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Published in Agron J 100:471-476 (2008)
DOI: 10.2134/agronj2007.0100
© 2008 American Society of Agronomy
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SOILS

Evaluation of Soil Environment after Saline Soil Reclamation of Xinjiang Oasis, China

Fan Huaa, Pan Xudonga, Li Yuyib, Chen Fub and Zhang Fenghuaa,*

a The Key Oasis Eco-Agriculture Laboratory of Xinjiang Production and Construction Group, P.O. Box 1286 ShiHezi City, 832003, Xinjiang,China
b College of Agronomy and Biotechnology, China Agric. Univ., Beijing 100094, China

* Corresponding author (zhangfenghua2008{at}yahoo.com.cn).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Land degradation resulting from soil salinity is a primary problem in arid farmland. This study clarifies the characteristics and regulation of reclaimed salinized farmlnd in a desert oasis. As a case study, we examined an alluvial fan oasis in the Manas River Valley using fixed position experiments, spacial substitutes, and a trend estimation model derived from principal component analysis. In this way, we analyzed the succession of plant and soil characteristics on farmland at the bottom of alluvial fan oasis. With increasing time following reclamation, perennial haloduric plants were replaced by annual plants. The area then entered a stable stage from 6 to 8 yr following reclamation. The total salt decreased gradually and then increased, while soil nutrients showed the opposite trend. The trend estimation model derived from principal components analysis revealed that the general soil quality improved over the first 10 yr before declining thereafter. By systematically analyzing the various indices of soil quantity, an early warning of changes in soil quality can be provided 10 to 15 yr in advance.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication March 21, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SALINIZATION AND DESERTIFICATION are the main types of land degradation (O'Hara, 1997). In terms of presenting a threat to global environmental quality, human survival, and agropastoral production, land degradation is becoming one of the most important ecoproblems being tackled by current research.

Arid areas with frail ecotopes make up one-third of the total area of China. Consequently, land degradation severely limits agricultural development and influences the stabilization of the oasis. Xinjiang is one of the most arid areas in China, within which the oasis occupies 4.27% of the total area and house 95% of the population. The oasis in Xinjiang is therefore the hub of the economy, resources, and population (Shen et al., 2002). Light, heat, water, and soil are the vital components of special agricultural areas in an oasis, ensuring high quality, high yield, and high efficiency. Mass salinization farmland (31.1% of the available cultivated land) forms under the unique climatic conditions in Xinjiang (Fan et al., 2002). Overuse and a lack of management have resulted in low to moderate yields and the abandonment of farmland (Tian et al., 2000). This has severely limited agricultural development in the region.

