Ameba community dynamics and diversity in a desert ecosystem

A field study was conducted to monitor the effect of different desert shrub ecophysiological adaptations on the composition, size, and diversity of soil free-living amebae. Population diversity was also analyzed using four morphological types. Samples were collected seasonally under the canopy of the common desert shrubs Artemisia herba alba, Reaumuria negevensis, and Noea mucronata. Control samples were taken from exposed interspace areas between shrubs. The composition and diversity of the ameba population were significantly affected by both season and plant species. Types 3 and 4 amebae were found to create a complementary system of adaptation in which type 3 was resistant and adapted to the harsh environment, whereas type 4 was much more vulnerable and existed for short periods of time when the environment allowed. The Reaumuria negevensis ecophysiological adaptation had a negative effect on type 4 amebae by creating a stressed environment.     

Substrate utilization patterns of desert soil microbial communities in response to xeric and mesic conditions

The Negev Desert is characterized by low soil moisture and organic matter content and an unpredictable rainfall amount, dispersion, and intensity. Water and nutrient availability are, therefore, the major limiting factors of biological activity in arid and semi-arid ecosystems. Plants have developed different ecophysiological adaptations in order to cope with the harsh conditions in this xeric environment, e.g., excretion of salt (Reaumuria negevensis) and chemical compounds (Artemisia sieberi) through the leaves. Microorganisms constitute a major part of these ecosystems' total biomass, and are diverse members of the soil food web, being primarily responsible for breaking down complex organic compounds, which are then recycled. They are also known to be very sensitive to abiotic changes and can time their activity to the environmental conditions.Soil samples were collected monthly from a 0 to 10 cm depth, under the canopies of A. sieberi, Noaea mucronata, and R. negevensis. Samples collected from inter-shrub spaces served as control. CO₂ evolution, microbial biomass, microbial functional diversity, and the physiological profile of the community, were determined by MicroResp™ analysis. A significant difference was found between the two dry periods in most of the examined parameters. The values of water, organic matter content, and total soluble nitrogen were higher in soil samples collected in the vicinity of R. negevensis than in samples collected in the vicinity of N. mucronata, A. sieberi, and the open area. A similar trend was found in CO₂ evolution, microbial biomass, and H' values, in which soil samples collected beneath the canopies of N. mucronata and R. negevensis and from open area had higher values during the wet periods (which were characterized by a mesic environment) and in samples collected beneath the A. sieberi in the wet 2006 and dry 2007 periods.     

Microbial biomass reflects a nitrogen and phosphorous economy of halophytes grown in salty desert soil

Seasonal variations in soil salinity were recorded under the canopy of two halophyte shrubs typical of the hot, dry Negev desert, Zygophyllum dumosum and Reaumuria negevensis. The effects of the fluctuating soil salinity levels on total soluble N and on microbial biomass N and P were also monitored. The microhabitat of the shrubs showed differences in trend and magnitude of soil mineral N, the NO _inf3 ^sup- :NH ₄ ⁺ ratio, and microbial N and P. The trends were assumed to be governed by the various mechanisms operating in the shrubs in order to survive salty environments. Data from the current study are discussed in terms of the assumption that the halophyte has developed ecophysiological strategies that force microbial communities coexisting in its microhabitat towards adaptation aimed at withstanding a fluctuating environment, and hence towards a beneficial plant-microorganism relationship.     

Temporal and shrub adaptation effect on soil microbial functional diversity in a desert system

