Development Of Soil Horizons Research Articles

MINERALOGY OF CONTINENTAL SEDIMENTS FROM THE ULAN-ZHALGA KEY SECTION (WESTERN TRANSBAIKALIA): RESPONSE TO QUATERNARY CLIMATIC CONDITIONS

We present the results of studying the material composition of Quaternary subaerial-slope sediments of the new key section Ulan-Zhalga (Western Transbaikalia) with a thickness of about 30 m. The research methods include X-ray diffractometry (XRD), infrared spectroscopy, laser grain size analysis, elemental analysis. Quartz, feldspar, clay minerals, carbonates, hematite and traces of amphibole were found in the mineral composition of the studied sediments. For the first time the composition and structure of carbonate minerals were shown as a reliable indicator of climatic changes in the Pleistocene and Holocene for the subaerial section. For correct identification and determination of quantitative ratios of carbonates the mathematical modeling of their XRD patterns by the Pearson VII function was used. It has been established that the structure of the section involves two thick sequences differing in lithological and mineralogical features corresponding to two major stages of sedimentation. The lower sequence was formed under conditions of moderate wind activity and relatively high humidity as evidenced by the widespread development of soil horizons and the presence of low-Mg carbonates. The upper sequence, formed under colder and drier conditions with increased dynamics of aeolian processes is characterized by the presence of calcite, high-Mg calcite and excess-Ca dolomite.

Developing and classifying urban biomes as a basis for nature-based solutions

Urbanization is a major driver of environmental change, which calls for multifunctional and comprehensive actions such as Nature-based Solutions (NbS). They are “inspired and supported by nature… and must benefit biodiversity and support the delivery of a range of ecosystem services”. But what nature should one aim for? We tested the hypothesis that local vegetation may not always be the best source of inspiration, as environmental changes impact both extant conditions and species suitability for restored ecosystems. We analyzed the megacity of São Paulo, where laws promote the use of species from the local Atlantic Forest biome. We trained a Linear Discriminant Analysis to classify the Brazilian biomes and predicted the biomes' correspondence considering city's vegetation cover and climate. With 80% accuracy, the model predicted correspondence with the Atlantic Forest in 57% of the city, while 43% is better represented by the Cerrado, a dense Tropical Savanna biome. Cerrado species are naturally adapted to higher insolation, temperature and more seasonal precipitation, and they can parallel the ecosystem services of the Atlantic Forest. To help guide NbS implementation, we consider four “urban biomes”: Atlantic Forest, Seasonally Flooded Atlantic Forest, Cerrado, and the Seasonally Flooded Cerrado, whose dynamics seem to depend mainly on changes in the proportion of dense vegetation cover. We then discuss possible examples of NbS in the city. ResultsAccording to the PCA (Tables S3-S6), the main difference between the classification of the green spaces classified is the higher proportion of forest cover in those classified as Mata Atlantica, with almost any influence from other vegetation cover types (Fig. 5E). This pattern is still consistent when vegetation cover is evaluated outside the largest green spaces where the presence of even small forest patches across the urban fabric results in the Mata Atlântica category DiscussionThe urban ecosystem restoration could benefit from such natural and dynamic processes in which the Cerrado could act as a transient urban biome in the way of restoring the urban forests through soil horizon development and species succession followed by the densification of the vegetation whenever this planting scheme is possible. ConclusionsThese transitions among urban biomes may occur as local conditions change with the implementation of NbS, and the results point to the effectiveness of increasing the vegetation density wherever possible.

Influence of saprophages (Isopoda, Diplopoda) on leaf litter decomposition under different levels of humidification and chemical loading

