Rock Fragment Content Research Articles

Impact of landslides on soil characteristics: Implications for estimating their age

The slopes of Mount Elgon, a complex volcano at the border between Uganda and Kenya, are frequently affected by landslides with disastrous effects on the livelihood of its population. Since local people greatly depend on the land for crop production, this paper examines if and how fast physico-chemical characteristics in landslide scars recover. A chronosequence of 18 landslides covering a period of 103years was sampled in order to explore differences between topsoil inside and outside landslide scars. For each landslide, two topsoil samples were taken within the landslide and two in nearby undisturbed soils to compare their physico-chemical characteristics. Samples inside the landslides were located at the transition zone between the depletion and accumulation zone, which is situated at the contact line between the plan concave and plan convex section of the landslide. No differences were found for available phosphorus, Ca2+, Mg2+ content or for the fine earth texture. Recent landslides had however lower content of soil organic carbon (OC) and K+, and higher content of rock fragments and Na+ than the adjacent soils. Soil OC content increased significantly with age and reached levels of the corresponding undisturbed soils after ca. 60years. Older landslides had even higher OC contents than soils adjacent to the landslide. Hence landslide scars act as local carbon sink. We suggest that the occurrence of rock fragments in the topsoil is a useful indicator for mapping past landslides. Moreover, the difference in soil OC content between landslide scars and adjacent soil could be used for estimating the age of landslides in data-poor regions.

Influence of rock fragments on hydraulic properties of Ultisols in Ratchaburi Province, Thailand

In Thailand, stony soils are mainly located in hillside areas. Even though they have physical limitations for agricultural use and they are exposed to a risk for soil erosion, they continue to be used for crop production. A good understanding of the hydraulic characteristics of soils containing an important fraction of rock fragments is crucial for soil and water management in these areas. The objective of this study was to investigate the influence of different rock fragment contents on effective hydraulic properties of skeletal soil with a clay content lower than 35% using measurements of the water retention curve and the hydraulic conductivity. A tension infiltrometer was used to determine the field hydraulic conductivity at four pressure heads (h) of 0, −30, −60 and −120mm. Soil water retention was determined on a pressure plate between −33 and −1500kPa. Finally, the Hydrus-1D was used to predict soil moisture dynamics using the obtained effective hydraulic parameters. The results show a decreasing water retention capacity with increasing rock fragment content. The saturated hydraulic conductivity decreased with increasing stone contents from 0 to 20%, but then increased for increasing stone content. Contradicting behavior can be observed using field and lab measurements, clearly exposing the need for a better understanding of the functioning of stony soils.

Evaluation of the spatial pattern of surface soil water content of a karst hillslope in Southwest China using a state-space approach

ABSTRACTThis study aims to evaluate the effects of soil physicochemical properties and environmental factors on the spatial patterns of surface soil water content (SWC) based on the state-space approach and linear regression analysis. For this purpose, based on a grid sampling scheme (10 m × 10 m) applied to a 90 m × 120 m plot located on a karst hillslope of Southwest China, the SWC at 0–16 cm depth was measured 3 times across 130 sampling points, and soil texture, bulk density (BD), saturated hydraulic conductivity (Ks), organic carbon (SOC), and rock fragment content as well as site elevation (SE) were also measured at these locations. Results showed that the distribution pattern of SWC could be more successfully predicted by the first-order state-space models (R2 = 67.5–99.9% and RMSE = 0.01–0.14) than the classic linear regression models (R2 = 10.8–79.3% and RMSE = 0.11–0.24). The input combination containing silt content (Silt), Ks, and SOC produced the best state-space model, explaining 99.9% of the variation in SWC. And Silt was identified as the first-order controlling factor that explained 98.7% of the variation. In contrast, the best linear regression model using all of the variables only explained 79.3% of variation.

Soil organic carbon stocks are systematically overestimated by misuse of the parameters bulk density and rock fragment content

Abstract. Estimation of soil organic carbon (SOC) stocks requires estimates of the carbon content, bulk density, rock fragment content and depth of a respective soil layer. However, different application of these parameters could introduce a considerable bias. Here, we explain why three out of four frequently applied methods overestimate SOC stocks. In soils rich in rock fragments (> 30 vol. %), SOC stocks could be overestimated by more than 100 %, as revealed by using German Agricultural Soil Inventory data. Due to relatively low rock fragments content, the mean systematic overestimation for German agricultural soils was 2.1–10.1 % for three different commonly used equations. The equation ensemble as re-formulated here might help to unify SOC stock determination and avoid overestimation in future studies.