The Negev Desert is characterized by spatial and temporal patterns of resource distribution, in which soil biota are considered to be among the most sensitive biological characteristics, easily influenced by changes related to soil and abiotic factors. Soil water availability and organic matter are among the most important factors, acting as triggers that determine the length of the period of activity. The main source of organic matter in this xeric environment is input from annual and perennial shrubs. In order to persist and propagate in this xeric environment, the plants have developed different ecophysiological adaptations (e.g. the excretion of salt (Reaumuria negevensis) and chemical compounds (Artemisia sieberi) via the leaves). We found that the values of soil moisture obtained for soil samples collected in the vicinity of R. negevensis were larger than for samples collected in the vicinity of Noaea mucronata and A. sieberi and in the open area. The maximum values of CO₂ evolution, microbial biomass and Shannon index (H′) were obtained for the samples collected from the vicinity of N. mucronata. Therefore, we assume that the vicinity of N. mucronata afforded the best conditions for the soil bacterial community. In the Negev Desert, we also found that water availability and pulses of rain compared with frequent rainfall influenced CO₂ evolution, microbial biomass, qCO₂ and the Shannon index (H′). The differences in water amount and availability between the two rainy seasons caused larger values in most of the properties during the first four seasons (December 2005–November 2006) compared with the last four seasons (December 2006–November 2007) for most of the samples.     

Energetic eco‐physiology of the soil microbiota in two landscapes of southern and northern Germany

Interactions between microbial communities and organic matter were analyzed for soils from the project regions ’︁Ecosystem Research in the Agricultural Landscape/FAM, Munich’ in southern Germany and ’︁Ecosystem Research in the Bornhöved Lake district’ from northern Germany using ratios between microbial biomass content (C_mic), microbial metabolic quotient (qCO₂) and organic carbon content (C_org). In the agricultural soils in southern Germany, the qCO₂/C_org ratio differed significantly with respect to agricultural management in contrast to ecophysiological C_mic/C_org ratio. In addition, C_mic/C_org ratio decreased from 39 to 21 mg C_mic g^—1 C_org and qCO₂/C_org ratio increased from 72 to 180 mg CO₂‐C g^—1 C_mic h^—1 (g C_org g^—1 soil)^—1 with increasing soil depth. For the upper soil horizons from the landscape in northern Germany the two quotients differed significantly with reference to land use showing highest microbial colonization under grassland and lowest under beech forest. In contrast, C use efficiency was lowest in arable field under maize monoculture and highest in a wet grassland having a high organic C content.     

Role of perennial plants in determining the activity of the microbial community in the Negev Desert ecosystem

Perennial plants are known to be one of the most influential parameters in desert ecosystems affecting microbial activity. In this study, we examined the importance of these perennial shrubs and attempted to determine the most influential factor that contributes the most to the ecosystem by separating the physical part and the organic contribution of perennial plants. The study site is located in the northern Negev Desert, Israel, where 50 Hammada scoparia shrubs and 50 artificial plants were randomly marked to be used as a tool for the above objectives. Soil samples were collected monthly in the vicinity of the canopies of both shrubs while control samples were collected from the open areas between the shrubs. All samples were collected from the upper (0–10 cm) and the deeper (10–20 cm) soil layers. The contribution to microbial activity was measured by evaluation of the microbial community functions in soil. The results of the research showed a trend of a strong influence of the perennial H. scoparia shrubs on microbial community function. The functional aspects of the microbial community that were measured were CO₂ evolution, microbial biomass, microbial functional diversity, and the physiological profile of the community. High values for all parameters were observed under the vicinity of the H. scoparia shrubs, while the artificial plants were found to have a weaker effect on the community according to soil depth. The upper-soil layer at both locations (artificial plant canopy and control samples) showed higher values of the functioning parameters than that of the deeper soil layer. The results indicate the importance of the organic contribution (plant litter) in comparison to the physical part of desert shrubs.     

Evaluation of the rates of soil organic matter mineralization in forest ecosystems of temperate continental,mediterranean, and tropical monsoon climates