The paper presents a study about the influence of two saprophage groups (Isopoda, Diplopoda) on leaf litter decomposition under different levels of humidification and chemical stress. Because of their worldwide distribution, we focused on the common pillbug Armadillidium vulgare (Latreille, 1804) (Isopoda, Armadillidiidae), and the common millipede species Rossiulus kessleri (Lohmander, 1927) (Julida, Julidae). The function of environment creation by the given saprophages, as destructors of dead plant matter, supporting such ecosystem services as soil fertility improvement and nutrients’ turnover, is highlighted. To conduct the experiment, the animals were collected manually and using pitfall trapping. In order to bring the experimental conditions closer to the natural, the individuals were not sexed. The maximum consumption of leaf litter by woodlice was recorded in the conditions with adequate moisture (0.5 mL of distilled water per box) and amounted to 2.52 mg/10 individuals per day, which exceeds its consumption with low and increased moisture, respectively, by 1.82 and 1.24 times. As for the effect of interaction, the consumption of maple litter with optimal moisture (4.77 mg/10 individuals per day) was the greatest. The largest absolute difference between broad-leaved tree species in the average weight of leaf litter consumed by woodlice was between maple leaf litter and oak leaf litter, the minimum – between robinia leaf litter and oak leaf litter. According to the results of the obtained data (Diplopoda), it can be stated that there is a statistically significant effect of chemical stress and discrepancy of the average zinc content in the object of study (in Diplopoda and their faecal pellets). We found that the diet provided did not affect the distribution of zinc in Diplopoda under conditions of chemical stress. According to the results of pairwise comparisons, we determined that the zinc content in the Diplopoda clearly differs for control and almost every concentration of zinc sulfate solution – 0.03 and 0.15 g/L, the samples of which do not form a homogeneous group. The species composition, abundance and distribution in space of soil invertebrates are informative indicators which reflect the ecological state of soils, intensity in development of soil horizons as well as intensity of processes occurring in them.

Evidence for Middle Palaeolithic occupation and landscape change in central Armenia at the open-air site of Alapars-1

AbstractHere we report the findings from excavations at the open-air Middle Palaeolithic site of Alapars-1 in central Armenia. Three stratified Palaeolithic artefact assemblages were found within a 6-m-thick alluvial-aeolian sequence, located on the flanks of an obsidian-bearing lava dome. Combined sedimentological and chronological analyses reveal three phases of sedimentation and soil development. During Marine Oxygen Isotope Stages 5–3, the manner of deposition changes from alluvial to aeolian, with a development of soil horizons. Techno-typological analysis and geochemical sourcing of the obsidian artefacts reveal differential discard patterns, source exploitation, and artefact densities within strata, suggesting variability in technological organization during the Middle Palaeolithic. Taken together, these results indicate changes in hominin occupation patterns from ephemeral to more persistent in relation to landscape dynamics during the last interglacial and glacial periods in central Armenia.

Characteristic of Soils Developed from Pyroclastic Materials Across the Dry Southwest Tambora’s Footslope

Development of soils from newly deposited materials are of interest for investigation, especially when the environment has specific conditions as the leading factors of the soil formation and development. The Southwest Tambora’s footslope is part of the last Tambora’s pyroclastic deposition but this part has the driest climate. Despite this Southwest Tambora’s footslope in fact at macro scale has homogeneous specific conditions, the results of toposequential soil observation showed that this area composed of soils having significant differences, even differed one another at the higher order of classification. The common characteristic of the soils in this area is that they have sandy texture with 70 to 90 percent sand mostly with limited soil horizon development, only minority of the soils already have B horizon. Regardless the sandy texture as the common strong characteristic of the soils here, the formation of B horizon is the strongest differentiae among the soils and yet there are some other characteristics to show that even the soils of the same taxonomic class are not necessarily homogeneous. Previous geological studies of this part of Tambora’s footslope depicted different modes of the deposition of the last erupted pyroclastic materials, among others by transportation and sortation processes after the material eruption. This difference has resulted in different stratification of particle size and the level of compaction of the pyroclastic materials. So the soil formation here have been taken place from not a completely homogeneous parent material condition, under the limited weathering rate due to the dry climate.

Soil horizon development and the tempo and modes of vegetation change during the Holocene in a Sonoran Desert basin, USA

Silt-rich and clay-rich vesicular horizons (V horizons) that develop beneath desert pavements limit infiltration and plant-available soil moisture. A radiocarbon date from buried charcoal constrains the timing of emplacement of a Sonoran Desert alluvial fan deposit to the very end of the Pleistocene (approximately 12.7 ka). The surface of the fan is dotted with 1.5–3 m light-colored plant scars that mark places once occupied by larger perennial plants. A few very large, ring-like clones of Larrea tridentata have persisted on the surface and represent plants that may have initially established as long ago as the early Holocene. The gravelly fan deposit initially would have possessed high infiltration capacities and fostered the establishment of vegetation. Two principal sets of factors have driven vegetation change over the course of the Holocene. During the first part of the Holocene, differences in life history attributes of various species (dispersal and colonization vs. competitive abilities) would have contributed to a succession of perennial plant occupants, culminating with the predominance of the slow-growing, long-lived, but competitively dominant L. tridentata. During the latter part of the Holocene and continuing to the present, continued development of the pavement and associated V horizon has increasingly diminished the capacity of the surface to absorb precipitation and is responsible for the more recent phase involving progressive elimination of vegetation. Development of soil features that markedly influence soil hydrology is a major component of vegetation change during the late Quaternary in the more arid portions of the Sonoran and Mojave deserts.

