黄土高原地区土壤干层的空间分布与影响因素
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摘要
黄土高原土壤干层是区域水文、气候、土壤、地形条件下土壤水分循环的一个综合结果,是土壤对植被过度消耗深层土壤水分、强烈蒸散发、长期水分供给不足等过程的一种响应。本论文针对黄土高原地区由于水分亏缺所导致的生态问题(特别是SVAT系统中因水分负循环而导致的土壤干燥化问题),以探明黄土高原地区土壤基本水分物理性质的空间分布特征、土壤干层的动态发育过程、区域尺度土壤干层的空间分布规律及其主导因素、深剖面土壤水分特征为主要目的。以高密度布点→野外实地考察→野外采样测定→室内分析为研究手段。在连续监测土壤干层发生、发育及其动态变化的同时,2008年在黄土高原地区共选择样地382块(间距约为40 km),2009年在黄土高原8个典型区进行分区研究共选择样地266块,测定所有样地的深剖面(> 5 m,最深21 m)土壤含水量;并分层采集各样地的原状(0-5 cm和20-25 cm)和扰动土样(0-20 cm和20-40 cm)测定其土壤物理性质,同时详细记录各样地的立地条件。在科学选择土壤干层的3个量化指标(干层起始形成深度、干层厚度、干层内平均土壤含水量)之后,借助经典统计学、地质统计学的基本理论和方法,结合Arcgis9.2、GS+(version 7.0)、SPSS13.0、SigmaPlot2001、Genstat12.1等相关软件,进行土壤干层量化指标的空间变异特征、影响因素、模型建立等分析,主要研究结果如下:
(1)黄土高原地区土壤基本水分物理性质(包括:土壤颗粒组成、田间持水量、凋萎系数、土壤有效水容量、土壤容重、饱和含水量、毛管上升水含量、饱和导水率等)在水平和垂直(分表层和底层)两个方向上均具有明显的空间变异特征与分布规律。除饱和导水率、表层砂粒、表层毛管上升水含量、表层田间持水量表现出强烈的空间依赖性外,其余指标均为中等的空间依赖性。颗粒组成和田间持水量的半方差结构可用高斯模型进行最优拟合,饱和含水量、200-800 cm不同土层深度的土壤含水量为球状模型,饱和导水率为指数模型。各个指标的变程均介于75 km(表层饱和导水率)和684 km(底层粘粒)之间。黄土高原地区各个土壤基本物理参数的空间分布格局是区域外部条件、内部因素、及人为活动长期共同作用的结果。
(2)在黄土高原水蚀风蚀交错带,土壤干层的动态发育过程及其干燥化强度受土地利用、植被类型、生长年限的显著影响。人工柠条林地的干燥化程度>人工草地>自然草地>农地;自然植被演替序列中出现的土壤干燥化程度比人工植被演替序列中的干燥化程度要轻;紫花苜蓿地从第2年开始就有干层的发育现象,而柠条林地从第3年开始,在干层发育初期,紫花苜蓿地的土壤干层厚度大于柠条地,而在植物生长后期,柠条植被下土壤干层的厚度(440 cm)将超过紫花苜蓿地(300 cm)。
雨季降雨对水蚀风蚀交错带土壤水分的补给深度在100 cm左右。大豆、长芒草、苜蓿和柠条4种植被的根长密度、根重密度、根表面积密度和平均直径均随土层深度的增加而减小。在土壤干层内部,土壤中的含水量在紫花苜蓿植被下与有机碳及根系平均直径状况的相关性较好,在柠条植被下与有机碳、平均根直径及根表面积密度相关性较好;在整个剖面上,土壤含水量与各指标的相关性比干层内的相关性好,显著性水平也较高。土壤的干燥化会降低土壤水分与土壤物理、化学、及植物指标的相关性。
(3)当前黄土高原地区的土壤干层分布广泛,且具有明显的空间变异性和独特的分布格局。土壤干层的平均厚度为160 cm,其在剖面上的起始形成深度平均为270 cm。区域尺度上土壤干层厚度的变异程度属于强变异(CV = 110%),土地利用对土壤干层具有极显著(P < 0.001)的影响。
地统计结果表明黄土高原地区土壤干层厚度具有强烈的空间依赖性,而干层起始形成深度呈中等空间依赖性。干层厚度和起始形成深度的变程、空间异质比分别为33.9 km和125 km、79%和50%。土壤干层的空间分布图表明,在黄土高原西部(即宁夏盐池→陕西定边→宁夏固原→甘肃静宁、甘谷沿线以西)和中部地区特别是陕西和山西交界的沿黄地区,土壤干层厚度较厚;而在黄土高原沿黄灌区(如宁夏、内蒙灌区)、内陆灌区、汾河灌区、南部关中平原等地,土壤干层厚度较薄。区域尺度上土壤干层厚度的主要控制因子有土地利用、降雨量、土壤类型和坡度,而干层起始形成深度的主控因子为土地利用、降雨量和土壤类型。
(4)黄土高原不同类型区的深层(21 m)土壤水分分布特征有所不同,并且土壤干层下界、深剖面土壤水分与土壤颗粒组成、有机碳含量的相互关系也有所差异。深剖面土壤水分随土层深度的增加具有一定的变化规律,特别是在0-600 cm深度,土壤水分大都呈先减小后增加的趋势,总体上呈波浪式变化趋势。根据植被类型、根系剖面分布及耗水特征的不同,可以对不同类型区各个样地2100 cm深剖面土壤水分的垂直变化特征进行分层描述。通过深剖面土壤水分的测定,可以确定各样地土壤干层的真实的下界深度。在本论文所调查的7个样地中,干层下界最深为1325 cm(陕西省吴起县刺槐林地)、最浅为450 cm(陕西省绥德县农地)。用同一的判定阈值去判别整个剖面上土壤的干层情况可能会造成干层“伪多层性”的出现。
