A preliminary investigation of the dynamic characteristics of dried soil layers on the Loess Plateau of China
Introduction
The Loess Plateau of China (35–41°N, 102–114°E), situated in the upper and middle reaches of the Yellow River, covers a total area of about 630,000 km2, has an elevation of 1200–1600 m above sea level, and is predominantly covered by loess deposits ranging from 30 to 80 m in thickness (Zhu et al., 1983, Chen et al., 2008a). This region has been prone to serious soil erosion that is a consequence of both natural factors (e.g., the unique geology and landforms, climate conditions, and low vegetation coverage due to water resource constraints) and anthropic factors (e.g., poor land use management, including cultivation of marginal lands and destruction of natural vegetation) (Qiu et al., 2001). Intensive soil erosion has resulted in the decline of land productivity, environmental degradation, and the elevation of the riverbed in the lower reaches of the Yellow River due to sedimentation (Shi and Shao, 2000, Chen et al., 2007, Lacombe et al., 2008). Establishing forests and grassland can be an effective measure to control serious soil erosion that is widely supported by many scientists, land managers, and policy-makers. However, intensive vegetation restoration over large areas might also aggravate soil water scarcity. Excessive use of limited soil water resources can lead to soil desiccation, and to the formation of a dried soil layer (DSL) in the soil profile. Generally, the presence of a DSL is the definitive indicator of severe soil desiccation on the Loess Plateau of China (Li, 1983; Wang et al., 2007; Wang et al., 2008a, Wang et al., 2008b, Zhu and Shao, 2008).
The formation of a DSL is a hydrological phenomenon typical of semi-arid and semi-humid regions. It mainly results from the excessive depletion of deep soil water by non-indigenous or natural vegetation through excessive evapotranspiration combined with long-term insufficient amounts of rainfall (Jipp et al., 1998, Chen et al., 2008b). Many studies have reported DSL formation as a phenomenon typical to the Loess Plateau of China (e.g., Li, 1983, Yang, 2001, Han et al., 1990, Wang et al., 2008a, Chen et al., 2008b). However, there are also reports that similar cases of desiccation in deep soil layers have been observed in other regions of the world such as Russia (Yang and Han, 1985) and eastern Amazonia (Jipp et al., 1998). Possibly soil desiccation on the Loess Plateau is more serious and typical than that occurring in other regions due to the unique nature of the region’s low rainfall combined with large water losses due to intense runoff, deep loess deposits, low water tables, topography and, to some extent, both past and present poor land management.
The DSL has been described as having the following three characteristics: (1) located at a certain soil depth, mainly in deeper layers that may extend to 10 m below the soil surface; (2) persistence, both spatially and temporally; and (3) having a soil water content (SWC) range between the permanent wilting point and the stable field capacity (SFC) (Li, 1983, Yang, 2001, Wang et al., 2008a). Generally, 60% of the field capacity (FC) has been considered to be equivalent to the SFC based on the textures of soils found on the northern Loess Plateau and a soil layer with a SWC lower than the SFC would thus be considered to be a DSL (Wang et al., 2004, Yang and Tian, 2004), although some studies have suggested that a SFC of 50% of the FC might be more appropriate for these soils (Yang and Han, 1985, Chen et al., 2008b). Using these criteria, many studies have reported that DSLs are widely distributed across the Loess Plateau (Han et al., 1990, Li, 2001, Wang et al., 2008a, Chen et al., 2008b). The presence of a DSL differs from the more general process of soil desiccation, in which the drying process may extend from the soil surface down into the profile rather than being confined to a deep layer, and usually only occurs seasonally or for short periods of time.
The presence of a DSL can potentially impact the soil-plant-atmosphere water cycle by blocking water interchanges between upper soil layers and the groundwater (Chen et al., 2008a). In addition, DSLs may lead to soil degradation, regional microclimate environment aridity, and further loss of land productivity. Afforestation may fail due to the lack of water at deeper depths resulting in reductions of vegetation biomass or stunted growth, localized and/or regional vegetation die-off, and poor renewal by natural germination (Breshears et al., 2005; Hou et al., 1991, Yang, 1996, Wang et al., 2004, Wang et al., 2008a).