The processes of the organic matter (OM) mineralization in forest soils developed under temperate continental (Moscow oblast, Russia), Mediterranean (the central and western parts of Spain), and tropical monsoon (southern Vietnam) climates were studied under laboratory conditions. The potential and specific rates of the OM mineralization (PR _min and PR _min/C_org, respectively), the ecophysiological parameters of the microbial communities status (C_mic, qCO₂, and C_mic/C_org), and the sensitivity of the rate of the OM mineralization to the rise in temperature were evaluated by the temperature coefficients (Q ₁₀) determined in the humus horizons (0–10 cm, without forest litter). The average values of PR _min for the climatic zones decreased in the following order: Mediterranean (57.1 ± 10.6 mg C/kg per day) > temperate continental (23.8 ± 7.1 mg C/kg per day) > tropical monsoon (10.4 ± 1.6 mg C/kg per day). The lowest resistance of the soil OM to mineralization as evaluated by the PR _min/C_org values was found in the Albeluvisol and Phaeozem of the temperate continental climate and in the Acrisol of the Mediterranean climate. The highest Q ₁₀ coefficients were attributed to the OM mineralization in the forest soils of the temperate continental climate. This allowed us to conclude that the observed and expected climate changes with an increase in the mean annual air temperature should lead to the maximum intensification of the OM mineralization processes in the forest soils of northern regions.     

Micro-patterns of nitrogen mineralization activity in soils of some shrub-dominated semi-desert ecosystems of Utah

Localization of litter fall beneath canopies of desert shrub species results in development of vertical and horizontal soil nitrogen distribution patterns. Concentration of these patterns increases towards the immediate surface of the profile and towards the under-canopy micro-habitat. Similar patterns of N mineralization activity were found in soils collected from shrub communities along an aridity gradient from northern to southern Utah. Mineralization rates were greatest in the 0–2.5 cm horizon of profiles and NO₃^?-N dominated. Below this level activity decreased abruptly and NH₄⁺-N became quantitatively more significant. Mineralization activity peaks in surface soil reflected greater susceptibility of organic N residues to decomposition as well as the higher concentrations of total N in the 0–2.5 cm layer. Results for Atriplex confertifolia communities indicated that N compounds in litter of this species may be particularly prone to microbial breakdown and mineralization. Mineralization of N proceeded at a greater rate in soils from beneath shrub canopies than in soils from interspace areas between shrubs. Development of higher concentrations of total N and mineralization activity in surface soil generally, and under-canopy soil in particular, is interpreted as a structural and functional organization of the N capital of shrub communities that enhances N availability and therefore water-use efficiency of the vegetation.     

How to Outgrow Your Native Neighbour? Belowground Changes under Native Shrubs at an Early Stage of Invasion

While it is acknowledged that invasive species are a global driver of land degradation, their effects are often only noticed when the invasion has been going on for a while. However, early stage processes must play a fundamental role in plant establishment until invasive plants are able to outgrow the native vegetation. In ten plots of 100 m² each, we tested the hypothesis that belowground properties are associated with early invasion processes aboveground. We examined the early stage of invasion by a woody legume (Acacia longifolia ), growing in the canopy of native dune shrubs (Corema album ) as a model system in oligotrophic primary dunes in southern Portugal. Biomass under canopies of invaded and non‐invaded C. album shrubs as well as organic matter (OM) distribution in various soil fractions was measured. In accordance with our hypothesis, A. longifolia presence was related to increased C. album foliar δ¹⁵N, a proxy for nitrogen derived from the invasive legume. Under invaded canopies, root and rhizosphere biomass were higher, as was OM in the silt‐clay fraction. Also, δ¹⁵N of the OM in the silt‐clay fraction under invaded canopies was enriched, while δ¹³C was depleted. Finally, we found that the ratio between OM in the biotic versus soil compartment could be a good early indicator for invasion. These findings suggest that even when aboveground invasion pressure on the system is low, it is imperative for ecosystem conservation to remove young plants, as they might alter soil functioning already at an early stage of invasion. Copyright © 2017 John Wiley & Sons, Ltd.     