Widespread loss of intermediate soil horizons in urban landscapes

Soils support terrestrial ecosystem function and therefore are critical urban infrastructure for generating ecosystem services. Urbanization processes modify ecosystem function by changing the layers of soils identified as soil horizons. Soil horizons are integrative proxies for suites of soil properties and as such can be used as an observable unit to track modifications within soil profiles. Here, in an analysis of 11 cities representing 10 of the 12 soil orders, we show that urban soils have ∼50% fewer soil horizons than preurban soils. Specifically, B horizons were much less common in urban soils and were replaced by a deepening of A horizons and a shallowing of C horizons. This shift is likely due to two processes: (i) local management, i.e., soil removal, mixing, and fill additions, and (ii) soil development timelines, i.e., urbanized soils are young and have had short time periods for soil horizon development since urbanization (decades to centuries) relative to soil formation before urbanization (centuries to millennia). Urban soils also deviated from the standard A-B-C horizon ordering at a much greater frequency than preurban soils. Overall, our finding of common shifts in urban soil profiles across soil orders and cities suggests that urban soils may function differently from their preurban antecedents. This work introduces a basis for improving our understanding of soil modifications by urbanization and its potential effects on ecosystem functioning and thereby has implications for ecosystem services derived from urban landscapes.

Effect of traditional agriculture technology on communities of soil invertebrates

The study of peculiarities specific for the spatial organization of communities of living organisms allows to develop principles of the rational and effective use of the biosphere natural resources and optimal adaptation of mankind to the natural environment. The aim of the research was to study communities of the soil mesofauna as an integral indicator of the state of soils under conditions of applying the traditional farming technology, to carry out the quantitative accounting of the soil mesofauna, and assessment of morphometric parameters of sunflower plants in places of selecting soil and zoological samples, to determine the species composition and abundance, as well as to analyze the ecological structure of the soil mesofauna community. Rheophilous species predominate on black steam, and mesophilic species predominate under sunflower. This can be explained by the fact that in the periodic cultivation of black steam, the evaporation from the soil surface is much higher. Ultra-mega-coenotrophs are dominant on black steam, and megacoenotrophs are dominant under sunflower. Since both demonstration trails are laid on one field, but have strategically been divided into a plot under black steam and a plot under sunflower, one can assume a different degree of saturation of the soil solution, as during the growth the crop being cultivated uses soil nutrients. Among topomorphs of soil animals, exactly soil animals are dominant, which is characteristic for both demonstration trails being studied. In the composition of trophomorphs of soil animals, phytophages are dominant in soil of the test demonstration trail on black steam, and in soil of the test demonstration trail, where sunflower was cultivated, phyto- and saprophages predominate in equal proportions. As a result of the correlation analysis, statistically reliable dependences are obtained: – numbers of soil animals in soil of the demonstration trail on black steam – on the distance from forest belt areas (-0.23) and length and width of sunflower leaves - on the distance from forest belt areas (0.53 and 0.53 respectively). The species composition, abundance and distribution in space of soil invertebrates are an informative indicator, which reflects the ecological state of soils, intensity in development of soil horizons as well as intensity of processes occurring in them.

Mapping Soil Salinity and Evaluation of Water Quality in Siwa Oasis Using GIS

Siwa Oasis represents a promising area for agricultural expansion projects in the western desert of Egypt due to the availability of water resources. However, these areas are suffering from land degradation problems, especially soil salinity. Accordingly, the main objectives of this work were to evaluate and map soil salinity as well as groundwater quality in Siwa Oasis. Therefore, twenty representative soil profiles were randomly distributed throughout the studied area. These profiles were sampled based on the development of soil horizons. A total of 46 soil-samples were collected and these samples were analyzed for their physical and chemical properties. Groundwater samples were collected from irrigation-wells and they were analyzed for their chemical parameters. Landsat images were also acquired at three different periods to monitor the changes in vegetation cover, soil salinity and water logging. The obtained results indicated a wide variability within soil physical and chemical properties in the Oasis. Most of the suited soils were very saline, non-sodic based on the EC and ESP values (EC= 77 dSm-1 and ESP= 14.62 in average). Agricultural areas were increased from 22 km2 in 1992 to 81 km2 in 2015. On the other hand, saline soils were increased from 35 km2 to 64 km2 and water logged areas were also increased from 19 to 51 km2 from 1992 to 2015, respectively. These results could be due to the expansion in reclamation-projects, increase in crop irrigation and poor drainage. The salinity of groundwater varied from 2.28 to 5.45 dSm-1 (3.76 in average), which indicates a degradation in its quality. In summary, soils in Siwa Oasis are salt-affected and they need a proper land reclamation program and development of effective irrigation and drainage systems.

