Soil water repellency and influencing factors of Nitraria tangutorun nebkhas at different succession stages

Abstract： Soil water repellency （WR） is an important physical characteristic of soil surface. It is capable of largely influencing the hydrological and geomorphological processes of soil, as well as affecting the ecological processes of plants, such as growth and seed germination, and has thus been a hot topic in recent research around the world. In this paper, the capillary rise method was used to study the soil WR characteristics of Nitraria tangutorun nebkhas. Soil water repellencies at different succession stages of Nitraria tangutorun were investigated, and the relationships between soil WR and soil organic matter, total N, and total P, soil texture, pH, and concentrations of CO32, HCO3-, CI, SO42-, Na~, K~, Ca2~ and Mg2＋ were discussed. Soil WR may be demonstrated at the following nebkhas dune evolvement stages： extremely degraded〉degraded〉stabilized〉well developed〉newly developed〉quick sand. Apart from some soil at the bottom, the WR of other soils （crest and slope of dune） was found to be largest at the topsoil, and decreased as the soil depth increased. The results showed that multiple factors affected soil WR characteristics e.g. WR increased significantly as the contents of soil organic matter and total N increased, but did not change as the total P content increased. Soil texture was a key factor affecting soil WR; soil WR increased significantly as clay content increased, and decreased significantly as sand content increased. Low pH was shown to be more suitable for the occurrence of soil WR. Four cations （Ca2＋, Mg2＋, K＋ and Na＋） and two anions （CI and SO42） enhanced soil WR, while CO32-decreased it. HCO3- did not show any observable effect. Finally, we established a best-fit general linear model （GLM） between soil-air-water contact angle （CA） and influencing factors （CA=5.606 sand＋6.496 （clay and silt）-2.353 pH＋470.089 CQ2＋11.346 Na＋-407.707 Cl--14.245 SO42-＋0.734 total N-519.521 ）. It was concluded that all soils contain subcritical WR （0°〈CA〈90°）. The development and succession of Nitraria tangutorun nebkhas may improve the formation of soil subcritical WR. There exist significant relationships between soils subcritical WR and soil physical or chemical properties.

Fertile island effect in the sedimentary process of Tetraena mongolica Maxim nebkhas in steppe–desert ecotones on the Inner Mongolia Plateau,China

Phytogenic mounds(nebkhas)formed by shrubs are a common phenomenon in arid and semiarid areas.The formation of nebkhas is accompanied by the appearance of the fertile island effect.Quantitative evaluation of the shrub sand compost island effect is a key link in preventing soil erosion and nutrient loss.This study took the typical shrub Tetraena mongolica in desert areas as the research object and quantified the sand-trapping capacity of the shrub.We revealed the influence of sediment texture and volumetric soil water content on fertile islands during the development of T.mongolica nebkhas.The results showed that(1)the single shrub intercepted large amounts of sediments due to the high density of branches of T.mongolica,forming nebkhas that were positively correlated with the shrub size.(2)The overall soil nutrient content below the shrub was greater than the soil nutrient content outside the shrub,forming a typical fertile island effect.The soil organic carbon(SOC),alkaline hydrolytic nitrogen(AHN),available phosphorus(AP),and available K(AK)content all increased gradually with increasing nebkha volume.Compared with the deep soil of nebkhas,the nutrient content of the surface soil was generally higher.(3)There was a positive correlation between the volumetric water content and nutrient content in nebkhas.(4)The semi-ellipsoid shape of T.mongolica enabled it to intercept large amounts of coarse-grained material.Fine sand(100–250μm)was the main particle size in the sediment aggregates inside the nebkhas.The fine sand content generally increased with increasing shrub size.Redundancy analysis(RDA)revealed that the fine sand content of the nebkha sediments had a strong positive correlation with the soil nutrient content.This paper provides an example for evaluating the fertile island effect during the deposition process of nebkhas in the desert transition zone.