The fate of soyabean photosynthetic carbon varies in Mollisols differing in organic carbon

The fate of photosynthetically‐fixed carbon (C) in the plant–soil–microbe continuum has received much interest because of its relevance to soil C and the global C cycle. However, information on the flow of this plant C below ground and its contribution to soil C sequestration in soils with contrasting organic C (C_org) is limited. In this study, soyabean (Glycine max L. Merr.) was grown in three Mollisols with low (1.04%), medium (2.90%) and high (5.05%) C_org, respectively. Plants were labelled with ¹³CO₂ to trace the photosynthetic C dynamics in the plant–soil system for up to 288 hours. The total amount of net fixed ¹³C by plants ranged from 66 to 78 mg pot^?1, and there was no difference between soils. The amount of ¹³C in soil organic matter (SOM) increased from 1.9 to 6.1 mg pot^?1 over time in the high‐C_org soil, while it showed a non‐significant change with 2.2 mg pot^?1 (on average) in the medium‐C_org soil, and decreased from 2.9 to 0.1 mg pot^?1 in the low‐C_org soil. In the low‐C_org soil, the amount of ¹³C in soil microbes decreased markedly over time, showing a fast turnover, and had a significant correlation (P ≤ 0.01) with ¹³C in the SOM pool. However, such a relationship was not significant in the soil with high or medium C_org. These results indicate that most of the root‐derived C in the low‐C_org soil is degraded quickly by microbial activity, while the greater input of the photosynthetic C to SOM in the high‐ and/or medium‐C_org soil can probably be attributed to physical sorption of root‐derived C by SOM and minerals, thus protecting it against microbial decomposition.     

石羊河流域干旱荒漠区人工梭梭林对土壤碳库的影响

采用野外调查与室内分析相结合的方法,研究石羊河流域民勤干旱沙区种植人工梭梭林4,13,36年后的土壤有机碳(Soil organic carbon,SOC)、无机碳(Soil inorganic carbon,SIC)、全氮(Total nitrogen,TN)和总碳(soil total carbon,TC)含量及储量变化特征。结果表明:流动沙地种植梭梭后,0-50cm层灌丛下和行间SOC和TN含量总体随造林年限增加而增加,5-50cm层灌丛下SIC含量在13年梭梭林地最高。36,13年林地0-50cm层灌丛下SOC和TN储量均高于行间,而13年灌丛下SIC储量低于行间,4年灌丛下5-50cm层SOC、TN和SIC储量均低于行间。0-50cm层土壤有机碳、无机碳、全氮储量增幅分别为102.44%,24.66%,54.55%,36年林地SOC和TN储量随土层加深先降低后增加,但4,13年和流动沙地SOC、SIC和TN储量均随土层加深而增加。土壤有机碳占总碳比例随造林年限增加而增加。相关分析结果表明,土壤颗粒组成、造林年限、土层深度等与土壤有机碳和全氮储量显著相关(P0.01)。民勤干旱沙区造林提高了土壤碳库截存量,并且随林龄增长而增长。     

Indications for soil organic matter quality in soils under different management

Changes in management practice are reflected by soil carbon and nitrogen status, in particular by the proportion of soil organic matter (SOM) being easily transformed (active SOM). We describe SOM quality for three management practices, Organic Farming system (OF), Integrated Crop Production (ICP) and pasture sites (G), which intend to achieve sustainable management practice. The experimental sites were conventionally farmed until 1992. SOM quality was examined by describing active SOM pools, such as the decomposed ‘young soil organic matter’ (YSOM), ratio of microbial biomass carbon (C_mic) to organic carbon (C_org), ecophysiological status of the microbial biomass (qCO₂), and the ratio of light particulate organic matter (POM-LF) to C_org. Ratios of soil microbial biomass (C_mic/C_org) and POM-LF (POM-LF/C_org) and the amount of decomposed YSOM were relatively similar to each other, despite differences in management practice and soil texture. Soil microbial parameters (C_mic, C_mic/C_org and qCO₂) were significantly (p<0.05) affected by the amount of decomposed YSOM and the silt content in the OF. In the ICP, soil microbial parameters depended only on the amount of decomposed YSOM, which was considered to be a consequence of the more heterogeneous texture at the OF-sites. Management effects were detectable for no-tillage in the ICP leading to an accumulation of active SOM in the surface soil (0–10 cm). The ratio POM-LF/C_org showed no difference between G and OF despite markedly higher C_org-contents at the G-sites. Conclusively, all methods used indicate comparable SOM qualities for the three management systems, despite differences in soil texture and soil management during 7 years. Management practices seem to be well adapted to the site conditions.     