Influence of landscape position and transient water table on soil development and carbon distribution in a steep, headwater catchment

Upland headwater catchments, such as those in the Appalachian Mountain region, are typified by coarse textured soils, flashy hydrologic response, and low baseflow of streams, suggesting well drained soils and minimal groundwater storage. Model formulations of soil genesis, nutrient cycling, critical loads and rainfall/runoff response are typically based on vertical percolation, development of soil horizons parallel to the land surface, mineral weathering inputs limited to the rooting zone and drainage from lumped catchment reservoirs (e.g., the subsoil) as the dominant source of stream flow. However, detailed study of the hydrologic reference catchment at Hubbard Brook Experimental Forest, NH, USA shows striking spatial patterns of soil development that reflect the influence of transient water tables within the solum in nearly all landscape positions. Shallow bedrock and variably low hydraulic conductivity in the subsoil promote lateral flow and development of soil horizons along hillslope flowpaths rather than in vertical profiles. We distinguished several morphologic units based on the presence of diagnostic horizons indicative of differing patterns of podzolization and carbon storage. The distribution of soils appears to be highly dependent on local drainability and frequency and duration of transient saturation within the solum. As such, monitoring of hydropedologic groups and transient water table fluctuations may prove to be a sentinel for the effects of climate change on spatial distribution of soils and retention/release of solutes from upland catchments.

Morphological and Physico-Chemical Characteristics and Classification of Vertisol Developed on Deltaic Plain

The name of Vertisol is derived from Latin “vertere” meaning to invert. This case restricts development of soil horizons in profile. These soils have the capacity to swell and shrink, inducing cracks in the upper parts of the soil and distinctive soil structure throughout the soil. The formation of these specific features are caused by a heavy texture, a dominance of swelling clay in the fine fraction and marked changes in moisture content. The swell-shrink behavior is attributed to the wetting and drying of the soil mass. In this study, morphology, physico-chemical characteristics and classification of vertisols that were formed on alluvial delta plains, were investigated. Those soils formed on the Bafra Plain found in the K?z?l?rmak Delta and located in the central Black Sea region of Turkey. All studied Vertisols are characterised by a dark colour in surface soil, a heavy clayey texture, hardpan formation under top soil (high bulk density a high compaction) and very high COLE values. In addition, they have deep wide-opened desiccation cracks at the surface, slickensides at the middle part of the profiles and a poor differentiation of their horizons. Physico-chemically, the studied soils are slightly basic to very basic, non-saline and poor in organic matter, which is slightly higher in the surface horizon. In addition, cation exchange capacity, sum of exchangeable bases and base saturation of soils are very high. On the basis of morphological and physicochemical analysis, soil profiles were classified as Sodic Haplustert, Typic Calciaquert, Sodic Calciustert according to Soil Taxonomy (Soil Survey Staff, 1975 and 1999) and as Sodic Vertisol and Calcic Vertisol according to FAO/ISRIC (2006) classification systems.

Impact of physico-chemical soil properties on erosion features in the Aliano area (Southern Italy)