在黄土高原不同类型区,除宁夏固原草地外,神木林地、绥德农地、吴旗林地的2100 cm剖面土壤水分与有机碳含量、粘粒、粉粒、砂粒含量的相关性均达到了极显著水平(P < 0.01),相关系数介于0.552和-0.873之间。对有机碳含量与粘粒、粉粒、砂粒含量之间的相关性而言,神木和吴起林地的相关性达到了极显著水平(P < 0.01),而绥德农地的相关性不显著;在宁夏固原草地,有机碳含量仅与粉粒含量呈显著的负相关关系(P < 0.05)。
(5)在黄土高原不同气候区(干旱、半干旱、半湿润区),土地利用和植被特征对土壤干层的影响程度不同:(a)在干旱区土地利用对干层无显著影响;半干旱区农地显著小于林地/草地,林地和草地无显著差别;半湿润区林地显著高于农地/草地,农地和草地无显著差别,表明在不同气候区进行土壤干层调控与植被恢复时,应该进行分区治理,而不是统一的管理政策;(b)不同乔木林、草地类型之间的土壤干层强度(干层厚度和干层内含水量)有所不同,但这种趋势在半干旱区和半湿润区内是一致的;黄土高原不同气候区干层内土壤含水量的空间分布格局是区域大尺度和小尺度因子共同作用的结果。
(6)黄土高原地区的林地普遍存在下伏土壤干层(125个样地中有102个样地形成土壤干层),并且土壤干层的发育较为严重(干层起始形成深度= 140 cm,干层厚度= 304 cm,干层内土壤含水量= 7.92% <田间持水量= 10.21%)。在获取28个相关变量的基础上,通过相关性分析、主成分分析、最小数据组及多元线性回归分析所建立的林地土壤干层3个量化指标的统计模型具有较高的预测精度,特别是对于预测干层内土壤含水量而言(Adjusted R~2 = 73%)。
田间持水量、容重、粘粒、坡度、干燥度5个指标对于土壤干层具有重要影响。在一定的时间尺度内,可作为一定置信水平上预测干层3个量化指标的预测变量。在整个黄土高原地区,利用这5个指标来预测林地土壤干层的强弱,可提高研究的工作效率。
(7)在大尺度因子一致的情况下,不同类型区的小尺度因子(如植被类型、生长年限、坡向、坡位、海拔)对土壤干层具有显著影响。根据内陆灌区(山西省万荣县)田间尺度上土壤干层“层面效应”的研究结果可知,在地形条件、植被特征、管理措施一致的情况下,土壤干层具有“层面效应”——在水平方向上表现为厚度基本一致的干燥化土层。
在丘陵沟壑区(陕西省安塞县)的研究表明,植被类型和生长年限对土壤干层具有重要影响。不同植被(大豆、蒿、柠条、刺槐)下的干层特征显著不同(由干层厚度、干层内土壤含水量来体现)。以柠条和刺槐为研究对象,随着生长年限的延长,剖面土壤水分的变化趋势均存在一个拐点(分别为26和15年);在这个拐点之前,土壤水分及其干燥化强度随年限增加而逐渐加剧,在拐点之后,土壤水分有恢复的趋势,但非常缓慢,在较长时期内土壤剖面仍属于干层的范畴。确定不同植物的这个“拐点”有助于植被建设、水分管理和土壤水分恢复。在丘陵沟壑区(陕西省绥德县)的研究表明,坡向和坡位对土壤干层特征具有重要影响——阴坡的干燥化强度比阳坡弱、而同一坡向下坡下的干燥化强度比坡中弱;在黄土台塬区(陕西省洛川县)的研究表明,土壤干层起始形成深度具有海拔梯度性。
在理解黄土高原地区土壤基本水分物理性质空间分布特征和土壤干层动态发育过程的基础上,充分认识和把握黄土高原地区土壤干层的空间变异特征、空间分布格局、主控因子以及统计预测模型,有助于在黄土高原不同气候、土壤、地形条件下进行科学的植被选种、布局与管理,有助于科学地维持不同气候区林草植被总耗水量与有效降雨量之间的平衡、确定土壤水分的植被承载力,进而缓减土壤的干燥化进程,这对黄土高原地区土壤干层的防止与治理、土壤侵蚀控制、植被恢复与生态环境重建的可持续发展具有重要的理论和实践意义。
The formation of a dried soil layer (DSL), on the Loess Plateau of China, is an integrated response of water cycle under the circumstance of regional climate, soils, topography, hydrological processes. 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. The occurrence of DSLs potentially limits the development and sustainability of the ecological environment on the Loess Plateau.