盐生荒漠植物群落土壤氮素含量及其组分特征

为了探讨盐生植物群落土壤氮素含量及组分特征,以新疆艾比湖流域盐生荒漠土壤为研究对象,分析了乔木、灌木和草本3种不同生长型盐生植物群落0～20、20～40和40～60 cm的土壤理化性质及各形态的氮素含量。结果表明,研究区不同生长型盐生植物群落土壤pH、电导率、有机质和C/N随土层深度增加而逐渐降低,而含水量与之相反。同一土层,土壤总氮、有机氮和氮密度呈现从乔木、灌木再到草本盐生植物群落逐渐增加的趋势。垂直分布方面,不同生长型盐生植物群落的土壤各形态氮素含量和氮密度均随着土层深度的增加而降低,表聚现象明显。除铵态氮以外,其他各种氮素间均表现为极显著正相关(P0.01),且与土壤有机质、含水量和容重呈极显著相关(P0.01)。土壤各形态氮素占总氮的比例对总氮的变化存在不同的响应,有机氮占总氮的比例相对稳定,有机氮与碱解氮占总氮的比例随总氮含量的增加而增加,无机氮、硝态氮和铵态氮占总氮的比例随总氮含量的增加而降低。     

Benefactor and allelopathic shrub species have different effects on the soil microbial community along an environmental severity gradient

Patches where shrubs have either positive or negative effects on their understory plant community are common in arid ecosystems. The intensity and balance of these effects change along environmental severity gradients but, despite the major role of soil microbes in plant interactions, little is known about the differences among soil microbial communities under these species and their possible influence on such contrasting shrub effects. We hypothesized that microbial communities associated to benefactor and allelopathic shrubs would differ among them and that differences would increase with environmental severity. To test these hypotheses we characterized soil microbial biomass, activity and community composition under a benefactor shrub species, Retama sphaerocarpa, an allelopathic shrub species, Thymus hyemalis, and in bare soil among plants (gaps) at three sites along an environmental severity gradient. Shrubs promoted an increase in soil bacterial diversity, being bacterial communities associated to benefactor shrubs, allelopathic shrubs and gaps different in composition. Microbial enzymatic activity and biomass increased under shrubs and under more mesic conditions; nonetheless, they were highest under benefactor shrubs at the most arid site and under allelopathic shrubs at the less severe site. Compared to gaps, the presence of shrubs induced changes in microbial activity and community composition that were larger at the most severe site than at the less severe site. Along the gradient, benefactor shrubs enhanced the abundance of bacterial groups involved in organic matter decomposition and N fixation as well as plant pathogens, which could contribute to Retama's outstanding positive effects on understory plant biomass and diversity. Plant patches mitigate the effects of extreme conditions on associated plant and soil microbial communities and promote soil biodiversity and ecosystem functioning in arid ecosystems, with shrubs actively selecting for specific microbial groups in their understory.     

THE INFLUENCE OF BLOWING SOIL TRAPPED BY SHRUBS ON FERTILITY IN TABERNAS DISTRICT (SE SPAIN)

The purpose of this study is to relate soil properties affected by the deposit of materials by the wind to the percentage of vegetation surface that facilitates their accumulation. To do this, we applied artificial wind and compared its effects to natural conditions by making use of the ‘fertility islands’ generated by some species of bushes under their canopies in semiarid environments and, which according to many publications, modify soil characteristics substantially. Two very common species in our flat semiarid study area, Hammada articulata and Artemisia barrelieri, were chosen for comparison. Soil was sampled in gaps between shrubs and under canopies. Some shrubs were subjected to extra wind, whereas others were kept under natural conditions. At the end of this first stage, the islands under the canopies were sampled again, and any differences found in the data were studied. Increased retention of material affects the fertility of soils. Thus, we observed, for example, an increase in biomass carbon showing greater biological activity. Finally, we performed a statistical analysis, which resolved our hypothesis on the influence of the percentage of plant surface on the soil parameters. Copyright © 2012 John Wiley & Sons, Ltd.     