Abstract This study has been carried out on three pairs of clayey-silt slopes, which are examples of typical badlands near Aliano (Southern Italy). These show features of erosion which are common to SE-exposed clayey-silt rocks outcropping in the same area. After mineralogical, geochemical and grain size analyses these slopes were compared with the SE-exposed adjacent ones, having however a different erosive action, and a partially-vegetated covering was found. Pedological analyses show a clear development of soil horizons where the slope is rich in organic matter, and this is wholly consistent with the presence of vegetation. A comparison between the eroded substrates and non-eroded substrates showed that the former have higher ( 16 µm). The grain size also distinguishes crusts from substrates, the latter having higher fractions ( TDS, pH, SAR and PS are parameters which distinguish the eroded from the non-eroded sides of the slopes. On average, the eroded substrates have pH, SAR and PS values higher than the non-eroded ones. The ESP values of the eroded slopes are slightly higher than those of the non-eroded ones. Up to now the relations between pH, TDS, SAR, PS and ESP have been considered as descriptors of dispersivity. However an improvement can be made regarding the predictability of diagrams (such as ESP–TDS) when taking mineralogical composition into consideration. The crusts are less dispersive than the eroded substrates, and the non-eroded ones show the same feature as crusts. This suggests that the most severely-weathered part of a slope tends to reach a stable condition when a strong decrease in SAR, PS and ESP occurs. This study shows that the erosion mechanism involves morphological and geographic exposure, climatic elements as well as grain size, mineralogy, chemistry and the exchange processes of soil.

Impact of small scale gold mining on soils of the wetland forests in east Usambara, Tanzania

Soil pits in disturbed and natural wetland forests of East Usambara were excavated and soil profile samples collected and analysed for physical and chemical properties. Their physical and chemical characteristics were found to be sharply different from those of the surrounding hills. Differences included lack of A Horizon, water saturation, presence of very high organic matter, very low pH and hence very poor in basic cations, and low microbial activity. Nutrient recycling, especially nitrogen and phosphorus, tended to shift from mineralization to immobilization. Effects of soil disturbance on both physical and chemical characteristics of the soils were also very vivid. The soils disturbed from gold mining possessed an altered soil structure, improper development of soil horizons and removal of organic matter on the surface. Burried A horizons were found in all plots at the disturbed site. Carbon:Nitrogen ratios were significantly higher in the undisturbed forests suggesting greater capacity in carbon sequestration. Both sites existed in the same general area but while the undisturbed one possessed all histosol (organic soil) characteristics, the disturbed site soils were heterogenous and possessed characteristics associated with mineral soils (inceptisols). The present study suggests that disturbance has led into a reduction in the supply capacity of minerals such as nitrogen and phosphorus, and the low supply of such minerals will inhibit regrowth of the natural vegetation.

Soil particles in agricultural landscapes of a derived savanna in southwestern Nigeria and implications for selected soil properties

The vast area of savanna ecology in Africa plays a significant role in food production, making a study of soils in this zone very important. Therefore, soil physical and chemical properties of 14 soil profiles were studied in a derived savanna zone of southwestern Nigeria on 2 toposequences at 2 locations (Ibadan and Alabata), which were 20 km apart. Six soil profiles were studied at Ibadan while 8 were studied at Alabata. Morphological descriptions of profiles were carried out. Data collected included particle size distribution, bulk density, clay dispersion, water retention characteristics, pH, organic carbon, exchangeable K, Ca, Mg and available P. Soil profiles along the toposequences were well-developed with depths exceeding 180 cm, except for one profile at the lower slope position where an indurated plinthic layer was found at 68 cm depth. The horizons were easily distinguished with color, texture and consistency. Total sand, with the main component being coarse sand, decreased with depth from 813 to 502 g kg − 1 at Ibadan and from 824 to 635 g kg − 1 at Alabata. The clay content increased with depth from 54 to 356 g kg − 1 at Ibadan and from 63 to 279 g kg − 1 at Alabata. Gravel concentration was highest for soil horizons found between 20 and 102 cm depth. Also, bulk density increased with soil depth from 1.35 to 1.51 g cm − 3 at Ibadan and from 1.38 to 1.64 g cm − 3 at Alabata, indicating that subsoil horizons were more compact due to higher clay and gravel contents, and sticky consistency. The A horizon had a significantly higher water content at water potentials > 2 kPa while the subsoils had higher water content at < 2 kPa. Soil organic C and total N decreased with soil depth at both locations with the A horizon having significantly higher organic C (7.10–12.69 g kg − 1 ) and total N (0.84–1.2 g kg − 1 ) than deeper horizons (1.9–4.47 g kg − 1 for organic C and 0.12–0.58 g kg − 1 for N). Particle size distribution was significantly different among the slope positions at both locations. Also, soil water retention, soil pH, total N and exchangeable K were distinguishing parameters among slope positions. The interaction of soil depth and slope position was, however, not significant suggesting that processes influencing soil horizon development acted independently in the vertical and lateral directions. Soil pH was the only attribute that distinguished the toposequences between the two locations.

