Soil CO2 Dynamics in a Tree Island Soil of the Pantanal: The Role of Soil Water Potential

The Pantanal is a biodiversity hotspot comprised of a mosaic of landforms that differ in vegetative assemblages and flooding dynamics. Tree islands provide refuge for terrestrial fauna during the flooding period and are particularly important to the regional ecosystem structure. Little soil CO₂ research has been conducted in this region. We evaluated soil CO₂ dynamics in relation to primary controlling environmental parameters (soil temperature and soil water). Soil respiration was computed using the gradient method using in situ infrared gas analyzers to directly measure CO₂ concentration within the soil profile. Due to the cost of the sensors and associated equipment, this study was unreplicated. Rather, we focus on the temporal relationships between soil CO₂ efflux and related environmental parameters. Soil CO₂ efflux during the study averaged 3.53 µmol CO₂ m⁻² s⁻¹, and was equivalent to an annual soil respiration of 1220 g C m⁻² y⁻¹. This efflux value, integrated over a year, is comparable to soil C stocks for 0–20 cm. Soil water potential was the measured parameter most strongly associated with soil CO₂ concentrations, with high CO₂ values observed only once soil water potential at the 10 cm depth approached zero. This relationship was exhibited across a spectrum of timescales and was found to be significant at a daily timescale across all seasons using conditional nonparametric spectral Granger causality analysis. Hydrology plays a significant role in controlling CO₂ efflux from the tree island soil, with soil CO₂ dynamics differing by wetting mechanism. During the wet-up period, direct precipitation infiltrates soil from above and results in pulses of CO₂ efflux from soil. The annual flood arrives later, and saturates soil from below. While CO₂ concentrations in soil grew very high under both wetting mechanisms, the change in soil CO₂ efflux was only significant when soils were wet from above.     

Population Dynamics of Soil and Vegetation Protozoa

Many fresh-water protozoa can be found in litters and soils,but the ubiquitous species are those which are able to copewith fluctuating moisture conditions. Terrestrial protozoa aremore characteristic of bryophyte-soil habitats than aquaticecosystems. Nutritionally, two groups have evolved in responseto the plant community: naked, predominantly bacterial feeders,whose abundance is determined by the decomposability of thelitter in which they live; and the slow growing, humusassociatedtestacea, which are more abundant in the litters of slow decomposability.Ubiquitous species comprise about 90% of the protozoa in soils.More continuous moisture conditions enhance the appearance ofadditional species. Hence species diversity indicates highermoisture content of a soil. Protozoa may contribute to the functioningof the soil ecosystem by inducing fiocculation of bacterialpopulations and recycling of minerals through ingestion of bacteriaand excretion of soluble products. The surface of vegetationappears to represent the most terrestrial habitat a protozoancan exploit, because in contrast to the litter-soil ecosystem,only one species, Colpoda cucullus, dominates the population.     

Soil Characteristics of Rehabilitating and Unmined Coastal Dunes at Richards Bay, KwaZulu-Natal, South Africa

The postmining rehabilitation of coastal sand dunes north of Richards Bay (28°43′S, 32°12′E), South Africa, is resulting in the development of a series of known-aged stands of vegetation dominated by Acacia karroo (sweet thorn). Other broad-leaved species are establishing themselves in rehabilitating areas more than 12 years of age. Soils from rehabilitating stands 3–5, 9–12, 13–15, and 16–18 years of age, from two disturbed, unmined stands 30 and 58 years of age, and from a mature unmined stand were examined to assess age-related trends in selected soil properties. Individually, these stands represent a series of different developmental stages of a coastal dune successional sere. Soil organic material, percentage organic carbon and concentrations of sodium, potassium, magnesium, calcium, and nitrogen increased with an increase in habitat regeneration age. Concentrations of most of these elements were lower than those recorded on the 58-year-old unmined and mature unmined stands. Multivariate analyses suggest, however, that the similarity of these values for rehabilitating stands to those for the unmined stands increased with an increase in regeneration age. The growth response of Raphanus sativus (radish) plants, based on mass attained under experimental growing conditions in soil collected from these stands, suggests an increase in soil fertility with an increase in regeneration age.     

