differences in soil resources and volume between SD and SCK, the external water supply was the key factor to determine the soil water content of two thickness of soil because of the extreme shortage of external water supply, which caused that there was no difference in soil water content between SD and SCK. Compared with SCK, SD provided more nutrition, space and water storage for plants, but it was not easy for plants to acquire water and nutrient in SD, since water was mostly distributed in deeper soil [10], and in arid soil, plants need to overcome large friction forces to expand their roots. At the same time, the soil water status would directly affect plant uptaking soil nutrients [42], so the advantage of habitat conditions in SD was not obvious compared to SCK. Despite all mentioned above, two species' root length increased to different extents in SD, but the increase of root length of L. perenne was not significant, which would be related to the difference in root architecture between two kinds of plant. Although both species were with fibrous root systems, F. arundinacea has a greater root depth than L. perenne [43,44], that would be conducive to its roots to a deeper level of soil. Therefore, it could be concluded that under the drought stress the larger soil thickness and underground space in SD could effectively induced the increase of the root depth of two species, which was consistent with the hypothesis mentioned previously. The two species suffered more serious drought stress in SD, compared with SCK. The root mass ratio of F. arundinacea gradually increased with soil thickness increase in D2 water treatment, and the above-ground growth and biomass accumulation of F. arundinacea in SD were slightly lower than in SCK, which would be attributed to the strong drought resistance of F. arundinacea [45]. In high-intensity drought stress habitats, it was still possible to reduce the inhibition of water deficit by actively adjusting the biomass allocation. The specific root mass ratio of L. perenne had shown a decline in SD, indicating that the L. perenne was subject to severe drought stress in SD, the root growth was seriously damaged, and plants allocated more biomass to shoots. The results also showed that the root surface area of L. perenne significantly decreased and the root biomass was lower than SCK, but there was no significant difference in above-ground growth and biomass accumulation between SD and SCK.

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5. Conclusion 1. In CK water treatment, the soil water content of SS was significantly higher than that of SCK, belonging to wet habitat. Two species had more energy to invest in the roots, and improved their absorption efficiency of water and nutrients by developing large root system, which resulted in that the root growth of two species in SS was not significantly different from that in SCK, but the above-ground growth and total biomass were inhibited. The SD and SCK both had sufficient soil moisture, nutrition resources and space, therefore, in SD two species only need to develop a small amount of root to obtain enough water for growth and transpiration, so both species reduced their investment in roots, and more energy was allocated to shoots, which promoted the growth of shoots. 2. Consistent with the hypothesis, under the water deficiency conditions, plants were subject to triple stresses of water, nutrition and space in SS, and the root and shoot growth of both grasses were seriously inhibited; while in SD, water reduction induced the increase of plants root depth to obtain more moisture and nutrients, thus reducing the effects of the dual stress of drought and soil resources reduction on the growth of the two kinds of grasses. 3. Although the two species under test are both with fibrous root system, F. arundinacea has larger root depth. As a result, no matter under water sufficient or insufficient conditions, F. arundinacea showed a stronger adaptability for deep soil than L. perenne. Acknowledgements This study was financially supported by grants from the National Natural Science Foundation of China (31500399), the Natural Science Foundation of Chongqing (cstc2014jcyjA80016) to J.C. Liu and Ministry of Education 49th batch of returned students to start research fund research projects. References [1] F.D. Fan, K.L. Wang, Y. Xiong, Y. Xuan, W. Zhang, Y.M. Yue, Assessment and spatial distribution of water and soil loss in karst regions, southwest China, Acta Ecol. Sin. 31 (21) (2011) 6353–6362. [2] M.D. Yang, On the fragility of karst environment, Yunnan Geogr. Environ. Res. 2 (1) (1990) 21–29. [3] K. Guo, C.C. Liu, M. Dong, Ecological adaptation of plants and control of rocky-desertification on karst region of Southwest China, Chin. J. Plant Ecol. 35 (10) (2011) 991–999. [4] C.H. Wadleigh, Soil science in relation to water resources development. III. Soil moisture conservation, Soil Sci. Soc. Am. J. 33 (1969) 480–482. [5] S.J. Wang, Concept deduction and its connotation of karst rocky desertification, Carsol. Sin. 21 (2) (2002) 101–105. [6] Y.C. Zhou, X.G. Pan, Adaptation and adjustment of Maolan forest ecosystem to karst environment, Carsol. Sin. 20 (1) (2001) 47–52. [7] A.D. Li, Y.F. Lu, X.L. Wei, L.F. Yu, Studies on the regime of soil moisture under different microhabitats in Huajiang karst valley, Carsol. Sin. 27 (1) (2008) 56–62. [8] H.S. Chen, K.L. Wang, Characteristics of Karst Drought and Its Countermeasures in Hunan Agricultural System Engineering Society Executive Council and Youth Academic Committee Meeting, Hunan Provincial Conference on Agricultural Systems Engineering, Changsha, 2004. [9] J. Chen, Z.H. Shi, L. Li, X. Luo, Effects of soil thickness on spatiotemporal pattern of soil moisture in catchment level, Chin. J. Appl. Ecol. 20 (7) (2009) 1565–1570. [10] C.P. Zhang, Influence of Nitrogen or Phosphorous on Water Metabolism of Juglans regia Seedlings, Chinese Academy of Forestry, Beijing, 2014. [11] W.J. Shen, J.F. Reynolds, D. Hui, Responses of dryland soil respiration and soil carbon pool size to abrupt vs. gradual and individual vs. combined changes in soil temperature, precipitation, and atmospheric [CO2]: a simulation analysis, Glob. Chang. Biol. 15 (9) (2009) 2274–2294. [12] IPCC, Climate Change 2007: The Physical Science Basis, Cambridge University Press, Cambridge, United Kingdom, 2007. [13] S.H. Wu, Z.C. Zhao, Updated understanding of climate change and water, Adv. Clim. Chang. Res. 5 (3) (2009) 125–133. [14] H. Zhou, B.G. Yang, B.Y. Chen, Analysis of characteristics of climate change over last 46 years in Chongqing, Chin. J. Agrometeorol. 29 (1) (2008) 23–27. [15] Y.W. Yin, Analysis of Climate Change Characteristics Over Southwest China Under the Background of Global Warming, Lanzhou University, Lanzhou, 2010. [16] Q. Zhang, Y.Q. Li, Climatic variation of rainfall and rain day in Southwest China for last 48 years, Plateau Meteorol. 33 (2) (2014) 372–383.

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