From Alluvial Fan to Playa: An Upper Jurassic Ephemeral Fluvial System, Neuquén Basin, Argentina

The Upper Jurassic fill of the back-arc Neuquén Basin includes a lowstand wedge known as the Tordillo Formation. The studied deposits crop out along a N-S oriented belt that runs parallel to the Andean magmatic arc. They are limited to the south by the east-west oriented positive structure of the Huincul arch formed as a result of Upper Jurassic tectonic inversion. The Tordillo deposits were formed in an arid fluvial-dominated system characterised by systematic downstream changes in architectural style. A gravelly and sandy bedload fluvial system is recognised in the southern upstream sector. The reduced thickness and the coarse grain size suggest steep gradients, excess of bedload supply and a low subsidence rate. Thicker and finer-grained deposits prevail to the north and northeast. They were formed under arid conditions in a wadi-sand flat-playa fluvial system. This distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates. These low-gradient deposits are characterised by cyclic alternations of mud-dominated and sand-dominated packages interpreted as high- and low-accommodation systems tracts. The overall fining upward stacking pattern of the Tordillo Formation suggests a change towards higher accommodation rates. This is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits. These attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the Tordillo lowstand wedge.

Ecological Responses of Two Mojave Desert Shrubs to Soil Horizon Development and Soil Water Dynamics

In the arid southwestern United States, subtle differences in soil horizon development affect seasonal soil hydrology and consequently influence plant performance and community structure. We measured canopy development, population structure, and seasonal ecophysiology (predawn water potential, ψpd, and midday net photosynthetic assimilation, Anet) of two co-dominant warm-desert shrubs, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa, in five Mojave Desert soils varying in surface and sub-surface soil development, and we used process-based soil hydrology modeling output to determine longer-term soil water dynamics underlying soil/plant responses. We hypothesized that ecophysiological performance would covary with plant development, which would reflect soil hydrological characteristics. Among three sites on alluvial fan deposits of different geological ages (Young Alluvial, <4000 yr BP; Intermediate Alluvial, ∼12 000 yr BP; Old Alluvial, ∼40 000 yr BP), total canopy volume of Larrea (cubic meters per 100 m2 ground area) was highest on the Young Alluvial site, in close agreement with soil modeling results showing that these coarse-textured, weakly developed soils permit deeper water infiltration. In older, stronger developed soils, infiltration and persistence of soil water was sharply reduced, which was reflected by lower individual Larrea plant volumes. However, during peak spring conditions, ψpd and Anet were highest in Larrea at the Intermediate Alluvial site (−4.2 ± 0.32 MPa and 3.2 ± 0.91 μmol·m−2·s−1), where soils had substantial surface and subsurface horizons, and at the Pavement site, where soils had strong surface layers but little subsurface development. Concurrent plant performance at the Young Alluvial site was unexpectedly low (−4.8 ± 0.49 MPa and 1.7 ± 0.56 μmol·m−2·s−1, respectively). During summer drought ψpd and Anet remained high in Intermediate Alluvial plants, but were extremely low in Pavement site Larrea (−8.17 MPa and −0.04 μmol·m−2·s−1, respectively), due to curtailed infiltration of summer precipitation. These findings suggest that Larrea growing in older soils experience greater mortality and reduced growth but are not subject to strong intra-specific competition resulting from the persistence of large individuals apparent in younger, coarser textured soils. In contrast to Larrea, density of Ambrosia increased with soil horizon development, but smaller plant sizes resulted in similar canopy volume per area, and identical ψpd and Anet across all soils where it occurred, suggesting greater plasticity to the transmittal of precipitation. These findings show that a strong geomorphology and soils context is essential for understanding the variation in plant responses and vegetation structure in desert environments.

