Vol.4, No.5B, 100-105 (2013) Agricultural Sciences doi:10.4236/as.2013.45B019 Soil water resources use limit in the loess plateau of China Ting Ning1, Zhongsheng Guo1,2*, Mancai Guo3, Bing Han1,2 1The State Key Laboratory of Soil Erosion and Dryland Farming in Loess Plateau, Institute of Soil and Water Conservation, CAS & MWR, Yangling, China; *Corresponding Author: zhongshengguo@sohu.com 2Institute of Soil and Water conservation, Northwestern A & F University, Yangling, China 3College of Science, Northwestern A & F University, Yangling, China Received 2013 ABSTRACT Soil water is a key factor limiting plant growth in water-limited regions. Without limit of soil water used by plants, soil degradation in the form of soil desiccation is easy to take place in the perennial fores tland and gras sland with too higher density or productivity. Soil water resources use limit (SWRUL) is the lowest control limit of soil water resources which is used by plant s in those regions. It can be defined as soil water storage within the maximum infiltration depth in which all of soil layers belong to dried soil layers. In this paper, after detailed discussion of characteristics of water resources and the relationship between soil water and plant growth in the Loess Plateau, the definition, quantitative method, and practical applications of SWRUL are introduced. Henceforth, we should strengthen the study of SWRUL and have a better underst anding of soil water resour- ces. All tho se are of grea t import ance for design- ing effective restoration project and sustainable management of soil water resources in water- limited regions in the future. Keywords: Infiltration Depth; Dried Soil Layer; Wilting Coefficient; Soil Water Resources Use Limit; Initial Stage to Regulate the Relationship between Soil Water and Plant Growth 1. INTRODUCTION Vegetation restoration is an effective measure to con- serve soil and water and improve ecological environment. Since 1960 s, large-scale afforestation has been carried out on the Loess Plateau in order to control serious soil erosion there [1]. Tree species, selected for their capacity to extend deep roots and fast growth, have been planted at initially high planting densities in order to rapidly establish higher degrees of ground cover, biomass and yields, and thereby to quickly realize ecological, eco- nomic and social benefits during vegetation restoration. To meet evapotranspiration needs, it is advantageous that the roots of these plants can grow quickly and thus take up water from considerable soil depths. Consequently, the combination of increased water used by plants and low water recharge rates has led to widespread soil dete- rioration occurring on the Loess Plateau in the form of excessive soil drying under both perennial grasses and forests. Such soil deterioration can adversely affect the stability of forest ecosystems and the ecological, eco- nomical and societal benefits of forest and other plant communities. Now trees and grasses species planted in the Loess Plateau are generally suitable for the local cli- mate[2]. Therefore, in order to regulate the relationship between plant growth and soil water, the following two issues should be solved: when we take effective meas- ures to regulate the relationship and how much the amount of trees and grass to be cut when regulating. This paper aims to introduce the SWRUL in order to better understand and use the soil water resources in wa- ter-limited regions. 2. WATER RESOURCES OF THE LOESS PLATEAU 2.1. Characteristics of Precipitation on the Loess Plateau The Loess Plat eau, l ocate d in t he ce ntral of Chi na, largely belongs to semi-arid area and water-limited regions. Precipitation plays an important role in the terrestrial water cycle, especially in the Loess Plateau. This is be- cause groundwater mostly lies 40 - 100 meters below the surface and the infiltration of precipitation is almost the only way to supplement soil water in the Loess Plateau [3]. Owing to a little account of the snowfall in winter, precipitation resources in this region can be represented by the rainwater resources which usually be defined as the sum of precipitation within a year in a place [4]. An- Copyright © 2013 SciRes. Openly accessible at http:// www.scirp.org/journal/as/