Aiming at the ecological problems caused by soil water shortage on the Loess Plateau Region, especially the soil desiccation phenomenon in the soil-vegetation-atmosphere transfer system, the main objectives of this dissertation were: (1) to explore the spatial distribution characteristics of soil basic physical parameters on the Loess Plateau; (2) to monitor the formation and development processes of DSLs on the Plateau; (3) to investigate the spatial variability and patterns of DSLs as well as the dominate factors of DSLs across the entire Plateau; and (4) to illustrate the distribution characteristics of deep soil water (21 m) in the different zones of the Loess Plateau. Based on the research approach of intensive sampling design→field survey→field sampling and measurement→laboratory analysis, first, we monitored the dynamic evolution processes of DSLs; then, we pre-selected 382 sampling sites across the Loess Plateau (~ 620 000 km~2) based on mapped information using a sampling grid of 40 km×40 km in 2008, and in the next year (2009), we selected 266 sampling sites in the eight zones of the Plateau. For all the sites, soil water samples were collected by using a soil auger (5 cm in diameter), in 10 cm increments down the soil profile to a sampling depth of > 5 m (the deepest depth was 21 m); moreover, undisturbed soil cores from the soil surface (0-5 cm) and subsurface layer (20-25 cm), and disturbed soil samples from the 0-20 cm and 20-40 cm depths were collected to determine soil properties potentially related to DSLs; in addition, the environmental conditions of the sampling sites were recorded in details, i.e., land use type, soil type, vegetation species and coverage, vegetation age, crop yields, and groundwater level, etc.
After determined scientifically the evaluation indice of DSLs (represented by three indices:①DSL thickness, DSLT;②DSL forming depth, DSLFD; and③mean SWC within the DSL, DSL-SWC), we conducted the research by using classical statistics and geostatistical methods with related software (i.e., Arcgis9.2, GS+ 7.0, SPSS13.0, SigmaPlot2001, Genstat12.1, etc). The main results are listed as follows:
(1) Soil basic physical properties (including soil particle composition, field capacity, permanent wilting point, available water capacity, bulk density, saturated soil water content, capillary soil water content, saturated hydraulic conductivity, deep soil water contents at different soil depth below 200 cm depth), at surface and subsurface layers, demonstrated a distinct spatial variation characteristics and distribution patterns both in horizontal and vertical directions, across the Loess Plateau. All parameters showed a moderate spatial dependence (except saturated hydraulic conductivity, and sand content, field capacity, capillary soil water content at upper layer, were strongly dependent). The semivarigrams of soil particle composition and field capacity can be best fitted with Gaussian model, and the best fitted model of saturated soil water content and the deep soil water content at 200-800 cm depths was spherical model, while for saturated hydraulic conductivity was exponential model. The values of range for all parameters varied from 75 km (surface saturated hydraulic conductivity) and 684 km (subsurface clay content). The spatial distribution patterns of soil hydraulic parameters on the Loess Plateau result from the long-term interactive influence of regional external conditions, interior elements, and human activities.
(2) The dynamic development process of DSLs and its desiccation extent were significant influenced by land use, vegetation types, and plant growth age, in the wind-water erosion crisscross region on the Loess Plateau. The extent of soil desiccation for different land use types followed the sequence of artificial Caragana korshinskii shrub land > artificial alfalfa land > natural grass land > farm land. The degree of soil desiccation under natural vegetation was generally less than that under non-indigenous plant species, and thus, the use of natural vegetation succession management principles would possibly reduce soil desiccation during vegetative restoration. Forming rate of DSLs thickness was dependent on vegetation type: DSLs formed after two years of alfalfa (Medicago sativa) growth and three years of Caragana korshinskii growth; after four years of growth, DSLs under alfalfa were thicker than those under C. korshinskii, but after 31 years the DSL thickness under C. korshinskii (440 cm) exceeded that formed under alfalfa (300 cm).
The more persistent DSLs occurred below a 100 cm thick upper soil layer that was seasonally dried and replenished by rainfall. Densities of root length, weight, and surface area, and the average root diameter of soybean (Glycine max), alfalfa, Stipa bubgeana, and C. korshinskii all decreased with increases in soil depths below 20 cm. Correlations between soil water content (SWC) and root indices, and various soil physical and chemical properties, were generally weaker within the DSL layers than within the whole soil profile. The only significant correlation was between soil organic carbon and SWC under alfalfa (r = 0.627, P < 0.05). Soil desiccation may thus interfere with these typical inter-relationships occurring within the whole soil profile.
(3) The dried soil layers, occurred on the Loess Plateau, distributed widely and demonstrated an obvious spatial variation and special distribution patterns. There was strong spatial variation (CV = 110%) in DSLs, which had a mean thickness of 160 cm occurring at a mean soil depth of 270 cm. Land use had a significant impact on both DSLT and DSLFD (P < 0.001).