Phytogenic Mounds (Nebkhas): Effect of Tricomaria usillo on Sand Entrapment in Central-West of Argentina

Nebkhas, developed by the trapping of sand within the body of a plant, were studied in the Médanos Grandes system, arid central Argentina, during the springs of 2009-2010. The dynamics of nebkhas was studied in three draas (megadunes), and considering both orientations: leeward and windward. The Drift Potential (DP) for the study area was 42, evidencing the inactivity of the sand dunes or the scarce activity only in crests. Dominant sand movement is in south-southeast direction, with deflation processes at the southern side of the nebkhas. All nebkhas showed uniformity in the morphometry. Tricomaria usillo is the dominant plant species in the nebkha formation process;and results showed a significant positive relationship between nebkha and canopy volumes.

Changes in morphology and soil nutrient patterns of nebkhas in arid regions along a precipitation gradient

Nebkhas, discrete mounds of sand and vegetation, are a common landscape feature critical to the stability of desert ecosystems and supported by limited precipitation. Nebkha morphology and spatial pattern vary in landscapes, but it is unclear how they change along precipitation gradients in arid and semi-arid regions. In this study we determined morphology and soil nutrient patterns of nebkha from different regions of northwestern China. The objective of this study was to understand zonal differences among nebkhas and how morphological characteristics and soil nutrient patterns of nebkha change along a precipitation gradient in northwestern China. Our results shows that mean annual precipitation(MAP) had significant effects on morphological characteristics of nebkhas such as height, area, and volume which significantly decreased with an increase in MAP. MAP had significant positive effects on shrub cover and species richness of nebkha. Soil nutrients such as soil organic matter(SOM), total carbon(TC), total nitrogen(TN), and total phosphorus(TP) in the 0-10 cm layer increased with an increase of MAP, and soil nutrient content within nebkhas was higher than in inter-nebkha areas.We concluded that nebkhas are "fertile islands" with an important role in ecosystem dynamics in study regions. Further,MAP is a key factor which determined zonal differences, morphological, and soil nutrients patterns of nebkhas. However,disturbance, such as animal grazing, and planted sand-stabilizing vegetation accelerated the degeneration of nebkha landscapes. We recommend implementation of protective measures for nebkhas in arid and semi-arid areas of China.

Distribution and Characteristics of Nitraria sphaerocarpa nebkhas in a Gobi habitat outside an oasis in Hexi Corridor region,China

Nitraria sphaerocarpa Maxim., a typical desert plant which often appears in nebkha form, is widely distributed along the periph- ery of oasis in arid regions of Northwest China, and plays an important role in desertification control around oases. In this paper, the characteristics and distribution patterns ofN. sphaerocarpa nebkhas were investigated in a Gobi environment outside an oasis. Results shows that about 200 N. sphaerocarpa nebkhas per hectare was recorded and total sand sediments was 25 m3/ha. Nebkhas vary in size, and the mean height （H） and horizontal composition （L） were 15 cm and 80 cm, respectively. There exists a signifi- cant linear relationship between H and L, suggesting that the investigated nebkhas are mostly in the growth stage. The numbers, height and sand deposit ofnebkhas decreased with increasing distance away from the oasis. Grain-size analysis indicates that the mean grain size of sediments at the crests of nebkhas increased with increasing nebkha height. Organic C and total N concentra- tions were related to the mean grain size. It was concluded that N. sphaerocarpa shrubs are efficient horizontal sand traps for wind-driven sediments. A better understanding of the development of nebkha can provide information for the design of biological sand control measures around oases in add regions.

The progress and prospects of nebkhas in arid areas

Nebkhas are often extensively distributed in arid and semi-arid areas, and play an important role in the stabilization of ecological environment in these areas. This paper reviews the following aspects, including formation and succession, spatial distribution, shape and surface airflow characteristics, balance between erosion and deposition, ecological and physiological characteristics of vegetation, response to precipitation and groundwater, soil properties as well as the protection measures. We found that nebkhas were studied without differentiation of succession periods, and shape characteristics, surface airflow, soil proper- ties as well as vegetation dynamics in different succession periods were not fully understood, which made it difficult to explain the succession dynamics of nebkhas and its affecting factors. Previous studies of nebkhas were over-emphasized for its role as an indicator of land deg- radation, while its ecological functions in degraded ecosystems were neglected, which was unfavorable for the ecological restorations in arid and semi-arid areas. Future studies should pay more attention to the variation of vegetation, soil as well as hydrological process in the succession of nebkhas and its interaction between different influencing factors. In addition, positive role of nebkhas in degraded ecosystems in arid and semi-arid areas should be fully discussed. According to the data got from the above, effective protection measures of nebkhas should be explored.