ECOLOGICAL RESPONSES OF TWO MOJAVE DESERT SHRUBS TO SOIL HORIZON DEVELOPMENT AND SOIL WATER DYNAMICS

In the arid southwestern United States, subtle differences in soil horizon development affect seasonal soil hydrology and consequently influence plant performance and community structure. We measured canopy development, population structure, and seasonal ecophysiology (predawn water potential, ψpd, and midday net photosynthetic assimilation, Anet) of two co-dominant warm-desert shrubs, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa, in five Mojave Desert soils varying in surface and sub-surface soil development, and we used process-based soil hydrology modeling output to determine longer-term soil water dynamics underlying soil/plant responses. We hypothesized that ecophysiological performance would covary with plant development, which would reflect soil hydrological characteristics. Among three sites on alluvial fan deposits of different geological ages (Young Alluvial, <4000 yr BP; Intermediate Alluvial, ∼12 000 yr BP; Old Alluvial, ∼40 000 yr BP), total canopy volume of Larrea (cubic meters per 100 m2 ground area) was highest on the Young Alluvial site, in close agreement with soil modeling results showing that these coarse-textured, weakly developed soils permit deeper water infiltration. In older, stronger developed soils, infiltration and persistence of soil water was sharply reduced, which was reflected by lower individual Larrea plant volumes. However, during peak spring conditions, ψpd and Anet were highest in Larrea at the Intermediate Alluvial site (−4.2 ± 0.32 MPa and 3.2 ± 0.91 μmol·m−2·s−1), where soils had substantial surface and subsurface horizons, and at the Pavement site, where soils had strong surface layers but little subsurface development. Concurrent plant performance at the Young Alluvial site was unexpectedly low (−4.8 ± 0.49 MPa and 1.7 ± 0.56 μmol·m−2·s−1, respectively). During summer drought ψpd and Anet remained high in Intermediate Alluvial plants, but were extremely low in Pavement site Larrea (−8.17 MPa and −0.04 μmol·m−2·s−1, respectively), due to curtailed infiltration of summer precipitation. These findings suggest that Larrea growing in older soils experience greater mortality and reduced growth but are not subject to strong intra-specific competition resulting from the persistence of large individuals apparent in younger, coarser textured soils. In contrast to Larrea, density of Ambrosia increased with soil horizon development, but smaller plant sizes resulted in similar canopy volume per area, and identical ψpd and Anet across all soils where it occurred, suggesting greater plasticity to the transmittal of precipitation. These findings show that a strong geomorphology and soils context is essential for understanding the variation in plant responses and vegetation structure in desert environments.

Effects of surface and sub-surface soil horizons on the seasonal performance of Larrea tridentata (creosotebush)

1. In warm arid and semiarid environments, the accumulation of clay minerals produces increasingly well developed soil horizons with the passage of time. Differences in the strength of development of two prominent soil horizons, silt- and clay-rich surface vesicular (Av), and clay-enriched subsurface argillic (Bt), may strongly influence the amount and seasonal continuity of plant-available water and the physiological activity of long-lived desert shrubs. Three sites were selected on an alluvial piedmont (bajada) in the Mojave Desert that varied in surface and subsurface horizon development. The first site, a deep deposit of stabilized dune sand, entirely lacked soil horizon development. The second site had a well developed surface stone pavement and underlying Av horizon, but lacked an argillic horizon in the sandy subsoil. The third had a well developed surface pavement and Av horizon, and a deeper, well developed clay-rich argillic horizon. Seasonal water potential and gas-exchange responses of the evergreen desert shrub Larrea tridentata[DC.] Cov., and volumetric soil water content (θ), were measured monthly in 1996 on these three soils in order to test the hypothesis that desert pavements, Av and subsurface Bt horizons differentially affect the effectiveness and utilization of seasonal precipitation. 2. Predawn and midday water potentials (ψpd and ψmid), net photosynthetic rates (Anet), and stomatal conductances (gs) in L. tridentata were highest in the deep, sandy dune soils lacking horizons that could restrict surface and subsurface infiltration. Plants growing in these soils also showed no physiological response to summer precipitation events. Following a single large precipitation event during the growing season (3·8 cm), the water potentials, Anet and gs in L. tridentata were similar in the first (sand dune) and second (pavement and Av horizon) sites. However, plant performance on these soil surfaces showed marked seasonal declines, and did not respond to a small pulse of summer rainfall. Plants growing at the third site (older soils with strongly developed pavement, Av and Bt horizons) had very low gas-exchange rates and water potentials. However, following convectional summer thunderstorms L. tridentata showed improved water relations and gas exchange in these soils. 3. Midday water potentials were frequently anomalously higher than predawn water potentials, up to +6 MPa late in the growing season, especially on soil surfaces with well developed soil horizons. This anomaly was due to seasonal decreases in ψpd accompanied by invariant midday ψ. In general, ψpd was correlated with θ across the depths measured. 4. Correlations between ψmid with θ at 35 cm increased dramatically with increasing Bt horizon development, suggesting that seasonal ψmid may have been due to the vertical translocation of water (‘hydraulic lift’). 5. Our results show that subsurface and surficial soil horizon development differentially affects the seasonal availability of water for desert plants. During wetter parts of the season, subsurface horizons limit the degree of water availability, while surface soil characteristics have the greatest influence on the effectiveness of summer precipitation. These findings suggest that the effectiveness of climatic precipitation and attendant plant utilization of water resources in warm desert systems may depend on the physical soil condition.