To control the negative impact of DSLs by gaining an understanding of the phenomenon, many studies have been conducted in the Loess Plateau region (e.g., Han et al., 1990, Yang, 1996, Yang, 2001, Huang and Gallichand, 2006, Li, 2001, Wang et al., 2008a). In particular, assessments of the basic characteristics of DSLs have been undertaken (Chen et al., 2008a, Han et al., 1990, Huang and Gallichand, 2006, Li, 2001, Shangguan, 2007, Wang et al., 2008a, Wang et al., 2008b), so that great progress has been made in defining and classifying them based, for example, on their spatial distribution and persistence within the soil profile (Chen et al., 2008b, Li, 1983, Shangguan, 2007, Yang and Tian, 2004). Studies on the mechanisms of DSL formation have identified the relationship between environmental aridization and SWC, from the aspect of the soil water energy status and soil water physical characteristics, by integrating the two dominant causal factors, i.e., poor land management and climate (Yang et al., 1999). Associations between vegetation and DSL indicated that the plant species was important in addition to other factors such as the age of the plantation, and the aspect and position of slopes, etc. (Han et al., 1990, Li, 2001, Yang, 1996). For example, Wang et al., 2008a, Wang et al., 2008b found that a substantial DSL was formed under an artificial Robinnia pseudoscacia forest but not under an indigenous Quercus liaotungensis forest, and that the SWC on the north-facing slope was significantly larger than on the south-facing slope.
Chen et al. (2008b) proposed some practical countermeasures to reduce DSL formation and made suggestions for accelerating vegetation rehabilitation. The possibility of soil water recharge under an appropriate land management system has also been suggested by others (e.g., Huang et al., 2004, Li and Huang, 2008, Wang et al., 2004). Simulations using the Simultaneous Heat and Water Transfer (SHAW) model indicated that a SWC recovery time would vary from 6.5 to 19.5 years (average = 13.7 years) for the 0–10 m soil layer, and from 4.4 to 8.4 years (average = 7.3 years) for the upper 0–3 m soil layer, for a soil that had been under an apple orchard for 32 years and that was then converted to winter wheat (Huang and Gallichand, 2006).
However, there remains the need for studies on the dynamics of the formation of, and changes to or within, DSLs under different succession periods for artificial and natural vegetation. This especially applies to analyses of the relationships between SWC and plant root indices or other soil properties within the DSLs. A better understanding of these processes should be helpful in order to prevent or alleviate the occurrence of DSLs, and also to effectively recover DSLs.
Therefore, the objectives of this study were: (1) to demonstrate the formation and development processes of DSLs on the Loess Plateau of China; (2) to explore the seasonal differences in the soil water profile under artificial and natural vegetation succession sequences; and (3) to analyze the correlations between SWC and plant root indices and/or soil physical and chemical properties within the DSLs.
Section snippets
Study site description
The study was conducted in the Liudaogou watershed (110°21′−110°23′E, 38°46′−38°51′N), located 14 km west of Shenmu County, in Shaanxi Province, China (Fig. 1a). The watershed has an area of 6.89 km2 and altitudes between 1081 m and 1274 m. The region has a semi-arid, continental climate with an average annual precipitation of 437 mm, 70% of which falls between June and September. According to data available for 1957–1989, the mean annual air temperature is 8.4 °C, the coldest being −9.7 °C in January
Formation and development characteristics of DSL
The profile distributions of SWC under C. korshinskii and alfalfa for different growth ages are presented in Fig. 2. Measurements were all made before the rainy season with the exception of the plots under 31-year-old C. korshinskii and alfalfa where measurements were made after the rainy season. The figure shows that SWCs under both C. korshinskii and alfalfa decreased gradually with the increase in growth age with corresponding changes down the profile, which implies that soil desiccation in
Conclusions
In this study, we investigated the formation and development of DSLs on a typical watershed on the northern Loess Plateau. The relationships between SWC and plant root indices, and soil physical and chemical properties, within both the DSL and the soil profile as a whole were compared. The following conclusions, which may provide pertinent information for eco-environmental restoration programs and for dryland farming on the Loess Plateau of China, and also for other arid and semi-arid regions
Acknowledgements
This research was supported by the Innovation Team Program of Chinese Academy of Sciences and the Key Project of Chinese National Programs for Fundamental Research and Development of China (No. 2007CB106803). We thank the editors of the journal and the reviewers for their useful comments and suggestions for this manuscript.
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