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Procedía
ELSEVIER
Environmental Sciences
Procedía Environmental Sciences 13 (2012) 967 - 981
The 18th Biennial Conference of International Society for Ecological Modelling
The impacts of grassland vegetation degradation on soil hydrological and ecological effects in the source region of the Yellow River-- A case study in Junmuchang region of Maqin country
a Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 11A, Datun Road, Chaoyang District, Beijing 100101, China; b Graduate University of Chinese Academy of Sciences, 11A, Datun Road, Chaoyang District, Beijing 100049, China; c School of Geography, Beijing Normal University, NO.19, Xinjiekouwai Street, Haidian District, Beijing 100875, China; dLaboratory of Regional Geography, Beijing Normal University, NO.19, Xinjiekouwai Street, Haidian
District,Beijing 100875, China
As one of the special structure layers of grassland ecosystem, soil has significant hydrological and ecological effects. However, the soil interior hydrological and ecological effects will be affected by the grassland vegetation degradation. This research was carried out in the source region of the Yellow River, where grassland vegetation was in severe degradation, with the methods of choosing typical areas and quadrates to collect soil samples and doing experiments in laboratory. Some important results were obtained from this research which mainly contained four aspects as follows. (1) With the increasing of grassland degradation degrees, the capillary water holding capacity and saturated water content decreased in all soil layers as a whole. However, the capillary water holding capacity and saturated water content increased when the grassland degradation from middle degree to heavy degree in all soil layers. (2) With the increasing of grassland degradation degrees, the field water capacity first increased and then decreased in the 0~10cm layer and decreased in the means of "increasing-decreasing- increasing-decreasing" manners in the 10~20cm, but it gradually increased in the 20~30cm layers. (3) Soil saturated water content was mainly affected by the soil bulk density and total phosphorus, and the soil capillary water holding capacity was mainly affected by the soil bulk density, while the soil field water capacity was mainly affected by the total nitrogen. (4) Soil water retention was not a simple process of decreasing during the grassland degradation, but it was a changing process of differences. It was very important to protect the original grassland vegetation for the hydrological process in the river source regions and this research could provide the scientific basis for revealing the impacts of grassland vegetation degradation on soil hydrological and ecological effects in the region scale and displaying the effects of grassland vegetation degradation on river runoff forming and regulation.
* Corresponding author. Tel.: +86-010-64889008; fax: +86-010-64889008. E-mail address: yixiaagsheag2004@163.com.
X.S. Yiab*, G.S. Lia, Y.Y. Yin1
Abstract
1878-0296 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of School of Environment, Beijing Normal University. doi:10.1016/j.proenv.2012.01.090
© 2011 PPbbshed by Elsevier B. V. Selection and/or peer-review under responsibility of School of Environment, Beijing Normal University.
Keywords: grassland vegetation degradation; hydrological and ecological effects; soil water retention ability; the source region of the Yellow River
1. Introduction
Soil is the main repository of ecosystem water conservation which can intercept and kept most or all infiltrating water [1, 2]. It reduces the possibility of infiltrating water transforming to underground runoff and further reduces the big recycle process of water and enhances the small recycle process of water [3]. So, like the river, lake, reservoir and underground aquifer, soil also has significant storing and regulating function. For this reason, people often call soil as soil reservoir [4-8]. Soil water retention is one of soil physical properties [9, 10] which can constrain the keeping, storing and supplying of soil water [11]. In addition, soil water retention is an important influencing factor for soil water recycle and terrestrial hydrological cycle [3]. So, the function of soil hydrological and ecological effect can be reflected on the difference of soil water retention [12].