Xylocopa Bees in Tropical Coastal Sand Dunes: Use of Resources and Their Floral Syndromes

Large bees such as species from Xylocopa Latreille are usually associated with pollination in tropical sand dune areas, which frequently present shrubby herbaceous vegetation adapted to conditions of high salinity, high solar radiation and strong winds. We report on the diversity of Xylocopa and the plants they visited to collect nectar and pollen, focusing on the floral syndromes they present in these plants and on the breadth of the trophic niche in a tropical sand dune fragment over the year. The field work was carried out monthly in Baixio (Bahia, Brazil; Northern Coast Environmental Protection Area) from April 2008 to March 2009, over two consecutive days, from 06:30?AM to 05:00?PM. The medium-large body sized Xylocopa (Neoxylocopa) cearensis Ducke and Xylocopa (Schonnherria) subcyanea Pérez were noticeable for their frequency, constancy on the flowers and sharing of plant species. Xylocopa spp. visited plants with flowers of different shapes, colors, inflorescence arrangement and syndromes. However, their resource collections were mainly concentrated on Cuphea brachiata, Waltheria cinerascens, Croton sellowii and Chamaecrista ramosa, which may be considered key species for Xylocopa spp. maintenance in coastal sand dune and restinga environments in Northeast Brazil.     

研究历史植被变化的新途径

介绍了近２０年来在国外生态学界应用较多的研究植被动态的一条比较新的途径：δ＾１３Ｃ分析法,并通过一些典型的应用例子,对这种方法的优缺点进行了概括分析。     

Ecohydrology of Dry Regions: Storage versus Pulse Soil Water Dynamics

Although arid and semiarid regions are defined by low precipitation, the seasonal timing of temperature and precipitation can influence net primary production and plant functional type composition. The importance of precipitation seasonality is evident in semiarid areas of the western U.S., which comprise the Intermountain (IM) zone, a region that receives important winter precipitation and is dominated by woody plants and the Great Plains (GP), a region that receives primarily summer precipitation and is dominated by perennial grasses. Although these general relationships are well recognized, specific differences in water cycling between these regions have not been well characterized. We used a daily time step soil water simulation model and twenty sites from each region to analyze differences in soil water dynamics and ecosystem water balance. IM soil water patterns are characterized by storage of water during fall, winter, and spring resulting in relatively reliable available water during spring and early summer, particularly in deep soil layers. By contrast, GP soil water patterns are driven by pulse precipitation events during the warm season, resulting in fluctuating water availability in all soil layers. These contrasting patterns of soil water—storage versus pulse dynamics—explain important differences between the two regions. Notably, the storage dynamics of the IN sites increases water availability in deep soil layers, favoring the deeper rooted woody plants in that region, whereas the pulse dynamics of the Great Plains sites provide water primarily in surface layers, favoring the shallow-rooted grasses in that region. In addition, because water received when plants are either not active or only partially so is more vulnerable to evaporation and sublimation than water delivered during the growing season, IM ecosystems use a smaller fraction of precipitation for transpiration (47%) than GP ecosystems (49%). Recognizing the pulse-storage dichotomy in soil water regimes between the IM and GP regions may be useful for understanding the potential influence of climate changes on soil water patterns and resulting dominant plant functional groups in both regions.     

A Vapour-pressure Instrument for the Measurement of Leaf and Soil Water Potential

A previously described vapour-pressure osmometer has been adaptedfor the measurement of depression of water potential (DPD) insamples of leaf tissue and soil. A pile of about 30 leaf annuliare contained in an air-tight metal capsule through the lidof which passes a pipette. A drop of water hangs from the tipof the pipette into the central cavity of the pile of leaf annuli.The drop is drawn bath into the pipette from time to time andthe rate of evaporation from it measured directly. The water-potentialdepression is linearly related to the rate of evaporation. Theinstrument is calibrated empincally. A period of a hours isnecessary to attain vapour equilibnum before readings are takenThe readings then occupy less than 30 minutes. For the calibration data, using filter paper moistened withNaCl solution, the 5 percent inverse tolerance limits for singlereadings of water-potential depression were ±0.26 atm.With leaf material the errors appear to be about the same, althoughrelease of cell sap at the cut edges of the disks may causean overestimation by as much as 0.5 atm. with turgid leaf material.Lining the capsule with leaf strips gave erroneous results.Using a suitably modified capsule the instrument can be usedfor soil-water potential measurement. Equilibration takes only30 minutes so that a reading can be completed within an hour.     

Comparative Field Summer Stress of Three Tree Species Co-occurring in Mediterranean Coastal Dunes

Chlorophyll a fluorescence, water potential (_s), and root system of Juniperus oxycedrus ssp. macrocarpa, Juniperus phoenicea ssp. turbinata, and Pinus pinea were studied in Mediterranean coastal dunes of SW Spain during summer drought and after fall rains in 1999, the driest year in the 90's. A strong and reversible depression in the photochemical efficiency of photosystem 2 of the three species was recorded, which happened concomitantly with the diurnal increase and decrease in radiation. J. phoenicea, with superficial root system, was the most affected species by summer drought. It showed high rates of down-regulation of photosynthesis by photoinhibition and positive correlation between _s and F_v/F_p, with _s lower than -7 MPa. However, it tolerated this high stress, showing a fast recovery of its physiological state after fall rains. On the other hand, J. oxycedrus and P. pinea, both with deep root systems, kept their _s values up to -3 MPa, showing lower stress during summer drought. On the other hand, J. oxycedrus and J. phoenicea were more sensible to changes in edaphic water content than P. pinea. These specific responses to summer drought would be determined by their root distributions and stomatal control of transpiration, conditioning the efficiency in getting and using the available water resources. Ecophysiological responses indicate that these species are well-adapted to long periods of drought in Mediterranean climate areas, developing different strategies: J. phoenicea tolerates high stress with a fast recovery after fall rains, while J. oxycedrus and P. pinea are less affected by summer drought since their deep root systems would allow them to reach deep water resources.     

