Effect of rock fragments on the percolation and evaporation of forest soil in Liupan Mountains, China

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1 ACTA ECOLOGICA SINICA Volume 28, Issue 12, November 2008 Online English edition of the Chinese language journal Cite this article as: Acta Ecologica Sinica, 2008, 28(12), RESEARCH PAPER Effect of rock fragments on the percolation and evaporation of forest soil in Liupan Mountains, China Shi Zhongjie 1,2, Wang Yanhui 1, *, Yu Pengtao 1, Xu Lihong 1, Xiong Wei 1, Guo Hao 1 1 Research Institute of Forestry Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing , China 2 Research Institute of Ecology, Chinese Research Academy of Environmental Sciences, Beijing , China Abstract: The water-retaining capacity, percolation and evaporation of stony soil in Liupan Mountains, China, were measured in order to understand the effect of rock fragments on soil hydrological processes. The results indicated that the effective water-retaining capacity of soil is positively related with the volumetric content of rock fragments, but there is no relation between saturated water-retaining capacity and rock fragment content. For the soil layers within 0 40 cm, the steady infiltration rate increases with increasing volumetric content of rock fragments until it reaches the range of 15% 20%, and then it decreases when the rock fragment content further increases. For the soil layers below 40 cm, the steady infiltration rate always increases with increasing rock fragment content. The soil evaporation rate decreases with increasing volumetric content of rock fragments when it varies in the range of 0 20%, while the soil evaporation rate keeps basically stable when the rock fragment content is higher than 20%. The soil evaporation rate shows a rising tendency with increasing size of rock fragments. Key Words: rock fragments; infiltration; soil evaporation; forest hydrology; Liupan Mountains The rock fragments are defined as the particles with diameters more than or equal to 2 mm [1]. The rock fragments embedded in the soil have an important effect on the soil physical properties, such as soil bulk density, porosity and water content, and also affect the soil hydrological processes, such as infiltration, evaporation and runoff [2 9]. Fu [10] summarized the effect of rock fragments in the soil on the infiltration, and indicated that the relation between rock fragment content and infiltration is very complex, and rock fragments may either increase or decrease infiltration and amounts. Cerdà [8] found that surface rock fragments retards ponding and surface runoff, increases steady-state infiltration rates, and diminishes runoff discharge, sediment concentrations and erosion rates. Rock fragments enhance water percolation and reduce erosion by curbing erodibility and runoff. Wilcox and Wood [11] considered that there is a positive correlation between infiltration and rock fragment content, and the existence of rock fragments induces to decrease the direct beating of soil surface and avoiding the crusting of soil surface. The infiltration is also related with the position and modes of rock fragments in the soil. If the rock fragments are randomly distributed to the soil surface, they can prevent the surface from crusting and increase the infiltration; if the rock fragments are imbedded into the soil, it is help to crust and decrease the infiltration [12]. The evaporation of soil can also be affected by the rock fragments. Kosmas and Danalatos [13] found that the rock fragments induce to decrease the soil evaporation after comparing the production of crop in the soil with and without rock fragments. Lu et al. [14] considered that the soil water evaporation declines when the rock fragments increase. Chen et al. [15] thought that the coverage of different sizes of rock fragments can induce the soil evaporation. So the effect of rock fragments on the hydrological process is one of the key problems of forest hydrology. In China, the forest is mostly distributed in the stony mountains, so the soil in the forest land contains large amounts of rock fragments, which affect the hydrological process. But the problem has not been paid enough attention in the previous forest hydrological research so as to limit the understanding of the process and mechanism of forest hydrology in the mountains. The Liupan Mountains, lying in the south of Ningxia Autonomous Region, China, is the headwater of some rivers such as Jing River, and has a very important effect on the hydrological Received date: August 10, 2007; Accepted date: April 9, 2008 *Corresponding author. wangyh@caf.ac.cn Copyright 2008, Ecological Society of China. Published by Elsevier BV. All rights reserved.