Soil and plant are two important components for the grassland ecosystem [13]. They have closely relationship [14-16] and they interact but can't be divided each other [17]. Any changes in one will induce to the changes of the other [14]. The grassland vegetation changes not only alter the vegetation in the ground and the root system underground, but also alter the land surface properties (such as slope, land roughness length and so on ) and further affect the soil physical and chemical properties[18-25]. So, the grassland vegetation changes will alter the water status stored in the soil and induce the changes of soil water [26]. Especially when the grassland vegetation coverage got down, soil would become dry and the soil water retention weakened [27]. For this reason, the changing of soil water retention has closely relationship with grassland vegetation changes. Grassland degradation and grassland restoration are two main aspects of the grassland vegetation changes. Many researches about the soil water retention changes during the grassland restoration process have been reported. The soil water retention would be improved for the soil organic matter accumulation during the grassland restoration process [28]. The abandoned cropland vegetation could affect the soil storage and retention function by enhancing the soil organic matter content to improve the soil structure, reducing the soil bulk density and increasing the soil porosity during its restoration [29]. Other scholars also got the similar results and they further pointed that sloping returned grain plots and enclosed grassland could improve the soil hydro-physical properties to enhance the soil water retention and erosion resistance ability [30]. However, there were few researches about the changes of soil water retention during the grassland degradation process [31, 32].
The source region of the Yellow river locates in the interior of the Qinghai-Tibetan Plateau. It is not only the major area of water conservation, but also is the ecological sensitive district and climate change promoter region [33]. Almost entirely alpine meadow grassland of the Qinghai-Tibetan Plateau distributes in this region and the environment changes in this region will affect the ecological security and social economy sustainable development of the whole watershed [32]. However, the alpine meadow grassland suffered different degradation degrees in the source region of the Yellow river for the reason of climate changes, human beings and rat damage [34-36]. Many scholars had studied the grassland degradation in the source region of the Yellow river[36-38], but the main researches concentrated on the relationship between grassland vegetation and soil water characteristics [39], the effects of grassland vegetation coverage changes on soil water recycle and so on[27]. There were few researches about the effect of grassland degradation on soil water retention, especially for the impacts of grassland degradation on soil hydrological and ecological effects.
So, this research chose the Jumuchang region of Maqin country in the source region of the Yellow river as the study area and carried out the impacts of grassland degradation on soil water retention by choosing typical areas and quadrates to collect soil samples and doing experiments in laboratory. The objectives of this study were: (1) to analysis the soil water retention characteristics in different grassland degradation degrees; (2) to point to the main influencing factors of soil water retention; (3) to generalize the different effects on soil water retention in different grassland degradation degrees. This research could supply the scientific basis for the research on the impacts of grassland degradation on soil hydrological and ecological effects in regional scale from revealing the relationship between soil water retention and grassland degradation.
2. Materials and methods
2.1. study site
The study area locates in the Junmuchang region (100°20.375'~100°33.080'E, 34°20.721'~34°25.701'N) of Maqin County in the sources region of the Yellow river (Fig.1). The altitude in the study area is from 3858 to 4183m above sea level, and the average altitude is 4004.18m. The climate in the study area is the typical plateau continental climate which is no different in the four seasons. The climate can be divided into two seasons which are warm season and cold season. The cold season is long, dry and cold, and the duration time has 7~8 months. The warm season is short, wetness and cool and only has 4~5 months. The temperature of annual range is small but the gap of temperature diurnal range is big. The sunshine in the study area is adequacy and the year average sunshine exceeds 2500 hours. The solar radiation is intensive and the total radiation energy in a year is between 623.8 and 629.9 kJ/cm2. The annual precipitation is between 420mm and 560mm, and the average annual precipitation is 423.2mm which is mainly from May to October[40]. The soil types in the study area are alpine meadow soil and alpine shrub meadow soil. The soil organic matter in the surface layer is abundance. The Kobresia humilis meadow is the main winter and spring pasture. In the Kobresia humilis meadow, the constructive specie is Kobresia humilis and the companion species contain Kobresiapygmaea, Scirpus distigmaticus, Elymus nutans, Poa spp., Stipa aliena Keng, Lagotis brachystachya, Leontopodium nanum, Ajania tenuif olia, Lancea tibetica, Saussurea superba, Pedicularis kansuensis Maxim and so on[41].