Geostatistic analysis showed that DSLT indicated strong spatial dependence while DSLFD had moderate spatial dependence. The values of range and nugget ratio for DSLT and DSLFD were 33.9 km and 125 km, 79% and 50%, respectively. The DSL was generally thicker (> 170 cm) in the western Loess Plateau region and in a central area (170 m to 220 cm) between Shaanxi and Shanxi boundary. Where irrigation was used along parts of the Yellow River (i.e., Ningxia and Neimeng irrigation districts) and near rivers in the interior (i.e., Fenhe irrigation district, Guanzhong plain), DSLT was considerably thinner or non-existent due to the higher water inputs. At regional scales, the dominate factors of DSLT were land use, rainfall, soil type and slope gradient, while for DSLFD were only land use, rainfall, and soil type.
(4) In different zones of the Loess Plateau, the profile distribution of soil water content at 0-21 m depth, and the relationships between soil water content and soil particle composition, soil organic carbon were different. At 0-600 cm depth, soil water content generally showed a trend of decreasing-increasing; while in the whole 21 m profile, the vertical trend of soil water content was waving change, however, it can be divided into different layers to describe the profile soil water characteristic based on the vegetation type and profile distribution of plant roots.
We can determine the lower bound of DSL by means of measuring deep soil water content (21 m in our study). Our investigation in seven typical zones of the Plateau showed that the maximum of low bound for DSL was 1325 cm (Wuqi County, Shaanxi province; forest land), while the minimum was 450 cm (Suide County, Shaanxi province; farm land).
For the 21 m soil profile in Shenmu (shrub land), Suide (farm land), and Wuqi (forest land), soil water content correlated significantly with soil organic carbon, clay, silt, and sand content (r > 0.552, P = 0.01); while in Guyuan (grass land), the correlation was not significant. Soil organic carbon significantly correlated with clay, silt, and sand content in Shenmu and Wuqi sites (P < 0.01), and it only correlated negatively with silt content in Guyuan site (P < 0.05). In Suide site, the correlations between soil organic carbon and clay, silt, and sand content were not significant.
(5) In the different climatic regions of the Loess Plateau—arid (P < 250 mm), semiarid (250 mm < P < 500 mm), and semihumid (500 mm < P < 800 mm), land use and plant characteristic had a significant impact on DSLs. (a) In the arid region, land use had no significant effect on DSLs but there were significant effects between farmland and grassland or forests (P < 0.05) in the semiarid region. In the semihumid region, DSLs under forests had a significantly greater DSLT and SFC than those under farmland and grassland (P < 0.05). Therefore, optimizing land use can mediate DSL formation and development in the semiarid and semihumid regions of the Loess Plateau and in similar regions elsewhere. (b) The development of DSLs under trees and grasses was generally more severe in the semiarid region than in the semihumid region. In each climatic region, the extent of DSLs depended on the plant species (e.g., native or exotic, tree or grass) and growth ages. The spatial pattern of DSL-SWC in the different climatic regions was an integrated result of large-scale and small-scale factors and their interactions.
(6) On the Loess Plateau, a dried soil layer under forest land has been generally formed (102 of 125 sampling sites), and the degree of soil desiccation was serious (DSLFD = 140 cm,DSLT = 304 cm,DSL-SWC = 7.92% < FC = 10.21%). Based on the potential 28 correlated variables of DSL, we developed the regression models for the three indices of DSL (DSLT, DSLFD, DSL-SWC), combining the methods of correlation analysis, principal component analysis, minimum data set, and multiple regression. The models had a high precision, especially for DSL-SWC (Adjusted R~2 = 73%).
Field capacity, bulk density, clay content, slope gradient, and aridity degree impacted DSL significantly, which can be used to predict the three indices of DSL at a certain confidence level. On the Loess Plateau, using these five variables to predict DSLs under forest land can improve the study efficiency.
(7) Small-scale factors (i.e., vegetation type, growth age, altitude, slope aspect and position) had a great impact on DSLs when the large-scale factors were uniform. (a) According to the study conducted in the interior district (Wanrong County, Shanxi province): under the circumstance of topography, vegetation characteristics, and management measure were uniform, DSLs indicated a“layer effect”—a desiccated soil layer with similar thickness in horizontal direction. (b) Based on the study conducted in the hilly and gully region (Ansai County, Shaanxi province), vegetation types and growth age had highly impact on DSLs. DSLT and DSL-SWC differed significantly under different vegetation types (soybean, Subgen. Artemisia, C. korshinskii, and Robinia pseudoacacia Linn). With the increasing of growth age for C. korshinskii, and Robinia pseudoacacia Linn, the profile change of soil water content existed an inflexion (26 and 15 years, respectively)—before the inflexion, soil water content and soil desiccation extent increased with the increasing of growth age; while after the inflexion, soil water content slightly increased and soil desiccation extent gradually alleviated, although the soil layer still belonged to the DSL. Ascertaining the inflexion of plants is very important to vegetation construction, water management, and soil water restoration. (c) The study conducted on Suide County, Shaanxi province, showed that slope aspect and position influenced DSL greatly—DSL was more severe in shady slope than in sunny slope; while in the same slope aspect, DSL was more severe in upslope than in down-slope and valley bottoms. (d) DSLFD showed an altitude gradient according to the study conducted on the Luochuan County, Shaanxi province.