Deep soil water extraction helps to drought avoidance but shallow soil water uptake during dry season controls the inter-annual variation in tree growth in four subtropical plantations

Some essential features of forest hydrologic cycles are based on observations of the soil water balance. However, measurements of soil water dynamics in subtropical forests have mostly been restricted to the upper 100cm of soil. The relative shallow depth in soil water measurement would bias the understanding how and to what extent soil water contributes to evapotranspiration. We investigated and compared the soil water dynamics down to 200cm depth over 7 years in four subtropical plantations, including one mixed stand type (Cunninghamia lanceolata mixed with Schima superba) and three pure stand types (C. lanceolata, Pinus elliottii, and Pinus massoniana). We also examined the variations in fine root distribution and tree basal area increment across stand types. We showed that, in contrast to the P. elliottii and P. massoniana stands, the C. lanceolata stand type extracted deep soil water when the top soil was still wet, depleted soil water over a longer season, but consumed less total soil water during the dry season. Our results revealed that the mixed stand type depleted more soil water during the dry season than the pure C. lanceolata stand type. In addition to the species-specific traits of water consumption, the different soil water depletion between stand types seems to be also accounted for by the different slope aspect and rock fragment content. During the entire dry season, the four stand types mainly relied on the soil water in the 100–200cm soil depth where fine root density was considerably low, however, the inter-annual variation in stem growth depended on the soil water depletion from 0 to 50cm soil layer. We conclude from these results that deep soil water is likely to make a significant contribution to drought avoidance over the dry period, but the available of shallow soil water to the fine roots during dry season may determine how well trees grow. This study highlights the important to consider the deep soil water extraction when explain the ecosystem evapotranspiration variations.

Soil control over the distribution of Mediterranean oak forests in the Montsec mountains (northeastern Spain)

Models of plant species distribution and response to global change rely mostly on climatic variables alone, whereas soil variables are usually not taken into account. Evergreen and marcescent oaks are therefore considered to share environmental niches in the Mediterranean region despite their functional differences. We studied the distribution of forests dominated by either Quercus ilex (QI plots) or Q. faginea/Q. subpyrenaica (QF plots) in 46 plots at an altitude between 570m and 980m on a north-facing slope in northeastern Spain. We used binomial logistic regression and classification tree analysis to explain the distribution of the two types of forest. Soils of the sample plots were mostly Lithic Xerorthents developed from limestone. Surface mineral horizons of QI forests had higher organic carbon (C), nitrogen (N), and NaOH-extractable phosphorus concentrations, while organic layers had smaller values of the C/N and C/P ratios. Soils of QF forests accumulated higher amounts of C despite the lower concentration in their surface mineral horizons. The distribution of QF and QI forests was significantly explained by the variability in soil available water-holding capacity and rock fragment content, QF forests appearing on soils with over 22mm of available water-holding capacity and <26% rock fragments. The presence of Acer monspessulanum, a secondary tree species, was related to soils with few rock fragments and high pH. Soil variability produces different patterns of water availability under homogenous macroclimatic conditions that are differently suited to the two types of forest. Information about whole soil profiles is required in the assessment of present vegetation distribution and future response to climate change.

Assessment of gravelly soil redistribution caused by a two-tooth harrow in mountainous landscapes of the Yunnan-Guizhou Plateau, China