Fig. 1 The location of study area
2.2. methods
For the reason of global climate changes, overgrazing and so on, the grassland in the sources region of the Yellow river occurred degradation of different degrees. This paper used the method of spatial sequence instead of time succession sequence to reflect the grassland degradation process in order to describe the soil water retention changes during the grassland degradation process. • The classification standard of grassland degradation
Alpine meadow grassland is the major grassland ecosystem and a wide distribution plant community in the source region of the Yellow river [39]. Many scholars in China had studied the plant community succession regulation and characteristics on the alpine meadow grassland during the different degradation phases in the Three-River Headwaters region of Qinghai province, China [42-50]. Their research conclusions revealed that the plant community of alpine meadow grassland undergone five steps during the degradation process which were short rhizome Cyperaceae plant community, short rhizome Cyperaceae and dense cluster Gramineae plant community, scatter cluster Gramineae, short rhizome Cyperaceae and weeds plant community, stolon weeds plant community, annual and biennial poisonous weeds plant community. So, by the means of summary and induction, this paper divided the alpine meadow grassland into five degrees which were no degradation, light degradation, middle degradation, heavy degradation and extreme degradation according to the five steps of alpine meadow succession process (Tab. 1). It was sure that the vegetation coverage and the number of plant species were also considered for the classification standard of grassland degradation.
Table 1 Classification standard of grassland degradation
Degradation degrees Number of plant species Vegetation coverage (%) Dominant species Companion species Plant community phases
No degradation 20~28 Above 90 Kobresia pygmaea, Kobresia humilis Stipa Sp., Festuca ovina, Saussurea superba, Gentiana straminea Short rhizome Cyperaceae plant community
Light degradation 24~32 80~90 Kobresia pygmaea, Kobresia humilis, Stipa Sp., Festuca ovina Saussurea superba, Gentiana straminea, Potentilla bifurca Linn. Short rhizome Cyperaceae and dense cluster Gramineae plant community
Middle degradation 18~29 60~80 Poa spp., Elymus dahuricus Turcz., Kobresia pygmaea Saussurea superba, Gentiana Lanrencei, Potentilla nivea Short rhizome Cyperaceae and weeds plant community, stolon weeds plant community
Heavy degradation 10~23 50~60 P. anserina, Lagotis brachystachya Lancea tibetica, Leontopodium nanum, Glaux maritima, Ajania tenuif olia, Polygonumsibiricum, Oxytropis Stolon weeds plant community
Extreme degradation 8~15 below 50 Ajuga lupulina, Ligularia virgaurea, Pedicularis sp., Morina chinensis Kobresia pygmaea, Saussurea superba Annual and biennial poisonous weeds plant community
• Sample plots design
We designed the sample plots in the grassland of different degradation degrees according to the alpine meadow vegetation degradation characteristics. Because the grassland in the study area occurred different degradation, we couldn't find the typical sample plot of no degradation. So, we divided the grassland in
the study area into five degradation degrees which were slight degradation, light degradation, middle degradation, heavy degradation and extreme degradation according to the degradation characteristics in Tab. 1. The detail descriptions of sample plots were displayed in the Tab. 2.
Table 2 Basic information of sample plots
Sample plots number Number of plant species Dominant species Companion species Degradation degrees
1 22 Kobresia pygmaea, Stipa Sp. Saussurea superba, Artemisia duthreuil, Artemisia frigida, Anaphalis lacteal, Ligularia virgaurea, Aconitum pendulum, Meconopsis integrifolia, Potentilla bifurca Linn. Slight degradation
2 24 Elymus nutans, Pedicularis kansuensis Maxim Puccinella tenuiflora, Poa spp., Ptilagrostis dichotoma, Oat, Festuca rubra Light degradation
3 26 Pedicularis kansuensis Maxim, Kobresia pygmaea Swertia bifolia, Microula pseudotrichocarpa, Aconitum gymnandrum, Pedicularis alaschanica, Lamiophlomis rotata, Elymus nutans, Ligularia virgaurea, Festuca ovina, Potentilla anserina Middle degradation
4 13 Artemisia tanacetifolia., Leontopodium nanum Aconitum pendulum, Oxytropis ochrantha, Lagotis brachystachya, Ligularia virgaurea Heavy degradation
5 9 Ligularia virgaurea, Aconitum pendulum Artemisia duthreuil, Ajuga lupulina, Ajania tenuif olia, Heracleum dissectifolium, Euphorbia fischeriana, Hierochloe odorata, Morina chinensis Extreme degradation
• Soil sampling
First of all, we dig soil profiles (approximately 60 X 50 X 50 cm in long, width and height) in all sample plots of different degradation degrees. Then, soil samples were collected from all sample plots at the depths of 0~10cm, 10~20cm and 20~30 cm using the cutting rings with the volume of 100 cm3. At the same time, the subsamples were collected using individual plastic bags in the same soil depth. Because the soil depth was not the same and individual soil profile was shallow, we just collected the soil samples in the depth of 0~10cm and 10~20cm. All the coordinates of the sampling locations were determined with a highly accurate global positioning system (GPS). • Experimental analysis
Soil physical properties analysis mainly contained soil particle-size fractions, soil bulk density and soil water retention and so on. Soil particle-size fractions were determined by the Laser Particle Sizer (Mastersizer 2000, Malvern Company, UK) which the measure range was 0.02~2000^m and the repetition measure errors were less than 2%. Soil bulk densities were determined using oven-dried weight and sample volume. Soil capillary water holding capacity, saturated water content and field water capacity were measured using the method of ring cuts immersion [51, 52].