On the basis of understanding the spatial distribution characteristics of soil basic physical parameters and the dynamic development of DSL, utilizing knowledge of the spatial variation characteristics of DSLs, spatial distribution patterns of DSLs, dominant factors affecting DSLs at the regional scale, and regression models of DSLs is benefit to the plant species selection, overall arrangement, and management measures under different climate, soils, and terrain on the Loess Plateau. This information also is helpful to keep the balance between rainfall (soil water input) and evaportranspiration (soil water output), and then enable scientifically based policies (e.g., sustainable land use management, distribution and/or nature of regional revegetation projects) to be made that would alleviate the process of soil desiccation and sustain development of the economy and restoration of the natural environment. Moreover, these results can also be useful to the modeling of the regional water cycle and related eco-hydrological processes. This study may have importance both in theory and in practices, for example, DSL avoiding and reclaim, soil erosion controlling, vegetation restoration and eco-environment reconstruction.
(1)黄土高原地区土壤基本水分物理性质(包括:土壤颗粒组成、田间持水量、凋萎系数、土壤有效水容量、土壤容重、饱和含水量、毛管上升水含量、饱和导水率等)在水平和垂直(分表层和底层)两个方向上均具有明显的空间变异特征与分布规律。除饱和导水率、表层砂粒、表层毛管上升水含量、表层田间持水量表现出强烈的空间依赖性外,其余指标均为中等的空间依赖性。颗粒组成和田间持水量的半方差结构可用高斯模型进行最优拟合,饱和含水量、200-800 cm不同土层深度的土壤含水量为球状模型,饱和导水率为指数模型。各个指标的变程均介于75 km(表层饱和导水率)和684 km(底层粘粒)之间。黄土高原地区各个土壤基本物理参数的空间分布格局是区域外部条件、内部因素、及人为活动长期共同作用的结果。
(2)在黄土高原水蚀风蚀交错带,土壤干层的动态发育过程及其干燥化强度受土地利用、植被类型、生长年限的显著影响。人工柠条林地的干燥化程度>人工草地>自然草地>农地;自然植被演替序列中出现的土壤干燥化程度比人工植被演替序列中的干燥化程度要轻;紫花苜蓿地从第2年开始就有干层的发育现象,而柠条林地从第3年开始,在干层发育初期,紫花苜蓿地的土壤干层厚度大于柠条地,而在植物生长后期,柠条植被下土壤干层的厚度(440 cm)将超过紫花苜蓿地(300 cm)。
雨季降雨对水蚀风蚀交错带土壤水分的补给深度在100 cm左右。大豆、长芒草、苜蓿和柠条4种植被的根长密度、根重密度、根表面积密度和平均直径均随土层深度的增加而减小。在土壤干层内部,土壤中的含水量在紫花苜蓿植被下与有机碳及根系平均直径状况的相关性较好,在柠条植被下与有机碳、平均根直径及根表面积密度相关性较好;在整个剖面上,土壤含水量与各指标的相关性比干层内的相关性好,显著性水平也较高。土壤的干燥化会降低土壤水分与土壤物理、化学、及植物指标的相关性。
(3)当前黄土高原地区的土壤干层分布广泛,且具有明显的空间变异性和独特的分布格局。土壤干层的平均厚度为160 cm,其在剖面上的起始形成深度平均为270 cm。区域尺度上土壤干层厚度的变异程度属于强变异(CV = 110%),土地利用对土壤干层具有极显著(P < 0.001)的影响。
地统计结果表明黄土高原地区土壤干层厚度具有强烈的空间依赖性,而干层起始形成深度呈中等空间依赖性。干层厚度和起始形成深度的变程、空间异质比分别为33.9 km和125 km、79%和50%。土壤干层的空间分布图表明,在黄土高原西部(即宁夏盐池→陕西定边→宁夏固原→甘肃静宁、甘谷沿线以西)和中部地区特别是陕西和山西交界的沿黄地区,土壤干层厚度较厚;而在黄土高原沿黄灌区(如宁夏、内蒙灌区)、内陆灌区、汾河灌区、南部关中平原等地,土壤干层厚度较薄。区域尺度上土壤干层厚度的主要控制因子有土地利用、降雨量、土壤类型和坡度,而干层起始形成深度的主控因子为土地利用、降雨量和土壤类型。