The processes of soil redistribution caused by hoeing tillage in hilly areas have been increasingly recognized for nearly two decades. Yet, few studies have examined the effects of hoeing tillage on soil redistribution in rocky mountainous areas. Due to a high rock fragments content of gravelly soils, a special type of hoe called the two-tooth harrow has been widely used in the Yunnan-Guizhou Plateau, China. In order to assess gravelly soil redistribution, a series of tillage experiments were set up and the subsequent data were compared with those acquired previously in hilly areas. The displacement distance of gravelly soils increased with slope gradient linearly in the range of 0.05 to 0.69mm−1; however, when the slope gradient extended further, an exponential equation could better describe the relationship between them. The repose angle of gravelly soils was approximately 0.69–0.70mm−1. Large-size rock contents and implement types are two important factors affecting tillage depth. Tillage transport coefficients (k3 and k4) of gravelly soil in rocky mountainous areas were 40 and 78kgm−1 tillage pass−1, respectively, displaying a slightly large k3 value but a significantly small k4 value compared with those in hilly areas. Due to its energy saving and applicability to gravelly soils, the two-tooth harrow is more attractive to farmers than the normal hoe in rocky mountainous areas. Moreover, the two-tooth harrow is proved to be efficient and effective in reducing tillage erosion than the normal hoe.

Comparison of different methods to determine wind-erodible fraction of soil with rock fragments under different tillage/management

Rock fragments (the coarse fraction >2mm in soil) widely exist in the surface soils of the agro-pastoral ecotone of northern China (APEC region) where is suffering severe soil degradation induced by wind erosion. The wind-erodible fraction of soil aggregate (<0.84mm dry soil aggregate) (EF) is an important parameter to evaluate soil management systems and estimate wind erosion. Rotary sieve is considered to be a standard device to determine EF. Flat sieves and EF estimation equation used in the Revised Wind Erosion Equation (RWEQ) are two alternatively methods to obtain EF. In this study, 74 soil samples for different soil management/tillage in a typical region of the APEC region were collected to compare rotary sieve, flat sieve and the EF estimation equation for determining EF. The results showed that (1) the EF determining from rotary sieve (EF-r) is closely related to the EF obtaining from flat sieve (EF-f) but the EF-r are generally lower than the EF-f, (2) the EF-r is significantly associated with surface soil rock fragments content for all soil management, (3) the EF-r is not related to soil textural fractions, organic matter and CaCO3 content for tree windbreak, conservation and conventional tillage farmland but is related to that for grassland, (4) the EF estimation equation in the Revised Wind Erosion Equation (RWEQ) cannot be used to predict the measured EF in the APEC region.

Using residual analysis in electromagnetic induction data interpretation to improve the prediction of soil properties

The electromagnetic induction (EMI) technique has been widely used to survey soil properties at intermediate spatial scales. However, EMI data interpretation remains a challenge for more accurate and robust mapping. Residual analysis is an alternative approach that can be used to improve the EMI data mining. On a tea garden (TG) hillslope and a bamboo forest (BF) hillslope, terrain indices were used to regress the apparent electrical conductivity (ECa) in ten repeated EMI surveys using stepwise multiple linear regressions (SMLR). Residuals of ECa in these regressions, which had terrain influence removed, were then calculated. The classification and regression tree (CART) model was adopted to quantify the relative contributions of terrain indices (elevation, slope, plane curvature–PLC, profile curvature–PRC, and topographic wetness index–TWI), static soil properties (rock fragment content–RFC, depth to bedrock–DB, contents of clay, silt and sand), and dynamic soil property (volumetric soil moisture–θ) to ECa and their residuals. Results show that contributions of terrain indices to ECa are around 50%. However, contributions of terrain indices to ECa residuals are <20%, while great contributions of different soil properties to ECa residuals can be observed in some cases. On both hillslopes, better predicting accuracies were achieved when using ECa residuals as independent variables in SMLRs to predict soil properties than using only terrain indices or ECa as independent variables. Similarly, on both hillslopes, using terrain indices plus ECa residuals as independent variables also yield better prediction of θ than using only terrain indices or using terrain indices plus ECa as independent variables. Findings of this study indicate that residual analysis can be a useful technique in improving EMI data interpretation for estimating the spatial variations of soil properties. In cases that relationship between target soil properties and ECa readings are weak, this approach can probably be used to improve the mapping accuracy of the target soil properties.