2 cycles of the Loess Plateau. The soil with large amounts of rock fragments is the important characteristics. The objectives of the paper are to study the effect of rock fragments on the soil water-retaining capacity, infiltration and evaporation in the mountain forests, to explore the function of rock fragments on the forest hydrological processes, to provide academic basis for the foundation of process-based distributed model and soil erosion model, and to instruct the construction of water-holding forest and the water resource management in the stony mountains. 1 Study area and plots The area, a representative zone of the Loess Plateau and one of the headwaters of the Jing River (a branch of the Yellow River), is situated in the Xiangshuihe watershed (106º09 106º30 E, 35º15 35º41 N) in the Liupan Nature Reserve in the southern region of the Ningxia Hui Autonomous Region, China. Its altitude ranges from 2060 m to 2931 m, and the area has a warm temperate continental climate with a transition from semi-humid and semi-arid climatic zones, characterized by hot and humid temperatures in summer, dry and cool temperatures in winter, and moderate temperatures in spring and autumn. The annual average precipitation is mm, and falls mainly in the summer and autumn. The annual mean temperature is 5.8 C, and the mean monthly temperature is 17.4ºC for the hottest month (July) and 7.0ºC for the coldest month (January). The vegetation coverage is 70% 80%, and the forest mainly consists of natural vegetation (such as Betula albo-sinensis, Pinus armandii, Quercus liaotungensis, Tilia paucicostata and Populus davidiana) and plantation forest vegetation (such as Larix principris-upprechtii and Pinus tabulaeformis) below 2700 m and dwarf shrubs and subalpine meadow above 2700 m. Several types of shrubs are predominant on the mountain slopes facing the sun. The soil is mainly gray-brown soil with large amounts of rock fragments below 2700 m and subalpine meadow soil above 2700 m. The parent materials of the soil are weathered sand mudstone, shale, limestone and remnants. In the soil of this area, there are large amounts of rock fragments with radius ranging from 2 mm to 30 mm, but basically ranging among 2 10 mm. Usually, rock fragments with radius < 10 mm are mainly of irregular polyhedron or round-grain shapes. Rock fragments with radius > 10 mm commonly are of thick-strip, round-flat or polyhedron shapes. The rock fragments are mostly randomly embedded in the soil. In contrast, there are fewer rock fragments in the subalpine meadow soil than in the brown soil. 13 plots were established in this study, including 6 natural arbor forest plots, 3 plantation plots, 3 shrub plots and 1 subalpine meadow plot (Table 1). 2 Methods 2.1 Soil physical properties To determine the physical characteristics of the soil, the soil samples were collected by using the ring method with a volume of 200 cm 3 (7 cm in diameter and 5.2 cm in height) at depths of 0 10, 10 20, 20 40, 40 60, 60 80, and cm. All the plots representing different vegetation types (Table 1) were investigated by collecting 3 repeated samples close to the edges of the plots. The soil infiltration, water-retaining capacity and bulk density were determined by using the ring method [16]. Firstly the soil water-retaining capacity was measured, secondly the soil infiltration was meas- Table 1 Basic characteristics of the vegetation and soil in different plots Plot Slope Volumetric Soil bulk density Slope Altitude Slope Soil depth grade Dominant species content of rock of rock fragments aspect (m) position (cm) (º) fragments (%) (g/cm 3 ) A 7 SE 2155 Lower Pinus tabulaeformis B 35 NW 2200 Middle Betula albo-sinensis C 35 SW 2200 Lower P. armandii D 10 E 2150 Lower Acer tetramerum Pax; Euonymus 100 sanguineus Loes E 35 E 2220 Lower B. platyphylla F 31 N 2060 Lower Quercus liaotungensis; Tilia 100 paucicostata G 45 SE 2286 Upper Larix principis-rupprechtii H 25 N 2900 Upper Subalpine meadow I 34 E 2080 Lower Quercus liaotungensis J 32 SE 2180 Mid-lower Larix principis-rupprechtii K 35 NE 2120 Lower Populus davidiana L 33 S 2160 Lower Syringa amurensis M 32 SE 2230 Middle Prunus spp

3 ured, then the bulk density was measured, and finally the rock fragment content was measured by a sieve net with diameter more than 2 mm. The volume of rock fragments was measured by draining water gradually. The weight of rock fragments was measured after drying. 2.2 Measurement of effects of rock fragments on soil evaporation The fine soil was sieved by the net with a diameter more than 2 mm in the Quercus liaotungensis and Tilia paucicostata plot (F plot). The rock fragments were selected with the diameters of 2 6 mm, 6 10 mm and > 10 mm, then mixed with sieved fine soil with different volumetric percentages and diameters, and finally loaded into lysimeters (20 cm in diameter and 30 cm in height). The volumetric content percentages and diameters of rock fragments were shown in Table 2. To avoid losing the fine soil particles in the lysimeters because of rainfall, the distance from the edge to the soil surface in the lysimeters was at least 5 cm. The volume of rock fragments was measured by draining water. The mixed soil of fine soil and rock fragments was pressed so that the bulk density was similar to the natural soil. The lysimeters were placed in the open area. After 3 4 rainfalls, the soil structure was similar to that of natural soil. Since then, the lysimeters were weighed with a regular period. If the rainfall happened, the infiltration volume was measured. The soil evaporation rate of the mixed fine soil and rock fragments was calculated by water balance. 