Nebkhas play important roles in desertification control and biodiversity protection in arid and semi-arid regions of China

Nebkhas are isolated mounds vegetated with burial-tolerant desert plants that trap windborne sediment within their canopies.Nebkhas are critical to desertification control and biodiversity protection of oases in arid and semi-arid regions of China,as they(ⅰ)trap windborne sediments and ensure oasis security;(ⅱ)as“fertile islands,”nebkhas can trap and provide a suitable micro-environment for seeds,birds,and small soil animals,and increase biodiversity;(ⅲ)increase the soil surface roughness and significantly reduce soil erosion following heavy rain.However,the establishment of sand-fixing vegetation and over-grazing significantly limits the development and accelerate the degradation of nebkhas,threatening oasis ecosystems.We provide background and recommendations for protective measures for nebkha landscapes in arid and semi-arid areas of China to aid managers in oasis protection.

Response of C:N:P in the plant-soil system and stoichiometric homeostasis of Nitraria tangutorum leaves in the oasis-desert ecotone,Northwest China

Nitraria tangutorum nebkhas are widely distributed in the arid and semi-arid desert areas of China.The formation and development of N.tangutorum nebkhas are the result of the interaction between vegetation and the surrounding environment in the process of community succession.Different successional stages of N.tangutorum nebkhas result in differences in the community structure and composition,thereby strongly affecting the distribution of soil nutrients and ecosystem stability.However,the ecological stoichiometry of N.tangutorum nebkhas in different successional stages remains poorly understood.Understanding the stoichiometric homeostasis of N.tangutorum could provide insights into its adaptability to the arid and semi-arid desert environments.Therefore,we analyzed the stoichiometric characteristics of N.tangutorum in four successional stages,i.e.,rudimental,developing,stabilizing,and degrading stages using a homeostasis model in an oasis-desert ecotone of Northwest China.The results showed that soil organic carbon(SOC),total nitrogen(TN),and total phosphorus(TP)contents and their ratios in the 0-100 cm soil depth were significantly lower than the averages at regional and global scales and were weakly influenced by successional stages in the oasis-desert ecotone.TN and TP contents and C:N:P in the soil showed similar trends.Total carbon(TC)and TN contents in leaves were 450.69-481.07 and 19.72-29.35 g/kg,respectively,indicating that leaves of N.tangutorum shrubs had a high storage capacity for C and N.Leaf TC and TN contents and N:P ratio increased from the rudimental stage to the stabilizing stage and then decreased in the degrading stage,while the reverse trend was found for leaf C:N.Leaf TP content decreased from the rudimental stage to the degrading stage and changed significantly in late successional stages.N:P ratio was above the theoretical limit of 14,indicating that the growth of N.tangutorum shrubs was limited by P during successional stages.Leaf N,P,and N:P homeostasis in four successional stages was identified as''strictly homeostasis''.Redundancy analysis(RDA)revealed that soil acidity(pH)and the maximum water holding capacity were the main factors affecting C:N:P stoichiometric characteristics in N.tangutorum leaves.Our study demonstrated that N.tangutorum with a high degree of stoichiometric homeostasis could better cope with the arid desert environment.

Nebkha （coppice dune） formation and significance to environmental change reconstructions in arid and semiarid areas

Environments in arid and semiarid regions are extremely sensitive to climate changes. High wind activity in these regions has resulted in an extensively developed arid geomorphology, but past environmental changes are poorly understood because of the absence of relatively high-resolution proxies. The accumulation characteristics of nebkhas, which have developed extensively in these regions, can be used as a method of reconstructing environmental changes. Here we summarized recent advances in research on the formation, development, and sediment characteristics of nebkhas and their significance to environmental changes in arid and semiarid regions. Based on the studies of our colleagues, we suggested that research on nebkha formation can provide distinct clues about environmental changes in arid and semiarid regions; however, continued studies are needed.