Surface soil water content and its controlling factors in a small karst catchment

Spatial distribution of soil water content (SWC) plays an important role in soil hydrological processes. Karst areas are especially heterogeneous with respect to soil distribution and properties and have not been adequately studied. The purpose of this study was to characterize the surface SWC and to discuss its controlling factors in a small karst catchment in Guangxi, China. SWC was measured using the gravimetric method along seven line-transects crossing farmland, shrub-grassland, shrubland and forestland, as well as hillslopes and depressions. Soil bulk density, capillary porosity, macroporosity, rock fragment content (RC), soil organic carbon and soil texture of the soil samples were also determined. Regression and sensitivity analysis methods were used to analyze the influence of soil properties on SWC in both the wet and dry season. Generally, SWC was significantly higher in depressions than on hillslopes. In the shrub-grassland and shrubland, SWC decreased with increasing elevation, whereas in the forestland, no obvious trend along the transect was found. Soil porosity, RC and soil texture were three main soil properties that influenced SWC significantly. Moreover, the distribution of SWC along the line-transects were coincidence with that of these soil properties. The results document the spatial distribution of SWC in a karst catchment in southwest China, laying a foundation for a better understanding and modeling of hydrological processes in this complex landform.

Temporal stability analysis of surface soil water content on two karst hillslopes in southwest China.

Knowledge of the temporal variability of soil water content (SWC) at the hillslope scale is essential for guiding rehabilitation strategies and for optimizing water resource management in the karst region of southwest China. This study aimed to use temporal stability analysis to upscale point-scale measurements to represent mean areal SWC on two typical karst hillslopes. Based on a grid sampling scheme (10 m × 10 m) applied to two 90 m × 120 m plots located on two hillslops, the SWC at a depth of 0-16 cm was measured 11-12 times across 259 sampling points, using time domain reflectometry (TDR) from April 2011 to October 2012. Soil texture, bulk density (BD), saturated hydraulic conductivity (K s ), organic carbon (SOC), rock fragment content (RFC), and site elevation (SE) were also measured at these locations. Results showed the hillslope with more shrub cover was wetter than the hillslope with mixed grass-shrub cover. This difference was related to the differences in soil texture, soil hydraulic permeability, and topography. Through a comparison of values obtained with the Spearman correlation coefficient (r s ), standard deviation of mean relative difference (SDRD), and mean absolute bias error (MABE), we inferred that there is a higher degree of temporal stability for SWC in wet conditions than in drier conditions on the two hillslopes. Based on the values of the index of temporal stability (ITS), which combine the mean relative difference (MRD) and SDRD, the two locations were determined to be representative of mean SWC on both hillslopes. Moreover, these locations captured changes in mean SWC (NSCE = 0.69, and 0.65, and RMSE = 1.96, and 1.96 %, respectively). This demonstrates the feasibility of using the temporal stability of SWC to acquire mean SWC on karst hillslopes of southwestern China. The indirect method, which estimates mean SWC by considering the offset between the mean and the measurement value at a time-stable location, predicted mean SWC (NSCE = 0.86, and 0.76, and RMSE = 1.29, and 1.63 %, respectively) more precisely than the direct method (mean SWC directly measured at a time-stable location), because it eliminates deviation by introducing a constant offset (MRD). We recommended the use of the indirect method to acquire mean SWC values, when an allowable bias of 5 % for both MRD and SDRD can not be achieved. In addition, we found that soil texture, RFC, and elevation affect the pattern of SWC on the shrub hillslope. These results are expected to be useful for monitoring soil water dynamics on karst hillslopes, especially for restoration purposes.

Lithologic characteristics and diagenesis of the Upper Triassic Xujiahe Formation, Yuanba area, northeastern Sichuan Basin

The Upper Triassic Xujiahe sandstone in the Yuanba area, northeastern Sichuan Basin, is a typical coal-bearing tight gas reservoir. Whether or not the commercial production of gas from these tight sandstones can be achieved largely depends on the quality of the reservoirs. In this study, a petrographic analysis using a thin section and a scanning electron microscope (SEM), combined with reservoir property data, was carried out to discuss the effects of primary mineral compositions on diagenetic processes and reservoir quality.The Xujiahe sandstones are mainly litharenite and feldspathic litharenite with minor sublitharenite, quartzarenite and calcarenaceous sandstone. Reservoir quality is poor, with porosity ranging mainly from 1% to 4% and permeability mainly from 0.01 md to 0.1 md, among which the relatively high values are limited to litharenite and feldspathic litharenite. The anomalously poor quality of sandstones is attributed to the high compaction and calcite cementation during the early diagenetic stage while the relatively high quality is associated with the precipitation of chlorite rims. These diagenetic processes are functions of the sandstones composition and the primary sandstones composition is intrinsic to the quality of the Xujiahe sandstones. The sandstones with high contents of matrix and ductile rock fragments (DRFs) and/or carbonate rock fragments (CRFs) commonly experience intense compaction or cementation, which are key factors for the poor quality of reservoirs. Although the contents of matrix, DRFs and CRFs are low in quartzarenite, intense compaction also occurs due to the finer grain size. Only litharenite and feldspathic litharenite with minor contents of matrix, DRFs, CRFs and volcanic rock fragments (VRFs) commonly experience the precipitation of chlorite rims, and are therefore recommended to be the favourable tight gas reservoirs in the Yuanba area.