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105 101 nual rainfall in the Loess Plateau is limited, merely rang- ing from 250 mm in the northwest (9.8 inch) to 600 mm in the southeast (23.6 inch). Because of the monsoon influence, rainfall here has great seasonal variability, and about 70% of rainwater fell in the months from June to September. At the same time, the relative variance of annual rainfall is also between 20% - 30% respectively. Furthermore, spatial variability of rainwater resources in this region is strong, too. It shows a total decreasing trend from southeast to northwest, which has a direct relationship with the amount of rainfall [5]. 2.2. Soil Water Resources in the Loess Plateau Soil water resource is a kind of renewable fresh water resource with the characteristics and properties of natu- ral resources [6]. Both rainfall and gro undwater can only be absorbed and used by plants after being transformed into soil water as most of the water used by plant is ob- tained from the soil by plant roots system. So under rainfed conditions, soil water plays a key role in the production of agriculture and forestry. Thinking highly of the maintenance and use of soil water resources in the Loess Plateau is of great significance. In general, soil water resources in the Loess Plateau have strong temporal and spatial variability. Multiple factors, including meteorological factors (such as rainfall, atmospheric evaporation) and soil factors (such as soil texture, land-use patterns, and soil water holding capac- ity) produce a combined effect on the distribution and dynamic of soil water [7]. Of which, rainfall plays a key role. Furthermore, groundwater of this region usually lies 40 - 100 m below the surface, leading to that groundwa- ter recharge including the side stream recharge of ground- water, is difficult to occur [8]. So water cycle in this region is relatively simple, and water contents at different time and depth depend on the redistribution of rainwater resources after infiltration [9]. Limited rainfall is intercepted by the topsoil firstly, then moving down slowly under the soil water potential gradient [10]. It is the reason why the infiltration depth is shallow, and gen- erally the depth will not exceed 3 m [11]. Furthermore, soil in the Loess Plateau mostly belongs to loam soil with a low bulk density of 1.0 - 1.3 g.cm-3. The total porosity of th is loess soil can be up to 50% and the water-holding por osity can also be up to 25% - 30%. So the loess soil in this region is often regarded as “soil reservoir” with a high water holding capacity. It is meas- ured that topsoil in the 0 - 200 cm soil profile (78.7 inch) can hold soil water of 551.1 mm (21.7 in ch) to 847.4 mm (33.4 inch) [5], which almost equals to the annual pre- cipitation. From another perspective, those features of loess soil lead to its high evaporation in turn and soil water can only be stayed for a short time. Of course, this phenomenon has a direct relationship with abundant light and heat of this region. Taking abandoned land for an example, even in the rainy years, the evapotranspiration calculated from water balance could account for about 80% of rainwater from natural rainfall [12]. Under par- ticipation of vegetation , maintainin g plant normal gr owth will require more water. This is the key reason why soil water content in this region is often at a low level. 3. RELATIONSHIP BETWEEN SOIL WA- TER AND PLANT GROWTH 3.1. Soil Water Conditi ons and Plant Growth Results of studies conducted in the Loess Plateau showed that the soil water condition is a key index of plant pro- ductivity [13, 14]. Soil water is divided into available water and non-available water. Filed cap acity and wilting coefficient are usually regarded as the upper and lower limit of available water. Under normal cases, soil water content in this region is always lower than the corre- sponding field capacity. Actual available water conten t in the Caragana(Caragana korshinskii Kom.) scrubland is less than 1/2 or 1/3 of the potential available water con- tent [15]. Then the non-available water storage equals to the residual water storage in the soil when soil water content is smaller than the wilting coefficient, and it can be vividly called as the “dead” storage of the “soil reser- voir”. In the light of available water, the optimum soil water contents for different plants are different. For a cer- tain plant, when the so il water conten t is within the range from wilting coefficient to the corresponding optimum soil water content, photosynthetic rate will increase to a certain extent with the increase of water content. Simi- larly, at the stage of soil drought, the increase of soil wa- ter content can also result in the improvement of plant’s leaf water content and then accelerate the transpiration [16]. It is notable that there is a threshold for plants to react to soil water deficit. Wilting coefficient is a small range rather than a point [17], which upper limit and lower limit are called as initial wilting coefficient and permanent wilting coefficient respectively. When the soil water content is less than the initial wiltin g coefficient, it is difficult for plant to uptake soil water. In this case, although it won’t immediately lead to the death of plant, plant’s normal growth and development will be inhibited. 3.2. Soil Desiccation and Dried Soil Layer The climate environment of “low rainfall” and “high evaporation” suggested that soil desiccation is easy to take place in most of the Loess Plateau [18]. Vegetation’s participation will greatly accelerate this process. Dried soil layer has been found in farmland, artificial gr assland Copyright © 2013 SciRes. Openly accessi ble at http:// www.scirp.org/journal/as/