Effect of Soil Water Potential on Survival of Meloidogyne javanica in Fallow Soil

A natural infestation of Meloidogyne javanica in an aggregated Oxisol declined at an exponential rate when aliquots of the soil were stored for 72 days in polyethylene bags at various soil water potentials (Ψ). Time periods required for reduction in soil infestations by 50% were 2.7, 4.9, 110, 10, and 2.6 days at Ψ of -0.16, -0.30, -1.1, -15, and -92 bars, respectively. In the wetter soils, at Ψ of -0.16, -0.30, and -1.1 bars, the predominant stage recovered was the second-stage larva. In the drier soils, at Ψ of -15 and -92 bars, both eggs and larvae were recovered with neither stage predominating. Incidence of coiled larvae was inversely related to the Ψ value of the soil, a greater incidence occurring in the drier soils. After 15-32 days, percentages of coiled larvae were 13, 27, 55, 65, and 88% in soil at Ψ of -0.17, -0.60, -1.9, -15, and -82 bars, respectively.     

Yellow Bush Lupine Invasion in Northern California Coastal Dunes II. Mechanical Restoration Techniques

Invasion of coastal dunes by Lupinus arboreus (yellow bush lupine) results in soil enrichment and displacement of native plants. Restoration by means of heavy equipment was tested as an alternative to costly manual techniques in a heavily invaded area of relatively flat terrain. Two experiments were conducted in consecutive years at the Eureka Dunes Protected Area in Humboldt County, California. Each experiment consisted of three plots subjected to one of three primary treatments: removal of vegetation with a brush rake, removal of vegetation with a brush rake followed by removal of litter and duff with a plough blade or bucket, and removal of vegetation with tractor-pulled chokers. Plots were then subdivided into smaller secondary treatment plots subjected to one of two treatments or a control. Secondary treatments consisted of weedmat placed for a 1- or 2-year duration. The goal of the treatments was to remove and prevent reestablishment of nonnative vegetation, including but not limited to bush lupine; success was measured by percent cover of recolonizing vegetation 1.5 years after treatment ended. In the first experiment, primary treatment (vegetation removal) but not secondary (prevention of reestablishment) resulted in significant differences in cover by the end of the experiment. The brush rake and plough blade treatment was most successful at preventing reestablishment of nonnative vegetation. In the second experiment, secondary treatment (prevention of reestablishment) but not primary (removal of vegetation) resulted in significant differences at the end of the experiment. Re-invasion increased with the amount of time subplots were left uncovered. The difference in the results of the first and second experiments was attributed to variation in rainfall and, to a lesser extent, to localized variation in species composition. Results suggest that mechanical restoration by means of combination of the brush rake and plough blade primary treatment with the 2-year weedmat secondary treatment would be most successful in meeting the dual goals of removal of nonnative vegetation and prevention of its reestablishment.     

Coastline Dune Vegetation Dynamics: Evidence of No Stability

Coastal dune ecosystems are subjected to severe stress and disturbance factors that are particularly high in the beach-foredune environment and generally decrease with distance from the coast. The present study aimed to link plant species composition of coastal foredunes with the physical dynamical processes of the coastline in central Italy. A random hierarchical sampling design, based on two spatial scales (quadrant and parcel), was applied to estimate the variation in plant community composition. Permutational multivariate analysis of variance (PERMANOVA) revealed an approximately similar amount of variation with respect to both the coastal dynamic class and the parcel level. In addition, principal coordinate analyses showed that three taxa ?C Ammophila arenaria, Elymus farctus and Otanthus maritimus subsp. maritimus ?C were mainly linked to the coastal dynamics: A. arenaria increases its abundance where erosion of the shoreline is very high, while E. farctus and O. maritimus are more abundant in the prograding coast. Finally, similarity percentages analysis (SIMPER) highlighted that where the erosive processes were strongest, the number of the species contributing to the total similarity was the highest. This is likely to indicate instability and a strong disturbance of plant communities that results in an unstable equilibrium. These findings have important implications for management and conservation actions.     