Geochemistry of the sandstones of Punagarh basin: Implications for two source terranes and Arabian - Nubian connection of Aravalli craton?

Sandstones of Punagarh basin of Trans Aravalli region, NW Indian shield were analyzed for their major and trace element contents in conjunction with petrographic modes. The Punagarh basin comprises four formations (Sojat, Bambolai, Khamal and Sowania), amongst which the Sojat Formation in uncoformable contact with overlying formations, is significantly older and enjoys separate status as rest of the formations constitute Punagarh Group. Petrographic attributes suggest that Sojat sandstones contain distinct modal abundances like high content of quartz and low content of feldspar, mica, matrix, rock fragments and cement. In general there is a decrease in the average modal abundance of quartz and mica with concomitant increase of feldspar, chert and rock fragments from base to top in Punagarh sandstones. Sojat sandstones are also geochemically distinct as they possess high SiO2/Al2O3, Th/U and Cr/Th ratios coupled with lowest Na2O/K2O ratio and Zr content, least fractionated LREE, more fractionated HREE and largest Eu* anomaly compared to Punagarh sandstones. The weathering indices suggest intense chemical weathering for Sojat sandstones and low to moderate for Punagarh sandstones. Compositionally all the sandstones of the Punagarh basin come under the category of quartzarenite. Compared to PAAS and UCC, both suite of sandstones are generally depleted in REE, HFSE (with exception of Zr, Hf and Ta), and enriched in ferromagnesian trace elements particularly Cr and Co. The chemical data indicate that the sediments were derived from the source(s) of mixed felsic - mafic composition, with the former being dominant. The source rocks were granites, TTG, basalts, and rhyolites. The Sojat sandstones owe their source form Archean crust while Punagarh sandstones got detritus from Mesoproterozic crust. Immobile element ratios and REE abundances of Sojat sandstones closely match with Paleoproterozoic metagreywackes of Ghana of African craton while Punagarh sandstones show near geochemical characteristic with Gogunda, Kumbalgarh and Vindhyan quartzites of Aravalli craton. This geochemical similarity of Sojat sandstones provides credence to the hypothesis that Trans–Aravalli region of India had been an integral part of Arabian - Nubian shield. The chemical data advocate the deposition of these sandstones in a tectonic setting comparable to modern back arc setting.

Sampling for Soil Carbon Stock Assessment in Rocky Agricultural Soils

Core Ideas Soil mass is the greatest source of uncertainty for C stocks on rocky soils. Coring methods underestimate soil rock fragment content. Hammer coring methods underestimate soil mass. Mass‐based C stock reporting can overcome coring method bias. Rotary corers are a cost‐competitive and less biased alternative to standard corers. Coring methods commonly employed in soil organic C (SOC) stock assessment may not accurately capture soil rock fragment (RF) content or soil bulk density (ρb) in rocky agricultural soils, potentially biasing SOC stock estimates. Quantitative pits are considered less biased than coring methods but are invasive and often cost‐prohibitive. We compared fixed‐depth and mass‐based estimates of SOC stocks (0.3‐m depth) for hammer, hydraulic push, and rotary coring methods relative to quantitative pits at four agricultural sites ranging in RF content from &lt;0.01 to 0.24 m3 m−3. Sampling costs were also compared. Coring methods significantly underestimated RF content at all rocky sites, but significant differences (p &lt; 0.05) in SOC stocks between pits and corers were only found with the hammer method using the fixed‐depth approach at the &lt;0.01 m3 m−3 RF site (pit, 5.80 kg C m−2; hammer, 4.74 g C m−2) and at the 0.14 m3 m−3 RF site (pit, 8.81 kg C m−2; hammer, 6.71 kg C m−2). The hammer corer also underestimated ρb at all sites as did the hydraulic push corer at the 0.21 m3 m−3 RF site. No significant differences in mass‐based SOC stock estimates were observed between pits and corers. Our results indicate that (i) calculating SOC stocks on a mass basis can overcome biases in RF and ρb estimates introduced by sampling equipment and (ii) a quantitative pit is the optimal sampling method for establishing reference soil masses, followed by rotary and then hydraulic push corers.