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105 102 Copyright © 2013 SciRes. Openly accessible at http:// www.scirp.org/journal/as/ and forestland in semi-humid and semi-arid regions of the Loess Plateau since 1960s [19]. It results from the negative balance in soil water cycle directly. The charac- teristics of the local plant resources, the features of the underlying surface and the eco-climatic zones have a combined effect on the formation of the dried soil layer [8]. Among those, the decrease in rainfall and the in- crease in temperature would be the direct reason, and improper vegetation type and exorbitant density or pro- ductivity would accelerate its development. The appear- ance of dried soil layer would seriously deteriorate the soil quality, and the self-regulating capacity of soil wou ld also be weakened [20]. Concerning the assessment standards of DSL, no con- sensus has been reached now. In most cases, it is often estimated according to the standards that water content is between stable water content and wilting coefficient [21]. As for the types of DSL, Li [22] d ivided it into temporary type and permanent type. The former refers to those dried soil layers which are located at the depth between land’s surface and the maximum infiltration depth, and it can be gradually restored by thinning, plowing and other measures. But permanent type located below the depth of soil affected by rainfall infiltration is relatively stable and soil water in these soil layers cannot be restored unless experiencing high-intensity precipitation. When DSL’s depth equals to the maximum depth which can be sup- plemented by rainfall infiltration, with the increasing forest age, strong depletion of soil water by plants will lead to serious soil degradation as the supplement amount of rainfall is limited. From this point, forestland and grassland will be further exacerbated by drou ght if effec- tive human intervention and regulation measures are not carried about. Thereby it will further affect the normal growth of plant as well as vegetation’s ecological bene- fits, even leading to the occurrence of desertification [23]. 4. SOIL WATER RESOURCES USE LIMIT 4.1. Concept and Definition In order to adapt to the dry climate coupled with low soil water content, perennials in the Loess Plateau usu- ally have highly developed root system which are able to take root into deep soil layers quick ly to uptake more soil water [24]. In the process of vegetation restoration, utili- zation depth of soil water by plants will increase with the age. For instance, result of study conducted in the semi- arid Loess Hilly Area showed that the utilization depth of soil water by Caragana increased from 2 cm (0.08 inch) to 200 cm (7.9 inch) during the first year after sowing [23]. The uneven distribution of plant roots in the soil profile coupled with the strong depletion of soil water by plants lead to the excessive consumption of soil water in the rhizosphere layer. Without timely and sufficient wa- ter supplement, soil water storage will reduce gradually from a higher storage at the beginning of afforestation to a very low storage at the adult Caragana scrubland be- cause soil water content itself is limited, and dried soil layer will take place eventually. In addition, the depletion and utilization of soil water resources by plants origin- nally are not unlimited. Non-available water of the soil cannot be used by plants theoretically. “Dead” storage capacity accounts for a considerable volume in the “soil reservoir”, especially in the Loess Plateau. So there must be an appropriate limit of soil water resources used by plants during the process of the vegetation restoration, which means soil water resources use limit (SWRUL) [25]. As precipitation is the only source of soil water supplement in this region, the maximum precipitation infiltration depth is also the maximum depth of soil water supplement. Dried soil layer and soil degradation will inevitably take place once soil water storage within the maximum infiltration depth lowered the limit. So, this concept can be defined as the soil water storage within the range from soil surface to the maximum infiltration depth in which all soil layers belong to dried soil layers. The standard of dried soil layer here is the initial wilting coefficient expressed by indicator plant, objective tree and grass species such as Caragana, Robinia pseudoacacia. 