(4)黄土高原不同类型区的深层(21 m)土壤水分分布特征有所不同,并且土壤干层下界、深剖面土壤水分与土壤颗粒组成、有机碳含量的相互关系也有所差异。深剖面土壤水分随土层深度的增加具有一定的变化规律,特别是在0-600 cm深度,土壤水分大都呈先减小后增加的趋势,总体上呈波浪式变化趋势。根据植被类型、根系剖面分布及耗水特征的不同,可以对不同类型区各个样地2100 cm深剖面土壤水分的垂直变化特征进行分层描述。通过深剖面土壤水分的测定,可以确定各样地土壤干层的真实的下界深度。在本论文所调查的7个样地中,干层下界最深为1325 cm(陕西省吴起县刺槐林地)、最浅为450 cm(陕西省绥德县农地)。用同一的判定阈值去判别整个剖面上土壤的干层情况可能会造成干层“伪多层性”的出现。
在黄土高原不同类型区,除宁夏固原草地外,神木林地、绥德农地、吴旗林地的2100 cm剖面土壤水分与有机碳含量、粘粒、粉粒、砂粒含量的相关性均达到了极显著水平(P < 0.01),相关系数介于0.552和-0.873之间。对有机碳含量与粘粒、粉粒、砂粒含量之间的相关性而言,神木和吴起林地的相关性达到了极显著水平(P < 0.01),而绥德农地的相关性不显著;在宁夏固原草地,有机碳含量仅与粉粒含量呈显著的负相关关系(P < 0.05)。
(5)在黄土高原不同气候区(干旱、半干旱、半湿润区),土地利用和植被特征对土壤干层的影响程度不同:(a)在干旱区土地利用对干层无显著影响;半干旱区农地显著小于林地/草地,林地和草地无显著差别;半湿润区林地显著高于农地/草地,农地和草地无显著差别,表明在不同气候区进行土壤干层调控与植被恢复时,应该进行分区治理,而不是统一的管理政策;(b)不同乔木林、草地类型之间的土壤干层强度(干层厚度和干层内含水量)有所不同,但这种趋势在半干旱区和半湿润区内是一致的;黄土高原不同气候区干层内土壤含水量的空间分布格局是区域大尺度和小尺度因子共同作用的结果。
(6)黄土高原地区的林地普遍存在下伏土壤干层(125个样地中有102个样地形成土壤干层),并且土壤干层的发育较为严重(干层起始形成深度= 140 cm,干层厚度= 304 cm,干层内土壤含水量= 7.92% <田间持水量= 10.21%)。在获取28个相关变量的基础上,通过相关性分析、主成分分析、最小数据组及多元线性回归分析所建立的林地土壤干层3个量化指标的统计模型具有较高的预测精度,特别是对于预测干层内土壤含水量而言(Adjusted R~2 = 73%)。
田间持水量、容重、粘粒、坡度、干燥度5个指标对于土壤干层具有重要影响。在一定的时间尺度内,可作为一定置信水平上预测干层3个量化指标的预测变量。在整个黄土高原地区,利用这5个指标来预测林地土壤干层的强弱,可提高研究的工作效率。
(7)在大尺度因子一致的情况下,不同类型区的小尺度因子(如植被类型、生长年限、坡向、坡位、海拔)对土壤干层具有显著影响。根据内陆灌区(山西省万荣县)田间尺度上土壤干层“层面效应”的研究结果可知,在地形条件、植被特征、管理措施一致的情况下,土壤干层具有“层面效应”——在水平方向上表现为厚度基本一致的干燥化土层。
在丘陵沟壑区(陕西省安塞县)的研究表明,植被类型和生长年限对土壤干层具有重要影响。不同植被(大豆、蒿、柠条、刺槐)下的干层特征显著不同(由干层厚度、干层内土壤含水量来体现)。以柠条和刺槐为研究对象,随着生长年限的延长,剖面土壤水分的变化趋势均存在一个拐点(分别为26和15年);在这个拐点之前,土壤水分及其干燥化强度随年限增加而逐渐加剧,在拐点之后,土壤水分有恢复的趋势,但非常缓慢,在较长时期内土壤剖面仍属于干层的范畴。确定不同植物的这个“拐点”有助于植被建设、水分管理和土壤水分恢复。在丘陵沟壑区(陕西省绥德县)的研究表明,坡向和坡位对土壤干层特征具有重要影响——阴坡的干燥化强度比阳坡弱、而同一坡向下坡下的干燥化强度比坡中弱;在黄土台塬区(陕西省洛川县)的研究表明,土壤干层起始形成深度具有海拔梯度性。
在理解黄土高原地区土壤基本水分物理性质空间分布特征和土壤干层动态发育过程的基础上,充分认识和把握黄土高原地区土壤干层的空间变异特征、空间分布格局、主控因子以及统计预测模型,有助于在黄土高原不同气候、土壤、地形条件下进行科学的植被选种、布局与管理,有助于科学地维持不同气候区林草植被总耗水量与有效降雨量之间的平衡、确定土壤水分的植被承载力,进而缓减土壤的干燥化进程,这对黄土高原地区土壤干层的防止与治理、土壤侵蚀控制、植被恢复与生态环境重建的可持续发展具有重要的理论和实践意义。
The formation of a dried soil layer (DSL), on the Loess Plateau of China, is an integrated response of water cycle under the circumstance of regional climate, soils, topography, hydrological processes. 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. The occurrence of DSLs potentially limits the development and sustainability of the ecological environment on the Loess Plateau.