Effect of rock fragments on soil porosity: a laboratory experiment with two physically degraded soils

There are still few studies on the role that rock fragments (RFs) have in the change in soil structure based on direct observation of the soil‐pore system. Physically degraded soil is of particular interest because small RFs might be considered a factor in its remediation. In our laboratory experiment we mixed five different proportions of 4–8‐mm sized RFs with a Luvisol and a Regosol that have poor ability to self‐structure and were characterized by a massive structure in the field. Nine wetting and drying cycles were applied to repacked samples (15 cm in diameter and height) of soil–RF mixtures to facilitate the formation of soil structure. Image analysis was used to quantify development of the pore system at varying RF contents. The physically degraded soils studied in this research initially showed a decrease and then an increase in porosity with increasing amounts of RFs in the soil–RF mixtures. The results indicated that the Regosol responded more than the Luvisol to RF content. Threshold values of RF content at which the mechanism of pore formation prevails over that of pore reduction depended upon the soil type and can be attributed reasonably to small differences in the coefficient of linear extensibility (COLE). We also identified a mechanism for the propagation of porosity downwards from the soil surface with increasing RF content, together with a vertical homogenization effect on porosity. Our results contribute to understanding the mechanisms by which small rock fragments embedded in physically degraded topsoil induce changes in the pore system and confirm the potential of rock fragments to protect soil structure in soil susceptible to compaction.HighlightsPhysical interaction between rock fragments and fine earth in degraded soil. Soil porosity examined by combined experimental laboratory approach with image analysis. Soil porosity first decreases then increases with increasing content of rock fragments. Coexistence of two opposing mechanisms: one of porosity reduction and one of formation of new pores.

Effect of rock fragments content on water consumption, biomass and water-use efficiency of plants under different water conditions

The proportion of rock fragments in soil affects water availability and therefore the characteristics of plants. The objective of this study was to evaluate the effect of rock-fragment content on plant water consumption, biomass, growth and water-use efficiency (WUE) under different water conditions. Four gravimetric treatments of rock-fragment contents (0, 10, 30 and 50%) and four treatments of water content were tested in sandy loamy soils. The water contents of the rock-free soil were 15–19% (80–100% of field capacity), 11–15% (60–80% of field capacity), 9–11% (47–60% of field capacity) and 6–9% (32–47% of field capacity). Transpiration, plant height, basal stem diameter and biomass of korshinsk peashrubs in the treatments were measured and compared. Plants grown in the soil with rock fragments transpired less, especially under well-watered conditions. The mean daily transpiration of plants in the soils with 30 and 50% rock-fragment contents was 18% (P=0.021) and 34% (P=0.001) lower, respectively, in 2014, and 25% (P=0.008) and 31% (P=0.002) lower, respectively, in 2015 relative to the soil without rock fragments and was not lower in the soil with 10% rock fragments. Plant height, basal stem diameter and biomass did not differ significantly between rock-fragment contents of 0 and 30% but were lower at 50%. WUE, the ratio between total transpiration and biomass, was highest at 30% and then decreased at 50%. Increasing plant water stress could thus improve WUE. The rock fragments in the soil had significant effects on plant water consumption, biomass, growth and WUE. Optimizing the rock-fragment content is necessary when the relationships between plants and water in stony ecosystems are evaluated.