Effects of surface and sub-surface soil horizons on the seasonal performance of Larrea tridentata (creosotebush)

1. In warm arid and semiarid environments, the accumulation of clay minerals produces increasingly well developed soil horizons with the passage of time. Differences in the strength of development of two prominent soil horizons, silt- and clay-rich surface vesicular (Av), and clay-enriched subsurface argillic (Bt), may strongly influence the amount and seasonal continuity of plant-available water and the physiological activity of long-lived desert shrubs. Three sites were selected on an alluvial piedmont (bajada) in the Mojave Desert that varied in surface and subsurface horizon development. The first site, a deep deposit of stabilized dune sand, entirely lacked soil horizon development. The second site had a well developed surface stone pavement and underlying Av horizon, but lacked an argillic horizon in the sandy subsoil. The third had a well developed surface pavement and Av horizon, and a deeper, well developed clay-rich argillic horizon. Seasonal water potential and gas-exchange responses of the evergreen desert shrub Larrea tridentata[DC.] Cov., and volumetric soil water content (θ), were measured monthly in 1996 on these three soils in order to test the hypothesis that desert pavements, Av and subsurface Bt horizons differentially affect the effectiveness and utilization of seasonal precipitation. 2. Predawn and midday water potentials (ψpd and ψmid), net photosynthetic rates (Anet), and stomatal conductances (gs) in L. tridentata were highest in the deep, sandy dune soils lacking horizons that could restrict surface and subsurface infiltration. Plants growing in these soils also showed no physiological response to summer precipitation events. Following a single large precipitation event during the growing season (3·8 cm), the water potentials, Anet and gs in L. tridentata were similar in the first (sand dune) and second (pavement and Av horizon) sites. However, plant performance on these soil surfaces showed marked seasonal declines, and did not respond to a small pulse of summer rainfall. Plants growing at the third site (older soils with strongly developed pavement, Av and Bt horizons) had very low gas-exchange rates and water potentials. However, following convectional summer thunderstorms L. tridentata showed improved water relations and gas exchange in these soils. 3. Midday water potentials were frequently anomalously higher than predawn water potentials, up to +6 MPa late in the growing season, especially on soil surfaces with well developed soil horizons. This anomaly was due to seasonal decreases in ψpd accompanied by invariant midday ψ. In general, ψpd was correlated with θ across the depths measured. 4. Correlations between ψmid with θ at 35 cm increased dramatically with increasing Bt horizon development, suggesting that seasonal ψmid may have been due to the vertical translocation of water (‘hydraulic lift’). 5. Our results show that subsurface and surficial soil horizon development differentially affects the seasonal availability of water for desert plants. During wetter parts of the season, subsurface horizons limit the degree of water availability, while surface soil characteristics have the greatest influence on the effectiveness of summer precipitation. These findings suggest that the effectiveness of climatic precipitation and attendant plant utilization of water resources in warm desert systems may depend on the physical soil condition.

Primary forest succession on poor sandy soilsas related to site factors

Three main succession lines are found to occur on poor sandy soils in a blown drift sand landscape in the Netherlands. Site factors – organic matter, parent material, soil moisture regime and relief – were determined. Site types, specific combinations of these site factors, are defined. For five site types succession was studied. The primary criterion for separating succession lines and for delineating the successional stages, was the site factor organic matter and the second criterion, groundwater level. Influence of organic matter on succession was significant on site types with a buried podzol. Species composition on these site types indicated richer sites than the species composition on the site types without a buried podzol. Groundwater level of about 1m with gleyzone was critical for species requiring moist sites. Within a succession line vegetation stages are characterized and the potential forest type is presumed. The stages were related to humus form and soil development. Litter of dominant species was suggested to be the main criterion for the humus form. Initial and older vegetation stages were linked to development of soil horizons. The defined site types, their soil development and relationships with the succession lines clarify spatial and temporal structure and diversity of vegetation, and herewith is the basic information for clarifying the heterogeneity and potential biodiversity of this blown drift sand landscape.