3 Results and analysis 3.1 Variation of volumetric content of rock fragments The rock fragment content in the soil of Liupan Mountains was high and varied with soil depth among different research plots. The mean volumetric content of rock fragments at each soil depth for all the sampled plots was used to describe the vertical distribution of rock fragments. The results indicated that the rock fragment content and its standard error increased with soil depth (Table 3). The depth-weighted mean volumetric content of rock fragments for the cm layer of different plots was compared. The highest volumetric content of rock fragments was 36.92% for plot C (Pinus armandii); the lowest was 0.64% for the cm layer of plot H (subalpine meadow), and the second lowest was 3.73% for plot G (Larix principis-rupprechtii) (Table 1). The mean volumetric content of rock fragments at the depth of cm for all plots was 20.00%, with a standard error of 10.53%. In the research, the volumetric content of rock fragments was the lowest in the upper slope with a mean of 2.19%, 21.2% in the middle slope and 23.92% in the lower slope. The particle diameter distribution of rock fragments in the soil was analyzed (Fig. 1), and the results indicated that the volumetric content of rock fragments with bigger diameter increased with the increase of soil depth. In different soil layers, the rock fragment content with smaller diameter (2 6 mm) was the highest, the second was the rock fragment content with middle diameter (6 10 mm) and the lowest was the rock fragment content with bigger diameter (> 16 mm). 3.2 Correlation of soil water-retaining capacity and volumetric content of rock fragments Soil water-retaining capacity was used to evaluate the adjusting capability of water resources and the self-restraint capability of hydrological cycles in different ecosystems. According to differences in the capability of absorbability and regulation, the soil water-retaining capacity comprises of saturated water-retaining capacity and effective water-retaining capacity. The saturated water-retaining capacity is defined as the water capacity when all pores are full of water, and the effective water-retaining capacity is defined as the water capacity in the non-capillary pores. The formula that can be used to calculate the water- retaining capacity is as follows: Table 2 Composing characteristics of soil and rock fragments in the lysimeters No. of lysimeters a b c d e f g i j k l m n o Volumetric content of rock fragments (%) Diameter classes of rock fragments (mm) >10 Mixed Mixed >10 Mixed Bulk density (g cm 3 ) The mixed in the table referred that the rock fragments with diameters 2 6 mm, 6 10 mm and > 10 mm were mixed with a volumetric proportion of 1:1:1. Table 3 Variance of the volumetric content of rock fragments in different soil depths Soil depth (cm) Mean (%) Std. Dev. (%) C.V. (%)

4 n S (10000 h p ) i 1 where S is the water-retaining capacity (t/hm 2 ), h i is the soil depth in the ith layer, p i is the total porosity (saturated water-retaining capacity, %) or non-capillary porosity (effective water-retaining capacity, %), and n is the number of soil layers. Table 4 shows the water-retaining capacity in the cm layer at different vegetations. The results show that the saturated water-retaining capacities range from 46.5 mm to mm in the cm layers, and the order at different vegetations is subalpine meadow (582.7 mm) > shrubs (564.0 mm) > natural forests (553.7 mm) > plantation (513.1 mm). The effective water-retaining capacities range from 61.0 mm to mm, and the order at different vegetations is shrubs (245.6 mm) > natural forests (184.7 mm) > plantation (126.2 mm) > subalpine meadow (85.2 mm). Based on the order, the water-retaining capacities are not completely decided by the vegetation types, and some other factors also play an effect on the water-retaining capacities. The volumetric content of rock fragments induces to increase the non-capillary porosity [17]. The rock fragment content imbedded in the soil at the watershed, as a non-biological factor, is very high and induces to increase the non-capillary porosity and also increase the effective water-retaining capacity. The research found that the effective water-retaining capacity is greatly affected by the rock fragments, but the effect is very little for the saturated water-retaining capacity. The regression between the volumetric content of rock fragments and the effective water-retaining