Aiming at the ecological problems caused by soil water shortage on the Loess Plateau Region, especially the soil desiccation phenomenon in the soil-vegetation-atmosphere transfer system, the main objectives of this dissertation were: (1) to explore the spatial distribution characteristics of soil basic physical parameters on the Loess Plateau; (2) to monitor the formation and development processes of DSLs on the Plateau; (3) to investigate the spatial variability and patterns of DSLs as well as the dominate factors of DSLs across the entire Plateau; and (4) to illustrate the distribution characteristics of deep soil water (21 m) in the different zones of the Loess Plateau. Based on the research approach of intensive sampling design→field survey→field sampling and measurement→laboratory analysis, first, we monitored the dynamic evolution processes of DSLs; then, we pre-selected 382 sampling sites across the Loess Plateau (~ 620 000 km~2) based on mapped information using a sampling grid of 40 km×40 km in 2008, and in the next year (2009), we selected 266 sampling sites in the eight zones of the Plateau. For all the sites, soil water samples were collected by using a soil auger (5 cm in diameter), in 10 cm increments down the soil profile to a sampling depth of > 5 m (the deepest depth was 21 m); moreover, undisturbed soil cores from the soil surface (0-5 cm) and subsurface layer (20-25 cm), and disturbed soil samples from the 0-20 cm and 20-40 cm depths were collected to determine soil properties potentially related to DSLs; in addition, the environmental conditions of the sampling sites were recorded in details, i.e., land use type, soil type, vegetation species and coverage, vegetation age, crop yields, and groundwater level, etc.
After determined scientifically the evaluation indice of DSLs (represented by three indices:①DSL thickness, DSLT;②DSL forming depth, DSLFD; and③mean SWC within the DSL, DSL-SWC), we conducted the research by using classical statistics and geostatistical methods with related software (i.e., Arcgis9.2, GS+ 7.0, SPSS13.0, SigmaPlot2001, Genstat12.1, etc). The main results are listed as follows:
(1) Soil basic physical properties (including soil particle composition, field capacity, permanent wilting point, available water capacity, bulk density, saturated soil water content, capillary soil water content, saturated hydraulic conductivity, deep soil water contents at different soil depth below 200 cm depth), at surface and subsurface layers, demonstrated a distinct spatial variation characteristics and distribution patterns both in horizontal and vertical directions, across the Loess Plateau. All parameters showed a moderate spatial dependence (except saturated hydraulic conductivity, and sand content, field capacity, capillary soil water content at upper layer, were strongly dependent). The semivarigrams of soil particle composition and field capacity can be best fitted with Gaussian model, and the best fitted model of saturated soil water content and the deep soil water content at 200-800 cm depths was spherical model, while for saturated hydraulic conductivity was exponential model. The values of range for all parameters varied from 75 km (surface saturated hydraulic conductivity) and 684 km (subsurface clay content). The spatial distribution patterns of soil hydraulic parameters on the Loess Plateau result from the long-term interactive influence of regional external conditions, interior elements, and human activities.
(2) The dynamic development process of DSLs and its desiccation extent were significant influenced by land use, vegetation types, and plant growth age, in the wind-water erosion crisscross region on the Loess Plateau. The extent of soil desiccation for different land use types followed the sequence of artificial Caragana korshinskii shrub land > artificial alfalfa land > natural grass land > farm land. The degree of soil desiccation under natural vegetation was generally less than that under non-indigenous plant species, and thus, the use of natural vegetation succession management principles would possibly reduce soil desiccation during vegetative restoration. Forming rate of DSLs thickness was dependent on vegetation type: DSLs formed after two years of alfalfa (Medicago sativa) growth and three years of Caragana korshinskii growth; after four years of growth, DSLs under alfalfa were thicker than those under C. korshinskii, but after 31 years the DSL thickness under C. korshinskii (440 cm) exceeded that formed under alfalfa (300 cm).
The more persistent DSLs occurred below a 100 cm thick upper soil layer that was seasonally dried and replenished by rainfall. Densities of root length, weight, and surface area, and the average root diameter of soybean (Glycine max), alfalfa, Stipa bubgeana, and C. korshinskii all decreased with increases in soil depths below 20 cm. Correlations between soil water content (SWC) and root indices, and various soil physical and chemical properties, were generally weaker within the DSL layers than within the whole soil profile. The only significant correlation was between soil organic carbon and SWC under alfalfa (r = 0.627, P < 0.05). Soil desiccation may thus interfere with these typical inter-relationships occurring within the whole soil profile.
(3) The dried soil layers, occurred on the Loess Plateau, distributed widely and demonstrated an obvious spatial variation and special distribution patterns. There was strong spatial variation (CV = 110%) in DSLs, which had a mean thickness of 160 cm occurring at a mean soil depth of 270 cm. Land use had a significant impact on both DSLT and DSLFD (P < 0.001).
Geostatistic analysis showed that DSLT indicated strong spatial dependence while DSLFD had moderate spatial dependence. The values of range and nugget ratio for DSLT and DSLFD were 33.9 km and 125 km, 79% and 50%, respectively. The DSL was generally thicker (> 170 cm) in the western Loess Plateau region and in a central area (170 m to 220 cm) between Shaanxi and Shanxi boundary. Where irrigation was used along parts of the Yellow River (i.e., Ningxia and Neimeng irrigation districts) and near rivers in the interior (i.e., Fenhe irrigation district, Guanzhong plain), DSLT was considerably thinner or non-existent due to the higher water inputs. At regional scales, the dominate factors of DSLT were land use, rainfall, soil type and slope gradient, while for DSLFD were only land use, rainfall, and soil type.
(4) In different zones of the Loess Plateau, the profile distribution of soil water content at 0-21 m depth, and the relationships between soil water content and soil particle composition, soil organic carbon were different. At 0-600 cm depth, soil water content generally showed a trend of decreasing-increasing; while in the whole 21 m profile, the vertical trend of soil water content was waving change, however, it can be divided into different layers to describe the profile soil water characteristic based on the vegetation type and profile distribution of plant roots.