4.2. Quantitative Method In order to determine the value of SWRUL in a certain region, it is necessary to choose the indicator plant in the local vegetation communities firstly. Here the indicator plant is usually the constructive species of natural vege- tation or the purpose plant species of artificial vegetation. The maximum precipitation infiltration depth and the wilting coefficient expressed by indicator plant are two key parameters for determining the limit. The former should be determined based on measurements of infiltra- tion depth in forestland or grassland under rainfed con- ditions for many years as inter-annual variability of pre- cipitation in this region is quite strong [26 ]. Wilting coef- ficient is the lowest limit of soil water use by plants, re- flecting the minimum requirement of soil water by plants. It can be got according to the direct field observations or other indoor meth ods. SWRUL numerically equals to the integral of soil wa- ter storage along soil profile from soil surface to the maxi- mum precipitation infiltration depth in which the soil water content equals to indicator plant’s initial wilting coefficient. Being similar to the calculation of soil water storage, the value of SWRUL is generally got by strati- fied calculation method. The corresponding theoretical calculation formula is as follows: 1 n ii i LH W (1)
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105 103 1 n i i H (2) where, L is the SWRUL, H is the maximum pr ecipitation infiltration, n is the number of subdivisions of dried soil layer, H(i) is the depth of a certain subdivided soil profile, W(i) is the initial wilting coefficient in the certain soil layer. 4.3. Significance and Application 4.3.1. The Standard of Measuring Whether the Use of Soil Water Resources by Plants is Excessive or Not As a common physical phenomenon in the Loess Pla- teau, DSL increasingly threatens achievements and stabil- ity of vegetation restoration. However, under the back- ground of climate drought and global warming, water restoration of dried soil layer is quite difficult to realize. The management of degraded land is facing with more suffering and challenges. Study results showed that in the semi-arid area of Loess Plateau, even the land use changed from alfalfa to annualcrops for 12 years, its water con- tent also couldn’t meet the needs of planting trees or perennial leguminous plants for their normal growth [27]. Therefore, the sustainable use of soil water in this region has an extremely important theoretical and practical sig- nificance. It is necessary to choose a reasonable index to evaluate local soil water conditions. SWRUL can be used as the indicator. It is mentioned that the essence of the dried soil layers is the excessive depletion of soil water use by plants. The depth and thickness of dried soil layer are increased with the age of plants. For instance, soil water content at the soil layer of 100 cm (39.4 inch) was smaller than the wilting coefficient of Caragana scrubland in the third year after sowing. After then, soil drying becoming more and more serious with times going by, finally extending to 60 cm (23.6 inch) to 300 cm (118.1 inch) at the fifth year [28]. Hence, the use of soil water by plants began to enter the “excessive use stage” as the maximum precipi- tation infiltration depth was only 290 cm [23]. In fact, for deep root plants, it often doesn’t need to take a long time. To sum up, according to SWRUL, we can identify whet her or not plants excessively use soil water re-sources at the initial stage of vegetation restoration. This is of impor- tant value for the sustainable use of soil water resources. 4.3.2. The Theoretic Foundation to Determine Initial Stage of Regulating the Relationship between Plant Growth and Soil Water Generally, there are disorder relationship between plant growth and soil water in forest ecosystems and grass ecosystems in the water –limited region. Regulating this relationship is an essential way to ensure healthy devel- opment of ecosystems. In water-limited regions of the Loess Plateau, the determination of the initial stage of regulating relationship between plant growth and soil water is critical. This is because if the time of regulating the relationship is earlier than the mention ed initial stage, it will result in the waste of soil water resources. Of course, if the time of regulating is later than the initial stage, it may lead to irreversible soil degradation [25]. SWRUL is the cordon of soil water use by plants and the theoretic foundation to determine initial stage of re- gulating the relationship between plant growth and soil water. Once the