capacities in the cm layers at different vegetations can be seen in Fig. 2. The results indicate that the volumetric content of rock fragments is one of the key factors that decide the effective soil water- retaining capacity, and the determined coefficient is Correlation between the soil percolation and rock fragments Fig. 3 shows the percolation process in the soil of Quercus liaotungensis plot. The results show that the percolation rates decrease rapidly in the initial stages, and then it diminishes gradually and becomes steady at last. Usually, the percolation rates between the initial stages and the steady stages change quickly in the surface or subsurface layer and gently in the depth of soil layers, and the intervals that the percolation rates become steady are shorter in the depth of soil layers than in the surface or subsurface layer. Because of great soil heterogeneity, the percolation rates change greatly in different soil i i Fig. 1 Particle diameter distribution of rock fragments in different soil depths A1 A7 in the abscissa denote the soil depths: A1, 0 10 cm; A2, cm; A3, cm; A4, cm; A5, cm; A6, cm; A7, cm Fig. 2 Effect of the volumetric content of rock fragments on the effective soil water-retaining capacity in the cm depths Fig. 3 Change of infiltration rate with time for Quercus liaotungensis Table 4 Water-retaining capacity in the cm layers for different vegetations No. of plots A B C D E F G H I J K L M Saturated water-retaining capacity (mm) Effective water-retaining capacity (mm)

5 depths between different vegetations. Compared with the initial percolation rates, the steady percolation rates can reflect the effects of soil macropores because they are not affected by the soil water content and the absorbing-water capacity of soil pores. The analyses in different soil depths among the 13 plots indicate that the steady percolation rates have a declining trend with increasing soil depth from the soil surface to the cm layer, but increase to some extent below the cm layers (Fig. 4). Fig. 4 Change of infiltration rate and rock fragment content with soil depth A, 0 10 cm; B, cm; C, cm; D, cm; E, cm; F, cm; G, cm The percolation rate can partially reflect and decide the soil infiltration capability and the formation of runoff in the soil surface. The mean soil steady percolation rate is mm min 1 in the shrubs, mm min 1 in the natural forests, 6.80 mm min 1 in the plantation and 2.71 mm min 1 in the subalpine meadow. The order of percolation rates is consistent with the soil effective water-retaining capacity between different vegetation types. Soil infiltration can be affected by many factors. The rock fragment is one of the most important factors that affect the soil infiltration of the stony soil in Liupan Mountains. Fig. 5 shows the effect of rock fragments on the soil steady infiltration rates. It is indicated that relationship between the volumetric content of rock fragments and the infiltration rates is a parabola curve in the soil layers of 0 10 cm, cm and cm. The steady percolation rates increase with the increasing volumetric content of rock fragments less than 15% 20%, e.g., a positive correlation, while the steady percolation rates decrease with the increasing volumetric content of rock fragments more than 15% 20%, e.g., a negative correlation. The relationship between the volumetric content of rock fragments and percolation rates is an exponential curve under the soil depth of 40 cm (i.e., 40 60, 60 80, and cm). The increase of the content of rock fragments induces to increase the percolation rates. 3.4 Effect of rock fragments on soil evaporation The research shows that the rock fragment content plays an important effect on the soil evaporation (Fig. 6). The soil evaporation rates decrease with increasing volumetric content of rock fragments among 0 20%. When the volumetric content of rock fragments is more than 20%, the soil evaporation rate almost does not change, indicating that the restraint of rock fragments on evaporation becomes very little. It is considered that the soil pores are comprised of capillary pores when the soil does not include rock fragments, so the water in the soil can be evaporated through the capillary pores. The increase of the volumetric content of rock fragments may induce to increase the non-capillary porosity and to decrease the connectivity among the capillary pores. So the daily soil evaporation rate becomes smaller gradually because the water can not be drawn out though the capillary pores easily. When the volumetric content of rock fragments is more than 20%, the soil evaporation rate is also very small. This may be affected systematically by the alignment modes of rock fragments, the ratio of the transverse area between the soil and rock fragments, the connectivity of capillary pores and the coverage of rock fragments on the soil surface. The diameter of rock fragments also plays some effect on the soil evaporation. Fig. 7 shows that the daily evaporation rate increases with increasing diameter of rock fragments when