Foreign researchers have studied degraded farmland from different perspectives and scales, concentrating on the classification, causative factors, and preventative measures related to soil degradation (Mehta et al., 2000; Alexander and Thomas, 2002). Darwish et al. (2005) declared that soil degradation depends on utilization time, causative factors, and nutrient conditions. Remote sensing has been used to monitor and evaluate the progression of land degradation in recent years (Okin et al., 2001; Metternicht and Zinckb, 2003). Analysis in China is mainly focused on red and yellow earth of tropical and subtropical regions (Chen and Li, 2004; Shui et al., 2003) and arid and semiarid regions in the west (Du, 2004). Su et al. (2002) declared that the causes of soil degradation in irrigated farmland include both gradual and rapid changes and human factors as the external driving force. Although these studies have made some progress, they have their limitations. For example, soil degradation is a long-term dynamic process following reclamation, yet many researchers consider degradation without any consideration of time scales. Time scale analyses of soil quality under complete irrigation in arid areas are rarely reported (Ashutosh et al., 2003; Gong et al., 2004), especially in terms of early warning systems that recognize soil degradation. The objectives of the present study are (i) to identify changes in vegetation and soil properties following reclamation of secondary salinization desert to farmland, (ii) to evaluate effects of land reclamation on soil properties and vegetation, and then to provide a detailed account of the nature of soil degradation and its succession.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Description of Study Area
This paper describes a case study of the Manas River Valley in Xinjiang, which contains a typical oasis suitable for study. The Manas River Valley lies on the mesomere of Xinjiang Tianshan Mountains, north of the Junggar Basin (84°43'–86°35' E, 43°21'–45°20' N). The total area is 3.1 x 104 km2, average range in temperature is 5.3 to 6.9°C, frost-free season is 160 to 180 d in length, range of daylight per day is 7.1 to 8.2 h, daily evaporation ranges from 4.7 to 6.0 mm, daily rainfall ranges from 0.44 to 0.58 mm, and farmland irrigation relies on riverflow derived from meltwater. Since the 1950s, 40 x 104 ha of farmland has been reclaimed from the Manas River Valley, which has become the largest oasis agricultural area in Xinjiang and the fourth largest irrigated agricultural area in China. The gross domestic product of the area reached $16.19 x 108 in 2002, which represents a major contribution to economic development in Xinjiang. Over time, the use of water resource in the Manas River Valley has experienced three stages: large-scale surface water exploration, surface water, and groundwater utilization stage, water saving irrigation stage. Land reclamation can be separated into four stages: large-scale reclamation, large-scale abandoning, recovery, and reconstruction. With the reclamation of soil and water resources, land degradation has become a serious problem. First, poor management of water canals and cropping practices resulted in secondary salinization in the region. Excessive irrigation without discharging the rising groundwater levels has destroyed the water and salt balance. The ratio of irrigation and drainage volume was 3:1 to 4:1 in normal farmland while it was 10:1, and even 20:1 in the secondary salinization desert. Second, because of evaporation, soil salinity migrated upward along with the rising water table and solutes accumulated near the surface soil. In this study area, water table was 5- to 8-m deep in the 1950s before land reclamation, while it was 0.6 to 2.5 m in the 1970s in the alluvial plains, thus introducing secondary salinization in many farmlands. Third, the lack of an external drainage system is also an important reason, because salinity in the river flows from mountainous regions and gradually deposits the salts in reservoirs and canal lining system in the middle reaches of the valley. Finally, poor cotton (Gossypium hirsutum L.) management aggravated soil deterioration. Cotton is a major crop, which accounts for 78% of total planting area in this study region. Soil nutrients decreased after cotton monoculture with large amounts of chemical fertilizers for some years.

Manas River Valley includes Shihezi, Shawan and Manas region (Fig. 1 ); the total area is 28.9 x 104 ha. Shihezi region reached 17.9 x 104 ha at present, which has become the largest oasis agricultural area in the Manas River Valley. Each region in the Manas River Valley experienced a similar soil and water reclamation process. Therefore, we chose Shihezi region as a study area to identify and evaluate soil environment after saline soil reclamation. It is estimated that about 1.73 x 104 ha of farmland has been reclaimed during the past 8 yr in the Shihezi region of the Manas River Valley. Farmland had been progressively cultivated, and subsequently, part of the cultivated lands had been abandoned by farmers due to secondary salinization. At present, the area of farmland subjected to secondary salinization has amounted to 5.9 x 104 ha, which is more than one-third of the total farmland in the Shihezi region. More than 50% of the farmland in this area is low in productivity.


Figure 1
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  		Fig. 1. Sketch map of the Manas River Valley and study region.

 
Therefore, a study of soil properties following reclamation, the early detection of undesirable changes in the soil, and the rational and efficient use of agricultural resources in the oasis may be helpful in aiding the recovery of degraded land (Henry and Le, 2000).