The effects of rock fragment shapes and positions on modeled hydraulic conductivities of stony soils

Mountainous soils usually contain a large number of rock fragments, particles with a diameter larger than 2mm, which can influence soil hydraulic properties that are required to quantitatively describe soil water movement in stony soils. The objective of this study was to numerically estimate both the saturated hydraulic conductivity of a stony soil and its dependence on a relative content of rock fragments (stoniness), and the shape, position and distribution of rock fragments in a soil matrix. The assessment method was based on a numerical version of Darcy's classic experiment that involved steady-state flow through a porous material under a unit hydraulic gradient. Our experiments, involving hypothetical stony soils in this particular case, were simulated using mainly the two-dimensional (2D) numerical model, HYDRUS-2D. A limited number of simulations were carried out using a three-dimensional HYDRUS model. Three different shapes of hypothetical rock fragments were used in the study: a sphere, an ellipsoid with two different positions, and a pyramid, all represented by their 2D cross-sections (i.e., a circle, an ellipse, and a triangle, respectively). The mean relative effective saturated hydraulic conductivity (Krs) for the same stoniness was almost the same for all simulated scenarios and fine soil textures. A stoniness between 0.07 and 0.5cm3cm−3 can cause a decrease of Krs in the range of 0.17–0.70. Numerical experiments were divided into 3 scenarios. The largest and the smallest values of Krs were different for different shapes of RFs (scenario A), different orientations of the slab-sided elliptical RFs (scenario B), and regular or irregular distributions of spherical RFs (scenario C). The largest difference between Krs values (0.26) was found in scenario B when the slab-sided elliptical RFs were oriented either horizontally or vertically for stoniness of 0.24 or 0.31 cm3cm−3. Simulated Krs values were underestimated in all scenarios as compared to the Ravina and Magier (1984) function. The smallest differences (−1.1%–2.5%) between numerically simulated and calculated (the Corring and Churchill (1961) method for a cylindrical shape of RFs) Krs values were found for scenario A with its 2D representation of spherical rock fragments. Calculated (the Corring and Churchill (1961) method for a spherical shape of RFs) Krs values corresponded well with those simulated using a 3D representation of spherical rock fragments. Numerical models provide a unique opportunity to evaluate the effects of different factors on the saturated hydraulic conductivity of stony soils that may be nearly impossible to measure in practice.

Effects of ‘red unit’ deposit on Acacia trees in the hyper-arid southern Israel

During the last few decades, populations of the Acacia genus across the hyper-arid Arava Valley and southern Negev have faced considerable demographic changes, with high mortality rates as a predominant trend. We suggest that, in addition to the decreasing precipitation rates and the resultant decrease in flood events during this period, these changes are not homogenous across the region but are related to the type of wadis' (ephemeral streams') riverbed. The largest part of the wadis across the region is covered by stony alluvium stratum. At the same time, a small share of the area and some of the wadis are covered by a hard (cement-like) paleo layer of fine-grained reddish sediments (‘red unit’ deposit, also named the Zehiha Formation). Of the most important observable differences is the high rock fragment (particles >2000μm) content in the alluvium as opposed to the practically no rock fragment content in the red unit. The Acacia trees and wadi riverbeds (alluvium stratum/red unit deposit) were studied in six wadis during the summer of 2015. Of these wadis, three contain an alluvium stratum and other three, a red unit deposit. The study results revealed very high variability in terms of overall (both alive and dead) tree density in the wadis. Yet, the mean overall tree density was (though not significantly, P=0.5463) 42% greater in the red unit deposit than that in the alluvium stratum. At the same time, mean percentage of alive trees was significantly (P=0.0437) and 9% greater in the red unit deposit than that in the alluvium stratum. Generally, characteristics of the red unit deposit indicated much better soil quality than those of the alluvium stratum. These included the texture (clayey in the red unit vs. sandy in the alluvium), hygroscopic moisture content (fivefold greater in the red unit), soil organic carbon concentration (69% greater in the red unit), and calcium carbonate content (24% smaller in the red unit). Above all, means of both water field capacity and permanent wilting point were approximately threefold greater in the red unit deposit than that in the alluvium stratum, resulting in the mean available water capacity to be threefold greater in the red unit than that in the alluvium. Strongly positive and significant (P<0.0001) correlation was found between the percentage of alive trees and available water capacity (r=0.91), but not between overall tree density and this variable. It is concluded that as long as precipitation regime and flood frequency are normal (similar to the long-term average) in these hyper-arid drylands, the Acacia similarly grow and establish under both the red unit deposit and alluvium stratum. However, once long-term droughts or climatic change occur, with the resultant decrease in precipitation rates and flood frequency, the red unit deposit alleviates water stress, considerably increasing Acacia vitality and survivability.