We can determine the lower bound of DSL by means of measuring deep soil water content (21 m in our study). Our investigation in seven typical zones of the Plateau showed that the maximum of low bound for DSL was 1325 cm (Wuqi County, Shaanxi province; forest land), while the minimum was 450 cm (Suide County, Shaanxi province; farm land).
For the 21 m soil profile in Shenmu (shrub land), Suide (farm land), and Wuqi (forest land), soil water content correlated significantly with soil organic carbon, clay, silt, and sand content (r > 0.552, P = 0.01); while in Guyuan (grass land), the correlation was not significant. Soil organic carbon significantly correlated with clay, silt, and sand content in Shenmu and Wuqi sites (P < 0.01), and it only correlated negatively with silt content in Guyuan site (P < 0.05). In Suide site, the correlations between soil organic carbon and clay, silt, and sand content were not significant.
(5) In the different climatic regions of the Loess Plateau—arid (P < 250 mm), semiarid (250 mm < P < 500 mm), and semihumid (500 mm < P < 800 mm), land use and plant characteristic had a significant impact on DSLs. (a) In the arid region, land use had no significant effect on DSLs but there were significant effects between farmland and grassland or forests (P < 0.05) in the semiarid region. In the semihumid region, DSLs under forests had a significantly greater DSLT and SFC than those under farmland and grassland (P < 0.05). Therefore, optimizing land use can mediate DSL formation and development in the semiarid and semihumid regions of the Loess Plateau and in similar regions elsewhere. (b) The development of DSLs under trees and grasses was generally more severe in the semiarid region than in the semihumid region. In each climatic region, the extent of DSLs depended on the plant species (e.g., native or exotic, tree or grass) and growth ages. The spatial pattern of DSL-SWC in the different climatic regions was an integrated result of large-scale and small-scale factors and their interactions.
(6) On the Loess Plateau, a dried soil layer under forest land has been generally formed (102 of 125 sampling sites), and the degree of soil desiccation was serious (DSLFD = 140 cm,DSLT = 304 cm,DSL-SWC = 7.92% < FC = 10.21%). Based on the potential 28 correlated variables of DSL, we developed the regression models for the three indices of DSL (DSLT, DSLFD, DSL-SWC), combining the methods of correlation analysis, principal component analysis, minimum data set, and multiple regression. The models had a high precision, especially for DSL-SWC (Adjusted R~2 = 73%).
Field capacity, bulk density, clay content, slope gradient, and aridity degree impacted DSL significantly, which can be used to predict the three indices of DSL at a certain confidence level. On the Loess Plateau, using these five variables to predict DSLs under forest land can improve the study efficiency.
(7) Small-scale factors (i.e., vegetation type, growth age, altitude, slope aspect and position) had a great impact on DSLs when the large-scale factors were uniform. (a) According to the study conducted in the interior district (Wanrong County, Shanxi province): under the circumstance of topography, vegetation characteristics, and management measure were uniform, DSLs indicated a“layer effect”—a desiccated soil layer with similar thickness in horizontal direction. (b) Based on the study conducted in the hilly and gully region (Ansai County, Shaanxi province), vegetation types and growth age had highly impact on DSLs. DSLT and DSL-SWC differed significantly under different vegetation types (soybean, Subgen. Artemisia, C. korshinskii, and Robinia pseudoacacia Linn). With the increasing of growth age for C. korshinskii, and Robinia pseudoacacia Linn, the profile change of soil water content existed an inflexion (26 and 15 years, respectively)—before the inflexion, soil water content and soil desiccation extent increased with the increasing of growth age; while after the inflexion, soil water content slightly increased and soil desiccation extent gradually alleviated, although the soil layer still belonged to the DSL. Ascertaining the inflexion of plants is very important to vegetation construction, water management, and soil water restoration. (c) The study conducted on Suide County, Shaanxi province, showed that slope aspect and position influenced DSL greatly—DSL was more severe in shady slope than in sunny slope; while in the same slope aspect, DSL was more severe in upslope than in down-slope and valley bottoms. (d) DSLFD showed an altitude gradient according to the study conducted on the Luochuan County, Shaanxi province.
On the basis of understanding the spatial distribution characteristics of soil basic physical parameters and the dynamic development of DSL, utilizing knowledge of the spatial variation characteristics of DSLs, spatial distribution patterns of DSLs, dominant factors affecting DSLs at the regional scale, and regression models of DSLs is benefit to the plant species selection, overall arrangement, and management measures under different climate, soils, and terrain on the Loess Plateau. This information also is helpful to keep the balance between rainfall (soil water input) and evaportranspiration (soil water output), and then enable scientifically based policies (e.g., sustainable land use management, distribution and/or nature of regional revegetation projects) to be made that would alleviate the process of soil desiccation and sustain development of the economy and restoration of the natural environment. Moreover, these results can also be useful to the modeling of the regional water cycle and related eco-hydrological processes. This study may have importance both in theory and in practices, for example, DSL avoiding and reclaim, soil erosion controlling, vegetation restoration and eco-environment reconstruction.
引文
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