Selection of the Study Area
The Manas River is the longest inland river within the Junggar Basin. Salinization within the upper portion of the alluvial fan has led to seriously degraded soil and water. Areas of concentrated saline sodic conditions also exist in Xinjiang (Yuan et al., 1995). The present study examined 142 farms (on alluvial fans) in the Shihezi region of the Manas River Valley during 2003 and 2004. We used the space for time substitution method (maintaining the same sampling environment), and chose farmlands with the following elapsed periods since reclamation: 1 to 2, 3 to 4, 6 to 8, 10, 15, 20, and 30 yr. Each study area was 10 to 15 ha. All of these farmlands were natural land before reclamation. The soils are formed from saltierra deposits. The fields that were reclaimed early were first planted with wheat, but are now planted with cotton using the same agricultural management practices. This experiment focused mainly on the successional regulation of vegetation colonization and the physical and chemical characteristics of the soil.

Study of Vegetation
Three plots were chosen and 21 sample sites (each area is 1 by 1 m) selected within each plot. We recorded indices (species, number, density, etc.) and environmental conditions. The Diversity Index, Uniformity Index, and Plentifulness Index were selected as diversity indices (Berg and Kellner, 2005). The relevant formulas are as follows (Zhao and Guo, 1990):

Simpson Index (Ma):

Formula 1[1]

Margalef Plentifulness Index (D):

Formula 2[2]

Pielou Uniformity Index (JP):

Formula 3[3]
where S is species number, Ln is natural logarithm, N is total individuals, Pi is species relative density.

Soil Tests
We selected five quincuncial plots and collected soil samples to a depth of 30 cm. We mixed the soil in each quincuncial plot for a total of 35 samples, each sample replicated three times. For the sampled soils, we measured total salt, organic C, available N, available P, total N, and total P.

Analytical methods (Bao, 2002) included total salt using the gravimetric method, organic C using the dichromate oxidation method, total N using the Kjeldahl method, total P using perchloric acid digestion and analyzed by the colorimetric method, available N using the alkaline hydrolysis diffusion method and available P using baking soda digestion and molybdenum antimony colorimetric analysis method.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Weed Succession in Farmland
Succession Sequence of Vegetation
The dominant species may reflect the interaction between soil properties and nutrient status (Kadmon and Harari-Kremer, 1999; Tzanopoulos et al., 2007). We used vegetation composition as the basis for analyzing the succession process. A remarkable change was observed in the weed community in farmlands as the years after reclamation increased. The succession sequence can be divided into three stages according to types and species, especially in the first few years after reclamation.

At 1 to 2 yr following reclamation, vegetation is mainly colonies of perennial haloduric weed. Community types were Alhagi sparsifolia and Karelinia caspica Less. These two dominant species made up more than 96% of the species in the first 2 yr after reclamation.

At 3 to 4 yr following reclamation, the community types were Alhagi sparsifolia and Setaria viridis. Perennials and therophytes made up 60% of vegetation. Other species included Echinochlea crusgalli Beauv., Portulaca oleracea L., and Solanum nigrum L.

At 6 to 8 yr following reclamation, the community type was Atriplex patens and Portulaca oleracea. Therophytes dominated the vegetation (69%). Other species identified were Echinochlea crusgalli, Atriplex patens, and Portulaca oleracea. As years passed, the community component remained in this stage and then became stable.

Vegetation Diversity Changes
Species diversity is an important colony characteristic that reflects the quantity index. Anthropogenic influences (e.g., cultivation, irrigation, and fertilization) caused significant changes in the structure and dynamics of plant communities (Pickett and White, 1985; El-Sheikh, 2005). The succession of the vegetation community brings about regular changes in species diversity following reclamation. We analyzed the Margalef Diversity index, the Simpson Plentifulness index, and the Pielou Uniformity index (Table 1 ). A strong correlation is observed between diversity and plentifulness; this index first increased and then decreased. The reason for this trend is as follows. As farmlands remained in natural land mode during the early stages, vegetation was mostly in salt tolerant wilding (bushes are cleared); improved farm environments combined with some salt tolerant wilding led to the advanced type and greater diversity 2 to 3 yr after reclamation. At 10 to 15 yr after reclamation, the vegetation of the farmlands was completely artificial; all original wilding had disappeared (only remaining outside the farmland areas) and weeds had been removed (farm management). At this stage, the diversity and quantity of the vegetation community had been greatly reduced.