Rootable depth controls height growth of Pinus halepensis Mill. in gypsiferous and non-gypsiferous soils

Pinus halepensis is generally considered a species adapted to soils with gypsum but there is hardly any data available to support such statement nor to assess the degree to which soil gypsum may constrain tree development. We studied fifty five 200m2-plots in a P. halepensis plantation in NE Spain, 23 on soils with gypsum and 32 on soils without gypsum. Trees were measured to estimate site index at age 40years (SI40). A soil pit was described in each plot to a depth of 1m or to a root-limiting layer, and samples of the various horizons analysed for pH, organic carbon (C), total nitrogen, Olsen phosphorus (P), exchangeable potassium (extracted with NH4OAc), calcium carbonate (calcimeter method), and gypsum concentration (Artieda method), and texture. We studied root development in the soil horizons of 15 of these plots by counting root numbers at depths of 0–30cm, 30–55cm, and 55–80cm in three 100cm2-squares per depth. Penetration resistance and bulk density were also measured in these horizons. Soils with gypsum were frequently less than 25cm deep, and had negligible concentrations of Olsen phosphorus. Values of SI40, with a maximum of 15.5m, were primarily determined, in all types of soils, by a positive effect of soil rootable depth, indicating the dominant influence of water availability, and to a lesser extent by the negative effect of the C/P ratio and rock fragment content in the upper 30cm of soil. Density of fine and very fine roots decreased in deeper soil horizons from a maximum value of 97 roots·dm−2 in the surface horizons. Mechanical impedance by increased penetration resistance was the main limitation for root development. Soil gypsum does not have a direct influence on growth but constrains the volume of soil that may be explored by roots.

Rapid weathering processes of a 120-year-old chronosequence in the Hailuogou Glacier foreland, Mt. Gongga, SW China

Increasing numbers of ‘young’ soil chronosequences have become exposed in global alpine areas as glaciers have retreated due to climate warming since the Industrial Revolution. However, knowledge on the initial stages of weathering at the decadal time scale is limited. Our aim is to reveal the weathering processes and influential factors in a 120-year-old chronosequence on Mt. Gongga in southwest China. Sixty-nine soil samples and 17 water samples were collected from 6 sites with different exposure ages (0, 30, 40, 52, 80 and 120 years). Mineral compositions were determined using X-ray diffractometry, and four mineral pools, i.e., (1) exchangeable cations, (2) carbonates and apatite, (3) biotite and hornblende and (4) refractory minerals, were separated using a sequential extraction method. The results reveal that the soil particle sizes of < 2 μm (clay) in the A horizons increased and the particle sizes of 20 – 2000 μm (sand) decreased significantly with time. The soil organic carbon accumulation rate was up to 14 g/m2·yr, which is relatively high compared with other chronosequences of similar age. The weathering can be generally divided into two stages: the first stage was dominated by the weathering of carbonates and was present at sites with an exposure age of ≤ 52 years; the second stage was characterized by the intense weathering of biotite, hornblende and apatite and was present at the 80- and 120-year-old sites, where coniferous trees were the dominant plants. The rapid weathering was a result of the interactions of the lithology, rapid vegetation succession and climate in the Hailuogou Chronosequence. Fine silt was produced in large quantities by the strong glacial abrasion and freeze-thaw cycles, providing rich materials for the subsequent chemical and biogeochemical weathering. The decarbonation and the rapid establishment of the conifer forest due to relatively high temperatures and precipitation promote mineral weathering by decreasing soil pH from 8.5 (0 years) to 4.8 (120 years). The high permeability of the soil horizons due to the high content of rock fragments and abundant precipitation allows water and organic acids to infiltrate and dissolve mineral soils above the impervious bedrock. This study highlights the importance of carbonate weathering in alpine biogeochemical cycling during the initial stage of weathering in granitoid rock areas. Furthermore, our results indicate that the weathering intensity increases with exposure age due to vegetation succession in the early stages of weathering.