Soil chemical properties analysis mainly contained soil organic matter, total carbon, total nitrogen, total phosphorus and total potassium. All the soil chemical properties were determined according to the regular analysis methods. Soil organic matter was determined using wet oxidation with K2Cr2O7. Total carbon and total nitrogen were measured by the elemental analyzer. The determination of total
phosphorus and total potassium was carried out using HNO3-HClO4-H2SO4 method microwave digestion, and then was determined in the ICP-OES. • Statistical analysis
The Excel 2003 software package was used for all the data processing in this paper. All the statistical analyses were calculated by SPSS software (version 18.0).
3. Results
Soil water retention is an important physical property for soil. We used soil capillary water holding capacity, saturated water content and field water capacity to reflect the soil water retention in this paper, and then we further analyzed the soil water retention characteristics in different degradation phases.
3.1. Soil capillary water holding capacity characteristic in different degradation phases
Capillary water is the main storing water in the soil that is in the soil capillary pore and is stored by the capillary force. Capillary water can be divided into two types which are capillary upload water and capillary hanging water according to the relationship between capillary water and groundwater. Capillary upload water is the water that is kept by the capillary force when the groundwater rises along the capillary pore. We called the maximum of capillary upload water soil capillary holding water capacity [53]. The Fig.2 shows that there were significant differences of soil capillary holding water capacity in different soil layers among the degradation degrees. The changing trend of soil capillary holding water capacity was similar. With the increasing of grassland degradation degree, soil capillary holding water capacity in different soil layers generally decreased, but soil capillary holding water capacity in different soil layers increased in the heavy degradation phase. As a whole, the soil capillary holding water capacity in the 0~10cm layer gradually decreased when grassland was from slight degradation to light degradation with the decreasing extent of 12.00%. The soil capillary holding water capacity increased slightly when grassland was from light degradation to heavy degradation with the increasing extent of 3.88%. The soil capillary holding water capacity significantly decreased from heavy degradation to extreme degradation with the decreasing extent of 7.78%.The changing regulation of soil capillary holding water capacity in the 10~20cm was similar with the 0~10cm layer. The soil capillary holding water capacity increased slightly from slight degradation to light degradation and decreased 5.44% from light degradation to middle degradation which was less significant than in the 0~10cm layer. But soil capillary holding water capacity decreased 11.10% from heavy degradation to extreme degradation which was more than in the 0~10cm layer. There were some differences among 0~30cm layer, 0~10cm layer and10~20cm layer. After decreasing of soil capillary holding water capacity from slight degradation to middle degradation in the 20~30cm layer, the soil capillary holding water capacity increased 10.54% from middle degradation to heavy degradation. In conclusion, there were significant differences of soil capillary holding water capacity in the layers among different degradation degrees. From slight degradation to extreme degradation, soil capillary holding water capacity decreased 15.90% in the 0~10cm layer, but it just decreased 8.49% in the 10~20cm layer. This phenomenon revealed that the impacts of grassland degradation on soil capillary holding water capacity was mainly in the 20cm layer, especially in the 0~10cm layer. The conclusion was similar with other scholars [39].