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  		Table 1. Diversity analysis of weed community in farmland following reclamation.

 
Changes in Salinity and Nutrients
Changes in Soil Salinity
Before reclamation, soil salinity in the Manas River Valley showed a zonal distribution according to climate, geomorphology, and groundwater. The zone of salt deposition was mostly at the edge of the alluvial fan in the midvalley area. This region has experienced slow but serious salification.

After reclamation, soil salinity changed in accordance with surface water and groundwater. Our results showed that soil salinity first decreased and then increased. The decrease then increase in soil salinity is due to the following. Excessive irrigation during the early stages led to the penetration of surface salinity deep into the soil. After long-term irrigation, salt rose along with the water table and accumulated at the surface. Using a polynomial approximate simulation, we found that the total salinity reached the lowest value (0.42%) at 9 yr after reclamation. The salinity then increased from this point (Fig. 2a ). Yoshinobu et al. (2006) reported similar results when they studied the secondary salinization in a rice (Oryza sativa L.)-based cropping system in arid land in the Aral Sea Basin.


Figure 2
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  		Fig. 2. Temporal changes in soil characteristics following reclamation.

 
Changes in Soil Nutrients
Zhao et al. (2005) studied soil degradation processes in the semiarid Bashang area, northern China and reported that after 50 yr of cultivation, soil organic C, total N, and total P concentrations had declined 73 to 79%, 60 to 70%, and 67 to 68% in the 0 to 20 cm plow layer, respectively. Over half of these losses occurred during the first 8 yr of cultivation, and the subsequent loss was slow. The results of the present study revealed that the nutrients such as total N, total P, available N, and available P showed the opposite trend to total salinity, with an initial increase followed by a decrease (Fig. 2b, 2c, 2d). Organic C increased steadily and reached a maximum (0.72%) at 15 yr after reclamation before decreasing (Fig. 2b).

Early-Warning Time Limit of Land Degradation
Evaluating cultivated land as years after reclamation as the independent variable and vegetation as the dependent variable, we performed a principal component analysis on soil quality (Table 2 ) (Niu et al., 2001; Santos et al., 2004). The accumulated contribution rate of the three principal components was 86% (≥85%). The first principal component included organic C, total N, and available N. This reflected soil nutrition and fertility. Being a measurement of nutrient availability, it was regarded as a fertility factor. The second principal component included unit weight, Simpson diversity index, and Margalef plentifulness index. This reflects the soil aging level and the water and fertility preservation ability, a fertility preservation factor. The total salt coefficient was negative (–0.791), which clearly reduced the value of the third principal component. In addition, total P concentration was greatly dependent on water and salt concentration. Accordingly, it can be regarded as an index of water condition in the soil.


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  		Table 2. Factor loadings, eigenvalues, contribution rate, and accumulated contribution rate of the principal component analysis.

 
A comprehensive evaluation (Table 3 ) on soil quality for years after soil was reclaimed shows that the optimal value increased and then decreased. Soon after reclamation (1–2 yr), the saline sodic nature of the land along with insufficient nutrients resulted in low values for the three principal components. The comprehensive evaluation appears negative.


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  		Table 3. Primary factor scores and comprehensive assessment values.