the volumetric content of rock fragments is lower such as 5% and 10%, which may be caused by the fact that the numbers of bigger rock fragments decrease at the same rock fragment content and induce to increase the connectivity between the capillary pores. The effect of rock fragments on soil evaporation is similar when the rock fragment content is 20%, 40% or 5%, but the trend becomes tiny because of different alignment modes. The evaporation rate diminishes remarkably with the mixed treatments (volume rate among the 3 diameter classes is 1 : 1 : 1) than that with the single diameter class of rock fragments, which may be due to different alignment modes of rock fragments. But the daily soil evaporation rate increases more weakly with the mixed treatments than that with the single treatment when the volumetric content of rock fragments is 5%, which shows that the effect of mixed treatments of different diameter classes on evaporation is not distinct when the rock fragment content is lower. 4 Discussion 4.1 Rock fragments and soil percolation Soil infiltration is a very complex process, and is affected by the soil physical properties and rock fragments. Cousin et al. [18] found that the infiltration rate is higher in the soil with rock fragments than without rock fragments. Cerdà [8] showed that rock fragments can induce to increase the infiltration by the field experimentation. But Abrahams and Parsons [19] found

6 Fig. 5 Effect of volumetric content of rock fragments on the steady-state infiltration rate in different soil depths that the soil infiltration rate is negatively related with the coverage of rock fragments by the manual precipitation experimentation. These results indicate that the rock fragments can either increase or decrease infiltration rate [6,10]. This paper indicates that the rock fragments can induce to increase the infiltration rate when the rock fragment content is smaller, and decrease the infiltration rate when the rock fragment content is bigger in the 0 40 cm layer. The rock fragments can induce to increase the infiltration rates under the soil depth of 40 cm (40 60, 60 80, and cm). These results show that the effect of rock fragments on infiltration is very complex. There are many factors, by which the rock fragments can affect the infiltration, such as coverage, content, position, size and shape [6]. Valentin [3] found that the infiltration increases with increasing small sizes of rock fragments in the stony soil. The research indicates that the infiltration rates have the trend of decreasing with increasing soil depths when the volumetric content of rock fragments with bigger size increases. The result is similar to the research by Valentin [3]. The alignment modes of rock fragments in the soil also play an important effect on the infiltration. The shape of rock fragments is flat or polyhedron in the research region, and the alignment modes are one of the most important factors that decide the soil infiltration. If the rock fragments are placed flatwise in the fine soil, the infiltration may be blocked off. If the rock fragments are placed erectly in the fine soil, the infiltration may be accelerated. But in this research the alignment modes of rock fragments are not considered, so the effect of rock fragments on the infiltration can not be measured, which is a very serious problem that should be studied in the future. Rawls et al. [20] found that the rock fragments are one of the factors that affect the soil macropores. The result reported by Eriksson and Holmgren [21] indicated that the rock fragments can affect the soil hydrological properties by affecting the macropores. Yang et al. [22] showed that the higher non-capillary porosity and connectivity are one of the main reasons that the saturated conductivity is higher in the stony mountains. Valentin [3] also found that the vesicular porosity is positively related with the embedded rock fragment coverage and nega-

7 Fig. 6 Effect of the volumetric content of rock fragments on the soil mean daily evaporation The equation in the figure is a formula when the volumetric content of rock fragments ranges from 0 to 20%. Fig. 7 Effect of the diameter class of rock fragments on the soil mean daily evaporation The rock fragment content in different lysimeters follows as: a, 0%; b e, 5%; f i, 10%; j k, 15%; n and o, 20%. tively related with the free rock fragment coverage. This research indicates that the mixed distribution of fine soil particles and rock fragments may induce to form a number of structural pores. The increase of the volumetric content of rock fragments induces to increase the mean radii and volumes of soil macropores, especially to increase the density of the macropores whose radii are more than 1.4 mm. Otherwise, the effect of rock fragments on the macroporosity has the trend of enhancing with increasing depth of soil layers [17]. These results show that the rock fragments possibly affect the soil infiltration to some extent by affecting the size and quantity of soil pores. The rock fragments also play an important effect on the connectivity of soil pores, which needed to strengthen study in the future. 