Fig.2 Soil capillary holding water capacity in the different grassland degradation phases
3.2. Soil saturated water content characteristic in different degradation phases
Soil saturated water content contained all the total water in the soil when all the soil porosity was filled with water [53]. It was an index of evaluating soil water storage capacity. That is to say, as the maximum of soil storage capacity, it not only reflects the potential ability of storage and regulating water, but also reflects the water conservation function [32]. From the Fig.3, we could see that there were significant differences of soil saturated water content in the layers among different degradation degrees. The changing trend was similar with the soil capillary holding water capacity. With the increasing of grassland degradation degree, soil saturated water content in different soil layers generally decreased, but soil saturated water content in different soil layers increased from middle degradation to heavy degradation phase. In the 0~10cm layer, the soil saturated water content decreased in general with the decreasing extent of 17.09%. Especially from slight degradation to light degradation and from heavy degradation to extreme degradation, the soil saturated water content decreased 9.95% and 8.73% respectively. However, the soil saturated water content increased and changed slightly when grassland degenerated from light to serious. The changing regulation of soil saturated water content in the 10~20 cm was similar with 0~10cm layer. It was less significant than 0~10 cm layer from slight degradation to heavy degradation phase. But the soil saturated water content decreased 14.33% in total from heavy degradation to extreme degradation phase which was more significant than 0~10 cm layer. There was significant difference of soil saturated water content in the 20~30cm layer among different grassland degradation phases. But the soil saturated water content in the 20~30cm layer changing characteristic was different from 0~10cm and 10~20cm layer. The soil saturated water content decreased from slight degradation to light degradation with the decreasing extent of 2.33%. But the soil saturated water content increased 8.71% from light degradation to extreme degradation. In addition, we couldn't determine whether the changing regulation of soil saturated water content in the 20~30 cm layer was the same as 0~10cm and 10~20cm layer or not because we only collected the 0~20 cm layer soil samples in the extreme degradation phases. In conclusion, there were significant differences of the soil saturated water content in the soil layers among grassland degradation phases. The average soil saturated water contents in the 0~30cm soil layer were 36.39%, 31.97%, 31.29%, 36.88% and 25.12% respectively. The decreasing extent of soil saturated water content was 11.27% from slight degradation to light degradation
in the 0~10cm layer. These phenomenons showed that the impacts of grassland degradation on soil saturated water content were mainly in the 0~10 cm layer.
Fig.3 Soil saturated water content characteristics in the different grassland degradation phases 3.3. Soil field water capacity characteristic in different degradation phases
Soil field water capacity was not only considered as the maximum of soil content which soil could maintain stably, but also was the maximum of capillary hanging water[53, 54]. Soil field water capacity was the main source of plant absorbing water in the mountain area, hilly area and mound land area, and it was mainly affected by the soil texture, soil organic content, structure and loose condition and so on[55].The changes of soil field water capacity in different grassland degradation phases were shown in the Fig. 4. With the increasing of grassland degradation degrees, soil field water capacity in the 0~10 cm layer first increased and then decreased with the changing range from 13.09% to 20.04%. The soil field water capacity was the maximum in the heavy degradation phase, and then was middle degradation phase, light degradation phase, slight degradation phase and extreme degradation phase. The soil field water capacity in the heavy degradation phase was 1.53 times than in the extreme degradation phase. The soil field water capacity in the 10~20cm layer changed fluctuant of increasing-decreasing- increasing-decreasing with the changing range from 10.53% to 20.50%. With the increasing of grassland degradation degrees, soil field water capacity in the 20~30 cm layer gradually increased. The soil field water capacity in the heavy degradation phase was 23.22% which was 2.04 times than in the light degradation phase. In conclusion, the soil field water capacity in all layers was the maximum in the heavy degradation phases, but it was the minimum in the extreme degradation in the 0~10cm layer and was the minimum in the slight degradation in the 10~20cm and 20~30cm layers. The soil field water capacity changing characteristics were different from soil capillary water holding capacity and soil saturated water content. Compared with soil capillary water holding capacity and soil saturated water content, the soil field water capacity might be affected by different factors.