 
Soil quality underwent steady improvements in the period of 3 to 8 yr after reclamation. Zhao et al. (2005) presented this result when they studied the soil nutrition affected by land use change in the semiarid Bashang area, northern China, which exhibits similar environmental conditions as the Manas River Valley in Xinjiang. Kong et al. (2006) also proved the effect of fertilizer application on soil nutrient contents in a land use change experiment. The three principal components of fertility, water condition, and the ability of fertility preservation attained high values at 6 to 8 yr and 10 yr after reclamation. At these times the comprehensive evaluation appeared positive. Over irrigation caused surface salinity to increase steadily along with the increased water table depth. Overuse and lack of management led to a reduction in organic C, and comprehensive factors resulted in land degradation. Soil quality degenerated clearly in the period of 10 to 15 yr after reclamation, and the comprehensive evaluation appeared negative. We can therefore regard 10 to 15 yr as the early warning time of reclaimed soil fertility changing to soil degradation. Timely and reasonable input may maintain the soil condition in the optimal state and extend its early warning time limit, thereby avoiding reduced yields and eventual abandonment resulting from deterioration.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Intensive agriculture accelerated the succession of the weed community in the field. Many authors came to the conclusion that secondary succession is constrained by physical conditions like topography (Pan et al., 1999), which is especially true for mountain areas (Del Barrio et al., 1997;Villalba et al., 1994). After a large disturbance, secondary succession is determined by the differential growth, survival, and colonization ability of plant species (Aragon and Morales, 2003; Mueller Dombois, 2000; Myster and Pickett, 1994). Our results suggested that the succession rate of vegetation was accelerated in abandoned fields following reclamation due to high disturbance (such as cultivation). The salt tolerance of the weed community was completely adapted to the field environment after just 6 to 8 yr before entering the therophyte stage where the weed community is dominated by Chenopodiaceae, Amaranthaceae, and Gramineae. Conversely, succession leading to stabilization required 20 yr after abandonment.

The total salinity content first increased and then decreased, while soil nutrients showed the opposite trend. Pronounced changes were observed in organic C content. These trends are mainly related to the long-term harvesting of a single crop, excessive irrigation, overuse, and lack of proper management. Soil degradation may occur drastically by inappropriate land use and management within a short time, while soil restoration for a degraded ecosystem may take a long period of time (Zhao et al., 2005). From the perspective of degradation control, conservational tillage and proper management for farmlands should be taken into account.

The principal component analysis was used to analyze indexes that reflect soil quality. The comprehensive soil quality reached a maximum value 6 to 10 yr after reclamation before deteriorating 10 to15 yr after reclamation. This provides an early warning value of comprehensive field quality in alluvial fan oasis. Therefore, we can take timely measures to maintain field quality in optimal conditions and avoid predeterioration and low yields that can lead to the abandonment of fields. This will aid the high quality produce, high yield, and sustainable development of oasis agriculture.

The present research area is located within an alluvial fan oasis in the Manas River Valley, where the saline sodic problem is severe. The results of the present study showed that the fields began to deteriorate 10 to 15 yr after reclamation. Compared with another oasis, for example, many cultivated lands will be abandoned in the Mediterranean countries, due not only to the EU Agrarian Policy, but also to the depletion and/or salinization of aquifers, salinization of soils and climatic change (Lasanta et al., 2000). The early warning times of field deterioration after reclamation would probably be different and the determination of their exact time limit requires further study.


   ACKNOWLEDGMENTS
 
This research was supported by the National Key Fundamental Research Council Project (2006CB708401) and (30760105). We gratefully acknowledge the helpful comments by the two anonymous referees who simulated significant improvements in the analysis and writing. We also appreciated Prof. Y.C. Liang of the Chinese Academy of Agricultural Sciences for his patient and detailed correction for the paper.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





This Article
Right arrow Abstract Freely available
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Right arrow Full Text (PDF) Free
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Right arrow Citing Articles via Google Scholar
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Right arrow Articles by Hua, F.
Right arrow Articles by Fenghua, Z.
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PubMed
Right arrow Articles by Hua, F.
Right arrow Articles by Fenghua, Z.
Agricola
Right arrow Articles by Hua, F.
Right arrow Articles by Fenghua, Z.
Related Collections
Right arrow Ecosystem Restoration
Right arrow Dryland Soils
Right arrow Soil Salinity
Right arrow Other Soil Management

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