4.2 Rock fragments and soil evaporation Pérez [5] considered that the rock fragments change the evaporative condition of the soil surface and affect the soil hydrological properties. Kemper et al. [23] found that the yearly evaporation reduces by 85% when the soil surface is covered by rock fragments with size of 5 cm. Chen et al. [15] considered that the coverage of rock fragments can reduce soil evaporation, and the soil evaporation is 40.7% of the gross evapotranspiration without coverage of rock fragments, but the value is only 17.8% 25% with the coverage of rock fragments. The evaporation rates reduce by 15.33%, 15.65%, 34.09% and 34.61% in the soil with the rock fragments of 5%, 10%, 20% and 40%, respectively, if compared with the fine soil in the research. The values are lower than those in other results, which is possibly due to different alignment modes of rock fragments and climate. How do the rock fragments affect the soil evaporation? Some research has shown that soil evaporation includes 3 stages: the stable stage in the surface (the soil water is full enough to evaporate, and the evaporation rate is mainly controlled by the environments), the changed stage with soil water content (the evaporation rate gradually reduces with the decrease of surface water content), and the vapor diffused stage (the evaporation rate is controlled by the diffused capability of vapor in the soil and the dried soil depths) [6]. Compared with the soil without rock fragments, there are more macropores, less capillary pores and higher saturated conductivity and temperature in the stony soil. These properties, with higher conductivity and smaller storage capacity, accelerate soil water drainage, shorten the stable stage of the evaporation, and reduce rapidly the water contents, so the evaporation rate also falls quickly. In the research, the higher rock fragment content can reduce the soil effective water-retaining capacity and capillary porosity, increase the macroporosity, and accordingly reduce the soil evaporation rate. But the rock fragment content only plays the effect on the soil evaporation rates to a certain extent; the evaporation rate is not affected evidently by rock fragments when the volumetric content of rock fragments is higher. The size of rock fragments also plays an effect on the soil evaporation rates. Chen et al. [15] found that the evaporation rate is evidently lower in the soil covered with rock fragments of the size of 2 5 mm than that of the size of 5 20 mm and mm, but not evidently different from the treatment without the cover of rock fragments. This is similar to our result. 5 Conclusions (1) The mean volumetric content of rock fragments is very high, reaching up to 20%, and increases with the depth of soil in Liupan Mountains. The content with a diameter class of 2 6 mm is the highest content. (2) The effective water-retaining capacity ranges from 61.0 mm to mm at the cm soil depth among different vegetations in the research region. The volumetric con-

8 tent of rock fragments has a positive relationship with the effective water-retaining capacity, but has no correlation with the saturated water-retaining capacity. (3) The rock fragment content plays an important effect on the soil infiltration. For the soil layers within 0 40 cm, the steady-infiltration rate increases with increasing volumetric content of rock fragments until it reaches the range of 15% 20%, and then it decreases when the rock fragment content further increases. For the soil layers below 40 cm, the steadyinfiltration rate always increases with increasing rock fragment content. (4) The soil evaporation rate decreases with increasing volumetric content of rock fragments when it varies in the range of 0 20%, while the soil evaporation rate keeps basically stable when the rock fragment content is higher than 20%. The soil evaporation rate shows a tendency of rising with increasing size of rock fragments. Acknowledgements The project was financially supported by National Basic Research Priorities Program of China (No.2002CB111501), the Key Project of the National Natural Science Foundation of China (No ), the National Key Technology R&D Program of the Eleventh Five-year Plan of the Ministry of Science and Technology of China (No. 2006BAD03A1803), the Special Project of Social Commonweal Research of the Ministry of Science and Technology (No.2004DIB3J102) and the project of Introducing International Advanced Technology of the State Forestry Ministry (No ) and the Forest Ecology and Environment Key Laboratory of the State Forestry Ministry References [ 1 ] Poesen J, Lavee H. Rock fragments in top soils: significance and processes. Catena, 1994, 23: [ 2 ] Torri D, Poesen J, Monaci F, et al. Rock fragment content and fine soil bulk density. Catena, 1994, 23: [ 3 ] Valentin C. Surface sealing as affected by various rock fragment covers in West Africa. 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