Persistence of Coastal Vegetation in Supratidal Zones of Northern China

Coastal vegetation comprises a number of coastal specialists and terrestrial generalists. It remains unclear how they persist on disturbed and undisturbed coastal conditions. We tested the hypothesis that coastal specialists may be superior to terrestrial generalists on supratidal zones of coasts, but their superiority can be influenced by human disturbances. Eight separate sandy coasts of the Shandong Peninsula were sampled, representing for disturbed and undisturbed sandy coasts. Plants growing on their supratidal zones were surveyed. On this basis, we compared the relative dominances, niche widths, and commonness of all species, and also analyzed species diversities of the coasts. Coastal specialists were found to be more common and widespread on supratidal zones of the sandy coasts than terrestrial generalists haphazardly invading from hinterlands. Coastal specialists exhibited lower Sørensen dissimilarities than terrestrial generalists among the coasts. Tourist trampling seemed more detrimental than pond fishery to coastal vegetation. Relative to terrestrial generalists, coastal specialists responded to human disturbances more deterministically, with steady decreases in species diversities. These evidences verify that coastal specialists are intrinsically superior to terrestrial generalists on supratidal zones of coasts, especially of undisturbed coasts, because their dispersal among coasts adapts well to local storm surge regime. They also validate that human disturbances can depress the superiority of coastal specialists, partly by inducing invasion of terrestrial generalists.     

Water Confined in Cylindrical Pores: A Molecular Dynamics Study

Molecular dynamics simulations of water confined in two hydrophilic cylindrical pores—PH and PL—that differ in their silanol surface concentration (7.6 and 3.0 nm⁻², respectively) have been performed at 300 K. A strong interaction of interfacial water molecules with the pore was systematically found and gives rise to a layering effect, a significant distortion of both the hydrogen bond network (HBN) and the tetrahedral structure of these water molecules, and a corresponding subdiffusive mean square displacement along the main axis of the pores. By contrast, water molecules in the inner part of both pores were found to behave similarly to bulk water. The HBN and the tetrahedral configuration of water were more gradually distorted in the PL pore given the larger heterogeneity and rugosity of the surface, and the number of water–pore hydrogen bonds did not scale linearly with the silanol surface concentration of the pores, in part because of the close proximity between silanols in the PH pore. With the PL pore, the dynamic slowing down of water was found consistent with the experiment, suggesting that it provides a better model for the cylindrical MCM-41 mesopores. The structural and dynamical properties of water vary little with the silica force field.     

A Robust Water Potential Parameterisation

Abstract

We compare molecular dynamics simulation results for the properties of liquid water predicted by four novel water potential models. These models are designed as a combination of parameters taken from the dedicated but brittle TIP3P water potential, and the more flexible but less accurate parameterisations such as the Dreiding and Universal force fields. We find that a hybrid of Dreiding and TIP3P delivers the best results, yielding a density, diffusion coefficient and radial distribution function in good agreement with experiment, performing in some respects even better than the dedicated reference TIP3P model. Another Dreiding based force field predicts semi-quantitative results for the water structure and dynamics while the Universal force field based models are incapable of simulating a condensed phase of water at all, continuing to expand indefinitely. These observations are useful for selecting and designing robust water force field parameterisations that can be used for general simulation purposes.     

Occurrence of Inverted Water Potential Gradients between Soil and Bean Roots

Measurements with a pressure chamber were made of the xylem water potential of leaves, shoots and roots from bean plants (Pkaseolus vulgaris L. cv. Processor) grown with a 12 hour dark period and natural or artificial light conditions during the day. The water potentials were measured at the end of a dark period and during the light period. Measurements taken at the end of the dark period indicated normal potential gradients within the soil/plant system (leaf < shoot < root < soil), when the matric potential of soil water was relatively high (above ?0.02 bar), and the gradients then also remained normal during the day (natural light). When the soil water potential was ?1 bar or lower in the morning, however, the root xylem water potential was higher than the soil water potential; at very low soil water potentials (< ?4 bar) it remained higher during most of the day. In this case also leaf and shoot xylem water potentials were higher than the soil water potential in the early morning, although decreasing rapidly in daylight. Under artificial light, both leaf and root water potentials were higher than the soil water potential throughout the whole diurnal cycle when the latter potential was below ?4 bar. From measurements of stomatal diffusion resistance, transpiration, relative water content of leaves and of changes in the matric potential of soil water, it was concluded that when the matric potential of soil water was low, water could be taken up by the plant against a water potential gradient. Because leaf xylem water potential was always lower than root xylem water potential, the mechanism involved in the inversion of water potential gradient must be localized in the roots, and probably related to ion uptake. Symbols and abbreviations used in the text: Ψ: Plant water potential (thermocouple psychrometer); Ψx: Xylem water potential (pressure chamber); Ψs: Osmotic potential of xylem sap; Ψm: Matric potential of soil water; RWC: Relative water content.