Na compound fertilizer stimulates growth and alleviates water deficit in the succulent xerophyte Nitraria tangutorum (Bobr) after breaking seed dormancy

Nitraria tangutorum (Bobr), a typical succulent xerophyte with high level of seed dormancy, is one of the few shrubs found to date that can develop and form fixed dunes in desert regions. Our studies have demonstrated that the strong drought tolerance of the succulent xerophytes was strongly linked to high sodium (Na⁺) accumulation in the photosynthesizing branches (PB) as well as leaves. The study is to explore a method that can rapidly promote the seed germination of N. tangutorum, and then investigate the positive effects of Na compound fertilizer (NaCF) on the growth and drought tolerance of N. tangutorum and ecological environment by short-term pot experiment in a greenhouse and long-term field and pot experiment in a desert environment. The results indicate that the germination rate of seeds obtained a maximum by 69% when seeds were treated with 150 mg L ^?1 gibberellic acid (GA₃) for 48 h followed by soaking in concentrated sulfuric acid (98% H₂SO₄) for 55 min, and then germinated (25/5°C) in darkness for 8 d. After breaking seed dormancy, the NaCF significantly stimulated growth of N. tangutorum and, concomitantly, improved its ability to cope with water deficit (30% of field water capacity) by increasing Na⁺ more than Potassium (K⁺) accumulation for osmotic adjustment in greenhouse and desert conditions. The contribution (take the pot experiment in the desert, for example) of Na⁺ to the osmotic potential (compared with control) varied from 13.9% in plants subjected to diammonium phosphate [(NH₄)₂HPO₄] to, surprisingly, 63.9% in plants grown in the presence of NaCF under water deficit. The distribution characteristics of the total Na⁺ (1620 mg) in the NaCF indicate that 691.2 mg (42.7%) is absorbed by plants, 848.8 mg (52.4%) remained in the pot and 80 mg (4.9%) leached, which accounted for 2.2% of the nursery soil, respectively. The positive effect of NaCF on the drought resistance of N. tangutorum and the ecological environment were also confirmed in the field experiments. These findings suggest that the rapid seed germination technology of N. tangutorum combined with the popularization and application of NaCF can shorten the seed germination period and make the seedling establishment much easier, which may be an effective strategy to restore and reconstruct degraded vegetation in many desert regions.     

内盖夫荒漠中小型土壤节肢动物群落的时空变化

Desert ecosystems are characterized by sparse vegetation that affects both abiotic parameters and soil biota along the soil profile.This study was conducted in 2010–2011 in a loess plain in the northern Negev Desert highlands, Israel, to test two main hypotheses:1) the abundance and diversity of microarthropods would vary seasonally in the top 30-cm soil layer, but would be relatively stable at soil depths between 30 and 50 cm and 2) soil microarthropods would be more abundant in soils under shrubs with large litter accumulations than under shrubs with less litter or bare soil. Soil samples were collected each season from the 0–50 cm profile at10-cm intervals under the canopies of Hammada scoparia and Zygophyllum dumosum and from the bare interspaces between them.Soil moisture and soil organic carbon in the top 30-cm layers varied seasonally, but there was little variation in the soil layers deeper than 30 cm. Soil mites were most abundant in the top 30-cm soil layer in autumn and winter, with the highest number of families found in winter. There were no differences in soil microarthropod abundance attributable to the presence or absence of shrubs of either species. The microarthropod communities of the microhabitats studied consisted of Acari, Psocoptera, and Collembola. The Acari were mostly identified to the family level and were dominated by Oribatida(55%) and Prostigmata(41%) in all seasons and microhabitats, while the psocopterans were most abundant in summer. These results are opposite to those obtained in other studies in similar xeric environments. Moreover, our findings were not in line with our hypothesis that a better microhabitat played a major role in microarthropod community composition, diversity, and density.     