Slight Light Middle Serious Extreme Degradation degrees
U L. 16
Slight Light Middle Serious Extreme Degradation degrees
Fig.4 Soil field water capacity characteristics in the different grassland degradation phases
4. Discussion
Soil was a loose material with many pores. Water will be stored in the soil pores by the forces of molecular attraction, capillary force and gravity force when the precipitation or irrigation water entered in the soil [53].Many researches' results [28, 56-62] shown that soil water retention had closely relationship with soil physical and chemical properties such as soil porosity, soil bulk density, soil texture and soil organic matter and so on. From the correlation analysis of soil capillary water holding capacity, saturated water content, field water capacity with soil physical and chemical properties (Tab.3), we found that the correlation coefficients between soil capillary water holding capacity and soil bulk density, soil organic matter were -0.806 and 0.675 respectively which were significant correlation at the significance level of 0.01. The correlation coefficients between soil capillary water holding capacity and total nitrogen, total carbon were 0.618 and 0.602 respectively which were significant positive correlation at the significance level of 0.05. All of these shown that soil bulk density, soil organic matter total nitrogen and total carbon were the main influencing factors for the soil capillary water holding capacity. The soil saturated water content not only had significant correlation with soil bulk density and soil organic matter at the significance level of 0.01, but also had significant correlation with total nitrogen, total carbon and total phosphorus at the significance level of 0.05. The correlation coefficient between soil saturated water content and soil bulk density was maximum (-0.906).This conclusion shown that the soil bulk density was the main influencing factor for the soil saturated water content and other soil physical and chemical properties affected the oil saturated water content slightly. Many scholars [63-66] also got the similar results with this paper. The correlation coefficients between soil field water capacity and total nitrogen, sand grain and silt content were 0.570, -0.506 and 0.503 respectively which were significant correlation at the significance level of 0.05. This revealed that the total nitrogen, sand grain and silt content were the main influencing factor for the soil field water capacity and other soil physical and chemical properties affected the soil field water capacity slightly. In conclusion, soil bulk density was the main influencing factor for the soil capillary water holding capacity and soil saturated water content, and then was soil organic matter. Total nitrogen and total carbon also affected the soil capillary water holding capacity and soil saturated water content significantly. The soil field water capacity was mainly affected by the total nitrogen, sand grain and silt content.
Table 3 The correlation coefficient among capillary water holding capacity, saturated water content, field water capacity and soil physical and chemical properties
Soil water retention Soil saturated water content Soil capillary water holding capacity Soil field water capacity
Soil bulk density -0.906** -0.806** -0.274
Soil organic matter 0.680** 0.675** 0.44
Total nitrogen 0.617* 0.618* 0.570*
Total carbon 0.587* 0.602* 0.37
C/N ratio -0.261 -0.225 -0.309
Total potassium 0.418 0.453 0.17
Total phosphorus 0.533* 0.439 0.019
Clay 0.129 0.106 0.49
Sand grain -0.209 -0.121 -0.506*
Silt 0.209 0.121 0.503*
Note: * and ** represent remarkable at the significance level of 0.05 and 0.01 respectively.
In the condition of multivariable, the relationship among variables was complex. Because the soil environment was complex and changeful, the simple correlation relationship among capillary water holding capacity, saturated water content, field water capacity and soil physical and chemical properties couldn't reflect the real relationship, while the partial correlation analysis could accurately evaluate the correlation degree of any tow variables. The Tab.4 showed that the partial correlation coefficients between soil bulk density and saturated water content, total phosphorus were -0.877 and -0.637 which were remarkable at the significance level of 0.01 and 0.1 respectively. In addition, The Tab.4 also revealed that the partial correlation coefficients between soil bulk density and soil capillary water holding capacity was -0.705 which was remarkable at the significance level of 0.1, while the partial correlation coefficients between total nitrogen and soil field water capacity was 0.746 which was remarkable at the significance level of 0.05. All of these showed that other soil physical and chemical properties affected the soil water retention slightly.