depletion of soil water by plants reached or was lowered than the limit, effective measures should be taken to regulate the plant-water relationship. These measures can be divided into the following two sorts: to increase soil water storage according to plants’ require- ment or to reduce evapotranspiration according to exist- ing soil water conditions. The form er is di fficult to achieve in water-limited regions and the later is mainly achieved through trim or cut trees. It is reported that soil water storage in maximum precip itation in filtration d epth in the soil under the 5-year-old Caragana scrubland in Shang- huang Ecoexperimental Station reached its limit, so the initial stage of regulating the plant water relationship was the fifth year [28]. Accordingly, SWRUL plays an im- portant theoretical role in gu iding the regulatio n of plant- water relationship at the population level. 4.4. Research Prospects Given the importance of SWRUL in theory and prac- tice, SWRUL in different site and vegetation types should be paid more attention. As the theory is initially proposed, many details remain to be improved, espe- cially the following two points. 4.4.1. Determination of Maximum Precipitation Infiltration Depth In bare lands of the Loess Plateau, the maximum pre- cipitation infiltration depth directly depends on the ini- tial soil water content and the amount of rainfall [29,30]. Infiltration and redistribution of precipitation will be- come complicated with vegetation’s participation [31]. On the one hand, the distribution of plant roots lead to non- uniform consumption of soil water. Water in the soil away from the rhizosphere area will flow to the rhizos- phere under soil water potential gradient [32]. On the other hand, vegetation coverage affects the characteristics of soil infiltration by canopies interception and weaken- ing the raindrop power hitting on the surface through improving the nature of underlying surface [33]. The vegetation coverage and biomass in a plant community usually increase with time in the progress of vegetation Copyright © 2013 SciRes. Openly accessi ble at http:// www.scirp.org/journal/as/
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105 104 succession. During this process there are significant im- provements of soil’s infiltration capacity. Biological holes such as root holes in the recovery woodland is likely to produce preferential flows, then the soil infiltration rate and maximum precipitation infiltration depth will in- crease to a certain extent [34]. Furthermore, watercon- suming capacity in different vegetation types as well as their influence on soil water redistribu tion is d ifferent. So it is necessary to determine maximum precipitation infil- tration depths in different vegetation types. Then the ap- plication scale of soil water resources use limit can be expanded. 4.4.2. Vertical Variability of Wilting Coefficient in the Soil Profile Wilting coefficient is a key element in the calculation of SWRUL. This valu e is usually ob tained by putting the critical leaf water potential of indicator plant into soil- water characteristic curve which describing the rela- tionship between soil water and soil suction. Taking Gardner empirical formula θ = a · S – b (θ is the volu- metric soil water content, S the soil suction, a, b are pa- rame- ters) to fit the curve, wilting coefficient can be ex- pressed as W= a · 1.5 - b. In this formula, parameter a re- flects soil’s water-holding capacity, and parameter b re- flects the decreasing speed of soil water content with the decrease of soil water suction [35]. Both parameter a and parameter b are mainly influenced by soil texture, or-ganic matter content and soil structure [36]. All fac- tors mentioned change with soil depth, so it will lead to vertical variability of wilting coefficient certainly. Re- sults of studies carried out in Shanghuang Ecoexperi- mental Station showed that wilting coefficient changed indeed with soil depth, the fitted values (volumetric soil water content) floated from 5.6% to 7.8%. The overall trend was that the wilting coefficient at the land surface was small and then increased to a certain level with the in-crease of soil depth. The variability of adjacent soil layers was quite strong within the depth that from soil sur-face to the maximum precipitation infiltration depth. In addition, the above analysis is based on the assump- tion that water suction is considered to be 1.5 MPa for temporary wilting coefficient in the Loess plateau [19]. Actually, the wilting coefficient has a relationship with soil water absorption capacity by p lant, so wilting coeffi- cients of different indicator plant types are different even in the same soil environment [37]. 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