Effects of the earthworm Pontoscolex corethrurus on banana plants infected or not with the plant-parasitic nematode Radopholus similis

Radopholus similis is a worldwide endoparasitic nematode that greatly hampers banana (Musa acuminata, Cavendish subgroup) productivity. Earthworms are known to closely interact with above-ground and under-ground soil biota and particularly with plants and microfaunal communities. This study was aimed at investigating, under greenhouse conditions, the effects of the earthworm Pontoscolex corethrurus on banana growth and nutrient uptake, and assessing the influences of this earthworm on the development of an inoculated population of R. similis. Six-week-old tissue culture banana plants were submitted to four treatments: with P. corethrurus, R. similis, P. corethrurus+R. similis, and a control with no earthworms or nematodes. At the end of the experiment, the P. corethrurus treatments showed significantly higher leaf surface areas, shoot dry root weights, and root fresh weights than those without earthworms. This root growth enhancement probably contributed to the evident but non-significant decrease in the density of nematodes in the roots, even though earthworms did not reduce the total number of nematodes per whole root system. Moreover, the presence of earthworms slightly alleviated the severity of root damage. N bioavailability in the soil, along with N, Ca, and Mg content of banana plants, were also significantly increased in the presence of earthworms. Our results demonstrated that banana plant growth and nutrition were positively influenced by earthworms. Cropping practices that boost the development of earthworm communities in soil should therefore be promoted to enhance sustainability and to naturally alleviate nematode impact.     

Heterogeneity of soil nutrients and subsurface biota in a dryland ecosystem

Dryland ecosystems have long been considered to have a highly heterogeneous distribution of nutrients and soil biota, with greater concentrations of both in soils under plants relative to interspace soils. We examined the distribution of soil resources in two plant communities (dominated by either the shrub Coleogyne ramosissima or the grass Stipa hymenoides) at two locations. Interspace soils were covered either by early successional biological soil crusts (BSCs) or by later successional BSCs (dominated by nitrogen (N)-fixing cyanobacteria and lichens). For each of the 8 plant type×crust type×locations, we sampled the stem, dripline, and 3 interspace distances around each of 3 plants. Soil analyses revealed that only available potassium (K_av) and ammonium concentrations were consistently greater under plants (7 of 8 sites and 6 of 8 sites, respectively). Nitrate and iron (Fe) were greater under plants at 4 sites, while all other nutrients were greater under plants at less than 50% of the sites. In contrast, calcium, copper, clay, phosphorus (P), and zinc were often greater in the interspace than under the plants. Soil microbial biomass was always greater under the plant compared to the interspace. The community composition of N-fixing bacteria was highly variable, with no distinguishable patterns among microsites. Bacterivorous nematodes and rotifers were consistently more abundant under plants (8 and 7 sites, respectively), and fungivorous and omnivorous nematodes were greater under plants at 5 of the 8 sites. Abundance of other soil biota was greater under plants at less than 50% of the sites, but highly correlated with the availability of N, P, K_av, and Fe. Unlike other ecosystems, the soil biota was only infrequently correlated with organic matter. Lack of plant-driven heterogeneity in soils of this ecosystem is likely due to (1) interspace soils covered with BSCs, (2) little incorporation of above-ground plant litter into soils, and/or (3) root deployment patterns.     

Modified topsoil islands within patchy Mediterranean vegetation in SE Spain

The spatial distribution of some chemical and physical soil properties was determined in the microenvironment of individual plants of three matorral species (Rosmarinus officinalis L., Stipa tenacissima L., Anthyllis cytisoides L.) in an area of patchy vegetation in south-east Spain. The influence of isolated plants on the chemical and physical status of the topsoil decreased significantly from the plant axis to the bare inter-plant area, giving rise to “islands” of improved topsoil properties beneath each plant (i.e., increased organic matter content and aggregate stability, and decreased bulk density, penetration resistance, shear strength and rock fragment content and cover). The degree and lateral extent of topsoil modification was significantly more pronounced under Rosmarinus and Stipa plants than under Anthyllis shrubs, mainly because of the differential influences of species morphology (i.e., above-ground structure) and components (i.e., canopy and litter covers) on soil properties and erosion/sedimentation processes.