The Fig.5a further showed that the soil saturated water content had well response relation with soil bulk density of all soil layers in different degradation degrees. That is to say, the bigger of soil bulk density, the smaller of soil saturated water content. In addition, the soil saturated water content also had well response relation with total phosphorus of all soil layers in different degradation degrees (Fig.5b). With the similar of Fig.5a, Fig.5c showed that the soil capillary water holding capacity had well response relation with soil bulk density of all soil layers in different degradation degrees. While the Fig.5d revealed that the soil field water capacity had well response relation with total nitrogen of all soil layers in different degradation degrees.
In conclusion, soil water retention had relationship with many factors. From the correlation coefficients, soil bulk density, soil organic matter had closest relationship with soil capillary water holding capacity and saturated water content, and the total nitrogen and total carbon also had closely relationship with capillary water holding capacity and saturated water content. While the soil field water capacity had closely relationship with total nitrogen, sand grain and silt content. From the partial correlation coefficients, the soil saturated water content was mainly affected by the soil bulk density and total phosphorus, and the soil capillary water holding capacity was mainly affected by the soil bulk density, while the soil field water capacity was mainly affected by the total nitrogen.
Table 4 The partial correlation coefficient among capillary water holding capacity, saturated water content, field water capacity and soil physical and chemical properties
Soil water retention
Soil saturated water content
Soil capillary water holding capacity
Soil field water capacity
Soil bulk density -0.877*** -0.705* -0.53
Soil organic matter 0.074 0 -0.604
Total nitrogen -0.019 -0.128 0.746**
Total carbon 0.036 0.166 0.336
C/N ratio -0.052 -0.135 -0.195
Total potassium 0.469 0.585 0.4
Total phosphorus -0.637* -0.587 -0.275
Clay 0.431 0.402 0.376
Sand grain 0.165 0.402 -0.129
Note: *, **and *** represent remarkable at the significance
—■— Soil hulk density . -- —•—Saturated water content
level of 0.1, 0.05 and 0.01 respectively.
—■— Total phosphorus
>-S 'I III î'I I i'c I wntpr I ' I > 111 > ' 111
SLMHE SLMHE SLMH Degradation degree —■— Soil bulk density —•—Capillary water holding capacity
SLMHE SLMHE SLMH Degradation degree —■— Total nitrogen —•— Field water capacity
SLMHE SLMHE SLMH Degradation degree
SLMHE SLMIIE S LMII Degradation degree
Fig.5 The relationship of soil water retention and its main influencing factors in different degradation phases
Note: S, L, M, H and E represent slight degradation, light degradation, middle degradation, heavy degradation and extreme degradation respectively.
5. Conclusions
(1) With the increasing of grassland degradation degrees, the soil capillary water holding capacity and saturated water content decreased as a whole. But they increased when the grassland degradation changed from middle degradation into heavy degradation. The changes in 0~10cm layer were more significant than in the 10~20cm and 20~30cm layer, and the changes in 10~20cm layer were also more significant than in the 20~30cm layer.
(2) With the increasing of grassland degradation degrees, soil field water capacity in the 0~10 cm layer first increased and then decreased. The soil field water capacity in the 10~20cm layer changed fluctuant by the means of increasing-decreasing- increasing-decreasing. With the increasing of grassland degradation degrees, soil field water capacity in the 20~30 cm layer gradually increased.
(3) Soil bulk density, soil organic matter, total nitrogen and total carbon had closest relationship with soil capillary water holding capacity and saturated water content. While the soil field water capacity had closely relationship with total nitrogen, sand grain and silt content. But the soil saturated water content was mainly affected by the soil bulk density and total phosphorus, and the soil capillary water holding capacity was mainly affected by the soil bulk density, while the soil field water capacity was mainly affected by the total nitrogen.
(4) The soil saturated water content would be changed during the grassland degradation process by changing the soil bulk density and total phosphorus, and the soil capillary water holding capacity would be changed by altering the soil bulk density, while the soil field water capacity was mainly affected by the total nitrogen. Soil water retention was not a simple process of decreasing during the grassland degradation, but it was a changing process of differences.
Acknowledgements
Authors thank anonymous reviews for valuable comments on the manuscript. This study was supported by National Key Technology R&D Program (NO. 2009BAC61B01).
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